New Mexico Geological Society Annual Spring Meeting — Abstracts

The Alabama mine is located along the north-trending Alabama fault in the eastern portion of the Steeple Rock district, Grant County, New Mexico. Production occurred sporadically from the late 1800s until 1951. Total production is estimated at about 4000 tons of ore averaging 0.3 oz/t Au, 13 oz/t Ag, and 0.03% Cu. In 1990 Great Lakes Exploration, Inc. conducted a surface sampling program and, in 1991, drilled two diamond drill holes as part of an exploration program for gold and silver.

The Alabama fault is formed by silicified breccia consisting of fragments of rhyolite and andesite porphyry. The host rocks consist of a rhyolite tuff overlain by andesite porphyry of Dark Thunder Canyon, both of which are younger than 28 Ma. Rhyolite dikes and plugs have been intruded along the fault. The breccia zone is about 7-10 ft wide at the shaft and varies along strike between 4 and 20 ft. The fault is about 1500 ft long and merges with northwest-trending faults. The ore at the Alabama mine is structurally controlled and offset by a northwest-trending fault south of the main shaft.

Ore consists of gold and silver with trace galena, sphalerite, and chalcopyrite in a gangue of quartz, hematite, pyrite, and sericite. Silicification and chloritization occurs near the vein. Seams of calcite cut the breccia at depth. Fluid inclusion analyses indicate an average T_H of 256°C (No. =60, STD= 16°) for quartz. Only one inclusion was found in sphalerite which had a T_H of 265°C. T_H probably approximates the temperature of formation. Salinities are low (average 1.9 eq. wt% NaCl, STD = 1.3, No. =50). Fluid inclusions are two-phase, liquid-dominated inclusions ranging in size from less than 1µ to 20µ.

Fifty-one surface samples were collected and assayed for Au, Ag, Sb, Hg, As, and F. Nineteen of these were analyzed for additional major and trace elements. Statistical analyses indicate a strong correlation between Au and Ag (pearson correlation coefficient = 0.83) and a weak correlation between Au and Hg (pearson corr. coef. = 0.48). No other elements correlated with Au or Ag, except SiO₂.

These data are consistent with an epithermal origin related to the rhyolites. Subsequent fluid migration along the northwest-trending faults may have depleted the Alabama orebody locally. Exploration for Au and Ag is difficult because Au and Ag do not correlate with typical pathfinder elements, although locally Hg correlates with Au. Additional drilling is recommended.