Abstract

Quartz veins in the Chloride mining district, Sierra County, New Mexico, host an unusual iron-poor calc-silicate and copper-rich, iron-poor sulfide mineral assemblage. The Hoosier vein, crosscutting Paleozoic sedimentary and Tertiary volcanic rocks, was studied in detail. Bornite, chalcocite, chalcopyrite, galena, sphalerite, and traces of pyrite and hematite are present. Silver occurs in chalcocite (less than 32 wt %) and bornite (less than 0.7 wt %). Calc-silicates include pyroxene (Jo (sub 18-91) , Di (sub 8-73) , Hd (sub 0-18) , garnet (Ad (sub 20-74) , epidote Ps (sub 30-40) , prehnite, and subcalcic amphibole. Three paragenetic stages have been recognized. Stage 1 brecciation, cemented by quartz, garnet-epidote, and sulfides, cuts pyroxene skarn replacing carbonate rocks and K feldspar alteration of volcanic rocks. Stage 2 silver-rich bornite-chalcocite overlaps the brecciation and banded vein stages. Stage 3 banded quartz veins with minor sulfides or calc-silicates cut earlier stages. Average quartz fluid inclusion T h values range from 284 degrees (200 m below the present surface) to 261 degrees C (present surface) and decrease to 241 degrees C in later, shallow veins. Stage 1 fluids were boiling, indicating a paleodepth of 500 m. Fluid salinity in early stages varied from 0.35 to 5.85 wt percent NaCl equiv. Later stage fluids had low salinities (less than 1 wt % NaCl equiv). Early skarn fluids evolved into stage 1 fluids with increasing a (sub O 2 ) and pH due to boiling. High a (sub O 2 ) and near-neutral pH suppressed iron solubility and decreased available reduced sulfur. Fluids from adjacent red beds possibly supplied metals for stage 2 mineralization.

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