Sulfur, oxygen, carbon, and strontium isotope geochemistry of the Sarrabus-Gerrei mining district, southeastern Sardinia, Italy

Abstract

The Sarrabus-Gerrei mining district consists of several hydrothermal ore deposits hosted within black shale, metasandstone, and metavolcanic ("porphyroid") and granitoid rocks of Paleozoic age. Ore consists of base metal sulfide minerals (chiefly galena and sphalerite) in a gangue of barite, fluorite, calcite, and quartz. Stable isotope compositions were determined for 69 base metal sulfide minerals, 33 barite, 23 calcite, and 3 quartz samples from 16 deposits throughout the district; some synsedimentary pyrite from the black shale also was analyzed. In addition, 87 Sr/ 86 Sr analyses of 14 barite, 6 calcite, and 11 different country-rock samples were performed.Barite displays delta 34 S = 14.1 to 21.9 per mil, delta 18 O = 9.3 to 18.8 per mil, and 87 Sr/ 86 Sr = 0.7103 to 0.7131. No significant difference in delta 34 S values exists among barite from the various host rocks, but barite from the black shale is depleted in 18 O with respect to barite from the sandstone, porphyroids, and granitoids. Barite from the granitoids has the highest (0.7128-0.7131) and the lowest (0.7103-0.7111) 87 Sr/ 86 Sr ratios; these latter values are close to the initial Sr isotope ratios of granitoids from the Sarrabus district. Moreover, barite always has a lower 87 Sr/ 86 Sr ratio than coexisting calcite, suggesting that the two minerals are not cogenetic. Sulfide minerals show delta 34 S values between -8.0 and +5.1 per mil. Galena and sphalerite from the Baccu Arrodas deposit within the black shale are appreciably depleted in 34 S compared to those from other shale-hosted deposits; galena and sphalerite from these latter deposits and all other deposits are quite uniform in 34 S. Sulfur isotope temperatures calculated for most barite-galena and barite-sphalerite pairs range from 247 degrees to 390 degrees C. At these temperatures, water in equilibrium with barite should have delta 18 O values in the following ranges: 6.2 to 7.9 per mil for the Baccu Arrodas deposit within the black shale, 9.7 to 16.0 per mil for deposits within the sandstone, and 10.5 to 17.9 per mil for deposits within the granitoid. Most calcite samples have delta 13 C values from -8.2 to -4.8 per mil, compatible with a magmatic origin of the carbon.The data are consistent with the following genetic model: the Baccu Arrodas ore deposit within the black shale formed from metamorphic fluids, probably related to the tectonism and metamorphism of the Hercynian orogeny about 365 m.y. ago. The main source of sulfur could have been pyrite present in the host rock. Barite in the other black shale-hosted deposits may have the same origin as at Baccu Arrodas, but the sulfide ore should have formed predominantly from magmatic sulfur at the time of emplacement of granitic plutons about 300 m.y. ago. The ore deposits within the other host rocks can be ascribed to this magmatic event. Barite and sulfide minerals were deposited from mostly magmatic water; sulfur was provided to the ore-forming fluids by granitic magmas, with a minor component from other wall rocks. Late-stage calcite and quartz samples may have crystallized from water with a meteoric component.