S.Chattopadhyay 1, S. Mukhopadhay 2, S.K. Sengupta 1, T. Pal 1 1

Transcription

1 Primary Ore Mineral Assemblage And Fluid Inclusion Studies of The Cu-Ag Mineralisation In Baniwala - Ki - Dhani Area, Rajasthan, India - A Product of Magmatic -Hydrothermal Milieu S.Chattopadhyay 1, S. Mukhopadhay 2, S.K. Sengupta 1, T. Pal 1 1 Central Petrological Laboratories, Geological Survey of India, Kolkata, 2 Geological Survey of India, Jaipur. The study area exposes low grade, high tonnage copper silver mineralisation, hosted within quartz - biotiteamphibole schist with intercalatory marble bands enveloped within poorly mineralised metapelites, belonging to the carbonate metapelite assemblage of the Meso-Proterozoic Ajabgarh Group of Delhi Supergroup. Bornite chalcopyrite - chalcocite - subordinate covellite are the dominant copper bearing minerals and occur within the remobilized quartz ± carbonate veins /veinlets pervading the host rocks. Coarse disseminations of bornite ± chalcopyrite occur both within the scapolite bearing calc silicates of the carbonate suite and scapolite bearing spotted schists of the metapelitic suite. Native Silver, Native bismuth and Silver- bismuth bearing phases, in the remobilized quartz- carbonate veins/veinlets, also occur in association with the major copper bearing phases. Significant ore textures as a) myrmekitic intergrowths of bornite-chalcocite, b) chalcocite lamellae in bornite (Fig1), c) bornite showing streaks of chalcopyrite as exsolved phases (Fig 2), implies a magmatic- hydrothermal condition of formation, the myrmekite being a product of eutectic intergrowth (Ramdohr, 1969). Figure 1. Chalcocite lamettae in Bornite Figure 2. Bornite showing streaks of exsolved chalcopyrite. Development of coarse biotite books and aggregates of tourmaline (schorlitic) in the calc silicates in proximity to the mineralized zone, defines a wall rock alteration phenomena. The association of native bismuth, native silver and silver-bismuth bearing phases marks a temperature constraint around 300 C for the mineralizing fluid, the melting point of native bismuth being 264 C to 271 C (Smirnov et al, 1983).

2 Primary fluid inclusions are both L + V and L + V+ S types while the secondary inclusions are L+ V types. L: V ratio in the primary inclusions varies from 80: 20 to 85:15 and L: V: S varying from 70: 15:15, 80:10:10 indicating a low vapour phase content. Fluid Inclusion thermometric studies reveal a gas poor, moderately high salinity fluid responsible for the copper silver mineralisation in the area. The temperatures of final ice melting (T M ) which measures the salinity of the fluid suggested a salinity variation from 10.2 wt% NaCl equivalent to 32.5 wt% NaCl equivalent approximately. The temperatures of first ice melting (T fm ) indicates the dissolved salts in the liquid component of the fluid inclusions to be Na, K, Ca, Mg, Fe in varying combinations. The temperatures of homogenisation (T H ), which is also, the minimum temperature of entrapment of fluids shows a range from 132ºC to 375ºC with the major mode being around 250º to 300ºC indicating an epigenetic hydrothermal condition of formation for the ore- bearing fluid. The Ts NaCl data of the daughter crystals indicate that they are halite in composition. Homogenisation of the vapour bubble was largely into the liquid state i.e. L+V liquid, thus implying that the medium of transport of the ore bearing fluid was liquid. The interpretative plots of temperature-salinity data (Fig 3) indicate that with progressive evolution there was a change in salinity of the mineralizing fluid possibly due to mixing with cooler fluids of variable salinity (Shepherd et al, 1985). The approximate pressure of the Cu-Ag bearing fluid at the time of entrapment shows a range from 420 bars (0.42Kb) to 1700 bars (1.7 Kb). On the bases of ore mineral assemblages disseminated in the host rock (bornite+chalcopyrite) and in remobilized quartz+calcite veins (bornite + chalcopyrite with subordinate covellite, native Ag, Bi and other Ab-Bi bearing phases), ore texture and pressure-temperature regime of the ore fluid, mineralization is inferred to have resulted from a hydrothermal fluid that evolved from orthomagmatic to low-temperature stages. References Figure 3. Bivariate plot of Salinity vs Temperature of Homogenisation Ramdohr, P. 1969: The Ore Minerals and their Intergrowths, Pergamon Press, 1174P. Shepherd, T., Rankin, A.H., Alderton, D.H.M. 1985; A Practical Guide to Fluid Inclusion Studies. Blackie and Sons, Glasgow, 239 P. Smirnov, V.I., Ginzburg, A.I., Grigoriev, V.M., Yakovlev, G.F. 1983: Studies of Mineral Deposits, Mir Publishers, Moscow, 288 P.

3 Primary Ore Mineral Assemblage and Fluid Inclusion Studies of the Cu-Ag Mineralisation In Baniwala - Ki - Dhani Area, Rajasthan, India A Product of Magmatic Hydrothermal Milieu Smt. S. Chattopadhyay, S. Mukhopadhyay, S.K. Sengupta and T. Pal Geological Survey of India

4 GEOLOGICAL MAP OF RAJASTHAN SHOWING MINERAL DISTRIBUTION km JAISALMER 26 BARMER 25 Agucha Rajpura-Dariba JALOR BIKANER JODHPUR PALI GANGANAGAR NAGAUR Rajpura-Dariba belt HANUMANGARH CHURU BHILWARA Pur Banera belt UDAIPUR AJMER JHUNJUNU SIKAR Sawar belt CHITTORGARH ALWAR DAUSA TONK SAWAIMADHOPUR BUNDI KOTA 1 BARAN JHALAWAR Copper Lead-Zinc Gold BHARATPUR Khetri Kho-Dariba DHAULPUR Kayar Sawar Pur-Banera GEOLOGICAL MAP OF RAJASTHAN (Map outline from 2 million map of GSI) 1 Study area Quaternary Deccan Trap Mesozoic & Tertiary of Jaisalmer Marwar Supergroup Vindhyan Supergroup Delhi Supergroup Khetri belt Pur-Banera, Rajpura-Dariba, Sawar Jahazpur, Jasma-Bhinder belts Lunavada Group Rakhabdev ultramafics Aravalli Supergroup Hindoli Group Granites & Granulites in Sandmata 24 Pindwara-Watera Zawar DUNGARPUR 74 BANSWARA Bhukia Berach Granite (2.5 Ga) Sandmata Complex Untala & Gingla granites (2.9 Ga) Mangalwar Complex Digitised by: D.J.Das Gupta, Geologist (Sr), W.R., Jaipur

5 Tectonostratigraphy Neoproterozoic ( Ma) Ma Ma 900Ma Mesoproterozoic Ma Marwar Supergroup Malani Igneous suite Sindreth/Punagarh Group Sirohi Group Erinpura Granite Delhi Supergroup South Delhi Fold belt Ma Palaeoproterozoic Ma Ma NeoArchean Ma North Delhi Fold belt Hindoli Group Sandmata Complex Aravalli Supergroup Mangalwar Complex Ajabgarh Group Alwar Group Raialo Group

6 Geological Map of Baniwala-Ki-Dhani area Sikar Dist, Rajasthan (27 46' ; 76 55') A-B and C-D are geological traverse lines for petrological sampling

7 Lithologically the area exposes a carbonate-metapelitic assemblage belonging to Ajabgarh Group of Delhi Supergroup The lithological variants include A) Metapelites: 1) Qtz - bt - grt ± staurolite ± kyanite schist ; 2) Scapolite bearing quartz-biotite schist. 3) Amphibole - carbonate- quartz- biotite schist B) Carbonate rock suite: 1) Amphibole ± biotite marble; 2) Calc silicates grading to impure marble; 3) Calc-gneiss. C) Amphibolites: 1) Amphibole-quartz rock ± grt ± plag.

8 The litho-contacts are gradational and show interfingering within short distances probably indicating facies variation. Attitude of lithounits is NE-SW with steep dips on either side but generally towards NW. These rocks are pervaded by quartz ± carbonate and quartz kyanite veins The calc-silicate rocks at places in the vicinity/ proximity to the mineralized zone shows development of coarse biotite books and sub-idiomorphic aggregates of tourmaline, probably indicating wall rock alteration phenomenon.

9 Qtz - bt - grt ± staurolite ± kyanite schist. Scapolite bearing schist. Biotite shows preferred orientation and wrapping around scapolite. Metapelites

10 Actinolitic marble with sulphides Biotite-carbonate rock with segregated carbonate clusters. The matrix shows a fabric. Carbonate suite of rocks

11 Amphibole ± biotite marble. Amphibole showing blue-green pleochroism. Amphibole showing sieve texture in impure Marble Carbonate suite of rocks

12 Sc Scapolite (Me % = 38.41%) -bearing calc-silicate.

13 Tour Development of Tourmaline (Tour) and Biotite as Wall Rock Alteration

14 Mineralisation Copper mineralisation has been intersected in bore holes BBH-1 to BBH-9. The host rocks are quartz-biotite ± amphibole schist and amphibole marble. Bornite, chalcopyrite, chalcocite are the main Cu minerals and are concentrated along quartz and calcite veins intruded within the host rocks. Disseminations of bornite within amphibole marble. Other minor sulphides are galena and covellite.

15 Petromineragraphic studies reveal A dominant sulphide assemblage comprising in order of abundance: Bornite Chalcocite Chalcopyrite Covellite Galena. with a subordinate oxide assemblage of Hematite - Magnetite Ilmenite Native silver, silver+ bismuth and native bismuth occur with sulphides. The textural relationships are as follows:

16 1. Coarse, subidiomorphic bornite grains occur intimately associated with chalcocite, chalcopyrite and subordinate galena Chalcocite lamellae in bornite probably indicates an exsolution feature. Streaks and crude star like chalcopyrite in bornite. Bornite-chalcocite bearing veins and veinlets and filling of hair like fractures pervade the rock at places in an anastomosing manner indicating a remobilised phenomena. 5. Irregular occurrence of bornite-chalcocite along intergranular boundaries of silicate-carbonate gangue and irregular chalcocites at bornite peripheries indicate replacement features.

17 6. Chalcopyrite-bornite with mutual boundary contact probably indicating contemporaneity in formation. 7. Anastomosing veinlets of chalcopyrite traversing the gangue. 8. Excellent myrmekitic intergrowths in various shapes and sizes of bornite and chalcocite is characteristic.

18 Bo Cc Bornite (Bo) Chalcocite (Cc) association

19 Bornite(Bo)- Chalcopyrite(Cp) association

20 Bornite(Bo) - Chalcocite (Cc)- Galena (Ga) association.

21 Bo Cc Bornite showing lamellar Chalcocite

22 Bornite-Chalcocite veinlets

23 (Gangue) Chalcocite(Cc) Bornite(Bo) Bornite-Chalcocite anastomosing veinlets pervading the gangue

24 Chalcopyrite-Bornite with mutual boundary contact indicating contemporaneity in formation

25 Bornite showing exsolved chalcopyrite lamellae

26 Streaks of chalcopyrite in bornite

27 Anastomosing veinlets of chalcopyrite traversing the gangue.

28 Coarse bornite-chalcocite intergrowth (graphic type)

29 Coarse bornite-chalcocite intergrowth (graphic type)

30 Bornite Chalcocite(Cc) Myrmekitic (graphic type) intergrowth of bornite and chalcocite.

31 Bornite-Chalcocite myrmekitic (graphic type) intergrowth indicating simultaneous crystallisation

32 Exsolved blebs of hematite in magnetite

33 Hematite associated with silicates

34 Ag Bi bearing phase Ag = Bi = Native Silver Ag = Bi Ag phase in native silver (BSE Image) in Qtz calcite veinlet.

35 Native Bi Bi = Native bismuth associated with chalcocite-bornite (BSE image)

36 Graphic/myrmekitic texture) Native silver associated with bornite-chalcocite intergrowth (BSE image)

37 Native Silver(n= 3): wt% Native silver associated with chalcocite

38 Native silver with chalcocite (Cc) and bornite (Bo)

39 Fluid Inclusion studies Primary fluid inclusions are of types (1) L+V (2) L+V+S type, secondary fluid inclusions are normally L+V type. The L:V ratio in L+V type is from 80:20 to 80:15 approximately with the vapour bubbles varying in its approx. diameter from 2.34 to 5.78 µm. In the L+V+S type, the L: V: S ratio is 70:15:15 to 80:10:10 approx., the daughter crystals are absolutely euhedral, cubic shaped, with squarish to rectangular outlines and are approx sq. µm in its area.

40 Bi-phase F.I. in quartz host.

41 Polyphase F.I. in quartz host.

42 Polyphase F.I. in quartz host.

43 Bi and polyphase inclusions in qtz-carbonate host.

44 Carbonic F.I. in quartz host

45 The temperatures of final ice melting (T M ) which measures the salinity of the fluid suggested a salinity variation from 10.2 wt% NaCl equivalent to 32.5 wt% NaCl equivalent approximately. The temperatures of first ice melting (T fm ) indicates the dissolved salts in the liquid component of the fluid inclusions to be Na, K, Ca, Mg, Fe in varying combinations. The T m CO 2 recorded in some inclusions indicated the vapour phase to be CO 2 along with other gas/vapour phases also. 4. The temperatures of homogenisation (T H ), which is also, the minimum temperature of entrapment of fluids shows a range from 132ºC to 375ºC with the major mode around 250º to 300ºC. The T S NaCl data of the daughter crystals indicate that these are halite in composition. 5. Homogenisation of the vapour bubble was largely into the liquid state i.e. L+V liquid, thus implying that the medium of transport of the ore bearing fluid was liquid.

46 F r e q u e n c y C Temperature of 1homogenisation Histogram showing temperature of homogenisation of inclusions against frequency in Cu-Ag mineralisation in Baniwala - Ki-Dhani, Sikar district, Rajasthan Histogram represents an unimodal to a more or less uniform distribution pattern. Such a distribution pattern could represent a single population of inclusions trapped over a wide range of P-T conditions implying a protracted history of fluid entrapment

47 PVTX Histogram showing Salinity vs. Frequency Histogram showing frequency distribution in terms of salinity indicates a unimodal distribution pattern representing a low to a moderately high salinity for the mineralizing fluid entrapped.

48 Bivariate plot of Salinity vs Temperature of Homogenisation. In a bivariate plot of T H (temperature of homogenization) Vs. salinity, for the biphase aqueous L+V type inclusions, the data plots when compared with the schematic model depicting the theoretical evolution path of the parental fluid, is indicative of a combination of more than one trend, which may be attributed to a somewhat mixing with cooler fluids of variable salinity.

49 The bivariate plot of temperature of homogenization (T H ) Vs. pressure. Pressure is calculated from the intersection of the fluid isochores (i.e. the equal density lines). The intersection is assumed to be an equivalent lithostatic load. The approximate pressure of the mineralizing Cu-Ag bearing fluid at the time of entrapment has been calculated and shows a range from 420 bars (0.42Kb) to 1700 bars (i.e. 1.7 Kb).

50 INFERENCES The association of native bismuth, native silver and Ag-Bi bearing phases indicates a comparatively higher temperature of the mineralizing fluid around 300 C, the melting point of native bismuth being 264 C to 271 C (Smirnov, 1983). Fluid inclusion studies have indicated a gas poor, moderately high salinity fluid, responsible for the Cu-Ag mineralisation in the area. The temperature of homogenization (T H ), which is also the minimum temperature of entrapment of the fluid, shows a range from 130 C to 375 C with the major mode around 250 C to 300 C. The approximate pressure of the mineralizing Cu-Ag bearing fluid at the time of entrapment has been calculated and shows a range from 420 bars (0.42 Kb) to 1700 bars (1.7 Kb). T H & the approx. pressure indicates an epigenetic hydrothermal condition of formation of the ore bearing fluid.

51 The biphase L+V type of fluid inclusions which form the dominant mode along with subordinate L+V+S (daughter crystal bearing) type envisage that, with progressive evolution, there was a change in the salinity of the mineralizing fluid vis-a-vis, to a somewhat mixing with cooler fluids of variable salinity. The ore mineral assemblages observed in the present area of study, right from its association in the calc silicate-impure marble rock suite as disseminations, to the epigenetic hydrothermal stage of both high and low temperatures, indicate, its formation in the magmatic series, in a wide range of temperaturesfrom as early as orthomagmatic stage, to both high and low temperature hydrothermal veins, rich in bornite (Ramdohr 1969).

52 Thus, from the association of bornitechalcopyrite-chalcocite, Native bismuth, Native silver and Ag-Bi bearing phases, and their textural relationship, a low temperature hydrothermal condition of formation of the ore bearing fluid is suggested. The temperature of homogenization (T H ), which is also the minimum temperature of entrapment of the fluids and the calculated pressure at the time of entrapment of the fluid, also supports the above contention.

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