Carlin-type Deposits Research Articles

Source and Evolution of the Ore-Forming Fluids of Carlin-Type Gold Deposit in the Youjiang Basin, South China: Evidences from Solute Data of Fluid Inclusion Extracts

The source and evolution of ore-forming fluids is important to understand the genesis of Carlin-type gold deposit. Constraints on the source and evolution of ore fluid components by the conventional geochemical methods have long been a challenge due to the very fine-grained nature and complex textures of hydrothermal minerals in these deposits. In this study, we present the crush-leach analyzed solute data of fluid inclusion extracts within quartz, calcite, realgar, and fluorite from the Shuiyindong, Nibao, and Yata gold deposits in the Youjiang Basin, providing new insights into the source and evolution of ore-forming fluids. The results show that the high molar Cl/Br ratios up to 2 508 in fluid inclusion extracts are indicative of a contribution of magmatic hydrothermal fluids. Fluids mixing between basinal and magmatic-hydrothermal fluids are evident on the plots of Cl/Br versus Na/K ratios, showing that ore-stage milky quartz near the magmatic-hydrothermal fluids reflects magma origin of the ore-forming fluids, whereas late ore-stage drusy quartz and realgar near the defined basinal fluids suggest the later input of basinal fluids in late-ore stage. Although the predominately host rocks in Shuiyindong, Nibao and Yata gold deposit are bioclastic limestone, sedimentary tuff, and calcareous siltstone, respectively, the solute data of fluid inclusion extracts records they underwent the similar fluid-rocks reactions between the Na-rich magmatic hydrothermal fluids and the Ca- and Mg-rich host rocks. This study highlights the solute data of fluid inclusion extracts obtained by crush-leach analyses have the potential to fingerprint the source and evolution of ore-forming fluids of the Carlin-type gold deposit.

The geophysical response of the Goldrush-Fourmile orebody and implications for camp-scale Carlin-type deposit exploration, Cortez District, Nevada

Since discovery of the Goldrush and Fourmile deposits, numerous geophysical surveys have been acquired over the footprint of mineralization and surrounding areas to focus exploration. The Goldrush-Fourmile system extends more than 7 km in strike length and averages approximately 300 m wide. Most of the orebody lies more than 300 m below ground surface and continues to depths of more than 900 m. Direct detection of the ore system using geophysical tools is impeded by several factors. The system is relatively flat lying and sits in the hinge of a doubly plunging anticline. This gives the shallowest zones a very small lateral footprint. The ore consists of thin strata-bound zones of silica-sulfide mineralization with a tight alteration selvage. Structural controls along faults are limited, with only small displacements. Strong petrophysical contrasts exist between unaltered rock units. Finally, the terrain is rugged. Resistivity inversions of airborne electromagnetic data show that mineralization sits within a complex zone of resistivity responses. Close to intrusions, the host stratigraphy is resistive. Away from intrusions, the same stratigraphic units show highly variable but commonly very low resistivities. This suggests the possibility of redistribution of carbon around intrusions during premineralization metasomatism. Within the orebody, sulfide content increases conductivity within individual formations related to the distribution of fine-grained sooty pyrite. The geometry of the Red Hill Anticline is imaged as a positive density contrast in the observed gravity and geologically constrained gravity inversions. The positive density contrast represents the gross geophysical signature of denser carbonates within the Paleozoic stratigraphy. Magnetic surveys are crucial for mapping the distribution of igneous rocks and potentially hornfelsed sedimentary rocks. The challenges associated with exploring for deeply buried Carlin mineralization notwithstanding, the application of innovative geophysical tools tuned to assess specific geologic questions, combined with best-practice geologic and geochemical modeling, is helping drive exploration for additional Carlin-type mineralization across the Cortez District.

Sm–Nd Isochron Age Constraints of Au and Sb Mineralization in Southwestern Guizhou Province, China

Southwestern Guizhou, China, is famous for hosting clusters of Carlin-type Au, Sb, and Hg-Tl deposits. These deposits are thought to be the products of a low-temperature hydrothermal metallogenic event. Calcite and fluorite are common and widespread gangue minerals in Au and Sb deposits, respectively. Ore-related calcite commonly coexists with stibnite, realgar, and orpiment at the periphery of high-grade orebodies in Au deposits, while ore-related fluorite is generally intergrown with stibnite in Sb deposits. In this study, ore-related calcite and fluorite samples from representative Au (Zimudang) and Sb (Dachang) deposits, respectively, were separated, and the rare earth element (REE) concentrations, Sm/Nd isotope ratios, and Sm–Nd isochron ages were analyzed. This study aims to determine the formation ages of the calcite and fluorite and to constrain the age of low-temperature metallogenic event in Southwestern Guizhou. The calcite and fluorite samples contain relatively high total concentrations of REEs (8.21–22.5 μg/g for calcite, 21.7–36.6 μg/g for fluorite), exhibit variable Sm/Nd ratios (0.51–1.01 for calcite, 0.35–0.49 for fluorite), and yield Sm–Nd isochron ages of 148.4 ± 4.8 and 141 ± 20 Ma, respectively. These ages are consistent with the age range constrained by the low-temperature thermochronology of zircon (132–160 Ma), crosscutting relationships of stratigraphy or intrusions (96–160 Ma), and previous dating results (135–150 Ma) in Southwestern Guizhou. Collectively, the ages obtained in this study add new evidence to previous geochronology studies, such that the low-temperature hydrothermal mineralization in Southwestern Guizhou can be constrained to 135–150 Ma, corresponding to the Yanshanian orogeny, which was associated with a weak extensional tectonic environment.

EOCENE MAGMA PLUMBING SYSTEM BENEATH CORTEZ HILLS CARLIN-TYPE GOLD DEPOSIT, NEVADA: IS THERE A DEEP-SEATED PLUTON?

Abstract The magmatic-hydrothermal conceptual model for Carlin-type gold deposit genesis calls upon deep-seated Eocene plutons as the primary source of gold-bearing fluids. However, geophysical surveys, geologic mapping, drilling, geochronology, isotopic tracers, and fluid inclusion chemistry have returned ambiguous evidence for the existence of such plutons. The high-grade Cortez Hills gold deposit in northern Nevada hosts shallow, Eocene syn- and postmineralization intrusions, offering an ideal site to investigate the existence of a deep-seated pluton beneath the district. Here, major and trace element analyses of quartz-hosted melt inclusions from four Eocene rhyolite dikes cropping out within the Cortez Hills pit and results from independent thermobarometers provide a window into the subsurface Eocene magmatic plumbing system to test the existence of a deep-seated source pluton. Dissolved volatile contents, melt inclusion entrapment pressures, and thermodynamic phase equilibria indicate that dike magmas were sourced from ~4- to ≥9-km depth from a polybaric magma reservoir residing as a physically and geochemically interconnected crystal mush with extractable or eruptible magma pockets. Magmas ascended adiabatically (nearly isothermally), exsolving fluids, evolving modestly by fractional crystallization, while trapping quartz-hosted melt inclusions steadily from depth to subvolcanic levels where they were emplaced. These data represent the first unequivocal evidence for a deep-seated magma reservoir from which fluid-saturated magma emanated and released magmatic fluids beneath the Cortez district during gold mineralization. However, further investigation into the specific metallogenic potential and metal budget of parental magmas and the partitioning of gold between silicate melt and aqueous fluids will be necessary to provide evidence that exsolved magmatic fluids may have been gold bearing.

Tennantite–Tetrahedrite-Series Minerals and Related Pyrite in the Nibao Carlin-Type Gold Deposit, Guizhou, SW China

A number of sediment-hosted, Carlin-type/-like gold deposits are distributed in the Youjiang basin of SW China. The gold ores are characterized by high As, Hg, and Sb contents but with low base metal contents (Cu+Pb+Zn < 500–1000 ppm). The Nibao deposit is unique among these gold deposits by having tennantite–tetrahedrite-series minerals in its ores. The deposit is also unique in being primarily hosted in the relatively unreactive siliceous pyroclastic rocks, unlike classic Carlin-type gold deposits that are hosted in carbonates or calcareous clastic rocks. In this study, we have identified tennantite-(Zn), tennantite-(Hg), and tetrahedrite-(Zn) from the tennantite–tetrahedrite-series mineral assemblage. The tennantite-(Zn) can be further divided into two sub-types of Tn-(Zn)-I; and Tn-(Zn)-II;. Tn-(Zn)-I; usually occurs in the core of a Tennantite–tetrahedrite composite and appears the darkest under the SEM image, whereas Tn-(Zn)-II overgrows on Tn-(Zn)-I and is overgrown by tetrahedrite-(Zn). Tennantite-(Hg) occasionally occurs as inclusions near the uneven boundary between Tn-(Zn)-I and Tn-(Zn)-II. An appreciable amount of Au (up to 3540 ppm) resides in the tennantite–tetrahedrite-series minerals, indicating that the latter is a major Au host at Nibao. The coexistence of tennantite–tetrahedrite-series minerals and Au-bearing pyrite indicates the Nibao ore fluids were more oxidized than the Carlin-type ore fluids. The tennantite–tetrahedrite series at Nibao evolved from Tn-(Zn)-I through Tn-(Zn)-II to tetrahedrite-(Zn), which is likely caused by Sb accumulation in the ore fluids. This indicates that the Nibao ore fluids may have become more reduced and less acidic during Au precipitation.

Quantitative Mineral Mapping of Drill Core Surfaces I: A Method for µXRF Mineral Calculation and Mapping of Hydrothermally Altered, Fine-Grained Sedimentary Rocks from a Carlin-Type Gold Deposit

Abstract Mineral distributions can be determined in drill core samples from a Carlin-type gold deposit, using micro-X-ray fluorescence (µXRF) raster data. Micro-XRF data were collected using a Bruker Tornado µXRF scanner on split drill core samples (~25 × 8 cm) with data collected at a spatial resolution of ~100 µm. Bruker AMICS software was used to identify mineral species from µXRF raster data, which revealed that many individual sample spots were mineral mixtures due to the fine-grained nature of the samples. In order to estimate the mineral abundances in each pixel, we used a linear programming (LP) approach on quantified µXRF data. Quantification of µXRF spectra was completed using a fundamental parameters (FP) standardless approach. Results of the FP method compared to standardized wavelength dispersive spectrometry (WDS)-XRF of the same samples showed that the FP method for quantification of µXRF spectra was precise (R2 values of 0.98–0.97) although the FP method gave a slight overestimate of Fe and K and an underestimate of Mg abundance. Accuracy of the quantified µXRF chemistry results was further improved by using the WDS-XRF data as a calibration correction before calculating mineralogy using LP. The LP mineral abundance predictions were compared to Rietveld refinement results using X-ray diffraction (XRD) patterns collected from powders of the same drill core samples. The root mean square error (RMSE) for LP-predicted mineralogy compared to quantitative XRD results ranges from 0.91 to 7.15% for quartz, potassium feldspar, pyrite, kaolinite, calcite, dolomite, and illite. The approaches outlined here demonstrates that µXRF maps can be used to determine mineralogy, mineral abundances, and mineralogical textures not visible with the naked eye from fine-grained sedimentary rocks associated with Carlin-type Au deposits. This approach is transferable to any ore deposit, but particularly useful in sedimentary-hosted ore deposits where ore and gangue minerals are often fine grained and difficult to distinguish in hand specimen.

Genetic relationship between hydrocarbon system evolution and Carlin-type gold mineralization: Insights from Re[sbnd]Os pyrobitumen and pyrite geochronology in the Nanpanjiang Basin, South China

The spatial association of hydrocarbons with metalliferous ore deposits is found worldwide and is particularly common to Carlin-type gold systems. Both liquid oil and pyrobitumen are found in Carlin-type gold deposits of North Nevada, USA and the Nanpanjiang Basin, South China. However, the temporal and genetic association of hydrocarbons and gold mineralization are still debated. To this end, using rhenium‑osmium (ReOs) geochronology of pyrobitumen and gold-bearing pyrite from the Laizishan and Banqi reservoirs and the Yata Carlin-type gold deposit in the Nanpanjiang Basin, we consider hydrocarbons played a critical role in the mineralization process.A ReOs age of 228 ± 16 Ma obtained for highly mature pyrobitumen suggests that liquid oil cracking occurred during the Late Triassic in the Laizishan and Banqi reservoirs. This age is in agreement with the modelled thermal history of the Nanpanjiang Basin. Additionally, a broadly identical gold-bearing pyrite ReOs age of 218 ± 25 Ma from Yata Carlin-type gold deposit which is in agreement with ages reported for other Carlin-type gold deposits in the Nanpanjiang Basin (e.g., in-situ SIMS UPb rutile = 213.6 ± 5.4 Ma, ReOs arsenopyrite = 204 ± 19 Ma - 235 ± 33 Ma and RbSr illite = 212.8 ± 4.6 Ma) suggests the auriferous Carlin-type systems of the Nanpanjiang Basin also formed during the Late Triassic. Integrating our ReOs data, with recent liquid hydrocarbon experimental data and fluid inclusion data from both reservoirs and gold deposits within the Nanpanjiang Basin, a methane (CH4) dominated thermochemical sulfate reduction (TSR) process, which introduced hydrogen sulfide (H2S) into basinal fluid and ultimately led to the deposition of gold-bearing pyrite by sulfidation, is considered to be the genetic link between of pyrobitumen and gold-bearing pyrite mineralization of the Carlin-type systems of the Nanpanjiang Basin.

Realization and application of geological cloud platform

ABSTRACT In recent years, with the progress of computer technology, some traditional industries such as geology are facing changes in industrial structure and application mode. So we try to apply big data and virtualization technology in the field of geoscience. This study aims at addressing the existing problems in geological applications, such as data sharing, data processing and computing performance. A Geological Cloud Platform has been designed and realized preliminarily with big data and virtualization technology. The application of the Geological Cloud Platform can be divided into two parts: 1) to nest the geological computing model in cloud platform and visualize the results and 2) to use relevant software to conduct data analysis and processing in virtual machines of Windows or Linux system. Finally, we prospect Carlin-type deposits in Nevada by using the spatial data model ArcSDM in the virtual machine.

Geology and in-situ sulfur and lead isotope analyses of the Jinlongshan Carlin-type gold deposit in the Southern Qinling Orogen, China: Implications for metal sources and ore genesis

The Jinlongshan gold deposit, Shaanxi Province, central China, is a large deposit (>150 t Au) in the Southern Qinling Orogen. Gold mineralization is hosted in Upper Devonian to Lower Carboniferous carbonaceous clastic-carbonate sequences. Four stages of pyrite and paragenetic sulfides have been identified based on morphology and paragenesis. Laser-ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA–MC–ICP–MS) analyses were conducted to determine the chemical and S–Pb isotopic composition of the sulfides. Pre-ore stage pyrites (Py1 and Py2) contain relatively high concentrations of Mn, Co, Ni, Cu, Zn, Sb, and Pb exhibiting a wide range of δ34S values, ranging from –29.9‰ to +48.4‰. These two pyrites were formed by bacterial and thermochemical sulfate reduction of Upper Devonian marine sulfate, respectively. Main-ore stage pyrite (Py3) and arsenopyrite (Apy) contain high concentrations of Au, As, Cu, Sb, Hg, and Tl with restricted ranges of δ34S values from 9.6‰ to 15.3‰, which is interpreted as resulting from the underlying low-grade metamorphic Paleozoic sedimentary rocks by the decomposition of pyrite and/or inorganic reduction of sulfate. Late- or post-ore stage pyrite (Py4) is characterized by low concentrations of Au, Ag, Sn, Hg, and Bi and exhibited variable δ34S values, ranging from –4.4‰ to +27.7‰, that may reflect the mixing of metamorphic fluids with meteoric water. Similarly, the Pb isotope composition of ore-stage sulfides resembles those of underlying low-grade metamorphic Paleozoic sedimentary rocks whereas differs from Precambrian metamorphic basement rocks and granitoids around the basin, indicating that the metals of the Jinlongshan gold deposit were sourced from the underlying Paleozoic sedimentary rocks.Detailed fieldwork revealed that the deposit has mainly undergone four deformation events. The ore bodies developed in NE-trending dextral oblique-slip reverse faults of D3 that crosscut WNW-trending shear zones of D2. The timing and corresponding stress regimes of the D3 event suggest that it formed in a local extensional environment of Yanshanian intracontinental orogeny of Qinling orogenic belt. Gold metallogenesis of the Jinlongshan gold deposit occurred during the transition of tectonic setting from compression to extension, which reactivated the shear zones and migrated deep ore-bearing fluids to NE-trending brittle faults. The geological and geochemical features of the Jinlongshan gold deposit are more analogous to the Carlin-type deposits than other orogenic gold deposits worldwide; therefore, we propose it is a typical Carlin-type gold deposit in South Qinling Orogen.

Mapping gold pathfinder metal ratios in Northern Nevada, USA: A compositional analysis approach

The use of pathfinder metals has been employed with relative success, to some extent, since low concentrations, weathering, deposit locations, among other factors, makes pinpointing targets harder. Classical methods, using multivariate statistics are limited and do not yield good results on harder-to-reach deposits, making the proper usage of the available information of utmost importance. In this work, compositional analysis is used to identify pathfinder elements for Carlin-type gold deposits more efficiently. Compositional geostatistics is then applied to obtain a variogram model and to krige the balance scores, producing heat maps showing possible gold prospects. The area of study focused on the northern part of the state of Nevada, which houses several Carlin-type gold districts. More specifically in the northern Carlin trend, where important mining complexes such as Meikle, Carlin and Bootstrap are located. The data was treated with the “compositions”, “Robcompositions” and “selbal” packages for R® and the basic steps were: 1. Exploratory analysis of the data; 2. Calculation of compositions; 3. Principal compositional component analysis (CoDaPCA) and Optimal balances analysis; 4. Selection of sub compositions; 5. Modeling the variogram; 6. Kriging; 7. Mapping the interpolation of compositions represented by the chosen balance score. The results have shown the importance of the balance (Sb)/(Ba, K, Na) as a gold pathfinder. The interpolation shows that strengthened signals from the balance are spatially close to the Carlin-type gold mining districts in both Area B and C. The methodology showed the potential to be used as part of a larger prospection study not just for gold, but for other commodities as well.

Recognition of two contrasting structural- and mineralogical-gold mineral systems in the Youjiang basin, China-Vietnam: Orogenic gold in the south and Carlin-type in the north

The gold deposits in the Youjiang basin, totaling ​> ​25 Moz gold, have traditionally been thought to be of Carlin-type, particularly those with extensional structural geometries in the northern basin dominated by platform sedimentary sequences. However, the structural geometries, mineralization styles and alteration types for the Jinya, Gaolong and Nakuang gold deposits in the south-central part of the basin are remarkably similar to those of unequivocal orogenic gold deposits. Structural studies show that gold mineralization in the three gold deposits was controlled by tight “locked-up” anticlines with NW−SE- to E−W-trending and/or concomitant thrusts and/or shear zones, which resulted from NE−SW- to N–S-trending compression or transpression following the Early Triassic closure of the Paleo-Tethyan Ocean. Alteration zones in these deposits are dominated by silicification (quartz), sericitization, sulfidation and carbonation. Zoned pyrites in these deposits comprise Au-poor cores and invisible Au-bearing rims with minor external free gold. Euhedral to subhedral auriferous arsenopyrites also contribute to the gold budget. These features indicate that the three gold deposits are sediment-hosted orogenic gold deposits that contrast markedly with the Carlin-type gold deposits in the northern part of the Youjiang basin in terms of structural geometry and timing, mineralization style and nature of associated alteration. Although additional reliable ages using robust methodologies are still required, the older isotopic ages of the gold deposits in the south-central Youjiang basin are also consistent with earlier formation during transpression that predated extension during orogenic collapse, the period of formation of the Carlin-type gold deposits in the northern Youjiang basin.

Process Mineralogical Study of the Arsenical Zone from a Carlin-type Gold Deposit

In this study, mineralogical and microanalysis characterization, as well as diagnostic leaching and cyanidation tests, were performed to gain a better understanding of the refractory nature of the arsenical zone of a Carlin-type gold deposit and also to evaluate possible treatment routes to maximize gold recovery. The results obtained showed that only 56% of the ore’s gold content is amenable to direct cyanidation. The majority of the remaining gold was found as submicroscopic gold and in association with sulfide minerals, such as orpiment, pyrite, and sphalerite. It was also found that despite the abundance of silicate and carbonate minerals in the arsenical zone, they do not play a key role in the refractory nature of the ore, and gold can be liberated from such minerals by appropriate grinding. The analyses further revealed that orpiment dissolves readily during cyanidation, and this dissolution retards the kinetics of gold extraction. Pyrite and sphalerite were generally found to occur in fine network texture and in association with quartz. Flotation of such auriferous sulfides may be difficult due to their oxidized surface. Fine grinding (−20 μm) was found to be partially effective in improving gold extraction.

Late Cretaceous-Cenozoic exhumation of Northwestern Guangxi (China) and tectonic implications: Evidence from apatite fission track dating

The study of the Late Cretaceous-Cenozoic uplift and tectonic evolution of the Northwestern Guangxi area in the Youjiang Basin is relevant for understanding the origin and preservation of hydrothermal deposits with ore-hosting potential. Apatite fission-track dating and time-temperature thermal history modelling were performed using six rock samples from the study area. The ages were found to range from 14 to 81 Ma, and in particular, three distinct time intervals could be defined using the apatite fission-track ages: 81–77 Ma, 58–35 Ma, and 17–14 Ma, each corresponding to a specific tectonic-thermal event. The 81–77 Ma event indicates the age of early magmatic activity in northwestern Guangxi. The earliest Himalayan tectonic phase could be attributed to the subduction–collision of the Indo-Asian plate at 58–35 Ma, followed by a second phase of rapid wedging of the Indian plate during the 17–14 Ma interval. We surmise that some hydrothermal deposits, including medium-temperature Cu, Pb, Zn deposits and high-temperature W, Sn, Mo, Bi deposits such as polymetallic Cu–Fe–Mo, and Cu–Mo–(Pb–Zn) porphyry types and skarn types, may be locally present below the low-temperature Carlin-type gold deposits, or at variable depths in northwestern Guangxi. And sand gold deposits may be a relevant prospecting target.

Multiple element mapping and in-situ S isotopes of Au-carrying pyrite of Shuiyindong gold deposit, southwestern China using NanoSIMS: Constraints on Au sources, ore fluids, and mineralization processes

This study employed high-sensitivity and high-resolution secondary ionizing mass spectrometry (NanoSIMS) to determine the distribution of Au, As, Cu, Se, and S isotopes of pyrite from the Shuiyindong Au deposit, southwestern China. NanoSIMS maps show that Au occurs as solid solution, and heterogeneously distributes at micron scale in the ore pyrite, which is the overgrowth of the preexisting synsedimentary or hydrothermal pyrite core, commonly rimmed again by the As- and Au-poor outermost zone. There is no detectable Au in the synsedimentary pyrite. Invisible Au was observed to fill in the submicron-scale cracks of the ore pyrite. Meanwhile, element line profile analysis shows that Au occurs in three discrete climaxes in the ore pyrite. In-situ S isotopic analyses identified two types of S sources. The synsedimentary pyrites have large negative δ34S values ranging from −38.3‰ to −18.8‰, which is a typical biogenetic S generated by bacterial reduction of sea water sulfate. The hydrothermal pyrites with zoned texture have a narrow δ34S range of −3.5‰～−7.4‰ with a median of 2.3‰, similar to the recently published sulfur isotope data of the Southwest China’s Carlin-type gold deposits except for the Lannigou Mine. Combining the distribution of Au and S isotopes in pyrite indicates that the Au-carrying zones have lower δ34S values than that of the ore-unrelated zones. These NanoSIMS observations indicate that the Au mineralization was closely associated with strong deformation of the host rock. The Au forming the deposit was exotic, instead of the reactivated Au in the preexisting pyrite formed in the synsedimentary basin. The mineralization progress was multistage. The distribution of Au and S isotopes is hard to interpret by the viewpoint of metamorphic fluid or fluid pressure fluctuation. Instead, it is more consistent with magmatic fluid, likely contaminated by sedimentary sulfur during its ascending to the surface. In addition, the multistage ore formation pattern discovered in this study poses new challenges to determine the mineralization age of the Au deposits in southwestern China.

Fluid properties and origins of the Lannigou Carlin-type gold deposit, SW China: Evidence from SHRIMP oxygen isotopes and LA-ICP-MS trace element compositions of hydrothermal quartz

Oxygen isotope and trace elements characteristics of ore-related quartz from Lannigou gold deposit, a typical fault-control second largest Carlin-type Au deposit in SW China, provide valuable information about the properties and origins of the ore-forming fluids. Previous works has focused on vein quartz instead of ore-related jasperoid-like quartz grain due to hard separation. Based upon different cathodoluminescence (CL) textural patterns and intensities observed within the quartz, four generations of quartz from this deposit were identified: Quartz I is sedimentary detrital quartz; Quartz II is syn-mineralization jasperoid-like quartz intergrown with and/or containing fine (<10 μm) As-bearing pyrite and arsenopyrite; Quartz III is late-mineralization veinlet quartz with a small of medium-coarse-grained (about 50–100 μm) As-bearing pyrite; and Quartz IV is the post-mineralization drusy thick-veined quartz. Analyses by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) were used to determine O isotope and trace elements compositions of quartz formed in different stages. The trace elemental (e.g., Al, Li, Ge, Ti) variation of quartz from Quartz II to IV reflect the effects of fluid saturation in CO2 as the result of decarbonization reactions, followed by a progressive fall in CO2, whereas the fluid pH and intensity of argillic alteration increase, progressively. The range of δ18OVSMOW values in Quartz II (12.1–24.8‰) is wide but narrows in Quartz III (24.1–27.8‰) and IV (24.3–26.9‰). The estimated fluid δ18O values of early- or main-stage (calculated from Quartz II) is 3.2–15.9‰, the late-stage (calculated from Quartz III) is 12.5–16.2‰, and the post-stage (calculated from Quartz IV) is 9.0–11.5‰. It is suggested that the ore-forming fluids of Lannigou gold deposit reflect a mixed source: a low O isotope end-member that was magmatic- or basement related, whereas the high O isotope end-member may have been fluid interacted with wall-rock, whose influence strengthened during the mineralization episode. The narrow δ18OVSMOW ranges of Quartz III and IV also suggest that any meteoric water influence on the late hydrothermal system was weak.

Identification and application of a Carlin-type gold mineral system for exploration targeting in the Takab geothermal basin, NW Iran

In this research, a mineral system approach applied in a GIS environment to predict favorable gold potential areas in a district to deposit scale at the north Takab geothermal basin, NW Iran. In a context of mineral system, initially the world class Zarshuran gold deposit studied and modeled to identify important conceptual controls on gold mineralization. Conceptual controls such as sources of heat, metals and mineralizing hydrothermal fluids, active fluid flow pathways, lithological and geochemical accumulation environments were identified in a deposit scale and converted to mappable spatial controls or targeting elements for district-scale gold exploration. Intermediate magma and metamorphosed ophiolites respectively considered as the main heat and gold sources. Major particular faults and their intersections with the axis of anticlines, strongly brecciated dacite lithic tuff and its contact with the favorable host lithologies presented as the most important indicators of active fluid flow pathways. The intersection of the main faults and anticline boundaries were determined as an important factor in fluid accumulation and gold mineralization. Silicified carbonate and black shale considered as the favorable chemical scrubber or reactive host. Dome-like structure such as Iman Khan anticline, formed through uplifting, was introduced as an important physical throttle. Finally, by introducing a mineral system model for the Zarshuran Carlin-type gold deposit, these critical processes converted to mappable gold exploration features by using the district-scale geological map and remotely- sensed hydrothermal alterations of the area in a GIS environment. Based on the proposed gold mineral system, the identified exploration features ranked by appropriate weighting for targeting gold mineralization. A final gold potential map of the area was created by using weighted index overlay data integration modeling to locate high gold potential areas for future exploration in the surrounding areas of the Zarshuran deposit.

The state of Au and As in pyrite studied by X-ray absorption spectroscopy of natural minerals and synthetic phases

In many hydrothermal deposits arsenian pyrite contains economic concentrations of Au in an “invisible” form, which is structurally bound in pyrite or has grain size <0.1 μm. Here we use X-ray absorption spectroscopy (XAS) of natural minerals and synthetic phases to reveal the forms of Au occurrence in pyrite and determine the effect of physicochemical-compositional parameters on the concentration and state of “invisible” Au. A sample of natural arsenian pyrite with 300 ppm Au and 0.34 wt% As (average value) from the Samolazovskoe Au-sulfide deposit (Yakutia, Russia) was used to study the states of Au and As, and two arsenian pyrite samples from the Vorontsovka Carlin-type deposit (North Urals, Russia) were used to characterize the state of As. The synthesis experiments were performed employing the hydrothermal method at 300 °C/Psat and 450 °C/1000 bar, at contrasting redox states – in oxidized (H2S/H2SO4 redox buffer) and reduced (H2S predominates) fluids, in As-free and As-bearing systems. As a result, a series of samples of As-free and arsenian pyrites was obtained with Au concentrations from ca. 100 to 300 ppm. The concentration of “invisible” Au, which was homogeneously distributed within the samples, was independent of As concentration but decreased with increasing synthesis temperature. The concentration of Au dissolved in acidic sulfide hydrothermal fluid was not affected by the presence of As. The X-ray absorption XANES/EXAFS spectra were recorded simultaneously at Au L3-edge and As K-edge. The use of the high energy resolution fluorescence detection (HERFD-XAS) mode made possible to acquire data for the trace amounts of Au in As-rich samples. According to XANES spectroscopy Au occurs in pyrite in the 1 + oxidation state. Two forms of Au were detected in the pyrite samples: the solid solution Au and Au2S-like clusters/inclusions. In the solid solution state Au replaces Fe in the structure of pyrite and is coordinated only with sulfur atoms (NS = 6, RAu-S = 2.41 ± 0.01 Å) in all pyrite samples independently of the conditions of their formation/synthesis and As content, no As atoms were detected in the local atomic environment of Au. Thus, the Au-S distance in the 1st coordination shell increased by 0.15 Å relatively to the Fe-S distance in pure pyrite. The distant coordination shells of the solid solution Au correspond to the pyrite crystal structure. The presence of the Au2S-like clusters leads to a significant decrease of the calculated values of coordination number of Au and Au-S interatomic distance. In natural pyrite samples from the Vorontsovka deposit As1− replaces S in the anionic sites, increasing the 1st shell As-S and As-Fe distances by 0.07–0.1 Å relatively to the pure pyrite structure. A contribution of As3+ oxide was detected in pyrite from the Vorontsovka deposit. The synthetic pyrite samples contain As in the form of As1− solid solution and As3+ and As5+ oxides. Results of our study show that neither the concentration and speciation of As nor the redox state of the system affect the state of “invisible” Au in the studied pyrites.

NOBLE GASES FINGERPRINT THE SOURCE AND EVOLUTION OF ORE-FORMING FLUIDS OF CARLIN-TYPE GOLD DEPOSITS IN THE GOLDEN TRIANGLE, SOUTH CHINA

Abstract Precise constraints on the source and evolution of ore-forming fluids of Carlin-type gold deposits in the Golden Triangle (south China) are of critical importance for a better understanding of the ore genesis and a refined genetic model for gold mineralization. However, constraints on the source of ore fluid components have long been a challenge due to the very fine grained nature of the ore and gangue minerals in the deposits. Here we present He, Ne, and Ar isotope data of fluid inclusion extracts from a variety of ore and gangue minerals (arsenian pyrite, realgar, quartz, calcite, and fluorite) representing the main and late ore stages of three well-characterized major gold deposits (Shuiyindong, Nibao, and Yata) to provide significant new insights into the source and evolution of ore-forming fluids of this important gold province. Measured He isotopes have R/RA ratios ranging from 0.01 to 0.4 that suggest a maximum of 5% mantle helium with an R/RA of 8. The Ne and Ar isotope compositions are broadly comparable to air-saturated water, with a few analyses indicating the presence of an external fluid containing nucleogenic 38Ar and radiogenic 40Ar. Plotted on the 20Ne/4He vs. helium R/RA and 3He/20Ne vs. 4He/20Ne diagrams, the results define two distinct arrays that emanate from a common sedimentary pore fluid or deeply sourced metamorphic fluid end-member containing crustal He. The main ore-stage fluids are interpreted as a mixture of magmatic fluid containing mantle He and sedimentary pore fluid or deeply sourced metamorphic fluid with predominantly crustal He, whereas the late ore-stage fluids are a mixture of sedimentary pore fluid or deeply sourced metamorphic fluid bearing crustal He and shallow meteoric groundwater containing atmospheric He. Results presented here, when combined with independent evidence, support a magmatic origin for the ore-forming fluids. The ascending magmatic fluid mixed with sedimentary pore fluid or deeply sourced metamorphic fluid in the ore stage and subsequently mixed with the meteoric groundwater in the late ore stage, eventually producing the Carlin-type gold deposits in the Golden Triangle.

Геофизическая модель земной коры, геодинамические обстановки и перспективы открытия месторождений золота карлинского типа в арктической зоне Республики Саха (Якутия)

Геофизическая модель земной коры, геодинамические обстановки и перспективы открытия месторождений золота карлинского типа в арктической зоне Республики Саха (Якутия)

Characteristics of Supergene Gold of Karst Cavities of the Khokhoy Gold Ore Field (Aldan Shield, East Russia)

Typomorphic features of supergene gold in karst cavities were studied in the recently discovered Au–Te–Sb–Tl deposit within the Khokhoy gold ore field of the Aldan-Stanovoy auriferous province (Aldan shield, East Russia). Two morphological types of supergene gold, massive and porous, are recognized there. The first type is represented by gold crystals and irregular mass, with the fineness ranging from 835 to 1000‰. They are closely associated with goethite, siderite, unnamed Fe, Te, and Tl carbonates, Tl tellurites/tellurates and antimonates, as well as avicennite with a Te impurity. The second type is represented by mustard gold of two types with different internal structure: microporous and dendritic. The supergene gold is characterized by persistently high fineness. Along with Ag, it invariably contains Hg (up to 5.78 wt%) and Bi, and, rarely, Pb, Cu, and Fe. The supergene gold is chemically homogeneous, and its particles are all marked by high fineness, without any rims or margins. The obtained characteristics made it possible to prove the existence of two genetic types of supergene gold. Mustard microporous gold is the result of the decomposition of the associated minerals—goethite, Tl oxides, tellurium, Fe, Mn and Tl carbonates and antimonates, containing microinclusions of gold. Massive gold and dendrites are newly formed. The decomposition, remobilization, and reprecipitation of residual gold nanoparticles and their aggregation led to the formation of dendrites, and with further crystal growth and filling of pores, to gold of massive morphology. In terms of morphology, internal structure, fineness, and trace element composition, supergene gold of the Khokhoy gold ore field is comparable to gold from the Kuranakh deposit (Russia) and the Carlin-type gold deposits. It also is similar to spungy and mustard gold from Au–Te and Au–Sb deposits, weathering crusts, and placers. Its main characteristic feature is a close paragenesis with Tl minerals.