ELUVIAL

Eluvial Deposits

The Kuranakh gold deposits found in 1959-1962 are mainly in Lower Cambrian bituminous limestones and dolomites in the Aldan anticline of southern Yakutia. There is considerable similarity between the primary gold occurrences and those disseminated deposits of north-central Nevada (Carlin, Cortez, etc.). The primary occurrences in the Kuranakh district are mainly potash feldspar (adularia) and quartz metasomatites with about 6 ppm gold occurring as irregular replacements of the limestones and dolomites in places in stratiform bodies up to 3 km in length, 300 to 500 m in breadth and 5 to 10 m in thickness. The gold in the adularia zones is very finely divided (2-5 µ) and mainly in pyrite. Thin kersantite dykes and sills of late Mesozoic age are also accompanied by adularization along their contacts both in the Cambrian strata and in Lower Jurassic sandstones, and these igneous rocks have a comparable tenor of gold within the ore zones. Oxidation of the primary gold-bearing zones and karstification of the Lower Cambrian limestones, in post-
Jurassic and recent times, has given rise to economic eluvial deposits consisting essentially of highly weathered material composed mainly of oxidized ore, limonite, clay and sandy materials in the karst cavities. The gold in the eluvial material occurs as very finely divided flakes ranging in size from 5 to 20 µ The ratio of concentration is not given, but presumably it is about 12 ppm.

In Luzon, Philippines, eluvial placers are common and were extensively mined in the past. In the Paracale-Mambulao district where the primary auriferous material is mainly veins and stock-works of quartz and pyritic quartz veins in granite intruding serpentinites. The auriferous eluvial material consisted mainly of weathered debris and disintegrated bedrock carrying free gold.

Certain eluvial auriferous deposits take the form of talus accumulations. A typical example of this type of placer occurs near the Belaya Gora deposit in the Lower Amur region of Russia. The bedrock deposit occurs near the summit of Belaya Gora in a strongly fractured, kaolinized and silicified Oligocene volcanic rock. The gold is mainly free and accompanied by about 0.5 % combined pyrite, arsenopyrite, sphalerite and argentiferous sulphosalts. The talus placer associated with the primary deposit lies below the deposit on a gentle slope and averages some 5 m. in thickness. In composition the talus material is largely variegated clay and sandy clay and oxidized rubble with a heavy mineral suite composed mainly of limonite, magnetite, ilmenite, chromite, epidote and small amounts of zircon, sulphides, manganese oxides and other minerals. The gold is disseminated through the talus material and also forms short streaks and enriched lenses. The gold in the talus is not much different from that in the bedrock. The gold grains range in size from 0.05 to 1.5 mm. Some adsorbed and very finely divided gold also occurs in concretions of kaolinite and quartz. Downhill from the talus placer there is a colluvial and alluvial placer system associated with springs and the river fed by the springs.

[ Placer Deposits 1 2 3 Eluvial 5 6 Alluvial 8 9 10 Examples 12 13 14 15 ]

Maps of alluvial gold deposits in: California, Western Canada, Eastern Canada, Russia, World
Maps of primary gold deposits in: Precambrian, Paleozoic, Mesozoic, Cenozoic Rocks