Discovery of a mineralized breccia pipe using gradient array induced polarization

Abstract

The Promontorio breccia pipe located in southern Sonora, Mexico, has been mined intermittently for silver since the turn of the century. Mineralization occurs as sulfides in the matrix of the breccia. Two to five percent pyrite also occurs in the premineral andesite that the breccia pipe has intruded. Geologic mapping led to the discovery of a small, unmineralized breccia in andesite 200 m west of Promontorio. The discovery of this breccia encouraged the search for other hidden mineralized breccias in areas obscured by postmineral alluvium. Exploration of the covered areas was carried out with an induced polarization (IP) and resistivity survey after a test line indicated that a distinct IP anomaly occurs over the Promontorio breccia pipe. The gradient array was chosen as the most effective electrode configuration to explore the covered areas because it can provide rapid detailed IP coverage necessary to locate small breccia pipes. Several anomalous IP trends were located. The most interesting one has an apparent response of 60 to 90 msec in low‐resistivity rocks similar to Promontorio. Additional information provided by dipole‐dipole profiles indicates that this anomalous zone is close to surface and about 20 m wide with a vertical to steep dip. Trenching and drilling confirmed the presence of a 20-m wide elongate breccia with sulfide mineralization similar to Promontorio. Silver grades were marginal for making ore in 1977.

DETECTION OF LATERAL VARIATIONS IN GEOLOGICAL STRUCTURES USING ELECTRICAL RESISTIVITY GRADIENT PROFILING1

“…For a comprehensive list of references see Bertin and Loeb (1976). Some recent works on the gradient array include Schulz (1985), West, Wieduwilt and Hall (1983) and Bhattacharya and Dutta (1982). Schulz (1985) has compared the theoretical anomalies detected by various arrays over a buried conductive sphere.…”

Caracterização geofísica de alvos rasos com aplicações no planejamento urbano e meio ambiente: estudo sobre o sítio controlado do IAG/USP

“…O objetivo é o de medir o potencial elétrico, gerado pelo meio, após a interrupção da corrente, em diferentes intervalos de tempo (SUMNER, 1976). É amplamente difundida por seus excelentes resultados na localização de depósitos minerais, principalmente jazidas de sulfetos (WEST et al, 1983).…”