Q: What are the signatures on surface, of a hidden copper-gold mineral resource, and which type of terrain should be looked into for it?

Answer: By A. Sundaramoorthy, former Director General, GSI

For copper the surface manifestation is development green or bluish green malachite stains and gossan, example Rajpura Dhariba, Rajasthan. For gold associated with other sulphides it is development of yellowish stains and development of gossan with boxwork. If gold occur in native form like Kolar Gold field no surface indication except destination and specks of gold in the host rock. Gold is a very tricky mineral. The sampling should be very close because concentration varies widely. Secondly the drilling also done very close may be 100 m intervel. The main thing is sample analysis should be done by fire assay.

5 thoughts on “Q: What are the signatures on surface, of a hidden copper-gold mineral resource, and which type of terrain should be looked into for it?

  1. Generally Copper-Gold mineralisation occurs in volcano-sedimentary packages of Archean-Proterozoic age as we get in Dharwar craton of Karnataka and in Aravalli craton of Rajasthan. In the case of copper mineralisation, surface manifestations in the form of Malachite – Azurite are generally seen in the outcrops as we get in Alwar Basin and Khetri basins of North Delhi Fold Belt of Rajasthan. Some evidences for ancient mining we do get in Rajasthan in the form of small old workings and some retorts indicating smelting. In the case of gold mineralisation, gossans are the best evidences on the surface if we are lucky to encounter in the field. If the rock which is being targeted for gold, shows presence of sulphides especially arsenopyrite, it is the good indication for mineralisation as it is considered as pathfinder for gold. Evidences for ancient mining activity in the form of old workings and small Mortar type depressions in the outcrop nearer to this OW will also help in locating the gold mineralised zones. Sampling of the dumps around such OWs and analysis by Fire assay will give an idea about the incidence of gold in that zone. Etymological evidences such as the names of places or mounds such as ” Bangaru gudda, (Bangaru means gold in Kannada), Sona pahar etc also give glues about the ancient gold mining activity in those areas. Another common feature one can observe in the areas where gold occurs is the panning activity by the local people in such areas during rainy season to collect gold grains from the stream sediments. A Geologist has to keep all these aspects in mind while carrying out investigation for copper or gold in new areas.

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    1. The copper mineralization can be related to magmatic concentration, contact metasomatism,hydrothermal or sedimentary.The suitable locales therefore have to be searched in such types of host rocks. Older schists belts, shear zones,conglomerates and breccia rocks etc are all potential targets where some surface indication of sulfide minerals are seen or ancient mine workings and even some local names of hills, rivers are mentioned e.g. Tama pahar (copper) or Suvarnrekha river (gold) in Singhbhum Shear Zone and around.Therefore all evidences have to be seen and correlated with geology,structure,known models of deposits and geochemical association of other minerals, pathfinder elements.All such evidences when converge together, a successful discovery can be made.As such there is no thumb rule and the saying that all that glitters is not gold, is very much true in exploration geology as well.

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  2. Badam Pahar Gorumahisani Belt (Archean Greenstone Belt) is a volcano-sedimentary sequence where search for gold has been intermittently carried out by GSI where panning activity was noticed since long. Some workers had even found small gold nuggets on the lateritic duricrust in the past. Scout drilling made in selected blocks could not help getting gold mineralization in the early 90s. Whereas Kunderkocha located in the NE of the same belt with similar set up is known to have a small gold deposit. What could be the surface indication in such areas, Dr.Nathan to comment pleaese.

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    1. I have confronted similar issue in Madagascar and Tanzania – finding nuggets in soil, stream sed and pits but no values in drilling. Both times it was coarse gold issue. As gold grain size increases the number of grains per unit volume reduces exponentially. Conventional drilling, therefore, becomes highly ineffective tool of exploration as in such cases the assay value becomes dependent of sample size. In such cases the typical 30g-50g pulverized assay is no longer “representative” of the drilled horizon. Well established method of dealing with coarse gold are:
      1. Screen fire assay of large sample volumes (not the regular 30g or 50g that are typically assayed) where the entire half core must be pulverized and tested with screen fire assay.
      2. Twin the DDH with large dia RC to collect large samples and pulverize large (several kg samples), split for FA/ICP and another part for screen fire assay. The tests must continue till ideal sample size is established
      3. Geochemical – Structural correlations can be established using 25 element assay of ALL samples in the area AND oriented drilling with structural measurement of ALL structures. These are to be used to define mineralized zones in terms of their chemistry and define the fertile structural trends.
      After doing a theoretical modelling, I applied this concept in other areas and the assay grades improved between 10% (for fine gold deposits) to over 100% (for coarse gold deposits) . In one deposit I was able to define an 800m long cylindrical high grade zones within an overall low grade envelope. I am available for any detailed discussion on this issue if required.

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  3. On behalf of Brij Bhushan Sharma
    Answer of q.1 Gossan, old workings, shear zone, slag dumps, presence of malachite and azurite staining for copper mineralisation and ancient artifacts as used by
    ancient miners for panning and crushing the metal are main signature on surface for hidden copper and gold mineralisation.

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