Ore Mineralogy, Geochemistry, and Dating (Re-Os, Ar-Ar) of the El Volcan Porphyry/Epithermal System, Maricunga Gold Belt, Northern Chile

The El Volcan gold project (8.9 Moz Au @ 0.71 g/t Au) is located in the Maricunga gold belt in northern Chile, on the flank of the large Cenozoic Copiapo Volcanic Complex. Precious metal mineralization is hosted in two zones (Dorado and Ojo de Agua) of (pervasively) altered Miocene porphyry intrusions and lava flows of andesitic to rhyolitic composition, and in breccias. The ore zones reflect an evolving magmatic-hydrothermal system with mineral assemblages of magnetite-ilmenite-pyrite-molybdenite (early), bornite-chalcopyrite-pyrite-rutile (stage I), chalcocite-chalcopyrite-enargite-fahlore-pyrite (stage II), and chalcopyrite-covellite-pyrite (stage III). Alteration is dominantly of Maricunga-style (illite-smectite-chlorite +/- kaolinite), partly obscured by quartzkaolinite-alunite +/- illite +/- smectite alteration. Powdery quartz-alunite-kaolinite alteration with native sulfur and cinnabar forms shallow steam-heated zones. Early K-feldspar +/- biotite alteration is preserved only in small porphyry cores and in deep drill holes. Most gold is submicrometer size and is in banded quartz veinlets, which are characteristic of the Maricunga gold belt. However, some gold is disseminated in zones of pervasive quartzkaolinite-alunite alteration, with and without banded quartz veinlets. Minor visible gold is related to disseminated chalcocite-chalcopyrite-enargite-fahlore-pyrite. The lithogeochemical database identifies a pronounced Au-Te-Re signature (>100x bulk crust) of the hydrothermal system. Molybdenum-rich bulk rock (100-400 ppm Mo) has an Re-Os age of 10.94 +/- 0.17 Ma (2 sigma). 40Ar39Ar ages on deep K-feldspar alteration and on alunite altered rock have the same age within error and yield a combined age of 11.20 +/- 0.25 Ma (2 sigma). The formation of the El Volcan gold deposit took place during the establishment of the Chilean flat-slab setting in a time of increasing crustal thickness when hydrous magmas were formed in a mature arc setting. The vigorous nature of the hydrothermal system is expressed by abundant one-phase vapor fluid inclusions recording magmatic vapor streaming through a large rock column with a vertical extent of >= 1,500 m.

Automated summary

The El Volcan gold project is located in the Maricunga gold belt in northern Chile, with mineralization hosted in altered Miocene porphyry intrusions and lava flows. The ore zones display an evolving magmatic-hydrothermal system with various mineral assemblages, and most gold is submicrometer size found in banded quartz veinlets. The lithogeochemical database shows a pronounced Au-Te-Re signature, indicating a strong hydrothermal system.