Geology of the Trout Lake molybdenum deposit, B.C.

CIM Bulletin, Vol. 76, No. 849, 1983

H. CRAIG BOYLE and CRAIG H.B. LEITCH, Newmont Exploration of Canada Limited Vancouver, B.C

The Trout Lake stockwork molybdenum deposit is located in the Selkirk Mountains of British Columbia, 50 km southeast of Revelstoke. The property is being explored by a joint venture between Newmont Exploration of Canada and Esso Minerals Canada. The deposit is associated with a small granodiorite stock of Upper Cretaceous age (76 Ma) which has variably altered the surrounding schists, argillites and marbles of the lower Paleozoic Lardeau Group to hornfelsic biotite schists and skarn. The intrusive is composed of a small stock and an intersecting network of northeast- and northwest-trending dykes at surface that coalesce downward into a larger stock. There are two main phases of intrusion, with an earlier quartz por-phyritic granodiorite cut by an infra-mineral "quartz diorite" porphyry. A strong sub-vertical north-trending fault controls the distribution of the mineralized stockwork and displays post-mineral movement. Molybdenite, accompanied by pyrite and pyrrhotite, is mainly present along the margins of veins in a well-developed quartz stockwork, but occasionally is strongly disseminated in microfractured intrusives. The stockwork is strongest in and around the contacts of the intrusive and its apophyses, and occurs over a vertical range of more than 1,000 m. As defined by the 0.10% MoS2 contour, the main mineralized zone is up to 300 m long by 200 m wide. Preliminary drill-indicated reserves, currently being revised, are approximately 50 million tonnes of 0.23% MoS2, within which are several zones of higher-grade material. Hydrothermal alteration, as defined by quantitative X-ray diffraction studies on composite core sections, is composed of a central quartz/'K-feldspar/albite/'minor biotite zone coincident with molybdenum mineralization, which is overlapped by a slightly later, antipathetic quartz/sericite/pyrite zone. Ankeritic carbonate is also a common alteration mineral, but only traces of fluorite, and no topaz or sulfosalts, have been observed. Analysis for trace elements such as Sn, W, Bi, Sb, As, Hg, U, Ag, Au, Mn, Cu, Pb, Zn and F has been limited except for Sn and W, which appear to be zoned inside and outside the Mo zone respectively; the other elements do not show detectable patterns thus far. A strong molybdenum soil geochemical anomaly is present immediately over and down-ice from the outcropping mineralization. No streams drain the area over the deposit, so it could not be detected by conventional stream silting. A proton magnetometer survey showed only a few scattered anomalies related to the skarns containing pyrrhotite.
Keywords: Economic geology, Molybdenum deposits, Trout Lake deposit, Stockwork deposits, Metamorphisrn, Alberation, Mineralization, Soil geochemistry.