The Granite Lake orebody is located west of Granite Mountain, approximately 11.5 kilometres east of the community of Marguerite on the Fraser River.
The area is underlain by the Stikine Terrane is dominantly oceanic and became amalgamated with the Quesnel Terrane to the east probably during Triassic times. The dominant rock types in the region are metabasalt, limestone and argillaceous metasediments of the Mississippian to Triassic Cache Creek Group. These are intruded by the dioritic to quartz dioritic Late Triassic Granite Mountain pluton and the (?)Cretaceous Sheridan Creek pluton. The Granite Mountain pluton has been dated at 204 +/- 6 Ma by potassium-argon dating of hornblende (CIM Special Volume 15 page 195). Jurassic sedimentary rocks overlap both the Cache Creek and Quesnel terranes to the north and east of the plutons. Older rocks are largely obscured by Plateau Basalt of probable Miocene age, to the west. The Granite Mountain pluton has been affected by regional metamorphism (greenschist facies) and deformation along with the enclosing Cache Creek Group. The Cache Creek Group and the margins of the Granite Mountain pluton record effects of ductile deformation. The main body of the pluton has been cataclastically deformed.
The Granite Lake deposit is one of the five orebodies that comprise the Gibraltar mine, the others are: Gibraltar West (093B 007), Pollyanna (093B 006), Gibraltar North (093B 011) and Gibraltar East (093B 012). The orebodies are hosted by the Granite Mountain pluton with ore mineralization almost entirely confined to the Mine Phase Tonalite portion of the Granite Mountain pluton. The Mine Phase Tonalite appears to form a thin outer shell about the main body of the pluton and contains approximately 30 per cent quartz, 50 per cent saussuritized plagioclase feldspar and 20 per cent chlorite. Varying degrees and types of alteration are present and readily visible in the Mine Phase Tonalite. Economic sulphide mineralization in the Mine Phase Tonalite is usually associated with sericitization and chloritization. The tonalite has been strongly deformed by shearing and mineralization is strongly associated with this deformation. Mineralization is generally accompanied by alteration and is confined to deformational structures. These structures comprise small and large shear zones, foliation planes, short veins and various dilatant structures.
The Granite Lake orebody is mainly hosted by the Mine Phase Tonalite, but the Granite Mountain Phase extends upward beneath the footwall in some places. The orebody exhibits several large offsets caused by block faulting. Pyrite, in a thick "blanket", overlies the main orebody which splits into two bodies towards the east. Concentrated adjacent to and inside the "blanket" are narrow zones of pod-like ore, mainly chalcopyrite, magnetite and minor bornite, hosted by bands of sericitic shearing. The main ore zone strikes 090 degrees and dips 30 degrees south.
Mineralization consists of pyrite, chalcopyrite, molybdenite, magnetite, bornite and cuprite. Associated alteration minerals are quartz, sericite, chlorite, epidote and carbonate. The Gibraltar deposits all show secondary oxidation and secondary enrichment with the formation of chalcocite as coatings and as replacement of pyrite and chalcopyrite.
As a whole, the Gibraltar mineralized system is comprised of numerous structural hosts for economic mineralization ranging from highly mineralized shear zones to complex sets of sheeted shear veins commonly referred to as oriented stockworks.
Production figures indicate that silver and minor gold mineralization is associated with these orebodies (see Gibraltar East (093B 012) for figures). Total measured recoverable reserves in 1988 for all the orebodies were 183.24 million tonnes, grading 0.32 per cent copper and 0.009 per cent molybdenum (Placer Dome Inc., Annual Report 1988). Total mineable reserves in 1995 for all ore deposits are 179.0 million tonnes grading 0.297 per cent copper and 0.009 per cent molybdenum (Gibraltar Mines Limited, Annual Report 1995). As of December 31, 1992, mining reserves (proven and probable at cutoff of 0.20 per cent copper) of the Granite Lake were 51,074,500 tonnes of 0.324 per cent copper and 0.008 per cent molybdenum. Mineral resources were 118,206,200 tonnes of 0.305 per cent copper and 0.009 per cent molybdenum (CIM Special Volume 46, page 202).
Combined (proven and probable) reserves for Granite Lake are 80.9 million tonnes, grading 0.305 per cent copper and 0.009 per cent molybdenum (Gibraltar Mines Limited, Annual Report 1995).
Granite Lake Reserves as of December 31, 1998 are:
Ore(tonnes) Cu(%) Mo(%) (%Cu)
Granite Lake proven 64,129,815 0.322 0.009 0.20
probable 6,297,678 0.321 0.007 0.20
combined 70,427,493 0.322 0.009 0.20
(Exploration in BC 1998, page A10; from Boliden Limited).
In mid-January 1999, Boliden began to shut down the mine, with full closure by February 1999. In April 1999, Taseko Mines Limited announced that they will acquire the Gibraltar mine. The same year, Gibraltar Mines Ltd. conducted a program of geochemical surveys and soil drilling on the Green Group (MINFILE 093B 058) to determine if additional mineralized structures existed between the Gibraltar and Sawmill mineralized trends. No significant anomalous values were found (Assessment Report 26064). In 2000, an induced polarization geophysical survey was conducted on the entire property.