The Copper Hill Cu-Ag-Sb deposit, Picuris Range, New Mexico; retrograde mineralization in a brittle-ductile trap

Abstract

The strata-bound copper-silver-antimony deposit at Copper Hill, New Mexico, developed in a distinctive structural here termed a brittle-ductile trap, during regional retrograde metamorphism of Early Proterozoic metasedimentary rocks. The deposit is localized near the contact between massive orthoquartzite (Ortega Formation) and the overlying schists (Rinconada Formation). Peak regional metamorphism at 4 kbars and 500 degrees C occurred near the end of a regional shortening event that formed the upright, west-plunging Copper Hill anticline. At Copper Hill, late metamorphic, vertical, north-south-striking faults and fractures cut the Ortega Formation quartzite and terminate against folded Rinconada Formation schists. Synmetamorphic quartz veins, several centimeters to 1 m thick, fill many of the fractures and terminate against the overlying schists. At the quartzite-schist contact, many of the veins merge into mushroom-shaped bodies that project along the contact. Fracture-controlled and disseminated Cu-Ag-Sb minerals occur within the quartz veins and in quartzite near the veins. The mineral assemblage includes malachite, chrysocolla, stibiconite, cuprite, and minor chalcocite and covellite, but some of these are late-stage alteration-oxidation minerals derived from an originally sulfide- or oxide-dominated suite. A syn- to late metamorphic age for the primary deposit is indicated by: (1) the crosscutting nature of the mineralized veins, (2) local replacements of kyanite and staurolite by economic minerals, and (3) evidence for postmineralization annealing of the veins and quartzite. During retrograde metamorphism, the quartz veins and massive quartzite behaved brittlely whereas the overlying schists deformed ductilely. SiO 2 -bearing and then Cu-Ag-Sb-bearing metamorphic fluids migrated through the fractured quartzite and ponded below the folded impermeable schists. Although the Copper Hill deposit is not currently economically viable, it presents a model for retrograde metamorphic mineralization in rocks with varying mechanical properties that may be important in other regions.