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

Abstract

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 AuAg 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.

Fluid Inclusion Studies of Comb Quartz and Stibnite at the Hishikari Au–Ag Epithermal Deposit, Japan

“…The waning of the hydrothermal activity during stibnite formation within the Hishikari deposit suggests that the ascending hot magmatic–hydrothermal fluids may have been minor but important contributors to stibnite deposition as the source of stibnite–sulfur was likely to be dominantly magmatic based on the sulfur isotopic study by Shimizu ().…”

“…Quartz and stibnite samples for microthermometric studies were collected at five locations: Locs. 1, 2, 3, 4, and 5 in Figure , which were also the sampling locations for a sulfur isotopic study conducted by Shimizu (). One or two doubly polished thin sections of quartz and/or stibnite with a thickness of ~150 μm were prepared from the samples for each location.…”

“…). The photo was taken after Shimizu (). (b) Single prismatic (bladed) crystal of stibnite from Shosen No.…”

“…Sampling locations plotted on the vein map at the Hishikari deposit after Shimizu (). The vein distribution is projected to be 40 ML (40 m mine level = 40 m above sea level).…”

Genesis and Evolution of Hydrothermal Fluids in the Formation of the High-Grade Hishikari Gold Deposit: Carbon, Oxygen, and Sulfur Isotopic Evidence

“…Ishihara et al [40] measured δ 34 S values of three vein ores and six basement sedimentary rocks from the Hishikari deposit by the conventional method, and the average δ 34 S value is around +0.3‰. The δ 34 S value of stibnite from the Hishikari deposit is −0.2‰ [41], and the δ 34 S values of seven stibnite samples, which are from the latest stage of the Hishikari mineralization, are narrow-ranged (from −0.2‰ to +0.7‰) among several different veins [42]. The near-zero values of δ 34 S from the Hishikari hydrothermal system are consistent with that from magnetite series magma [43].…”