THE GEOLOGY AND ALTERATION PATTERNS OF A DISSEMINATED, SHEAR ZONE-HOSTED MESOTHERMAL GOLD DEPOSIT - THE FRANCOEUR-3 DEPOSIT, ROUYN-NORANDA,QUEBEC

The Francoeur 3 gold deposit, located approximately 15 km southwest of Rouyn-Noranda, Quebec, is one of several geologically similar Archean mesothermal gold deposits occurring along the Francoeur-Wasa shear zo ne, a second-order reverse structure possibly related to the Cadillac- Larder Lake break. The principal gold mineralization, which is complet ely hosted by the Francoeur-Wasa shear zone, is associated with dissem inated pyrite and occurs in tabular zones of intense hydrothermal repl acement. The mineralized zones consist of a buff- or gray-colored, ver y fine grained, carbonate-albite-pyrite assemblage which occurs in ver y different lithologies, including sheared andesite, red albitite dike s, and gabbro. Three types of hydrothermal alteration occur in the dep osit. In chronological order, they are (1) a widespread pervasive preo re carbonate-hematite-muscovite alteration associated with the intrusi on of late kinematic albitite dikes, (2) an albite-pyrite alteration c losely associated with the gold mineralization, and (3) a postore sulf ate-bearing alteration. The distribution of alteration assemblages and replacement textures suggests evolution of fluid-rock interaction. Th e initial hydrothermal alteration involved the hydrolysis of metamorph ic silicates and oxides to form carbonate, muscovite, and hematite wit h net additions in CO2, K, Rb, and TiO2. Subsequently, muscovite and h ematite were replaced in the ore zones by albite and pyrite, respectiv ely, resulting in K2O and Rb depletion and large enrichments in NaO, S , W, and Au. These two alteration types are crosscut by late gypsum an d anhydrite veinlets which are associated with late brittle extensiona l faults and possibly mark the last product of hydrothermal activity i n the Francoeur-Wasa shear zone in this area. It is proposed that gold deposition occurred as a result of changes in physicochemical conditi ons of ore-forming fluids in response to progressive fluid-rock intera ction. The combined effects of an increase in f(O2) and a(Na+ or K+) a nd reductions in both f(H2S) and pH controlled Au solubility, and very efficiently partitioned its deposition in the albite-pyrite alteratio n facies. This interpretation is consistent with preliminary sulfur is otope data which indicate that pyrite coexisting with gold is highly d epleted in deltaS-34 (-10.5 +/- 0.8 parts per thousand) compared with pyrite elsewhere in the shear zone, as a result of reduced S oxidation . The absence of an auriferous quartz-carbonate vein system and the oc currence of disseminated replacement mineralization at Francoeur sugge st that this hydrothermal system remained relatively permeable through much of the hydrothermal process such that fluid flow operated under near-constant hydraulic gradient. Such a fluid regime is very differen t from that generally proposed for the formation of mesothermal gold q uartz veins in which episodic fluid-pressure elevations above lithosta tic pressure are thought to control fluid migration, wall-rock alterat ion, and gold deposition.