A Geochemical Drainage Survey and its Implications for Metallogenesis, Central Coast Mountains, British Columbia

Abstract

Two thousand stream sediment samples from 3,300 square miles of the central Coast Mountains of British Columbia were analyzed for Mo, Cu, Zn, and cold-extractible copper (cxCu). The data were grouped according to the lithologies underlying the drainage basins, and the distributions and interrelationships of the trace metals were examined using cumulative frequency plots and relatively simple statistical methods. Multimodal metal populations may be caused by analytical bias, heterogeneity of lithologic units, and sulfide mineralization. Positive correlations between mean Cu and Zn contents of stream sediment samples and mean specific gravity of associated plutonic rocks are interpreted as resulting principally from variations in the mafic mineral content of the plutonic rocks.Samples containing anomalous metal values were obtained from streams draining most known skarn, massive sulfide, and pegmatitic deposits in the areas sampled. Sulfide mineralization associated with skarn, and many Mo anomalies, are spatially associated with major faults and are interpreted as having resulted from fault-controlled metallization that may be genetically unrelated to nearby plutonic rocks. Massive sulfide deposits and Cu, cxCu, and Zn anomalies from the Ecstall Belt of metasedimentary rocks are thought to be the results of metallization that was restricted to the immediate vicinity of the anomalies. Hypothermal quartz veins of the Surf Point Mine and many of the Mo anomalies on Porcher Island are related genetically to the forcible intrusion of a nearby pluton.Relatively simple statistical techniques are useful in interpreting geochemical prospecting data in some cases where more sophisticated techniques cannot usefully be applied. Reconnaissance stream sediment surveys in the central Coast Mountains are useful both in prospecting for massive sulfide deposits and in metallogenic studies.