Sediment-hosted Disseminated Gold Deposits Research Articles

Sediment-hosted disseminated gold deposits in orogenic belts: An example from the giant Jiagantan gold deposit in the West Qinling orogen, China

Sediment-hosted disseminated gold deposits in orogenic belts share many similarities to both epizonal orogenic and Carlin-type gold deposits, leading to a lot of difficulties in their genetic classification. The giant Jiagantan gold deposit was newly discovered in the West Qinling orogen, central China, and was selected to provide a better understanding of this issue. Gold mineralization at Jiagantan is hosted within sandstone and silty slate, and the gold is intimately associated to disseminated pyrite and arsenopyrite. Three generations of pyrite (Py0, Py1, and Py2) and two generations of arsenopyrite (Apy1 and Apy2) are recognized and used to perform in situ textural, chemical, and sulfur isotopic analyses.

Source of ore-forming fluids of the Yangshan gold field, western Qinling orogen, China: Evidence from microthermometry, noble gas isotopes and in situ sulfur isotopes of Au-carrying pyrite

Abstract The Yangshan gold ore belt contains a series of sediment-hosted disseminated gold deposits in the southern subzone of the western Qinling orogen. The ore deposits formed in the Middle Devonian carbonaceous clastic-carbonate sequences that underwent strong deformation and low greenschist facies regional metamorphism. The shapes and distribution of the ore bodies are strictly confined by the E-W-trending Anchanghe-Guanyinba thrust fault zone. Pyrite and arsenopyrite are the major Au-carrying minerals. Microthermometric results show that the ore-forming fluids are CO2-rich aqueous solutions. The homogenization temperatures range from 221 to 303.5 °C, and salinities range from 2.0 to 7.2 wt% NaCl equiv. These characteristics are consistent with orogenic gold deposit. Noble gas isotopic measurement shows that 3He/4He ratios of pyrite and quartz grains inherited from the mineralized carbonaceous phyllite, ranging from 0.033 to 0.081 Ra, implying that the origins of ore fluids are restricted within the crustal regime. The 40Ar/36Ar ratios range from 434 to 863, indicating 34 to 68% crust-derived radiogenic 40Ar* in the ore fluids. Based on the backscattered electron (BSE) maps of pyrite, three generations (i.e. core, mantle, rim) are distinguished: pre-ore pyrite including framboidal pyrites and the cores of zoned pyrites both in the phyllite and plagiogranite dikes; ore stage pyrites including the middle zones of the three-zoned pyrites in the granitic dikes and the outer zones of the zoned pyrites in the phyllite; post-ore pyrites, i.e. the outermost rims of the zoned pyrites in the dikes. The arsenopyrite in the phyllite has similar texture to the pyrite, and is regarded as the peak of the mineralization due to its highest Au concentrations. High spatial resolution NanoSIMS multi-elemental maps of the pyrites reveal that (i) gold is distributed heterogeneously as lattice-bounded Au in the zoned pyrites with higher Au (As) in the middle zones and lower Au (As) in the inner and outermost zones. No nano-particles of native gold was identified in pyrite; (ii) gold mineralization occurred in a single event instead of multistage ore formation; (iii) gold precipitation event is followed by an overgrown zone with low Au and As concentrations. In situ sulfur isotopes of Au-bearing pyrite identified two types of sulfur sources: one is framboidal pyrite and the barren core of pyrite in the carbonaceous phyllite, which have very negative δ34S values ( Combining Au concentrations with the corresponding δ34S values in the pyrites reveals that the zones with high Au have lower δ34S values (and vice versa). This coupling relation between Au and S isotope has been also identified in other orogenic and Carlin-type gold deposits. The oxidation of ore fluid is taken as the key for gold precipitation during formation of orogenic gold deposit. However, the oxidation of ore fluids accompanied by the escape of reduced gas (e.g. H2S) should result in high Au coupled with high δ34S. Therefore, we propose that the Yangshan gold deposit forms through partitioning aqueous vapor with high Au, As, and lower δ34S from a deep magma chamber under the ore field, which leads to overgrowth of the pre-existing pyrites. Based on the geological features, texture of pyrite, and sulfur isotopic characteristics, we suggested that the Yangshan gold deposits have the characteristics between Carlin-type and orogenic gold deposits. They are the product of the western Qinling orogeny during the Late Triassic.

Mineralogical and morphological studies of gold-bearing arsenopyrite and pyrite minerals of Bakyrchik and Bolshevik gold black shale deposits (Eastern Kazakhstan)

Abstract Bakyrchik and Bolshevik both are sediment-hosted disseminated gold deposits. They are located in Eastern Kazakhstan (4 km in distance between deposits) and include in the Western Qalba metallogenic zone. They originated in late Paleozoic age along structured line between palaeocontinents of Kazakhstan and Altai-Mongol. The purpose of this paper is designation of typomorphic features of gold-bearing arsenopyrite and pyrite minerals, determination of chemical composition of these two sulfides minerals. The arsenopyrite and pyrite are the main objects for analytical study of gold-bearing sulfide minerals, their different textures, morphology of the crystals and aggregates. The studies of ore samples and minerals were carried out by methods of optical microscope, X-ray diffraction, X-ray fluorescence and electron microprobe analyses. The analytical study of sulphide minerals has shown that the arsenopyrite is acicular and tabular, pyrite has three varieties - globular, hexahedral and pentahedral forms in three rock samples of the carbonaceous-terrigenous formation in the deposits.

Sediment-Hosted Disseminated Gold Deposits in Southwest Guizhou, PRC: Their Geological Setting and Origin in Relation to Mineralogical, Fluid Inclusion, and Stable-Isotope Characteristics

The sediment-hosted disseminated gold deposits in Southwest Guizhou, People's Republic of China (PRC) are located in faults on the flanks of anticlines or domes in clastic sedimentary rocks of Late Permian to Middle Triassic age on the southwestern edge of the Yangtze paraplatform. Lamprophyres crop out in the vicinity of the gold deposits. Mineralization in the area coincides with belts of weak Bouguer gravity and magnetic anomalies. The Lannigou and Yata deposits, described in detail in the present study, together with the Baidi deposit, are situated in the southeastern domain where mineralization was emplaced in fine turbidites of basinal facies of Middle Triassic age. The structures guiding this mineralization are high-angle reverse faults on domes or anticlines. To the northwest, the Getang deposit is one of a group of deposits including Zimudang, Sanchahe, Dayakou, and Xiongwu, which were emplaced in silicified breccias in impure carbonates or marls of Permian to Lower Triassic age. They are controlled by low-angle and bedding-parallel faults on anticlines. The clastic sedimentary host rocks are rich in illite and organic matter. Mineralization takes the forms of pervasive silicification, veinlets of quartz and disseminated auriferous arsenic-bearing pyrite and arsenopyrite, veins of quartz and calcite, and veinlets of realgar, cinnabar, and stibnite. Gold is mainly associated with arsenic-rich pyrite. The main-stage gold mineralization in pyrite is accompanied by pervasive silicification of host rocks. The Permian Emeishan basalts, widely distributed in the northwestern area, contain high average gold contents and may have been the primary source of the gold in the sediment-hosted deposits in Southwest Guizhou. The distribution of arsenic, antimony, and mercury in the host rocks and country rocks shows a pattern similar to that of gold. Gold is found mainly in pyrite and partly in illite. High-resolution electron-probe microanalysis (EPMA) of samples from the Lannigou deposit revealed that gold is located in pyrite rims in zones of intermediate arsenic content (3-5 wt%). It is deduced that gold probably occurs as discrete submicron-sized particles rather than as a charged Au species in a coupled diadochic substitution with arsenic in the pyrite structure. The auriferous fluids at the Lannigou and Yata deposits are shown to be CO2-rich (Xco2 > 0.05) and of low salinity (<5 wt% equiv. NaCl), with relatively high homogenization temperatures (mainly 240° to 300°C) and were probably trapped under high confining pressures (1.5 to 2.3 kbar). They are not typical epithermal fluids. At Lannigou, the δ34SVCDT values of sulfides range from +8.4 to +12.5‰, the δ13CVPDB of carbon in calcite ranges from -0.1 to -3.6‰, and the δ18OVSMOW of quartz and calcite are mainly around +17.6 and around +25.8‰, respectively. At Getang, the isotopic compositions of hydrothermal minerals are in the range δ34SVCDT of -14.3 to +4.4‰ for sulfides, δ13CVPDB of -3.2 to -0.6‰ for calcite and σ18OVSMOW of +14.0 to +15.3‰ for calcite and quartz. These isotope analyses show that sulfur was probably derived mostly from the sedimentary country rocks, and therefore inherited from the marine reservoir in which they were deposited, although part of the sulfur in the Getang deposit could be from altered or weathered basalt. Most of the carbon in the hydrothermal fluids was probably derived from the dissolution of carbonates in sedimentary rocks, although decarbonation reactions caused by low-grade metamorphism at deeper levels could have contributed some of the CO2. The original hydrothermal fluids responsible for the gold mineralization are deduced to have formed by burial metamorphism at depths of 6-8 km with addition of meteoric water through deep fractures. Mineralization probably took place as a result of decompression when impermeable shales in the cover sequence were structurally breached during the Yanshanian tectonic cycle, allowing fluids to escape from confinement at near lithostatic pressures in permeable clastic horizons. Mixing between evolved formation water/burial metamorphic water and meteoric waters was an important process during the late stage of the hydrothermal evolution. The tectonic setting, structural control, hydrothermal alteration, and ore and gangue mineral assemblages of the deposits in Southwest Guizhou show many features in common with those of the Carlintype gold deposits in Nevada, United States, although the host rocks, relationship to igneous rocks, and the timing of mineralization are different.

Syngenetic origin for the sediment-hosted disseminated gold deposits in NW Sichuan, China: ore fabric evidence

Sediment-hosted disseminated gold deposits in NW Sichuan China have many features in common with the well-known Carlin-type deposits in the western United States. They are hosted by Middle–Upper Triassic turbidites composed of 1300–4300 m of rhythmically interbedded, slightly metamorphosed calcareous sandstone , siltstone , and slate. The ore bodies are typically layer- or lens-like in shape and generally extend parallel to the stratification of the host sedimentary rocks, with a strike length of tens to several hundreds of meters. The immediate host rocks consist mainly of calcareous slate and siltstone characterized by high contents of organic matter and diagenetic pyrite . The main primary ore minerals associated with gold mineralization include pyrite, arsenopyrite , realgar , and stibnite . Gangue minerals comprise mostly quartz, calcite and dolomite. Gold is extremely fine-grained, usually less than 1 μm, and cannot be seen with an electron microscope . Two types of ore mineralization have been recognized in the deposits. The stratiform ores are composed of rhythmical interbeds of sulfides (e.g., pyrite, arsenopyrite, realgar, stibnite) interpreted to be authigenic and detrital quartz, quartzite , sericite, and graphite of allogenic origin. They were folded and deformed concordantly with host rocks, and grade both vertically and laterally into normal country rocks. Another type of ore forms a network of numerous gold-bearing veins and veinlets of quartz–calcite–sulfides of millimeter-, centimeter-, decimeter-, and even meter-scale in width. The network ore randomly fills fissures, microfissures, and cleavages , but still is stratabound in character. Detailed studies on ore fabrics show abundant evidence for synsedimentary origins, although subsequent diagenesis , metamorphism , tectonic deformation, and epigenetic hydrothermal activity have significantly remolded the primary fabrics. Primary fabrics are shown either by rhythmical interbeds of different mineral components parallel to the bedding, or by the change of grain size of the same minerals such as pyrite, realgar, and stibnite. The layer inhomogeneity of the stratiform ore is clarified by parallel overprints of later schistosity planes, resulting in distinct grain orientation and elongation, aggregate polarization, and undulating extinction of ore minerals, especially of mechanically and chemically extremely mobile ones, such as realgar and stibnite. It is proposed that the stratiform ores in these Chinese deposits were most probably formed concurrently with their host Middle–Upper Triassic turbidites in submarine, hot spring environments, while the network mineralization was formed as a result of complicated processes such as diagenesis, weak metamorphism, tectonic deformation, and epigenetic hydrothermal activity, responsible for the remobilization or reworking of the pre-existing stratiform ores. Geochemical data also support this genetic model.

Petrography, mineralogy and geochemistry of the Zarshuran Carlin-like gold deposit, northwest Iran

Gold mineralisation at Zarshuran, northwestern Iran, is hosted by Precambrian carbonate and black shale formations which have been intruded by a weakly mineralised granitoid. Granitoid intrusion fractured the sedimentary rocks, thereby improving conditions for hydrothermal alteration and mineralisation. Silicification is the principal hydrothermal alteration along with decalcification and argillisation. Three hydrothermal sulphide mineral assemblages have been identified: an early assemblage of pyrrhotite, pyrite and chalcopyrite; then widespread base metal sulphides, lead-sulphosalts and zoned euhedral arsenical pyrite; and finally late network arsenical pyrite, massive and colloform arsenical pyrite, colloform sphalerite, coloradoite, and arsenic–antimony–mercury–thallium-bearing sulphides including orpiment, realgar, stibnite, getchellite, cinnabar, lorandite and a Tl-mineral, probably christite. Most of the gold at Zarshuran is detectable only by quantitative electron microprobe and bulk chemical analyses. Gold occurs mainly in arsenical pyrite and colloform sphalerite as solid solution or as nanometre-sized native gold. Metallic gold is found rarely in hydrothermal quartz and orpiment. Pure microcrystalline orpiment, carbon-rich shale, silicified shale with visible pyrite grains and arsenic minerals contain the highest concentrations of gold. In many ways Zarshuran appears to be similar to the classic Carlin-type sediment-hosted disseminated gold deposits. However, relatively high concentrations of tellurium at Zarshuran, evidenced by the occurrence of coloradoite (HgTe), imply a greater magmatic contribution in the mineralising hydrothermal solutions than is typical of Carlin-type gold deposits.

Sediment-hosted disseminated gold mineralisation at Zarshuran, NW Iran

Mineralisation at the Zarshuran, NW Iran, occurs on the flank of an inlier of Precambrian rocks hosted in black silty calcareous and carbonaceous shale with interbedded dolomite and limestone varying in thickness from 5 to 60 m and extending along strike for approximately 5–6 km. Two major, steeply dipping sets of faults with distinct trends occur in the Zarshuran: (1) northwest (310–325) and (2) southwest (255–265). The main arsenic mineralisation occurs at the intersection of these faults. The mineral assemblage includes micron to angstrom-size gold, orpiment, realgar, stibnite, getchellite, cinnabar, thallium minerals, barite, Au-As-bearing pyrite, base metal sulphides and sulphosalts. Hydrothermal alteration features are developed in black shale and limestone around the mineralisation Types of alteration include: (1) decalcification, (2) silicification, (3) argillisation, (4) dolomitisation, (5) oxidation and acid leaching and (6) supergene alteration. The early stage of mineralisation involved removal of carbonates from the host rocks, followed by quartz precipitation. The main stage includes massive silicification associated with argillic alteration. In the late stage veining became more dominant and the main arsenic ore was deposited along fault cross cuts and gouge. These characteristics are typical of Carlin-type sediment-hosted disseminated gold deposits. The early stage of mineralisation contains only two-phase aqueous fluid inclusions. The main stage has two groups of three-phase CO2-bearing inclusions with minor CH4 ± N2, associated with high temperature, two-phase aqueous inclusions. During the late stage, fluids exhibit a wide range in composition, salinity and temperature, and CH4 becomes the dominant carbonic fluid with minor CO2 associated with a variety of two-phase aqueous fluid inclusions. The characteristics of fluids at the Zarshuran imply the presence of at least two separate fluids during mineralisation. The intersections of coexisting carbonic and aqueous inclusion isochores, together with stratigraphic and mineral stability evidence, indicate that mineralisation occurred at 945 ± 445 bar and 243 ± 59 °C, implying a depth for mineralisation of at least 3.8 ± 1.8 km (assuming a lithostatic pressure gradient). Fluid density fluctuations and the inferred depth of formation suggest that the mineralisation occurred at the transition between overpressured and normally pressured regimes. Geochronologic studies utilising K/Ar and Ar/Ar techniques on hydrothermal argillic alteration (whole rock and separated clay size fractions) and on volcanic rocks, indicates that mineralisation at Zarshuran formed at 14.2 ± 0.4 Ma, and was contemporaneous with nearby Miocene volcanic activity, 13.7 ± 2.9 Ma. It is proposed that mineralisation was the result of the infiltration of hydrothermal fluids containing a magmatic gas component, and that it was localised in the Zarshuran Unit because of the redox boundary that it provided and/or because it lay between an overpressured region at depth and a zone of circulating, hydrostatically pressured fluids above.

Carlin-type gold deposits in Qinling and some related problems

The sediment-hosted disseminated gold deposits in the Qinling region are of sedimentation-slight-metamorphic origin superimposed by hydrothermal reworking at moderate-low temperatures and are well comparable with the typical Carlin gold deposits in the United States. In view of the confusing concept concerning the “sediment-hosted” and “Carlin-type” gold deposits, the authors propose that the term “sediment-hosted gold deposit” should be used in a broad sense which encompasses at least the four subtypes, i.e., the Carlin type, the metamorphic fine clastic type, the hydrothermal sedimentary type and the vein type. In other words, the “Carlin-type” should not be used as a synonym for “sediment-hosted” but is recommended as a subtype under the general category of “sediment-hosted gold deposits”.

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 δ18O 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.

Genetic analogy between sediment-hosted gold deposits and sediment-hosted copper deposits in Southwest China

The sediment-hosted copper deposits, according to the authors’ study, were formed from connate formation water during the post-sedimentary or diagenetic stage while the sediment-hosted disseminated gold deposits are unanimously considered to be of post-sedimentary hydrothermal origin. Therefore, apart from their own individualities, these two types of deposits must share some characters in common. Comparisons are attempted, in this paper, between the sediment-hosted copper deposits in southwest China and the Triassic sediment-hosted disseminated gold deposits in the Yunnan-Guizhou-Guangxi Triangle in terms of geological and geochemical features.

Characterization and dating of argillic alteration in the Mercur gold district, Utah

The Mercur gold district of north-central Utah includes several sediment-hosted disseminated gold deposits which are located in the lower member of the Mississippian Great Blue Limestone. Argillic alteration of host limestone consists of illite (R3 illite-smectite <10% S) + kaolinite + quartz + or - Fe oxides or pyrite. Argillized limestone has identical clay mineralogy in both oxidized and unoxidized rock. Unlike some other sediment-hosted disseminated gold deposits, variations in the Kubler index and illite/kaolinite ratios show no spatial relationship to faults or to gold distribution within the mineralized areas. Hydrothermally altered Long Trail Shale, the middle member of the Great Blue Limestone, also occurs within the Mercur district and surrounding region. The shale varies in mineralogy from an assemblage consisting of R1-type illite-smectite (20-35% smectite) + chlorite + kaolinite + quartz + pyrite or Fe oxides to an assemblage consisting of R3-type illite-smectite (5-10% smectite) + kaolinite + quartz + pyrite or Fe oxides. Shale with R3 illite-smectite is most highly altered and is mineralogically and chemically very similar to the highly altered limestone within the gold deposits. Both the altered shale and the altered limestone contain veins of R3 illite-smectite + kaolinite + or - quartz and pyrite or Fe oxides. Additional hydrothermal features exhibited by both shale and limestone are felted and interlocking clay mineral textures, radiating clay mineral habits, crosscutting and replacement clay mineral textures, and fossil replacement by clay minerals and quartz. Ten K-Ar ages of illite-rich clay-size separates of argillically altered and mineralized limestone from the Mercur deposits range from 98.4 to 226 Ma. None of the hydrothermal illites in the deposit have Tertiary ages. These ages are similar to a range of Mesozoic ages previously obtained for vein and matrix clay in the Long Trail Shale. Most of these ages are interpreted as mixed ages resulting from overprinted thermal and/ or mass transfer events. Completely argillized and highly mineralized limestone obtained from the Carrie Steele fault yields an age of 152 + or - 4 Ma (0.03-0.3-mu m-size fraction) that is interpreted as the best estimate...

40 Ar/ 39 Ar, K/Ar, and fission-track geochronology of sediment-hosted disseminated gold deposits at Post-Betze, Carlin Trend, northeastern Nevada; reply

40 Ar/ 39 Ar, K/Ar, and fission-track geochronology of sediment-hosted disseminated gold deposits at Post-Betze, Carlin Trend, northeastern Nevada; reply

40 Ar/ 39 Ar, K/Ar, and fission track geochronology of sediment-hosted disseminated gold deposits at Post-Betze, Carlin Trend, northeastern Nevada; discussion

40 Ar/ 39 Ar, K/Ar, and fission track geochronology of sediment-hosted disseminated gold deposits at Post-Betze, Carlin Trend, northeastern Nevada; discussion

40 Ar/ 39 Ar, K/Ar, and fission track geochronology of sediment-hosted disseminated gold deposits at Post-Betze, Carlin Trend, northeastern Nevada

The Post-Betze deposit of Nevada is the largest sediment-hosted disseminated gold deposit presently known, both dimensionally and in terms of contained metal. Ore occurs primarily as submicron-sized gold that is disseminated in altered sedimentary rocks of the Lower Paleozoic Roberts Mountains Formation. However, significant portions of the ore are present in altered monzonite of the Goldstrike stock. Alteration and mineralization were controlled by both structure and stratigraphy. Alteration began with early decarbonatization and was followed by silicification and, finally, argillization. Phyllosilicate mineral zoning grades from proximal kaolinitc to kaolinitc + sericite to unaltered rock.Based on geochronologic studies utilizing the K/Ar, 40 Ar/ 39 Ar, and fission track techniques, we suggest that the age of gold mineralization is approximately 117 Ma. The premineralization Goldstrike stock was emplaced at about 158 Ma and a postmineralization sill was emplaced at 39 Ma; these events place clear limits on the age of mineralization. Fine-grained sericite that is interpreted to have formed during hydrothermal events which also generated gold ore was dated by either the K/Ar method or a modification of the standard 40 Ar/ 39 Ar fusion technique. Age determinations on several samples of fine-grained sericite from altered host rocks (both sedimentary and igneous) yield ages near 117 Ma. Younger dates are interpreted to be the result of either thermal disturbance of the K/Ar system or mixing of sericites of two ages. One sample of coarser grained sericite that was stepheated gave discordant spectra.Fission track analyses of zircon and apatite from the postore sill clearly document the lack of significant thermal or hydrothermal activity younger than 39 Ma. Although less definitive, fission track data on zircon from preore sedimentary and igneous hosts suggest that no widespread hydrothermal activity having temperatures above about 100 degrees C has occurred since approximately 110 Ma.The 117 Ma age is consistent with one of the major magmatic pulses recognized in the northern Great Basin. It is also similar to, within analytic uncertainty, a K/Ar date of 109 Ma from the Welches Canyon stock (20 km south of Post-Betze). An intrusion of similar age, unexposed at present, could have been the driving force for the hydrothermal system at Post-Betze. Published geochronologic data, though less well tied to mineralization, suggest similar mid-Cretaceous ages for other sediment-hosted disseminated gold deposits in Nevada. Based on these ages, we suggest that sediment-hosted disseminated gold deposits are not necessarily products of extensional environments and, in fact, appear to be associated with compressional environments in Nevada during Cretaceous time. The alignment of sediment-hosted disseminated gold deposits (and other features) in the Basin and Range province of the western United States suggests that zones of long-term crustal weakness may have controlled igneous intrusion and consequent hydrothermal gold mineralization, although not all igneous episodes necessarily generated sediment-hosted disseminated gold ore deposits. The strong association between sediment-hosted disseminated gold deposits, the Precambrian craton, and these deep-seated structures also suggests the possibility of a deep source for gold and other components of these systems.

Gold and arsenic in iron sulfides from sediment-hosted disseminated gold deposits; implications for depositional processes

Secondary ion microscopic elemental distribution maps and quantitative measurements of Au and As concentrations in samples from five sediment-hosted disseminated gold deposits in Nevada present a consistent picture of the location and nature of gold in these samples. Gold is present in arsenian pyrite that forms overgrowth rims on, and narrow veinlets in, gold-free pyrite (and possibly other minerals, but only to a minor extent). Auriferous overgrowth rims that may exhibit compositional zoning, either simple or oscillatory, range from approximately 25 mu m in thickness to the limits of microscopic resolution. Because all of the observed gold-bearing pyrite contains sufficient As to be metastable (i.e., in excess of approximately 0.5%), it is suggested that small native gold grains occasionally observed in unweathered sedimenthosted disseminated gold ores resulted from exsolution of gold from the arsenian pyrite lattice.Available evidence suggests that gold in arsenian pyrite is most likely present as a charged species (Au (super +3) ) and that gold probably was deposited with arsenic as coupled solid solution substitutions in pyrite. The codeposition of gold and arsenic in pyrite in these and many other deposits may be a function of a redox reaction involving oxidation of gold and concurrent reduction of arsenic.Commonly proposed reactions involving gold deposition in sediment-hosted disseminated gold deposits usually involve gold precipitation as a neutral species from a bisulfide complex. It is proposed that such reactions are inappropriate for gold deposition in arsenian pyrite but that reactions involving oxidation of gold and concurrent reduction of arsenic are probably more appropriate. Mechanisms effecting gold deposition as native metal also will effect deposition as Au (super +3) if it is deposited along with arsenic (in either arsenopyrite or arsenian pyrite).

The Association of Gold and Hydrocarbons in Sediment-Hosted Disseminated Gold Deposits

The Association of Gold and Hydrocarbons in Sediment-Hosted Disseminated Gold Deposits

Fluid inclusion gas chemistry as a potential minerals exploration tool: Case studies from Creede, CO, Jerritt Canyon, NV, Coeur d'Alene district, ID and MT, southern Alaska mesothermal veins, and mid-continent MVT's

Recent advances in instrumentation now permit quantitative analysis of gas species from individual fluid inclusions. Fluid inclusion gas data can be applied to minerals exploration empirically to establish chemical (gas composition) signatures of the ore fluids, and conceptually through the development of genetic models of ore formation from a framework of integrated geologic, geochemical, and isotopic investigations. Case studies of fluid inclusion gas chemistry from ore deposits representing a spectrum of ore-forming processes and environments are presented to illustrate both the empirical and conceptual approaches. We consider epithermal silver-gold deposits of Creede, Colorado, Carlin-type sediment-hosted disseminated gold deposits of Jerritt Canyon, Nevada, metamorphic silver-base-metal veins of the Coeur d'Alene district, Idaho and Montana, gold-quartz veins in accreted terranes of southern Alaska, and the mid-continent base-metal sulfide deposits of Mississippi Valley-Type (MVT's). Variations in gas chemistry determine the redox state of the ore fluids, provide compositional input for gas geothermometers, characterize ore fluid chemistry (e.g., CH4CO2, H2SSO2, CO2/H2S, organic-rich fluids, gas-rich and gas-poor fluids), identify magmatic, meteoric, metamorphic, shallow and deep basin fluids in ore systems, locate upwelling plumes of magmatic-derived volatiles, zones of boiling and volatile separation, interfaces between contrasting fluids, and important zones of fluid mixing. Present techniques are immediately applicable to exploration programsas empirical studies that monitor fluid inclusion gas threshold concentration levels, presence or absence of certain gases, or changes in gas ratios. We suggest that the greater contribution of fluid inclusion gas analysis is in the integrated and comprehensive chemical dimension that gas data impart to genetic models, and in the exploration concepts based on processes and environments of ore formation derived from these genetic models.

Genesis of sediment-hosted disseminated-gold deposits by fluid mixing and sulfidization: Chemical-reaction-path modeling of ore-depositional processes documented in the Jerritt Canyon district, Nevada

Integrated geologic, geochemical, fluid-inclusion, and stableisotope studies of the gold deposits in the Jerritt Canyon district, Nevada, provide evidence that gold deposition was a consequence of both fluid mixing and sulfidization of host-rock iron. Chemical-reaction-path models of these ore-depositional processes confirm that the combination of fluid mixing, including simultaneous cooling, dilution, and oxidation of the ore fluid, and wall-rock reaction, with sulfidization of reactive iron in the host rock, explains the disseminated nature and small size of the gold and the alteration zonation, mineralogy, and geochemistry observed at Jerritt Canyon and at many other sediment-hosted disseminated gold deposits.