Tungsten–molybdenum skarn and stockwork mineralization, Mount Reed – Mount Haskin district, northern British Columbia, Canada

Abstract

In the Mount Reed – Mount Haskin area, intrusive- and calc-hornfels-hosted stockwork Mo(−W) mineralization, oxidized W–Mo and Zn–Pb–Cu skarn mineralization, and Ag–Zn–Pb vein mineralization are spatially associated with multiphase, epizonal, magnetite-bearing granite stocks of Eocene age, emplaced into the Lower Cambrian Atan Group of the Cordilleran miogeocline.Mo(>W)-stockwork mineralization partly predated the main exposed intrusions and was related to bodies of aplitic syenogranite, vestigially preserved. Two extensive, mineralogically and texturally distinctive, but probably coeval, zoned exoskarn systems constituting the main stage (stage I) are recognised at Mount Reed. The B- and F-rich inner system was controlled by fracture permeability in dolomitic marble. It is zoned from massive magnetite skarn, at the intrusive contact, through concentrically banded ("wrigglite"-textured), diopside–phlogopite–magnetite–chondrodite–vesuvianite–(Mo-rich) scheelite skarn, to stockwork veinlets with diopside cores and serpentine–chondrodite–humite–ludwigite–magnetite marginal zones. The calcic outer system was controlled mainly by bedding-plane permeability at a gradational marble – metapelitic hornfels contact and consists of banded skarn zoned from garnet-, through ferrosalite-, to wollastonite-dominant assemblages, with fluorite as a minor but persistent constituent.Superimposed on both systems are dykelike bodies of vesuvianite–garnet skarn (stage II) and amphibole–biotite–sulphide skarn (stage III) associated with quartz veins. Scheelite is markedly enriched in the latter, retrograde, skarn facies, which is, however, of only limited development. Because the main stage skarns appear to be truncated by the Mount Reed monzogranite, it is tentatively proposed that they formed adjacent to the early aplitic syenogranite and may thus have been coeval with the Mo-stockwork system.The unusually varied exoskarn assemblages at Mount Reed well illustrate the influence of protolith composition and structure and of fluid–rock interaction on skarn development.