Structural geology of the Vangorda PbZnAg orebody, Yukon, Canada

Abstract

The Vangorda PbZnAg orebody is a 7.1 M tonne, polydeformed, polymetamorphosed stratiform massive sulphide orebody in the Anvil mining district, Yukon, Canada. The orebody consists of fine- to medium-grained semi-massive and massive sulphides with a dominant sulphide mineralogy of pyrite, pyrrhotite, sphalerite, galena and minor chalcopyrite. Five sulphide lithofacies have been identified within the orebody; ribbon-banded, carbonaceous, pyritic quartzite; pyritic quartzite; massive pyritic sulphides; baritic, massive pyritic sulphides; and pyrrhotite-sphalerite massive sulphides. The orebody is elongate in a northwest-southeast orientation and plunges shallowly towards the northwest. The host rocks and the various sulphide lithofacies have been complexly deformed during two penetrative phases of deformation (D 1 and D 2, respectively) and associated metamorphism (M 1 and M 2). M 2 metamorphism resulted in extensive recrystallisation and subsequent grain growth of sulphide minerals. M 2 metamorphic grade recorded by arsenopyrite and chlorite thermometry and sphalerite+hexagonal pyrrhotite barometry is of mid-greenschist facies with temperatures of 336° to 363°C, respectively, and a pressure of 4 kb. The earliest structure that can be recognised in the orebody is a penetrative S 1 spaced cleavage that is well-developed in the host phyllites and a differentiated layering in the semi-massive and massive sulphides. However, F 1 folds have not been identified in the orebody, although they do occur within the Anvil District. The S 1 cleavage has been deformed into downward-facing, southwest-verging, recumbent F 2 folds that plunge shallowly northwest-southeast. A penetrative axial planar S 2 crenulation cleavage is developed in the phyllites and ribbon-banded, carbonaceous, pyritic quartzite, but is found only locally in the massive sulphide lithofacies. In the baritic lithofacies, F 2 folds have thickened hinges suggesting remobilisation of the ore. Along the overturned limbs of the F 2 folds and at lithofacies contacts centimetric to metric-scale zones enriched in pyrrhotite, sphalerite and galena are developed. These areas commonly have a significantly reduced grain size, a well-developed sulphide foliation and numerous shear indicators such as winged and rotated clasts. F 2 folds and the S 2 form surfaces are cut by syn- to post-D 2 NW-SE-dipping extensional faults. Minor strike-slip faults cut D 1 and D 2 structures.