Abstract

The Ansil Cu-Zn volcanic-hosted massive sulfide deposit is located approximately 14 km north-northwest of Rouyn-Noranda, Quebec, within the Blake River Group of the Archean southern Abitibi greenstone belt. The deposit is the second lowest within the Central Mine sequence, a series of bimodal rhyolite-andesite formations that fill in the Noranda cauldron, host to 17 volcanic-hosted massive sulfide deposits. The deposit consists of a single massive sulfide lens situated at the contact between rhyolite of the Northwest formation and overlying andesite of the Rusty Ridge formation. The rhyolite flow forms a ridge up to 500 m high which was inundated by the andesite flows. The orebody is hosted within a unit of quartz porphyritic, finely layered, felsic volcaniclastite (Cranston tuff) that fills in an east-west-trending graben formed on the upper surface of the rhyolite flow. At the east end of the graben the Cranston tuff is interlayered with a thin unit of dacite flows. The deposit lies within an extensive discordant alteration zone characterized by feldspar destruction and Na depletion. The zone extends from 400 m below the deposit at the contact of the synvolcanic Flavrian Intrusive Complex to over 400 m above the deposit. This alteration zone is controlled by a fault system that served as a focus for hydrothermal fluid upflow from the beginning of cauldron formation until the end of the first cauldron cycle. Within the Na depletion zone there are three distinct alteration facies associated with the orebody. Early hydrothermal activity is defined by hydrothermal explosion breccias that formed along the base of the east-west trending graben walls. They were filled with finely banded quartz-albite- sphalerite-pyrite, and the wall rock altered to sericite-quartz, with finely disseminated sphalerite and pyrite- pyrrhotite. The graben was then filled with the Cranston tuff, which was selectively altered and mineralized by the Zn-rich fluid. Subsequent development of northerly trending faults was accompanied by the upflow of Cu-rich fluid that selectively chloritized the footwall rhyolite below the graben floor and formed a ...

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