New Mexico Geological Society Annual Spring Meeting — Abstracts

Arsenide five-element vein deposits are Ag-Co-Ni-Bi-As bearing vein systems with local 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, and is from a supply chain that is vulnerable to global and national disruption. The arsenide five-element vein deposits are unusual deposits and consist of high-grade silver (1000’s g/t Ag), but are low tonnage (<1 Mt). These elements have different chemical properties and aren’t normally found in the same environment. Ag, Bi, and As occur as native elements and Co and Ni as arsenides and/or sulfides. Ag in most other types of deposits are typically found with Cu and Au, which are rare in these deposits. Carbonates such as calcite and siderite are the most common gangue (non-economic) 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 in Grant County, New México. From 1881 to 1960 the district produced approximately 1,286,000 million ounces Ag, along with 1000 lbs Cu, 4000 lbs Pb, 1000 oz Au, and some tungsten and fluorite. The Black Hawk deposits are 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, typically in zones having strike lengths of more than 30 m and widths of less than 1 meter with a vertical extent of 183 m. A sample of altered material adjacent to a vein was dated as 65.3±1.2 Ma (K-Ar, Gerwe, 1986). A 100 lb sample taken from below the 8^th level of the Black Hawk mine assayed 8.82% Ni, 0.9% Co, 8.8% Zn, and 2,542 oz/ton Ag (H. Schmidt, unpublished report, 1/7/1958). Early studies in the district identified Ni, Co, Ag, and U minerals. Major minerals observed in our study suggests a preliminary paragenesis of early, brecciated pyrite, locally as black, reduced clasts. Sulfides, which formed during the earliest mineralization satge, 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. The deposit represent an environment with Ag, which is represent as pure metal and incorporated in otherphases such as sulfides (e.g., acanthite and jalpaite) 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 latest vein brecciation and banded, multi-stage carbonate precipitation.Our efforts contribute to the understanding of mineralogy and geochemistry of these potentially-important sources of critical minerals.