Ore mineralogy, geochemistry and dating (Re-Os, Ar-Ar) of the El Volcán porphyry/epithermal system, Maricunga gold belt, N-Chile

Stephanie Lohmeier, Bernd Lehmann, Albrecht Schneider, Andrew Hodgkin, Raymond Burgess

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The El Volcán 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 Copiapó 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 martitized 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 quartz-kaolinite-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 sub-μm size and is in banded quartz veinlets, which are characteristic of the Maricunga gold belt. However, some gold is disseminated in zones of pervasive quartz-kaolinite-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 (>100 times bulk crust) of the hydrothermal system. Molybdenum rich bulk rock (100-400 ppm Mo) has a Re-Os age of 10.94 ± 0.17 Ma (2σ). 40Ar-39Ar 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 (2σ). The formation of the El Volcán 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 ≥1500 m.
Original languageEnglish
JournalEconomic Geology
Publication statusAccepted/In press - 31 Mar 2021


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