THE SUBSEA-FLOOR FORMATION OF VOLCANIC-HOSTED MASSIVE SULFIDE - EVIDENCE FROM THE ANSIL DEPOSIT, ROUYN-NORANDA, CANADA

The Ansil Cu-Zn volcanic-hosted massive sulfide deposit is located app roximately 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 seri es of bimodal rhyolite-andesite formations that fill in the Noranda ca uldron, host to 17 volcanic-hosted massive sulfide deposits. The depos it consists of a single massive sulfide lens situated at the contact b etween rhyolite of the Northwest formation and overlying andesite of t he 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 volcanicla stite (Cranston tuff) that fills in an east-west-trending graben forme d on the upper surface of the rhyolite flow. At the east end of the gr aben the Cranston tuff is interlayered with a thin unit of dacite flow s. The deposit lies within an extensive discordant alteration zone cha racterized by feldspar destruction and Na depletion. The zone extends from 400 m below the deposit at the contact of the synvolcanic Flavria n Intrusive Complex to over 400 m above the deposit. This alteration z one is controlled by a fault system that served as a focus for hydroth ermal fluid upflow from the beginning of cauldron formation until the end of the first cauldron cycle. Within the Na depletion zone there ar e three distinct alteration facies associated with the orebody. Early hydrothermal activity is defined by hydrothermal explosion breccias th at formed along the base of the east-west trending graben walls. They were filled with finely banded quartz-albite-sphalerite-pyrite, and th e wall rock altered to sericite-quartz, with finely disseminated sphal erite and pyrite-pyrrhotite. The graben was then filled with the Crans ton tuff, which was selectively altered and mineralized by the Zn-rich fluid. Subsequent development of northerly trending faults was accomp anied by the upflow of Cu-rich fluid that selectively chloritized the footwall rhyolite below the graben floor and formed a morphologically complex pyrrhotite-chalcopyrite-rich vein stockwork. The Cu-rich miner al assemblage progressively replaced the Cranston tuff in the center o f the graben, overprinting previously deposited sphalerite-mineralized tuff. The Cu-rich fluids also selectively altered overlying andesitic flow breccia and hyaloclastite, replacing parts of the unit with mass ive pyrrhotite-chalcopyrite. The replaced Cranston tuff forms the laye red eastern half of the orebody, whereas the steep-sided dome at the w est end is a product of andesite flow replacement. This hydrothermal s tage was also responsible for the continued replacement of interflow h yaloclastite for up to 400 m above the orebody. The third stage of hyd rothermal activity resulted in the formation of calc-silicate alterati on assemblages, and then a footwall magnetite vein stockwork and repla cement of sections of the massive sulfide orebody by massive magnetite . The Ansil deposit is the product of shallow subsea-floor replacement , with the primary permeability of the host rocks controlling the morp hology of the orebody and associated vein stockworks. It began to form in shallow water (200-300 m) on the graben floor and continued to evo lve during infilling of the graben by felsic mass flows, and burial of the footwall rhyolite by andesitic flows. Late-stage development of t he ore took place under a 500-m thickness of volcanic flows at an unkn own water depth. Three compositionally distinct fluid phases were resp onsible for the development of the Ansil system: an Si-Zn-K-rich fluid responsible for the hydrothermal brecciation of the graben floor and sphalerite precipitation within the quartz porphyritic mass flow; a hi gher temperature Fe-Cu-Au-Ag-rich fluid which formed extensive footwal l and hanging wall, chlorite-rich, alteration zones, as well as the py rrhotite-chalcopyrite-rich vein stockwork and massive sulfide lens; an d finally, a Ca-Fe-Co-2-rich fluid which formed calc-silicate alterati on assemblages, and then partially replaced the massive sulfide lens w ith massive magnetite.