New Mexico Geological Society Annual Spring Meeting — Abstracts

New geochemical, geochronological, and geological data, combined with earlier studies, have provided a refinement of the evolution of the mineralization in the Hillsboro (Las Animas) district in central New Mexico. Laramide (polymetallic) vein, placer Au, carbonate-hosted Ag-Mn, Laramide skarn, and porphyry-Cu deposits are found in this district. Past production has been predominantly from the Laramide veins, although minor production has occurred from the carbonate-hosted deposits. During 1877-1982 an estimated 270,000 troy oz Au (lode and placer), 78,000 troy oz Ag, 24 million Ibs Cu, and 153,387 lbs Pb were produced. The Copper Flat porphyry-Cu deposit in the central part of the district was discovered in 1975. Quintana Minerals Corp. produced approximately 7 million Ibs of Cu in March-June 1982, prior to closure of the open-pit mine. Alta Gold Co. is applying for mining permits to reopen the Copper Flat mine.

The geology of the Hillsboro district is dominated by Cretaceous andesite flows (7S.4±3.5 Ma, ⁴⁰Ar/³⁹Ar), breccias, and volcanoclastic rocks that were erupted from an andesite volcano. The Copper Flat quartz monzonite porphyry (74.93±0.66 Ma, ⁴⁰Ar/³⁹Ar) intruded the vent of the volcano. The unmineralized Warm Springs quartz monzonite (74.4±2.6 Ma, ⁴⁰Ar/³⁹Ar) is south of the Copper Flat porphyry. A third altered, unmineralized quartz diorite crops out in the northern part of the district. These two intrusions most likely represent small, satellite stocks that intruded along fracture zones on the flanks of the volcano. Latite and quartz latite dikes intruded the andesite and Copper Flat porphyry and radiate outwards from the Copper Flat porphyry. A ⁴⁰Ar.³⁹Ar plateau age of 70.21±0.20 Ma of a latite dike is distinctly younger than the andesite and quartz monzonite and could represent the true cooling age or was reset by younger hydrothermal activity. The igneous rocks are part of a differentiated comagmatic suite. The andesites are metaluminous and alkaline; the quartz monzonites and latites are peraluminous and alkaline to subalkaline. The linear variation in Nb/Zr, Zr/TiO₂, V/ TiO₂, and various major elements suggests that the igneous rocks are comagmatic. Alteration of the igneous rocks consists of locally intense silicification and K-metasomatism. Large jasperoid bodies have replaced limestones belonging to the EI Paso Fonnation, Fusselman Dolomite and Lake Valley Limestone in the southern part of the district. The Sugarlump Tuff (35 Ma) unconformably overlies some of the jasperoids, indicating jasperoid formation prior to formation of the Emory caldera.

The Copper Flat porphyry-copper deposit consists of Cu, Au, Mo, and Ag disseminations and quartz veins in a breccia-pipe in the quartz monzonite stock. It is predominantly a low-grade hypogene deposit that is concentrated within a breccia pipe containing pyrite, chalcocite, chalcopyrite, azurite, malachite, and cuprite. Copper Flat has reported reserves (Dec. 31, 1997) of 56,549,000 tons of ore grading 0.432% Cu, 0.14 ppm Au, 2.19 ppm Ag and 0.014% Mo.

Many workers in the district have recognized district zoning. The Copper Flat porphyryCu deposit forms the center. Propagating outward radially from the Copper Flat porphyry are Laramide Au-Ag-Cu veins hosted by many of the latite dikes. Chemical analyses range from 864,600 ppb Au, <0.2-590 ppm Ag, 40-57,337 ppm Cu, <1-475 ppm Mo, 57-8906 ppm Pb, and 138-17,026 ppm Zn. Carbonate-hosted replacement deposits (Ag, Pb, Mn, V, Mo, Zn) are found in the southern and northern parts of the district, distal from the center. Chemical analyses range from <5-99 ppb Au, 1-<50 ppm Ag, 131-173 ppm Cu, 2-140 ppm Mo, 30-> 10,000 ppm Pb, and 123->20,000 ppm Zn. Collectively, the evidence suggests that the deposits found in the Hillsboro district were formed by large, convective hydrothermal systems related to the Copper Flat volcano and subsequent intrusion of the quartz monzonite and latite dikes.