All primary gold deposits are formed usually during the last stages of the chilling of the magma, which rise along fissures to the upper layers of the earth’s crust from great depths. The gold is transported together with the magma from the depths of the earth in hot aqueous solution and vapours. In due course, these solutions get solidified with the falling of temperature, giving rise to ore bodies mostly in the form of veins filling fissures in rocks.
These veins generally consist of quartz with a small admixture of other minerals and particles of gold in the form of fine grains, platelets and flakes and sometimes crystals, wires, filaments and so on. Big chunks of gold are called ‘nuggets’. The tellurides and auriferous sulphides also occur in veins.
In the course of time, these rocks and veins under the action of various geological agents undergo mechanical disintegration and chemical decomposition. The weathered products while being carried down to the depositional site undergo the processes of mechanical concentration and all these processes give rise to placer gold deposits. The factors contributing to the concentration of gold m placers are its high specific gravity and resistance to chemical alteration through weathering.
Mode of occurrence:
Most of the primary gold deposits occur in the vicinity of acidic igneous intrusions and have been formed by hydrothermal solutions as replacement or cavity-filling and a few have been formed by contact metasomatism.
In India, gold occurs as primary veins or lodes or as placers.
The other modes of occurrence of gold deposits in various parts of the world are as follows:
(i) Magmatic deposits:
South Africa (Waarkral).
(ii) Contact metasomatic:
(iii) Residual concentration:
Brazil, Australia. Distribution in India:
It is situated in the state of Karnataka and has four productive mines-Nandydroog, Champion Reef, Mysore, and Ooregaum. The Champion Reef mine is the deepest mine in the world. In these cases, the lodes are localised along or in vicinity of stratigraphic contacts of granular, fibrous and amphibolites of the schist belt. The lodes are made up of quartz-vein zones in altered and mineralised wall rock. The mineralising solutions responsible for the development of the auriferous veins of South India, were probably derived from the magma which gave rise to champion gneisses. The principal process, which led to the development of the auriferous veins, was cavity-filling and metasomatic replacement of the wall-rocks is said to have assumed a less significant role.
2. Hutti gold-field:
In Hyderabad, auriferous veins occur in the Hutti-gold-field within the schistose rocks of Dharwarian age.
3. Ramgiri gold-field (Andhra Pradesh).
4. Wynaad gold-field (Partly Tamilnadu and Partly Kerala).
5. Chotanagpur region, Kundra-Kocha Gold-field.
The alluvial placer gold deposits occur in Assam (Subansiri River), Bihar (Subernarekha), the Son, Deo and lb-rivers of Madhya Pradesh, in Koraput and Sambalpur districts of Odisha etc.
(i) Gold is a precious metal and constitutes the international standard of exchange.
(ii) In alloy-state with other metals, it is mostly used in ornamentation.
(iii)The industrial uses of gold alloys are in dentistry, chemical plants, thermocouples, watches, X-ray equipment, photography and in some medicines.