New Mexico Geological Society Annual Spring Meeting — Abstracts

The Black Hawk district of southwestern New Mexico contains veins of the Ag-Ni-Co-U type. Mineralization consists of native silver, nickel and cobalt arsenides and sulpharsenides in carbonate gangue. The sequence of ore mineralization is pitchblende-native silver-Ni and Co arsenides and sulpharsenides-sulfides. The sequence of gangue mineralization is calcite-siderite-ankerite-rhodochrosite. These sequences are interpreted to indicate decreasing oxygen fugacity, and constant, low temperatures. Intermediate argillic alteration developed next to the veins suggests H⁺ metasomatism adjacent to the veins, while the development of propylitic alteration farther away indicates that CO₂ metasomatism is more significant out into the wall-rock. This corresponds to acidic consitions in the fluids, and increasing pH out into the wall-rocks.

The veins are inferred to have formed as a result of the intrusion of a dioritic porphyry stock into a volcanic/sedimentary rock sequence. Thermal convection of meteoric waters, probably mixed with a magmatic component, leached ore elements from the country rocks, and reaction of the oxidized fluids with reducing wall-rocks initiated the precipitation of the veins.