Carlin Gold Research Articles

Mineral Assemblages and Ore-Forming Physicochemical Conditions of the La’erma and Qiongmo Au–Se Deposits in the Western Qinling Orogen, Central China

Mineral Assemblages and Ore-Forming Physicochemical Conditions of the La’erma and Qiongmo Au–Se Deposits in the Western Qinling Orogen, Central China

Mineral Phase Evolution during Oxidation Roasting Pretreatment of Typical Carlin Gold Ore and Effects on Gold Leaching Efficiency

Arsenious and sulphur-bearing micro-disseminated gold ore is a kind of typical refractory Carlin-Type. The gold in Carlin-Type gold ore grains is distributed finely, existing as invisible or submicroscopic gold, encapsulated in arsenopyrite and pyrite. The technical difficulty of treatment Carlin-Type gold ore lies in how to release the fine gold wrapped in pyrite and arsenopyrite. In this study, the oxidation roasting pre-treatment technique was used to treat the Carlin-Type gold ore. This included a two-stage roasting process: the arsenic was removed in the first roasting process, and the sulphur was removed in the second roasting process. The thermodynamic of the roasting process was analyzed, and the mineral phase evolution of the roasting process was investigated by using XRD, SEM and EDS. Finally, the influence of sodium cyanide dosage and leaching time on leaching efficiency was investigated. The results suggest that for the first roasting temperature at 550 °C, and the second roasting at temperature 700 °C with air flow 2.5 L/min, the sodium cyanide dosage is 1.75 kg/t and leaching time is 27 h; a good leaching efficiency is obtained with 83.85%.

Karst Hydrogeochemical Characteristics and Controlling Factors of Carlin-type Gold Mining Area Based on Hydrochemistry and Sulfur Isotope

Mining activities change the groundwater level and flow conditions through pumping and drainage, which enhances the interaction between groundwater and aquifer rocks; mine drainage is discharged into the surface water system, which affects the whole karst water hydrogeochemical process. Based on hydrogeochemistry and the δ34S isotope, the hydrogeochemical processes, characteristics, and main controlling factors for waste water, karst groundwater, and surface water in a typical Carlin gold mining area and its surrounding areas were revealed. The results showed that:chemical compositions of groundwater and surface water unaffected by gold mining activities were mainly controlled by the weathering of limestone and dolomitic limestone; Ca2+, Mg2+, and HCO3- were main ions; and the water chemical types were Ca-HCO3. The mine wastewater and its downstream receiving water were affected by the dissolution of carbonate and silicate minerals, and cation exchange also played a role; the main ions were Ca2+, Mg2+, Na+, and SO42-, and the hydrochemical type gradually evolved from Ca-HCO3 to Ca-SO4. SO42- was the characteristic component in various water bodies affected by mining, and the concentration of SO42- gradually decreased from top to bottom in the well. The values of δ34S for unaffected groundwater and surface water were positive, and SO42- was mainly derived from realgar oxidation. Conversely, mine wastewater and downstream water were negative, SO42- was mainly influenced by the mixing action of realgar oxidation and meteoric precipitation, and pyrite also contributed to a certain extent. At the same time, NO3- came from agricultural fertilizer and rural domestic sewage discharge directly. Principal component analysis (PCA) further demonstrated:sulfide mineral oxidation and mining activities were the main controlling factors for the water chemical composition of mine wastewater and downstream water, whereas unaffected groundwater and surface water were mainly influenced by water-rock (carbonate rock) interactions. Agricultural fertilizer and rural sewage discharge also had a certain influence. Therefore, the study area should strengthen the interception of surface water, control-block-management of sulfide oxidation, rural domestic sewage treatment, and agricultural fertilizer.

Mineral Phase Evolution During Oxidation Roasting Pretreatment of Typical Carlin Gold Ore and Effects on Gold Leaching Efficiency

Mineral Phase Evolution During Oxidation Roasting Pretreatment of Typical Carlin Gold Ore and Effects on Gold Leaching Efficiency

Extraction of gold from typical Carlin gold concentrate by pressure oxidation pretreatment - Sodium jarosite decomposition and polysulfide leaching

Most of the gold is encapsulated in sulfides and arsenopyrite in Carlin type gold concentrate; the gold leaching efficiency is less than 8% without pretreatment. Cyanide process is a traditional gold leaching method with the disadvantages of several environmental pollution incidents occurred due the breach of tailings disposal. In view of the difficulties in gold extraction and serious pollution incidents of typical Carlin type gold concentrate, this paper proposes pressurized oxidation - sodium jarosite decomposition - polysulfide leaching method to recover gold from typical Carlin type gold concentrate. Standard characterization/analytical methods of XRD, ICP, MLA, XPS, and SEM-EDS were employed in the experiment. In the pressurized oxidation pretreatment process with high temperature and ferric sulfate concentration, jarosite minerals with stable properties were easily formed due to the appropriate amounts of K+ introduced in the flotation of the ore and a small amount of mica minerals containing Na+. The results show that pressurized oxidation pretreatment can efficiently liberate gold from the mineral, but causes secondary inclusion of sodium jarosite. The one-step sodium jarosite decomposition treatment of the residue produced by oxygen pressure leach, can effectively liberate gold from the sodium jarosite pHase. Thus, the influence of factors on extracting gold from sodium jarosite decomposition residue using polysulfide were systematically studied. Under the most suitable conditions, the leaching efficiency of gold increased up to 90.2%, which was 8.4% higher than that of the direct cyanidation of oxygen pressure leach residue. This study provides an efficient, clean, and environmentally friendly way for the resource utilization of refractory gold concentrate, with a potential industrial application.

Seismic‐impedance inversion with fuzzy clustering constraints: an example from the Carlin Gold District, Nevada, USA

ABSTRACTThe seismic reflection method provides high‐resolution data that are especially useful for discovering mineral deposits under deep cover. A hindrance to the wider adoption of the seismic reflection method in mineral exploration is that the data are often interpreted differently and independently of other geophysical data unless common earth models are used to link the methods during geological interpretation. Model‐based inversion of post‐stack seismic data allows rock units with common petrophysical properties to be identified and permits increased bandwidth to enhance the spatial resolution of the acoustic‐impedance model. However, as seismic reflection data are naturally bandlimited, any inversion scheme depends upon an initial model, and must deal with non‐unique solutions for the inversion. Both issues can be largely overcome by using constraints and integrating prior information. We exploit the abilities of fuzzy c‐means clustering to constrain and to include prior information in the inversion. The use of a clustering constraint for petrophysical values pushes the inversion process to select models that are primarily composed of several discrete rock units and the fuzzy c‐means algorithm allows some properties to overlap by varying degrees. Imposing the fuzzy clustering techniques in the inversion process allows solutions that are similar to the natural geologic patterns that often have a few rock units represented by distinct combinations of petrophysical characteristics. Our tests on synthetic models, with clear and distinct boundaries, show that our methodology effectively recovers the true model. Accurate model recovery can be obtained even when the data are highly contaminated by random noise, where the initial model is homogeneous, or there is minimal prior petrophysical information available. We demonstrate the abilities of fuzzy c‐means clustering to constrain and to include prior information in the acoustic‐impedance inversion of a challenging magnetotelluric/seismic data set from the Carlin Gold District, USA. Using fuzzy c‐means guided inversion of magnetotelluric data to create a starting model for acoustic‐impedance proved important in obtaining the best result. Our inversion results correlate with borehole data and provided a better basis for geological interpretation than the seismic reflection images alone. Low values of the acoustic impedance in the basement rocks were shown to be prospective by geochemical analysis of rock cores, as would be predicted for later gold mineralization in weak, decalcified rocks.

Geostatistical estimation analysis of typical Carlin gold deposit in Hunan province, China

In this paper, 3D mining modelling software was used to construct the 3D geological model of Typical Carlin gold deposit in Hunan province of China, and the geostatistics theories are also involved for the resource estimation which offer references for additional exploration and mining plans. Access database was constructed for gold deposit, topo DTM, mineralised body models and grade models are constructed by Surpac, and also analysis of the spatial grade distribution in reality was completed with statistic softwares. By using of geostatistics, assay samples are composited and analysed statistically and outliers influence was reduced to certail level. Experimental Variation functions were constructed for the strike, dip and vertical directions. Certain attributes are estimated by Ordinary Kriging estimation method. Comparing to the traditional estimation methods, estimation under 3D block model can obtain more reliable analysis results, which have significant reference values for dynamic management of mine operation.

Alteration and mineralization styles of the orogenic disseminated Zhenyuan gold deposit, southeastern Tibet: Contrast with carlin gold deposit

Orogenic disseminated and Carlin gold deposits share much similarity in alteration and mineralization. The disseminated orogenic Zhenyuan Au deposit along the Ailaoshan shear zone, southeastern Tibet, was selected to clarify their difference. The alteration and mineralization from the different lithologies, including meta-quartz sandstone, carbonaceous slate, meta-(ultra)mafic rock, quartz porphyry and lamprophyre were researched. According to the mineral assemblage and replacement relationship in all types of host rocks, two reactions show general control on gold deposition: (1) replacement of earlier magnetite by pyrite and carbonaceous material; (2) alteration of biotite and phlogopite phenocrysts in quartz porphyry and lamprophyre into dolomite/ankerite and sericite. Despite the lamprophyre is volumetrically minor and much less fractured than other host rocks, it contains a large portion of Au reserve, indicating that the chemically active lithology has played a more important role in gold precipitation compared to structure. LA-ICP-MS analysis shows that Au mainly occurs as invisible gold in fine-grained pyrite disseminated in the host rocks, with Au content reaching to 258.95 ppm. The diagenetic core of pyrite in meta-quartz sandstone enriched in Co, Ni, Mo, Ag and Hg is wrapped by hydrothermal pyrite enriched in Cu, As, Sb, Au, Tl, Pb and Bi.Different host rock lithology has much impact on the alteration and mineralization features. Carbonate and sericite in altered lamprophyre show they have higher Mg than those developed in other of host rocks denoting that the carbonate and sericite incorporated Mg from phlogopite phenocrysts in the primary lamprophyre during alteration. The ore fluid activated the diagenetic pyrite in meta-quartz sandstone leading the hydrothermal pyrite enriched in Cu, Mo, Ag, Sb, Te, Hg, Tl, Pb and Bi, but the hydrothermal pyrite in meta-(ultra)mafic rock is enriched in Co and Ni as the meta-(ultra)mafic rock host rock contain high content of Co and Ni. However, Au and As shear similar range in both types of host rocks indicating that these two elements most likely come from the deep source fluid rather than the host rocks. It was shown in the disseminated orogenic gold deposit that similar hydrothermal alteration with mineral assemblage of carbonate (mainly dolomite and ankerite), sericite, pyrite and arsenopyrite develops in all types of host rocks. This is different from the Nevada Carlin type, in which alteration is mainly dissolution and silicification of carbonate host rock. On the other hand, Au mainly occur as invisible gold in both disseminated orogenic and Carlin gold deposits.

Semiautomatic and Automatic Cooperative Inversion of Seismic and Magnetotelluric Data

Natural source electromagnetic methods have the potential to recover rock property distributions from the surface to great depths. Unfortunately, results in complex 3D geo-electrical settings can be disappointing, especially where significant near-surface conductivity variations exist. In such settings, unconstrained inversion of magnetotelluric data is inexorably non-unique. We believe that: (1) correctly introduced information from seismic reflection can substantially improve MT inversion, (2) a cooperative inversion approach can be automated, and (3) massively parallel computing can make such a process viable. Nine inversion strategies including baseline unconstrained inversion and new automated/semiautomated cooperative inversion approaches are applied to industry-scale co-located 3D seismic and magnetotelluric data sets. These data sets were acquired in one of the Carlin gold deposit districts in north-central Nevada, USA. In our approach, seismic information feeds directly into the creation of sets of prior conductivity model and covariance coefficient distributions. We demonstrate how statistical analysis of the distribution of selected seismic attributes can be used to automatically extract subvolumes that form the framework for prior model 3D conductivity distribution. Our cooperative inversion strategies result in detailed subsurface conductivity distributions that are consistent with seismic, electrical logs and geochemical analysis of cores. Such 3D conductivity distributions would be expected to provide clues to 3D velocity structures that could feed back into full seismic inversion for an iterative practical and truly cooperative inversion process. We anticipate that, with the aid of parallel computing, cooperative inversion of seismic and magnetotelluric data can be fully automated, and we hold confidence that significant and practical advances in this direction have been accomplished.

Decratonic gold deposits: a new concept and new opportunities

Early Cretaceous gold deposits in the North China Craton and the famous Carlin-type gold deposits in Nevada are given the name ‘decratonic gold deposits’ [1]. This is the first new type of ore deposit proposed in the last 20 years. This new type of deposit introduces a concept that may change perspectives and provide fresh opportunities for gold deposit research and exploration. Cratons are stable ancient crustal domains characterized by thick lithospheric mantle roots (>150 km), weak tectonism, and little or no magmatic activity within their boundaries. Since their stabilization, the formation of endogenic mineral deposits in such ancient cratonic blocks is very rare owing to a lack of significant magmatism, deformation, or generation of hydrothermal fluids. Several cratons, however, including the North China, the North American and the Brazilian cratons, have experienced destruction to differing degrees [1]. The eastern part of the North China Craton was extensively destructed in the Early Cretaceous [2]. Meanwhile, gold deposits with a total reserve of 4000– 5000 tons formed there within a few millionyears (130–120Ma), coevalwith craton destruction [1]. Similarly, the western part of the North American Craton was destabilized in theCenozoic and that destructionmay be responsible for the famous Nevada Carlin-type gold deposits that host a total of over 6000 tons of gold reserves [1,3]. It is suggested that all these deposits nowbe termed ‘decratonic type’ gold deposits, deposits formed by magma-derived fluids under extensional tectonics conditions associated with craton destruction [1]. The model further postulates that the westward subduction of the western Pacific Plate in the Early Cretaceous and the eastward subduction the eastern Pacific Plate in the Cenozoic were responsible for these twomajor gold mineralization events. Dehydration of the subducted and stagnant slabs led to continuous hydration andmetasomatism of the overlying mantle wedges and the enrichment of gold and other chalcophile elements. Subsequent partial melting of the mantle wedge produced ore-forming fluids (Fig. 1) [1]. Gold deposits in the North China Craton have been classified into two types: ‘altered-rock’ type and ‘quartzvein’ type. The altered-rock-type gold deposits are characterized by disseminated and stockwork goldmineralization, which is usually controlled by brittle or ductile to brittle fault zones. These types of deposits have commonly been intensively overprinted by hydrothermal alteration and suffered later brittle deformation. The quartz-vein-type gold deposits are hosted in the Archean amphibolite facies rocks or earlier granitoids and typically have large proportions of visible native gold. In contrast, the Carlin-type gold deposits in Nevada typically contain invisible gold hosted in fine-grained sulfides disseminated in Paleozoic marine sedimentary rocks.The North China Craton and Carlin gold deposits are seemingly entirely different, but both types are genetically related to craton destruction. Carlin-type gold deposits are abundant in Nevada but have not been found in the North China Craton. The decratonic gold deposit model ascribes the contrast in mineralization styles to host rocks, controlling structures, and ore-forming depths [1]. Decratonic gold deposits in both the North China Craton andNevada formed within a relatively short time interval (<10Ma). In both areas, the deposits become progressively younger toward the subduction zones. Such temporal trends may plausibly be explained by subduction retreat or rollback of the subducted slab at rates of 8.8 and 3.3 cm y−1 for the western and eastern Pacific plates, respectively [1]. Craton destruction starts deep in the mantle with mantle-derived melts and fluids, implying that decratonic gold deposits may extend to great depths. This suggests newopportunities for future discoveries. It is proposed that gold mineralization belts extending approximately NNE in the eastern and the western portions of the Eastern Block of the North China Craton are the most favorable areas for future exploration [1]. ‘Decratonic gold deposit’ is a new term. It will inevitably face reactions that will affirm or challenge it, and that may improve or refute it. Major questions are: the (paleo)-Pacific plate has subducted westward underneath the whole of east China followed by slab rollback during three periods since the Jurassic [4,5].Why did goldmineralization in the North China Craton take place mainly within a short period in the Early Cretaceous? Episodes of tectonic extensions in the North China Craton also have lasted for long periods. What made the Early Cretaceous so different? If craton destruction was the controlling factor, then what made craton destruction unique? Alternatively, are craton

Bryophyte Diversity and Their Monitoring for Heavy Metal Pollution from Xinlu Carlin Gold Deposit in Guizhou Province

Twenty-eight bryophyte taxa in 17 genera of 10 families from Xinlu Carlin gold field were investigated.Nine species belong to Pottiaceae and 8 species belong to Bryaceae.Among them,Pottiaceae and Bryaceae are the dominate families.By analyzing β diversity of four sites in this area,we found that β diversity difference between barren rock field-waste residue field and relatively polluted area-clear area is the largest,bryophyte community structure is the biggest difference,and similarity is the lowest.A particular regularity was proved by monitoring 6 kinds of heavy metals Pb,Zn,Cu,Cd,Hg,As of bryophytes and their substrates from different areas in Xinlu Carlin gold deposit,that is waste residue fieldbarren rock fieldrelatively polluted areaclear area.It might be closely related to the mining and smelting.Obviously,further researches on bryophytes can be used to monitor the pollution of heavy metals in gold deposits.

Gold exploration success

Discovery of a goldfield is a rare and difficult event, which contrasts with the normal outcome of exploration that is non-discovery. As there are many gold provinces globally and numerous discoveries annually, the recent and historical records of success are a great source of ideas to learn from and to enhance the rate of discovery.Over the last two decades, important gold discoveries have been made in well-established gold provinces such as Australia, Canada and USA, regions with a long history of production like Ghana, and some further countries such as China, Mali, Tanzania and Peru that received lesser attention during the twentieth century. In contrast to the dominance of gold-only deposits throughout the last millennium of gold production, a significant proportion of recent discoveries are gold-plus, especially Cu–Au.There have been some golden periods of exploration success in various parts of the world and as well as providing inspiration for exploration, they are sources of learning. Examples include the discoveries in Victoria (1850s), in the Witwatersrand (1886) including Carletonville (1930s) and Welkom (1940s), in the Carlin gold province of Nevada (1961 and 1980s), in the Yilgarn Craton (1980–1990s), and SW Pacific (1980s). Any area that is the focus of one of these golden periods attracts increased exploration activity and funding at the expense of less productive areas.There is additional knowledge to glean from non-success. Despite being the world’s major source of gold for the twentieth century, South Africa stands out for its lack of recent exploration success with no new goldfields discovered since Evander in 1951. This lack of exploration success has led to an 80% fall in gold production since the 1970 peak, and one significant consequence is a major decrease in revenue, and hence employment, in that industry.Important ingredients in exploration success are area selection, appropriate exploration technologies and models, and skilled and motivated people. Some commentators have added luck to this list though this may be mis-guided. Instead, the thesis presented here is that doing exploration is like doing science; mineral discovery – like scientific breakthrough – is a rare event, and the way teams think and interact is a very important determinant of success in both mineral exploration and in science. If this thesis is valid, then teaching explorers to think and creating environments in which they can do so should favour discovery. It cannot be taken for granted that systematic thinking processes will be either taught or learned at all institutions, but rather there are places and people who enhance the development of thinking skills.Exploration teams need to safe-guard against groupthink by the involvement of self- and external critical evaluation. Unorthodoxy will always have an important place in exploration while discovery remains a rare event well-removed from the norm; this is parallel to scientific revolutions which often owe their origin to unorthodox thinking and attention to minor anomalies. Introducing simple practices such as a fostering of a single-minded focus on discovery, maintaining a line-of sight from activities back to one’s aims, encouraging unorthodoxy and avoiding the pernicious mentality of groupthink can be adopted at little cost.

Yangshan Gold Deposit: The Largest Carlin and Carlin‐Iike Type Gold Deposit in China

AbstractGold Headquarter of the CAPF has discovered the Yangshan super large‐scale gold deposit in Gansu Province, which is a great breakthrough of gold exploration and prospecting in Western Qinling Mountains of China. The gold resources of this deposit achieved 308 tons with increasing potentials. Preliminary geological investigations indicate that the Yangshan gold deposit is located in the intra‐continental collision orogenic belt; and the deposit was formed during the continent‐continent collision orogenic processes. The geological characteristics of the deposit are similar to that of the typical Carlin‐type gold deposits, while differences still exist. The ore‐forming background is notably different from the Carlin gold deposit province in the United States; and the ore‐forming fluids are similar with that of the orogenic‐type gold deposit. Accordingly, the Yangshan gold deposit is a transitional type between the Carlin‐type and the orogenic type gold deposits. At present, the Yangshan gold deposit is the largest Carlin and Carlin‐like type gold deposit that is ever discovered in China. Researches on metallogeny, metallogenic model and ore‐enrichment regularities of the Yangshan gold deposit are crucial to meet the pressing needs of the current geological investigation and ore exploration of the deposit.

The giant Carlin gold province: a protracted interplay of orogenic, basinal, and hydrothermal processes above a lithospheric boundary

Northern Nevada hosts the only province that contains multiple world-class Carlin-type gold deposits. The first-order control on the uniqueness of this province is its anomalous far back-arc tectonic setting over the rifted North American paleocontinental margin that separates Precambrian from Phanerozoic subcontinental lithospheric mantle. Globally, most other significant gold provinces form in volcanic arcs and accreted terranes proximal to convergent margins. In northern Nevada, periodic reactivation of basement faults along this margin focused and amplified subsequent geological events. Early basement faults localized Devonian synsedimentary extension and normal faulting. These controlled the geometry of the Devonian sedimentary basin architecture and focused the discharge of basinal brines that deposited syngenetic gold along the basin margins. Inversion of these basins and faults during subsequent contraction produced the complex elongate structural culminations that characterize the anomalous mineral deposit “trends.” Subsequently, these features localized repeated episodes of shallow magmatic and hydrothermal activity that also deposited some gold. During a pulse of Eocene extension, these faults focused advection of Carlin-type fluids, which had the opportunity to leach gold from gold-enriched sequences and deposit it in reactive miogeoclinal host rocks below the hydrologic seal at the Roberts Mountain thrust contact. Hence, the vast endowment of the Carlin province resulted from the conjunction of spatially superposed events localized by long-lived basement structures in a highly anomalous tectonic setting, rather than by the sole operation of special magmatic or fluid-related processes. An important indicator of the longevity of this basement control is the superposition of different gold deposit types (e.g., Sedex, porphyry, Carlin-type, epithermal, and hot spring deposits) that formed repeatedly between the Devonian and Miocene time along the trends. Interestingly, the large Cretaceous Alaska–Yukon intrusion-related gold deposits (e.g., Fort Knox) are associated with the northern extension of the same lithospheric margin in the Selwyn basin, which experienced an analogous series of geologic events.

The Tuscarora Au-Ag District: Eocene Volcanic-Hosted Epithermal Deposits in the Carlin Gold Region, Nevada

The Tuscarora Au-Ag District: Eocene Volcanic-Hosted Epithermal Deposits in the Carlin Gold Region, Nevada

Geological-geochemical characteristics of Carlin- and Carlin-like-type gold deposits in South Qinling mountains

In this paper an integrated geological and geochemical comparison is made of Carlintype and Carlin-like-type gold deposits in the region of South Qinling with their associated hydrothermal sedimentary lead-zinc deposits, the Carlin-type deposits in the Yunnan-Guizhou-Guangxi gold triangle, China, the Carlin gold deposits, Carlin, western United States, and the Muruntau-type gold deposits in Uzbekistan. The comparative study indicates that the formation of these deposits has an intimate coupling relation with the mechanism of the tectonic movement of modern plates. Therefore, these deposits belong to those that were formed in the orogenic stage. They have the following similar characteristics: they are all sediment-hosted, occur in a tectonic environment of extensional rifting nature and show the characteristics of syngenetically modified mineralization. On the other hand, they also have noticeable differences: they are different in respect to their tectonic settings, association and structure of orogenic belts and complexity and strength of the modified mineralization. The regional tectonic evolution and extensive mineralization in orogenic belts are different expressions of the same continental dynamic process.

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”.

Carlin gold deposits, Nevada; origin in a deep zone of mixing between normally pressured and overpressured fluids

Gold mineralization at Carlin is clearly younger than hydrocarbon maturation (pre-Cretaceous) and felsic dike intrusion (Cretaceous), and older than deep oxidation (late Tertiary). Within the episode of gold mineralization, the main gold ore (MGO) stage and late gold ore (EGO) stage are distinguished paragenetically, with a variety of vein and mineralization types in each. MGO stage fluids contained 5 to 10 mole percent CO 2 , appreciable H 2 S, and 3 + or - 1 wt percent NaCl equiv. At least portions of MGO stage mineralization were characterized by two-phase boiling (CO 2 exsolution) at 215 degrees + or - 30 degrees C and 800 + or - 400 bars. In contrast, EGO stage fluids were gas poor with salinities 18 O (sub H 2 O) values of 5 to 9 per mil, whereas EGO stage fluids resembled unevolved meteoric water with delta 18 O (sub H 2 O) values 18 O values shifted from near whole-rock values of 12 + or -3 per mil to around 0 + or -1 per mil as EGO stage fluids flooded the system. Jasperoids also record a large range (9-22%) in delta 18 O (sub H 2 O) values. These data indicate the involvement of two very different fluids in ore deposition. Because MGO and EGO stage features are closely associated spatially with each other and with Au, As, Sb, Hg, and other ore elements, both fluids are believed to have both been present during most stages of ore deposition. At pressures of 80 to 85 percent lithostatic, depths of 3.8 + or - 1.9 km are required to accommodate the 800 + or - 400 bars of pressure recorded in MGO stage fluid inclusions. Carlin, therefore, is not an epithermal or hot spring deposit. Carbon dioxide in gas-rich MGO stage fluids may have originated either directly from buried intrusions or their contact aureoles, or from low-grade regional metamorphism at depth. The water may have been originally meteoric, and Au may be magmatic or derived from leaching of deep metamorphic or sedimentary rocks. Ore deposition appears to have occurred in zones of throttling at a pressure seal between normally pressured and overpressured regimes, where fluids experienced a change from near-lithostatic to hydrostatic conditions. Such pressure seals are common in deep sedimentary basins and may be a key to highly localized gold deposition. Mixing of two fluids and interaction with host rocks along thin permeable bioclastic horizons are believed to have been the major factors in depositing ore.

Geology and geochemistry of wall-rock alteration at the Carlin gold deposit, Nevada

The Carlin disseminated gold deposit occurs in an autochthonous sequence of Paleozoic sedimentary rocks exposed in a structural window in the Roberts Mountains thrust in north-central Nevada. The upper 175 m of the Silurian Roberts Mountains Formation hosts the majority of ore at Carlin and is characterized by laminated, fine-grained, calcareous and/or dolomitic argillaceous siltstone with local coarser grained siltstones and 50-cm-thick lenticular interbeds of sand- and granule-sized calcareous bioclastic debris or fossil hash. Detailed studies of drill core and exposures in the East pit of the Carlin mine show that alteration and mineralization are zoned away from crosscutting fault conduits and these more permeable bioclastic beds, indicating that these two features were major inflow zones for hydrothermal fluid.In unoxidized rocks, unaltered calcareous siltstone (1) containing quartz, dolomite, calcite, illite, K feldspar, and pyrite is progressively converted to assemblages of (2) quartz + dolomite + calcite + illite + pyrite, (3) quartz + dolomite + illite-K mica + pyrite, (4) quartz + illite-K mica + pyrite, and (5) quartz + kaolinite-dickite + pyrite adjacent to inflow zones where jasperoids are developed. Gold most consistently enriches the zone of calcite and dolomite removal (3 and 4 above), though it occurs in all zones, locally in high concentrations. This zoned alteration was accomplished by a CO 2 -rich acidic fluid. This acidic alteration enhanced the passage of fluids by extensive carbonate removal to form zones of higher permeability.Oxidation is wholly a supergene effect related to deep weathering, because the oxidation is superimposed on both mineralized and altered rocks with only minor effect on the major element chemistry; it has produced low-temperature goethitic Fe oxides rather than higher temperature hematite and is not spatially related to Au distribution at the mine or on a district scale.Because of extensive carbonate removal leading to local volume reduction through collapse and/or compaction, geochemical effects are examined using ratios to relatively immobile elements such as Al and Ti. Extensive depletion of Ca, Mg, aud CO 2 and introduction of Si, Au, and S have occurred. Potassium is depleted in the conversion of illite to dickite-kaolinite in proximal silicified inflow zones, and Fe enriches some pyritized rock. Carbonate removal and silicification are two separate processes, both of which are spatially associated with mineralization. Mineralized decarbonated rocks and barren footwall rocks commonly are not silicified, and intensely silicified proximal alteration zones are generally low grade.

Thermal infrared exploration in the Carlin trend, northern Nevada

Experimental Thermal Infrared Multispectral Scanner (TIMS) aircraft data have been acquired for the Rodeo Creek NE 7 1/2 minute quadrangle, Eureka County, northern Nevada, covering the Carlin gold mine. A simple model has been developed to extract spectral emissivities for mapping surface lithology and alteration based on the physical properties of geologic materials. Emissivity‐ratio images were prepared that allow generalized lithologic discrimination, identification of areas with high silica content, and the first reported detection of the carbonate secondary reststrahlen feature. Together with thermal‐inertia images, they permit identification of areas of subtle lithologic variation not shown on conventional geologic maps. Emissivity‐ratio spectra extracted from the data compare favorably with modeled laboratory spectra and establish for the first time the link among theory, measured physical properties, and color‐composited TIMS images. Identification of previously unknown silicification in the Tertiary volcanic rocks and mapping of a silicified structure and possible extensions in alluvium indicate that TIMS data can be a valuable tool for detecting subtle alteration associated with mineralization, particularly in less accessible or less well‐known areas.