New Mexico Geological Society Annual Spring Meeting — Abstracts

This study integrates new concepts on the structural evolution of the Picuris Mountains with hypotheses on the metamorphic origin of copper mineralization at Copper Hill. This mineralization is characterized by the coincidence of: (1) a structural trap (the Copper Hill anticline); (2) brittle structures that postdate folding, and; (3) the differing mechanical behavior of schists versus quartzites during regional metamorphism.

At Copper Hill, quartzites and schists of the Ortega Group are tightly folded by the overturned, west-plunging Copper Hill anticline. The anticline is cut on its south side by a slightly cross-cutting ductile fault along which older metasedimentarymetavolcanic rocks have been transported northward over the Ortega Group. Up-plunge (east) of Copper Hill, on Copper Mtn, lower structural levels of the anticline are exposed where the hinge region and south limb of the anticline have been eroded.

Peak metamorphic conditions that were slightly below the alumino-silicate triple point closely followed folding. Kyanite, andalusite, and staurolite are present on Copper Hill, and sillimanite is found nearby.

A system of north-to northwest-trending vertical fractures post-dates folding. These structures range in scale from the >85 km-Iong Picuris-Pecos fault to outcrop-scale fractures. Although macroscopic faults cut all Precambrian rock types, at the outcrop scale on copper Hill, pure quartzites are highly fractured, whereas immediately overlying schistose horizons are unbroken.

Abundant syn-metamorphic quartz veins, which range in thickness from cms to 1 m, fill the north-trending fractures. These veins cut the relatively pure quartzites but not the overlying schistose quartzites and metapelites. Commonly, quartz veins are thickest at quartzite-schist contacts. Associated with the veins are a suite of copper minerals including malachite, chrysocolla, stibiconite, chalcocite, cuprite, and covellite. Mineralized samples also contain anomalous silver, arsenic, antimony, and gold. Locally, copper minerals have replaced kyanite in quartz veins. Although disseminated mineralization is also present in quartzites, it appears to occur as replacements related to veins rather than as original, syngenetic stratabound mineralization. We suggest that during retrograde metamorphism, ore-bearing fluids migrated upward through fractured quartzite and ponded below impermeable schists after folding and faulting. Stratigraphic and structural work in the Picuris Mountains within the last 10 years has resulted in renewed economic interest in Copper Hill. Exploration efforts were mainly based on the premise that high-grade are might exist at depth. However, a 700 ft drillhole on the hinge of the anticline showed that copper mineralization decreases with depth. Although the exact source of the metals remains unknown, one possibility is that older metavolcanic rocks, similar to those found to the south, are in fault contact with ortega Group rocks beneath Copper Hill.