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Geochemical study of the Guichon Creek batholith, British Columbia Brabec, Dragan

Abstract

Aqua regia-extractable copper in 330 fresh rock samples from Guichon Creek batholith, B.C., shows a multimodal distribution with values ranging widely from 1 to 1600 ppm. Aqua regia-extractable zinc, as determined in 235 samples, has a much more limited range of values from 5 to 77 ppm. The observed distributions, particularly that of copper, are believed to be a result of the presence of both syngenetic and eplgenetic metal in the batholithic rocks. Trend surfaces computed for copper and zinc show patterns that correlate with the zonal arrangement of major rock phases, with a general decrease in metal values from the marginal Hybrid phase inward. This trend is much better expressed for copper than for zinc. Also, the areas of lowest content of the two metals occur in different rock phases. Mean copper content of the two central phases, Bethlehem and Bethsaida, is significantly lower than the means of other phases at 0.05 t-probability level. Distribution of copper in the granitic rocks, particularly the Hybrid phase, versus relatively copper-low pre-batholithic rocks does not favour the hypothesis that the latter rocks were the source of copper in the batholith. On a regional scale, the central rock phases, containing the main porphyry copper deposits in the area, are the lowest in copper. Therefore, a relatively high copper content of an intrusive phase is not necessarily indicative of its superior ore potential. Zinc in the rocks analysed correlates significantly with magnesium and iron, but such a relationship was not established for copper. Significant correlation between the metal contents and the mineral modes was found only for the pair whole-rock zinc - biotlte mode. The proportion of total copper extractable by the acid attack is considerably higher than that of zinc. All these relations, as well as the distribution of copper among rock-forming minerals, Indicate that the two metals differ in their form of occurrence. Copper and sulphur are correlated significantly only when epigenetic metal is present, that is at concentrations above 150 ppm Cu. By comparison with other intrusions composed of similar rock types, the granitic rocks of Guichon Creek batholith are higher in copper and mercury, but lower in zinc and possibly lead and sulphur.

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