PORPHYRY CU-STYLE MINERALIZATION POTENTIAL IN THE SAN FRANCISCO DISTRICT, UT

The "Frisco Summit Sulfide System", San Francisco Mining District, UT, exhibits characteristics of a very large, high-level porphyry copper-type system. An enormous area (>9.3 Km2) of pyrite-white phyllosilicate alteration is exposed. Drilling indicates that an enormous volume of Horn Sivler Andesite (perhaps in excess of 15 km3) has been subjected to alteration by sulfur-bearing fluids producing pyrite and anhydrite/gypsum. Surrounded this phyllic zone is a zone of propyllitic alteration that hosts two of the largest historic Ag-Pb-Zn mines in Utah, the Beaver Carbonate Mine and the Horn Silver Mine. These mines lie on a continuous, semi-circular "Ring fault" that, in part, forms the contact between the phyllic and propyllitic zones. The Ring fault down drops the Horn Silver Andesite up to 700 m against the Cactus stock and other rocks to the north and west. Intrusions of quartz monzonite porphyry, alteration and mineralization appear to postdate movement on the ring fault. The Cactus mine, 2.3 km northwest of the Ring fault, produced copper from a nearly vertical breccia zone in the Cactus Stock. An outer gold zone in sedimentary rocks occurs to the north.

A variety of alteration assemblages have been identified locally within the phyllic zone: argillic (celadonite, kaolinite, dickite), advanced argillic (alunite, pyrophyllite), silicification (a variety of jasperoid types), potassic (biotite and K-feldspar). Chalcopyrite occurs as a relic sulfide in a few surface samples. A well-developed zone of stockwork veins is also present. Breccia pipes are common throughout the district.

Biotite from unaltered Horn Silver Andesite yields an age of 30.8±0.8 Ma and sericite from altered Horn Silver Andesite (phyllic zone) yields a K/Ar age of 28.6±0.7 Ma. Biotite from the Cactus Granodiorite (propyllitic zone) yields a K/Ar age of 28.7±1.0 Ma (Best et al., 1989).

The large amount of pyrite indicates that a vigorous hydrothermal system operated in the district; discovery of an economic porphyry deposit here will require persistent effort.