Greg B. Arehart

Characteristics and origin of sediment-hosted disseminated gold deposits: a review

Abstract Sediment-hosted disseminated gold (SHDG) deposits comprise a major portion of the gold production and reserves in the US. Although presently known to be common only in western North America, SHDG deposits are a significant source of world gold production. These deposits are characterized by extremely fine-grained disseminated gold, hosted primarily by arsenian pyrite. Other metals show very little enrichment although in addition to As, anomalous concentrations of elements such as Sb, Hg, Tl and Ba are utilized as exploration tools. The host rocks are dominantly silty carbonates, but ore concentrations are also present in siliceous and silicified rocks as well as intrusive rocks. Alteration consists of decarbonatization, silicification (jasperoid formation) and argillization, which are arranged both spatially and temporally in that order. Argillic alteration is zoned from kaolinite-dominated cores to sericite-dominated margins. The deposits commonly exhibit significant structural (faults) and stratigraphic (composition/permeability) controls. Until the last few years, SHDG deposits were considered as near-surface, epithermal type deposits in origin. Because of their fine-grained nature and the lack of macroscopic features such as veins, it has proven quite difficult to extract geochemical data that are clearly related to their genesis. However, fluid inclusion data indicate pressures corresponding to depths of 2–4 km under lithostatic conditions. Temperatures are constrained by fluid inclusions and phase equilibria to near 225°C. Stable isotope data from alteration minerals and fluid inclusions indicate that the ore fluids were dominated by meteoric waters, some of which had clearly exchanged oxygen with wallrocks during their passage through the crust. Although the data vary, most ore fluids probably had δD values near −150‰ and δ 18 O values ranging from −10 to +5‰. Sulfur isotope values reported from SHDG deposits span a wide range, from −30 to +20‰ (sulfides) and 0 to >45‰ (sulfates). Ore-related sulfides (pyrite, realgar) fall at the upper end of the range reported for sulfides. The alteration and mineral assemblage indicate the ore fluids were probably near neutral and gold was likely carried as a bisulfide complex. The depositional mechanism(s) probably included mixing, cooling and oxidation. These mechanisms are consistent with the observed alteration features, i.e. quartz precipitation, calcite dissolution and sericite-kaolinite coexistence. It also explains the presence of both siliceous ores containing native Au and sulfide ores containing Au in pyrite. The extreme variations in sulfur isotopes as seen at Post and fluid inclusion data from Carlin may be indicative of some phase separation (‘boiling’), but such relations have not been documented in other deposits and the importance of phase separation to gold deposition appears minimal.

40Ar39Ar dating of very fine-grained samples: An encapsulated-vial procedure to overcome the problem of 39Ar recoil loss

40Ar39Ar dating of very fine-grained materials is compromised by the loss of 39Ar by recoil, and also sometimes other Ar isotopes, during irradiation. An encapsulated sample 40Ar39Ar procedure is described which overcomes this problem and which produces a K-Ar equivalent date. Measurements indicate that the procedure yields reliable and reproducible results. It offers the potential for dating small amounts of material of many types, for example very fine-grained authigenic sheet silicates from rock cores, which may suffer Ar loss during irradiation.

Exceptionally fast growth rate of <100-yr-old tufa, Big Soda Lake, Nevada: Implications for using tufa as a paleoclimate proxy

Large tufa mounds (>3 m tall, with a basal circumference of 5 m) have been discovered on the margin of Big Soda Lake, Nevada, USA. These tufa mounds are rooted at a maximum of 4 m below the current lake surface and are actively forming from groundwater seepage, which can be seen emanating from the top of the tufa mounds. Big Soda Lake is a volcanic crater lake whose water level is maintained exclusively by groundwater. The age of the tufa mounds is well constrained because prior to the development of the Newlands Irrigation Project in 1907, the water level was ∼18 m lower than the current lake level. The vertical columnar nature of the tufa mounds indicates that they formed under the lake and not subaerially. Thus, the tufa mounds are <100 yr old and have grown at a rate ≥30 mm/yr. Stable oxygen and carbon isotope analyses of tufa carbonate compared to isotopic analyses of groundwater and lake water and hydrochemical data indicate that the fluids responsible for their precipitation are a simple mixture of modern groundwater and lake water and do not reflect a recent climate signature. The exceptionally fast growth of the tufa mounds indicates that large tufa deposits may form almost instantaneously in geologic time. Given this potential for rapid growth and the fact that variations in isotopic compositions of tufa deposits have been interpreted in terms of changes in paleoclimate and changes in the composition of recharge water over thousands of years, care should be taken when trying to determine the significance of variations in isotopic or chemical compositions of tufas that may have been caused by mixing with groundwater.

Dating gold deposition in a Carlin-type gold deposit using Rb/Sr methods on the mineral galkhaite

Significant effort has been expended in an attempt to date hydrothermal activity that generated Carlin-type gold deposits (CTDs) in the Great Basin of Nevada. Thus far, these efforts have been only partially successful, because the relationship(s) between the dated mineral and hydrothermal activity are equivocal in many cases. Galkhaite, a trace component of at least four CTDs in Nevada, contains significant amounts of Rb and virtually no Sr, making it an ideal candidate for radiometric dating. At the Getchell deposit, galkhaite is paragenetically late, but clearly associated with gold mineralization. Our data place gold mineralization at Getchell at 39.0 ± 2.1 Ma. This is the first unequivocally gold-related date produced for any of the Carlin-type systems. Galkhaite also has been reported at the Carlin, Rodeo, and Betze deposits and is likely present in other CTDs in Nevada. This mineral may provide a solution to the conundrum of dating of CTDs.

Ion microprobe determination of sulfur isotope variations in iron sulfides from the Post/ Betze sediment-hosted disseminated gold deposit, Nevada, USA

Abstract Secondary ion mass spectrometric analyses of ore samples from the Post/Betze sedimenthosted disseminated gold deposit were utilized to constrain elemental distribution of Au and As in iron sulfide phases. Most of the Au was deposited very early in the paragenetic sequence. Although Au and As are covariant in arsenian pyrite, Au apparently was depleted much more rapidly from the hydrothermal solutions than was As. Sensitive high-resolution ion microprobe (SHRIMP) sulfur isotope analyses of iron sulfides from the Post/Betze deposit vary widely from δ 34 S = −29 to 23‰ and provide important information on the origin of sulfur and constraints on depositional mechanisms. Ore solutions had high δ34S values and were most likely derived, at least in part, from thermochemical reduction of lower Paleozoic seawater-derived sulfate, possibly bedded barite. Late-stage ore fluids ( δ 34 S sulfide = −12 to −29‰ ) are extremely depleted in 34S relative to main-stage ore fluids ( δ 34 S sulfide = 16–23‰ ). Although such low δ34S values can be generated hypothetically from the original ore fluids by oxidation (possibly boiling), the stability of pyrite is compromised. Introduction of and mixing with Fe-rich fluids is necessary to deposit pyrite having low isotopic values.

Sulfur Isotopes in Plutonic Rocks of the Great Basin as Indicators of Crustal Architecture

Reconstructions of geologic provinces in the crust are an important component of understanding ancient plate tectonic processes and crustal evolution. Isotopic data from plutonic rocks, primarily Sr, Nd, Pb, and O, have been commonly utilized to delineate ancient geological provinces. Sulfur isotopes have not previously been utilized for province reconstruction, perhaps because of the difficulty in analysis and interpretation. We present here the results of the first large-scale effort at utilizing sulfur isotopes as an indicator of crustal architecture and demonstrate that sulfur is complementary to other isotopic measurements. We also show that, because of mass balance considerations, sulfur can be potentially a far more sensitive indicator of magma-crust interaction processes during magma migration through the crust than are other isotope systems. We anticipate that addition of sulfur isotope measurements to other studies of crustal evolution will provide significant insights into plate tectonics and crustal evolution in many areas of the world.

Platinum-Group Element, Stable Isotope, and Fluid Inclusion Investigation of the Ultramafic Rock-Hosted Gunes-Sogucak Ni-Cu-Sulfide Mineralization in the Gunes Ophiolite, East-Central Turkey

Gunes-Sogucak Ni-Cu-sulfide mineralization in the Divrigi district of Sivas Province is genetically related to Late Cretaceous ultramafic rocks. Disseminated mineralization is scattered through the ultramafic rocks, whereas vein-type mineralization is present in fracture zones in them. Both disseminated and vein-type Ni-Cu-sulfide mineralization show enrichment of platinum-group elements (PGE), Au, and Ag relative to primitive mantle. Average metal concentrations of the Cu-Ni-sulfide mineralization are: Ni: 106, Cu: 263 ppm; Ru: 293, Pt: 26, Pd: 126, Au: 87, Ag: 316 ppb. Enrichments in PGE, Au, and Ag may have resulted from the chemical evolution of hydrothermal fluids. δ34SΣS values range from 7.9 to 14.6‰, with most values at the higher end. Judging by the range of sulfur isotope values, a component of sedimentary sulfate evidently was present in the hydrothermal fluids. δD values of waters trapped in quartz crystals range from -69 to -90‰, and δ18O values of quartz range from 11 to 15‰. Based on δ18O and δD data, combined with the geologic setting of the deposits, we propose that the ore fluids were dominated by magmatic waters. Sulfur isotopes indicate that ultramafic rocks assimilated a portion of their sulfur from sedimentary sulfates during ascent through the crust. Fluid inclusion data indicate that hydrothermal mineralization with PGE enrichment formed at a minimum temperature of around 280-300°C during percolation of high-salinity fluids through the ultramafic rocks.

Geology, geochemistry, stable isotope, and fluid inclusion investigation of the iron oxide-gold mineralization in Bakir Tepe, Kangal-Sivas, East-Central Turkey

The Bakir Tepe area is characterized by four different lithologic units of Devonian-Carboniferous age. These units, from bottom to top, are metasiltstone, metacarbonate, metasandstone, and metaquartz sandstone. Metamorphic rocks in the study area are tectonically overlain by the recrystallized Mesozoic Munzur limestone. Iron oxide-gold mineralization in the Bakir Tepe area occurs in two different localities. The first is an alteration zone, which was formed at the thrust contact of metasiltstone with metacarbonate and metasandstone; the second locality is in metaquartz sandstone. The dominant minerals are quartz + hematite/magnetite ± pyrite ± gold in veins of various lengths and widths. Magnetite occurred after hematite, along fractures in hematite. Oxygen isotope data (δ18O = 6.2-13.3‰) from fluids in equilibrium with quartz, and hydrogen isotope data (δD = -55 to -75‰) from fluid inclusions trapped in quartz, indicate that the hydrothermal fluids responsible for formation of the gold mineralization were dominated by metamorphicderived waters. Hydrothermal solutions were of the NaCl-CO2-H2O type. Salinity values as NaCl equivalent wt% range between 34 and 39. Total homogenization temperatures range between 132 and 382°C in secondary L+V inclusions, and 237 to 324°C in primary L+V+H inclusions. Calculated temperatures of vein formation using oxygen isotopes in quartz-hematite mineral pairs yields formation temperatures between 200 and 330°C, consistent with the fluid inclusion measurements.