ALTERATION AND MINERALIZATION ASSOCIATED WITH A LARAMIDE AGE COPPER PORPHYRY DEPOSIT, GILA COUNTY ARIZONA

The Chilito porphyry Cu-Mo prospect is located in southwestern Gila County, Arizona, and comprises favorable porphyry copper deposit terrain along a trend between two large, producing copper-molybdenum ore deposits: the Christmas Mine, closed in 1979 and the Ray Mine, currently in operation. The prospect area consists of a horst block bound on the west by the Keystone Fault and on the southeast by the O’Carroll Fault. There are several historic copper, lead-zinc and manganese replacement deposits adjacent to the Chilito property, peripheral to the horst block.

During the Laramide Orogeny, a felsic to intermediate-composition stock intruded host rock Proterozoic-age sedimentary rocks; this stock produced fluids that yielded the copper mineralization occurring in the Chilito prospect. ³⁹Ar/⁴⁰Ar geochronology has yielded late Cretaceous ages on biotite and potassium feldspars related to the dioritic stock and sericitic alteration consistent with other deposits in this part of Arizona. Detailed mapping of several thousand feet of core reveals that the composition of the stock evolved from a crowded quartz diorite porphyry to an increasingly mafic plagioclase pheric biotite porphyry. Importantly, copper and molybdenum mineralization is well-developed at the margins of the stock and at depth. Continued drilling-evaluation of the property is underway, and suggests that the Chilito occurrence is part of a general porphyry copper belt in this part of Arizona.