The southwest Guizhou Province, China, contains numerous sediment-hosted Au deposits with Au reserves greater than 700 tonnes. To date, the source of ore fluids that formed the Guizhou sediment-hosted Au deposits is controversial, hampering the formulation of genetic models. In this study, we selected the Shuiyindong and Jinfeng Au deposits, the largest strata-bound and fault-controlled deposits in Guizhou, respectively, for detailed research on pyrite chemistry and S isotope composition using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and laser ablation-multicollector-inductively coupled plasma-mass spectrometry ( LA- MC-ICP-MS), respectively.

Petrography and pyrite chemistry studies distinguished five generations of pyrite. Among these, pre-ore pyrite 2 and ore pyrite are the most abundant types in the deposits. Pre-ore pyrite 2 is anhedral to euhedral and with similar to 2,639 ppm As and wider ranges of Cu, Sb, and Pb (<similar to 22-4,837 ppm, <similar to 6 to 532 ppm, and <similar to 4 to 1,344 ppm, respectively). Gold in pre-ore pyrite 2 is below the detection limit of LA-ICP-MS (similar to 2 ppm). Pre-ore pyrite 2 is interpreted to have a sedimentary (syngenetic or diagenetic) origin. Ore pyrite commonly rims the four identified pre-ore pyrites or occurs as individual, anhedral to euhedral crystals. Ore pyrite is enriched in Au (similar to 641 ppm), As (similar to 9,147 ppm), Cu (similar to 1,043 ppm), Sb (similar to 188 ppm), Hg (similar to 43 ppm), and Tl (similar to 22 ppm) in both deposits. Ore pyrite formed mainly by sulfidation of Fe in Fe-bearing host rocks, mainly Fe dolomite, and As, Cu, Sb, Hg, and Tl, also in ore fluids, were incorporated into ore pyrite.

In situ delta S-34 isotope ratios for pre-ore pyrite 2 and ore pyrite were measured by LA-MC-ICP-MS. Pre-ore pyrite 2 from Shuiyindong and Jinfeng deposits resulted in delta S-34 values ranging from -0.8 to +3.4%o and from 5.1 to 10.5 parts per thousand, respectively. Analyses of ore pyrite from the Shuiyindong have delta S-34 values that vary from -3.3 to +2.5 parts per thousand, with a median of 0.7 parts per thousand; analyses of ore pyrite from the Jinfeng range from 8.9 to 11.2 parts per thousand, with a median at 10.3 parts per thousand. Available bulk and in situ delta S-34 data in the literature for pre-ore pyrites 1 and 2 and ore-related sulfide minerals including ore pyrite, arsenopyrite, and late ore-stage stibnite, realgar, orpiment, and cinnabar from several Guizhou sediment-hosted Au deposits were compiled for comparison. Pre-ore-stage pyrites from Guizhou sediment-hosted Au deposits have a broad range of delta S-34 values, from -33.8 to +17.99 parts per thousand (including in situ and available bulk delta S-34 data). Ore-related sulfide minerals in all Guizhou sediment-hosted Au deposits, except Jinfeng, have very similar delta S-34 values, and most data plot between similar to-5 and +5 parts per thousand. In the Jinfeng deposit, the ore-related sulfide minerals exhibit delta S-34 values ranging from 1.9 to 18.1, with most data plotting between 6 and 12 parts per thousand.

The broad range of S isotope compositions for the sedimentary pyrites (pre-ore pyrites 1 and 2) indicate that S in these pre-ore pyrites was most likely generated by bacterial reduction from marine sulfate. The narrow range of delta S-34 values (similar to-5-+5 parts per thousand) for ore-related sulfide minerals in all Guizhou sediment-hosted Au deposits, excepting the Jinfeng deposit, suggests that the deposits may have formed in response to a single widespread metallogenic event. As the S isotope fractionation between hydrothermal fluids and sulfide minerals in a sulfide-dominated system is small (<2 parts per thousand) at similar to 250 degrees C, the initial ore fluids that formed the Guizhou sediment-hosted Au deposits would have had delta S-34 values similar to the ore-related sulfide minerals, between similar to-5 and +5 parts per thousand. At Jinfeng, initial ore fluids may have mixed with local fluids with heavier delta S-34, possibly basin brine (delta S-34(basin brine) >18 parts per thousand), resulting in elevated delta S-34 values of ore-related sulfide minerals and especially late ore-stage sulfide minerals.

Although few igneous rocks are exposed in the mining area around these deposits, there is evidence of magmatic activity similar to 20 km away. Furthermore, gravity and magnetic geophysical investigations indicate the presence of a pluton similar to 5 km below the surface of the Shuiyindong district. Based on in situ S isotope results and recent data indicating proximal intrusions, we interpret a deep magmatic S source for the ore fluids that formed the Guizhou sediment-hosted Au deposits. However, as the age for Au mineralization of Guizhou sediment-hosted Au deposits is still debated, the mineralization-magma connection remains hypothetical. Identifying an ore fluid source and time frame for Guizhou Au mineralization continues to be a critically important research goal for this district.