The Malanjkhand copper ( + molybdenum) deposit, India: mineralization from a low-temperature ore-fluid of granitoid affiliation |
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| Authors: | M K Panigrahi A Mookherjee |
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| Institution: | (1) Department of Geological Sciences, Jadavpur University, Calcutta 700 032, India, IN |
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| 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(Laq + Lcarb ± Vcarb); (3) type-III – pure-carbonic(Lcarb ± Vcarb) – type-II and III being restricted to stringer and pegmatitic ores, and (4) rare polyphase (Laq + Vaq + 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.
Received: (10 August 1994), 15 August 1995 / Accepted: 12 January 1996 |
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