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Department of Earth Sciences, Indian Institute of Technology (Bombay), Powai, Mumbai 400,076, India
Corresponding author: e-mail,
pandalai{at}geos.iitb.ernet.in
The Proterozoic, carbonate-hosted, Zawarmala Pb-Zn deposit has stratiform, banded pyrite-sphalerite ore, as well as discordant, sphalerite-galena veins and rich massive galena ore shoots. The mechanisms of deposition of discordant vein and massive ores have been studied using speciation and solubility calculations based on ore fluid characteristics and physical parameters inferred from petrography, fluid inclusion studies, and leachate analysis.
Massive galena ore, which is the economic ore type, cuts
across the earlier, F2-related, vein-type ore and is distinct
from the vein-type ore in terms of mineralogy, texture, and fluid inclusion
characteristics. Fluid inclusions in quartz associated with the ores show
that fluids forming vein-type ore are low-salinity (4.3–14.7
wt % NaCl equiv) H2O-NaCl fluids. The trapping temperature of
vein-type ore fluid was estimated to be in the range of 395°
to 290°C at a
pressure of about 1,450 bars. Fluids associated with massive galena ore are
H2O-CO2-NaCl fluids of lower salinity (about 3–4
wt % NaCl equiv). The wide variation in XCO2 of fluid inclusions
that occur in close proximity to each other within the same sample of
massive ore suggests heterogeneous entrapment of the low-salinity H2O-CO2-NaCl
fluids. From the intersection of isochores of local, homogeneously entrapped
CO2 and aqueous biphase fluids of unmixed CO2-H2O-NaCl
fluid, trapping temperature fluid was estimated to be between 250°
and 150°C at
pressures of 2,000 to 750 bars. Despite the low salinity of the ore fluids,
speciation studies show that chloride complexes are dominant over most of
the temperature range of interest in both ore fluids. The only bisulfide
complex of importance is 
, which is dominant below 175°C
in fluids associated with massive galena ore.
Data from fluid inclusions and solubility calculations reveal that cooling is the important mechanism of vein-type ore deposition. In contrast, physicochemical data indicate that an increase in reduced sulfur, by mixing a regional, high metal content, low sulfur fluid with a local, high sulfur fluid at the site of deposition, was the mechanism for massive galena ore deposition. The difference in oxidation state (and therefore also in metal content) of the two fluids is inferred to be a significant factor that caused differences in the modes of deposition of the two ore types and the relatively high value of XCO2 (0.2033) in fluids associated with massive galena ore.
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