Geology of a disseminated copper deposit near Cerrillos, New Mexico

Abstract

A small, primary disseminated copper deposit is located in the Cerrillos mining district, Santa Fe County, New Mexico, about one mile north of the town of Cerrillos.Rocks in the Cerrillos district consist of latitic flows and monzonitic and syenitic intrusives of probable Early Tertiary age. The disseminated copper mineralization is located between the Cash Entry and Mina del Tiro mines, around a small, circular coarse-grained monzonite stock that has invaded monzonite porphyry and pink syenite. Spectrographic analysis of 1,600 surface rock samples shows that a copper anomaly forms an annular ring up to 600 feet wide nearly encircling the coarse-grained monzonite stock. The anomalous area contains more than 2,000 ppm copper-in-rock.In the area of the copper anomaly, the mineralization at the surface consists of green copper carbonates and silicates. Drilling has revealed that the mineralization at depth consists of thin veinlets and disseminated grains of chalcopyrite and rare bornite. The best grades of copper mineralization are found in silicified monzonite that also contains secondary biotite. The grade of the oxidized portion of the deposit is essentially the same as that in the unoxidized portion and virtually no secondary enrichment has occurred. As a consequence, the deposit is considered to be a primary disseminated copper deposit, the upper part of which has been slightly modified by surface oxidation.A zone of pyritization associated with strong argillic alteration and turquoise is found outside of the zone of copper mineralization.