A: by Dr. M.K. Devarajan
Exploration for deep mineral deposits is carried by indirect methods such as geophysical methods (deep IP, magneto-telluric, seismic etc) and by deep drilling. Before the advent of geophysics mineral deposits were delineated on the basis of geological, structural and geo-chemical aspects.
A: by Sri Pramod Kumar, AMD
Geological modelling, concept based ideas and lessons learnt from other places.. . Geophysics now is the third eye n very helpful in exploration.
A: by Sri Om Prakash Somani, MSPL
Normally it is very difficult to assess the depth in the absence of any geophysical model or drilling, but one can draw inference of the depth if any nearby mine exists in the similar geological set up.
Mineral Exploration Academy 
Selection of suitable area for a particular commodity( mineral/metal) is based on the Geological considerations as there is a close link between the minerals and the rock type in which they occur. For e.g. Gold generally occurs in greenstones, Platinum Group of elements in Ultramafic- mafic complexes, REE in carbonatites and alkali granites, RM in pegmatites and granites. Major shear zones do play a significant role in remobilising and concentration of the mineralization in structurally weak zones. Taking such geological and structural aspects in to consideration, GSI has identified Obvious Geological Potential (OGP) areas for each commodity and the geological ma of these OGP areas are in GSI portal. So we can select the target area for a particular commodity from these OGP area maps. Common Exploration methodologies include Detailed Geological mapping of the target area aided with Photogeology and Remote sensing studies which will help in identifying different lithologies and structural features in the study area. If the area exposes good amount of outcrops, surface samples can be collected from all the lithologies within the complex and analyse for the metals which we want to explore, as a part of Orientation survey. This would help us in narrowing down the target for detailed sampling. The host lithlogy thus identified can be focussed in our Detailed mapping on larger scale to delineate the entire strike length and width of the target rock. Then the entire strike length of the target lithology has to be sampled at regular interval ( 100m.) across the strike keeping the sample length at not more than 1m. This exercise would certainly help us in delineating the mineralized zone at a chosen cut-off value (e.g. 0.5 ppm for gold and PGE). Having delineated the mineralised zone on the surface, its depth persistence has to be proved by drilling to get the grade and width of the mineralised zone at the proposed depth at which it is intersected in the borehole. With all these three dimensional data as well as the grade, available resources of that metal can be estimated by the conventional methods which are in practice. Yes if we do not have good outcrops and the area is mostly soil covered, we have to resort to integrated Geophysical surveys- gravity, magnetic , IP/SP and radiometric methods, to delineate the causative ore body based on the physical properties of the rock or minerals we intend to delineate ( e.g. gravity survey helps in delineating Ultramafic bodies which host PGE, IP/ SP for sulphides, magnetic for delineating shear zones, dykes etc., Radiometric for U-Th mineralization). These geophysical surveys give a rough estimate of the presence of such causative bodies and the prognostigated depth at which it is present. But actual delineation of these bodies is possible by drilling at closer interval followed by sampling. So final word about the depth extension of such mineralised bodies can be said only after drilling and studying the cores recovered from these drill holes. Thus mineral exploration involves an integrated approach involving geology, geophysics and chemical analysis.
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On behalf of Saurabh Priyadarshi, GeoExplorers
“…One needs to understand the depositional geology of ores & minerals. Thereafter, one needs to understand the stratigraphic sequence by going through country specific regional exploration reports available.One can also look at river sections or adjacent operating mines to have back of the envelope idea about the depth of mineralisation. Looking at the attitude of the mineral prospects and mapping it over a specific grid will also enable depth estimation if the dips of the ore body vary.Of course extrapolation is good way but no substitute for DC or RC drilling..”
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On behalf of Dr.V.Ganeshan, former Director, GSI
“…a.Prior to the geophysical survey, Geologists can locate the probable area for mineral exploration studies based on the following criteria:
IN AN EXPOSURE: If any lineament structure exhibiting intense fracture/shears running for >100mts distance with intermittent or continuous injection of silica/carbonate/ baryte veins bearing some incidences of dissemination of sulphides (Chalcopyrite-CuFeS2, Pyrite-FeS2/Bornite/ Cuprite/ Malachite etc) or very good development of gossanised zones (oxidation zone with distinct Fe encrustation like limonitisation / varied boxwork development) on the surface may indicate the area of interest. Since it is a linear feature exposed >100mts or more distance, definitely it will have a depth persistence even beyond > 150mts.
b.If there is no exposure but only SOIL COVER, just like a plain land, showing floats of concentrated limonitised / ferruginised quartz / or box work or highly sheared/ fractured altered rock fragments with incidence of sulphides may give a clue for your area of interest in that zone. Which may have definitely a strike extension and quite different from the adjoining land. This may be confined through any well sections and the dug out materials. The periodical finite interval of soil sampling from the zone of B and C horizon ( bottom of the upper weathered soil and top of the oxidized zone below) will analyse the anomaly zone in geochemical soil sampling. Based on the anomaly zone of contours for different minerals (ex: Cu, Pb, Zn, Mo, Ag etc.,), the exploration zone may be targeted. If rock exposures are there, the bed rock sampling at finite interval along and across the strike may indicate the anomaly zone which is our area of interest. Trenching and sampling in periodical intervals and their chemical analyses will support in identifying the geochemical anomaly zones of our interest of exploration. Subsequently, the Geophysical studies may help to delineate the strike extension and depth persistence of the ore body followed by drilling and sampling techniques may prove the deposit existence…”
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