Sulfur isotopic ratios and mode of occurrence of stibnite at the Hishikari epithermal Au-Ag deposit, Japan

Abstract

Toru Shimizu (2017) Sulfur isotopic ratios and mode of occurrence of stibnite at the Hishikari epithermal Au-Ag deposit, Japan. Bull. Geol. Surv. Japan, vol. 68 (3), p. 111-117, 5 figs, 1 table. Abstract: Mode of occurrence and sulfur isotopic ratios (δ 34 S) of stibnite were investigated to understand the characteristics of stibnite mineralization, and the source of stibnite sulfur at the Hishikari epithermal Au-Ag deposit. Stibnite occurs as prismatic or acicular crystals forming radiating aggregates in quartz druses, indicating that stibnite precipitated during the last stage of sequential mineralization. Stibnite samples were collected from several veins at 40 and 10 ML (mine meter level) within the bonanza (high-grade Au) zone, located between 150 and-50 ML. Sulfur isotopic values (δ 34 S) of stibnite samples yielded a narrow range:-0.2 to +0.7‰ (n = 7), and are similar to previous δ 34 S data from ginguro (an Au-Ag-rich black sulfide band) in earlier mineralization (+0.4 to +0.6‰), and to δ 34 S data from pyrite in veins and hydrothermally-altered host rocks (Shimanto Supergroup sedimentary rocks)(-1.1 to +1.8‰). The presence of stibnite within the bonanza zone indicates that stibnite mineralization overprinted Au-Ag mineralization at the same depth. Considering that stibnite commonly forms in the peripheral portions of the epithermal environment, e.g., stibnite deposition on the top of an Au-Ag orebody near the surface, the overprinting of stibnite mineralization on Au-Ag mineralization at depth in the Hishikari deposit may reflect mineralogically exceptional circumstances. The narrow isotopic range of stibnite sulfur could be due to a spatially homogeneous source of stibnite sulfur. The isotopic similarity between stibnite and ginguro sulfur suggests that the source of sulfide sulfur may have been the same throughout both early and late vein formation. The similarity between sulfur isotopic values in stibnite and other sulfides (ginguro and pyrite) also suggests that the source of stibnite sulfur is likely to be dominantly magmatic, as with the source of other sulfide sulfur discussed in previous studies of this deposit.