Opinion – by Dr. V. Ganesan, fmr Director, GSI: on “Mineral System” based deposit classification for earth mapping resources

Relationship exists between mineral systems/ore deposits, principal commodities and critical minerals. Very good illustration have been made in clear cut pictures explaining the various tectonic domain wherein what type of deposits are formed. By repeated explanations with world / Indian  examples have to be given to the Geologists community by showing the models where one can look for the expected ore mineral deposits and what type of tectonic set up is favorable for their formation.

I have been requested to offer my Opinion on a recent report by the US Geological Survey suggesting a new method of classification of “mineral system” based deposit classification FOR EARTH MAPPING RESOURCES.

Yes, This new classification would provide better clarity of the complex science of mineral exploration and equip them with better understanding to be a more successful explorer.

Yes, the emphasis has to be given while teaching the students in line with System-Deposits-Commodities-Critical mineral table Open File Report 2020-1-42 for the Earth Mapping resources Initiative”.

The table has clearly indicated the relationship existing between mineral systems/ore deposits, principal commodities and critical minerals and the very good illustration have been made in clear cut pictures explaining the various tectonic domain wherein what type of deposits are formed. By repeated explanations with world / Indian examples have to be given to the Geologists community by showing the models where one can look for the expected ore mineral deposits and what type of tectonic set up is favorable for their formation.

Though they have given priority of Phases I (REE), II (Al, C, Co, Li, Nb, PGE, Sn, Ta, Ti and W), and III ( to locate As, BaSO4, Be, Bi, CaF2, Cr, Cs, Ga, Ge, He, Hf, In, KCl, Mg, Mn, Re, Rb, Sb, Sc, Sr, Te, U, V and Zr) for exploring the different set up of minerals, it can be taken up as a necessity based at this juncture.

Both Table-I and different schematic models are quite exhaustive, I suggest some of the following points for your kind observation.

Prior to the initiation of the above said view, that the budding geologists have to be given an valuable exposure to the geochemistry wherein the great affinity of certain elements with other elements have to be explained. So when we are looking for certain ore bearing minerals, one has to bear in mind that a mineral has the affinity for certain other minerals which commonly occur together in good quantity or in negligible amount. Sometimes those associated minerals may act the role of indicators to locate the minerals what we look for. Eg: Geochemical indicators for gold include: silver, copper, lead, zinc, cobalt, nickel, arsenic, antimony, tellurium, selenium and mercury. Similarly the compositions of PGM and micro inclusions in them, and the dominant associated mineral assemblages of silicates, oxides, sulphides and type of texture, morphology composition and alteration testify to several possible primary source indicators of PGE mineralization., Similarly for locating diamond-presence of omphacite pyroxene and pyrope garnet are path finders).

Hence it is very essential to deal with the various forms of mineral compound  viz.,– sulphides, oxides, carbonates, tellurides, etc., and the nature of formation has to be known in detail which will help in effectively narrow down the search area for many different types of ore deposits in mineral exploration.

Moreover, I draw your kind attention to the following:

The publication of Indian Bureau of mines FNO: 215(1)/UNFC/ME(I)2003 dt.3.6.2003 deals with the guidelines under MCDR for UNFC of mineral reserves/resources  (see rule 4591)© and form H-1 to H-8 and H-9), it is understood  that  the field guidelines which  are simply formatted  to adopt the UNFC are in seven categories;  ie all mineral/ ore formations or deposits  fall under  these seven categories which may deal with specific mineral assemblages exclusive to this  deposit nature:  they are

  1. Stratiform, strata bound, tabular deposit of regular habits :  Principal kinds of minerals involved are  coal seams, lignite beds, iron ore formations, manganese horizons, bauxite cappings, chromite in ultramafics, dolomite, limestone, baryts, gypsum, evaporites ( including postash and salt beds), chalk, fineclay and fullers, earth;
  2. Stratiform, strata bound, tabular deposit of irregular habits: Principal kinds of minerals  involved.. in addition to the above said minerals, gold in banded iron formation, platinum group of elements in chromite bearing rocks and molybdenum in shear- controlled  zones.
  3. Lenticular bodies of dimensions including bodies occurring in enechelon, Silicified linear zones composite veins: Principal kinds of minerals in this category are: Base metal sulphides, supergene iron and manganese bodies in lateritised bodies, bauxite-cobalt and nickel lateroids, auriferous quartz reefs, PGM in association with sulphides, graphite lenses, porphyry deposit of copper, molybdenum, tin, pyrite and pyrrhotite bodies.
  4. Lenses, veins and pockets; stock works, irregular shaped, modest to small size bodies: Multimetal sulphide bodies of Cu, Pb, Zn, Sb, Hg, Cr, Sn-Ag, tektite bodies, skarn bodies of sheelite, powellite, wollastonite, fluorite etc., semiprecious minerals,  veins of apatite, barite and asbestos veins, magnesite lenses, vermiculite, magnesite lenses, mica in pegmatite, pyrophyllite lenses and veins, high grade bauxite , clay pockets, ochre and bentonite clays and diamond pipes.
  5. Gem stones and rare metal pegmatites, reefs and veins: Tin-tungston-tantalum-molybdenum veins and pegmatite, beryl, topaz, emerald, cesium deposits, mineralization associated alkaline rocks, veins and plugs of carbonatite.
  6. Placer and residual mineral deposits of Hills and valley wash: Placer tin and gold deposits, monazite, garnet, ilmenite, rutile, diamonds in conglomerate, floats of corundum, kyanite, sillimanite and  magnetite etc.,
  7. Dimension stones: Colored varieties including granite, syenite, schists, gneisses, marbles, slate, sandstones, migmatites etc., black varieties of diorite, dolerite, gabbro etc, sills/ batholiths of anorthosite / gabbro etc.,

in which  the principal associated minerals/ores are grouped and systematically prospected  from Reconnaissance, Prospecting  to General and  further Detail stages of exploration like G4, G3, G2 and G1 categories by assigning the quantum of different surveys like aerial reconnaissance, geological, geochemical, geophysical, technological studies (including pitting , trenching, sampling, drilling, core sampling- chemical analysis), petrographic  study, minerographic study, geostatistical analysis, to delineate exact nature of the deposit reserve/ resource.

In such cases, while assigning the programme, the fixation of the target and execution of methodology have been formulated. This special knowledge has to be brought to the knowledge of the students/ professionals which help them to understand the clear cut objective of the projects and the methodology adopted to achieve the cent percent results on any assigned programme.

Moreover in the year 2011, in the Secretariat of Mission Head II, GSI, Nagpur where the undersigned worked, had implemented the practice and while the Geologists writing their technical report, they are advised to follow this UNFC classification wherein they will aware in which type of deposits and what stage of exploration of work they do and what exactly expected further for improvement and achievement of the objective.

This methodology without any iota of doubt, helps to bring out all the world resources without any lacunae in the scientific advancement of works to elevate the mineral deposit from G4 to G1 category with all economic feasibility and viability of exploration.

An attempt has been made to prepare an abstract of the Table-I of Hofstra and Kreiner (2020)

TABLE-I SHOWS THE ABSTRACT OF SYSTEMS-DEPOSITS-COMMODITIES-CRITICAL MINERALS TABLE FOR THE EARTH MAPPING RESOURCES INITIATIVE:

NoSystem nameGeological environment /set upMAIN CRITICAL MINERALS ASSOCIATED
1PLACER (RIVERINE,MARINE, RESIDUAL,ELUVIAL,ALLUVIAL, SHORLINE, PALAEO)DRAINAGE BASINS, SHORLINE,TIDAL AND WIND DRIVENAU, PGE, U,Sn ,W, FLUORITE,REE, Nb, Ta,Ti Mn ETC.,
2CHEMICAL WEATHERINGSTABLE AREAS OF LOW TO MODERATE RELIEF BY CHEMICAL LEACHING—BAUXITE, Ni-LATERITE-CARBONATE LATERITENi, Al, Co, REE, Ga, U,PGE, Mn
3METEORITIC RECHARGEOXIDISED DESCENDING WATER SCAVENGES  U,V,Sr,REE,Re
4LACUSTRINE EVAPORITECLOSED DRAINAGE BASINS WATER FROM METEORITE,SURFACE, GROUND, GEOTHERMAL RECHARGE WATER EVAPORATE AND PRODUCESSALT,GYPSUM,POTASH BORATE, NITRATE, Li ETC.,
5MARINE EVAPORITESEA WATER EVAPORATESALT, GYPSUM, POTASH ETC.,
6BASIN BRINE PATHMARINE EVAPORITES CONNECTED WITH LOWERGROUND AND LATERAL PERMEABLE STRATA—ORE FLUIDS SCAVENGING METALSCu, Pb,Zn,POTASH, Li,Hg,U,V, Ba,REE,Co, Sr,PGE, Ge,Ga,
7Marine chemocline rimLOCATION WHEREIN BASIN BRINES DISCHARGE INTO OCEAN—CHEMICAL DESIMENTATIONPHOSPHATE,V,Ni,PGE, REE,Co
8HYBRID MAGMATIC REE/ BASIN BRINE PATHHYBRID SYSTEM WITH CO2 AND HF BEARING MAGMATIC VOLATILES– REPLACE CARBONATES WITH FLUORITEFLUORITE, REE, Ti, Nb,Ba,Be ETC.,
9ARSENIDECONTINENTAL RIFT SYSTEMS-DEEP SEATED- ASCENDING OXIDISED-METAL RICH BRINES-PRECIPITATION OF NATIVE ELEMENTSAG,As,Co,Bi,Ni,U,Sb
10VOLCANOGENIC SEA FLOORVOLCANOGENEIC SEA FLOOR MAGMATIC ARC, BACK ARC BASINCu,Zn POLY METALLIC SULPHIDES, Ba, Mn ETC.
11OROGENICFLUID FLOW DURING EXHUMATION OF VOLCANIC SULPHIDES, CARBONACEOUS, CALCAREOUS SILICATESAu,Ag,MERCURY, GRAPHITE, Sb, W
12COEUR –DE- ALENE TYPEDEWATERING OF OXIDISED SEQUENCES DURING EXHUMATION / METASEDIMENTARY HOST ROCKS/POLYMETALLIC SULPHIDES& ANITMONY (Cu,Pb Zn,Sb,Co,Ge, Ga
13METAMORPHICRECRYSTALLISED ORGANIC CARBON/REE AND PHOSPHATE MINERALSGRAPHITE, REE, U,TH,Y
14PORPHYRY Cu-Mo-AuPOLYMATALLIC SULPHIDES, GREISSEN, SKARN DEPOSITSMo,W,Sn,PGE, Te,Sb,As
15ALKALIC PORPHYRYOCEANIC, CONTINENTAL MAGMATIC ARC,CONTINENTAL RIFT ZONES FLUIDS EXSOLVED FROM FRACTIONATED ALKALIC PLUTONMo,Bi,W, PGE,Au, Ag,Cu,V,F ETC.,
16PORPHYRY Sn (GRANITE RELATED)GRANITE RELATED S-TYPE PORPHYRY TYPE PERALUMIOUS PLUTONSENRICHED IN W,Au,Ag,Cu,Pb,Zn ETC.,,
17REDUCED INTRUSION RELATEDCONTINENTAL MAGMATIC ARCS—FLUIDS EXSOLVED FROM CALC ALKALINE PLUTONSENRICHED IN W,Au,Ag,Te,Bi, AND As
18CARLIN TYPECONTINENTAL MAGMATIC ARCSAu,Ag,Sb,As
19CLIMAX TYPECONTINENTAL RIFTS WITH HYDROUS BIMODAL MAGMATISMNb,Ta,Be  Mo, W,Au, ,Te,Bi, AND As, FLUORITE,,U,Sn
20IOA-IOCGSUBDUCTION AND RIFT RELATED PROVINCEU,REE, Co,Re, Mn,Bi,Te
21MAGMATIC REECONTINENTAL RIFT SYSTEM OR ALONG TRANSLITHOSPERIC STRUCTURE.REE IN MANTLE DERIVED ULTRTABASIC, ALKALINE, PERALKALINE INTRUSIONSCARBONATE, P, REE, Zr, Hf,Nb,Y,Ba, Zr, MAGNETITE,VERMICULITE
22MAFIC MAGMATICLARGE IGNEOUS PROVINCES RELATED TO MANTLE/ METEORITE AND LAYERED INTRUSIONSCr, Co, PGE, Ni, Cu, Ag, Au, Ti, V, REE
Modified after Hofstra and Kreiner (2020), USGS OFR 2020-1-42

The above, twenty type of systems falling under different environments of geological set up and the same thing was explained in different models by Hofstra and Kreiner (2020) it is quite exhaustive and clearly understandable.

The MCDR 7 stage of classification is a simplified form for the execution of  exploration work in systematic manner to achieve the target.

Hence it is an efficient approach globally to streamline the USGS earth mapping resources initiative to achieve the objective in a short span of time and no other type of deposits are left and the UNFC procedure may be adopted while exploring the mineral deposits.

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