The structure and geological development of the Erickson gold mine, Cassiar District, British Columbia, with implications for the origin of mother-lode-type gold deposits

Abstract

The Erickson gold mine is a typical gold quartz vein deposit. The veins are hosted by a thrust-imbricated, gently dipping, synformal allochthon of low-grade metamorphic, Devonian to Upper Triassic basalts, argillites, and peridotites of oceancrustal origin belonging to the Sylvester Group, part of the Slide Mountain assemblage. The Sylvester allochthon lies concordantly on Devonian miogeoclinal sedimentary rocks of the North American continental margin and was emplaced in the Middle Jurassic as a result of the collision of the Quesnel arc with North America. The veins in the mine are hosted mainly by a moderately dipping system of shear zones with approximately orthorhombic symmetry, indicating a triaxial bulk, inhomogeneous strain pattern superimposed on the earlier formed, gently dipping thrusts. Steeply dipping extension veinlets, rotation of schistosity, and downdip slickenlines indicate the maximum shortening axis was subvertical. The veins display complex superimposed ribbon and breccia textures, indicating incremental growth. Most of the gold occurs in association with tetrahedrite, sphalerite, and chalcopyrite in steeply dipping, late, grey quartz veinlets localized within and striking perpendicular to the main veins. The vein-forming event, dated at 130 Ma, appears to have been related to extension and high heat flow associated with the rise of the Omenica geanticline, in turn the result of crustal thickening caused by the collision of the amalgamated Quesnel arc – North America plate with Stikinia in the Middle Jurassic.