Geochemistry of Accessory Apatite from the Cu–Ni–Sulfide-Bearing Ultramafic–Mafic Rocks of the Khudolaz Complex (South Urals) as a Monitor of Magmatic and Metasomatic Processes


 —We present results of mineralogical and geochemical studies (optical and electron microscopy, probe microanalysis, and LA-ICP-MS) of accessory apatite from the rocks of four ore-bearing massifs of the Khudolaz Complex: Vostochnyi Buskun, Severnyi Buskun, Malyutka, and Tashly-Tau. Two groups of apatite are recognized by morphology and chemical composition: (1) “magmatic” unaltered apatite (Ap-1) in unaltered and slightly altered rocks and (2) “metasomatized” apatite (Ap-2) surrounded by secondary silicates that replaced primary minerals. Ap-1 is represented by euhedral and subhedral hexagonal crystals with a high content of chlorine (0.7–1.2 wt.% Cl) and an extremely low content of sulfur (<0.05 wt.% SO3). It is enriched in REE (ΣREE = 2.2–3.0 wt.%), shows a pronounced negative Eu anomaly (Eu/Eu* = 0.36–0.58), and is depleted in Co, Ni, and chalcophile trace elements (Zn, Pb, and Bi). The distribution of major and trace elements in Ap-1 made it possible to identify its two generations, early and late, in the Khudolaz Complex. The early generation, characterized by a low content of Ca (Т ≥ 1000 °C, CaO < 54 wt.%), crystallized almost simultaneously with Ca-plagioclase (An81–61). The late generation, characterized by a high content of Ca (Т ≥ 700 °C, CaO > 55 wt.%), is associated with Na-plagioclase (An28–8), amphibole, and phlogopite. Ap-2 is represented by highly cracked grains and is characterized by a low content of chlorine (0.0n–0.5 wt.% Cl) and a high content of sulfur (0.06–0.93 wt.% SO3). The content of fluorine in both groups of apatite varies in the same range (0.6–1.7 wt.% F), which determines its relative inertness during metasomatic processes. Ap-2 is less enriched in REE (ΣREE = 1.1–2.1 wt.%) and shows a less pronounced Eu anomaly (Eu/Eu* = 0.61–0.77) but has high contents of Co, Ni, and chalcophile trace elements. The Ap-1 and Ap-2 show different variations in contents of major elements (Fe, Mg, Na, etc.), which is due to the impact of hydrothermal fluid on the mineral at the postmagmatic stage. It is shown that the low content of sulfur in apatite from unaltered rocks does not indicate a low sulfide ore potential of the Khudolaz Complex. The Cu–Ni sulfide signature of the complex is more clearly evidenced by the composition of metasomatized apatite enriched in S, Co, Ni, and chalcophile elements as a result of their hydrothermal removal from primary sulfide minerals. The high Eu/Eu* value indicates an increase in oxygen fugacity at the postmagmatic stage.