Lead- and strontium-isotope geochemistry of the Tertiary Catface intrusions and related mineralization, Vancouver Island, British Columbia

Abstract

Lead- and strontium-isotope ratios from the Catface Tertiary intrusions in Vancouver Island occupy a narrow range of values, indicating a homogeneous or well-mixed source. Within this range, the lead-isotope data fall into two distinct clusters centred about initial 206Pb/204Pb of 18.70 and 18.95. These two clusters correlate, respectively, with the eastern and western belts of Catface intrusions. Initial strontium ratios for these intrusions show little variation about a mean value of 0.7039.The source of these intrusions cannot be identified uniquely using lead- and strontium-isotope data alone, but two possibilities can be ruled out. 207Pb/204Pb, 208Pb/204Pb, and initial strontium ratios are all too high to be solely from mantle-derived magmas. Initial strontium ratios are too low and the 208Pb/204Pb and 232Th/238U ratios are too high for the Catface intrusions to be the products of melting of pre-existing Sicker Group rocks.Galena from quartz-gold veins related to the Zeballos pluton has a lead isotopic composition similar to that of the Zeballos pluton. Consequently, if lead and gold have the same origin, the source of the gold is the intrusion, rather than the host rocks.