2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 10
Presentation Time: 10:35 AM


GRAYBEAL, Frederick T., Exploration Management LLC, 37 Wynwood Road, Chatham, NJ 07928, FLECK, Robert J., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 and VIKRE, Peter G., U. S. Geol Survey, Mail Stop 176, University of Nevada, Reno, Reno, NV 89557, ftgraybeal@aol.com

The Patagonia Mountains are cored by Laramide granodiorite that covers 60 square miles. Mineral deposits in and adjacent to the granodiorite define a central Cu-Mo zone, an intermediate Zn-Pb-Ag zone, and a peripheral Ag-Mn zone. Detailed mapping and gradients delineated by metal ratios, fluid inclusion microthermometry, and Fe/Mg and dD in igneous hornblende and igneous and hydrothermal biotite and K-mica suggest that zoning is related to several igneous-hydrothermal centers.

40Ar/39Ar ages support at least two intrusive and related hydrothermal events. The older event involved emplacement of the granodiorite and formation of associated metal deposits at c.65 Ma. This was followed by intrusion of quartz feldspar porphyry (qfp) and concurrent formation of the Sunnyside porphyry copper system with its hydrothermal alteration and mineralization zones and additional surrounding metal deposits at c. 59 Ma.

Alunite alteration is symmetrically distributed above both qfp and the Sunnyside chalcopyrite zone, suggesting that alunite is genetically linked to both. However, 40Ar/39Ar ages of alunite are 54.9 and 51.5 Ma. This requires that alteration continued for 4-7.5 my following emplacement of qfp and formation of the porphyry copper deposit, an unusually long interval of time, but there is no evidence for an intrusive heat source younger than qfp. Geologic reconstruction suggests a thick premineral ignimbrite that covers the Red Mountain porphyry copper deposit 3 miles to the north may have also covered and preserved the full vertical extent of the Sunnyside porphyry copper system. Alunite alteration may have been prolonged by heat from and the insulating effect of this ignimbrite.