Bussey, SD; Kakarieka, A; Meinert, LD; ,
The El Toqui District is located in the Aysen Region of Southern Chile, 1350 km south of Santiago. Mineralization was recognized in 1970 and since then exploration has discovered eight skarn bodies, a carbonate replacement deposit and a system of quartz-base metal veins. Total geological resource for the district is 20 Mt grading 8.2% Zn and 1.5 g/t Au. The mine is an underground room and pillar operation and has average annual production of 500,000 tonnes per year.Regional host rocks include Jurassic andesitic rocks of the Ibanez Formation, 200 m of volcanic sandstone and tuff of the Cretaceous Toqui Formation with a basal 15 m thick limestone unit rich in oyster fossils that is the main ore host, and 800m of Katterfeld Formation black shale. The Katterfeld Formation is overlain by andesitic lava/breccia of the Cretaceous lower Divisadero Group which is overlain unconformably by rhyolite ignimbrite of the upper Divisadero Group. Intrusive rocks in the district include: andesite dikes, plugs and associated peperite intrusion breccias emplaced into the Katterfeld Formation at about 124 Ma; widespread rhyolite sills and an andesite sill emplaced into the Toqui and Katterfeld Formations at 120 Ma; and dacite sills intruded into the Toqui and Katterfeld Formations at 113 Ma. Age dates for intrusions overlap those on hydrothermal mineralization: molybdenite Re-Os date of 120.1 +/- 0.4Ma in the Mallines-Cerro Elefantes area and 118.0 +/- 0.4 Ma in the Altazor area, and a 110Ma Ar-40/Ar-39 date on skarn actinolite at Aserradero. Rhyolite and dacite sills have local areas with abundant dioritic xenoliths interpreted as magma-mixing textures. Narrow andesite dikes cross cut mineralization and appear to be post-mineral in age. The emerging picture is of a large porphyry-skarn district with multiple pulses of intrusion and alteration from 120-110 Ma, resulting in the multiple orebodies and mineralization styles.The district is a large zoned skarn system that forms a NW-SE elongated area of 40 km(2). At district scale, Fe, As, Au, Bi and Co are highest in the SE associated with garnet, pyroxene and amphibole alteration, whereas Pb and Ag are highest in the NW associated with chlorite and sericite. Sphalerite is present across the district zoned from high-Fe in the SE to low-Fe in the NW. Other sulfide-oxide minerals include pyrrhotite, pyrite, magnetite, arsenopyrite with minor chalcopyrite, galena and locally abundant electrum, native bismuth, cobaltite and a variety of sulfosalts. In the SE part of the district, mineralization occurs along the crest and on the south side of a broad E-W trending anticlinal uplift. At deposit scale mineral zonation, including garnet-pyroxene ratios and garnet color, illustrates the importance of NW-trending structures as principle conduits for mineralizing fluids. Deep drilling has encountered two areas of subeconomic pyrite-chalcopyrite molybdenite stockworks. One beneath skarn orebodies in the SE part of the district and one is beneath carbonate-replacement mineralization in the NW part. The limit of skarn mineralization is open to the east, south, and west of known mineralized drill intercepts.