Circular 132—Trace base metals, petrography, and alterations of Très Hermanas stock, Luna County, New Mexico

By P. Doraibabu and P. D. Proctor, 1973, 29 pp., 11 tables, 25 figs.

Spatial relationships exist between geochemical anomalies of zinc, lead, and copper with respect to surrounding sedimentary rocks and alluvium, known mineralized areas, petrographic textures, and alteration. Potential mineralized zones for exploring for zinc and lead are suggested. The exposed Tertiary granite-quartz monzonite stock, comprising almost half of the Tres Hermanas Mountains, and some of the alluvial area to the north, was studied in respect to its content of trace elements of copper, lead, and zinc, and its petrography and alteration. Purpose of the study was to determine the spatial inter-relationships of these features within the stock and to the known metallic mineralization around the stock. Discussing in detail the geology, ore microscopy, mineralogy of ore bodies, and ore bodies per se was not the purpose of this study. These data are already published. The Tres Hermanas mining district has produced moderate amounts of lead and zinc and minor amounts of silver, gold, and copper.

The current study suggests that certain trace concentrations of lead, zinc, and copper, and rock alteration types within the stock may relate to known and possible zones of mineralization around the periphery of the body. The mineral composition of the granite-quartz monzonite porphyry, grain size, and isolated hydrothermal alteration zones are related to the trace element trends noted. Confirmation of such relationships in other productive stocks could yield an exploration tool that would be most useful in identifying productive stocks and predicting general locations of possible mineralized areas about them. Zinc, lead, and minor copper-bearing replacement and vein-type deposits occur in Paleozoic sedimentary rocks and Tertiary volcanic rocks adjoining the Tres Hermanas granite-quartz monzonite stock, Luna County, New Mexico. In an attempt to prove or disprove a possible genetic relationship between these mineral deposits and the stock, 99 grid samples from the stock and 22 soil samples from a re-entrant alluvial valley were analyzed for trace contents of zinc, lead, and copper. Thirty-four grid-rock samples were petrographically studied and modal analyses calculated and alteration products estimated. Trace metal zinc, lead, and copper anomalies are spatially related to each other.

Zinc and lead anomalies are almost congruent with less agreement with low-level copper anomalies. A close spatial relationship exists between anomalous trace concentrations of zinc-lead-copper in the stock and external hydrothermal deposits in Paleozoic sedimentary rocks and Tertiary volcanics. The ratios of highest zinc, lead, and copper concentrations in the anomalies of the stock show general agreement to the ratios of production of these same metals from the adjacent mineral deposits. Alteration appears to be a general requisite for trace metal anomalies within the stock. A relationship also appears to exist between certain primary petrographic textures and the anomalies of zinc, lead, and copper suggesting a possible late magmatic or deuteric origin for at least one stage of the development of the anomalies. Yet, in combination with rock alteration, a two-phase or continuous-phase origin of the anomalies appears likely. The isolated character of the trace metal anomalies, their patterns within the stock, the decrease in the magnitude of the anomalies near the stock contacts marginal to mineralized zones, and primary petrographic textures and alteration features associated with the anomalies, suggest that the source of the geochemical anomalies and the mineral deposits was the stock. In the alluvial area north of the stock, geochemical anomalies of zinc, lead, and copper suggest possible nearby sources of metal in the bedrock.