Page 301 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 265

Because of the relatively short time of exposure and the low gradient
topography, the development of gold placers derived from now submerged
source rocks is possible but unlikely. Few primary gold source rocks are known
to occur on the shelves themselves; any streams that may have serviced them
during periods of emergence were probably small. Furthermore, while some
areas such as the Sahul shelf, northwestern Australia and the coastal shelves of
Canada and northeastern USA are extensions of metallogenic belts onshore, the
generally thick cover of marine sediments and excessive water depths puts them
beyond the reach of present-day exploration and mining techniques. Down
warping has submerged other shelf areas to presently unmineable depths.

Beach placers at Nome, Alaska

The best-known examples of drowned strandline gold placers of glacial origin
are in California and Nome, Alaska. The Nome deposits were laid down on a flat
alluvial plain over which twelve or more beaches were developed successively
during the Quaternary and earlier periods of fluctuating sea levels. They are
classified separately as offshore, modern, submarine, second, intermediate,
Monroeville, third and fourth placers (Fig. 4.39).

4.40 General geological map showing trends of gold content in surface
sediment in Nome near shore area (after Nelson and Hopkins, 1972).