EVALUATION OF A PROSPECT AT TEBISCO, MOCTEZUMA DISTRICT, SONORA, MEXICO

A spectacular bright red gossanous outcrop at Rancho Tebisco, 5 km north of Moctezuma, Sonora, Mexico appears to present an attractive target for exploration. Is the outcrop worth drilling? Detailed mapping reveals that the strongly mineralized area was tilted 25°NE, and cut by a NW-trending normal fault, dipping 45° SW, with an offset of 1600 m. The hydrothermal system is exposed at the surface over two originally oblique sections, each with an area of about 0.25 km². The deeper part lies at the surface east of the fault, and the shallower part to the west. The sections represent a paleodepth interval of 900 m vertically, and 1 km laterally. Strongly bleached alteration, principally sericite, kaolinite and montmorillonite, all with disseminated pyrite (now oxidized) accompany comb quartz veins. Trace element analysis indicates an original vertical zonation of epithermal deposits, from base metals (Cu, Zn, Pb) at depth to bonanza silver deposits (Ag, Au, Sb, As, Mn) to high-level veins (Au, As, Te, Ag) nearest the paleosurface. All the hanging wall outcrops contain only sparse, very thin (<10cm) gold-bearing veins. Deposits in the footwall of the fault are bonanza silver veins, one of which was mined for gold, with poor results. This vein reached only 30 cm in thickness. Since faulting and erosion have exposed an extensive cross section of the system, trace-element analysis should reveal some attractive anomalies. However, the extent of high grades is so limited that the volume of any minable ore must be very small. The spectacular mineralized hill at Rancho Tebisco suggests a rich underlying mineral deposit, but study of the volcanic stratigraphy indicates that the hill consists of basaltic andesite, a rock type with a high mafic content, susceptible to pyritization, but not strongly mineralized in economic metals. It might have been above the paleo water table, pyritized by sulfur-rich vapors, with ore below, but the exposure of the entire system by tilting and erosion suggests that no viable deposits underlie the bright colors. It therefore appears that the bright outcrop is misleading.