New Mexico Geological Society Annual Spring Meeting — Abstracts

A detailed study of skarn mineralization in the Lake Valley Formation and Magdalena Group in the Central mining district indicates the presence of two different types of copper skarns. One type is associated with porphyry copper stocks at Santa Rita and the Continental Mine and the other a barren stock copper skarn at Pinos Altos. The porphyry types are characterized by massive texture with extensive cross-cutting mineralization resulting from contemporaneous deformation and mineralization. Barren stock copper skarns are coarse grained, mineralogically zoned, and show a preservation of early stage skarn minerals.

Three stages of skarn growth can be recognized: 1) a thermal stage characterized by isochemical recrystallization, 2) a metasomatic stage represented by extensive garnet and pyroxene zones resulting form the addition of chemical components, and 3) a retrograde stage evidenced by ore mineral deposition and hydrous alteration of previously formed minerals. Porphyry skarns show greater development of garnet, cross-cutting mineralized quartz veins, and partial to total destruction of thermal stage mineralization.

Fluid inclusion data and detailed mineralogy of the metasomatic phases (derived from electron microprobe analysis) are being performed to determine physiochemical parameters responsible for any systematic differences in mineralization in the two types of copper skarns. Previous work indicates an increase in fluid inclusion temperatures with time in porphyry skarns (Ahmad and Rose, 1978). Petrographic work shows the lack of extensive metasomatic and retrograde phases at Pinos Altos. This may be the result of a cooling of hydrothermal fluids with time or the or the lack of an adequate fracture system for passage of late-stage hydrothermal fluids.