THE COPPER-HILL CU-AG-SB DEPOSIT, PICURIS RANGE, NEW-MEXICO - RETROGRADE MINERALIZATION IN A BRITTLE-DUCTILE TRAP

The strata-bound copper-silver-antimony deposit at Copper Hill, New Me xico, developed in a distinctive structural trap, here termed a ''brit tle-ductile trap,'' during regional retrograde metamorphism of Early P roterozoic metasedimentary rocks. The deposit is localized near the co ntact between massive orthoquartzite (Ortega Formation) and the overly ing schists (Rinconada Formation). Peak regional metamorphism at 4 kba rs and 500 degrees C occurred near the end of a regional shortening ev ent that formed the upright, west-plunging Copper Hill anticline. At C opper Hill, late metamorphic, vertical, north-south-striking faults an d fractures cut the Ortega Formation quartzite and terminate against f olded Rinconada Formation schists. Synmetamorphic quartz veins, severa l centimeters to 1 m thick, fill many of the fractures and terminate a gainst the overlying schists. At the quartzite-schist contact, many of the veins merge into mushroom-shaped bodies that project along the co ntact. Fracture-controlled and disseminated Cu-Ag-Sb minerals occur wi thin the quartz veins and in quartzite near the veins. The mineral ass emblage includes malachite, chrysocolla, stibiconite, cuprite, and min or chalcocite and covellite, but some of these are late-stage alterati on-oxidation minerals derived from an originally sulfide- or oxide-dom inated suite. A syn- to late metamorphic age for the primary deposit i s 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 qu artzite. During retrograde metamorphism, the quartz veins and massive quartzite behaved brittlely whereas the overlying schists deformed duc tilely. SiO2-bearing and then Cu-Ag-Sb-bearing metamorphic fluids migr ated through the fractured quartzite and ponded below the folded imper meable schists. Although the Copper Hill deposit is not currently econ omically viable, it presents a model for retrograde metamorphic minera lization in rocks with varying mechanical properties that may be impor tant in other regions.