135342012
Epigenetic gold occurrence in a Paleoproterozoic meta-evaporitic sequence in the Rompas-Rajapalot Au system, Peräpohja belt, northern Finland
The Rompas-Rajapalot gold prospect is located in the northern part of the Paleoproterozoic Peräpohja belt. It covers an area of at least 10 x 10 km and comprises various styles of gold mineralization ranging from localized high-grade Au pockets in uraniniteand pyrobitumen-bearing calcsilicate-carbonate-quartz veins in mafic metavolcanic rocks (Rompas area) to disseminated gold grains in Fe-Mg-rich metasediments and quartz-tourmaline-sulfide-native gold veins (Palokas area). This study deals with the petrography and mineral chemistry of the gold mineralization at Palokas, which occurs in the eastern part of the Rompas-Rajapalot prospect. Major and trace element data and fluid inclusion characteristics of tourmaline are used to evaluate the origin and the pressure-temperature-fluid composition parameters of hydrothermal fluids. Whole-rock geochemical analyses are utilized to evaluate the nature of the protolith of the host rocks. Gold occurs in a native form in at least two different textural settings: 1) single, relatively coarse grains disseminated among the rock-forming silicates in cordierite-orthoamphibole rocks and 2) smaller grains occurring in fractures of tourmaline in quartz-sulfide-tourmaline breccias and in fractures of chloritized cordierite-orthoamphibole rocks adjacent to the tourmaline-rich breccias. Fracture-related gold is associated with Bi-Se-S-bearing tellurides, native Bi, molybdenite, chalcopyrite, and pyrrhotite. Coarser-grained disseminated gold were not found to be clearly associated with sulfides nor any fractures. Statistical correlations show that the Au concentration correlates strongly with Te, Cu, Co, Se, Bi, Mo, and Ag (ρ = 0.730–0.619) whereas Au correlates moderately with As, Fe, W (ρ = 0.523–0.511) and to a lesser extent with U, Pb, and Ni (ρ = 0.492–0.407). Gold has the strongest negative correlations with Sr and Ca. The chondrite-normalized REE patterns of tourmaline from the Au-mineralized rocks (both vein type and host-rock tourmaline) and the lateto post-orogenic granite partly overlap and show similar LREE-enriched trends, with the enrichment being lower in tourmaline from the granite. Fluid inclusion studies from tourmaline in gold-bearing Bulletin of the Geological Society of Finland, Vol. 90, 2018, pp 69–108, https://doi.org/10.17741/bgsf/90.1.004 70 J.-P. Ranta, F. Molnár, E. Hanski and N. Cook quartz-tourmaline-sulfide veins indicate that the veins were formed from H2O-Na2O-CO2CH4-(H2S) fluids in a boiling system under pressure conditions ranging from lithostatic to hydrostatic, with the depth being ~5 km and the temperature ~300°C. The properties of the ore-forming fluids support the genetic link between the lateto post-orogenic granitoid magmatism at ~1.78 Ga and the formation of the fracture-hosted gold mineralization, suggested based on earlier studies (including Re-Os-molybdenite age and boron isotope data from tourmaline). Based on the whole-rock geochemistry, it is highly plausible that the cordierite-orthoamphibole rocks and interlayered calcsilicatealbite rocks are part of a basin-wide lacustrine, at least partly evaporitic, sequence. The protolith of the cordierite-orthoamphibole rock was most probably a lake-margin sedimentary pile with abundant Mg-rich clays.
131409509
Association of gold with uraninite and pyrobitumen in the metavolcanic rock hosted hydrothermal Au-U mineralisation at Rompas, Peräpohja Schist Belt, northern Finland
The Peräpohja Schist Belt comprises a supracrustal sequence of quartzites, mafic volcanics and volcaniclastics, carbonate rocks, black shales, mica schists and greywackes which were deposited from ca. 2.44 to ~1.91 Ga, during the rifting of the Archaean basement in the eastern part of the Fennoscandian shield. Metamorphism and multiple folding of the basin fill took place during the Svecofennian orogeny (1.9–1.8 Ga) followed by intrusions of late-orogenic (1.84–1.80 Ga) and post-orogenic granitoids (1.79–1.76 Ga). The Rompas Au-U mineralisation is hosted by deformed calcsilicate veins in mafic volcanic rocks and locally contains very high grade (>10,000 g/t Au) gold pockets with strict spatial association of gold minerals to uraninite and pyrobitumen. Chemical ages from the unaltered domains in the structure of uraninite indicate a 1.95–1.90 Ga age for the deposition of the primary, high temperature (e.g. U/Th < 100 in uraninite) hydrothermal uranium mineralisation. These data are in agreement with the results of previous U-Pb dating of uraninite by SIMS. Textural evidence suggests that metamorphic recrystallisation of the uraninite-bearing quartz-dolomite veins into calcsilicate mineral assemblages during the Svecofennian orogeny (1.9–1.8 Ga) was followed by a hydrocarbon-bearing fluid flow event and radiolytic polymerisation of hydrocarbons around grains of uraninite. Gold precipitated during a subsequent hydrothermal process in the fractures of uraninite, as well as in the cracks and on the botryoidal surfaces of uraninite-pyrobitumen nodules. Remobilisation and redeposition of uranium by these hydrothermal events produced secondary uraninite grains with chemical ages between 1.85 and 1.65 Ga. Native gold is associated with galena, altaite, hunchunite, nickeline and rare cobaltite, Pb-bearing maldonite, pyrite, pyrrhotite, chalcopyrite, molybdenite and titanite. Raman spectra show disordered structure of undeformed pyrobitumen nodules in contrast with the well-ordered graphite in calcsilicate veins. Mean random reflectance data for pyrobitumen indicate 270–340 °C maximum temperature of thermal maturation—this temperature range is also considered as the temperature of gold deposition. Results of multiple sulphur isotope analyses of organic material-, pyrite- and acid-volatile-bound sulphur show distinct ranges of δ34S values for SORG and SCRS in uraninite-pyrobitumen (from −6.99 to −3.55‰ and from −10.02 to −4.41‰, respectively) and uraninite-pyrobitumen-native gold mineral associations (from +1.36 to +6.87‰ and from +0.42 to +9.7‰, respectively). Δ33S data indicate local occurrence of nonmass-dependent sulphur isotope fractionation owing to interaction of fluids with organic material. Concentration of lead in uraninite is depleted along the gold mineral filled fractures whereas the uranogenic lead isotope contents of galena, altaite and hunchuite deposited in the same fractures are extremely high, suggesting that the dominant source of lead for the crystallisation of these minerals was the radiogenic lead content of uraninite. Taking into account this source of radiogenic lead, the calculated Pb-Pb model ages for the lead minerals are between 1.75 and 1.70 Ga. Sulphur and tellurium removal from the fluid by reaction with radiogenic lead released by uraninite appears to be an important mechanism in the strongly localised deposition of gold minerals. Scavenging of sulphur by pyrobitumen nodules from gold transporting fluids was an additional process triggering precipitation of gold. Carbon particles and organic functional groups in pyrobitumen probably acted as nucleation and adsorption centres for gold minerals.
132167467
Source of boron in the Palokas gold deposit, northern Finland: evidence from boron isotopes and major element composition of tourmaline
The recently discovered Palokas gold deposit is part of the larger Rompas-Rajapalot gold-mineralized system located in the Paleoproterozoic Peräpohja Belt, northern Finland. Tourmaline is an important gangue mineral in the Palokas gold mineralization. It occurs as tourmalinite veins and as tourmaline crystals in sulfide-rich metasomatized gold-bearing rocks. In order to understand the origin of tourmaline in the gold-mineralized rocks, we have investigated the major element chemistry and boron isotope composition of tourmaline from three areas: (1) the Palokas gold mineralization, (2) a pegmatitic tourmaline granite, and (3) the evaporitic Petäjäskoski Formation. Based on textural evidence, tourmaline in gold mineralization is divided into two different types. Type 1 is located within the host rock and is cut by rock-forming anthophyllite crystals. Type 2 occurs in late veins and/or breccia zones consisting of approximately 80% tourmaline and 20% sulfides, commonly adjacent to quartz veins. All the studied tourmaline samples belong to the alkali-group tourmaline and can be classified as dravite and schorl. The δ11B values of the three localities lie in the same range, from 0 to −4‰. Tourmaline from the Au mineralization and from the Petäjäskoski Formation has similar compositional trends. Mg is the major substituent for Al; inferred low Fe3+/Fe2+ ratios and Na values (<0.8 atoms per formula unit (apfu)) of all tourmaline samples suggest that they precipitated from reduced, low-salinity fluids. Based on the similar chemical and boron isotope composition and the Re–Os age of molybdenite related to the tourmaline-sulfide-quartz veins, we propose that the tourmaline-forming process is a result of a single magmatic-hydrothermal event related to the extensive granite magmatism at around 1.79–1.77 Ga. Tourmaline was crystallized throughout the hydrothermal process, which resulted in the paragenetic variation between type 1 and type 2. The close association of tourmaline and gold suggests that the gold precipitated from the same boron-rich source as tourmaline.
132092540
Overview on Gold Deposits in Finland
Genetic types of gold deposits identified in Finland include Au-rich volcanogenic massive sulfide (VMS), metamorphosed high-sulfidation epithermal, porphyry gold-copper, orogenic gold, placer, and paleoplacer deposits. There also are gold deposits and occurrences whose genetic type is ambiguous, such as the Au-Co ± Cu occurrences in the Kuusamo schist belt, the Au-U occurrences in the Perapohja schist belt, and the Pahtavaara gold deposit. In contrast to many other Precambrian shield areas, most of the known gold occurrences and resources in Finland are hosted by Paleoproterozoic sequences. All gold-mineralized environments in Finland, except the possible Archean epithermal deposits and the recent placers, can be related to supercontinent evolution of the region between ∼2.75–1.77 Ga. Most of the gold deposits were formed during the main stages of crustal growth globally, at ∼2.72–2.64 Ga and 1.91–1.77 Ga, during the Neoarchean and Svecofennian orogenies, respectively.
132910444
Optimizing a Knowledge-driven Prospectivity Model for Gold Deposits Within Peräpohja Belt, Northern Finland
This paper combines knowledge- and data-driven prospectivity mapping approaches by using the receiver operating characteristics (ROC) spatial statistical technique to optimize the process of rescaling input datasets and the process of data integration when using a fuzzy logic prospectivity mapping method. The methodology is tested in an active mineral exploration terrain within the Paleoproterozoic Peräpohja Belt (PB) in the Northern Fennoscandian Shield, Finland. The PB comprises a greenschist to amphibolite facies, complexly deformed supracrustal sequence of variable quartzites, mafic volcanic rocks and volcaniclastic rocks, carbonate rocks, black shales, mica schists and graywackes. These formations were deposited on Archean basement and 2.44 Ga layered intrusions, during the multiple rifting of the Archean basement (2.44–1.92 Ga). Younger intrusive units in the PB comprise 2.20–2.13 Ga gabbroic sills or dikes and 1.98 Ga A-type granites. Metamorphism and complex deformation of the PB took place during the Svecofennian orogeny (1.9–1.8 Ga) and were followed by intrusions of post-orogenic granitoids (1.81–1.77 Ga). The recent mineral exploration activities have indicated several gold-bearing mineral occurrences within the PB. The Rompas Au-U mineralization is hosted within deformed and metamorphosed calc-silicate veins enclosed within mafic volcanic rocks and contains uranium-bearing zones without gold and very high-grade (>10,000 g/t Au) gold pockets with uraninite and uraninite-pyrobitumen nodules. In the vicinity of the Rompas, a magnesium skarn hosted disseminated-stockwork gold mineralization was also recognized at the Palokas-Rajapalot prospect. The exploration criteria translated into a fuzzy logic prospectivity model included data derived from regional till geochemistry (Fe, Cu, Co, Ni, Au, Te, K), high-resolution airborne geophysics (magnetic field total intensity, electromagnetic, gamma radiation), ground gravity and regional bedrock map (structures). The current exploration licenses and exploration drilling sites for gold were used to validate the knowledge-driven mineral prospectivity model.
46909821
Orogenic gold deposits : A proposed classification in the context of their crustal distribution and relationship to other gold deposit types
Abstract The so-called `mesothermal' gold deposits are associated with regionally metamorphosed terranes of all ages. Ores were formed during compressional to transpressional deformation processes at convergent plate margins in accretionary and collisional orogens. In both types of orogen, hydrated marine sedimentary and volcanic rocks have been added to continental margins during tens to some 100 million years of collision. Subduction-related thermal events, episodically raising geothermal gradients within the hydrated accretionary sequences, initiate and drive long-distance hydrothermal fluid migration. The resulting gold-bearing quartz veins are emplaced over a unique depth range for hydrothermal ore deposits, with gold deposition from 15–20 km to the near surface environment. On the basis of this broad depth range of formation, the term `mesothermal' is not applicable to this deposit type as a whole. Instead, the unique temporal and spatial association of this deposit type with orogeny means that the vein systems are best termed orogenic gold deposits . Most ores are post-orogenic with respect to tectonism of their immediate host rocks, but are simultaneously syn-orogenic with respect to ongoing deep-crustal, subduction-related thermal processes and the prefix orogenic satisfies both these conditions. On the basis of their depth of formation, the orogenic deposits are best subdivided into epizonal ( mesozonal (6–12 km) and hypozonal (>12 km) classes.
140621668
Orogenic gold and geologic time: a global synthesis
AbstractOrogenic gold deposits have formed over more than 3 billion years of Earth’s history, episodically during the MiddleArchean to younger Precambrian, and continuously throughout the Phanerozoic. This class of gold deposit is characteristi-cally associated with deformed and metamorphosed mid-crustal blocks, particularly in spatial association with major crustalstructures. A consistent spatial and temporal association with granitoids of a variety of compositions indicates that melts andfluids were both inherent products of thermal events during orogenesis. Including placer accumulations, which arecommonly intimately associated with this mineral deposit type, recognized production and resources from economicPhanerozoic orogenic-gold deposits are estimated at just over one billion ounces gold. Exclusive of the still-controversialWitwatersrand ores, known Precambrian gold concentrations are about half this amount.The recent increased applicability of global paleo-reconstructions, coupled with improved geochronology from most ofthe world’s major gold camps, allows for an improved understanding of the distribution pattern of orogenic gold in spaceand time. There are few well-preserved blocks of Middle Archean mid-crustal rocks with gold-favorable, high-strain shearzones in generally low-strain belts. The exception is the Kaapvaal craton where a number of orogenic gold deposits arescattered through the Barberton greenstone belt. A few )3.0 Ga crustal fragments also contain smaller gold systems in theUkrainian shield and the Pilbara craton. If the placer model is correct for the Witwatersrand goldfields, then it is possiblethat an exceptional Middle Archean orogenic-gold lode-system existed in the Kaapvaal craton at one time. The latter half ofthe Late Archean ca. 2.8–2.55 Ga was an extremely favorable period for orogenic gold-vein formation, and resulting oresŽ.preserved in mid-crustal rocks contain a high percentage of the world’s gold resource. Preserved major goldfields occur ingreenstone belts of the Yilgarn craton e.g., Kalgoorlie , Superior province e.g., Timmins , Dharwar craton e.g., Kolar ,Ž. Ž . Ž.Zimbabwe craton e.g., Kwekwe , Slave craton e.g., Yellowknife , Sao Francisco craton e.g., Quadrilatero Ferrifero , andŽ. Ž . Ž .Tanzania craton e.g., Bulyanhulu , with smaller deposits exposed in the Wyoming craton and Fennoscandian shield. SomeŽ.workers also suggest that the Witwatersrand ores were formed from hydrothermal fluids in this period.The third global episode of orogenic gold-vein formation occurred at ca. 2.1–1.8 Ga, as supracrustal sedimentary rocksequences became as significant hosts as greenstones for the gold ores. Greenstone–sedimentary rock sequences nowexposed in interior Australia, northwestern Africarnorthern South America, Svecofennia, and the Canadian shield were the
126404634
Orogenic gold: Common or evolving fluid and metal sources through time
Abstract Orogenic gold deposits of all ages, from Paleoarchean to Tertiary, show consistency in chemical composition. They are the products of aqueous-carbonic fluids, with typically 5–20 mol% CO 2 , although unmixing during extreme pressure fluctuation can lead to entrapment of much more CO 2 -rich fluid inclusions in some cases. Ore fluids are typically characterized by significant concentrations of CH 4 and/or N 2 , common estimates of 0.01–0.36 mol% H 2 S, a near-neutral pH of 5.5, and salinities of 3–7 wt.% NaCl equiv., with Na > K > > Ca,Mg. This fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation. Nevertheless, there are broad ranges in more robust fluid-inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths. Much of the reported isotopic and noble gas data is inconsistent between deposits, leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces. Fluid stable isotope values are commonly characterized by the following ranges: (1) δ 18 O for Precambrian ores of + 6 to + 11‰ and for Phanerozoic ores of + 7 to + 13‰; (2) δD and δ 34 S values that are extremely variable; (3) δ 13 C values that range from − 11 to − 2‰; and (4) δ 15 N of + 10 to + 24‰ for the Neoarchean, + 6.5 to + 12‰ for the Paleoproterozoic, and + 1.5 to + 10‰ for the Phanerozoic. Secular variations in large-scale Earth processes appear to best explain some of the broad ranges in the O, S, and N data. Fluid:rock interaction, particularly in ore trap areas, may cause important local shifts in the O, S, and C ratios. The extreme variations in δD mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations, many which are not related to ore formation. Radiogenic isotopes, such as those of Pb, Sr, Nd, Sm, and Os, measured on hydrothermal minerals are even more difficult to interpret for defining metal source, particularly as the low-salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition. Noble gas and halogen data are equally equivocal. Fluid exsolution from granitoids emplaced into the upper and middle crust, metamorphism of the crust, or fluids entering trans-crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation, as the abundant geochemical data are equivocal. However, geological and geochronological data weigh heavily against a magmatic-hydrothermal model in the upper to middle crust. There is no universal temporal association between orogenic gold and magmatism, and where there is an overlap in age, there is no specific type of magmatism consistently associated with gold formation, nor element zonation around any specific pluton. A crustal metamorphic model for fluid and metal sources is very consistent with geological, geochronological, and geochemical data, although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins. These can include crustal thickening and radiogenic heating, slab rollback and heating during crustal extension, or subduction of a spreading ridge heating the base of an accretionary prism. In rare examples where Phanerozoic orogenic gold deposits are hosted in Precambrian high-grade metamorphic terranes, fluids and metals must, however, enter a transcrustal fault system from a sub-crustal source. This could either be a devolatilized, subducted, relatively flat, perhaps stalled slab and its overlying sediment, or the corner of the fertilized mantle wedge that releases a fluid during a thermal event.
128170256
The Rompas Prospect, Peräpohja Schist Belt, Northern Finland
The Rompas prospect in the Ylitornio municipality is one of the most exciting recent discoveries in Finland. Spectacular bonanza grades, with abundant visible gold in association with uraninite, are hosted within deformed dolomite–calc-silicate–quartz veins in metabasalts along a 6-km-long ridgeline. The highest gold grades are associated with texturally late, sulfur-bearing pyrobitumen inferred to have formed at least 150° cooler than peak metamorphic conditions, approximately 150 million years after the crystallization of the vein-hosted uraninite.
128820642
Sm−Nd and Pb isotopic study of mafic rocks associated with early Proterozoic continental rifting: the Peröpohja schist belt in northern Finland
The Peräpohja schist belt in northern Finland rests unconformably on Archaean granitoids, and marks the early stages of Proterozoic crustal evolution in the Fennoscandian (Baltic) shield. 2440 Ma old layered mafic intrusions predate the supracrustal , and ca. 2200 Ma old sills of the gabbro-wehrlite association intrude the lowest quartzites and volcanics (Runkaus) of the sequence. The Sm-Nd mineral isochron of the Penikat layered intrusion gives an age of 2410±64 Ma. The initial ɛNd-values of the Penikat intrusion (ɛNd(2440) = −1.6) and the Runkausvaara sill (ɛNd(2200) ≈ 0) suggest that these mafic magmas were contaminated by older crustal material. The Sm-Nd and Pb isotopic results on the 2.44–2.2 Ga old Runkaus volcanics indicate mobility of Pb, fractionation of Sm/Nd during “late” greenschist facies metamorphism, and crustal contamination. The Pb-Pb data provide an age of 1972±80 Ma with a high initial 207Pb/204Pb ratio (μ1 = 8.49), while scattered Sm-Nd data result in an imprecise age of 2330±180 Ma, with an initial ɛNd-value of about zero. Secondary titanite gives an U-Pb age of ca. 2250 Ma. The Jouttiaapa basalts, in contrast, ascended from the mantle without interaction with older crust. These LREE depleted tholeiites mark a break in continental sedimentation, and yield a Sm-Nd age of 2090±70 Ma. Their initial ɛNd = + 4.2 ±0.5 implies that the subcontinental early Proterozoic mantle had been depleted in LREE for a long period of time. The first lava flows are strongly depleted in LREE, suggesting that their source was significantly more depleted than the source of mid-ocean ridge basalts today.
126754052
U–Pb and Sm–Nd isotopic constraints on the evolution of the Paleoproterozoic Peräpohja Belt, northern Finland
Ten localities, representing metasedimentary and igneous rocks from the northern part of the Paleoproterozoic Perapohja Belt, Fennoscandian Shield, were sampled for zircon U–Pb dating by LA-MC-ICP-MS and Sm–Nd analysis by TIMS. The results revealed two phases of granitic magmatism that took place at 1.99 Ga and 1.79–1.77 Ga. The former age (1989 ± 6 Ma) was given by the Kierovaara granite pluton representing the first example of felsic plutonism of this age in northern Finland, though A-type felsic volcanic rocks of the same age have previously been recognized. The strongly negative initial ɛnd values obtained for granitoids, ranging from −3.9 to −11.4, are compatible with the previous Sm–Nd data, indicating the presence of Archean basement in northern Finland at the time of granite emplacement. A garnet-whole rock Sm–Nd age of 1752 ± 14 Ma was measured for a mica gneiss, constraining the time of metamorphic cooling down to the closure temperature of garnet. 
 
Detrital zircon grains from quartzite samples belonging to the Jatulian Palokivalo Formation and a stratigraphically unassigned lithodemic unit, the Mellajoki Suite, yielded solely Archean zircon populations. In contrast, our sample from the Martimo Formation representing Kalevian sedimentation revealed a 66% Paleoproterozoic detrital zircon component with the youngest zircon grains showing an age of ca. 1.91 Ga. The new age data allow us to constrain the depositional time of this sedimentary unit between ca. 1.91 and 1.88 Ga, the latter being the time of the synorogenic plutonism. Our geochronological data coupled with recent data from other Karelian belts suggest that the depositional time period for rocks assigned to the Kaleva spans ca. 150 Ma and the simple division of these rocks into two non-coeval units, the Lower and Upper Kaleva, has become problematic.
46194680
Closure temperatures of the Sm—Nd system in metamorphic garnets
Abstract Garnet-whole rock and garnet-mineral isochrons were determined on granulite facies gneisses and amphibolites from the Archean Pikwitonei Granulite Domain of the Superior Province, and the Proterozoic Central Gneiss Belt and Adirondack Highlands of the Grenville orogen. The Sm—Nd ages obtained from Archean garnets 0.1–0.5 cm in length are 30–110 Ma younger than the U—Pb ages obtained on the same garnets and also younger than the time of the last regional metamorphism, as determined by the growth ages of the youngest metamorphic garnets and zircons. Similarly, the Sm—Nd ages obtained from Proterozoic garnets with a diameter of 0.1–5 cm are younger than the time of the last regional metamorphism and similar or younger than cooling ages obtained on sphenes from the same sample or from the same geologic setting. Only the core of a garnet with a diameter of ca. 30 cm and without abundant inclusions may record the time of garnet growth. Comparison of the Sm—Nd ages with other geochronologic data and temperature estimates leads to the conclusion that the closure temperature for the Sm—Nd system in garnets analyzed in this study is ca. 600 ± 30°C. Only garnets with radii much larger than 5 cm may record Sm—Nd growth ages in upper amphibolite facies rocks from slowly cooled terranes. Garnets from higher grade terranes yield cooling ages that define the retrograde history of metamorphic terranes.
131091508
Retention of isotopic memory in garnets partially broken down during an overprinting granulite-facies metamorphism: Implications for the Sm-Nd closure temperature
Garnets from an early granulite event that have partially broken down to orthopyroxene-bearing assemblages during a second regional granulite-facies metamorphism were used to test the closure temperature of the Sm-Nd system in garnet. The garnets from Sostrene Island, Prydz Bay, Antarctica, retain a memory of both the physical conditions of their formation and their Sm-Nd age, notwithstanding an effective diameter for diffusion of only 1 mm. Because only minor isotopic resetting of these garnets has taken place during the overprinting granulite event, the closure temperature for the Sm-Nd system in garnet in the present case must be >700–750 ° C. The results show that chronological constraints are essential for the unambiguous interpretation of reaction textures in terms of thermotectonic evolution. The garnet Sm-Nd dating method is particularly well suited for this purpose.
56509177
On the ages of some granitoid and schist complexes in northern Finland
Contact observations and most radiometric datings indicate that the large complex of granitoid rocks and migmatites in Central Lapland is late Svecokarelian (Proterozoic) in age. The geological setting of the complex suggests, however, that it previously formed the basement for the Proterozoic sedimentation and a source of some of the sedimentary material. About 1800 Ma ago it apparently underwent partial anatexis, especially near the Proterozoic schist belts. The concept that some parts of the schist areas of Kuusamo, Salla and Central Lapland are of Archean age is disputable. The Salla schist area, which some authors maintain is an Archean greenstone belt, is obviously Proterozoic in age. U-Pb ages reported for zircons, titanites and monazites from granitoid rocks from six localities in Kuusamo, Salla, Kemijärvi and Rovaniemi areas range from 2738 Ma to 1770 Ma.
133500228
SIMS zircon ages and Nd isotope systematics of the 2.2 Ga mafic intrusions in northern and eastern Finland
Using the SIMS, ID-TIMS and Sm-Nd isotopic methods and the electron microprobe, we have studied several differentiated mafic intrusions of the c. 2.2 Ga gabbro-wehrlite association (GWA) from four Paleoproterozoic schist belts and the Archean Kuhmo Greenstone Belt. Back-scattered electron images and electron microprobe analyses revealed that zircon crystals vary from well-preserved to turbid and highly altered with individual grains often displaying irregular, hydrated, CaO-bearing domains. In the most pristine domains, suitable for establishing the crystallization ages, SIMS 207Pb/206Pb ages fall in the range of 2210–2220 Ma, which is consistent with the most concordant ID-TIMS UPb ages. One of the studied intrusions that had previously yielded a conventional U-Pb date of less than 2.0 Ga, could be shown by spot analysis to belong to the 2.2 Ga family. In contrast to the well-preserved zircon domains, altered domains exhibit a variable and often strong U-Pb discordance up to 70 % and have distinctly lower 207Pb/206Pb ages. Some zircon grains record isotopic resetting at the time of the Svecofennian orogeny (ca. 1.8–1.9 Ga), while the most discordant ones project in the concordia diagram to late Paleozoic lower intercept ages indicating a relative recent Pb loss. The mineral chemistry of zircon suggests that the leakage of radiogenic Pb can be ascribed to an opensystem behavior related to hydrothermal alteration via action of CaCl2-bearing fluids. Common albitization of plagioclase in the GWA intrusions has caused this mineral to behave as an open system with regard to the Sm-Nd isotopic systematics. Despite this uncertainty, our Nd isotopic data indicate that the magma that produced the GWA intrusions in various parts of northern and eastern Finland was isotopically homogeneous and had an initial εNd(2220 Ma) value of c. +0.6 precluding significant upper crustal contamination upon emplacement and subsequent fractional crystallization.
131736533
Diverse Tourmaline Compositions from Orogenic Gold Deposits in the Hutti-Maski Greenstone Belt, India: Implications for Sources of Ore-Forming Fluids
The Late Archean Hutti-Maski greenstone belt hosts the largest operational gold mine in India at Hutti, as well as two satellite mines at Hira-Buddini and Uti. Tourmaline is a common accessory mineral in the auriferous altered wall rocks, proximal to the gold-bearing veins of these orogenic gold deposits. We report here major, rare earth element (REE), and selected trace element compositions of tourmalines from these alteration zones at Hutti and Hira-Buddini, and from spatially associated volcaniclastic rocks at Uti, in order to compare and evaluate possible fluid sources related to tourmaline precipitation and gold mineralization. Tourmalines at Hutti occur as tourmaline-calcite microveins within proximal alteration zones, and as disseminated grains along the biotite-chlorite–defined mylonitic foliation. These tourmalines belong to the dravite/oxydravite group, with an evolutionary trend towards magnesio-foitite, and commonly are Mg rich (up to 2.06 apfu) with low Na (<0.61 apfu) and up to 53% X-site vacancies. Characteristics are low abundances of total REE (0.25–0.67× chondrite) and a flat trend, with minor HREE enrichment accompanied by strong positive Eu anomalies. At Hira-Buddini, tourmalines of the oxydravite-povondraite series replace amphiboles and hydrothermal biotite. Based on textural and chemical characteristics, three generations of tourmalines are present at Hira-Buddini. Earliest type I tourmaline has moderate Fe content (up to 1.46 apfu), high Na (average 0.80 apfu), and moderate X-site vacancies up to 22%. Type II tends to be relatively enriched in Fe (up to 2.08 apfu), have similar Na contents (average 0.79 apfu), and low X-site vacancies (up to 8%). These tourmalines contain higher total REE (0.93–2.23× chondrite) and display a flat trend with slight middle rare earth element (MREE) depletion accompanied by a small positive Eu anomaly. The latest tourmalines (type III) are Fe-poor (up to 1.52 apfu) and Na-poor (average 0.67 apfu), with relatively high X-site vacancies (up to 21%). This tourmaline type has relatively low total REEs (0.21–1.05× chondrite) and similar abundance patterns to those of Hutti tourmalines, accompanied by large positive Eu anomalies. Similarity in tourmaline compositions with evolutionary trends in compositional space from different textural associations suggests that a single low-salinity and reduced metamorphogenic fluid, with inherently low ∑REE, was responsible for proximal alteration and gold mineralization at Hutti. The type I and type II tourmalines from Hira-Buddini display oxydravite-povondraite trends, with compositional overlap that suggests fluid mixing at a paragenetically early stage. The early hydrothermal fluid was more saline, oxidizing, and likely had a granite-derived component with high ∑REE contents. A later evolved, relatively low-salinity and reduced fluid is recorded by the last generation (type III) tourmalines at Hira-Buddini. The tourmalines from Uti metapelites are characterized by dravitic composition, low Na contents, high X-site vacancies (31–47%), and an evolutionary trend toward magnesio-foitite. These tourmalines have higher ∑REE contents (1.16–2.08× chondrite), LREE-enriched patterns together with large positive Eu anomalies, and low concentrations of most trace elements, all features that suggest a metamorphic origin.
134690156
Major and trace element geochemistry of tourmalines from Archean orogenic gold deposits: Proxies for the origin of gold mineralizing fluids?
Abstract Orogenic style gold mineralizations in the Archean Hattu Schist belt (E Finland) are present in all major host rock lithologies including epiclastic sedimentary and volcanogenic rocks, as well as felsic intrusives. The gold mineralizations occur as dissemination in altered wall rocks and within hydrothermal quartz veins. Hydrothermal tourmalines are often associated with the gold mineralizations occurring in the quartz veins, as alteration minerals in contacts of the metasedimentary host rocks and quartz veins, but also within mineral assemblages of the metasedimentary and magmatic rocks. We characterize and compare the major, trace and rare earth element chemistry of these tourmalines in order to evaluate their suitability as a petrogenetic tool for tracing the fluid sources of the hydrothermal system. By comparing the chemistry of tourmalines from gold mineralization with those from metamorphic and magmatic host rocks, we test whether tourmaline composition can be used to identify the source rocks of the gold transporting hydrothermal fluids. All analyzed tourmalines belong to the alkali super group and plot along the schorl-dravite join. The hydrothermal tourmalines have Li, Sr and V concentrations comparable to metamorphic tourmalines and clearly distinct from magmatic tourmalines. Because the concentrations of Ni, Pb, Cr, Mn, Ga, Zn and Sn show a wide overlap between magmatic, metamorphic and hydrothermal tourmalines, these elements do not permit to discriminate between different source rocks. The co-variations of many elements in hydrothermal tourmalines, when plotted against Li, show more similarity to metamorphic tourmalines and the whole-rock compositions of metasedimentary and metavolcanic host rocks than to their magmatic counterparts. This indicates that the major and trace element composition of hydrothermal tourmalines in the Hattu Schist Belt is predominantly controlled by the host rocks and local fluid-rock interactions, and does not reflect the distal fluid sources. The effect of local fluid-rock interaction is also manifested by the REE patterns of tourmalines. The magmatic tourmalines have distinct LREE enriched patterns resembling the whole-rock REE patterns of granitic intrusives, while the metamorphic and hydrothermal tourmalines have flat or weakly fractionated patterns similar to metavolcanic and metasedimentary host rocks. Taken together, the tourmaline data suggest that hydrothermal tourmalines associated to gold mineralizing fluids are most likely genetically related to metamorphic rock sources without important contributions of magmatic fluids, and that local fluid-rock interaction exerted a major control on tourmaline chemistry.
60160905
Statistical data analysis explained : applied environmental statics with R
Preface. Acknowledgements. About the Authors. 1. Introduction. 1.1 The Kola Ecogeochemistry Project. 2. Preparing the Data for Use in R and DAS+R. 2.1 Required data format for import into R and DAS+R. 2.2 The detection limit problem. 2.3 Missing Values. 2.4 Some "typical" problems encountered when editing a laboratory data report file to a DAS+R file. 2.5 Appending and linking data files. 2.6 Requirements for a geochemical database. 2.7 Summary. 3. Graphics to Display the Data Distribution. 3.1 The one-dimensional scatterplot. 3.2 The histogram. 3.3 The density trace. 3.4 Plots of the distribution function. 3.5 Boxplots. 3.6 Combination of histogram, density trace, one-dimensional scatterplot, boxplot, and ECDF-plot. 3.7 Combination of histogram, boxplot or box-and-whisker plot, ECDF-plot, and CP-plot. 3.8 Summary. 4. Statistical Distribution Measures. 4.1 Central value. 4.2 Measures of spread. 4.3 Quartiles, quantiles and percentiles. 4.4 Skewness. 4.5 Kurtosis. 4.6 Summary table of statistical distribution measures. 4.7 Summary. 5. Mapping Spatial Data. 5.1 Map coordinate systems (map projection). 5.2 Map scale. 5.3 Choice of the base map for geochemical mapping 5.4 Mapping geochemical data with proportional dots. 5.5 Mapping geochemical data using classes. 5.6 Surface maps constructed with smoothing techniques. 5.7 Surface maps constructed with kriging. 5.8 Colour maps. 5.9 Some common mistakes in geochemical mapping. 5.10 Summary. 6. Further Graphics for Exploratory Data Analysis. 6.1 Scatterplots (xy-plots). 6.2 Linear regression lines. 6.3 Time trends. 6.4 Spatial trends. 6.5 Spatial distance plot. 6.6 Spiderplots (normalized multi-element diagrams). 6.7 Scatterplot matrix. 6.8 Ternary plots. 6.9 Summary. 7. Defining Background and Threshold, Identification of Data Outliers and Element Sources. 7.1 Statistical methods to identify extreme values and data outliers. 7.2 Detecting outliers and extreme values in the ECDF- or CP-plot. 7.3 Including the spatial distribution in the definition of background. 7.4 Methods to distinguish geogenic from anthropogenic element sources. 7.5 Summary. 8. Comparing Data in Tables and Graphics. 8.1 Comparing data in tables. 8.2 Graphical comparison of the data distributions of several data sets. 8.3 Comparing the spatial data structure. 8.4 Subset creation - a mighty tool in graphical data analysis. 8.5 Data subsets in scatterplots. 8.6 Data subsets in time and spatial trend diagrams. 8.7 Data subsets in ternary plots. 8.8 Data subsets in the scatterplot matrix. 8.9 Data subsets in maps. 8.10 Summary. 9. Comparing Data Using Statistical Tests. 9.1 Tests for distribution (Kolmogorov-Smirnov and Shapiro-Wilk tests). 9.2 The one-sample t-test (test for the central value). 9.3 Wilcoxon signed-rank test. 9.4 Comparing two central values of the distributions of independent data groups. 9.5 Comparing two central values of matched pairs of data. 9.6 Comparing the variance of two test. 9.7 Comparing several central values. 9.8 Comparing the variance of several data groups. 9.9 Comparing several central values of dependent groups. 9.10 Summary. 10. Improving Data Behaviour for Statistical Analysis: Ranking and Transformations. 10.1 Ranking/sorting. 10.2 Non-linear transformations. 10.3 Linear transformations. 10.4 Preparing a data set for multivariate data analysis. 10.5 Transformations for closed number systems. 10.6 Summary. 11. Correlation. 11.1 Pearson correlation. 11.2 Spearman rank correlation. 11.3 Kendall-tau correlation. 11.4 Robust correlation coefficients. 11.5 When is a correlation coefficient significant? 11.6 Working with many variables. 11.7 Correlation analysis and inhomogeneous data. 11.8 Correlation results following addictive logratio or central logratio transformations. 11.9 Summary. 12. Multivariate Graphics . 12.1 Profiles. 12.2 Stars. 12.3 Segments. 12.4 Boxes. 12.5 Castles and trees. 12.6 Parallel coordinates plot. 12.7 Summary. 13. Multivariate Outlier Detection. 13.1 Univariate versus multivariate outlier detection. 13.2 Robust versus non-robust outlier detection. 13.3 The chi-square plot. 13.4 Automated multivariate outlier detection and visualization. 13.5 Other graphical approaches for identifying outliers and groups. 13.6 Summary. 14. Principal Component Analysis (PCA) and Factor Analysis (FA). 14.1 Conditioning the data for PCA and FA. 14.2 Principal component analysis (PCA). 14.3 Factor Analysis. 14.4 Summary. 15. Cluster Analysis. 15.1 Possible data problems in the context of cluster analysis. 15.2 Distance measures. 15.3 Clustering samples. 15.4 Clustering variables. 15.5 Evaluation of cluster validity. 15.6 Selection of variables for cluster analysis. 15.7 Summary. 16. Regression Analysis (RA). 16.1 Data requirements for regression analysis. 16.2 Multiple regression. 16.3 Classical least squares (LS) regression. 16.4 Robust regression. 16.5 Model selection in regression analysis. 16.6 Other regression methods. 16.7 Summary. 17. Discriminant Analysis (DA) and Other Knowledge-Based Classification Methods. 17.1 Methods for discriminant analysis. 17.2 Data requirements for discriminant analysis. 17.3 Visualisation of the discriminant function. 17.4 Prediction with discriminant analysis. 17.5 Exploring for similar data structures. 17.6 Other knowledge-based classification methods/ 17.7 Summary. 18. Quality Control (QC). 18.1 Randomised samples. 18.2 Trueness. 18.3 Accuracy. 18.4 Precision. 18.5 Analysis of variance (ANOVA) 18.6 Using Maps to assess data quality. 18.7 Variables analysed by two different analytical techniques. 18.8 Working with censored data - a practical example. 18.9 Summary. 19. Introduction to R and Structure of the DAS+R Graphical User Interface. 19.1 R. 19.2 R-scripts. 19.3 A brief overview of relevant R commands. 19.4 DAS+R. 19.5 Summary. References. Index.
129667550
The geochemical evolution of the continental crust
A survey is given of the dimensions and composition of the present continental crust. The abundances of immobile elements in sedimentary rocks are used to establish upper crustal composition. The present upper crustal composition is attributed largely to intracrustal differentiation resulting in the production of granites senso lato. Underplating of the crust by ponded basaltic magmas is probably a major source of heat for intracrustal differentiation. The contrast between the present upper crustal composition and that of the Archean upper crust is emphasized. The nature of the lower crust is examined in the light of evidence from granulites and xenoliths of lower crustal origin. It appears that the protoliths of most granulite facies exposures are more representative of upper or middle crust and that the lower crust has a much more basic composition than the exposed upper crust. There is growing consensus that the crust grows episodically, and it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons, or 2.7 Ga). There appears to be a relationship between episodes of continental growth and differentiation and supercontinental cycles, probably dating back at least to the late Archean. However, such cycles do not explain the contrast in crustal compositions between Archean and post-Archean. Mechanisms for deriving the crust from the mantle are considered, including the role of present-day plate tectonics and subduction zones. It is concluded that a somewhat different tectonic regime operated in the Archean and was responsible for the growth of much of the continental crust. Archean tonalites and trond-hjemites may have resulted from slab melting and/or from melting of the Archean mantle wedge but at low pressures and high temperatures analogous to modern boninites. In contrast, most andesites and subduction-related rocks, now the main contributors to crustal growth, are derived ultimately from the mantle wedge above subduction zones. The cause of the contrast between the processes responsible for Archean and post-Archean crustal growth is attributed to faster subduction of younger, hotter oceanic crust in the Archean (ultimately due to higher heat flow) compared with subduction of older, cooler oceanic crust in more recent times. A brief survey of the causes of continental breakup reveals that neither plume nor lithospheric stretching is a totally satisfactory explanation. Speculations are presented about crustal development before 4000 m.y. ago. The terrestrial continental crust appears to be unique compared with crusts on other planets and satellites in the solar system, ultimately a consequence of the abundant free water on the Earth.
206380287
An Excel spreadsheet to classify chemical analyses of amphiboles following the IMA 2012 recommendations
A Microsoft Excel spreadsheet has been programmed to assist with classification of chemical analyses of orthorhombic and monoclinic amphiboles following the 2012 nomenclature recommended by the International Mineralogical Association. The spreadsheet is intended for use only with compositional data (wt% oxides and halogens, rather than atomic proportions) and provides options for the estimation of Fe3+/ΣFe and Mn3+/ΣMn ratios and OH content. Various cation normalization schemes can be automatically or manually selected. For each analysis, the output includes the group, subgroup (or B-occupancy for the oxo-amphiboles), and species name including any mandatory chemical prefixes, along with a formula based on 24 anions. The formula results can be exported in a form suitable for the AMPH2012 program. Prefixes related to space groups (proto-) and suffixes (-P21/m) are not assigned in the spreadsheet. Large data sets (up to 200 analyses at a time) can be accommodated by the spreadsheet, which is accompanied by results calculated for more than 650 amphibole analyses taken from the literature. .Display Omitted A spreadsheet to classify amphiboles using the IMA 2012 nomenclature is presented.Output includes group, subgroup, species name and formula.Up to 200 analyses can be classified.Results for more than 650 analyses are tabulated.
32057489
Nomenclature of the amphibole supergroup
A new classification and nomenclature scheme for the amphibole-supergroup minerals is described, based on the general formula AB 2 C 5 T 8 O 22 W 2 , where A = □, Na, K, Ca, Pb, Li; B = Na, Ca, Mn 2+ , Fe 2+ , Mg, Li; C = Mg, Fe 2+ , Mn 2+ , Al, Fe 3+ , Mn 3+ , Ti 4+ , Li; T = Si, Al, Ti 4+ , Be; W = (OH), F, Cl, O 2− . Distinct arrangements of formal charges at the sites (or groups of sites) in the amphibole structure warrant distinct root names , and are, by implication, distinct species; for a specific root name, different homovalent cations (e.g., Mg vs. Fe 2+ ) or anions (e.g., OH vs. F) are indicated by prefixes (e.g., ferro-, fluoro-). The classification is based on the A, B, and C groups of cations and the W group of anions, as these groups show the maximum compositional variability in the amphibole structure. The amphibole supergroup is divided into two groups according to the dominant W species: W (OH,F,Cl)-dominant amphiboles and W O-dominant amphiboles (oxo-amphiboles). Amphiboles with (OH, F, Cl) dominant at W are divided into eight subgroups according to the dominant charge-arrangements and type of B-group cations: magnesium-iron-manganese amphiboles, calcium amphiboles, sodium-calcium amphiboles, sodium amphiboles, lithium amphiboles, sodium-(magnesium-iron-manganese) amphiboles, lithium-(magnesium-iron-manganese) amphiboles and lithium-calcium amphiboles. Within each of these subgroups, the A- and C-group cations are used to assign specific names to specific compositional ranges and root compositions. Root names are assigned to distinct arrangements of formal charges at the sites, and prefixes are assigned to describe homovalent variation in the dominant ion of the root composition. For amphiboles with O dominant at W, distinct root-compositions are currently known for four (calcium and sodium) amphiboles, and homovalent variation in the dominant cation is handled as for the W (OH,F,Cl)-dominant amphiboles. With this classification, we attempt to recognize the concerns of each constituent community interested in amphiboles and incorporate these into this classification scheme. Where such concerns conflict, we have attempted to act in accord with the more important concerns of each community.
36476063
The Ti-saturation surface for low-to-medium pressure metapelitic biotites: Implications for geothermometry and Ti-substitution mechanisms
Abstract The Ti content of biotite can serve as a geothermometer for graphitic, peraluminous metapelites that contain ilmenite or rutile and have equilibrated at roughly 4-6 kbar. The relationship between Ti- coutent, temperature, and Mg/(Mg + Fe) value was calibrated empirically using an extensive natural biotite data set (529 samples) from western Maine and south-central Massachusetts in combination with the petrogenetic grid of Spear et al. (1999). The calculated Ti-saturation surface is curved such that for a given Mg/(Mg + Fe) value, Ti concentration increases as a function of temperature in a nonlinear' fashion, and for a given temperature Ti concentrations decrease with an increase in Mg/(Mg + Fe). The fit to the Ti-saturation surface can be reformulated as the geothermometric expression: T = {[In(Ti) - a - c(XMg)3]/b}0.333, in which T is temperature in degrees Celsius, Ti is the number of atoms per formula unit (apfu) normalized on the basis of 22 O atoms, XMg is Mg/(Mg + Fe), a = -2.3594, b = 4.6482 x 1O-9 and c = -1.7283. The calibration range for this expression is XMg = 0.275-1.000, Ti = 0.04-0.60 apfu, and T = 480-800 0C. Precision of the Ti-in-biotite geothermometer is estimated to be ±24 0C at the lower temperature range and improves to ±12 °C at higher temperatures. Application of the Ti-in-biotite geothermometer to ilmenite- or rutile-bearing, graphitic, peraluminous metapelites equilibrated at 3-6 kbar is generally consistent with independent temperature determinations, but with some deviations that represent local reequilibration. Consequently, the Ti systematics in biotite can also serve as the basis of a very sensitive indicator of chemical equilibrium, or lack thereof. Application of the geothermometer to metapelites not containing the requisite mineral assemblages can lead to minor-to-significant errors in estimated temperatures. Biotite Ti-substitution mechanisms are controlled by several factors. Based on the biotite calibration data set, magnesian biotites (XMg> 0.65) incorporate Ti in accordance with the exchange vector TiAl2R-1Si-2, where R is the sum of the divalent cations Mg + Fe + Mn. This substitution mechanism is primarily a response to misfit of the octahedral and tetrahedral layers in magnesian biotites. Intermediate biotites (XMg <0.65), particularly at higher temperatures, exhibit enhanced Ti concentrations, most consistent with the Ti-deprotonation TiO2R-1(OH)-2, exchange vector. Dominance of Ti-deprotonation substitution is largely a function of reduction of H2O activity at higher metamorphic grades. Supplementary biotite data from metaluminous amphibolites and mafic granulites, metamorphosed isothermally with variable H2O activities, reveal that Iow-Al biotite incorporates significantly higher concentrations of Ti relative to peraluminous biotite as a result of a combination of the exchange vectors Ti02R-1(0H)-2 and RSiAl-2, substituting in roughly an 8:1 ratio.
140163807
A new review of the chlorites
The chlorites form an extensive isostructural series with a high degree of isomorphous substitution, within which it has always been difficult to define or delimit species; until recently, owing to the lack of adequate X-ray studies; several structurally distinct species were included with the group, further complicating the problem. The group comprises aluminosilicates of magnesium and iron (ferrous and ferric); a few contain appreciable amounts of chromium, nickel, or manganese, and one (pennantite) contains manganese as a major constituent. The first step towards the understanding of the relations of the chlorites was taken by G. Tschermak (1890, 1891), who divided them into two groups: the orthochlorites, with compositions between (Mg,Fe")2Al2SiO5(OH)4 and (Mg,Fe")2Si2O5(OH)4, and the leptochlorites, with compositions not explicable on this basis, and in general richer in trivalent ions (often including considerable Fe"′) relative to silicon and divalent ions. The two orthochlorite end-members have the composition of amesite and serpentine respectively, but it is now known that neither of these two minerals has the chlorite structure.
128913952
Europium redox equilibria in aqueous solution
Thermochemical data and recent advances in theoretical aqueous solution chemistry enable prediction of the relative stabilities of aqueous Eu2+ and Eu3+ over a wide range of temperatures and pressures. At low temperatures, near earth surface conditions, the aqueous geochemistry of europium should be dominated by the trivalent state, except possibly in the most reducing, alkaline pore waters of anoxic marine sediments. However, at temperatures greater than about 250°C and elevated pressures, divalent europium should predominate. Even with significant amounts of complexing, trivalent europium is not stable at elevated temperatures and pressures. Consequently, under most hydrothermal and metamorphic conditions, europium in aqueous solution should be divalent. At intermediate temperatures, around 100°C, significant activities of both Eu2+ and Eu3+, and related complexes, can occur in aqueous solutions, depending on the oxidation state and the pH of the solutions, and the activities of potential ligands such as sulfate, carbonate and chloride. The predicted stability of divalent europium in aqueous solution at elevated temperatures is consistent with the large positive europium anomalies in rare earth element patterns of high-temperature barites of hydrothermal and metamorphic origin reported by Guichard et al. [5] and with the observed depletion of europium as a result of high-temperature sericitization of feldspar-bearing assemblages discovered by Alderton et al. [1]. It is suggested that significant fractionation of europium relative to the other rare earth elements may take place during high-temperature hydrothermal alteration processes, such as in the mid-oceanic ridge systems, because of the expected stability of divalent europium during these processes.
94881947
Rare earth elements in hydrothermal systems: Estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures
Abstract Standard partial molal thermodynamic properties including association constants for 246 inorganic aqueous rare earth element (REE) complexes with chloride, fluoride, hydroxide, carbonate, sulfate, bicarbonate, nitrate, and orthophosphate can be calculated at pressures from 1 to 5000 bars and temperatures from 0 to 1000°C, using experimental data from the literature and correlation algorithms. Predicted association constants for REE complexes are used to calculate the speciation of the REEs in simulated and natural fluid compositions over ranges of pH, temperature, and pressure. Our results demonstrate that in a generalized chloride-rich hydrothermal fluid, REE transport may be facilitated by formation of chloride, fluoride, and hydroxide complexes at acidic, neutral, and basic pH conditions, respectively. The HREEs (GdLu) are complexed more strongly by fluoride, and less strongly by chloride, than the LREEs (LaSm), whereas at basic pH conditions HREEs and LREEs strongly associate with hydroxide to an equivalent degree. Estimates of REE speciation in natural hydrothermal solutions that differ in composition and geologic setting demonstrate that different REE-complexes predominate in different environments. It follows that ad hoc assumptions about the identity of complexes which are responsible for REE mobility in a given geologic setting are not necessary.
96901101
Chemical evolution and mineral deposition in boiling hydrothermal systems
A thermodynamic model of boiling hydrothermal solutions is developed and applied over a wide range of physical and chemical conditions. Within the range of conditions observed in natural boiling hydrothermal systems the processes of liquid-vapor partitioning and the resultant effects on mineral solubilities are highly varied and complex. Metals that are complexed by chloride are deposited largely as a result of the decreasing proton concentration associated with CO 2 exsolution during boiling. Metal bisulfide complexes are destabilized most when the decrease in proton concentration is sluggish relative to the loss of H 2 S.Vaporization of only a few percent of a solution can decrease the proton concentration by several orders of magnitude when the CO 2 /H (super +) and CO 2 /Sigma SO 4 concentration ratios are initially high. The relationship between the proton, CO 2 , and Sigma SO 4 concentrations prior to boiling to the proton concentration after boiling is defined explicitly by a few simple equations. These equations along with the solubilities of calcite and anhydrite constitute the chemical boundary conditions for significant mineral deposition by boiling. Typical hydrothermal fluids lose most of their volatile components to the vapor phase and most of their metals to mineral phases by the time boiling has proceeded to the point where the volumes of the vapor and liquid phases are equal.Physical variables such as the heat budget and the restrictions on the partitioning of mass between liquid and vapor, although significant, are subordinate to the compositional variables in determining the chemical evolution of a boiling hydrothermal solution. Mineral deposition is most vigorous when the volatile components partition from the solution to the vapor phase in a manner resembling perfect fractional (Rayleigh) distillation. As temperature decreases, the efficiency of boiling for depositing metals from solution increases, and the amount of metals in solution typically decreases such that the net effect of boiling is most favorable for ore formation at temperatures around 300 degrees C. Mineral and metal complex stoichiometries in combination with the relative volatilities of CO 2 and H 2 S determine the general sequence of mineral deposition during boiling. These major variables, many other minor variables, and the multiple interactions thereof are accounted for rigorously. The amount and paragenesis of ore and gangue minerals deposited by boiling are presented for numerous hypothetical hydrothermal systems. Analysis of these results suggests that boiling is perhaps the most generally effective ore depositional mechanism at the conditions operative in many boiling hydrothermal systems.
127890655
Precambrian geology of Finland : key to the evolution of the fennoscandian shield
Focusing on the Precambrian in the central part of the Fennoscandian Shield, the book combines the results from modern geological and geophysical research into a detailed petrologic, lithologic, and structural synthesis and interpretation of the Archean and Proterozoic of Finland. It will be of value to anyone interested in the evolution of the shield in particular and in Precambrian geology in general.
135171178
Temporal and Hf isotope geochemical evolution of southern Finnish Lapland from 2.77 Ga to 1.76 Ga
The southern Finnish Lapland area in the central part of the Fennoscandian shield is a geologically complex zone comprising several Archean blocks and Paleoproterozoic supracrustal belts all of which are intruded by voluminous Paleoproterozoic granites (the central Lapland granitoid complex, CLGC). New in-situ single crystal zircon U–Pb age determinations coupled with Lu–Hf isotope data from the same zircons were acquired from five granitoid rocks and one amphibolitic rock sample from the southern Lapland area. The samples represent at least four distinct magmatic events (at ca. 2.77 Ga, 2.12 Ga, 1.81 Ga, and 1.76 Ga). The 2.77 Ga and the 1.81-1.76 Ga events have initial Hf isotope signatures implying that local Archean rocks represent the source for the younger granites. The 2.12 Ga event has a slightly more juvenile Hf isotope composition suggesting either that the source for the 2.12 Ga granites represents a different Archean block or that the source is composed of mixed Archean and Paleoproterozoic components. The Neoarchean source for the Paleoproterozoic granites may be traced through the CLGC all the way to the Jokkmokk area in Sweden and possibly to the Lofoten area in Norway.
130626915
The role of metamorphic fluids in the formation of ore deposits
Abstract Many ore deposits are hosted by metamorphic rocks, and metamorphic fluids have been invoked as a source for various deposits, especially gold deposits. Metamorphic fluid compositions reflect original sedimentary environment: continental shelf sequences yield saline metamorphic fluids with little dissolved gas while metasediments from accretionary and oceanic settings host less saline fluids with significant CO2 contents. The principal difficulty in reconciling ore deposits with a metamorphic origin is that many form quickly (c. 1 Ma), whereas metamorphic heating is slow (c. 10–20 °/Ma). Gravitational instability means that fluid cannot be retained. Metamorphic ores may nevertheless form by: (a) segregation leading to enrichment of pre-existing concentrations; (b) infiltration of water-rich fluids from schists into marbles at high temperature overstepping decarbonation reactions and allowing fast reaction that locally draws down temperature; and (c) rapid uplift driving dehydration reactions owing to pressure drop. Some orogenic lode gold deposits fit well with a purely metamorphic origin during rapid uplift, but others are problematic. At Sunrise Dam, Western Australia, anomalies in Sr-isotope ratios and in apatite compositions indicate a partial mantle/magmatic source. Low salinity, H2O–CO2 fluids commonly associated with hydrothermal gold reflect the effect of salt on gas solubility, not the origin of the fluid.
134301957
6.3 The Onega Basin
The main geological and stratigraphic features of the Onega Basin are discussed in Chap. 4.3. Given here is a brief geological outline to provide a scientific context and background information for the FAR-DEEP implemented in this area.
128188891
Distribution, character and genesis of gold deposits in metamorphic terranes
Epigenetic gold deposits in metamorphic terranes include those of the Precambrian shields (approx 23,000–25,000 t Au), particularly the Late Archean greenstone belts and Paleoproterozoic fold belts, and of the late Neoproterozoic and younger Cordilleran-style orogens (approx 22,000 t lode and 15,500 t placer Au), mainly along the margins of Gondwana, Laurentia, and the more recent circum-Pacific. Ore formation was concentrated during the time intervals of 2.8 to 2.55 Ga, 2.1 to 1.8 Ga, and 600 to 50 Ma. Prior to the last 25 years, ores were defined by grades of 5 to 10 g/t Au in underground mines; present-day economics, open-pit 
mining, and improved mineral processing procedures allow recovery of ores of ≤1 g/t Au, which has commonly 
led to the recent reworking of lower grade zones in many historic orebodies. Most of these deposits formed 
synchronously with late stages of orogeny and are best classified as orogenic gold deposits, which may be subdivided into epizonal, mesozonal, and hypozonal subtypes based on pressure-temperature conditions of ore formation. 
A second type of deposit, termed intrusion-related gold deposits, developed landward of Phanerozoic accreted terranes in the Paleozoic of eastern Australia and the Mesozoic of the northern North American Cordillera. These have an overall global distribution that is still equivocal and are characterized by an intimate genetic association with relatively reduced granitoids. The majority of gold deposits in metamorphic terranes are located adjacent to first-order, deep-crustal fault zones, which show complex structural histories and may extend along strike for hundreds of kilometers with widths of as much as a few thousand meters. Fluid migration along such zones was driven by episodes of major pressure fluctuations during seismic events. Ores formed as vein fill of second- and third-order shears and faults, particularly at jogs or changes in strike along the crustal fault zones. Mineralization styles vary from stockworks and breccias in shallow, brittle regimes, through laminated crack-seal veins and sigmoidal vein arrays in brittle-ductile crustal regions, to replacement- and disseminated-type orebodies in deeper, ductile environments (i.e., a continuum model). Most orogenic gold deposits occur in greenschist facies rocks, but significant orebodies can be present in lower and higher grade rocks. Deposits typically formed on retrograde portions of pressure-temperature-time paths and thus are discordant to metamorphic features within host 
rocks. Spatial association between gold ores and granitoids of all compositions reflects a locally favorable structural 
trap, except in the case of the intrusion-related gold deposits where there is a clearer genetic association. 
World-class orebodies are generally 2 to 10 km long, about 1 km wide, and are mined downdip to depths of 2 to 3 km. Most orogenic gold deposits contain 2 to 5 percent sulfide minerals and have gold/silver ratios from 5 to 10 and gold fineness >900. Arsenopyrite and pyrite are the dominant sulfide minerals, whereas pyrrhotite is more important in higher temperature ores and base metals are not highly anomalous. Tungsten-, Bi-, and Te-bearing mineral phases can be common and are dominant in the relatively sulfide poor intrusion-related gold deposits. Alteration intensity, width, and assemblage vary with the host rock, but carbonates, sulfides,muscovite, chlorite, K-feldspar, biotite, tourmaline, and albite are generally present, except in high-temperature systems where alteration halos are dominated by skarnlike assemblages. The vein-forming fluids for gold deposits in metamorphic environments are uniquely CO2 and 18O rich, with low to moderate salinities. Phanerozoic and Paleoproterozic ores show a mode of formation temperatures at 250° to 350°C, whereas Late Archean deposits cluster at about 325° to 400°C. However, there are also many important lower and higher temperature deposits deposited throughout the continuum of depths that range between 2 and 20 km. Ore fluids were, in most cases, near-neutral pH, slightly reduced, and dominated by sulfide 
complexes. Globally consistent ore-fluid δ18O values of 6 to 13 per mil and δD values of –80 to –20 per mil 
generally rule out a significant meteoric water component in the gold-bearing hydrothermal systems. Sulfur 
isotope measurements on ore-related sulfide minerals are concentrated between 0 and 10 per mil, but with many higher and much lower exceptions, indicating variable sulfur sources and an unlikely dominant role for mantle sulfur. Drastic pressure fluctuations with associated fluid unmixing and/o rdesulfidation during water/rock interaction are the two most commonly called-upon ore precipitation mechanisms. The specific model(s) for gold ore genesis remains controversial. Although the direct syngenetic models of the 1970s are no longer applicable, the gold itself may be initially added into the volcanic and sedimentary crustal rock sequences, probably within marine pyrite, during sea-floor hydrothermal events. Gold transport 
and concentration are most commonly suggested to be associated with metamorphic processes, as indicated by 
the volatile composition of the hydrothermal fluids, the progressive decrease in concentration of elements enriched 
in the gold deposits with increasing metamorphic grade of the country rocks, and the common association of ores with medium-grade metamorphic environments. Gold deposits of typically relatively low grade, which formed directly from fluid exsolution during granitoid emplacement within metamorphic rocks, are now also clearly recognized (i.e., intrusion-related gold deposits), but there are limited definitive data to implicate such an exsolved fluid source for most gold deposits within orogenic provinces. The fact that orogenic gold deposits are associated with all types of igneous rocks is a problem to a pure magmatic model. Hybrid models, where slab-derived fluids may generate rising melts that drive devolatilization reactions in the lower crust, are also feasible. Although involvement of a direct mantle fluid presents geochemical difficulties, the presence of lamprophyres and deep-crustal faults in many districts suggests potential mantle influence in the overall, largescale tectonic event controlling the hydrothermal flow system.
140187298
Intrusion-related gold deposits
A broad spectrum of gold mineralization styles is found in the epizonal intrusive environment, and a number of these give rise to world-class gold deposits (Figure 6.1). Here the spectrum is subdivided into: (a) intrusion-hosted stock-work/disseminated deposits of both porphyry and non-porphyry types, the former possessing all the essential geological attributes (especially multidirectional stock-works) of typical porphyry copper and/or molybdenum deposits; (b) skarn and non-skarn replacement deposits in carbonate wallrocks; (c) stockwork, disseminated and replacement deposits in non-carbonate wallrocks; (d) wallrock-hosted breccia pipes; and (e) veins in both intrusions and wallrocks. This spectrum of deposits is transitional upwards to the volcanic-hosted, epithermal gold environment, which is mentioned only briefly in this chapter but discussed at length by Henley (this volume).
128828443
Geochemical and Oxygen Isotope Signature of Sea-Floor Alteration Associated with a Polydeformed and Highly Metamorphosed Massive Sulfide Deposit, Ruostesuo, Central Finland
Massive sulfide deposits in the Vihanti-Pyhasalmi district, central Finland, are hosted by a highly deformed and metamorphosed bimodal volcanic sequence. Alteration zones that envelop these deposits are composed of cordierite + orthoamphibole ± garnet-bearing rocks (referred to as COR rocks) belonging to upper amphibolite to granulite facies. Outcrops of mafic and felsic COR rocks (MCOR and FCOR) in the vicinity of the Ruostesuo Zn-Cu deposit define mappable layers with distinctive metamorphic mineral assemblages: orthoamphibole + cordierite ± plagioclase ± quartz (M1COR); garnet + orthoamphibole + cordierite ± plagioclase ± quartz (M2COR); quartz + cordierite + biotite ± orthoamphibole ± cummingtonite ± garnet ± plagioclase (F1COR); and quartz + cordierite + garnet + sillimanite (F2COR). These rocks are interlayered with, or grade into, typical amphibolites (i.e., hornblende + plagioclase) and quartz + plagioclase-bearing rocks that are interpreted as least altered mafic and felsic metavolcanic rocks, respectively. 
 
Whole-rock geochemical data for least altered metavolcanic rocks and COR rocks at Ruostesuo define three mafic suites (primitive basalts, basaltic andesites, and ferrobasalts) and two felsic suites (R1 and R2 rhyolites). Using a combination of high field strength element ratios and rare earth element profiles, COR rocks can be assigned to all five volcanic suites. 
 
Calculated mass changes for the majority of exposed COR rocks at Ruostesuo are similar to those reported for chlorite + quartz assemblages found within the inner core of alteration pipes located below some sea-floor massive sulfide deposits. Relative to least altered precursors, mafic COR rocks have gained Fe, Mg ± S and lost variable amounts of Si, Ca, Na, K, Rb, Ba, Sr, and Eu, which resulted in an average net mass loss of about 10 percent. The bulk composition of most altered mafic COR rocks (near total loss of Ca, Na, K) reflects the proportion of chlorite to quartz within the premetamorphic alteration assemblages and the composition of the chlorite. Mass-change results for altered mafic rocks indicate that basalts at Ruostesuo were altered by an evolved, Fe-rich hydrothermal fluid at high water/rock ratios. 
 
Mass changes for COR rocks of the R2 felsic volcanic suite are similar to those of the mafic rocks (enriched in Fe ± Mg and depleted in Ca, Na, K, Rb, Ba, Sr, and Eu) except for significant additions of Si that resulted in net mass gains of 20 to 30 percent. Felsic COR rocks of the R1 volcanic suite show gains in K, Rb, and Ba that are attributed to the formation of premetamorphic sericite during an alteration stage dominated by hydrothermal seawater. 
 
In comparison to least altered metavolcanic rocks, mafic and felsic COR rocks are depleted in 18O, with {Delta}18O values 2 to 5 per mil lower than precursor rocks. Estimated temperatures of alteration were 270° to 350°C. These data indicate that the signature of a high-temperature (~300°C) sea-floor hydrothermal upflow zone is preserved within upper amphibolite to granulite facies rocks at Ruostesuo. 
 
The stratigraphic boundary between strongly chloritized mafic volcanic rocks and silicified felsic volcanic rocks marks a transition from net mass losses associated with hydrothermal fluid-dominated alteration to net mass gains associated with seawater-dominated alteration. Silicification at or near the boundary between mafic and felsic volcanic rocks may have been the result of conductive cooling or mixing with seawater from above. In the absence of textural or crosscutting relationships indicative of a younging direction, a stratigraphic model is proposed on the basis of the above geochemical constraints. Altered basalts at Ruostesuo represent the exposed base of a chloritic stockwork zone that passes upsection into altered felsic volcanic rocks that host massive Zn-Cu sulfides. 
 
This stratigraphic model has significance for the interpretation of other massive sulfide deposits in the Vihanti-Pyhasalmi district and exploration strategies that couple chemostratigraphy with mass-change results. More significantly, the preservation of premetamorphic oxygen isotope values for strongly metamorphosed and locally migmatitic COR rocks indicate that oxygen isotope mapping is a viable exploration tool in the district and in other mineralized upper amphibolite to granulite facies terranes worldwide.
129195717
Geochemistry of evaporite-bearing series: A tentative guide for the identification of metaevaporites
Literature on metasedimentary rocks indicates that former evaporites have been assumed where brines are present as fluid inclusions or minerals such as scapolite, tourmaline and lazurite, which are considered as evidence of equilibria with brines. Very few works take into account the chemical composition of rocks, because chemical data on evaporite-bearing sedimentary series are scarce.This paper is a contribution to a geochemical approach: 172 analyses of major and trace elements are given on argillites, dolomitic marls and mixed sulphate rocks from some large evaporitic series, mainly from the Triassic of France, the Devonian of the Canning Basin and the Proterozoīc of the Amadeus Basin in Australia. Compared with rocks from common platform series, these samples clearly exhibit specific chemical features: high Mg contents, high Mg/Ca ratios except for Ca-sulphate-rich rocks and relatively low iron contents caused by the widespread occurrence of Mg-rich clay minerals. Though sulphate-bearing rocks often show low Mg/Ca ratios, their chemical composition clearly indicates Mg-rich clay fractions. K content is high while Na is low, except in a few halite-bearing rocks. Argillites are characterized by high Li and F contents, and high Li/Mg and B/A1 ratios, while major amounts of Sr are recorded in sulphatebearing rocks.Application to metamorphic series is discussed with some illustrative examples. Though chemical changes during metamorphism are probably frequent in series of evaporitic origin, some relations between Al, Fe, Mg, Ca are often maintained. Under nearly isochemical conditions, the resulting metasediments exhibit abundant phlogopite through a large part of the metamorphic column, owing to high Mg, K and F contents. Such rocks are, for instance, well developed in the Zambian Copperbelt, Central Africa. Although further investigations are needed both on nonmetamorphic and metamorphic evaporite sequences, the efficiency of this approach for identifying an evaporitic origin is well established.
160569113
Handbook of Geochemistry
Submission information at the series homepage and springer.com/authors Order online at springer.com ▶ or for the Americas call (toll free) 1-800-SPRINGER ▶ or email us at: customerservice@springer.com. ▶ For outside the Americas call +49 (0) 6221-345-4301 ▶ or email us at: customerservice@springer.com. Handbook of Geochemistry Editor-in-chief: K.H. Wedepohl Series Editors: C.W. Correns, D.M. Shaw, K.K. Turekian, J. Zemann
130125705
Stability of tourmaline in peraluminous granite systems; the boron cycle from anatexis to hydrothermal aureoles
Peraluminous granites and pegmatites derived from the anatexis of metasedimentary rocks commonly pos­ sess elevated B contents, which are manifested as disseminated accessory tourmaline within the igneous bodies or as locally abundant tourmaline or other borateslborosilicates at their margins, in miarolitic cavities, within internal brec­ cias, or in the host wallrocks. Because of the incompatibility of B in all but a few phases, the history of B accumula­ tion and depletion in granites and pegmatites is difficult to establish from whole-rock chemistry. A new experimentally based model provides understanding of how tourmaline (principally schorlldravite-olenite-foitite solid solutions) will behave during anatexis of metapelites, through the fractionation of peraluminous S-type melt, to the magmatic-hydrothermal transition in evolved melts. High B contents of melts at saturation in tourmaline (togeth­ er with other AFM phases) imply that tourmaline should be completely consumed with > 5 % melting of metapelites. Such melts are initially saturated in biotite, cordierite, etc. , and the eventual saturation in tourmaline entails the back-reaction of earlier AFM phases with the B-enriched melt. The B content of melts will not normally remain buffered because of the limited quantity of AFM silicates to form tourmaline. Consequently, B increases to the end stages of melt fractionation. If Li content increases sufficiently, then elbaite may form. Whether late-stage elbaite crystallizes or not, most magmas do not conserve B. Much of the boron originally present in magma is lost to wallrocks, wherein B-precipitating reactions (of tourmaline, axinite, danburite, etc. ) proceed. Key-words: tourmaline, boron, granite, pegmatite, metapelite, geochemistry.
129946674
Chemical and boron-isotope variations in tourmalines from an S-type granite and its source rocks: the Erongo granite and tourmalinites in the Damara Belt, Namibia
Tourmaline is widespread in metapelites and pegmatites from the Neoproterozoic Damara Belt, which form the basement and potential source rocks of the Cretaceous Erongo granite. This study traces the B-isotope variations in tourmalines from the basement, from the Erongo granite and from its hydrothermal stage. Tourmalines from the basement are alkali-deficient schorl-dravites, with B-isotope ratios typical for continental crust (δ11B average −8.4‰ ± 1.4, n = 11; one sample at −13‰, n = 2). Virtually all tourmaline in the Erongo granite occurs in distinctive tourmaline-quartz orbicules. This “main-stage” tourmaline is alkali-deficient schorl (20–30% X-site vacancy, Fe/(Fe + Mg) 0.8–1), with uniform B-isotope compositions (δ11B −8.7‰ ± 1.5, n = 49) that are indistinguishable from the basement average, suggesting that boron was derived from anatexis of the local basement rocks with no significant shift in isotopic composition. Secondary, hydrothermal tourmaline in the granite has a bimodal B-isotope distribution with one peak at about −9‰, like the main-stage tourmaline, and a second at −2‰. We propose that the tourmaline-rich orbicules formed late in the crystallization history from an immiscible Na–B–Fe-rich hydrous melt. The massive precipitation of orbicular tourmaline nearly exhausted the melt in boron and the shift of δ11B to −2‰ in secondary tourmaline can be explained by Rayleigh fractionation after about 90% B-depletion in the residual fluid.
128914521
Meta-evaporite sequence, exhalative chemical sediments and associated rocks in the Proterozoic Willyama Supergroup, South Australia: implications for metallogenesis
Abstract The Willyama Supergroup comprises Proterozoic meta-sedimentary and meta-igneous rocks in western New South Wales and eastern South Australia, and contains the giant Broken Hill PbZnAg deposit, in addition to numerous small stratiform, stratabound and vein mineral deposits. New lithological, geochemical and fluid inclusion data from the Olary Block, west of Broken Hill, suggest the presence of former evaporite rocks now metamorphosed to a variety of chemically and mineralogically unusual lithologies. These evaporite rocks overlie relatively oxidised strata, and are in turn overlain by reduced, graphite-bearing pelites. Rocks of sodic composition abound, represented by albite-quartz-rich rocks (+calcite, actinolite, epidote, garnet, clinopyroxene), inferred to represent metamorphosed analcime-rich strata of felsic volcaniclastic provenance, with variable proportions of carbonates; for example calcite, dolomite, trona and shortite. These rocks are interspersed with potassic rocks, possibly also with an evaporitic origin, in addition to “normal” clastic meta-sediments. The presence of halogen-rich minerals in a distinctive mineral assemblage, hypersaline fluid inclusions, pseudomorphic voids and minerals after gypsum and carbonate, in addition to tourmalines with isotopically light boron isotopes is interpreted as evidence for a non-marine evaporite parent rock for portions of the Olary Block. Types of mineral occurrences in the Olary Block form a continuum between chemical sediments formed by the exhalations of hydrothermal fluids to intrasediment deposits, inferred to have been formed due to the chemical changes imposed on these and/or diagenetic fluids during their passage through the sedimentary pile. Disseminated stratabound pyrite (including cobaltian types) and chalcopyrite extend along strike for many kilometres and are interpreted to have formed by a diagenetic mechanism. The presence of massive sulphide-bearing strata, iron formations, barite, tourmalinite and manganiferous rocks are typical of chemical sediments derived from hot spring sources. The Olary Block meta-evaporites have similar whole-rock compositions to Tertiary analcime-rich rocks of the Green River Formation, Wyoming. These new data imply that portions of the adjacent Broken Hill Block may also comprise meta-evaporites, for example the quartz-albite-rich strata of the Thackaringa Group and the calcsilicates of the lower Broken Hill Group, and that the disseminated cobaltian pyrite mineralisation in the Thackaringa Group may not be of exhalative origin.
133284294
Sedimentary-volcanic successions of the Alta–Kvænangen Tectonic Window in the northern Norwegian Caledonides: Multiple constraints on deposition and correlation with complexes on the Fennoscandian Shield
Airborne geophysical data, sedimentological and geochemical characteristics of carbonate rocks, geochemical features of igneous rocks, carbon isotope chemostratigraphy and radiometric dating form a multiple approach applied for the reconstruction of depositional environments and the age of the sedimentary-volcanic succession exposed in the Alta–Kvænangen Tectonic Window (AKTW) in the northern Norwegian Caledonides. Aeromagnetic geophysical data confirm that the AKTW succession continues beneath the Caledonian nappe complexes and connects with the Kautokeino Greenstone Belt in the main part of the Fennoscandian Shield. The carbonate rocks, mainly dolostones, of the Kvenvik formation are markedly enriched in 13C (d13C = +7.4 ± 0.7‰, n = 51) and record a global positive excursion of carbonate carbon isotopes in sedimentary carbonates known as the Lomagundi –Jatuli isotopic event whose duration was constrained in the Fennoscandian Shield between c. 2220 and 2060 Ma. A radiometric date of 2146 ± 5 Ma (U–Pb, zircon) obtained from a gabbro comagmatic with mafic lavas provides a minimum age for the deposition of the 13C-rich, Lower and Upper dolostones and the accumulation age of the 13C-rich Uppermost dolostone. The carbonate rocks of the structurally overlying Storviknes formation show near-zero d13C (+1.1 ± 1.2‰, n = 41). Carbon isotope chemostratigraphy suggests that their deposition post-dated 2060 Ma which, together with the above mentioned radiometric date, indicates a non-depositional break/hiatus of over 80 Myr separating accumulation of the two neighbouring formations. Sedimentological features of the carbonate rocks are consistent with deposition in a carbonate platform/shelf, whereas the depositional features of shales suggest accumulation in a shallow-water epeiric sea. The 13C-rich dolostones of the Kvenvik formation contain plentiful halite casts and sulphate pseudomorphs reflecting formation of abundant CaSO4 and NaCl in time-equivalent successions across the Fennoscandian Shield, which may represent a source for Na and Cl metasomatism that affected Palaeoproterozoic rocks in the AKTW and across northern Fennoscandia.
129150968
A model for tectonic control of lacustrine stratigraphic sequences in continental rift basins
Lake deposits have similar sedimentary characteristics and occupy similar positions within stratigraphic sequences of numerous continental rift basins that represent a wide range of geologic age, climate, and geographic location. Each lacustrine unit deposited during a phase of large lake development corresponds to a distinct rifting episode. New models for the structural evolution of continental rifts, together with changing depositional patterns through time, suggest that the topography required for large lake development only occurs early in rift history. Temporal changes in relative rates of subsidence and deposition can terminate lake occurrence by filling topographic lows, and preclude further large lake development. Thus, structural evolution, and resulting deposit onal patterns, limit large lake development to a specific interval in a rift's history, and are a primary control on large lake occurrence.