Orogenic-type Gold Deposit Research Articles

Gold mineralisations in the Canan area, Lepaguare District, east-central Honduras: Fluid inclusions and geochemical constraints on gold deposition

Gold mineralisations in the Canan area (Honduras) occur within altered metamorphic rocks crossed by quartz veins which filled the fractures where the hydrothermal fluids carrying gold flowed. Quartz crystals of the veins contain abundant fluid inclusions which have been divided into four main types on the basis of the petrographic features and microthermometric data. The association of L-rich and V-rich inclusions with the same major components supports boiling at the time of entrapment. The trapped hydrothermal fluid consists of an aqueous solution (0.9–4.8wt.% NaCl equivalent) plus a CO2–CH4-bearing bubble. Some fluid inclusions contain graphite of hydrothermal origin. Microthermometric and spectroscopic data on fluid inclusions indicate that hydrothermal fluids carrying gold were at T=300°C, P=500–1400bar, log fO2~−37.6±2.5 and with a pH value of 2.9±0.4. Large amounts of sulphides (mainly pyrite) are associated with gold. We infer that Au was transported as Au–S complex, in particular as Au(HS)°. The activity of sulphur in the hydrothermal fluids was at least 10−2. The precipitation of gold can be related to several processes that reduced the stability of AuHS°: 1) boiling and vapour loss following the pressure drop caused both by fracture opening and fluid uprise, 2) sulphidation as a result of the presence of Fe2+ (and other metal cations) from the mineralogical alteration (mainly of biotite and chlorite) in the wall-rocks and 3) hydrothermal alteration of feldspars and micas into kaolinite and diaspore with increase in pH of the hydrothermal fluids. The mineralogical, petrological, geochemical features and the geological setting are broadly consistent with those of orogenic-type gold deposits.

Genesis of the hydrothermal gold deposits in the Canan area, Lepaguare District, Honduras

The Canan area (Honduras) is characterized by a gold-bearing ore deposit that is associated with quartz-veined shear zones. Gold mineralization occurs in low-to medium-grade metamorphic host-rocks (graphitic and sericitic schists). Hydrothermal fluids, which are associated with the emplacement of Cretaceous-Tertiary granodioritic intrusions, are responsible for the formation of quartz veins and the hydrothermal alteration of wall-rocks. Three main altered zones have been detected in the wall-rocks as far as 150 cm from the quartz veins. The distal zone (up to 50-cm thick) contains quartz, chlorite and illite. The intermediate zone is the thickest (up to 80 cm) and is marked by quartz, muscovite, sulphides, kaolinite and native elements such as Au and Ag. The proximal zone, which is close to the quartz veins, is rather thin (up to 25 cm) and contains clay minerals, Al-oxides-hydroxides and sulphides. The transition from the distal to the proximal zone is accompanied by the enrichment of SiO2 and the depletion of all other major elements, except for Fe2O3(tot). Precious metals occur in the highest concentrations in the intermediate zone (Au up to 7.6 ppm and Ag up to 11 ppm). We suggest that gold was transported as a reduced sulphur complex and was precipitated from the hydrothermal solution by the reaction of the sulphur complexes with Fe2+ from the alteration of the mafic minerals of the host-rock. Fluid–wall-rock interactions seem to be the main cause of gold mineralization. Genetic relationships with a strike-slip fault system, hydrothermal alteration zones within the metamorphic wall-rocks, and an entire set of geochemical anomalies are consistent with orogenic-type gold deposits of the epizonal class.

Multiple fluid sources/pathways and severe thermal gradients during formation of the Jílové orogenic gold deposit, Bohemian Massif, Czech Republic

The Jílové deposit in the central part of the Bohemian Massif represents a vein to stockwork type of orogenic-type gold deposit. It is hosted by Neoproterozoic rocks of the Jílové Belt and by various magmatic dikes related to the ~355 to ~335Ma Central Bohemian Plutonic Complex. The deposit is situated along the terrane boundary of the Teplá Barrandian and Moldanubian units.The deposit offered an exceptional opportunity to trace O, C, S and Sr stable isotope evolution of parent fluids based on combined mineralogical and geochemical study of carbonate, quartz, scheelite, and sulfide minerals, which represent six stages of mineralization, including the gold-bearing event.Stable isotope data and mineral and isotope thermometry indicate gangue and ore mineral formation between ~350°C and <100°C, which can be divided into 6 stages. Scheelite-bearing assemblages (stages 2–3) precipitated at 292±8°C from a fluid with calculated values: δ18OSMOW=+4.2±0.5‰ and δ13CPDB=−11±1‰. Gold precipitation (stage 5) probably started at about 300°C, but the major event probably occurred at 230±30°C from a fluid with more variable isotope values (δ18OSMOW=+2.5 to +5‰ and δ13CPDB=−9 to −13.5‰). The carbon speciation was characterized by predomination of dissolved CO2 (H2CO3ap.) in the fluids. Some gold, however, undoubtedly precipitated from bicarbonate dominated fluids even at <120°C.Extreme variations in the δ18O values of carbonate minerals, obtained from sampling profiles across individual veins with macroscopic gold, revealed severe thermal gradients during vein formation (~50 to ~100°C difference of crystallization temperatures between the vein margin and core).The sulfur stable isotope composition of sulfide minerals indicates the dominant role of sulfur remobilization from Neoproterozoic rocks and stratiform mineralizations of the Jílové Belt by Variscan hydrothermal fluids. Similarly, the Sr-isotope composition of carbonates indicates both relatively primitive (87Sr/86Sr=0.7055) and more evolved (87Sr/86Sr~0.7090) fluid compositions, probably indicating fluid exchange with the Jílové Belt and the Central Bohemian Plutonic Complex rocks, respectively.Age determination of hydrothermal muscovite (related to stage 2) via 40Ar/39Ar indicated an age of 339.0±1.5Ma for the quartz veins. The mineralization is essentially coeval with the late intrusive phases of the Central Bohemian Plutonic Complex (i.e. the ultrapotassic suite) and with late-orogenic large-scale tectonic movements at the boundary between the two crustal terranes (Teplá-Barrandian and Moldanubian).Based on evaluation of the available age data on the hydrothermal and magmatic activity within the broader area of the Central Bohemian Plutonic Complex, we suggest two intervals of gold mineralization: 347 to 341Ma and 340 to 337Ma. The former interval overlaps with the intrusive activity of the Blatná high-K suite (granodiorite). The associated gold deposits (Mokrsko and Petráčkova hora) exhibit strong affiliation to the intrusion-related-gold-type deposit. The later interval overlaps with the ultrapotassic magmatism and is associated with more or less “classical” orogenic-gold-type deposits (Jílové, Bělčice, Libčice deposits).

A two-stage evolution model for the Amantaytau orogenic-type gold deposit in Uzbekistan

A lithogeochemical, mineral chemical, isotopic, and fluid inclusion study of barren, low-, and high-grade Au-mineralized samples from the shear zone-hosted Amantaytau gold deposit, Uzbekistan, shows that the local host rocks, Late Ordovician–Earlz Silurian carbonacous shales, are likely to have been an important source of Au, As, Ni, and S in the formation of the deposit. Syn-depositional pyrite in these shales contains on average 0.23 ppm Au, 1,083 ppm As, and 861 ppm Ni. The distribution of rare earth elements (REE) indicates a homogeneous source of light REE, whereas the heavy REE distribution reflects most likely primary variations in the sediments. The mineralized zone is marked by a positive Eu anomaly, which supports reducing conditions during the mineralization. A hydrothermal overprint by an aqueous–carbonic fluid is reflected in a high-grade Au-mineralized sample by δ13C values of −13.0 ‰ (V-PDB). The δ34S values in pyrite (−0.13 to +7.30 ‰ CDT) from barren and mineralized samples are consistent with marine sulfate being the principal source of the ore sulfur. Assuming a formation temperature of between 300 and 400 °C for the main stage of mineralization, as indicated by the alteration mineral assemblage, the calculated δ18Ofluid is between 9.5 and 13.4 ‰ V-SMOW, which points at a metamorphic origin of the ore fluid.

Geology and fluid evolution of the Wangfeng orogenic-type gold deposit, Western Tian Shan, China

The Wangfeng gold deposit is located in Western Tian Shan and the central section of the Central Asian Orogenic Belt (CAOB). The deposit is mainly hosted in Precambrian metamorphic rocks and Caledonian granites and is structurally controlled by the Shenglidaban ductile shear zone. The gold orebodies consist of gold-bearing quartz veins and altered mylonite. The mineralization can be divided into three stages: quartz–pyrite veins in the early stage, sulfide–quartz veins in the middle stage, and quartz–carbonate veins or veinlets in the late stage. Ore minerals and native gold mainly formed in the middle stage. Four types of fluid inclusions were identified based on petrography and laser Raman spectroscopy: CO2–H2O inclusions (C-type), pure CO2 inclusions (PC-type), NaCl–H2O inclusions (W-type), and daughter mineral-bearing inclusions (S-type). The early-stage quartz contains only primary CO2–H2O fluid inclusions with salinities of 1.62 to 8.03wt.% NaCl equivalent, bulk densities of 0.73 to 0.89g/cm3, and homogenization temperatures of 256°C–390°C. Vapor bubbles are composed of CO2. The middle-stage quartz contains all four types of fluid inclusions, of which the CO2–H2O and NaCl–H2O types yield homogenization temperatures of 210°C–340°C and 230°C–300°C, respectively. The CO2–H2O fluid inclusions have salinities of 0.83 to 9.59wt.% NaCl equivalent and bulk densities of 0.77 to 0.95g/cm3, with vapor bubbles composed of CO2, CH4, and N2. Fluid inclusions in the late-stage quartz are NaCl–H2O solution with low salinities (0.35–3.87wt.% NaCl equivalent) and low homogenization temperatures (122°C–214°C). The coexistence of inclusions of these four types in middle-stage quartz suggests that fluid boiling occurred in the middle-stage mineralization. Trapping pressures estimated from CO2–H2O inclusions are 110–300MPa and 90–250MPa for the early and middle stages, respectively, suggesting that gold mineralization mainly occurred at depths of about 10km. In general, the Wangfeng gold deposit originated from a metamorphic fluid system characterized by low salinity, low density, and enrichment of CO2. Depressurized fluid boiling caused gold precipitation. Given the regional geology, ore geology, fluid-inclusion features, and ore-forming age, the Wangfeng gold deposit can be classified as a hypozonal orogenic gold deposit.

Isotopic geochemistry of the Sawayaerdun orogenic-type gold deposit, Tianshan, northwest China: Implications for ore genesis and mineral exploration

The Sawayaerdun gold deposit is hosted by Carbonaceous metasediments and is considered to be the largest Muruntau-type gold deposit in the Chinese Tianshan metallogenic belt. Gold mineralization at Sawayaerdun occurs in quartz veins associated with three major hydrothermal events: an early, barren quartz vein stage, middle stage mineralized quartz veins with pyrite and late carbonate (-quartz) veins.The isotopic compositions of quartz and sulfides from the Sawayaerdun gold deposit show some variation but are generally comparable to those of other orogenic-type gold deposits. Fluids trapped in early-stage quartz have a δ18O range of 13.6‰ to 15.4‰, δD of −48‰ to −75‰, δ13C of 0.5‰ to 4.2‰ and δ30Si of −0.2‰ to 0‰. In contrast, isotopic compositions of fluids trapped in middle-stage quartz have δ18O values of 6.7‰ to 14.7‰, δD of −56‰ to −110‰, δ13C of 0.4‰ to 10.1‰ and δ30Si of −0.3‰ to 0‰. Diagenetic and hydrothermal pyrite have similar sulfur (−1.8‰ to 0.9‰) and Pb isotopic values that are associated with host rock compositions. The early-stage, 18O and 13C-rich fluids are probably derived from metamorphic decarbonation of the sedimentary host rock at depth, leading to the precipitation of early barren quartz veins. In the middle stage, a decrease in the regional pressure and temperature regime could have resulted in the incorporation of external fluids into the ore-forming system. These external fluids with isotopic signatures similar to that of the host rock and generally rich in 34S and radiogenic Pb mixed with original ore-forming fluids to generate extensive metal precipitation. Late-stage fluids trapped by calcite veins show isotopic compositions similar to meteoric water, indicating the cessation of hydrothermal fluid circulation at Sawayaerdun occurred at this time. The metallogenetic model illustrated by stable and Pb isotopes is also consistent with fluid inclusion studies in Sawayaerdun.The development of mineralization at Sawayaerdun is strongly linked to fluid mixing, as witnessed by the isotopic signatures of fluids from identified ore-bearing zones. The isotopic compositions of other anomalous zones at Sawayaerdun are similar to those of the mineralized zones, suggesting a high potential for further exploration.

LA-ICP-MS trace element analysis of pyrite from the Xiaoqinling gold district, China: Implications for ore genesis

The Xiaoqinling district, the second largest gold producing district in China, is located on the southern margin of the North China Craton. It consists of three ore belts, namely, the northern ore belt, the middle ore belt and the southern ore belt. Pyrite from the Dahu gold deposit in the northern ore belt and Wenyu and Yinxin gold deposits in the southern ore belt were investigated using a combination of ore microscopy and in-situ laser-ablation inductively-coupled plasma-mass spectrometry (LA-ICP-MS). A range of trace elements was analyzed, including Au, Te, Ag, Pb, Bi, Cu, Co, Ni, Zn, Mo, Hg, As and Si. The results show that there are no systematic differences between the trace element compositions of pyrite in the three deposits from different ore belts. In general, Au concentrations in pyrite are low (from < 0.01 ppm to 2.2 ppm) but Ni concentrations are rather high (up to 8425 ppm). A four-stage mineralization process is indicated by microscopic and field observations and this can be related to the systematic trace element differences between distinct generations of pyrite. Stage I precedes the main gold mineralization stage; pyrite of this stage has the lowest Au concentrations. Stages II and III contributed most of the gold to the ore-forming system. The corresponding pyrite yielded the highest concentrations of Au and Ni. Our microscopic observations suggest that pyrite in the main gold mineralization stage precipitated simultaneously with molybdenite that has been previously dated as Indosinian (~ 218 Ma by Re–Os molybdenite dating), indicating the Indosinian as the main gold mineralization stage. The Indosinian mineralization age and the geological and geochemical features of these gold deposits (e.g., low salinity, CO 2-rich ore fluids; spatial association with large-scale compressional structures of the Qinling orogen; δ 18O and δD data suggestive of mixing between metamorphic and meteoric waters; δ 34S and Pb-isotopic data that point to a mixed crustal-mantle source) all point to typical orogenic-type gold deposits. High Ni concentrations (up to 8425 ppm) of pyrite possibly linked to deep-seated mafic/ultramafic metamorphic rocks provide further evidence on the orogenic gold deposit affinity, but against the model of a granitic derivation of the mineralizing fluid as previously suggested by some workers. Generally low Au concentration in pyrite is also consistent with those from worldwide orogenic gold deposits. Therefore, the gold mineralization in the Xiaoqinling district is described as orogenic type, and is probably related to Indosinian collision between the North China Craton and the Yangtze Craton.

Bicarbonate-rich fluid inclusions and hydrogen diffusion in quartz from the Libčice orogenic gold deposit, Bohemian Massif

Unusual paleofluid composition is reported for the Libčice orogenic-type gold deposit located in the Central Bohemian Plutonic Complex (CBPC), Czech Republic. Unexpected bicarbonate-rich fluids and their complex chemistry variations characterize primary fluid inclusions from the main gold-bearing quartz vein. A detailed microthermometry, Laser Raman Micro Spectroscopy (LRMS) and SEM cathodoluminescence study was used to understand the complex chemistry and P–T–X history. The results indicate the presence of H 2O and H 2O–CO 2–CH 4 (± N 2; H 2S) fluids, the latter displaying variations of the CO 2/CH 4 ratio in the gaseous phase from 6.8 to 0.06. Variation of the CH 4 content across single grains and between different levels of the mine was recorded. The presence of nahcolite, H 2 (up to 6 mol%) and ethane (0–0.2 mol%) in the fluids were also discovered by LRMS. Potential models for the formation of different types of fluids present in the deposit are discussed, including the genesis of HCO 3 − rich fluids as well as H 2 and C 2H 6 presence in the primary fluid inclusions. The potential influence of organic matter-bearing sediments, as well as the impact of the intrusion of CBPC, re-equilibration and/or re-speciation of fluid inclusions during the post-entrapment history is considered to have the main impact on the complex paleofluid chemistry. Based on the thermodynamic modeling, H 2 diffusion into the fluid inclusions was shown to be the main reason for the CH 4 variation on the scale of a single grain, as well as across the whole vein. Although the exact processes of production/formation of HCO 3 − and H 2 at the Libčice deposit remain open to discussion, reactions in the C–O–H system are considered to be a possible formation mechanism. This work also contributes to our understanding of the importance of post-entrapment modifications and reactions in the C–O–H system on interpretation/deciphering the processes in orogenic-type deposits.

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.

Gold metallogeny in the Birimian craton of Burkina Faso (West Africa)

Primary gold deposits in Burkina Faso occur in Paleoproterozoic Birimian belt formations (2.0 Ga). Mineralization was synchronous with regional metamorphism and deformation, and is either hosted within, or is adjacent to, quartz-bearing veins. These are classical characteristics of epigenetic gold deposits in Precambrian metamorphic terranes and permit to classify the mineralized sites from Burkina Faso as orogenic-type gold deposits. A review of data collected over the past decade by our team permits to recognize two main styles of gold mineralization: (1) Quartz-vein hosted; this style occurs in all lithologies, the veins are deformed and gold is principally concentrated within the veins, associated with either sulfides or tourmaline. (2) Disseminated; this style occurs exclusively in albitites (and to a lesser extent listvenites) with gold occurring mainly within alteration halos of generally undeformed quartz-albite-carbonate vein. Quartz-vein and disseminated styles of mineralization can be associated within the same deposit. Albitites and listvenites are alteration products of mainly calc-alkaline igneous rocks of felsic to ultramafic composition, respectively. The predominant alteration assemblage consists of chlorite, albite, carbonate, and pyrite. Sulfides occur as fine masses commonly in the alteration halos close to vein margins and consist mainly of pyrite and arsenopyrite, depending on host-rock composition. Gold occurs as free native metal and, locally, in form of tellurides, in fissures or as inclusions within pyrite and arsenopyrite. Two main populations of fluid inclusions are associated with the gold deposits, independently of the mineralization style: (1) carbonic inclusions consisting of up to 90 mol% CO 2 (plus N 2 and CH 4) and (2) aqueous-carbonic fluid inclusions with moderate salinities. Interestingly, the disseminated gold style deposits of Burkina Faso, which have the highest economic potential, show strong similarities with the world-class Ashanti deposit, in neighboring Ghana.

Multiple mineralization events in the eastern Tianshan district, NW China: Isotopic geochronology and geological significance

The eastern Tianshan district occupies the middle of the Central Asian Orogenic Belt and has great economic potential for polymetallic mineralization. More than 20 medium to large ore deposits have been discovered in this region since the late 1980s. The timing of mineralization was previously thought to be Hercynian, but, Mesozoic ages for intrusions and mineralization have been repeatedly reported in recent years. Based on geochronological data and regional geological evidence, it is suggested that the mineralization in the eastern Tianshan was formed during multiple events. The mineralization processes are related to an subduction-island arc stage (∼360–320 Ma) with porphyry-type and volcano-sedimentary copper deposits, a collisional-accretionary stage (∼300–280 Ma) with orogenic-type gold deposits, a post-collisional-extension stage (∼280–245 Ma) with mafic–ultramafic copper–nickel and epithermal gold deposits, and an intracontinental extensional stage (∼240–220 Ma) that gave rise to the Jinwuozi gold deposit, Xiaobaishitou skarn W–Mo deposit and Baishan porphyry Mo–Re deposit.