Q: What are the main sources of hydrothermal gold deposits

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  1. McQueen (2009) putforth 4 basic requirements for the formation of any type of mineral/ore deposits. The metallic elements (Au, Cu, Pb, Zn etc.,) present in the basic, intermediate, felsic magmas/or in tholeiitic/ andesitic basalts will be in association with any of the gaseous mass of either sulphur, oxygen, carbon, chlorine and/or hydrogen ( ie a bonding affinity (ligands) between metals and gases) and they try to move from stress zone to stress free zone by using the hot liquid/water ( hydrothermal solutions) solution as a transporter from deeper level to higher level in the crust due to the prevailing convection current energy or due to the induced energy from different tectonic -deformational events. The ascending hydrothermal fluids may move upwardly or outwardly through the available vents of prevailing fractures / shear zones and slowly alter the wall rocks got solidified / precipitated and deposited the ore bearing minerals in different favourable zones at different levels.
    By this process, various types of mineral deposits are identified globally and each deposit has got its own characteristic nature and different sets of mineral assemblages as shown below.
    1.Mafic volcanic rocks associated hydrothermal deposits-(Canadian shield Greenstone belt- Au, Cu, Ag, As); 2.Felsic to intermediate pluton ( granite, granodiorite or diorite) associated hydrothermal deposits, in the zone of island arc forming- Porphyry type sulphides of Cu, Ag, Pb, Zn, rare Au, As) (Sillitoe-2010); 3.Meta sedimentary exhalative mineral deposits near subduction zones (SEDEX- Cu, Pb, Zn); 4. Low temperature hydrothermal solutions deposit in higher level as epigenetic Mississippi type valley deposits (MVP); 5. Apart from these, on the sea floor around /under sea volcanoes , midoceanic ridges, within back arc basin and fore arc /rift zones also the precipitation of sulphides from hydrotherm fluids are recorded and in which presence of massive Cu, Zn, Pb, Au, Ag, Co, Tin, barium, selenium, Mn, Cd, Bi, tellurium, Ge are reported and they are known as volcanogenic massive sulphide deposits (VHMS) of volcano sedimentary assemblage. 6. In some situation where the hydrothermal fluids react with the wall rocks of carbonate and by metasomatic replacement process, forming the ore minerals and calc silicate minerals as contact aureoles (Skarn type of deposits) . It is to note that, below the crust, the hydrous magma undergoes polybaric differentiation (PCD) and forms different plutons. The saturation of sulphide minerals show good bonding relation (ligands) with C, Cl, S, O, H gaseous material is a routine nature. And one has to remember that this process is not a single episodic, it may happen in different pulses in different period of time and because of that the network of veins/ ramification of veins of quartz with precipitated/remobilized and reprecipitated sulphide minerals are noticeable in such zones with varying density of concentration. Ore deposits get concentrated and enriched in some zones and depleted in most of the zones. In addition to that, there may be the subsequent mineralized hydrothermal solutions cutting across the already formed deposit and it may scavenge them and bring to the further higher level and deposit and get concentrated in different environment/ geological set up as noticed on the earth planet. Because of these reasons, each mineral deposit will have its own characteristic feature but over all nature offset up of the formation has to be considered and look for the minerals we interested.
    Though there are more than 60 mineral deposits only a few are mentioned for the hydrothermally originated gold deposits. 1. Kolar gold field, Kolar District 2. Hutti gold fiield,Raichur district, Karnataka.3.Maharajakadai-Veppanapalli- Addkonda Gold fields Krishnagiri district, TN:4.Devala-Pandalur gold fields, Gudalur taluk ,Nilgiri district,:5.Adathurai gold fields, Kothagiri Taluk, Nilgiri district. 6.Hot spring epithermal gold deposit in Puga, Ladakh, Jammu and Kashmir 7.Dhani Basri in Mangalwar complex (in Archaean Greenstone assemblage) 8. Attapadi valley, Nilambur valley, Kerala state 11.Mallappakonda-Chigarakunta, Andhra Pradesh etc.,

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