New Mexico Geological Society Annual Spring Meeting — Abstracts

The Copper Flat deposit, located in southwestern New Mexico, is a Laramide porphyry copper deposit with associated molybdenum, gold, and minor silver. The hydrothermal evolution of this deposit is associated with an andesite that has been intruded by the mineralized Copper Flat quartz monzonite (CFQM) and a series of dikes that radiate out from the CFQM. The CFQM contains a breccia pipe that hosts high-grade mineralization.

Preliminary petrographic analyses indicate the evolution of the porphyry system is characterized by three alteration assemblages: A biotite-potassic, a potassic, and a quartz-sericite assemblage. The biotite-potassic assemblage is characterized by potassium feldspar and biotite flooding in which plagioclase is replaced by orthoclase and igneous biotite is replaced by hydrothermal biotite. The potassic is characterized by the presence of potassium feldspar rimming plagioclase and a minor to moderate amount of chloritization of the biotite. The quartz-sericite suite is distinguished by the replacement of plagioclase and biotite by white phyllosilicates. Minor argillic alteration exists locally and is fracture-, vein-, and fault-controlled. This suite is recognized by the replacement of feldspars by and the filling of veins with clays. The biotite-potassic and potassic assemblages are associated with the majority of copper mineralization.

The breccia pipe consists of quartz monzonite fragments surrounded by a matrix of mainly quartz, biotite, pyrite, chalcopyrite, and molybdenum. Two types of breccias within the pipe are distinguished, a biotite breccia and a quartz breccia, both classified depending on the relative amounts of biotite and quartz present. Alteration of the breccia occurred both pre- and syn-brecciation. This is evidenced by breccia clasts of mineralized and altered quartz monzonite exhibited in core. δ³⁴S values of chalcopyrite and pyrite extracted from the breccia matrix range from 0.9 to 1.8 per mil and from -0.1 to 2.2 per mil, respectively, which is consistent with a magmatic origin for the breccia.

Ongoing investigations focus on the sources of hydrothermal alteration, determining temperatures and pressures at the time of brecciation, and documenting the distribution of gold throughout the deposit.