Summary:
The El Toqui deposits consist of skarn-type stratified deposits and manto-type mineralized bodies.
The Toqui deposits were discovered in the early 1970's. The three original deposits at Toqui, Doña Rosa, Mallín-Monica and San Antonio adjoin each other but are separated by faults. The Toqui property encompasses an area of approximately 1,800 square kilometres. This area includes the Doña Rosa zinc-gold mine, Rosa West zinc-lead mine, the San Antonio East and Mallín-Mónica and Mallín South zinc mines, the Aserradero and Mina Profunda gold-zinc mines.
Zinc/gold mineralisation at Toqui occurs as manto deposits hosted by an 8 metre to 12 metre thick bed of fossiliferous limestone, known as the Main Manto unit. The Toqui deposits all dip gently to the south, and most deposits are amenable to room and pillar mining, with 11 metre rooms around 8 metre pillars.
Mining is normally carried out in a single horizon from flat lying mineralisation varying in thickness from four to 12 metres, except where localised faulting may offset the manto into smaller stopes accessed via mine ramps. In Estatuas, there is a lower secondary manto which has economic zinc grades and is being mined.
The Toqui district contains a series of skarn and replacement orebodies within a 24 km2 area. Oldest rocks include Jurassic andesite and Cretaceous volcanic sandstone and tuff of the Toqui Formation, with a basal 5- to 30-m-thick limestone unit, rich in oyster fossils and forming the main ore host. Above these units is 800 m of black shale of the Katterfeld Formation, overlain by andesite of the Cretaceous lower Divisadero Group, which is then overlain unconformably by rhyolite ignimbrite of the upper Divisadero Group. Intrusive rocks include rhyolite, dacite, and andesite sills emplaced into all the Cretaceous rock units. Multiple periods of magmatic and hydrothermal activity have been documented from 120 to 105 Ma.
At district scale, Fe, As, Au, Bi, and Co are highest in the southeast, associated with garnet, pyroxene, and amphibole alteration, whereas Pb and Ag are highest in the northwest, associated with chlorite and sericite. Zinc grades are fairly uniform across the district, but sphalerite is zoned from high Fe in the southeast to low Fe in the northwest. Economically significant gold mineralization was superimposed on earlier base metal-rich skarn in the southeastern part of the district. Late hydrothermal fluids entered the skarn system along preexisting northwest-trending structures. Gold occurs as electrum associated with native bismuth, cobaltite, and a variety of sulfosalts. Gold-rich ore generally contains abundant arsenopyrite, but arsenopyrite-rich ores are not necessarily gold rich. Gold and cobaltite deposition was accompanied by extensive retrograde amphibole formation, with clay minerals more abundant at the periphery of the gold zones. Deep drilling has encountered two areas of subeconomic pyrite-chalcopyrite-molybdenite stockworks. One is beneath the skarn orebodies in the southeastern part of the district and the other is beneath mineralization in the northwestern part. The emerging picture is one of a large porphyry-skarn district with multiple pulses of intrusion and alteration, resulting in multiple orebodies and mineralization styles.