The Malanjkhand copper (+ molybdenum) deposit, India: mineralization from a low-temperature ore-fluid of granitoid affiliation

Abstract

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 and sulfide disseminations, all within the pink-granitoid, represent other minor modes of occurrences. Despite this diversity in mode of occurrence, the mineralogy of ores is quite consistent and conform to a common paragenetic sequence comprising an early 'ferrous' stage of precipitation of magnetite(I) and pyrite(I) and, the main-stage chalcopyrite mineralization with minor sphalerite, pyrite(II), magnetite(II), molybdenite and hematite. Both stages witnessed continuous precipitation of quartz ±microcline ±(chlorite, biotite and epidote). The enclosing pink-granitoid and the regional grey-granitoids display alteration features such as saussuritization of plagioclase, breakdown of hornblende and chloritization of biotite on a regional scale, indicating interaction with a pervasive fluid. Quartz and microcline precipitation mostly restricted within the pink granitoid, postdates this alteration. Four types of primary inclusions were encountered in quartz from ore samples: (1) type-I - aqueous-biphase(L+V) inclusions, the commonest variety in all ore types; (2) type-II - aqueous-carbonic(L_aq+L_carb±V_carb); (3) type-III - pure-carbonic(Lcarb±Vcarb) - type-II and III being restricted to stringer and pegmatitic ores, and (4) rare polyphase (L_aq+V_aq+calcite/gypsum) inclusions. Quartz in granitoids contain primary type-I inclusions only. Type-I inclusions from ore samples furnish a temperature range (after a rough pressure correction to the T _H -maxima of 140-180 °C) of 150-275 °C and a moderately low salinity of 4-12 wt.% NaCl equivalent. This is inferred 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 °C) and low salinity and density represent the other (F1) identifiable component of the ore fluid present in low proportion. The T _H -maxima and the total range in salinity of type-I inclusions in quartz from granitoids are strikingly similar to those from the ore samples. Composition of syn-ore chlorites furnished a temperature range of 185-327 °C, which conforms to the fluid inclusion microthermometric data. Pressure estimates using standard fluid inclusion geobarometric methods, vary from 550 to 1790 bar in the stringer ores. Observed temperature-salinity/density relationships are best explained by a two-stage evolution model of the ore fluid: the first stage witnessed mixing of the two components, F1 and F2 in unequal proportion, bringing about mineralization. The second stage of evolution was marked by the separation of a carbonic component on continued sulfide precipitation and attendant increase in salinity of the fluid. The F1 component emerged as a distinct, heated and (CO₂+S)-charged entity due to steam-heating and contamination of the early-ingressed F2 fluid at the fracture zone. The pervasive fluid phase in the surrounding granitoids contributed the F2 component.