Geology, mineralization and alteration of the battle zone, Buttle Lake camp, central Vancouver Island, southwestern British Columbia

Volcanogenic massive suiphide deposits within the Buttle Lake mining camp are associated with andesitic and felsic rocks of the Price and Myra formation in the Paleozoic Sicker Group. The Battle zone occurs within H-W horizon, the lowest member of the Myra formation. H-W horizon is a 15 to 200 m thick felsic package that occurs throughout the camp, immediately above the Price andesite, which is the lowest known unit within the Sicker Group in the Buttle Lake area. H-W horizon consists of seven members, two of which represent periods of massive sulphide deposition. From the stratigraphic base to top these members are: Battle and Gap zone massive suiphide lenses, fine rhyolitic tuffaceous deposits, H-W mafic sills, coarse rhyolite pyroclastic deposits, rhyolite tuffaceous sediments, upper zone massive suiphides, and the rhyolite flow-dome complex. Thin section petrography and Pearce element ratio analysis of lithogeochemical data from samples of least altered rocks from the Price formation and rhyolite flow dome complex support the following conclusions: (i) lavas in the Price andesite are comagmatic and are related by sorting of plagioclase feldspar, pyroxene, olivine and Fe-Ti oxides, and (ii) the quartz porphyritic rhyolite (QP), quartz feldspar porphyritic rhyolite (QFP) and green quartz feldspar porphyritic rhyolite (GQFP) units of the rhyolite flow dome complex are comagmatic and are related by sorting of quartz and feldspar. Alteration in the Battle zone is marked by addition of silica in both the footwafi and immediate hangingwall. A signfficant amount of iron in the form of stringers and disseminated pyrite has also been added in the footwall. A broader zone dominated by sericite alteration that occurs both above and below the orebody corresponds to loss of sodium and calcium, and addition of potassium, barium and rubidium. Magnesium does not show any systematic spatial trends that clearly are related to hydrothermal alteration. The first cycle of mineralization was the most voluminous, and formed main Battle massive suiphide lens, the H-W main lens and probably the Gap lens. The main Battle massive sulphide lens is localized in a fault-bounded basin developed in the Price formation andesite. From a laterally extensive footwall stringer zone, it varies upwards through: (i) massive pyrite and chalcopyrite, (ii) banded massive suiphide in the central region, to (iii) pale yellow massive sphalerite at the top and periphery. This zonal distribution appears to have formed by a process of progressive zone replacement as a result of continued reaction between upweffing fluid and previously deposited suiphides. A second cycle of upper zone mineralization is represented by small, discontinuous lenses of baritic sphalerite and tetrahedrite rich massive sulphide lenses above the rhyolite tuffaceous sediments. Galena lead isotope data from the Buttle Lake mining camp indicate that lead evolved in an orogene or island arc environment. The linear trend in the galena lead data can be explained as a mixing line. Positions of data along this trend do not relate to age differences among the ore lenses, but appears to represent either: (i) varied mixing of upper crustal and mantle components, as might be expected in an orogene or island arc environment, or (ii) variable selective leaching of lead isotope components from footwall source rocks. A combination of these processes is not excluded. The less radiogenic end member appears to be spatially related to rhyolitic host rocks. The more radiogenic deposits occur immediately above major discharge stockworks in andesite. In H-W horizon, markedly radiogenic lead is characteristic of deposits that define the main lens trend (i.e. the H-W and Battle main lenses). Lenses in the main trend are among the largest; consequently a more radiogenic lead isotopic composition may identify favourable exploration targets.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat