Glacially derived, shallow-marine gold placers of the Cape Yakataga district, Gulf of Alaska

Abstract

Economic gold-bearing coastal sediment of Holocene age (10 ka) occurs along the Gulf of Alaska coastal margin from Cape Yakataga in the west to Icy Bay in the east, a distance of 50 km. Deposits have been worked since 1898 and total production since then is about 15,000 ounces. Gold is recovered from discontinuous sheets of “ruby” garnet-rich beach sand exposed at low tide along the high-energy storm-dominated coastline. Similar auriferous sand facies are also present within raised marine terraces cut across outwash fans along the narrow mountainous coastal margin. The area has been uplifted by glacio-isostatic rebound following the last glacial maximum and by high-magnitude earthquakes; the area lies along the collision zone between the North American and Pacific plates. Study of uplifted and well-exposed Late Cenozoic strata (the glaciomarine Yakataga Formation) that outcrop along the coastal margin confirms this succession as the source of placer gold. The Yakataga Formation records the rapid accumulation of over 6 km of gold-bearing glaciomarine sediments in a rapidly subsiding forearc basin. These strata have in turn been uplifted as coastal mountains and eroded by glaciers; the White River Glacier and Yakataga Glacier drainage basins can be identified as the principal source of detrital gold along the Yakataga coastline. Auriferous beach sediment is the product of westward reworking of glacial outwash from the White and Yakataga rivers and is accompanied by a well-defined downcurrent reduction in the size and number of gold particles.