Tourmaline in Proterozoic Massive Sulfide Deposits from Rajasthan, India

We have analyzed the chemical composition and boron isotope composition of tourmaline from tourmalinites, granite and a quartz-tourmaline vein from the Deri ore zone and from a pegmatitic band in the Rampura-Agucha ore body. These two Proterozoic massive sulfide deposits occur in the Aravalli-Delhi orogenic belt, Rajasthan, northwest India. Tourmaline from stratiform tourmalinites closely associated with the massive sulfides in the Deri deposit have preserved their original chemical compositions despite regional and thermal metamorphism in the area. These tourmalines have low Fe/(Fe + Mg) ratios (0.19–0.30; mean 0.26) that suggest formation close to the sediment-sea water interface. The δ¹¹B values (−15.5 and −16.4‰) are compatible with boron derived from leaching of argillaceous sediments and/or felsic volcanics underlying the original massive sulfide deposit during its formation. Boron isotope compositions measured in tourmaline from a post-ore granite and quartz-tourmaline vein in the Deri deposit indicate that boron in these tourmalines was derived from the tourmalinites produced during ore formation. The boron isotope systematics of a coarse brown tourmaline crystal from a pegmatitic band on the hanging wall contact of the Rampura-Agucha deposit indicate that 45 ± 25% of the boron within the original tourmaline was lost during upper amphibolite facies regional metamorphism. Received: 3 April 1996 / Accepted: 11 April 1996