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Geochemical dispersion in bedrock and glacial overburden around a copper property in south central British Columbia Hoffman, Stanley J.

Abstract

The Rayfield River copper property, 14 miles east of 70 Mile House in south central British Columbia, is underlain by a zoned syenite intrusion of late Triassic or early Jurassic age. The batholith was introduced into Mesozoic volcanics of the Nicola Group and has since been partially buried by Tertiary Basalt flows. The syenite grades from several hybrid phases at the country rock contact, through hornblende syenite, to leucocratic syenite and finally, to several centrally disposed pegmatitic bodies. Chalcopyrite and bornite, the major sulphides present, occur as inclusions within hornblende, along feldspar veinlets and along fractures in a concentric zone surrounding the leucrocratic syenite. On the property, the Rayfield River dissects to 400 feet, a gently rolling plateau. Glacial deposits are rare along the valley but commonly approach 50 feet in thickness on the plateau. Soil, lake sediment and water samples collected from these young and relatively unweathered surficial deposits are alkaline. Talus soils along the valley, however, are slightly more acidic, due to oxidation of primary sulphide minerals. The value of geochemical patterns in locating bedrock mineralization may be gauged by the success attained where sources of copper in bedrock are known. Three copper rich areas have been found. The most striking bedrock anomaly, near the center of the property, is outlined by copper-rich syenite float, stream sediment, lake water and sediment and talus soils. The second, along the northern half of the river valley, was found by stream sediment and talus surveys. The third bedrock anomaly, on the southeastern fraction of the property, is surrounded by float blocks relatively high in copper. Copper enrichment within glacial overburden is usually detectable over twice the area underlain by bedrock mineralization. Most secondary anomalies overly batholithic rocks, except in the south where rounded syenite float blocks, mineralogically and structurally similar to the most striking bedrock anomaly, were transported by a glacial Bonaparte River to where they now overly Nicola Volcanics. On a regional survey, boulder tracing and lake sediment or lake water sampling are most likely to indicate the presence of a mineralized intrusive. Detailed sampling reveals anomalous stream sediments of the Rayfield River and copper-rich talus along the valley sides of the northern half of the property. Detailed soil sampling is not suitable for outlining copper mineralization, as alkaline soil and thick overburden restrict movement of copper ions. Erratic high copper values are usually related to mineralized float or bedrock. Analysis of second year growth of Douglas fir or lodgepole pine apparently does not detect mineralization in bedrock.

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