Sulphur isotope microanalysis of sphalerite by SIMS : constraints on the genesis of Mississippi Valley-type mineralization from the Mascot-Jefferson City District, East Tennessee

Author

Jana L. Peevler

Date of Award

8-2001

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Kula C. Misra

Committee Members

Claudia I. Mora, Harry Y. McSween Jr.

Abstract

The Mascot-Jefferson City district is the most productive zinc district in East Tennessee. The deposits are of the 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 in sphalerite and analyses of associated pyrite. Two types of pyrite were noted: pre-sphalerite, diagenetic pyrite (δ³⁴S) = - 16.1 %o and -20.0 %o) and syn-sphalerite pyrite that is intergrown with sphalerite (δ³⁴S = 31.3%o to 33.7%o). Sulfur isotope geothermometry on one sphalerite-pyrite pair yielded a temperature of 149° ± 31°C, which is consistent with fluid inclusion estimates of 141° ± 54°C (average 139.2 °C). Two textural varieties of sphalerite mineralization (zoned and unzoned) were characterized. Zoned sphalerite exhibits fine (μm to cm) banding that has grown around a carbonate substrate. Zoned sphalerite has δ³⁴S values from 27.8%o to 51.0%o, high Cd contents (up to 0.96 wt. %) and dark areas that are likely due to minute inclusions of organic carbon. The unzoned sphalerite has δ³⁴S values from 20.2%o to 39.5%o, high Fe content and no organic inclusions. Regardless of the textural variety of sphalerite mineralization, our results show that the sulfur isotopic composition of sphalerite within a single polished thin section is heterogeneous and can vary by as much as 15 %o. The δ³⁴S values recorded in this study are among the heaviest ever reported for MVT sphalerite. The micro-scale δ³⁴S variations and presence of such high δ³⁴S values have not been previously documented for east Tennessee. The data presented here suggests multiple sulfur sources and complex precipitation mechanisms. Several possible mechanisms are examined. The most probable scenario involves significant sulfur input from a sulfate- and metal-bearing fluid of variable δ³⁴S composition mixing with a gas cap containing H₂S of relatively homogeneous δ³⁴S 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 δ³⁴S (sphalerite).

Recommended Citation

Peevler, Jana L., "Sulphur isotope microanalysis of sphalerite by SIMS : constraints on the genesis of Mississippi Valley-type mineralization from the Mascot-Jefferson City District, East Tennessee. " Master's Thesis, University of Tennessee, 2001.
https://trace.tennessee.edu/utk_gradthes/9707