Petrochemistry of subvolcanic dike swarms associated with the Golden Revenue Au–Cu and the Stoddart Mo–Cu±W mineralizations (Dawson Range, Yukon Territory, Canada) and implications for ore genesis

摘要

Petrologic and geochemical features enabled us to constrain the petrogenesis of dikes spatially associated with the Golden Revenue (GR) Au–Cu and the Stoddart Mo–Cu±W mineralization, in the Mountain Freegold, Yukon Territory, Canada. Both dike sets are aplitic and porphyritic and characterized by broken and embayed feldspar and quartz phenocrysts/phenoclasts set in a fine-grained groundmass. In both GR and Stoddart mineralized zones, aplitic dikes are typically related to the lower T magma relative to the porphyritic dikes, as indicated by zircon and apatite saturation temperatures. GR dikes are essentially dacitic to rhyolitic in compositions, whereas dikes from Stoddart display a wide compositional range spanning from basaltic–andesitic to rhyolitic compositions. GR and Stoddart dike suites are classified as high-K calc–alkaline units and exhibit adakitic affinities with their depletion in HREE+Y relative to LREE and subsequently high Sr/Y and La/Yb ratios. Dikes associated with the Stoddart Mo–Cu±W mineral occurrence are oxidized (as suggested by the elevated X_(Mg) of biotite and hornblende, and magnetic susceptibility data) relative to dikes associated with Au–Cu mineralization, and the former were fed by a high-level magma chamber, whereas the latter are associated with a deep-seated magma chamber. ~(87)Sr/~(86)Sr_i values from GR and Stoddart dikes are near equal (0.7052–0.7083 and 0.7054–0.7076, respectively), and the εNd_i of both dike suites is equal as well (-0.1). While Pb isotope compositions of GR dikes are heterogeneous and span a wide composition range (~(206)Pb/~(204)Pb_i: 18.823–19.577; ~(207)Pb/~(204)Pb_i: 15.593–15.681, and ~(208)Pb/~(204)Pb_i: 38.539–39.017), by contrast dikes from Stoddart yielded relatively homogeneous Pb isotope compositions (~(206)Pb/~(204)Pb_i: 19.103–19.389; ~(207)Pb/~(204)Pb_i: 15.606–15.667, and ~(208)Pb/~(204)Pb_i: 38.672 to 38.99). The difference in fO_2 and in isotope compositions, as well as in the depth of magma emplacement between the two dike sets indicates magma derived from different sources, and it is suggested that those differences further controlled the variation in styles of mineralization. The preservation of petrographic features, such as miarolitic cavities, quartz phenocrysts and phenoclasts, and the presence of intermineral porphyry dikes suggest a close temporal relationship between dike emplacement and the hydrothermal activity, thus consistent with the magmatic–hydrothermal transition.