Abstract The Kayad Zn-Pb deposit, situated within the Proterozoic Aravalli-Delhi fold belt in western India, is primarily characterized by sphalerite and galena along with pyrrhotite and chalcopyrite. The mineralization occurs as disseminated ores in quartzite, disseminated/laminated and massive ores in quartz-mica schist, and in pegmatite and quartz veins. The laminated ores conform to the regional schistosity and folding, whereas the massive Zn-Pb ores postdate the pervasive tectonic fabric, accumulating at the fold hinges. The massive ore is characterized by durchbewegung texture, discrete blebs of galena and chalcopyrite in a sphalerite matrix with low interfacial angles, and discrete intergrowths of sulfides and sulfosalts such as pyrargyrite, gudmundite, Ag-tetrahedrite, and breithauptite. Geochemical analyses of sulfides reveal microinclusions of sulfosalts comprising Ag, Sb, Cu, Tl, and As, which are regarded as low-melting chalcophile elements (LMCEs). Hydrothermal alteration is insignificant in the laminated and massive ores but prominent around Fe-Cu ± Zn-Pb and Zn-Pb ± Fe-Cu veins. The alteration assemblages in these veins evince a pervasive K + Na ± Fe alteration, later overprinted by a subsidiary Ca ± Na alteration. We interpret the laminated/disseminated ores to be of syndiagenetic sedimentary-exhalative (SedEx) origin formed within an euxinic basin. Conversely, the textural features, mineralogical composition, lack of associated hydrothermal alterations, and evident structural influence on the emplacement of the massive ores suggest they have been remobilized both via plastic flow and by sulfide partial melting. Temperature estimates of up to 650°C, derived from Ti-in-biotite geothermometry of the metamorphosed host rocks, indicate lower-middle amphibolite facies conditions during regional metamorphism. The initiation of melting at these temperatures was promoted by the desulfurization of pyrite to pyrrhotite in quartz-mica schist, aided by melting point depression due to the presence of LMCEs like Ag, Sb, and As.
<p>The Kayad Zn-Pb deposit in Ajmer, Rajasthan is a Proterozoic SEDEX deposit located in the Aravalli-Delhi fold belt of western India. The ore mineralization comprising predominantly of sphalerite and galena and subordinate chalcopyrite and pyrrhotite occurs in quartz mica schist (QMS), calc-silicate, quartzite and pegmatite, of which QMS hosts the majority of it. Other minerals such as arsenopyrite, lollingite and sulfosalts such as pyrargyrite, gudmundite and breithauptite are commonly associated with the massive ores in QMS.</p><p>The mineralization occurs as dissemination in calcsilicate and quartzite, in veins intruding pegmatite or on the wall rock of pegmatite, and occurs as lamination and massive ores in QMS. The laminated ores conform to the schistosity whereas the coarse, massive ores disrupt and overprint the metamorphic fabric. The massive sphalerite and galena (&#177; chalcopyrite and pyrrhotite) ores are commonly associated with one or more of the hydrothermal minerals such as prehnite, Al-pumpellyite, albite and allanite replacing K-feldspar and plagioclase which indicates episode of Ca-Na metasomatism. On the other hand, pyrrhotite and chalcopyrite are mostly associated with chamosite, albite and potash-feldspar replacing other minerals in the host rock suggesting&#160; -Fe-Na-K metasomatism. Sphalerite, galena, and arsenopyrite have been analysed by the SHRIMP SI ion microprobe for &#948;<sup>34</sup>S while multiple sulfur isotope (<sup>32</sup>S, <sup>33</sup>S, <sup>34</sup>S, and <sup>36</sup>S) study has been attempted on pyrrhotite and chalcopyrite. &#948;<sup>34</sup>S of chalcopyrite (+6.4 to +8.8&#8240;), pyrrhotite (+6.1 to +11.3&#8240;) and arsenopyrite (+7.1 to +9.4&#8240;) are relatively compact and consistent while sphalerite shows a larger variation from +2.7&#8240; to +8.9&#8240; across host rocks. Galena, however, shows the highest &#948;<sup>34</sup>S values ranging from +7.8 to +24.3&#8240;. Such high variations for both sphalerite and galena can result partly from crystal orientation effect during analysis. Average &#8710;<sup>33</sup>S and &#8710;<sup>36</sup>S of pyrrhotite are -0.01&#177;0.06&#8240; (2 S.D.) and 0.03&#177;0.02&#8240; (2 S.D.) respectively that show no MIF-S signatures. However, in the case of chalcopyrite, a few &#8710;<sup>33</sup>S values deviate up to 0.33&#8240; from the mean of 0.11&#177;0.15 (2 S.D.)&#8240;.</p><p>Various microscale and mesoscale textures in massive sulfides, like attenuation of fold limbs of QMS and accumulation of sulfides at fold hinges, discrete blebs of galena and chalcopyrite in a matrix of sphalerite and extremely low dihedral angles among them, and prominent durchbewegung textures indicate the ores have been mobilized. Mineralogy (presence of sulfosalts) and geochemical analysis of the massive sulfides show enriched concentrations of low chalcophile elements like Ag, Sb, As, Bi, Se, Tl which indicate metamorphism-induced sulfide melting might have been an important process in migration of pre-existing ore. However, presence of hydrothermal alterations in close proximity with the mobilized massive ores suggests that fluid-mediated chemical mobilization also played a crucial role in such remobilization. Consistently high positive values of &#948;<sup>34</sup>S hint at a thermochemical reduction of seawater sulfate during SEDEX mineralization and recycling of the sulfur during remobilization that formed massive ores.</p>
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