Abstract Most gold in sediment-hosted micron gold (SHMG) deposits cannot be imaged by conventional reflected light, secondary electron (SEM), or transmitted electron (TEM) microscopy. Gold particles with a diameter of about 200 Å were observed by Bakken et al. (1989) as inclusions in pyrite, cinnabar, quartz and illite from the Carlin mine, although considerable gold could not be accounted for. Arehart et al. (1993) showed that much of this “invisible” gold at Post/Betze and Gold Quarry is in As-rich growth zones in pyrite, possibly in solid solution. At Twin Creeks, we have observed mat the gold content of ores appears to vary with the texture of pyrite. We report here results of a preliminary study on the relation between gold values and pyrite textures at the Twin Creeks SHMG deposit, based in part on ion probe measurements of gold concentrations in pyrite and X-ray Absorption Near-Edge Spectroscopy (XANES) measurements to determine the oxidation state of gold in the pyrite. SHMG mineralization in the Megapit (see Fig. 1 of Hallet al., this volume) area of the Twin Creeks deposit, where our work was concentrated, is hosted by calcareous black shales and interlayered volcanic rocks of the Ordovician Comus Formation, which form a large fold. Gold mineralization is concentrated in the nose of this fold and in favorable stratigraphic horizons in the limbs of the fold. Gold is associated with arsenic-bearing pyrite (arsenian pyrite) that was deposited along with various combinations of quartz, adularia, sericite, realgar, orpiment, and stibnite. The Megapit deposit is

Abstract Despite important advances in our understanding of sediment-hosted micron gold deposits, debate continues over the processes that deposited their gold. Possible processes include cooling or boiling of the ore fluid, dilution, oxidation or other changes caused by mixing two fluids, or wallrock reactions such as carbonate dissolution and sulfidation. Several recent studies have called attention to the role of sulfidation in deposition of SHMG ores. At Jerritt Canyon, support for fluid mixing and sulfidation was provided by the relation between δ 18 O values and gold grades in ore-stage jasperoids (Northrop et al, 1987) and chemical reaction-path modeling of ore and alteration mineralogy (Hofstra et al, 1991). At Carlin, studies of fluid inclusions by Kuehn and Rose (1995) suggested that mineralization accompanied by sulfidation took place when an acidic, CO 2 -rich, mineralizing fluid mixed with cooler, gas-poor meteoric(?) water as it underwent episodic leakage from a deep overpressured reservoir. The best way to assess the role of sulfidation in SHMG deposits is through comparison of the abundance and distribution of elements at the scale of a deposit. We report here the results of comparisons of this type based on a database comprised of 20-foot composite samples from hundreds of drillholes at Twin Creeks, all of which were analyzed for Au, Fe, S (total and reduced) and other elements of interest and labeled according to rock type and degree of oxidation. From the standpoint of chemical compositions, one of the most important differences among the depositional mechanisms listed above is the degree to which

Abstract This paper will explore the significance of thermochemical sulfate reduction (TSR) to the origin of sedimenthosted disseminated gold (SHDG) deposits. TSR provides a means of generating abnormally high concentrations of H2S at moderate temperatures (e.g., 100 to 200°C). A high H2S content is a prerequisite for mobilizing gold and other metals associated with SHDG deposits (Ag, Hg, Sb, As), and is also necessary to form ore bodies with a high Au/Ag ratio. Field and sulfur-isotopic evidence supporting the TSR hypothesis will be reviewed. The discussion will focus on SHDG deposits in the Carlin Trend and similar deposits in China, as well as the Hemlo gold deposit, Ontario, a possible Archean analog of this deposit type. If valid, the TSR hypothesis has significance to the minerals exploration industry as a means to discriminate between favorable vs. unfavorable source rocks. Mineralization at Twin Creeks mine is hosted by rocks of the Ordovician Comus Formation, which consists of dolomitic to calcareous black shales with tuffaceous interbeds and interlayered basalt flows and sills. These rocks have been deformed into a northwest-trending fold system with a known length of about 4.5 km (3 miles). Gold mineralization post -dates this folding, and is concentrated in the nose of this fold and in favorable stratigraphic horizons in the limbs of the fold. In the most intensely mineralized part of the deposit, the Megapit, the fold is cut into three sections by right-lateral the TC (northern) and DZ (southern) faults trending NE-SW (Fig. 1) and dipping steeply to

THE NEWMONT MINING Corporation Twin Creeks mine is located 45 mi northeast of the city of Winnemucca, Nevada in Humboldt County. Access to the property is east from Win-nemucca, along Interstate 80, 16 mi to Golconda then northeast out of Golconda along State Route 789 for 17 mi. At the Midas-Getchell intersection follow the well-maintained dirt road for 12 mi past the Pinson and Getchell mines to the Twin Creeks property (Fig. 1 ). The Twin Creeks mine is the third largest primary gold-producing mine in North America and includes the Mega, Vista and West pit disseminated Carlin-type gold deposits. In 1999, the mine produced 760,574 oz from an oxide and sulfide milling and oxide leaching operation. Twin Creeks currently employs approximately 650 employees. Fig. 1. Northern Nevada Mine Location. Oxide ores at Twin Creeks are processed at a rate of 2,800 tpd through the Juniper Mill. The leach-grade material is processed through three leach pads. Sulfide ores at Twin Creeks are processed through the $250 million Sage Mill, which incorporates two of the largest autoclaves used in the gold industry. Start-up of the first 4,000 tpd autoclave occurred in May of 1997; the second autoclave, of the same size, began operation in November of 1997. The sulfide project enables Twin Creeks to process refractory ores that contain approximately 4.05 Moz of gold, about 59% of the total Twin Creeks’ reserves. Current reserves at Twin Creeks are 87,112,000 tons at an average grade of 0.079 oz/st and contains 6,857,000 oz of gold.