Geology Of Placer Gem Deposits

Here is an experts view on placer deposits and its commercial significance in gem producing countries around the world + the geological and gemological characteristics that are important in identifying the particularities of placer deposits.

James M Prudden (Prudden GeoScience Services, Nevada, USA) writes:

Placer gem depositional environments consist of colluvial, fluvial, and beach deposits. The weathering of primary gem-bearing deposits forms overlying eluvial deposits, and the down-slope migration of the residual gems by both gravity and water creates colluvial deposits. Fluvial systems range from youthful through mature and old-age sedimentological regimes with associated channel geometrics that determine the hydraulic energy and therefore the locations of gem deposition. Fluvial systems commence with straight steep-channel gradients, with low depth-to width ratios containing unsorted clasts and large gems. This evolves into to the downstream, low-energy, old-age fluvial systems with low channel gradients that host bedded, well-sorted smaller clasts deposited in a meandering fashion within a broad flood plane. Gems in this environment are smaller and more rounded. At the point where the river enters a marine or lacustrine environment, the resulting abrupt gradient change is very favorable for gem deposition. Wave energy and long shore currents further winnow and transport gems in beach environments. Alpine and continental glaciers are nature’s bulldozers and the braded fluvial streams that are fed from their melt water effectively concentrate the contained gems from the glacial rubble.

Gem characteristics such as specific gravity, hardness, shape and durability will influence their related depositional environments and survivability, thus favoring the economic concentrations of certain gems in the fluvial milling environment.

Select case histories of a variety of placer deposits illustrate the practicality of applying detailed geology and sedimentology to placer gem exploration:

1. Australian Tertiary modified paleo-colluvial type sapphire deposits, derived from the weathering of alkaline basalts, have been a major global source of sapphires.

2. Namibian long-shore diamond distribution along the Atlantic Ocean coast constitutes the world’s most valuable diamond deposit, extending westward 100km to the continental shelf edge and 200km northward. The diamonds were originally liberated from the South African kimberlites (and possibly more distant sources) by post-Gondawana erosion of the southern African craton, which commenced in the humid Middle Cretaceous with the formation of the ancient Karoo and Kalahari Rivers. Subsequent erosion of these diamondiferous placers was accomplished by the Orange River in the Miocene. Prolonged winnowing of the diamonds increased their value by about 500%.

3. Fluvial reworking of glacial sediments in British Colombia, Canada, concentrated sapphires and garnets from several cubic kilometers of glacial material.

4. A fluvial diamond deposit in China’s Hunan province was deposited on completely weathered karst bedrock, which presents challenges to sampling and mining.