LEAD ISOTOPE AND OTHER EVIDENCE ON BEDROCK SOURCE OF PLACER GOLD AT HAHNS-PEAK, COLORADO

Isotopic analyses of lead from galena of two different Precambrian ages, from galena and feldspar of Tertiary age, and from placer gold of unknown origin on two sides of Hahns Peak, Routt County, Colorado, are compatible with gold mineralization associated with Tertiary igneous activity. An isotopic mixing curve shows the leads to be related to each other, possibly reflecting a common ultimate origin in the underlying basement. Tertiary feldspar lead is less radiogenic than Tertiary galena lead, and lead from gold lies between them on the mixing curve. Analysis of the gold for uranium and thorium showed too little uranium and thorium to account for the radiogenic component of the lead in gold. Spectrographic analyses of the gold and geochemical sampling data also favor a relationship of gold mineralization to Tertiary igneous activity rather than to a source in Precambrian rocks or conglomeratic strata of Mesozoic or Tertiary age. Metamorphic rocks of Precambrian age, sedimentary rocks of Mesozoic and Cenozoic age, and igneous rocks of Tertiary age have all been proposed as the bedrock source of gold in placer deposits in the vicinity of Hahns Peak, northwest Colorado (Parker, 1961). This region seemed to be a desirable one in which to test the possibility of extracting lead from placer gold and analyzing it isotopically, because two episodes of mineralization occurred in Precambrian time (Antweiler, 1965) and at least one episode of mineralization occurred in Tertiary time. The long time span guaranteed isotopic variability. If a lead isotope analysis would indicate to which of these episodes gold mineralization was related, as is possible, an added tool would thus be available in exploration for the lode gold source. © 1972 Society of Economic Geologists, Inc.