Carlin-type Deposits Research Articles

Partitioning of arsenic between hydrothermal fluid and pyrite during experimental siderite replacement

Arsenian pyrite is the major host mineral of gold in Carlin type gold deposit (CTGD) and forms during the interaction of H2S bearing hydrothermal fluids with Fe-bearing carbonates (e.g., siderite, ankerite, Fe-rich dolomite) in the wall rock. Experimental replacement of siderite at hydrothermal conditions (i.e., 200 °C, Psat) was used to study the parameters that control the formation of CTGD pyrite (i.e., pH, time, aH2S) and the partitioning of As between the hydrothermal fluid and newly formed pyrite.Hydrothermal pyrite formed via replacement of siderite in experiments mainly occurs as two types: 1) porous, massive pyrite at alkaline pH conditions or 2) clusters of dense euhedral to anhedral pyrite grains at slightly acidic conditions (pH <6). Longer experimental run times (2 weeks) lead to formation of type 2 pyrite at alkaline conditions and an increase in euhedral grain sizes in all conditions.Concentration of As in newly formed pyrite depends strongly on the As concentration of the hydrothermal fluid used in the experiment. Calculated partition coefficients vary between 300 and 1700 with lowest D values at high As concentrations of the fluid. In the studied system As−1 substitutes for S−2 as indicated by the correlation of high As with low S concentrations. Experimental findings agree well with previous studies in natural CTGD on As substitution and the partitioning of As between ore stage pyrite and hydrothermal fluids trapped as fluid inclusions.The newly constrained partition coefficients for As help to explain the large heterogeneity in natural CTGD pyrite compositions which is caused by changes in the fluid composition due to the strong partitioning of As into the forming pyrite.

Carlin-style gold deposits, Youjiang Basin, China: tectono-thermal and structural analogues of the Carlin-type gold deposits, Nevada, USA

The Triassic gold deposits of the Youjiang Basin, southern China, have been variously correlated to Carlin-style and orogenic gold deposits or classified as a new intermediate deposit type. However, in terms of a multi-scale mineral system approach, they show remarkable similarities to the Tertiary Carlin-type deposits of Nevada and distinct contrasts to orogenic gold deposits. Both the Nevada and Youjiang deposit groups formed in a continent-scale post-orogenic extension event on fragmented continental crust underlain by metasomatized lithosphere. Both form roughly orthogonal deposit trends that subparallel near-orthogonal margins of a continental crustal block, with deposits controlled by gentle anticlines, monoclines or half-horsts and extensional faults, not tight, “locked-up” anticlines, and shear zones. The mineralogy and ore geochemistry of the two groups are similar, with differences consistent with slightly deeper and higher temperature of formation of the older Chinese deposits, commensurate with deeper erosional levels. The Youjiang gold deposits should be classified as Carlin-type, rather than Carlin-like or other terminologies, with their lower gold endowment probably related to a more distal thermal and fluid source than the Nevada Carlin-type deposits.

Integrating geologic and Landsat-8 and ASTER remote sensing data for gold exploration: a case study from Zarshuran Carlin-type gold deposit, NW Iran

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and the Landsat-8 data were integrated with geological data to map lithologic units, lineament features, hydrothermal alteration zones, and distribution of Fe3+-bearing minerals in the Zarshuran Carlin-type gold deposit. The dominant hydrothermal alterations related to the gold mineralization in the study area, consist of argillic, alunite, silicified, and decalcified alterations of carbonate host rocks. The alteration assemblages are accompanied by ferric iron minerals such as jarosite and hematite. The red-green-blue (RGB) combination of bands 5, 2, and 1 of the ASTER sensor provides excellent criteria for the separation of the host rock lithologic boundaries, particularly for the Oligo-Miocene andesitic volcanic rocks covering the vast parts of the study area. The ASTER lithologic map derived from RGB combination of bands 5, 2, and 1 is represented by a set of lineaments in which both the thrust and normal faults were enhanced. The band ratios obtained by the Landsat-8 and the ASTER datasets have been effectively used to delineate the alteration halos characterized by kaolinite, alunite, quartz, and ferric iron minerals both in regional and local (the Zarshuran gold mine) scales. In addition, methods such as the principal component analysis (PCA) and the selective principal component analysis (Crosta) were exerted on the ASTER data to identify the hydrothermal alteration zones. The spectral angle mapper (SAM) algorithm using the ASTER data could better differentiate the various alteration (e.g., argillic, alunite, and silicified) zones existing in the study area. The matched filtering (MF) technique processed on the Landsat-8 data was applied to characterize the ferric iron minerals (jarosite and hematite) and separate the hydrothermal alteration zones as well. The integration and combination of various image processing techniques indicated that the alteration zones at Zarshuran are affiliated with normal faults within the carbonate host rocks and can be used for prospection mapping purposes in regional scale.

Pyrite from Zarshuran Carlin-type gold deposit: Characterization, alkaline oxidation pretreatment, and cyanidation

Different types of pyrite exist within a refractory ore body and certain types possess a relative abundance of gold compared to other forms. Accordingly, identification and characterization of the different types of pyrite may represent a beneficial effect on the gold recovery by downstream processes. The main purpose of this study was to establish a connection between the mineralogy, leaching behavior and gold recovery of such gold bearing pyrites during alkaline pretreatment and cyanidation processes. The pyrite concentrate used in this study was obtained from the refractory Carlin-type gold deposit of Zarshuran, Iran. Characterization studies were done using SEM/EDX and EPMA analyses which revealed that pyrite was generally in the two form of euhedral pyrite including pyritodrone and cubic pyrite and subhedral to anhedral pyrite including large particles or fine-grained aggregates. The later type of pyrite was identified as the best host with gold occurring more likely in rims rather than the center of pyrite particles. A positive correlation of Au and As, Ni, and Pb was also found in gold-bearing rims. Alkaline oxidation experiments were conducted to liberate gold from pyrite context. The results showed that leaching rate increases with pH, temperature, oxygen overpressure and surface area where orders of reaction equal to 0.24 and 0.22 were calculated with respect to [OH−] and PO2 respectively. Kinetic analysis showed that pyrite leaching fitted well to the shrinking core model with surface chemical reaction control with the activation energy of 38.41 kJ/mol. Gold cyanidation experiments after alkaline oxidation pretreatments correlated well with the results of characterization and leaching studies in which heterogeneous distribution of Au was identified in pyrite context. It was found that oxidative pretreatment is required for the gold-bearing pyrite to improve gold recovery. However, because much of the gold is in the rims, high gold recovery is possible by the application of only partial oxidation.

Geochemistry of Au-bearing pyrite from the Sepon Mineral District, Laos DPR, Southeast Asia: Implications for ore genesis

The Sepon gold-copper district occurs along the Truong Son Foldbelt in a 10 km × 40 km east-trending corridor in southern Laos. This study has investigated the nature of gold in pyrite and ore-forming fluid characteristics using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Proton Induced X-ray Emission (PIXE) techniques. Gold is present as invisible gold (Au+1) and as nanoparticles of gold (Au0) in both sedimentary-rock hosted gold and skarn Cu-Au deposits. Gold is present in an early syngenetic pyrite generation (pyrite 2B) with concentrations to 0.5 ppm. Post-sedimentation processes included formation of quartz veins which host pyrite 3B with concentrations to 1.8 ppm Au. The highest concentrations of gold were found in the rims of hydrothermal vein-hosted pyrite 4A and in skarn related pyrite, suggesting gold concentration at a later stage during the ore-forming processes and not during sedimentation.As rhyodacite-porphyry is proximal to calcareous shale and dolomite that host ores, it is plausible that the emplacement of rhyodacite-porphyry introduced significant amount of hydrothermal fluids that circulated through fractures and favoured high grade (to 293 ppm Au) in pyrite 4A and pyrite SKN2. Gold is preferentially concentrated in As-rich zones and in areas that underwent brittle deformation, as evidenced by fracture-fill coarse-grained pyrite 4A, indicating a structural control to ore formation. The predominance of Se, Bi and Te in pyrite SKN1 and the elevated concentrations of Bi, Cu, Se, Sn and Mo in pyrite SKN2 raise the possibility of derivation from a magmatic source. The presence of primary hematite associated with the skarn suggests prevailing oxidising fluid conditions during Au mineralisation in pyrite SKN2.There is a striking chemical similarity between pyrite 4A core and py-SKN1 and rimmed zones of pyrite 4A and pyrite SKN2, which indicates possibility of two types of hydrothermal fluids. The As-rich fluids with elevated concentrations of Au, Se, Sb, Tl, and Cu recorded in pyrite 4A rims and pyrite SKN2. The As-poor fluids with elevated concentrations of Ni and Pb yielded in pyrite 4A cores and pyrite SKN1. The chemistry of pyrite has also shown that both ore-forming fluids are undersaturated in Au similar to Au-bearing pyrite in the Carlin-type deposits, Nevada, USA.

Application and effects of singularity analysis in evaluating the denudation degree of Carlin-type gold deposits in southwest Guizhou, China

Denudation is a critical factor controlling the post-ore change and preservation of deposits, and the study of the degree of post-ore denudation is a common practice in mineral exploration. This paper introduces the use of singularity analysis and its efficacy in evaluating the degree of denudation of Carlin-type gold deposits in southwest Guizhou. The paper specifically focuses on investigating the geological effects before and after the introduction of the singularity analysis. Compared to the results obtained by means of the element concentration contrast, the results of the singularity analysis effectively estimate the relative degrees of denudation of the deposits and ore fields. In addition the results also accord well with findings from studies of the geology of ore deposits, the geochemical primary-halo and the apatite fission track. These outcomes suggest that using singularity analysis to determine the degree of denudation can yield significant geological information and support accurate interpretation of the data. Thus, the evaluation results based on such analysis have a higher reference value for expanding the understanding of post-ore deformation of deposits and exploring unexposed orebodies in southwest Guizhou.

Chemically-assisted phytoextraction from metal(loid)s-polluted soil at a typical carlin-type gold mining area in southwest China

Abstract Laowuchang (LWC) gold mine, located in Southwest China, is a typical Carlin-type gold deposit with the characteristic Au–As–Sb–Tl trace elements assemblage. This study focuses on the mobilization of arsenic (As), antimony (Sb), and thallium (Tl) in polluted soils collected from LWC mining sites and the influence of adding ammonium thiosulfate in different chelant to soil ratio (0, 0.5 and 2 g kg−1) on the efficacy of As, Sb and T1 phytoextraction by Brassica juncea. The soluble metal(loid)s in the soils after harvesting plants was studied to understand the potential of phytoextraction in managing the risk of metal(loid)s dispersion. Results show that the soils contained relatively high concentration of soluble As (3.6～5.5 mg kg−1) and low concentration of soluble Sb (40～128 μg kg−1) and T1 (6.2～10 μg kg−1). The level of soluble As in the soils is toxic to the environment according to the Chinese environmental quality standard for irrigation water (GB 5084-92). The key results from phytoextraction experiment were that addition of thiosulfate enhanced As and Sb accumulation in B. juncea compared to the control, and the maximum As and Sb uptake in the roots and shoots occurred in the treatment of 2 g thiosulfate kg−1 of soil. However, application of thiosulfate inhibited plant uptake of Tl. The concentration of soluble As in 2 g kg−1 thiosulfate treatment was decreased significantly compared with the control and initial soils. Compared with the initial soils, soluble Sb was increased both in non-treated (control) and treated soils. However, no noticeable difference was observed between the soils of non-treated and treated plants. Both plant growth and thiosulfate amendment increased the soluble T1 in the soil, when compared to the initial soil. We conclude that thiosulfate-assisted phytoextraction might be used to reduce the environmental risk apparent for LWC soil through reducing the soluble As fractions, while managing the soluble fractions of Sb (compared with control soil). Meantime, further studies are required to manage the T1 risk in the soils at LWC mining site.

Data for: Distinguishing Generations of Quartz and a Distinct Gas Signature of Deep High-Grade Carlin-Type Gold Mineralization Using Quadrupole Mass Spectrometry

Abstract Carlin-type gold deposits in north-central Nevada occur in a region that was the locus of igneous and/or hydrothermal activity over time and the overprinting of silicification associated with metamorphism, base metal mineralization, barren jasperoid, and post-ore alteration hinders exploration efforts. Therefore, the potential to distinguish unrelated generations of quartz using the results of quadrupole mass spectrometer (QMS) gas analyses was evaluated for the Getchell trend, and compared to results from the Goldstrike property in the Carlin trend and a Carlin-type gold occurrence in the Huijiabao trend, China. The results of fluid inclusion petrography, QMS analyses, and interpretation of gas data using N 2 –Ar–He ternary diagrams and CO 2 /CH 4 vs. N 2 /Ar plots indicate that different generations of quartz in the Getchell trend were deposited by hydrothermal fluids having distinct characteristics. Metamorphic quartz containing one-phase CH 4 and halite-bearing liquid–water dominant vapor-poor inclusions has the highest gas content (37.6 mol%) and highest N 2 /Ar values (3250), which are suggestive of an evolved magmatic fluid. Pre-ore and quartz–base metal veins contain CO 2 -bearing inclusions. Base metal veins have N 2 /Ar values ≥675 and a positive correlation between H 2 S/Ar vs. N 2 /Ar that is suggestive of a magmatic fluid. Carlin-type quartz contains liquid-dominant vapor-poor inclusions and is characterized by total gas contents >10 mol% and N 2 /Ar values >300 that are suggestive of mixed crustal–meteoric–magmatic fluid sources. Post-ore quartz and barren jasperoid formed from mixtures of gas-poor meteoric water and crustal fluids. Samples of deep high-grade mineralization from the Goldstrike property have total gas contents up to 10.5 mol% and N 2 /Ar values >300. Fluid sources include crustal, meteoric, and magmatic. Gas data from the Goldstrike property, Turquoise Ridge deposit, and metamorphic quartz of the Getchell trend plot near the N 2 apex of N 2 –Ar–He diagrams, which is indicative of a magmatic fluid end member. Carlin-type mineralization in the Huijiabao trend is unique because fluids were CH 4 -rich and there is a correlation between CH 4 and hydrocarbons. Gas analyses commonly record 2 is the dominant gas specie. Together, the results show that fluid inclusion gases can be used to distinguish different generations of quartz in mining districts with complex origins.

Distinguishing generations of quartz and a distinct gas signature of deep high-grade Carlin-type gold mineralization using quadrupole mass spectrometry

Carlin-type gold deposits in north-central Nevada occur in a region that was the locus of igneous and/or hydrothermal activity over time and the overprinting of silicification associated with metamorphism, base metal mineralization, barren jasperoid, and post-ore alteration hinders exploration efforts. Therefore, the potential to distinguish unrelated generations of quartz using the results of quadrupole mass spectrometer (QMS) gas analyses was evaluated for the Getchell trend, and compared to results from the Goldstrike property in the Carlin trend and a Carlin-type gold occurrence in the Huijiabao trend, China.The results of fluid inclusion petrography, QMS analyses, and interpretation of gas data using N2–Ar–He ternary diagrams and CO2/CH4 vs. N2/Ar plots indicate that different generations of quartz in the Getchell trend were deposited by hydrothermal fluids having distinct characteristics. Metamorphic quartz containing one-phase CH4 and halite-bearing liquid–water dominant vapor-poor inclusions has the highest gas content (37.6 mol%) and highest N2/Ar values (3250), which are suggestive of an evolved magmatic fluid. Pre-ore and quartz–base metal veins contain CO2-bearing inclusions. Base metal veins have N2/Ar values ≥675 and a positive correlation between H2S/Ar vs. N2/Ar that is suggestive of a magmatic fluid. Carlin-type quartz contains liquid-dominant vapor-poor inclusions and is characterized by total gas contents >10 mol% and N2/Ar values >300 that are suggestive of mixed crustal–meteoric–magmatic fluid sources. Post-ore quartz and barren jasperoid formed from mixtures of gas-poor meteoric water and crustal fluids.Samples of deep high-grade mineralization from the Goldstrike property have total gas contents up to 10.5 mol% and N2/Ar values >300. Fluid sources include crustal, meteoric, and magmatic. Gas data from the Goldstrike property, Turquoise Ridge deposit, and metamorphic quartz of the Getchell trend plot near the N2 apex of N2–Ar–He diagrams, which is indicative of a magmatic fluid end member.Carlin-type mineralization in the Huijiabao trend is unique because fluids were CH4-rich and there is a correlation between CH4 and hydrocarbons. Gas analyses commonly record <1 mol% total gas for evolved (crustal) and organic fluids. Fluid inclusions in post-ore quartz and barren jasperoid are also gas poor, but CO2 is the dominant gas specie.Together, the results show that fluid inclusion gases can be used to distinguish different generations of quartz in mining districts with complex origins.

Alkaline leaching pretreatment and cyanidation of arsenical gold ore from the Carlin-type Zarshuran deposit

ABSTRACTCarlin-type refractory gold deposits are complex bodies which only occur in certain regions such as the western United States, southwest China, and northwest Iran. It is important to investigate the most suitable treatment option for each deposit from the point of the mineralogical composition. The present study investigates the effect of atmospheric NaOH alkaline pretreatment on the cyanidation gold recovery of the arsenical Carlin-type gold ore from the Zarshuran deposit, Iran (4.2 g t−1 Au feed grade). The results are explained based on the mineralogical considerations and with the aid of thermodynamic and kinetic investigations of the leaching behaviour of the main gold-bearing phases including orpiment, pyrite, and sphalerite. It was found that the high arsenic content of Zarshuran (3.7%) has some implications on the pretreatment and cyanidation processes. Solution potential and pH decreased significantly by the oxidation of arsenic sulphides which resulted in the decreasing oxidation extent of the other sulphides, specifically sphalerite. It was also found that pretreatment only in highly alkaline media (pH > 13) can be effective in gold recovery. Gold and silver extractions during subsequent cyanidation were improved from 69.4 to 80.7% for gold and 44.4 to 88.9% for silver, where the extraction yields of arsenic, iron, and zinc were 93.7, 75.9, and 26.9%, respectively.

The use of hyperspectral remote sensing for mineral exploration: a review

The hyperspectral remote sensing technology has been available to the research community for more than three decades. Since in its first steps the hyperspectral technology was also promoted as a tool for mineral exploration. Numerous mineral exploration applications of hyperspectral remote sensing have been reported. This paper provides an up-to-date and focused review of the applications of the hyperspectral remote sensing to mineral exploration. The ore deposits are grouped based on major processes of formation (magmatic, hydrothermal, sedimentary, supergene). The review shows that the hyperspectral remote sensing technology has found application to the study and exploration of a number of ore deposits including kimberlites (host-rocks of diamonds), carbonatites (host-rock of rare earth elements deposits), porphyry deposits, epithermal gold and silver deposits, skarn deposits, volcanic-hosted massive sulfide (VHMS) deposits, orogenic gold deposits, Carlin-type gold deposits, SEDEX Pb-Zn-Ag deposits. On the other hand, the possibilities of the hyperspectral technology remain still underexplored for the study and exploration of chromite deposits, Ni-sulfide deposits in mafic and ultramafic rocks, rare-metal pegmatites, greisen and related deposits, Mississippi Valley-type (MVT) Pb-Zn deposits, Kupferschiefer deposits and uranium deposits in sedimentary basins, iron ores in banded-iron formations, laterites and bauxites. A special attention has been paid in this review to the applications in mineral exploration of the emerging airborne hyperspectral thermal infrared technology. In addition, the possibilities and limitations of spaceborne hyperspectral imagery of moderate spatial resolution for detailed characterization and detection of mineralized systems are discussed for each major deposit type. The spatial resolution of the hyperspectral data is noted to be a key factor on the success of a hyperspectral exploration project. By providing a full up-to-date picture of the applications and contribution of the hyperspectral imagery to the exploration and characterization of ore deposits this review paper should be useful to the interested geological remote sensing researcher and practitioner, and to the mineral exploration manager as well.

Mineralogy and Geochemistry Characteristics and Genetic Implications for Stratabound Carlin-Type Gold Deposits in Southwest Guizhou, China

Mineralogy and Geochemistry Characteristics and Genetic Implications for Stratabound Carlin-Type Gold Deposits in Southwest Guizhou, China

Carbon-oxygen isotopes and rare earth elements as an exploration vector for Carlin-type gold deposits: A case study of the Shuiyindong gold deposit, Guizhou Province, SW China

The Shuiyindong gold deposit is a deeply concealed strata-bound Carlin-type deposit in southwestern Guizhou Province, China. The deposit lies on the eastern limb of the Huijiabao anticline with ores mainly along the anticline axis and hosted in bioclastic limestone, containing calcite veins, of the Permian Longtan Formation units. In this study, we measured carbon and oxygen isotopic ratios and rare earth element (REE) concentrations of the host rocks and calcite veins along a profile across the Shuiyindong deposit. Orebodies in the upper unit of the Longtan Formation have higher δ18O values (20.6–22.4‰) and lower δ13C values (−3.7 to −0.5‰) than the country rocks (δ18O: 18.8–21.4‰; δ13C: −0.7 to 0.8‰). However, there are no obvious trends of δ18O and δ13C values from the country rocks to the orebodies in the middle unit of the Longtan Formation. The spatial distribution of the calcite veins displays distinct halos of δ13C and δ18O values and REE concentrations. Calcite veins along the anticlinal axis and major reverse fault are enriched in Middle REE (Sm, Eu, Gd, and Tb) and 18O and depleted in 13C. Surficial veining calcite-filled fractures/faults that connect to deep concealed strata-bound gold mineralization systems can be vectors toward deep ores in southwestern Guizhou Province, China.

High-Grade Gold Deposition and Collapse Breccia Formation, Cortez Hills Carlin-Type Gold Deposit, Nevada, USA

The Cortez Hills breccia zone, a Nevada Carlin-type gold deposit located in the Cortez district on the Battle Mountain-Eureka trend, formed within a high-grade polylithic breccia, and grades at the center of the breccia zone are locally in excess of 1 oz/ton Au. Sample transects from low grade or below detection (<5 ppb) to high grade were collected and analyzed using petrography, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses to correlate changes in ore pyrite geochemistry and alteration mineralogy with grade and to relate ore deposition to breccia formation. Ore-related minerals, textures, and analyses are consistent with fluid-rock reaction and replacement at relatively low hydrothermal temperatures (<~260°C) during a single heating and cooling event and over a relatively short time frame, coincident in time and space with regional magmatism. Prior to Carlin mineralization, Roberts Mountains, Wenban, and Horse Canyon Formations were affected by burial diagenesis and low-grade contact metasomatism and metamorphism, producing host rocks composed of calcite, dolomite, and quartz with generally minor dickite, pyrite, chalcopyrite, sphalerite, clinochlore, tremolite, and organic material, and minor to trace K-feldspar, muscovite, and biotite. Acidic ore fluids sulfidized Fe in carbonate rock to form rims of Au- and trace element-rich pyrite on earlier formed pyrite or produced small, generally <10- μ m, spheres of fuzzy ore pyrite with no apparent core. Trace elements in ore-stage pyrite are dominated by As with strongly correlated Hg, Tl, and Cu, all greater than Au; lesser and variable Sb; and minor to undetected Ag, Bi, Ti, W, and Te. Ore fluids also variably dissolved calcite and dolomite and replaced carbonate minerals with jasperoid or produced minor pore space. Ore pyrite and jasperoid after calcite are enclosed in illite that replaced dickite, clinochlore, and tremolite and filled minor open space, primarily between ~227° and 277°C for most analyzed samples based on illite chemistry. As hydrothermal collapse began, ore pyrite chemistry transitioned to an evolved ore-stage chemistry with abundant As, low to undetected Au, lesser Pb + Cu > Sb, and diminished Hg and Tl compared to the ore stage. Evolved ore-stage fluids coincidently precipitated sulfosalt minerals—initially, unusual Cu- and Hg-rich aktashite (Cu6Hg3As4S12) and Tl- and Hg-rich christite (TlHgAsS3) and, later, realgar as Cu and Zn in hydrothermal fluids—and chalcopyrite and sphalerite in the host rocks, respectively, were consumed. Calcite-only veins indicate termination of the hydrothermal system. Small fragmented clasts of realgar with chemically distinct Au-bearing, evolved ore-stage pyrite rims found in brecciated rock along a faulted dike-Wenban contact provide the first evidence in a Carlin-type gold deposit of precipitation of an Au-bearing pyrite rim on a late ore-stage mineral. These rare rims likely formed in response to fault movement along the dike-Wenban contact or by dike injection that allowed deep, Au-bearing fluids to access a higher and cooler part of the system where late ore-stage minerals had begun to form. These rims further highlight the normal textures in Carlin-type gold deposits where ore-stage pyrite was overgrown by late ore-stage sulfosalt minerals, demonstrating a single hydrothermal event followed by a single cooling event. This paragenesis is consistent with previously modeled deposit formation time frames of ~45,000 to 15,000 years. Two styles of breccia were identified within the deposit. Replacement, matrix-supported breccia, in which an illite-rich, punky matrix encloses Au-bearing pyrite, jasperoid, and variable relict calcite, formed during intense ore-stage alteration and mineralization. The fine-grained ore and alteration replacement minerals did not require significant open space to form. Later evolved and late ore-stage sulfosalt minerals and calcite that are visible at the hand-sample scale provide the earliest evidence for the development of significant open space. Hydrothermal collapse allowed the incursion of cooler meteoric water into the system, which—along with the retrograde solubility of calcite—provided coupled mechanisms driving significant calcite dissolution, collapse brecciation, and formation of the Cortez Hills breccia zone. Dissolution and collapse brecciation increasingly concentrated ore-stage pyrite as the hydrothermal system transitioned to the late ore stage, contributing to high Au grades.

Global Se and Te systematics in hydrothermal pyrite from different ore deposits: a review

Pyrite is one of the most important constituents in hydrothermal base and precious metal ores of Carlin-type, epithermal-porphyry and orogenic Au deposits. The trace elements Se and Te are enriched...

STABLE ISOTOPE (δ13C, δ18O) ANALYSIS OF SULFIDE-BEARING CARBONATE SAMPLES USING LASER ABSORPTION SPECTROMETRY

Hydrothermally altered country rocks surrounding hydrothermal ore deposits are commonly characterized by altered δ 18 O and δ 13 C values relative to distal, unaltered rocks. Stable isotopes have been recognized as a powerful tool to complement traditional geochemical and geophysical approaches during mineral exploration, particularly in carbonate replacement and Carlin-type deposits. However, wall-rock alteration and carbonate veining during ore formation is often accompanied by the precipitation of sulfide minerals, complicating stable isotope analysis. The classic phosphoric acid extraction technique for carbonate minerals results in the release of additional H 2 S gas, interfering with isotope ratio mass spectrometry (IRMS), and laser absorption spectrometry (LAS) analysis of CO 2 . Here we present the first results from a novel off-axis integrated cavity output spectrometer (OA-ICOS) instrument, capable of determining the stable isotope composition ( δ 13 C, δ 18 O) of mixed CO 2 -H 2 S gas evolved from carbonate-sulfide mixtures. Instrument calibration was performed by analyzing in-house and commercially available CaCO 3 stable isotope reference materials. Analyses of mixed CO 2 -H 2 S tank gas and sulfide-spiked stable isotope reference materials containing up to 40 wt % chalcopyrite and sphalerite demonstrate that the method outlined here is capable of effectively analyzing previously problematic high sulfide samples. The analytical capabilities, together with the robust nature of the OA-ICOS instrument, may be of direct practical use for mineral exploration in carbonate-hosted hydrothermal systems including ore deposits.

Gold mineralization in the Jiangnan Orogenic Belt of South China: Geological, geochemical and geochronological characteristics, ore deposit-type and geodynamic setting

Located in the southeastern margin of the Yangtze Block and generally interpreted as the Neoproterozoic collisional product of the Yangtze with the Cathaysia Blocks of South China, the Jiangnan Orogenic Belt (JOB) contains a number of gold (Au) (-polymetallic) ore deposits and mineral showings, mostly hosted by Neoproterozoic low-grade metamorphic volcaniclastic and sedimentary rocks. The mineralization styles mainly include auriferous quartz veins and disseminated mineralization in altered mylonite and cataclasite that are developed along shear zones, fracture zones and inter- or intra-formational fault zones closely related to regional folding and shearing deformation. Three gold mineralizing epochs are recognized in the JOB. The ca. 423–397Ma mineralization was associated with the early Paleozoic tectonothermal event(s), which induced widespread emplacement of Silurian S-type granites, low-grade metamorphism and enrichment of gold in the Neoproterozoic rocks (i.e., forming Au source beds). The second Au mineralization epoch, occurring at ca. 176–170Ma (Jurassic), was related to the subduction of the Paleo-Pacific plate beneath the South China continental margin. The third and most important Au mineralization epoch took place at ca. 144–130Ma (early Cretaceous), when a Basin-and-Range tectonic pattern was developed, characterized by NE–NNE-trending strike-slip faults, granitic domes and metamorphic core complexes (MCC), and basins filled with red bed lithologies. C, H, O, He-Ar, S and Pb isotopic and fluid-inclusion data suggest that the ore fluids were predominantly metamorphic and/or magmatic, with variable input of mantle-derived fluids and the progressive involvement of meteoric waters in the later stages of mineralization. Ore materials were mostly contributed by the Neoproterozoic source beds, plus a possible contribution from mantle- or magma-derived components. The Au (-polymetallic) deposits in the JOB, particularly those formed in the early Cretaceous, share many geological and geochemical features with the orogenic-type and Carlin-type deposits. In the context of tectonic evolution of South China, the gold mineralization in the JOB may be considered an “intracontinental reactivation type”, characterized by synchronous development of Au–polymetallic mineralization, reactivation of stuctures developed in Neoproterozoic metamorphic rocks, and widespread granite emplacement in the late Mesozoic.

In situ SIMS U-Pb dating of hydrothermal rutile: reliable age for the Zhesang Carlin-type gold deposit in the golden triangle region, SW China

The contiguous region between Guangxi, Guizhou, and Yunnan, commonly referred to as the Golden Triangle region in SW China, hosts many Carlin-type gold deposits. Previously, the ages of the gold mineralization in this region have not been well constrained due to the lack of suitable minerals for radiometric dating. This paper reports the first SIMS U–Pb age of hydrothermal rutile crystals for the Zhesang Carlin-type gold deposit in the region. The hydrothermal U-bearing rutile associated with gold-bearing sulfides in the deposit yields an U-Pb age of 213.6 ± 5.4 Ma, which is within the range of the previously reported arsenopyrite Re–Os isochron ages (204 ± 19 to 235 ± 33 Ma) for three other Carlin-type gold deposits in the region. Our new and more precise rutile U-Pb age confirms that the gold mineralization was contemporaneous with the Triassic W–Sn mineralization and associated granitic magmatism in the surrounding regions. Based on the temporal correlation, we postulate that coeval granitic plutons may be present at greater depths in the Golden Triangle region and that the formation of the Carlin-type gold deposits is probably linked to the coeval granitic magmatism in the region. This study clearly demonstrates that in situ rutile U–Pb dating is a robust tool for the geochronogical study of hydrothermal deposits that contain hydrothermal rutile.

Geology, geochronology, geochemistry and ore genesis of the Wangu gold deposit in northeastern Hunan Province, Jiangnan Orogen, South China

The Wangu gold deposit in northeastern Hunan, South China, is one of many structurally controlled gold deposits in the Jiangnan Orogen. The host rocks (slates of the Lengjiaxi Group) are of Neoproterozoic age, but the area is characterized by a number of Late Jurassic–Cretaceous granites and NE-trending faults. The timing of mineralization, tectonic setting and ore genesis of this deposit and many similar deposits in the Jiangnan Orogen are not well understood. The orebodies in the Wangu deposit include quartz veins and altered slates and breccias, and are controlled by WNW-trending faults. The principal ore minerals are arsenopyrite and pyrite, and the major gangue minerals are quartz and calcite. Alteration is developed around the auriferous veins, including silicification, pyritic, arsenopyritic and carbonate alterations. Field work and thin section observations indicate that the hydrothermal processes related to the Wangu gold mineralization can be divided into five stages: 1) quartz, 2) scheelite–quartz, 3) arsenopyrite–pyrite–quartz, 4) poly-sulfides–quartz, and, 5) quartz–calcite. The Lianyunshan S-type granite, which is in an emplacement contact with the NE-trending Changsha-Pingjiang fracture zone, has a zircon LA-ICPMS U–Pb age of 142±2Ma. The Dayan gold occurrence in the Changsha-Pingjiang fracture zone, which shares similar mineral assemblages with the Wangu deposit, is crosscut by a silicified rock that contains muscovite with a ca. 130Ma 40Ar–39Ar age. The gold mineralization age of the Wangu deposit is thus confined between 142Ma and 130Ma. This age of mineralization suggests that the deposit was formed simultaneously with or subsequently to the development of NE-trending extensional faults, the emplacement of Late Jurassic–Cretaceous granites and the formation of Cretaceous basins filled with red-bed clastic rocks in northeastern Hunan, which forms part of the Basin and Range-like province in South China. EMPA analysis shows that the average As content in arsenopyrite is 28.7 atom %, and the mineralization temperature of the arsenopyrite–pyrite–quartz stage is estimated to be 245±20°C from arsenopyrite thermometry. The high but variable Au/As molar ratios (>0.02) of pyrite suggest that there are nanoparticles of native Au in the sulfides. An integration of S–Pb–H–O–He–Ar isotope systematics suggests that the ore fluids are mainly metamorphic fluids originated from host rocks, possibly driven by hydraulic potential gradient created by reactivation of the WNW-trending faults initially formed in Paleozoic, with possible involvement of magmatic and mantle components channeled through regional fault networks. The Wangu gold deposit shares many geological and geochemical similarities as well as differences with typical orogenic, epithermal and Carlin-type gold deposits, and may be better classified as an “intracontinental reactivation” type as proposed for many other gold deposits in the Jiangnan Orogen.

No genetic link between Late Cretaceous felsic dikes and Carlin-type Au deposits in the Youjiang basin, Southwest China

The Youjiang basin in southwest China is distinguished by preserving a number of Carlin-type gold deposits. Tens of felsic dikes occur in this region, and some are present within the ore fields. A genetic link between these felsic dikes and ore-formation has previously been proposed, but detailed field investigation shows that the dikes crosscut the main ore body, and clearly postdate gold mineralization. Three felsic dikes in the Youjiang basin have been dated, and SIMS zircon U-Pb dating results show that the Liaotun and Xiabaha dikes were emplaced at 97.2±1.1Ma (MSWD=2.9) and 95.4±2.4Ma (MSWD=0.56), respectively, broadly consistent with a LA-ICP-MS zircon U-Pb age for the Bama dike (99.4±0.37Ma, MSWD=1.4). Combined with published dates of the other felsic dikes in this region, it is suggested that the felsic magmatic event in the Youjiang basin occurred between ∼100 and 95Ma. Although a wide range of ages for gold mineralization (275–46Ma) has been reported by a variety of methods, the best available ages suggest a timing between 235 and 193 (Direct Re-Os dating on Fe-sulfides; Chen et al., 2015, Re–Os isochron ages for arsenopyrite from Carlin-like gold deposits in the Yunnan–Guizhou–Guangxi “golden triangle”, southwestern China), significantly older than the emplacement age of the felsic dikes. Taken together, we conclude that there is no genetic link between these Late Cretaceous felsic dikes and Carlin-type Au mineralization in the Youjiang basin.