Geology, gold mineralization and alteration of the Horne West Property Rouyn-Noranda

The Horne deposit in the Noranda Mining Camp, northwestern Quebec, is one of the largest volcanic-hosted massive sulfide deposits in the world. Nearly all historical production came from massive sulfide orebodies; however, the occurrence of widespread disseminated gold mineralization has been well established. A new exploration program by Xstrata Copper Canada and Alexis Minerals Corporation was initiated in late 2006 to evaluate the mineral potential of the Horne West occurrence, located ∼1km to the west of the Horne deposit. Gold grades in historical exploration drill core from Horne West included 4.56 g/t Au over 14.63 m, 4.27g/t Au over 9.32 m, 5.49 g/t Au over 20.63 m and 3.37 g/t Au over 15.58 m. In addition, significant zinc mineralization was encountered in some of the drill holes. Integration of historical assay data with the results of new drill-core logging indicates two distinct zones of gold mineralization, at the stratigraphic base and in the upper part of the Horne West succession. Surface and subsurface mapping suggests that the mineralization may have formed in a localized basin adjacent to a synvolcanic structure. Sulfide mineralization in the gold zone close to the stratigraphic base of the succession is characterized by sulfide veining and sulfide impregnations in aphyric coherent rhyolite and associated volcaniclastic rocks. The mineralized rhyolite facies association is overlain by a mass-flow derived, fining-upward succession of normally-graded wispy clast-rich breccia and sandstone that contains conspicuous pyrite-rich sulfide clasts at the base of coarse-grained beds. A second zone of gold mineralization is located within the upper portion of the stratigraphic succession within lithic clast-dominated sandstone and breccia, coherent xenolith-bearing rhyolite, and a distinct quartz-phyric rhyolite- and sulfide clast-bearing lithic breccia. The mineralized zone steps up stratigraphically with depth below surface and is typically overlain by, or contained within, a sulfide-clast-bearing volcaniclastic rock interval. Gold mineralization occurs mainly in discordant sulfide stingers and disseminations. Although field observations indicate that elevated precious metal contents occur in coarse-grained volcaniclastic rocks containing abundant sulfide clasts, the sulfide clasts themselves do not contribute significantly to gold grade. These sulfide clast-bearing mass-flow derived volcaniclastic deposits define a broad paleo-seafloor position within the volcanic succession where massive sulfide mineralization occurred down dip or along strike. The gold-rich disseminated and stinger sulfide mineralization and alteration at Horne West occurred in spatial and temporal association with the seafloor massive sulfides that were the source of the sulfide clasts. Distribution of gold is controlled in part by the shallowly emplaced synvolcanic coherent rhyolite intrusions that may have intruded along, or acted as barriers to, an area of hydrothermal upflow and mineralization. Late mafic intrusions, which occur more prominently in the mineralized zones and possibly along synvolcanic structures, have remobilized gold locally.