Abstract

Massive, stratiform, zinc-lead-copper ores occur at Ambaji and Deri, located 8 km apart, in Gujarat and Rajasthan, respectively. The ores are concentrated in lensoid bodies of metamorphosed magnesian and calc-magnesian rocks, such as cordierite-anthophyllite-chlorite rocks, quartz-chlorite-tremolite schists, and diopside-forsterite marble, enclosed within argillaceous and arenaceous metasediments or orthoamphibolites. The rocks belong to the lower part of the Ajabgarh series of Precambrian Delhi system. The host rocks and ores were subjected to an earlier low-grade regional metamorphism under greenschist facies conditions and a superimposed hornblende hornfels facies thermal metamorphism, apparently related to the intrusion of granite and alkali syenite plutons at Ambaji and Deri, respectively.The P-T conditions of the last metamorphic episode have been estimated by comparing the mineral assemblages having a hornfelsic fabric with experimentally determined mineral reactions. Based on the stability relations of Mg-Al chlorites, anthophyllite, magnesian cordierite, and the assemblage diopside-tremolite-calcite-quartz, it is inferred that thermal metamorphism took place in the temperature range of 525 degrees to 625 degrees C at 2 to 4 kb pressure, with a mean condition of 575 degrees C/3 kb. The average mole percent FeS in sphalerite from the buffered assemblage sphalerite-pyrrhotite-pyrite, in well-annealed Deri ores, is 16.3 + or - 0.2, which for the temperature range mentioned corresponds to a pressure range of 3.8 to 3.1 kb, according to Scott's (1973) geobarometer.The fugacities of O 2 and S 2 are determined by the mineral assemblage and vary considerably from rock to rock. A log f (sub O 2 ) -- log f (sub S 2 ) diagram at the mean P-T of thermal metamorphism (3.45 kb and 575 degrees C) has been prepared and contoured with the sphalerite composition in chlorite-anthophyllite rock from the Ambaji-Deri area.The compositions of coexisting sulfides and silicates from the magnesian assemblages were studied with the electron microprobe. The iron sulfide coexisting with Mg chlorite, anthophyllite, and Zn-rich pleonaste is pyrite and the Mg enrichment appears to be due to the original bulk composition of these rocks rather than the iron extracted during metamorphism.From a number of geological features it is inferred that the host rocks of sulfide ores were formed essentially by isochemical metamorphism of sedimentary calc-magnesian and magnesian concentration accumulated in a transitional shelf environment. The metals were derived from basalt, which was subsequently metamorphosed to amphibolite, and were concentrated syngenetically and/or diagenetically with the magnesian sediments. A model of consanguinous origin is presented which invokes the derivation of the metals, the concentration of the magnesia, the nonbacteriogenic reduction of seawater sulfate, and the concentration of stratiform sulfides by sea-water-basalt interactions under near-shore peritidal conditions.

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