The Mineralogy of the Black Hawk Arsenide 5-Element Deposit

The unusual arsenide five-element-vein deposits of the Black Hawk district in the Burro Mountains, Grant County, New Mexico is one of only a few examples of these types of deposits in the United States. These are unusual deposits due to their scarcity, unusual metal association, and uncommon mineral textures. The typical metal assemblage consists of silver-nickel-cobalt-arsenic-bismuth mineralization, with varying amounts of uranium, copper, antimony, mercury, and zinc. These deposits have been long produced for high grades of silver, and more recently cobalt, nickel, and bismuth, but they are not well studied. The Black Hawk deposits appear to be late Cretaceous in age, and occur within faults of Proterozoic granites, diorites, and metamorphics. Production from the Black Hawk district from 1881-1960 amounts to 1,286,000 oz Ag, 3,000 lbs. Cu, 1,000 oz Au, 4,000 lbs. Pb, and minor tungsten and fluorite. The mineralogical and textural relationships are very similar to those observed from the 5-element system in Cobalt, Ontario, Canada. These include early precipitation of dendritic and skeletal native silver, followed by nickel and cobalt arsenides, such as nickeline, skutterudite, nickelskutterudite, safflorite, and rammelsbergite. This is followed by a sulfide stage and precipitation of minor base metals, such as galena, sphalerite, and chalcopyrite. The last minerals to precipitate are gangue minerals, typically calcite or siderite, with some quartz. The Black Hawk district shows early uraninite precipitation, whereas the Cobalt, Ontario system shows no such uranium mineralization. This assembly of highly reduced metallic phases indicates a reducing agent component to precipitation, and it is theorized that methane or other organic fluids could have caused this rapid crystallization, leading to the development of the observed dendritic and uneven vein filling mineral