Supergene Copper Enrichment at Hanover Mountain, New Mexico

Abstract

Hanover Mountain is a supergene enriched chalcocite deposit located in the Central Mining district, southwest New Mexico, near Silver City. The Central Mining district is a complex, polymetallic district from which Cu, Fe, Zn, Pb, and Ag have been mined. Deposit types present in the district include porphyry, skarn and vein deposits. Primary mineralization in the district is Laramide in age. Hanover Mountain is near, but separate from other deposits in the district, but is believed to be a part of a larger hypogene system related to igneous intrusion. Detailed surface mapping of Hanover Mountain and measurement of preferred structural orientations, leached capping and alteration were completed as part of this research project. Hanover Mountain is somewhat unusual compared to the other deposits because the mineralization is located in the Colorado Formation, an Upper Cretaceous, heterolithic sedimentary unit composed of shale, siltstone, and sandstone. The Colorado Formation was deposited during a regressive period and ranges from shallow marine to fluvial in origin. In the study area the Colorado Formation has a weak metamorphic overprint. Some of the heterogeneity in the Colorado Formation at Hanover Mountain has been interpreted as facies changes. Rocks on the south side of the mountain are more sandy and silicified. Finer-grained facies predominate on the north side of the mountain and at lower elevations. The predominant structural grains at Hanover Mountain are N-S, NE and ENE. The mountain is bounded on the southeast by the Barringer fault, a NE-striking, district-scale fault with approximately 1500' of displacement. The Barringer Fault drops sediments down to the northwest, juxtaposing Cretaceous clastic rocks with Paleozoic limestone. Two types of dikes crop out on the surface of Hanover Mountain. Early, mafic plagioclase-hornblende porphyry dikes occupy E-W structures; younger granodiorite porphyry dikes are found in N-S structures. The leached cap on Hanover Mountain is dominantly goethitic and ranges from 20-200' thick. Mapping of the distribution of hematite, goethite, and limonite indicates the existence of at least one previous enrichment blanket. In the study area the Colorado Formation is pervasively sericitized. The enrichment blanket is up to about 100' thick and mimics topography. Relict primary pyrite and chalcopyrite incompletely replaced by chalcocite indicate an immature enrichment blanket.