THE MALANJKHAND COPPER (PLUS MOLYBDENUM) DEPOSIT, INDIA - MINERALIZATION FROM A LOW-TEMPERATURE ORE-FLUID OF GRANITOID AFFILIATION

Copper and subordinate molybdenum mineralization at Malanjkhand occurs within a fracture-controlled quartz-reef enclosed in a pink granitoid body surrounded by grey-granitoids constituting the regional matrix. Sulfide-bearing stringers, pegmatites with only quartz + microcline an d sulfide disseminations, all within the pink-granitoid, represent oth er minor modes of occurrences. Despite this diversity in mode of occur rence, the mineralogy of ores is quite consistent and conform to a com mon paragenetic sequence comprising an early 'ferrous' stage of precip itation of magnetite(I) and pyrite(I) and, the main-stage chalcopyrite mineralization with minor sphalerite, pyrite(II), magnetite(II), moly bdenite and hematite. Both stages witnessed continuous precipitation o f quartz + microcline + (chlorite, biotite and epidote). The enclosing pink-granitoid and the regional grey-granitoids display alteration fe atures such as saussuritization of plagioclase, breakdown of hornblend e and chloritization of biotite on a regional scale, indicating intera ction with a pervasive fluid. Quartz and microcline precipitation most ly restricted within the pink granitoid, postdates this alteration. Fo ur types of primary inclusions were encountered in quartz from ore sam ples: (1) type-I - aqueous-biphase(L + V) inclusions, the commonest va riety in all ore types; (2) type-II - aqueous-carbonic(L(aq) + L(carb) +/- V-carb); (3) type-III - pure-carbonic(L(carb) +/- V-carb) - type- II and III being restricted to stringer and pegmatitic ores, and (4) r are polyphase (L(aq) + V-aq + calcite/gypsum) inclusions. Quartz in gr anitoids contain primary type-I inclusions only. Type-I inclusions fro m ore samples furnish a temperature range (after a rough pressure corr ection to the T-H-maxima of 140-180 degrees C) of 150-275 degrees C an d a moderately low salinity of 4-12 wt.% NaCl equivalent. This is infe rred to represent the signature of the major component (F2) of the ore fluid. A few type-I inclusions of higher T-M (up to 380 degrees C) an d low salinity and density represent the other (F1) identifiable compo nent of the ore fluid present in low proportion. The T-H-maxima and th e total range in salinity of type-I inclusions in quartz from granitoi ds are strikingly similar to those from the ore samples. Composition o f syn-ore chlorites furnished a temperature range of 185-327 degrees C , which conforms to the fluid inclusion microthermometric data. Pressu re estimates using standard fluid inclusion geobarometric methods, var y from 550 to 1790 bar in the stringer ores. Observed temperature-sali nity/density relationships are best explained by a two-stage evolution model of the ore fluid: the first stage witnessed mixing of the two c omponents, F1 and F2 in unequal proportion, bringing about mineralizat ion. The second stage of evolution was marked by the separation of a c arbonic component on continued sulfide precipitation and attendant inc rease in salinity of the fluid. The F1 component emerged as a distinct , heated and (CO2 + S)-charged entity due to steam-heating and contami nation of the early-ingressed F2 fluid at the fracture zone. The perva sive fluid phase in the surrounding granitoids contributed the F2 comp onent.