Geochemistry of the Copper Flat porphyry and associated deposits in the Hillsboro mining district, Sierra County, New Mexico, USA

Abstract

New geochemical, geochronological, and geological data, combined with earlier studies, have provided a refinement of the evolution of mineralization in the Hillsboro district in central New Mexico. Laramide (polymetallic) vein, placer gold, carbonate-hosted Ag–Mn and Pb–Zn, and porphyry-copper deposits are found in this district. The Hillsboro district is dominated by Cretaceous andesite flows (75.4±3.5 Ma), breccias, and volcaniclastic rocks that were erupted from a volcano. The mineralized Copper Flat quartz-monzonite porphyry (CFQM, 74.93±0.66 Ma) intruded the vent of the volcano. The unmineralized Warm Springs quartz monzonite (74.4±2.6 Ma) and a third altered, unmineralized quartz monzonite intruded along fracture zones on the flanks of the volcano. Younger latite and quartz-latite dikes intruded the andesite and CFQM and radiate outwards from the CFQM; the polymetallic vein deposits are associated with these dikes. The igneous rocks are part of a differentiated comagmatic suite. Alteration of the igneous rocks consists of locally intense silicification, biotite, potassic, phyllic, and argillic alteration. Large jasperoid bodies have replaced the El Paso Formation, Fusselman Dolomite, Lake Valley Limestone, and Percha Shale in the southern part of the district. Many workers in the district have recognized district zoning. The low-sulfur (<7%) porphyry-copper deposit forms the center. Trending radially from the CFQM are Laramide Au–Ag–Cu veins. Carbonate-hosted replacement deposits (Ag, Pb, Mn, V, Mo, Zn) are located in the southern and northern parts of the district, distal from the center. Collectively, the evidence suggests that the deposits found in the Hillsboro district were formed by multiple convective hydrothermal systems related to the Copper Flat volcanic/intrusive complex.