New Mexico Mineral Symposium — Abstracts

The Chino (Santa Rita) deposit is a porphyry copper ore body that includes intrusive and skarn¬hosted copper mineralization. The largest copper-producing mine in New Mexico, and among the oldest in the United States, Chino is set in a complex geological setting in the transition zone between the Colorado Plateau and Basin and Range physiographic provinces. Early Tertiary mineralization is associated with a generally porphyritic composite intrusive, varying in composition from granodiorite to quartz monzonite, that has intruded and domed surrounding Paleozoic and Cretaceous sedimentary rocks. Sedimentary rocks in the district include Paleozoic sandstone, limestone, dolomite, and shale. The upper Cretaceous rocks include sandstone, siltstone, shale, and minor shaley limestone of the Colorado and Beartooth Formations. The sedimentary section was also intruded by Late Cretaceous quartz diorite sills that predate the main stock intrusive but are not believed to be associated with mineralization. Three ages and compositions of dikes cut the intrusive, including granodiorite, quartz monzonite, and quartz latite. The granodiorite may occasionally be mineralized, while the latite is unmineralized and the quartz monzonite only rarely, but is seldom of ore grade. Post-mineral (mid-Tertiary) volcanic rocks overlie the southern and southeastern part of the deposit. These rocks, extruded about 35 m.y. ago, include the Sugarlump and Kneeling Nun tuffs, and probably initially overlaid the entire deposit. The tuffs are overlain, in places, by basaltic andesite flows.

Hydrothermal alteration and mineralization is associated with the intrusion of the Santa Rita composite stock, which has been dated by ⁴⁰Ar/³⁹Ar methods at between 58.1 and 59 m.y. The stock was intruded at the junction of three sets of faults including pronounced northwest and northeast sets and a less prominent easterly set. The intrusion is elongate in a northwest-southeast direction. Abundant mineralized fractures show the stock was intensively fractured closely following solidification and attending hydrothermal alteration. Non-skarn alteration includes k-spar, biotite, quartz-sericite-pyrite, and argillic alteration. Pennsylvanian and Mississippian sedimentary rocks, primarily limestone, that were invaded by the magma were completely altered and replaced by calc-silicates and associated contact metasomatic minerals. Common skarn minerals include magnetite, pyrite, quartz, garnet, epidote, actinolite, chalcopyrite, and others in lesser amounts. The alteration minerals are generally controlled by their proximity to the stock. However, some, such as magnetite, are also controlled laterally by the composition of the Paleozoic host rocks.

Chalcopyrite is the primary hypogene copper mineral in both skarn and unenriched intrusive rocks. Without secondary chalcocite enrichment, however, little other than the skarns would be of ore grade. Cretaceous elastic sedimentary rocks, and the quartz diorite porphyry sills within surrounding sedimentary rocks, are also secondarily enriched, frequently to ore grade. The upper Paleozoic rocks, though extensively altered, were typically reactive to descending supergene solutions and, consequently, were not greatly susceptible to secondary enrichment. Subsequent oxidation of the chalcocite enrichment blanket has resulted in the formation of some of the better-known secondary copper minerals in the deposit, including native copper, cuprite, libethenite, and turquoise (see list). Specimen-grade native copper and cuprite were extensively collected in late 1993 and early 1994, when numerous samples of coarsely crystalline copper with coatings of cubic cuprite crystals were found during mining in the East Pit area of the mine.