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Bedrock geochemistry of porphry copper deposits, Highland Valley, British Columbia Olade, Moses Ayodele Deleson


The feasibility of utilizing bedrock and mineral geochemistry in the exploration for porphyry copper deposits has been investigated in the Highland Valley copper district. More than 1500 bedrock samples collected from the vicinity of the Valley Copper, Bethlehem-JA, Lornex, Highmont and Skeena deposits together with 60 fresh unmineralized samples covering the Guichon Creek batholith (Northcote, 1968) were analyzed for more than 20 elements using total and partial digestion. An efficacious sulphide-selective technique, not used previously in bedrock geochemistry was developed during this investigation. Chemical variations in fresh rocks of the Guichon Creek batholith are consistent with a model of fractional crystallization of a calc-alkaline dioritic magma, Cu, like other femic elements (Zn, Mn, V, Ti, Ni, Co, Fe, Mg), generally decreases with increasing magmatic fractionation. This geochemical pattern is commonly characteristic of unmineralized intrusions suggesting that ore metals were not derived by differentiation of a Cu-rich Guichon Creek magma as proposed by previous workers, Results of isotopic studies (Field et al., 1973) are however, consistent with a model of derivation of ore metals from a subcrustal source, most probably subducted oceanic crust or upper mantle. Detailed bedrock geochemistry around mineralization reveals that S and Cu show the highest geochemical contrast, with halos extending up to 0.5km from mineralized zones. Of these two elements, S shows the more consistent pattern. Dispersions of the litho-phile elements (Rb, Srf Ba, K, Ca, Na) are controlled "by type and intensity of wall-rock alteration, with halos extending slightly beyond ore zones but within the alteration envelope. Distribution of the femic elements (Zn, Mn, V, Ti, Ni, Co, Fe, Mg) is controlled principally by primary lithology, although minor hydrothermal redistribution is apparent. Hg defines a broad anomaly at Bethlehem-JA but is absent at Valley Copper. B anomalies are well developed at Lornex and Highmont but less prominent at Valley Copper and Bethlehem-JA. Results of factor analysis are consistent with subjective interpretations of metal associations and ore-forming processes. Mineral geochemistry indicates that biotite, magnetite and quartz-feldspar phases from mineralized samples are enriched in Cu but depleted in Ni, Zn, Mn, Co and Mg, relative to background samples. Better geochemical contrast is obtained with whole-rock than mineral analysis, consequently the use of mineral phases offers no advantages for exploration in the Highland Valley, In exploration for porphyry copper deposits of the Highland Valley type, S, Cu, Rb, Sr, Ba, K, Na, B and Hg in bedrock can be useful in delineating intensely altered and mineralized zones.

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