Geology, lithogeochemistry, and alteration of the Battle volcanogenic massive sulfide zone, Buttle Lake mining camp, Vancouver Island, British Columbia

Abstract

Volcanogenic, polymetallic massive sulfide deposits within the Buttle Lake mining camp occur within the felsic volcanic and volcaniclastic rocks of the Myra formation that immediately overlies Price formation andesite. These units comprise the lower exposed part of the Paleozoic Sicker Group in the Buttle Lake uplift in central Vancouver Island. The Sicker Group represents a volcanic island arc that forms the base of the allochthonous Wrangell terrane.The Battle Main zone contains 2.3 million tons (Mt) of proven and probable reserves grading 1.3 g/t Au, 25.3 g/t Ag, 2.5 percent Cu, 0.5 percent Pb, and 13.2 percent Zn. Overall, the Buttle Lake camp hosts geologic reserves of about 12.5 Mt. The Battle Main massive sulfide zone occurs within the H-W horizon, the lowest member of the Myra formation, immediately above the Price formation andesite.Price formation andesite is over 300 m thick in the vicinity of the camp and consists of feldspar + or - pyroxene porphyritic basaltic andesite flows. Synvolcanic block faulting of this unit formed the regional-scale Buttle Lake camp basin with a strike length of at least 10 km. Local sub-basins in this major structure contain the sulfides and felsic volcanic rocks of the H-W horizon.The H-W horizon is a 15- to 200-m-thick felsic package that extends throughout the basin. It consists of eight members, three of which represent periods of massive sulfide deposition. From the stratigraphic base to the top of these members are (1) the Battle Main massive sulfide lens, (2) fine rhyolitic tuffaceous deposits, (3) H-W mafic sills, (4) Gap massive sulfide lens, (5) coarse rhyolite pyroclastic deposits, (6) rhyolite tuffaceous and cherry sediments, (7) Battle Upper massive sulfide lenses, and (8) a tripartite rhyolite flow-dome complex consisting of quartz porphyritic rhyolite, quartz-feldspar porphyritic rhyolite, and green quartz-feldspar porphyritic rhyolite. Volcaniclastic units in the felsic package evolved with interfingering of subaerial and submarine pyroclastic processes. Flow-dome units evolved by fractionation of quartz and feldspar.Alteration, evaluated petrographically and with Pearce element ratios, is most intense close to synvolcanic and synmineral feeder faults that channeled solutions through the Price formation andesite proximal to the Battle Main massive sulfide lens. Envelopes to the feeder faults have distinctive Mg addition anomalies and contain the mineral assemblage pyrite > sericite > quartz > chlorite. Discharge feeder stockworks immediately below the Battle Main massive sulfide lens contain varying proportions of sericite, quartz, and pyrite but lack significant chlorite. Deposition of the H-W horizon rhyolitic units on top of the Battle Main massive sulfide lens did not halt mineralization. Rather, fluids continued to percolate upward through the newly deposited rhyolitic units and deposited the Battle Upper lenses close to the new sediment-water interface. Alteration below the Battle Upper lenses consists of quartz, sericite, and pyrite with minor galena, sphalerite, and tennantite. Overall, hydrolysis of feldspar in both the Price formation and the H-W horizon resulted in a halo surrounding the ore lenses marked by addition of K and concomitant Na and Ca depletion.