Evolution of the sulfide ores of Saladipura, Rajasthan, India

Abstract

At Saladipura stratiform sulfide ores occur in the Ajabgarh group of the middle Proterozoic Delhi Supergroup. Two coaxial buckle folds and another transverse one have deformed the rocks in the area, but no pronounced fault or shear zone is developed. The rocks evolved through a low-pressure intermediate facies series of metamorphism to a stage where the condition for the coexistence of the triple alumino-silicate polymorphs was closely approached, at about 600 degrees C and 5.5 kb. The origin of the amphibolitic rocks associated with the ores is indeterminate. They may represent a metamorphosed pile of volcanic and avolcanic rocks. Granite in the area, anatectic in origin, was emplaced at a late kinematic stage.Several orebodies occur in the area, of which the largest, defining a plunging synform (syncline), is traced out over a strike about 7 km long. The orebodies have been affected by all the deformation episodes. Different small structures in the ores are discussed. Pyrite and pyrrhotite are the dominant sulfides followed by sphalerite. Coexisting assemblages of ores and their metamorphic fabrics are critically analyzed. The wall rocks of the orebodies are composed of either cordierite + orthoamphibole(s) + quartz + or - scapolite, or muscovite + biotite + quartz + or - andalusite. The following features inter alia suggest the ores to be of sedimentary-diagenetic origin: very thin lamination in the ores, parallelism of the orebodies with the primary laminations in the associated (meta-) sediments, slump structures in ores, buckle-folding in the ore layers with the development of pyritic axial planar schistosity, virtual absence of replacement features between the ores and the silicate, and close association with carbonaceous matter. Sedimentation took place in a near-shore anoxic environment.The ores are metamorphosed. The invariant assemblage, pyrite + pyrrhotite + arsenopyrite (+ (As-S) l + V), and mole percent FeS in sphalerite coexisting with pyrrhotite suggest that the temperature and pressure of ore metamorphism were about 600 degrees C and 5.5 kb, respectively. These data agree well with those obtained from the metamorphosed host rocks. The growing idea that sulfur is internally controlled during metamorphism is upheld and it is concluded that, in ores rich in pyrite, a pyrite [rightharpoons] pyrrhotite (+ S 2 ) break-down should be negligible even in high-grade metamorphism. In the equilibration of Fe-silicates coexistent with sulfides, sulfidation was unimportant. Pegmatoid ores result from the circulation of volatiles emanated during granitic-pegmatitic activity.