Geophysical Case History of North Silver Bell, Pima Co., Arizona
A Supergene-Enriched Porphyry Copper Deposit

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 the period 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 powerlines 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 exploration 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 (ZERO) personnel 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 from World Geoscience, all data were processed at ZERO's office in Tucson, Arizona. TEM, IP and CSAMT data were modeled with proprietary smooth-model inversions.