Sulfur isotope microanalysis of sphalerite by SIMS: constraints on the genesis of Mississippi valley-type mineralization, from the Mascot-Jefferson City district, East Tennessee

Abstract

The Mascot-Jefferson City (MJC) district is the most productive zinc district in East Tennessee. The deposits are of Mississippi Valley-type (MVT), hosted by carbonate rocks and dominated by sphalerite mineralization in strata-bound breccia bodies. We have utilized the high spatial resolution (20–30 μm) of the ion microprobe to obtain in situ sulfur isotopic analyses from discrete growth zones of sphalerite and analyses of associated pyrite. Two types of pyrite were noted: pre-sphalerite, diagenetic pyrite ( δ 34S of −16.1‰ and −20.0‰) and syn-sphalerite pyrite that is intergrown with sphalerite ( δ 34S of 31.3‰ to 33.7‰). Two textural varieties of sphalerite mineralization (banded and non-banded) were characterized. Banded sphalerite exhibits fine (μm to cm) banding that has grown around a carbonate substrate. Banded sphalerite has δ 34S values from 27.8‰ to 51.0‰, high Cd contents (up to 0.96 wt.%) and dark areas that are likely due to minute inclusions of organic carbon. The non-banded sphalerite has δ 34S values from 20.2‰ to 39.5‰, high Fe content and no organic inclusions. Regardless of the textural variety of sphalerite mineralization, our results show that the sulfur isotopic composition within a single polished thin section is heterogeneous and can vary by as much as 15‰. The δ 34S values recorded in this study are among the heaviest ever reported for MVT deposits. The microscale δ 34S variations and presence of high δ 34S values have been previously undocumented for East Tennessee. The data presented here suggest multiple sulfur sources and sulfide precipitation by fluid mixing. The most probable scenario involves significant sulfur input from a sulfate- and metal-bearing fluid of variable δ 34S composition mixing with a gas cap containing H 2S of relatively homogeneous δ 34S composition. The gas cap provided lesser amounts of sulfur to the system. Mixing of two isotopically different sulfur sources of variable proportions can account for the observed microscale variation in δ 34S.