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Geophysical Case History of North Silver Bell, Arizona: a Supergene-Enriched Porphyry Copper Deposit

Publisher –
Zonge, 1996.

Authors –
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
The Silver Bell district is within the porphyry-copper province of southwestern North America, located 35 miles northwest of Tucson, Arizona on the south side of the Silver Bell Mountains. Mineralization in the district consists of at least three distinct disseminated porphyry copper deposits and several skarn replacement deposits. Disseminated primary and supergene-enriched porphyry copper mineralization were mined in two open pits, El Tiro and Oxide, by ASARCO, mainly during 1954-1977. Total production for that period is reported at 75.66 million tonnes (Mt) at 0.80% copper (Graybeal, 1982). The North Silver Bell deposit is located at the north end of the district and represents a leachable resource of in excess of 80 Mt at an average grade of 0.40% copper contained mostly within an enrichment blanket of chalcocite. When the geophysical work was being done, in 1993-1994 and again in 1996, the deposit was not being mined. Mining of North Silver Bell by ASARCO began in 1997.

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 master’s thesis at the University of Arizona by K. C. Foreman (1994).

Geophysical surveys conducted over the deposit by Zonge Engineering and Research Organization included: ground magnetics, dipole-dipole complex resistivity (CR), reconnaissance induced polarization (RIP), controlled source audio-frequency magnetotellurics (CSAMT) and transient electromagnetics (TEM and NanoTEM). Additional data include CR rock measurements on core specimens from drill holes within the deposit and airborne magnetics and EM flown by World Geoscience in 1993. Except for the airborne data, all data were processed at Zonge’’s office in Tucson, Arizona. TEM, IP and CSAMT data were modeled with proprietary [Zonge] smooth-model inversions.

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.