Geology, petrology, and geochemistry of andesite flows and associated intrusive porphyry in the Spences Bridge Belt, Lytton, British Columbia: Implications for precious metal exploration

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University of New Brunswick


The Spences Bridge Gold Belt is host to the Skoonka Creek precious-metal mineral deposit in Lytton. British Columbia. The main mineralization is epithermal style Au-Ag, which is hosted in Cretaceous subaerial malfic to intermediate volcanic rocks. This precious metal mineralization is hosted in the Pimanius Formation in multi-stage massive veins, and stock work vein lets with associated breccia zones with siIica, albite, and argillic alteration round throughout the Skoonka Creek property. The mineralization on the property is highlighted at the J.J showing where Au values grade as high as 20.2 g/t over 12.8 m and 28.6 g/r O\U 3.3 1m. Two outcrops and the Discovery showing diamond drill holes show a geological contact relationship between an amygdaloidal microcrystalline basaltic andesite and an intrusive hornblende porphyry. There is no petrogenetic relationship between the two units; I) because of the distinctly different nature of plagioclase zoning evident; the basaltic andesite has reverse zoning with a sodic interior and a calcic rim, whereas the hornblende porphyry displays normal zoning with a calcic interior and a sodic rim, 2 ) both units have similar groundmass mineralogy containing plagioclase, quartz, calcite and biotite but differ with the presence of clinopyroxene (7%) in the microcrystalline to cryptocrystalline basaltic andesite, and amphibole (Y%), and potassium feldspar (5%) in the microcrystalline hornblende porphyry. Evidence that the extrusive basaltic andesite is older than the hornblende porphyry is supported by the observation of a quenched contact of hornblende porphyry (An content 43-59 wt.%) next to the basaltic andesite (An content 45-56 wt.%). The intrusive hornblende porphyry may be directly genetically associated with the epithermal precious-metal forming hydrothermal system.