Stratigraphic control of Cu-Fe skarn ore distribution and genesis at Craigmont, British Columbia

CIM Bulletin, Vol. 73, No. 820, 1980

GREGG W. MORRISON, Geology Department, University of Western Ontario, London, Ontario

margin of the late Triassic Guichon Creek batholith in south-central British Columbia. Host to the ore are sedimentary and volcanic rocks of the western calc-alkaline belt of the Upper Triassic Nicola Group. In the mine area, the Nicola Group rocks are a stratigraphically complex sequence, at least 1800 feet thick, which have been subdivided into Basalt, Rhyolite, Carbonate and Clastic Sediment units. All the units strike and dip parallel to the margin of the Guichon Creek batholith and are within the batholith contact aureole. However, only the Clastic Sediment unit is in contact with the batholith.Most of the skarn and ore is within the Carbonate unit and more specifically within an Interbedded fades that consists of lime sandstone, lime siltstone, quartzo-feldspathic siltstone and argillite. The Interbedded fades and the adjacent barren Massive Limestone fades are interpreted as part of a carbonate reef complex. The distribution of skarn and ore within the Interbedded fades suggests that the fades boundary was a physical and chemical barrier that resulted in concentration of metals adjacent to the Massive Limestone fades.Two stages of skarn formation have been recognized at Craigmont. In Stage I, ore-bearing magnetite-rich and actinolite-epidote-magnetite skarns developed both in basaltic rocks and in Fe-bearing siltstone and argillite of the Interbedded fades, while barren grossular garnet-epidote-caldte-pyrite skarns developed in Fe-poor rhyolitic bands within the Interbedded and Massive Limestone fades. In Stage II, massive, barren grandite garnet skarn replaced mineralized Stage I skarns in the vicinity of diorite plugs and specularite ore formed in brecdated Stage I skarn and clastic rocks away from the plugs. The formation of Stage I skarns required no addition of material, only redistribution and concentration of elements already present within the Interbedded fades. However, the concentration of Cu in Stage I ore requires a mechanism for more efficient leaching of Cu from the host rocks, an anomalous concentration of Cu at the site of the Stage I orebody or an additional input of Cu from an external source. The formation of Stage II skarn and ore requires only remobilization and recrystallization of Stage I skarn and ore.Copper and iron are the only elements in economic concentrations in the ore and chalcopyrite is the only significant copper-bearing mineral. Chalcopyrite is disseminated and interstitial to magnetite in Stage I mineralized skarns and is interstitial to specularite in the matrix of Stage II breccia ores. Chalcopyrite also occurs with pyrite as stringer ore in bleached -sediments and rhyolite tuffs in the Stratigraphic foot wall of the main orebodies.The Craigmont deposit has more features in common with Cu-Fe deposits hosted in metavolcanic and metasedimentary rocks than it has with typical skarn deposits associated with porphyry copper deposits. The orebody is more likely a product of concentration of metal contained within the Nicola Group rocks than of the mineralizing event that produced the Highland Valley Cu-Mo porphyry deposits in the central part of the Guichon Creek batholith.
Keywords: Stratigraphy, Copper, Iron, Skarn, Craigmont mine, Nicola Group, Guichon Creek batholith, Contact aureoles.