THE ORIGIN OF POLYMETALLIC ARSENIDE FIVE-ELEMENT VEINS IN THE BLACK HAWK DISTRICT, GRANT COUNTY, NEW MEXICO

Arsenide five-element vein deposits are Ag-Co-Ni-Bi-As bearing vein systems with additional elements such as U, Cu, Pb, Zn, Sb, Hg, and others. Many of these elements are critical minerals. Critical minerals are nonfuel mineral commodities that are essential to the economic and national security of the United States that have potential supply disruptions. The arsenide five-element vein deposits are unusual deposits that consist of high-grade silver. Ag, Bi, and As typically occur as native elements, and Co and Ni as arsenides and/or sulfides. Carbonates such as calcite, dolomite, and siderite are the most common gangue minerals, although barite, quartz, and fluorite are locally present. Our study characterizes the arsenide five-element vein deposits in the Black Hawk district in the Burro Mountains, Grant County, New México. From 1881 to 1960 the district produced approximately 1,286,000 ounces Ag. The Black Hawk deposit is hosted within north-trending faults in Mesoproterozoic calc-alkaline granites and metamorphic rocks and comprises open-space fillings within numerous fissure-style veins and breccias. Early studies in the district identified Ni, Co, Ag, and U minerals. Major minerals observed in our study suggest a preliminary paragenesis of early, brecciated pyrite, locally as black, reduced clasts. Sulfides, which formed during the earliest mineralization stage, are represented mainly by pyrite, chalcopyrite, sphalerite, and galena. Fluorite is present as local, pre-sulfide, euhedral microcrysts. Following brecciation, pyrite is surrounded and partially replaced by a series of Ag minerals and, locally, Ni- and Co-arsenides. Ag is present as native metal and incorporated in other phases such as sulfides (e.g., acanthite) and sulfosalts (e.g., freibergite). At least two generations of native silver are observed, with acanthite as the primary phase. Skutterudite and acanthite display replacement textures showing that Co-Ni-Ag-As-S activities alternated substantially following early pyrite development and prior to the latest vein brecciation and banded, multi-stage carbonate precipitation. Our efforts contribute to the understanding of the mineralogy and geochemistry of these potentially important sources of critical minerals.