New Mexico Geological Society Annual Spring Meeting — Abstracts

The Black Hawk mining district in the Burro Mountains near Silver City, New Mexico was studied by field mapping, petrography, whole rock major and minor element geochemistry, fluid inclusion microthermometry, and K-Ar dating methods.

The predominant lithologies in the district are a Precambrian monzodiorite gneiss and a late Cretaceous monzonite porphyry stock (72.5 ± 4.7 m.y.), which intrudes the gneiss. The gneiss contains 35 ppm Co and 23 ppm Ni, while the monzonite contains 20 ppm Co and 11 ppm Ni. Ag-Ni-Co-U mineralization occurs in NE trending fissure veins, dominantly in the monzodiorite gneiss. The major are minerals, according to Von Bargen (1979), are native silver, argentite, skutterudite, rammelsbergite, and uraninite within a carbonate gangue. Pyrite, quartz, clay, and carbonate are the most commonly occurring alteration minerals along the vein. Away from the vein, the mafic minerals are altered to Chlorite. feldspars to sericite, and disseminated pyrite occurs along fractures. A whole rock sample of the vein alteration assemblage yielded a date of 65.3 ± 1.2 m.y.. There is a bimodal distribution of homogenization temperatures from the Black Hawk mine; one population with an average temperature of 357 degrees Celsius (299-404) and another with an average of 189 degrees Celsius (155-210). The salinities of the fluids were 0.0 to 2.6 eq. wt. % NaCl. Boiling occurred as evidenced by the coexistance of vapor and liquid filled inclusions.

The petrography, geochemistry, fluid inclusion, and K-Ar age data allow the following conclusions to be made. First, the source for the Ni and Co was probably the monzodiorite since it has the largest aerial distribution in the district and the highest Ni and Co concentrations of the predominant lithologies. Second, the low salinities of the fluids suggest that the water was meteoric in origin. Third, the high temperatures and the relatively similiar ages of the intrusion and alteration suggests that the monzonite porphyry intrusion was the heat source for the hydrothermal system. Finally, boiling and cooling of the fluids along the northeastern fractures were responsible for the Ag -Ni -Co-U mineralization.