GEOCHEMISTRY AND GENESIS OF THE COPPER SULPHIDE DEPOSIT OF MALANJKHAND, BALAGHAT DISTRICT (M.P).

Abstract

A copper sulphide-bearing quartz reef (length : 2.6 km, average width ; 60 m; strike N355°, dip 60 a) intrud-s into Pr^cambrian granodiorite suite of rocks in Malanjkhand, Central India (21° 01' 30" Nand 22° 01' 18" H, • 80° 40» E and 80° 42' E). A pole plots of the joint plans show that -Lhe mineralised quartz reef follows a modal joint plant trending I\E W, which was the possibl avenu- for the or-r-b: aring fluid. Reference to the Ca0-ite20-X20 ternary plot (following Condie and Hunter, 1976) of the basement rocks of this arra, suggests that they can he classified into (1) tonalitc, (2) granodioiite, (3) quartz monzonite and (4) granite. 'She modal analysis of the rocks also confirm this geoch-=mical classification. Plot of such elements as Si, Al, Fe, Ti, Mg, Ca, Na, K, V, Li, Ni, Co, Sr and Ba with respect to the differen tiation index (1/3 Si + K) - (Ca + Mg) shows the usual trend of crystallization as expected in acidic plutonic rocks. The presence of (1) epidote +green biotite +chlorite + muscovite at Pauni, Anditola, Bhandarpur and Chartola area, and (2) the development of staurolite and amphibole with other phases in Bhimjori, Tingipur and north of North Hill, shows that the P-T condition of metamorphism of the basement

rocks ranged from quartz-muscovite-chlorite-albite-epidote subfacies (greenschist fancies) to staurolite-almandine subfacies (almandine-amphibolite facias). The studies on the stabilities of staurolite with respect to variable P-T-f02 conditions suggest that the oxygen fugacity of the basement rocks during metamorphism was above the IMQ buffer , but lying somewhere between WNO and HM buffers. It should be mentioned here that the Cu concentration is low in the basement rocks except at the vicinity of the quartz reef_,where : it is high due to metasomatic effect. Absence of zoning in Malanjkhand granite-granodioritic rocks, low concentration of Cu in comparison to a porphyry copper deposit and copper mineralization in stable cratonic environ ment goes against the view that it is a porphyry deposit as suggested by Tripathi (1979). Major and minor element geochemistry of the amphibolites show that chemically they are similar to tholeiites and they are devoid of copper mineralization. The study of the ore assemblage under reflected light shows that, of the primary minerals chalcopyrite, pyrite, sphalerite and magnetite are essential, while molybdenite and coballite occur as accessories. The secondary minerals are covellite, bornite, chalcocite, hematite, cuprite, (iv) malachite and azurite, Based on textural study the following paragenetic sequence is drawn : Chalcopyrite •—• : Pyrite ——— Magnetite I Sphalerite — ___—_——_——— Molybdenite Cobalt.ite Magnetite II It is interesting to note that the chalcopyrite shows deformation and transformation twinning. Transformation twins indicate that it crystallized above 547 + 5 C, in cubic system and later inverted to tetragonal symmetry which is also substantiated by X-ray study. Further, X-ray study of chalcopyrite and pyrite show that there is no change in th-'-ir cell volumes with depth. Ore chemistry shows that the average composition of chalcopyrite is Cu0#g8 ^0t956 32.061 and the 0a/Pe rati° ±S 0.914 - 1.087 , while average composition of pyrite is Pe0.917 V

Trace element partitioning of 9 elements among the coexisting chalcopyrite and pyrite was plotted in Roozeboom diagrams. On the basis of thermodynamic treatment it is shown that the plots of these elements should be linear or curvilinear. It is established that equilibrium condition was attained during the crystallization of the pyritechalcopyrite pair. Further, ratios of these elements (RT y~ ^y) wore plotted in relation to the concentration of the same element in either pyrite or chalcopyJftte, it was found that these curves (are linear or curvilinear) follow Mclntire equation, indicating thereby the presence of 'induced vacancies' in the pyrite and chalcopyrite structure. Eh-pH condition of the oxidized zone, and the fg_ and fQ and temperature of crystallization of the primary mineralized zone was also established. It is shown that cuprite and malachite might have been formed under an Eh of + 0.25 to - 0.08 and pH of 1 to 12.8.Possible explanation of replacement of chalcopyrite by chalcocite, covellite and hematite and replacement of pyrite by hematite in the intermediate zone has been attanpted. Using thermodynamic equation univariant curves were drawn. It is suggested that the crystallization of the primary ores of chalcopyrite-pyrite-magnetite assemblage was restricted to following T, fn , fg ranges : 580°-820°K, u2 2 (vi) -9 to -10 and -3 to -4 (log fs ) and -28 to -30 and -18 to -21 2 (log fo ) respectively. Analysis of bulk materials of the sulphide (3 samples) taken from different depths were plotted in the Cu-Fe-S join, on different isothermal sections (700°, 400° and 300°C) to learn about the cooling history. The plot shows that chalcopyrite solid solution should crystallize first from such a composition which will be in equilibrium with sulphur-rich liquid. But pyrite would coprecipitate with chalcopyrite as the temperature drops. It is suggested that the ore-bearing fluid along with * silica-rich material was produced at depth in the lower crust. • Fluid carrying Cu, Fe etc., as different types of chloride and sulphide species at greater depths of the crust moved upward along fracture systems in the basement rocks. With the fall in temperatur-:.- the solubility of these species were reduced and resulted in precipitation of the ore minerals near the surface.