Hydrothermal Gold Mineralization Research Articles

Occurrence of State of Gold in Crude Oil and Its Economic Significance

Gold and petroleum are also strategic resources of great importance to national security. With the increasing demand for energy, multi-energy cooperative exploration has become an inevitable trend of resource development and utilization. Petroleum and hydrothermal gold deposits may form together, with similar evolutionary trends in their formation, migration, and enrichment. Petroleum reservoirs and gold deposits are closely coupled under certain geological conditions. The solubility of gold in crude oil and its forms of occurrence are important in determining the mechanisms of interaction between gold and petroleum and in facilitating the recovery of gold from gold-bearing petroleum. In this study, the occurrence of gold in crude oil from the Linnan Depression in the Bohai Bay Basin, China, was studied using inductively coupled plasma–mass spectrometry and X-ray photoelectron spectroscopy. Concentrations of gold in crude oil from the Linpan and Shanghe oilfields averaged 44.5 ppb, which is well above the minimum concentration required for hydrothermal gold mineralization. Gold has an affinity with carbon, oxygen, and sulfur, and its concentration in crude oil is positively correlated with total acid and sulfur contents. We speculate that gold may exist in crude oil as complexes with organic acids or thiols, with crude oil thus being a transport medium for gold.

Elemental Gains and Losses during Hydrothermal Alteration in Awak Mas Gold Deposit, Sulawesi Island, Indonesia: Constraints from Balanced Mineral Reactions

Hydrothermal gold mineralization is commonly associated with metasomatic processes resulting from interaction of hostrock with infiltrating hot aqueous fluids. Understanding of the alteration mechanism requires quantification of element changes in altered rock, relative to the unaltered or least-altered rock, representing the protolith. Balanced mineral reactions are used to gain quantitative insight into the alteration process associated with gold mineralization at the Awak Mas deposit. Three representative samples were carefully selected from the least-altered pyllite and the two alteration zones bordering the mineralization. Mineral mode, textural features, and mineral compositions were studied by microscopy and electron microprobe analyzer (EMPA). Quantitative modal analysis was performed with a Quanta 650 F QEMSCAN® system. The hydrothermal alteration sequence around the mineralization starts with the proximal albite–ankerite–pyrite alteration zone via the distal albite–chlorite alteration zone to the least-altered phyllite wall-rock. Balanced mineral reaction calculations were performed to evaluate elemental gains and losses. Most noticeable is the addition of Si, Na and Ca to each alteration zone. This alteration is represented by the almost complete replacement of muscovite by albite. The addition of Fe and S was highest in the albite–ankerite–pyrite alteration zone. Alteration of the least altered phyllite to the albite–chlorite zone involved a mass increase of 14.5% and a neglectable volume increase of 0.6%. The mass and a volume increase from the least altered phyllite to the albite–ankerite–pyrite zone was 40.5% and 0.47%, respectively. The very low volume change during alteration is also corroborated by the textural preservation indicating isovolumetric metasomatic reactions. The replacement of muscovite by albite may have had an important effect on the change of the rock failure mode from ductile to brittle, with consequences for the focusing of fluid flow.

A TRIASSIC OROGENIC GOLD MINERALIZATION EVENT IN THE PALEOPROTEROZOIC METAMORPHIC ROCKS: EVIDENCE FROM TWO TYPES OF RUTILE IN THE BAIYUN GOLD DEPOSIT, LIAODONG PENINSULA, NORTH CHINA CRATON

Abstract Rutile grains often occur in different types of gold deposits, and their U-Pb ages have been widely used to determine the formation time of gold mineralization. However, the origin of rutile grains in the gold deposits remains controversial. In this paper, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of U-Pb ages and trace elements on rutile grains were applied to investigate the metamorphic and hydrothermal processes of the Baiyun gold deposit (70 t Au, avg grade: 3 g/t) in the Liaodong Peninsula in the northeastern part of the North China craton. Rutile grains in the hydrothermal altered gold schist from the Baiyun deposit yielded two group ages of 1924 ± 18 and 237.0 ± 1.8 Ma, respectively. Combined with our systematic U-Pb zircon geochronological results of the ore-hosting schists and post-ore dikes, we suggest these rutile ages record a Paleoproterozoic metamorphic event and a Triassic hydrothermal gold mineralization event, respectively. The metamorphic and hydrothermal rutile grains have no obvious textural differences, but they show distinct trace element contents of Zr, W, Nb, and Ta. Combined with previous published data, we propose that high W (>1,000 ppm) and low Zr (<200 ppm) contents in rutile can be used to distinguish hydrothermal rutile from metamorphic and magmatic rutile. The newly identified ca. 237 Ma hydrothermal event is much older than the ca. 227 to 210 Ma Triassic magmatic rocks in the region, which precludes a temporal and genetic link between the Baiyun gold mineralization and the regional Mesozoic magmatism. Rather, the ca. 237 Ma gold mineralization may be associated with the Triassic orogenic metamorphism, and Baiyun is an orogenic gold deposit. The Triassic gold deposits in the northern margin of the North China craton formed by orogenesis between the Siberian craton and the North China craton. After a hiatus, the large-scale gold deposits formed during the Early Cretaceous in the North China craton due to a westward subduction of the paleo-Pacific plate beneath the craton since the Early Jurassic. Our study highlights that rutile in gold deposits may be inherited from the host rocks and/or formed by hydrothermal fluids. Distinguishing between these two different rutile generations requires a combination of in situ age dating and trace element geochemistry in petrogenetic context.

Ore fluid characteristics and depositional processes of the hydrothermal gold mineralisation at Damang, Southwestern Ghana

Gold mineralisation at Damang in the southwestern part of Ghana occurs in two types of geological settings: (1) as paleoplacer in the Banket Series conglomerates of the Tarkwaian system and (2) in hydrothermal sheeted quartz vein system that crosscuts all the Tarkwaian lithologies including the palaeoplacer units. This paper focused on the second type because not much research has be conducted on the fluid inclusion and stable isotope characteristics of the Damang deposit to reveal the nature of the hydrothermal ore-forming associated the quartz veins and gold deposition. Microthermometry measurements of fluid inclusions in mineralised quartz veins indicated two sets of temperature-salinity variations: a relatively low temperature (142–292 °C) and moderate salinity aqueous fluids that range from 2.4 to 8.9 wt% NaCl equivalent, and a high temperature (210–393.6 °C) with low salinity aqueous-carbonic fluids with the salinity ranging from 1.0 to 2.6 wt% NaCl equivalent. These fluids are inferred to have unmixed from a homogeneous fluid with an average salinity of 3.85 ± 2.42 wt % NaCl equivalent. Laser Raman spectroscopic data on the fluid inclusions confirmed an aqueous-carbonic (H2O–CO2–NaCl) composition of the fluids with minor amounts of H2S, CH4 and SO2 as volatile species. The temperature of gold precipitation based on overlapped temperatures of chlorite, fluid inclusions and stable isotope geothermometers, indicated a mean temperature of 324.7 ± 20 °C and corresponding pressures from 1.1 to 3.5 kbar. The calculated oxygen isotopic composition of the ore fluids, δ18Ofluid using the estimated mineralisation temperature indicated average values of 9.3 ± 1.2‰. This isotopic range and the low salinity of the fluids (<10 wt % NaCl equiv.) are suggestive of metamorphic origin, which may be attributed to devolatisation reactions in the underlying Birimian greenstones. This is consistent with orogenic lode gold mineralisation in many Precambrian greenstone belts worldwide. The close association of gold with sulphides is consistent with gold transport as bisulphide complexes. A combination of factors including sulphidation and phase separation, concomitant with temperature decrease are inferred to be the principal factors that controlled the hydrothermal gold precipitation at Damang.

U-Pb dating reveals multiple Paleoproterozoic orogenic events (Hamersley orogenic cycle) along the southern Pilbara margin (Australia) spanning the onset of atmospheric oxygenation

Abstract The early Paleoproterozoic was a time of unprecedented change in Earth's climate and surface environment. The key to resolving some of the controversies surrounding the timing and causes of these changes lies with supracrustal sequences, such as the 2.45–2.22 Ga Turee Creek Group in the southern Pilbara craton, northwestern Australia. The group preserves a predominantly siliciclastic sequence; however, its precise age, tectonic setting, and postdepositional history are disputed. Although it is interpreted to have been deposited in a foreland basin setting shortly after 2.45 Ga, the oldest well-recognized deformational event, marked by northward folding and thrusting, is the 2.20–2.15 Ga Ophthalmia orogeny. Evidence for a pre-Ophthalmia fabric-forming tectonic event north of the Archean Sylvania Inlier, southeast Pilbara craton, which is marked by tight to isoclinal folding, has been largely overlooked. In this area, we report in situ U-Pb geochronology of authigenic monazite and xenotime in shale with a well-developed tectonic cleavage from the ca. 2.63 Ga Jeerinah Formation. Monazite porphyroblasts, which are locally wrapped by strain fringes aligned in a tectonic cleavage, yielded weighted mean 207Pb/206Pb ages at 2370 ± 11 Ma and 2312 ± 8 Ma, whereas xenotime, which overprints a crenulation cleavage, gave a weighted mean 207Pb/206Pb age of 2291 ± 11 Ma, constraining fabric development to between 2.31 Ga and 2.29 Ga. Our results confirm the existence of a pre-Ophthalmia deformational event in the southeastern Pilbara craton, herein referred to as the Sylvania orogeny, which is part of an ~300 m.y. interval (2.45–2.15 Ga) of northward-directed compression (“Hamersley orogenic cycle”). This orogenic cycle is marked by east-west and northwest-southeast folding, cleavage development, veining, hydrothermal gold mineralization, and isotopic resetting across the southern Pilbara craton. Our results indicate that the syn–Great Oxidation Event Turee Creek Group was deposited in one or more foreland basins after 2.45 Ga. Our results provide a new tectonostratigraphic and geodynamic framework for understanding the timing and origin of geochemical records in a key succession deposited during an interval of global environmental change.

Multivariate Statistical Evaluation of Geochemical Data from Quartz Vein-Associated Gold Mineralisation at the Badukrom Prospect within the Tarkwaian Base

The Kawere conglomerate at the base of the Tarkwaian Group in the Iduapriem Mine appears to show good prospect for hydrothermal gold mineralisation evidenced by major structures mapped from magnetic and Airborne Electromagnetic (AEM) geophysical data. Detailed geological mapping and sampling at Badukrom target which lies within the Kawere rocks confirm the presence of multiple structures as well as quartz veins indicative of potential hydrothermal gold mineralisation. Follow up soil sampling conducted within the area revealed a strong anomalous trend from the southern portion. Two diamond drilled (DD) holes pushed to delineate the extent of the anomalous zones returned narrow veins of significant gold intersections. Twenty-five (25) samples of the Badukrom hydrothermal Au mineralisation intersects were sent for analysis to ascertain the optimum pathfinder element suite for the hydrothermal targets within the Iduapriem concession. A total of fifty-nine elements were analysed via ICP-MS and ICP-OES. Multivariate statistical evaluation was carried out on the results to understand the geochemical associations of Au and the other elements to aid future exploration programmes planned for the hydrothermal targets. Multivariate analysis of the geochemical data using Pearson product-moment of correlation suggests a stronger positive correlation of gold with Iron, Antimony, Sulphur, Chromium, Vanadium respectively. However, the Spearman Rank correlation data indicates that, the optimum pathfinder elements for hydrothermal Au are: As, Sb and Te.&#x0D; &#x0D; Keywords: Tarkwaian Base, Statistical Evaluation, Hydrothermal Mineralisation, Geochemical Data

Genesis of hydrothermal gold mineralization in the Qianhe deposit, central China: Constraints from in situ sulphur isotope and trace elements of pyrite

The Qianhe gold deposit, located at Xiong'ershan in Eastern Qinling, with estimated gold reserves of 22 tons, hosts six orebodies within auriferous quartz veins and hydrothermally‐altered rocks along E–W trending faults. Here we present results from an integrated study involving field investigation, ore petrology, in situ sulphur isotopes, and trace elements analysis of gold‐bearing pyrite with a view to understanding the genesis of the hydrothermal gold mineralization. Three generations of pyrite are identified in the Qianhe Au deposit: Py1 is composed of coarse‐grained, euhedral cubic grains occurring at the margin of stage I quartz + pyrite vein; Py2 is occurs as porous grains coexisting with other sulphide minerals in stage II quartz + polymetallic sulphide veins; and Py3 is mainly composed of anhedral grains in stage III quartz + calcite + fluorite vein. Among the three generations, Py2 has the highest contents of Au (mean = 3.5 ppm), Ag (mean = 46.0 ppm), Zn (mean = 303 ppm), and Cu (mean = 346 ppm). The distribution and concentration of these trace elements are likely to be controlled by micro/nano‐inclusions in pyrite. The Co/Ni ratios range from 0.010 to 296, with most values lying between 0.1 and 10. The δ34S values of the various pyrite generations show a wide range between −17.9‰ and 4.5‰ with obviously higher values in Py3 (2.0‰ to 4.5‰) than those in Py1 (−16.7‰ to −13.5‰) and Py2 (−17.9‰ to −9.0‰). The negative δ34S values in Py1 and Py2 possibly resulted from sulphur isotope fractionation between sulphide and sulphate minerals. As sulphate minerals are absent in stage III, the slightly positive δ34S values in Py3 suggest a mixed source from mantle‐derived material and wall rocks of the Xiong'er Group. We correlate the genesis of gold mineralization in the Qianhe deposit with the extension and lithospheric thinning during Early Cretaceous, when ore‐bearing fluids channelled along fault zones induced hydrothermal alteration and Au mineralization.

Structural Control of Gold Mineralization of the Central Part of the Malouralsky Volcano–Plutonic Belt (the Polar Urals), Based on an Analysis of Multispectral Images from the Landsat 8 Spacecraft

The central part of the Malouralsky volcanic–plutonic belt, which includes the Manyukuyu–Vorchatinsky ore cluster, is analyzed using multispectral imagery from the Landsat-8 Earth remote sounding satellite with the aim of identifying gold deposits and compared with the Toupugol–Khanmeishor ore region. Similar features in the geological structure and location of deposits and ore deposits in the system of morphological structures are found that should help to develop predictive and prospecting indicators (geological and morphological) of gold mineralization. Three structures are distinguished via analysis of satellite images: the monogenic Novogodnenskaya structure (the Toupugol–Khanmeishorsky ore region) formed under the influence of one main geological process and morphostructures Nos. 1 and 2 (the central part of the Malouralsky volcanic–plutonic belt). These two structures are larger and more complex, with long multistage development. Magmatic chambers are located at depths of ~12 and ~16 km, respectively, for morphostructures Nos. 1 and 2; their root (focal) parts extended southward and then southwestward, respectively, during magmatic development. Prospecting for hydrothermal gold mineralization at the north of the eastern slope of the Polar Urals should be done at the periphery of the centers of large complex morphostructures with long and multistage development with signs of telescoping. Mineralization should be localized in areas of concentration of northwestern and/or northwestern lineaments and in regional fault zones that can be hundreds of kilometers long.

Phase equilibria, thermodynamic properties, and solubility of quartz in saline-aqueous-carbonic fluids: Application to orogenic and intrusion-related gold deposits

Hydrothermal fluids in orogenic and intrusion-related (mesothermal) gold deposits are dominated by saline-aqueous-carbonic fluids, commonly represented by the ternary system H2O-NaCl-CO2. The phase equilibria, thermodynamic properties (PVTx, density, compositions of fluid phases) and quartz dissolution-precipitation behavior in the H2O-NaCl-CO2 system, which are still poorly constrained, are of great importance to understanding the process of hydrothermal gold mineralization. Here, we conducted thermodynamic modeling to constrain the fluid properties under single- and two-phase conditions in the H2O-NaCl-CO2 system at temperatures of 300 to 500 °C and pressures of 0.001 to 3.5 kbar. Our results illustrate thermodynamic controls rooted in the equilibria of the H2O-NaCl-CO2 system. Increasing CO2 and/or NaCl contents shift the solvus to higher pressures and temperatures, expanding the pressure–temperature region of L + V immiscibility. Calculated isopleths of CO2 content in coexisting, immiscible vapor and liquid describe the maximum amount of CO2 that may be present in fluid inclusions through the studied P-T-x ranges. Quartz solubility in the H2O-NaCl-CO2 fluids shows strong dependence on temperature, pressure, and CO2 content, with several potential triggers for vein mineral deposition. Specifically, solubility of quartz generally decreases with decreasing temperature, pressure, and increasing CO2 content both in the single- and two-phase fluids, but exhibits retrograde behavior in the L + V field or at the phase-transition boundary. In detail, the dependence of quartz solubility on pressure is weak at low temperatures (300 °C), and becomes progressively stronger at high temperatures (400 °C and 500 °C), vice versa for temperature dependency at different pressures.The present study reviews two mechanisms of fluid immiscibility and constrain the conditions at which distinct fluid inclusion types, bedding-parallel shear veins and fault-related extension veins are formed in mesothermal gold deposits. Thermodynamic modeling of the solvus of the H2O-NaCl-CO2 system is consistent with the hypothesis that decompression is an efficient mechanism driving fluid immiscibility at specific ranges of temperature and composition, producing coexisting liquid- and vapor-phase fluids, both with low density and relatively low content of CO2. For fluids with high CO2 contents (>10 mol. %), cooling at relatively high pressure may also lead to fluid immiscibility, producing a liquid with low CO2 content in equilibrium with a vapor of medium to high CO2 content. Very CO2-rich, or “pure CO2” fluid inclusions (>90 mol. % CO2, type I) cannot be produced by immiscibility under these conditions, but may rather be the result of decrepitation of primary H2O-NaCl-CO2 fluid inclusions. Within the P-T-x ranges studied here, cooling-dominated phase separation accounts for the formation of fluid inclusions with moderate to high CO2 contents (50–80 mol. % CO2, type IIa). Fluid inclusions with low to moderate CO2 contents (5–30 mol. % CO2, type IIb) can represent the original ore-forming fluids, or can be produced by decompression-dominated phase separation. And H2O-NaCl fluid inclusions (type III) generally represent the latest-stage ore-forming fluids, or can be produced by decompression-induced immiscibility at high temperature. In orogenic (mesothermal) gold deposits, decompression-induced quartz precipitation during pressure fluctuation is dominant in bedding-parallel shear veins. Fault-related extension veins are associated with initial decompression-induced quartz precipitation and subsequent cooling-dominated deposition.

Vein Mineralogy and Hydrothermal Gold Mineralization at the Kyaw Soe Thu Prospect in the Wabo Deposit, Mandalay Region, Central Myanmar

The Kyaw Soe Thu Prospect is a part of the Wabo deposit which is situated along Mogok Metamorphic Belt (MMB) in Central Myanmar. Gold bearing veins are hosted in the banded garnet-biotite gneiss, calc-silicate, marble and granitoid of the Wabo area. The granitic rocks intruded into banded garnet-biotite gneiss. The MMB was intensely deformed and it is a highly fractured metamorphic terrain in which metamorphic facies ranges from amphibolite to granulite facies. Gold mineralization at the Kyaw Soe Thu Prospect in the Wabo deposit occurred in the forms of gold-quartz veins. The quartz veins are a few centimeters in width. The color of the quartz vein at the Kyaw Soe Thu Prospect is commonly smoky and milky. The quartz vein exhibits chalcedonic, saccharoidal and granular textures. Three mineralizations were indicated. Stage I is the thin layered crustiform chalcedonic quartz. Stage II is chalcedonic quartz vein and stage III is Ankerite-quartz-sericite vein. Stage II and stage III are gold rich hydrothermal stages, and Ankerite-quartz-sericite veins were formed with a small amount of sericites in the Kyaw Soe Thu Prospect. The Ankerite-quartz-sericite veinlets range from a centimeter up to three centimeter in width within the stage II chalcedonic quartz vein. Textures of mineralized quartz vein samples were petrographically examined. The textures include a feathery texture that is most closely associated with electrum in both stage II chalcedonic quartz veins and stage III Ankerite-quartz-sericite veins. Ankerite varies in color from pink, white to buff. Among them, pink color is more common in the Kyaw Soe Thu Prospect. Ankerite in the Kyaw Soe Thu Prospect exhibits rhombohedral idiomorphic, which exhibits growth patterns. Ore minerals of the veins are electrum, pyrite, galena, and trace amount of very fine-grained chalcopyrite. On the basis of textures of the chalcedonic quartz veins, the Kyaw Soe Thu Prospect is ascribed to be a low-sulfidation epithermal deposit.

Geology and U–Pb geochronology of the Amani Region, southwestern Tanzania

Large geographic regions of the Paleoproterozoic Ubendian Belt in southwestern Tanzania remain relatively understudied, hindering mineral exploration efforts in these remote areas. One example of such an understudied area is the Amani Region, a ~48 km2 remote location in the Upangwa terrane which is characterized by significant alluvial gold endowment. Here, we present the first geochemical and petrographical characterization of lithologies encountered within the Amani Region. We propose original nomenclature for these lithologies and document the occurrence of four distinct lithological groups (Kabelege Group, Ulembo Suite, Rafiki Group and Amani Group). Lithostratigraphic correlations are made between the Kabelege Group and the Ubendian Supergroup. This study also documents a hitherto unrecognised package of greenschist facies metasediments (Amani Group), which seem to be lithologically and temporally equivalent to the Txitonga Group in NW Mozambique. New U–Pb zircon and rutile data collected from metasediments in the Amani Region are presented, which constrain the maximum depositional age of their sedimentary protoliths, thereby providing new time constraints on the tectonic evolution of the SE Ubendian Belt. The U–Pb zircon data substantiate a ~1080-990 Ma regional metamorphic event in the bedrock sources that provided detritus to the Upangwa terrane, likely associated with the Kibaran-Irumide orogeny during the Mesoproterozoic. Our U–Pb rutile and zircon data from the Amani Group metasediments that host auriferous quartz veins, suggest that hydrothermal gold mineralization (responsible for the alluvial gold occurrence) in the Amani Region is younger than ~944 Ma.

Umicore gains Hazari pre-catalysts

The Jiangnan Orogen, located between the Yangtze and Cathaysia Blocks in southern China, hosts significant gold and antimony resources. Long-standing controversies over the number and precise timing of gold, gold-antimony and antimony mineralization event(s), and the genesis of these deposits, limit the understanding and exploration of this giant gold and antimony system. Together with geological evidence, a critical review of the published geochronological data of these deposits suggest that there were two gold events in the Triassic (~235 Ma) and Early Cretaceous (~142–130 Ma), two gold-antimony events in the Early Devonian (~402 Ma) and Late Triassic-Middle Jurassic (~224–163 Ma: possibly equivalent to the two gold events), and one antimony event in the Early Cretaceous (~130 Ma). There are other possible gold events in the Neoproterozoic or at an even older age, and Ordovician to Early Devonian, which are constrained only by limited geological evidence and a few non-robust isotopic ages. The regional distribution of the gold, gold-antimony and antimony districts, and deposits therein, reveal a first-order control on the mineralization by crustal scale faults which acted as ore-forming fluid pathways connected to deep fluid and metal source areas. Second- and third-order faults that are situated along the jogs of the first-order faults, especially fault corridors defined by NNE-NE-trending second-order faults and third-order NW to E-W-trending discontinuous oblique faults, provided favorable locations for Pre-Cretaceous gold-(antimony) mineralization. In addition, NE-trending open anticlines, linked to deep crustal levels by first-order faults, host significant antimony and minor gold mineralization, and the curvilinear Fanjingshan detachment Fault hosts some gold and antimony deposits. Locally, at deposit to orebody-scales, pre-ore barren quartz veins and magmatic dikes were the loci for mineralization in some gold-antimony deposits. The structural geometry of the pre-Cretaceous gold and gold-antimony deposits suggests an Early Devonian orogenic gold-antimony mineralization event during a transpressional tectonic regime related to coeval intracontinental orogeny between the Yangtze and Cathaysian Blocks. In contrast, Triassic orogenic gold and Triassic-Middle Jurassic orogenic gold-antimony events are interpreted to relate to distal effects of the collision between the North and South China blocks. The structurally-contrasting major Early Cretaceous epizonal antimony mineralization event, together with contemporaneous hydrothermal gold mineralization, is interpreted to have been controlled by normal faults and pre-ore open folds in an extensional tectonic regime related to the distal effects of rollback of the paleo-Pacific Plate after ~135 Ma. For future geoscience-based exploration in the Orogen, documentation and interpretation of critical structural geometries of gold, gold-antimony and antimony deposits is a vital step towards successful target generation.

The effects of mineralization on the lognormal distribution and exponential fluctuations of a hydrothermal gold deposit in Jiaodong Peninsula, China

Statistical analysis of the mineralization intensity in the No. І ore body of the Xincheng gold deposit in Jiaodong Peninsula, China, including grade and linear productivity, was used to study the mineralization characteristics of the hydrothermal gold deposit based on a lognormal distribution pattern and exponential fluctuations. The results show that the mineralization intensity of the hydrothermal gold deposit follows a lognormal distribution pattern. The grade and linear productivity in terms of the strike, dip, and thickness aspects are not linear, while the mineralization intensity fluctuations in exponential function were introduced between the peaks and valleys. This means the gold deposition mineralization process in the hydrothermal gold deposit takes place in accord with the mass action law of the first-order chemical reaction. The logarithmic averaging method is proposed to calculate the average grade and linear productivity of hypothermal gold deposits in accordance with the law of mineral fluctuations that are expressed as exponential functions. These research results have both academic and practical significance for further studies on hydrothermal gold mineralization, extra high-grade processing, improving traditional methods of estimation, and ultimately utilization of gold mineral reserves.

Hydrothermal gold mineralization and structural controls near May Hibey, Northwestern Tigray, Northern Ethiopia

Ezana Mining Development Company has recently reported presence of gold mineralization near May Hibey area, western Tigray, northern Ethiopia. It is in association with hydrothermal quartz veins within the Neoproterozoic low grade basement rocks, metavolcanics and metasedimentary rocks. Structural data suggests four phases of deformations and NE-SW trending foliation is the prominent one among others. Among different generations of veins, NNE, NS and NNW trending third generation quartz veins are gold-bearing and presence of a narrow shear zone with dextral sense of movement trending NW-SE is significant interms of mineralization. Hydrothermal alterations are dominated by chloritization and sericitization. Core samples from shear zone within the sericite-quartz schist and quartz porphyry rocks shows presence of pyrite, pyrrhotite, sphalerite, chalcopyrite, galena and gold in decreasing order of abundance. Gold mineralization is closely associated with the sericite and carbonization alteration zone. Based on mineral association, alteration, deformation, texture, and paragenetic scheme, it is suggested that the mineralization is a syn-orogenic variety formed in an island arc tectonic setting.Keywords: Quartz veins, Shear zones, Gold, Petrography, Geochemistry, MayHibey, Ethiopia.

Regional Hydrogeochemical Mapping for Environmental Studies in the Mineralized Lom Basin, East Cameroon: A Pre-industrial Mining Survey

The present study is the first appraisal of regional hydrogeochemical programme for environmental assessment of the mineralized Lom Basin in East Cameroon. Fifty-two streamwater samples were collected and analysed for major cations (Ca2+, Mg2+, Na+, K+) and major anions (HCO3–, F–, Cl–, NO2–, NO3–, Br–, PO43–, SO42–). Calcium and HCO3– were the dominant ions. The chemical facies for the surface water were CaHCO3 and NaHCO3 types indicating a fresh water source. Major ion geochemistry demonstrated the potential to discriminate between natural and anthropogenic origins. Calcium ion, Mg2+, Na+, K+and HCO3– had similar distribution trends reflecting weathering from the parent rocks. Sulphate distribution correlated with the occurrence of sulphide minerals associated with hydrothermal gold mineralization in the area while the distribution patterns of NO3–and Cl– reflected pollution from domestic activities within the municipality. Overall, the chemistry of stream water in the Lom Basin is mainly controlled by silicate weathering with only minor anthropogenic influence. This study forms an analogue for hydrogeochemical mapping in other mineralized terrains in Cameroon.

Chemical dating of zircon from granitic pegmatite of the Shartash Massif (Central Urals)

Using techniques of microprobe analysis and Raman microspectroscopy, a study of the composition, structure, and age of high-uranium zircon from the Sharatash intrusion pegmatite was performed. The presence of a number of zones in crystals having different physicochemical characteristics has been revealed. The Id-e zones affected by a radiation dose of (30–130) × 1018 α-dec/g were completely amorphized, and their chemical isochron age was estimated at 311.4 ± 7.1 Ma: this is interpreted as the time of zircon formation in the pegmatite. The age of uraninite coupled with zircon is 317.1 ± 2.3 Ma. These dating results clarify the formation time of the final phase (pegmatite) of the Shartash intrusion and determine the upper age limit of hydrothermal gold mineralization (Berezovsk deposit) associated with the granite massif.

A review of the Birimian Supergroup- and Tarkwaian Group-hosted gold deposits of Ghana

Ghana is the largest producer of gold in West Africa, a region with over 2,500 years of history with regards to gold production and trade. Modern exploration for and mining of gold in Ghana dates from 1874 with the establishment of the British Gold Coast Colony, which was followed in 1957 by the independence of Ghana and increased gold production since the early 1980s through Ghana's Economic Recovery Plan. At the time of writing, gold production (108.2 tonnes or 3.48 million ounces [Moz] in 2014) accounted for approximately one-third of Ghana's export revenues, with 36% of gold production coming from small-scale mining. The majority of the gold occurs in two styles of mineralisation, namely mesothermal quartz vein-hosted and associated gold in metavolcanics and metasediments and modified palaeoplacer gold in conglomerates. These styles of mineralisation occur in the Palaeoproterozoic Birimian Supergroup and Tarkwaian Group that make up Ghana's mainly southwest to northeast trending Birimian belts. Significant gold resources also occur as hydrothermal mineralisation in basement-type granitoids which show some geological association with the Birimian Supergroup-hosted mesothermal mineralisation. The majority of the gold mineralisation is believed to have formed between approximately 2.15 and 2.06 Ga during the Eburnean orogeny. The mesothermal quartz vein gold mineralisation is usually confined to tectonic corridors within the Birimian belts and is strongly associated with shear zones and fault systems. The quartz veins show multiple stages of formation and are steeply dipping, with the gold mineralisation occurring either as free gold within fractures in the veins or as invisible gold within disseminated sulphides in the host rocks surrounding the veins. The vein- and sulphide-hosted gold is strongly associated with deformational fabrics formed by the Eburnean extensional and compressional events, respectively, suggesting that disseminated sulphide mineralisation predates quartz vein-hosted mineralisation. The fluid from which the gold precipitated is believed to have been of metamorphic origin and carbon dioxide (CO 2 ) dominated, with lesser water (H 2 O) and nitrogen (N 2 ) and minor methane (CH 4 ). Gold precipitation was probably caused by decrease in pressure, temperature and CO 2 -H 2 O immiscibility, at depths of between 7 and 11 km. The palaeoplacer gold mineralisation shows some hydrothermal modification and occurs mostly within the conglomerates and to a lesser extent within the interbedded quartzite of the Banket Series of the Tarkwaian Group. The gold occurs as free gold in the matrix of the conglomerates and as intergrowths and overgrowths on and inclusions in, other heavy mineral grains. Based on strong evidence for palaeoplacer mineralisation, the gold was likely deposited with the conglomerates in alluvial fans and braided tributary channels. Because the Eburnean compressional event associated with the Birimian Supergroup-hosted mesothermal gold mineralisation also overprinted the gold-bearing Banket conglomerates, the former gold occurrences could not have acted as the source for the palaeoplacer gold. Hydrothermal gold mineralisation occurs in the Palaeoproterozoic belts and basin granitoids that intrude the Birimian belts, as well as in the sedimentary basins occurring between the belts. Gold mineralisation within the granitoids occurs as micro-inclusions in sulphides in small, steeply dipping stockworks and as sulphide disseminations concordant with regional faults and shears. A gold-bearing fluid similar to that for the Birimian Supergroup-hosted quartz vein gold mineralisation, but with a larger H2O component, is proposed to have formed the granitoid-hosted gold mineralisation.

Hypozonal lode gold deposits: A genetic concept based on a review of the New Consort, Renco, Hutti, Hira Buddini, Navachab, Nevoria and The Granites deposits

The mineral system of hypozonal lode gold deposits has been a matter of scientific discussion in recent years, mainly centring around two models: (1) syn- to post-peak metamorphic hydrothermal mineralization (hypozonal orogenic gold) and (2) pre-peak metamorphic mesothermal mineralization and subsequent metamorphic overprint.Here we review the evidence for and against the existence of hydrothermal gold mineralization in high-temperature metamorphic terranes. The New Consort, Renco, Hutti, Hira Buddini, Navachab, Nevoria and The Granite deposits are considered to represent hypozonal orogenic gold deposits based on petrographic and field observations. Gold mineralization is hosted in syn- to post-peak metamorphic shear zones. Ore and hydrothermal alteration assemblages replace the metamorphic mineral assemblages of the host rocks and follow the same retrograde PT path. This indicates that the mineralization forms an integral part of the terrane evolution from syn- to post-peak metamorphism to final exhumation.Comparison with other hypozonal deposits indicates that the hypozonal orogenic gold system is active at ca. 500–700°C and 2–7kbar, and forms a Ca–Se–Cu–Ni–Co–S-dominated enrichment compared to the mesozonal counterparts. The hypozonal deposits appear to be restricted to Precambrian terranes in variable host rocks ranging from amphibolite and banded-iron formation to marble and granite. The fluid source for hypozonal gold is metamorphic, magmatic or a combination of both. A complex tectonometamorphic evolution of the high-grade metamorphic host terranes is regarded critical for forming the deposits by: (1) generating permeability in shear zones; (2) creating PT gradients that promote fluid migration; (3) inverting the metamorphic gradient by thrusting or extensional shearing; and (4) initiating crustal anatexis and granite emplacement as source of heat and ore fluid. Hypozonal orogenic gold deposits form in syn- to post-peak metamorphic shear zones of high-grade terranes in the centre or foreland of Precambrian accretionary and collisional orogens during the evolved collision stage, when active subduction ceased or moved outboard and the terranes are uplifted.

Hydrothermal Alteration and Gold Mineralization in the Jiaojia Gold Deposit, Jiaodong Peninsula, China

Hydrothermal Alteration and Gold Mineralization in the Jiaojia Gold Deposit, Jiaodong Peninsula, China

Constraints on the timing of late-Eburnean metamorphism, gold mineralisation and regional exhumation at Damang mine, Ghana

The Damang gold deposit in southwest Ghana is unique among known deposits in Ghana, comprising gold mineralisation in two distinct styles. Early gold, hosted in a stratigraphically controlled, auriferous quartz-pebble conglomerate horizon, is overprinted by later gold mineralisation contained in a sub-horizontal fault-fracture quartz vein array. A multi-system geochronological study is used to constrain the timing of igneous activity, regional metamorphism, gold mineralisation and the thermal history at Damang. U/Pb analysis of zircons from Birimian volcaniclastic and intrusive rocks constrain volcanism and associated intrusive activity at 2178.0±9.3Ma and 2164.6±8.0Ma respectively, which is consistent with previous studies. The age of formation of staurolite-grade, amphibolite facies peak metamorphic mineral assemblages at 2005±26Ma is provided by U–Th–total Pb EPMA analysis of metamorphic monazite grains in the Tarkwa Phyllite. Measured 40Ar/39Ar biotite ages range between 1980±9Ma and 1898±11Ma. Argon diffusion modelling with the programme DIFFARG suggests that this age range could be achieved by a period of rapid cooling, at a rate of approximately 17°C/Ma, followed by a prolonged period of much slower cooling, at a rate of 0.15°C/Ma. The period of rapid cooling is interpreted to represent localised exhumation of the Damang host rocks during the latest stage of the Eburnean orogeny at the time of hydrothermal gold mineralisation. Given these age constraints, hydrothermal gold mineralisation is inferred to have occurred between approximately 2030Ma and 1980Ma. These ages constrain metamorphism, fluid flow and gold mineralisation at Damang and are the youngest currently recognised in the Birimian of SW Ghana.