Alteration and fluid inclusion study of the Groundhog vein system, Central mining district, New Mexico

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Polymetallic vein and skarn orebodies in the Groundhog mine, Central district, New Mexico, produced more than $590 million in silver, zinc, copper, lead, and gold between 1870 and 1977 (1990 prices). Mineralization occurs along the northeast-trending Groundhog fault zone, and underground workings extend to within 2 km of the Santa Rita porphyry copper deposit. Vein orebodies were mined from the upper 200 m of the deposit whereas extensive skarn and replacement orebodies were mined between 400 and 700 m below the surface.In the upper 200 m of the deposit, early Tertiary granodiorite porphyry dikes, injected along the Groundhog fault zone, crosscut Upper Cretaceous sandstone, shale, and two stages of diorite sills. Propylitic alteration characterized by albite, chlorite, calcite, epidote, and sericite accompanied each igneous intrusion.Hydrothermal fluid circulation followed a stage of normal faulting, resulting in quartz-base metal-silver precipitation in open fissures such as the Crescent ore shoot (330 X 120 X 7 m in size). Next to veins, diorite sills are altered to quartz-sericite-pyrite-clay, with local silicification. Away from veins, sericite decreases and chlorite-pyrite becomes more abundant. Vein-related chlorite is 5 to 16 times more manganese rich (2.8-8 mole % MnO) than chlorite formed in the propylitic alteration stage (0.4-1.4 mole % MnO).Paragenetic vein relationships indicate that early quartz and pyrite were followed by mainstage base metal mineralization, consisting dominantly of sphalerite, galena, chalcopyrite, and pyrite. Sphalerite is more iron and manganese rich at depth in the vein system; FeS increases from 0.8 to 7.3 mole percent and MnS increases from 0.2 to 1.2 mole percent over a 350-m vertical sampling range.Fluid inclusions in vein quartz homogenize between 270 degrees and 405 degrees C, with evidence for local boiling at 390 degrees C. Vapor-rich inclusions were only observed in preore quartz. Primary inclusions in sphalerite homogenize between 270 degrees and 340 degrees C, with a peak at 305 degrees C. The hydrothermal fluid contained an average of 7 equiv wt percent NaCl. Daughter minerals, possibly including dawsonite, anhydrite, and carbonate, are present in some inclusions.Alteration zoning, fluid inclusion data, and element zoning within Groundhog vein orebodies suggest that mineralization occurred during mixing with late-stage meteoric water. Fluid flow in a southwest direction has been documented in the underlying skarn ore zone (Meinert, 1987), whereas fluid flow in the vein ore zone appears to have been primarily vertical, with a smaller component of lateral flow. A pressure-depth estimate determined from fluid inclusion analyses suggests that 2 km of volcanic rocks may have been present above the Groundhog area at the time of mineralization and has since been removed by erosion. Comparison with other base and precious metal vein systems suggests that precious metal mineralization may have been present in the overlying, now-eroded volcanic rocks and may still constitute an unexplored target in the peripheral parts of the district.