The Mineralogy, Paragenesis, and Alteration of the Camino Rojo Deposit, Zacatecas, Mexico
Geological Sciences and Engineering
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ABSTRACTThe Camino Rojo Zn-Pb-Ag-Au deposit, located in north-central Mexico, has characteristics of intermediate to low sulfidation epithermal deposits, skarns, and carbonate replacement deposits. Mineralization is hosted in sulfide-rich vein networks and manto-style replacements within calcareous, Cretaceous sedimentary rocks. U-Pb and fission track dating of zircon grains from a mineralized dike constrained the maximum age of the deposit to about 73Ma. Primary sulfides at the deposit include pyrite, sphalerite, arsenopyrite, galena, marcasite, chalcopyrite, electrum, acanthite, pyrargyrite, and tennantite-tetrahedrite. The deposit veins and manto replacement bodies are composed of massive-textured, intergranular sulfides and are concentrated in the carbonaceous siltstone and sandstone of the Caracol Formation. Disseminated sulfides commonly spread outward along bedding planes. Gangue mineralogy includes calcite and potassium feldspar, with lesser amounts of quartz, sericite, and fluorite. Two stages of mineralization were identified using optical petrography. Stage 1 mineralization consists of extensive pyrite-sphalerite-arsenopyrite-galena- chalcopyrite deposited as veins and mantos. Stage 2 mineralization was further divided into two parts: (2a) includes electrum± galena± chalcopyrite± acanthite filling fractures within stage 1 sulfides; and (2b) includes calcite, galena, pyrargyrite, acanthite, and argentiferous tennantite-tetrahedrite within veins which can cross-cut stage 1 mineralization. Over 80% of the electrum at the deposit formed during Stage 2a. Stage 2a and 2b have not been observed to be temporally or spatially associated. Three alteration events were recognized at Camino Rojo. First, early potassic metasomatism and decarbonization bleached and hardened the sedimentary host rocks through K-feldspar flooding of the host matrix. Second, an ore-stage calcite -sericite± pyrite ± quartz alteration overprinted the potassic metasomatism. The final alteration stage is characterized by late calcite dissemination and multiple generations of cross-cutting calcite veins and veinlets. Hornfels texture within the host rocks indicates proximity to a deeper intrusion. Skarn formed within the limestone units underlying the bulk of deposit mineralization. The Camino Rojo deposit is likely part of a larger porphyry system, and the veining and manto replacement which host mineralization may be a transition between distal skarn and the epithermal environment.