Geophysical Case History of North Silver Bell, Arizona: a Supergene-Enriched Porphyry Copper Deposit
Mark W. Thoman, Regional Geologist, Minera Phelps Dodge Mexico, Tucson, Arizona
Kenneth L. Zonge, President, Zonge Engineering and Research Organization, Tucson, Arizona
Dexin Liu, Geophysicist, Zonge Engineering and Research Organization, Tucson, Arizona
Paper – [pdf] Silver Bell Case History
Introduction and Summary
In early 1993, the area was suggested by J. M. Guilbert of the University of Arizona as a good site for a baseline geophysical study over a porphyry copper deposit. The deposit was well defined by drilling and surface mapping and did not have any significant surface disturbance or excessive cultural contamination such as numerous power lines and fences. The approach was to survey the area with a variety of geophysical techniques and develop a comprehensive geophysical signature of the deposit that would have relevance to explora-tion for porphyry copper deposits elsewhere. ASARCO was essential to the project by allowing access to the deposit and making available company information regarding the deposit. The study was used as the basis for a masters thesis at the University of Arizona by K. C. Foreman (1994).
The magnetic signature of the deposit is a low within a regional magnetic high. At the scale of the deposit, ground magnetics distinguishes the alteration zoning, with weak local highs in the potassic zone, lower responses in the phyllic zone and higher increasing values in the propylitic zone. Sulfide mineralization, mostly pyrite with lesser chalcopyrite and chalcocite, is characterized by moderate to high induced polarization (30-60 milliradians). The strongest IP response is correlated with quartz-sericite-pyrite in the phyllic alteration zone. The chalcocite mineralization has an anomalous, but lower, IP response that is partially masked by the laterally adjacent or underlying stronger pyrite response. Decoupled CR or spectral IP responses from the field survey when compared with those obtained for laboratory rock measurements indicate different type responses for pyrite as compared to those for mixed sulfides such as chalcocite-pyrite and pyrite-chalcopyrite. These differences in spectral responses may reflect a combination of changes in grain sizes as well as sulfide species.