Bulletin 59: Geology of Copper Mountain
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Bulletin 59 describes the geology of a 50-square-mile area centered on Copper Mountain, 10 miles south of Princeton, British Columbia.
Particular attention is paid to the structure, stratigraphy, and alteration of volcanic and sedimentary rocks of the Nicola Group, to the Copper Mountain intrusions, which cut them, and to the setting of copper deposits in this environment.
Nicola Group rocks consist of a volcanic succession that includes massive flow units, coarse to very fine-grained pyroclastic units and some pillow lava, and of a sedimentary succession that includes siltstone, argillite, conglomerate, and some reefoid limestone. The volcanic rocks are generally andesite to basaltic andesite in composition. Their age, as indicated by fossils, is Upper Triassic.
The Copper Mountain intrusions cut the Nicola rocks and consist of a series of differentiated quartz-poor calc-alkalic rocks that range in composition from pyroxenite to syenite. These intrusions include the Copper Mountain, Smelter Lake, and Voigt stocks and the Lost Horse intrusions, and have been dated radiometrically at 193±6 million years.
Copper deposits are found near Copper Mountain in a narrow belt of strongly altered and fractured Nicola rocks that is bounded on the south by the Copper Mountain stock and on the north by the Lost Horse intrusions. Mineralization is controlled by faulting and fracturing, suitable alteration, and, in most cases, by the proximity of rocks of the Lost Horse intrusions, which appear to have been the immediate source of hydrothermal and mineralizing fluids. The copper deposits are regarded by some as being pyrometasomatic, and by others as being partly skarns (Ingerbelle) and partly complex porphyry copper gold deposits (Copper Mountain). Typical rock alteration includes an early development of biotite followed by extensive albite-epidote replacement and later veining by potash feldspar and scapolite.
Post-mineral intrusions include a quartz monzonite mass that has been radiometrically dated at 99.5±4 million years and a later swarm of northerly trending felsitic dykes.
Middle Eocene sedimentary, volcanic, and intrusive rocks of the Princeton Group unconformably overlie and cut all the previously mentioned rock units.
Deformation in the area consisted of mostly broad northerly to northwesterly trending folds, and intense faulting. At Copper Mountain faults may be separated into distinct sets which, in order of decreasing relative age, trend east-west, northwest, and northeast. Later northerly trending normal faults dissected the area and divided it into two distinct blocks, the western of which has been downdropped and appears to be devoid of intrusive rocks of the Copper Mountain type at the present level of erosion.
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