Geology of the Fresnillo area, Zacatecas, Mexico
The Fresnillo area, in the state of Zacatecas in north-central Mexico, comprises a gently rolling terrain near the western edge of the Mexican Central Plateau. It is underlain by 1,800 m of Lower Cretaceous marine beds that are subdivided into four lithostratigraphic units; this sequence is mostly clastic in the lower part and becomes progressively more limy upward. Lower Cretaceous strata rest unconformably on deformed Upper Triassic andesitic pillow lava and greenstone, considered to be allochthonous, and are covered by Upper Cretaceous flysch-type sedimentary rock. Folded Mesozoic strata are overlain unconformably by lower Tertiary continental clastic and associated volcanic rocks, remnants of middle Tertiary rhyolitic pyroclastic flows, and upper Tertiary basalt flows. Quaternary deposits consist of alluvium in valleys and caliche on hillslopes. Early Tertiary deformation produced open north-northwest–trending folds that were intruded at two localities by small stocks of granodiorite and quartz monzonite. This was followed by block faulting, erosion, rhyolitic volcanism, further erosion, and finally basaltic volcanism. Caliche developed over wide areas in late Tertiary time, and subsequent erosion resulted in the accumulation of alluvium in valleys. Mining operations at Fresnillo started in 1553 and to date have produced more than 12 tons of gold, 775 tons of silver, and about 1,000,000 tons of combined lead and zinc. Ore bodies are folded mantos, fissure veins, and related stockworks. Ore minerals are pyrite, sphalerite (marmatite), galena, chalcopyrite with some pyrrhotite, arsenopyrite, pyrargyrite, tetrahedrite, polybasite, matildite, and pavonite. Gangue minerals are quartz, calcite, scheelite, siderite, and graphite.