Stratigraphic, lithogeochemical and tectonic setting of the Kutcho Creek massive sulfide deposit, northern British Columbia.

Barrett, T.J., Thompson, J.F.H. and Sherlock, R.L., 1996. Exploration and Mining Geology, v. 5, p. 309-338.
The Kutcho Creek Cu-Zn deposit in northern British Columbia occurs within the King Salmon allochthon, between the Stikine and Cache Creek-Quesnellia terranes. The stratigraphically lower portion of the King Salmon allochthon includes the volcanic-dominated Kutcho Formation; the upper part comprises sedimentary formations. The southern portion of the Kutcho Formation, which may be the stratigraphically lower part, comprises massive and volcaniclastic mafic volcanic rocks, with lesser felsic volcaniclastic rocks, and shallow-level intrusions of trondhjemite and rhyolite porphyry. The upper Kutcho Formation consists of dacitic volcaniclastic rocks, and overlying sequence of rhyolitic pyroclastic rocks which hosts the Kutcho Creek deposit, and finally, felsic volcaniclastic turbidites and black mudstones representing a transition from felsic volcanism to marine basin sedimentation. The ages of the felsic intrusions and the rhyolites immediately hosting the deposit are close to the Permo-Triassic boundary (Childe and Thompson, 1995).

The Kutcho Creek deposit consists of three lenses of massive sulfide occurring along essentially same stratigraphic horizon over a strike length of 3.5 km, with reserves in the main Kutcho lens of 17 Mt grading 1.6% Cu, 2.3% Zn, 29 g/t Ag and 0.3 g/t Au (Bridge et al., 1986). The lenses consist mainly of pyrite with subordinate sphalerite and chalcopyrite, and are locally cut by bornite-rich veins. Intercalations of laminated ferroan dolomite occur within the sulfide lenses. The mineralized horizon lies close to the contact between footwall rhyolite lapilli tuffs, and hangingwall quartz-plagioclase-rich pyroclastic rhyolites. The main alteration assemblage in the footwall and lower hangingwall is quartz-sericite-dolomite-pyrite. Chemical alteration effects, mainly Na depletion and variable additions of Mg, Si and Fe, extend at least 100 metres below the mineralized horizon lenses and 10-20 metres above. The upper footwall rhyolite is commonly cut by pyrite-quartz veins which are interpreted as representing tectonically flattened stockwork veins.

The Kutcho Creek rhyolites have a distinctive primary chemistry characterized by low contents of high-field-strength elements (Zr, Y, Nb), including the rare-earth elements (REE). Least altered rhyolites contain 100-120 ppm Zr, with tholeiitic Zr/Y ratios of 2-4. REE patterns are flat to light REE-depleted. The lithogeochemical data and field relations suggest that the massive sulfides and exhalites were deposited during a hiatus between eruptions of chemically and physically different rhyolite types (although they may have been derived from the same magma chamber). The linear trend of the massive sulfides and the rapid facies changes in the rhyolitic pyroclastic rocks suggest that hydrothermal activity and fissure-related felsic eruptions occurred in an extensional submarine setting with significant seafloor relief. Basalt and basaltic andesite lower in the Kutcho Formation are of tholeiitic affinity, with high (15-19%) Al2O3 values, and very low contents of Zr, Y and Nb, and flat REE patterns. Dacitic volcaniclastic rocks lying between the lower mafic and upper rhyolitic parts of the Kutcho Formation are also of tholeiitic affinity. All of the lavas and volcaniclastic rocks have similar immobile trace element ratios and REE features.

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