LARAMIDE ALTERATION OF PROTEROZOIC DIABASE - A LIKELY CONTRIBUTOR OF COPPER TO PORPHYRY SYSTEMS IN THE DRIPPING SPRING MOUNTAINS AREA, SOUTHEASTERN ARIZONA

Proterozoic diabase of the Dripping Spring range occurs as sills in th e Proterozoic Apache Group and the Troy Quartzite and as intrusive she ets in basement rocks. The aggregate thickness of the diabase sills an d intrusive sheets averages about 450 m in the part of the range showi ng little mid-Tertiary extension. Laramide alteration is of two types, dominated by chlorite and actinolite, respectively, and formed mostly from clinopyroxene. Actinolite-dominated assemblages are higher in Na and Ca. Hydrothermal biotite is common in the central areas of both a lteration types. Laramide alteration forms two distribution patterns: a subequant pattern centered on Laramide intrusions and small porphyry deposits, characterized by actinolitic alteration, and a more extensi ve branching linear pattern that follows Laramide structures, centered on tile larger Ray porphyry deposit, extending toward other Laramide districts and showing both alteration types. Alteration has apparently mobilized copper and other metals from diabase. The freshest diabase samples average about 120 ppm copper with little variation. In chlorit ic alteration, about 100 ppm of this copper is expelled in the most co mpletely altered rocks. In actinolitic alteration, diabase may either gain or lose copper during alteration. Chloritic alteration constitute s roughly 70 percent of the diabase alteration in the study area, wher e alteration averages 41 percent complete. This implies liberation of about 9 x 10(6) tons (t) copper from diabase alteration, significantly less than the 16 x 10(6) t copper in Laramide mineral deposits of the superdistrict (Ray, Superior, Chilito, Christmas). However, diabase a lteration may have been a significant component of the supply of coppe r to the Laramide mineral districts of the area. Synmineral magmatic s ources of copper are not documented in this area. The distribution of Proterozoic diabase coincides with the central part of the southeaster n Arizona copper province, which may thus owe much copper availability to an unusual abundance of diabase. However, many unanswered question s remain about metal supply from altering diabase.