Ore-bearing Veins Research Articles

Millerite from the Madan ore field

Millerite is discovered in samples from Krushev Dol and Mogilata deposits in the Madan ore field. It was found in two associations different in appearance and mineral composition. One of them is with light green ferro­dolomite, and the other with bright green mariposite (chromium-bearing phengite). Only cobalt and iron are detected as impurity elements in some analyses. They are in small concentrations (below 0.01 apfu). The Ni/S ratio is 0.95. The nickel source is associated with bodies of serpentinized ultramafics cut by ore-bearing veins.

Zircon as a pathfinder for ore exploration

Despite its wide application in dating geological events like magmatism, diagenesis, and metamorphic processes, zircon morphological, geochronological, and geochemical implications in ore systems have hitherto received little attention. In this study, we used textural, chemical, and isotopic data of zircons from variable granite-related deposits and other deposit types to emphasize zircon robustness as a pathfinder for ore exploration. The results show that zircons from W–Sn-related granites are rich in U, P, Ti, Y, Nb, Ta, Pb, Hf, Th, and REE compared to Cu–Mo-related granite. For the vein-type deposits, ore-bearing veins have significant zircons captured from geological bodies through which ore-forming fluid originated and migrated (e.g., basement rocks), whereas ore-free veins possess fewer zircons and are mostly from country rocks. Finally, in skarn-type deposits, ore-bearing skarns have zircons from granitic rocks and country rocks, whereas ore-free skarns have zircons only from one end member, either from granitic rocks or from country rocks. The variability in zircon abundance in these deposit types results from differences in the degree of magmatic differentiation of granitic rocks, variable zircon incorporation processes during veining and degree of contact-metasomatism during skarnization. We proposed a series of diagrams and employed partial least square discrimination analysis (PLS-DA) to differentiate the metallogenetic potential of granite-related, vein-type, and skarn-type deposits. The zircon criteria summarized by this study can be applied to similar ore deposits worldwide, which are significant for exploring economic minerals.

Garnets in the Yinachang Fe-Cu deposit, Kangdian region, China: Mineralogical and geochemical insights into fluid-rock dominated iron oxide-copper–gold (IOCG) mineralization

Garnet is an important metasomatic mineral in iron oxide-copper–gold (IOCG) ore systems, but its crystallization mechanisms and relationship to fluid evolution in IOCG are poorly understood. In this study, major and trace element analyses were conducted on garnets from the Yinachang IOCG deposit in the Kangdian region, southwest China, in order to reveal their characteristics and forming process.Garnet commonly occurs in the K-Fe ± REE mineralization stage and Cu ± Au mineralization stage of the Yinachang deposit. Three types of garnets are identified from this deposit, which generally have high almandine but low grossular and spessartine contents (Alm65-78Gro5-17Sp5-17). The type 1 garnet (Grt 1) in the ore bodies and the type 2 garnet (Grt 2) in the proximal silicate alteration zones are pink to purple, euhedral to subhedral grains. Grt 1 exhibit zoning pattern, and Grt 2 were altered into chlorite and was mainly crosscut by ore-bearing veins. They are both extremely depleted in LILEs and enriched in HFSEs, with low total rare earth element ∑REE contents but variable LREE vs HREE ratios. The type 3 garnets (Grt 3) in the intermediate garnet-biotite-quartz zones are purple-brown, anhedral grains with high LILE, HFSE, and ∑REE contents, and exhibit LREE-depleted and HREE-enriched patterns, with marked positive Eu anomalies. All three types garnets that formed during alteration and mineralization are hydrothermal, but their composition is controlled by different mechanisms due to different environments. Compositions of Grt 1 and Grt 2 are mainly related to substitution mechanism controlled by crystal chemistry, physicochemical conditions of the closed hydrothermal system, and buffering effects of carbonate wall rocks during slow infiltration metasomatism, whereas the composition of Grt 3 is mainly controlled by surface adsorption and related to the composition of external fluid during later rapid garnet growth. Decreases in temperature, pressure, and fO2 triggered by mixing of fluids could be responsible for the distinct mineralogical and geochemical characteristics of the three types of garnets.Garnets from IOCG deposits show various textual and geochemical characteristics worldwide. Generally, almandine and andradite are dominating end members for IOCGs. Andradite is typical for metasomatic origin. Although almandine garnet is generally considered to form during metamorphism, the occurrence of hydrothermal almandine in the alteration zones of IOCG deposits reveal a metasomatism between Fe-bearing fluid and Al-enriched pelite protolith in a sufficiently reduced environment. Also, it is possible that the metasomatism during mineralization has enriched the protolith with Fe, and the post-ore metamorphism formed almandine by providing high temperature and pressure.

The role of fluid mixing in the formation of vein-type Zn-Pb deposits in eastern Guizhou Province, SW China: Insights from elemental compositions and chlorine isotopes of fluid inclusions

• Multiple fluid sources are identified by variation range of Cl isotopes and Cl/Br ratios from. • Vein-type Zn-Pb mineralization is triggered by mixing of evaporated seawater and magmatic hydrothermal fluids . • Quantitative model shows that the mixing volume ratios of both sources are between 1:9 and 4:6. Numerous hydrothermal vein-type Zn–Pb deposits worldwide are characterized by low-temperature metallogeny, no spatial relationship with plutons, and strong water-rock interaction, which result in the difficulty of constraining the fluid origin and the process of ore formation by traditional geochemistry. In this study, we employ chemical and chlorine isotopic composition of bulk fluid inclusions from ore-bearing and ore-barren quartz veins to constrain the possible sources of ore-forming fluids and the process of metal precipitation in the hydrothermal systems of the economic vein-type mineralization in eastern Guizhou Province, SW China. Our new geochemical data are indicative of two end-members in the systems, one identified in the ore-bearing veins with high K/Na, Li/Na, Cl/Br ratios, and δ 37 Cl values representing magmatic hydrothermal fluids, another distinguished in the ore-barren veins with low Cl/Br ratios and δ 37 Cl values representing evaporated seawater. We conclude there is a systematic variation of these geochemical data as the results of the mixing between the former and the latter end-member. In the meantime, the mixing is quantitively assessed to study the mixing volume ratios of both end-members. Based on our data and our own previous study, a genetic fluid mixing model involving the magmatic hydrothermal fluids related to deep magma activity and the evaporated seawater is established for the formation of the vein-type Zn–Pb deposits. This study provides a novel geochemical approach for recognizing hydrothermal mineralizing systems.

Application of ground magnetics and geoelectrical methods in delineating sulphide deposit in oshiri area, southeastern nigeria

The Asu River Group Shale in the Oshiri area was evaluated using ground magnetic and geoelectrical methods within existing mine. Some exploration programs in the area have resulted to failure due to the fact that project managers have not fully understood the use and importance of geophysics in enhancing the search for mineral deposits. The aim of the study was to delineate the sulphide deposits using ground magnetics and geoelectrical methods. Result of the ground magnetic survey delineated some shallow tectonic structures like fractures and faults which are capable of hosting metallic sulphide deposits. The residual magnetic intensity of the study area indicates very low magnetic field intensities around the central and southern portions of the study area. These low values indicate absence of magnetite in the near surface materials as suggested by previous studies in the area, but could be referred to as zones with shallow tectonic structures which could be faults and/or fractures capable of housing sulphide deposits. The ore-bearing veins within the sedimentary rocks have susceptibilities of ≤ -1.8 nT. The low, negative residual susceptibility zone in the southeastern portion of the study area was targeted for the induced polarization IP and electrical resistivity tomography. Four traverses of induced polarization IP and resistivity surveys were carried out with all the profile lines were in the NE-SW direction. Isolated chargeable bodies with values greater than 340msec and a correspondingly low resistivity values (10Ωm-50Ωm), were encountered across some of these profiles. This zone may represent zones with sulphide mineralization. These identified chargeable bodies occur at approximate depth and width range of between 25-30m and 50-70m respectively. The 1D stratigraphic models reveal an average thickness estimate of 11.0m. Using scalar geometric approach, an estimate of 103,513.75tonnes was obtained, representing a relatively economically viable quantity and a worthy target for investors.

Sources of metals and fluids for the Taijiying gold deposit on the northern margin of the North China Craton

Recognition of the sources of metals and fluids is crucial to determine the genesis of gold deposits, especially in distinguishing the orogenic and magmatic hydrothermal deposits. There is no consensus on the source and genetic style of the Mesozoic gold deposits in the North China Craton (NCC). The Taijiying gold deposit in the Chifeng-Chaoyang gold district on the northern margin of the NCC is a medium-sized gold deposit and worth studying for its sources of metals and fluids. It is hosted by amphibole- to granulite-facies metamorphic rocks and Mesozoic intrusions, and controlled by NNE-, NE-trending compressional reverse faults and NW-trending extensional normal faults. The gold mineralization types include auriferous quartz veins and wall rock alterations of phyllic mineralization, pyritization, chloritization, and carbonatization. According to the crosscutting relationships of the veins and the mineral textures within the ore-bearing veins, four mineralization stages are recognized: clouded-white quartz ± pyrite (stage I), gray quartz + pyrite (stage II), quartz + polymetallic sulfides (stage III), and calcite ± quartz (stage IV), among which the stages II and III represent the main gold mineralization stages. The timing of the gold mineralization is constrained to the Middle Triassic by zircon U-Pb ages of a pre-metallogenic diorite (258.0 ± 1.9 Ma) and a ore-related granitic porphyry dike (241.5 ± 2.2 Ma). Three types of fluid inclusions (FIs) in the quartz veins of four stages are identified: vapor-rich (type 1), CO2-H2O three-phase (type 2), and liquid-rich FIs (type 3). Their trapping temperatures gradually decrease from 352 ℃ (stage I) to 124 ℃ (stage IV), with salinities decreasing from 9.8 to 2.0 wt% NaCl eqv., indicating that the ore-forming fluids belong to a H2O-NaCl-CO2 system. The first three stages were characterized by moderate-high temperatures, a large variation in salinities, akin to those of the magmatic hydrothermal fluids. The H-O isotope compositions from the stage II (δ18Ow = 4.1–4.8‰ and δD = -110 to −87‰) to the stage III (δ18Ow = 3.5 to 3.7‰ and δD = -101 to −98‰), indicate that the ore-forming fluids were mainly magmatic fluids. The δ34SV-CDT (1.7–2.1‰), Pb isotopes (206Pb/204Pb = 15.09–15.90, 207Pb/204Pb = 15.00–15.15, 208Pb/204Pb = 35.76–37.97), and Os isotopes (initial 187Os/188Os = 2.3 ± 1.0) of the Au-bearing pyrite samples at the stages II and III, similar to those of the crust-derived magma, suggest that the gold and other metals were extracted from the partial melting of the lower crust. Accordingly, the Taijiying gold deposit is probably a magmatic hydrothermal deposit. The areas recording the Triassic batholith with a developed fault system would be prospective targets for gold mineralization in the Chifeng-Chaoyang district on the northern margin of the NCC.

Early Neoproterozoic Gold Deposits of the Alto Guaporé Province, Southwestern Amazon Craton, Western Brazil

Abstract The Alto Guaporé gold province, southwestern Amazon craton, contains gold deposits that have been mined since the beginning of the 18th century and these deposits, together, have modern-day, pre-mining gold resources of at least 1.8 Moz. The ore is associated with quartz vein systems along the southeastern part of the Aguapei belt, a ~35-km-wide and ~500-km-long, NNW-trending shear zone formed due to reactivation of a terrane-bounding suture. The Aguapei belt evolved by ca. 1150 to 1100 Ma rifting and deposition of siliciclastic sediments in an aulacogen basin, followed by deformation and low-grade metamorphism of the sedimentary sequences during 1100 to 900 Ma terrane collision along the craton margin. The deformation was characterized by a compressional regime until ca. 950 Ma and transition to a transpressional setting during the final 50 m.y. The gold deposits are hosted in a variety of structures that are second-order to the main Aguapei shear zone. The Ernesto and Pau-a-Pique deposits are located ~40 km apart and at jogs along the Aguapei belt. They are marginal to pre-ore igneous rocks, with Ernesto hosted in the basal part of the metasedimentary Fortuna Formation that overlies tonalite and Pau-a-Pique at the contact between metasedimentary rocks and diorite. Three deformational phases comprise the compressional (D1 to D2) to transpressional (D3) tectonic events. In the Pau-a-Pique deposit and the deeper level of the Ernesto deposit, the ore-bearing veins are bedding parallel and follow D2 strike-slip and reverse fault zones, respectively. However, the veins formed during D3 reactivation of the older structures by an array of oblique accommodation faults. In contrast, ores at shallower levels of Ernesto, both in discordant and bedding-parallel veins, are hosted within a ~20-m-thick rigid metaconglomerate with associated dilation due to the structural complexity as sedimentary rocks of the Aguapei Group were folded around the dome-shaped roof of the pre-ore tonalite. The ores in both deposits, as well as in many other deposits of the province, are characterized by disseminated and vein-hosted pyrite. Gold occurs mainly as inclusions in the pyrite, with other hydrothermal phases comprising muscovite, Fe-Ti oxides, and minor apatite, chalcopyrite, and galena. Fluid inclusion data, coupled with stable isotope geochemistry and geothermometry, indicate that gold precipitated from a low-salinity, CO2-rich fluid at ~300°C and ~2.5 kbar. The source for the fluid and gold was the interbedded pelites during devolatilization of the Aguapei Group sequence. The aqueous-carbonic fluid inclusions and the narrow range of δ18O values of quartz (12 ± 1‰) from many auriferous veins from the central part of the province represent a regional ore-forming fluid. The broad range of δD for hydrous minerals (–116 to –55‰) reflects influx of small amounts of meteoric water into the steeply dipping shear zones during postgold exhumation. The 40Ar/39Ar geochronology from hydrothermal muscovite indicates a widespread hydrothermal event along the belt between 928 and 920 Ma. Collectively, the geological, geochronological, and geochemical data suggest that metamorphic fluids migrated laterally into and then upward along the Aguapei belt and deposited gold in lower-order structures where strain gradients existed between lithounits. The province has many characteristics of large orogenic gold provinces worldwide and represents a highly prospective and underexplored target region for early Neoproterozoic gold, a time period that generally is not well endowed in gold ores.

Progressively released gases from fluid inclusions reveal new insights on W-Sn mineralization of the Yaogangxian tungsten deposit, South China

The gentle stepwise crushing technique permits progressive extraction of gases from large, irregular, secondary fluid inclusions (SFIs) along micro-cracks to small, regular, liquid-rich primary fluid inclusions (PFIs). Previous studies have verified this general gas release pattern in different hydrothermal minerals based on the agreements between 40Ar/39Ar ages of the released gases from different crushing stages and the most likely corresponding geological events dated with other methods; and based on the excellent agreements between gas chemistry measured by QMS (quadrupole mass spectrometer) during stepwise crushing and the expected characteristics of SFIs and PFIs in the same samples acquired using Raman spectroscopy. In this study, we applied this QMS-based gentle stepwise crushing technique on ore and gangue minerals from the world-class Yaogangxian tungsten deposit in South China. Gases released from PFIs in cassiterite, wolframite, and quartz from ore-bearing veins show high CH4, N2, C3H8, and low CO2, C4H10 contents, low CO2/CH4 (≪1) and high N2/Ar (327–861 for ore minerals and 60.7–943 for quartzs from ore veins) ratios, along with multiple organic gaseous species. Their compositions resemble those of gases released from PFIs in quartzs from the Yaogangxian granites which represent magmatic-hydrothermal fluids that exsolved from the granitic magma. However, compared with these exsolved magmatic-hydrothermal fluids, ore mineral PFIs have relatively high He contents which indicate contributions from non-magmatic crustal fluids. This is consistent with high Ca2+, low F− concentrations, and low K+/Na+ (0.21–0.32) ratios observed in fluids in the quartz samples, as well as their relatively low water hydrogen isotopes (δD-H2O = −102 to −59‰). These chemical characteristics, coupled with their magmatic sulfur (δ34S-molybdenite and arsenopyrite = −0.2–1.4‰), methane hydrogen (δD-CH4 = −58 to −42‰), and fluid oxygen (δ18O-H2O = 3.9–5.1‰) isotopic compositions, indicate both magmatic and non-magmatic crustal components in the mineralizing fluids. Based on these observations and the low methane carbon isotope values (δ13C-CH4) in the studied quartz samples, we propose that fluid-rock interactions between the exsolved magmatic-hydrothermal fluids and the metasedimentary wall rocks played an important role in promoting W-Sn precipitation in this ore deposit. This study demonstrates the power of combining QMS-based stepwise crushing technique with traditional geochemical techniques in investigating ore-forming processes.

Tectonics, geochronology, and mineralization of the Shuidegou gold deposit in the Mianlue belt, western Qinling Mountains, China

The gold mineralization in the Mianlue belt, western Qinling Mountains, has been a hotspot in geoscience research in recent years. In particular, important breakthroughs have been achieved in terms of prospecting in the medium-sized Shuidegou gold deposit, located in the middle of the Mianlue belt. Here, we present the results of systematic tectonic and chronological analyses conducted on the Shuidegou gold deposit. The reconstructed zircon U–Pb chronologies of the ore-bearing altered rocks and of the ore-bearing vein demonstrated that the Shuidegou gold deposit formed at ~183 Ma and was further enriched at ~ 150 Ma. The macro- and micro-structures identified in deformed geologic bodies and quartz c-axis fabrics in the mining area suggest a division of the deformation sequence into four phases, corresponding to: (1) a Mid–Late Triassic collision and convergence of middle–deep deformation structures during the main orogenic period (D1); (2) a Late Triassic–Early Jurassic collision and convergence during the late-stage thrust-nappe tectonic deformation (D2); (3) a Mid–Late Mesozoic shallow strike-slip shear structure stage during intracontinental orogeny (D3); (4) post-Cretaceous, shallow brittle thrust extrusion strike-slip structure (D4). Based on the deformation and zircon U–Pb chronological features of the Shuidegou gold deposit, we believe that the main metallogenic age (~183 Ma) corresponds to the thrust-shear deformation stage (D2) and that it is genetically connected with magmatism. Notably, the late-stage further enrichment(~150 Ma) occurred in the sinistral strike-slip stage. Ore-bearing fluids may have been restricted by the far-field effect of the Pacific plate subduction along the eastern edge of East Asia. Based on the macro- and micro-deformation features, the chronological data relative to the key geologic bodies, and the regional geologic background, we subdivided the mineralizing process of the Shuidegou gold deposit is composed into three stages: (1) the formation of an ore-guiding channel, (2) the deposition and mineralization of ore-forming elements, and (3) a later further enrichment. The study of ore-controlling structure showed that the development of the Shuidegou gold deposit was mainly controlled by WNW- (E–W) and NNE-trending structures during the two metallogenic periods (~183 Ma and ~ 150 Ma, respectively). Concerning this aspect, the Shuidegou gold deposit contrasts with some typical gold deposits in the same region.

A High-Arctic flow-through lake system hydrochemical changes: Revvatnet, southwestern Svalbard (years 2010–2018)

Lake ecosystems are strongly coupled to features of their surrounding landscapes such as geomorphology, lithology, vegetation and hydrological characteristics. In the 2010–2018 summer seasons, we investigated an Arctic flow-through lake system Revvatnet, located in the vicinity of the coastal zone of Hornsund fjord in Svalbard, characterising its hydrological properties and the chemical composition of its waters. The lake system comprises of a small upper lake and a large lower one, the latter cone-shaped, with −29.1 m maximum depth. With near-neutral pH (full range 6.5–8.4) and low EC (7–147 μS cm-1), the lake has rather similar characteristics to many Arctic lakes. Metal and metalloid concentrations were either similar across the lake system or increased downstream (except Zn, which has important ore-bearing veins in the upper part of the catchment), which is consistent with the likely slow dissolution of suspended particles within the lakes. The ∑PAHs concentrations ranged from <MDL to 2151 ng L-1, and according to the indicator PAHs concentration ratios, they originated from a mixture of combustion processes (they were not petrogenic). Principal component analysis showed that seasonal variability was the most characteristic feature of the chemical composition of these waters, although there appear to be consistent changes with time (sampling year) as well. Future research should explore the occurrence of high maxima in the concentrations of priority pollutants, such as PAHs, metals and metalloids (e.g. As).

Evidence of iron oxide-copper–gold mineralization in the Torud-Chahshirin Magmatic Belt, northern Iran: Insight from the Robaie area

The Torud-Chahshirin Magmatic Belt in northern Iran is defined by Tertiary porphyry, IOCG, Skarn, Manto, MVT, and Au, Ag and base metal-rich epithermal systems, and it is a part of the eastern Alborz Orogenic Belt of Iran. Mineralization in the Torud-Chahshirin Magmatic Belt occurs as veins, veinlet, and disseminated mineralization hosted by the Eocene-Oligocene volcanic, subvolcanic, and volcanoclastic rocks. Mineralization prospects of the Torud-Chahshirin Mgmatic Belt are spatially and temporally accompanied with magmatic rocks of early to middle Eocene (∼55–37 Ma.) calc-alkaline and early to late Oligocene (∼34–24 Ma.) alkaline rocks that have continually evolved through assimilation and fractional crystallization (AFC) processes at shallow crustal levels. The Robaie area is located in the Torud-Chahshirin Magmatic Belt. The dominant stratigraphic unit is composed of Eocene volcanic rocks (trachyandesitic to andesitic in composition), which are intruded by subvolcanic rocks. The subvolcanic rocks have features typical of shoshonitic affinity and are metaluminous to weakly peraluminous. These rocks are characterized by enrichment in large-ion-lithophile elements (LILEs) and light rare-earth-elements (LREEs), depletion of heavy rare-earth-elements (REEs, LaN/YbN ≈ 9.65–24.34), and high-field-strength-elements (HFSEs). Values of εNd(i) commonly show a small variation between +0.2 and +0.7, whilst 86Sr/87Sr (i) goes from 0.7056 to 0.7063. The whole set of evidence agrees with derivation of intrusive rocks of Robaie from primitive magma resulting from partial melting in a mantle wedge above a subduction zone. A sample from a dyke of biotite-hornblende diorite porphyry, with a negative εNd(i) value (−1.7), probably corresponds to a magma that underwent assimilation of crustal material. U-Pb data of zircons from dioritic stock and dyke show ages of 49.99 Ma and 50.43 Ma, respectively. Mineralization in Robaie is related to the biotite-hornblende diorite intrusions and is associated with hydrothermal alterations including argillic, argillic-silicification, argillic-sericitic, and propylitic alteration. The ore-bearing veins, veinlets, and disseminated mineralization are controlled by structures. There are two types of veins and veinlets mineralization in the Robaie area including: 1) quartz-chalcopyrite-pyrite-specularite ± gold veins, and 2) quartz-chalcopyrite-pyrite-specularite-magnetite ± gold veinlets. The highest amount of copper (>1%) and gold (>5 g/t) is associated with quartz-chalcopyrite-pyrite-specularite ± gold veins. Microthermometric study of fluid inclusions shows that homogenization temperatures are 181–422 °C and salinities of ore-forming fluids ranged from 11.1 to 17 wt% NaCl equivalent. The temperature and salinity of ore-forming fluids were lower in the formation of the quartz-chalcopyrite-pyrite-specularite ± gold veins than in the quartz-chalcopyrite-pyrite-specularite-magnetite ± gold veinlets, and fluid mixing was the most important mechanism for mineralization in the area. The mineralogy, alteration, geochemistry, fluid inclusion studies, and petrogenesis of subvolcanic rocks of the Robaie area indicate that the Robaie deposit could be classified as a member of the Iron Oxide-Copper-Gold (IOCG) deposits.

Lamprophyres and mineralization of the Koytash ore field (Southern Tien Shan)

Research subject. The Koytash ore field is located in the potentially productive Au, Ag, W, Mo, Ti, Fe, Cu, Pb, Zn and REE North Nuratau mineralisation zone of Tien Shan. The authors undertook a study of the composition of dikes breaking through the Paleozoic ore-bearing formations, as well as their petro- and ore-generating role in the formation of the Koytash-Ugat sulphide-rare-metal (W, Mo, Fe) specialised mineralisation. Materials and methods. A study of rock and mineral composition was performed at the Institute of Geology and Geophysics named after Kh.M. Abdullaev. The content of petrogenic and rare elements in rocks and sulphides was determined by ICP-MS using an ICPE-9000 mass-spec trometer in the Central Laboratory of the State Committee for Geology of the Republic of Uzbekistan. The chemical analysis of minerals was performed using a Jeol-8800Rh electronic microanalyser at the Institute of Geology and Geophysics named after Kh.M. Abdullaev. The micrographs of transparent sections were obtained using Nikon Optiphot 2 Pol and Polam R-311 microscopes. Results and conclusions. The conducted study showed that, in terms of their structure, the Koytash ore field dike formations can be regarded as lamprophyres. In terms of their chemical composition, these formations are mafic and intermediate rocks of the subalkaline series. It was found that the composition of lamprophyre dikes correlates with the size of the erosion section. Their melanocratic varieties are confined to the southern part of the intrusion (absolute elevations are 1000–1200 m), and leucocratic – to the northern (about 1900 m). This is assumed to be the result of crystallization differentiation of a single initial melt. The dikes of the Koytash ore field lamprophyres break through not only sulphide-rare-metal bodies of the Koytash-Ugat strip, but also skarn and carbonate rocks and, in turn, are broken through by quartz-polymetallic ore-bearing veins, which testifies to their inter-ore character.

Geology and structural controls of the Ag–Sn–Zn Pirquitas deposit, northwestern Argentina

Pirquitas is a polymetallic silver, tin, and zinc deposit, located in the Altiplano–Puna plateau, northwestern Argentina. The mineralization occurs in veins and hydrothermal breccia hosted in Ordovician meta–sedimentary rocks, but there is a lack of data on the structural geology of the deposit. In this paper we use detailed field mapping accompanied by structural analysis and mineralogy (petrography and XRD) to infer: (i) the mineralization structural controls, (ii) the influence of regional structures on the deposit formation, and (iii) characteristics of the Pirquitas deposit as compared with those from the Bolivian Tin–Silver Belt.The main stage of deformation observed at Pirquitas is associated with the Late Ordovician–Early Silurian Oclóyic orogeny. However, the ore deposition occurs as recurrent hydrothermal pulses related to the reactivation of Ordovician structures during the upper Miocene Quechua orogeny, which was partially contemporaneous with volcanic activity. The NW–SE and WNW–ESE ore–bearing veins of the Pirquitas deposit represent, respectively, second–order R– and T–fractures associated with major regional–scale, sinistral, brittle shear zones such as the Lípez and the Olacapato–El Toro faults.Similarities in mineralogy, structural control and association with magmatic activity indicate that Pirquitas is the southernmost deposit of the Bolivian Tin–Silver Belt. As such, subvolcanic intrusions associated with the Granada Volcano and the Coranzuli Caldera that are aligned along regional structures should be investigated by future exploratory works looking for porphyry to epithermal mineralization in the region.

Discrete Jurassic and Cretaceous Mineralization Events at the Xiangdong W(-Sn) Deposit, Nanling Range, South China

Abstract The Xiangdong W(-Sn) deposit is hosted in the Dengfuxian multiphase granites (biotite, two-mica, and muscovite granites) within the Nanling Range metallogenic belt in south China. Previous studies suggested that the W(-Sn) mineralization in the Xiangdong deposit is related to the Late Jurassic two-mica granite, whereas recently W-Sn–bearing quartz veins have also been identified in muscovite granite. We present new cassiterite and zircon U-Pb ages to constrain the timing of W-Sn mineralization and related granitic magmatism. Our laser ablation-inductively coupled plasma-mass spectrometry U-Pb dating of zircon grains, combined with previous zircon ages, in addition to the trace element composition of the muscovite granite, suggest the muscovite granite in the Dengfuxian pluton was emplaced at 145 to 142 Ma and shows highly evolved features. Cassiterite grains from the ore-bearing veins in two-mica granite yielded U-Pb ages of 151.6 ± 3.7 and 141 to 138 Ma, whereas cassiterite grains from quartz veins occurring in muscovite granite yielded a U-Pb age of 136.8 ± 3.3 Ma. The new ages and detailed geologic evidence indicate that the Early Cretaceous muscovite granite is also genetically related to W-Sn mineralization. Combining this with previously published data from the Late Jurassic two-mica granites and related mineralization, we suggest that there were two stages of W-Sn mineralization at Xiangdong. Arsenopyrite geothermometry from the two stages suggests temperatures of 300° to 491° and 308° to 450°C in stage I and stage II, respectively. Wolframite grains from the two stages also show different characteristics and patterns for their major and trace elements. The enrichment in Sc in wolframite suggests low-pH and low-Eh conditions for a fluid containing F– and/or PO43− complexes during stage I, whereas higher contents of Nb and Ta and lower contents of Sc in wolframite from stage II indicate relatively lower pH and higher Eh conditions. In combination with data from other recent studies, we propose that the 145 to 130 Ma interval represents a newly recognized W-Sn metallogenic period linked with highly evolved granites in the Nanling Range metallogenic belt. The Early Cretaceous muscovite granite is an important new target for W and Sn resources in south China.

Valunistoe Epithermal Au–Ag Deposit (East Chukotka, Russia): Geological Structure, Mineralogical–Geochemical Peculiarities and Mineralization Conditions

The Valunistoe Au–Ag deposit is the third largest among epithermal deposits in Chukotka after the Kupol and Dvoinoe. It is located at the western closing of the East Chukotka flank zone of the Okhotsk-Chukotka volcanic belt. Volcanic domes (Pravogornenskaya, Zhil’ninskaya, Shakhskaya, Valunistaya, Shalaya, and Oranzhevaya, each is 3–6 km in diameter) have the main ore-controlling significance in the area; they form a chain elongated to the northeast, along the Kanchalan fault zone. Near the deposit, Upper Cretaceous volcanics are widespread: ignimbrites, lavas and tuffs ranging from rhyolite to basaltic composition, and lenses and interbeds of sedimentary rocks, subvolcanic bodies and dikes of andesites, basalts, and dacites. The structure of the deposit is caused by its localization within the limits of the eponymous (Valunistaya) volcanic dome. Twelve ore-bearing vein zones with thicknesses ranging from several to several tens of meters have been revealed at the deposit. The Glavnaya (Main) and Novaya (New) vein zones have been studied in detail; they are traced along their strikes to a distance of more than 1500 m and consist of en echelon veins 1.0 m thick on average, with lengths varying from 100 to 400 m. Based on the sampling data, Au and Ag contents in ores are 0–474.3 and 0–3794.23 g/t, respectively. Colloform-banded structures are frequently encountered, often combined with breccia structures. The main vein minerals are quartz and adularia; calcite, chlorite, fluorite, sericite, pyrophyllite, kaolinite, montmorillonite, gypsum, and epidote are less frequent. The main ore minerals are pyrite, acanthite, chalcopyrite, galena, sphalerite; secondary ore minerals are native Au and Ag and polybasite; rare ore minerals are pearceite, magnetite, hematite, marcasite, freibergite, tetrahedrite, bournonite, hessite, matildite, and others. Ores are characterized by an Au/Ag ratio from 1 : 5 to 1 : 10 and sulfidity (0.5–5%). Ores are enriched in many elements (Au, Ag, Sb, Cd, Pb, Cu, Zn, As, Se, Mo, Te, and Cr), with enrichment factors ranging from several times (Se, Mo, Te, and Cr), to tenfold (Cd, Pb, Cu, and Zn) and hundredfold (Sb) levels, reaching an excess of tens and hundreds thousand times for Au and Ag (Fig. 7). Ores are characterized by a low total REE and demonstrate positive Eu anomalies. Geochemical features are consistent with the mineral composition of ores. Full homogenization of fluid inclusions in quartz occurs at temperatures of 203–284°C and 174–237°C in calcite, while the salt concentration in both cases is from 0.2 to 0.7 wt % NaCl equiv. Fluid density changes from 0.87 to 0.56 g/cm3. The results give grounds to attribute the Valunistoe deposit to the low-sulfidized epithermal class. The data provided in the article are of practical value for regional forecast–metallogenic maps and can be used in searching for and appraising epithermal Au–Ag deposits.

Geology, geochronology and geochemistry of the Miocene Jiaoxi quartz vein-type W deposit in the western part of the Lhasa Terrane, Tibet: Implications for ore genesis

The Jiaoxi deposit is the first Miocene quartz vein-type W deposit discovered in the western part of the Lhasa Terrane comprising with 39,000 metric tons of WO3. The mineralization occurs in wolframite-bearing hydrothermal tensional quartz veins and is associated with Miocene granites. The mineralization event can be divided into an early quartz-oxide stage succeeded by a sulfide stage, and a final fluorite-carbonate stage. Magmatic intrusions at the deposit include biotite monzogranite porphyry, biotite monzogranite, granite porphyry, and muscovite granite at depth and show crystallization ages ranging between 14.5 and 13.7 Ma. Whole-rock geochemistry shows that the intrusions are alkali-rich peraluminous granites that experienced strong fractional crystallization. The most evolved muscovite granite has high W, Nb and Cs contents and is considered to be the source for mineralization. During the late crystallization stage of the highly fractionated muscovite granite, exsolved hydrothermal fluids interacted with the muscovite granite magma and migrated along extensional fractures forming the Jiaoxi deposit. Zircon grains from the granites show uniform εHf (t) values with a wide range between −7.08 and +3.87. The granites have initial Sr isotopic compositions ranging between 0.7094 and 0.7124 and negative εNd (t) values ranging between −4.9 and −9.4. The Sr-Nd-Hf isotope data indicate that the granites were derived from partial melting of a crustal source with minor contributions from the lithospheric mantle. The 3He/4He ratios for wolframite and pyrite vary between 0.30 and 0.74 Ra (Ra = 1.4 × 10–6 for air), confirming that W was mainly sourced from the crust. Collectively, the regional geology, geochronology and geochemistry, indicate that the Miocene post-collisional peraluminous granites and associated W mineralization were triggered by the roll-back and gradual break-off of the northward-subducting Indian continental lithosphere slab. The less evolved biotite monzogranite porphyry, which contains ore-bearing veins and local greisen alteration, appears to be unrelated to W mineralization.

The Lukoganskoe Au–Cu Skarn Deposit (Eastern Transbaikalia): Mineral Composition, Age, and Formation Conditions

Abstract —The Lugokanskoe deposit is located in southeastern Transbaikalia and has been studied for a long time by many researchers. However, the type of its formation is still debatable. In this paper we study the mineral composition of ores by modern methods, recognize and describe the main gold mineral assemblages, and present detailed data on the chemical composition of native gold and sulfide minerals and their isotope composition. We have established that gold–pyrite–chalcopyrite–arsenopyrite and gold–bismuth parageneses localized in skarn deposits are the main productive assemblages. Study of the sulfur isotope composition of sulfide minerals has shown an endogenous source of sulfur of the ore minerals. The carbon and oxygen isotope compositions of carbonates of ore-bearing veins indicate the participation of a magmatic fluid. The established age of the gold mineralization and igneous rocks of the Shakhtama complex, together with direct geological observations, points to their spatial, temporal, and genetic relationships. According to their petrochemical and geochemical characteristics, the igneous rocks of the Shakhtama complex are I-type ilmenite (reduced) granitoids. Study of fluid inclusions by heating and cooling and Raman spectroscopy has shown that the mineral formation was accompanied by a gradual decrease in the content of salts in the ore-forming fluids and by a decrease in their homogenization temperatures. Optical observations demonstrate that the fluid was heterogeneous at the early stages of the mineral formation. The evolution of the ore system was accompanied by a change in the gas phase composition of fluid inclusions from predominantly nitrogen–carbon dioxide to essentially aqueous, with carbon dioxide impurity (H2O + CO2 ± N2 → H2O ± CO2). The research data testify to the magmatic nature of fluids and the participation of meteoric waters at the late stages of the ore-forming process. The data obtained have led to the conclusion that the Lugokanskoe gold deposit is related to reduced intrusions formed at a shallow depth.

Double-vein (ore-bearing vs. ore-free) structures in the Xitian ore field, South China: Implications for fluid evolution and mineral exploration

Two types of spatially and temporally associated hydrothermal veins, ore-bearing and ore-free, occur as double vein structures in the Xitian ore field, South China. Here, we investigated the genetic relationship, material provenance, fluid properties, and prospecting significance of the co-developed veins in the granite-related, fault-controlled vein-type Xiangdong W, Goudanlan W, Chaling Pb–Zn and Xinggao fluorite deposits of the Xitian ore field, focusing on two common minerals: quartz and zircon. Petrographic, CL, microthermometric and H–O isotopic analyses of Stage I and Stage II quartz in both the ore-bearing and ore-free veins of these deposits document two episodes of hydrothermal activity. The first episode involved a high-temperature (up to 370 ℃), high-salinity (up to 28% NaCl equiv.), high-δ18O (–2.1‰ to +7.2‰), and high-δ2H (–82‰ to –54‰, mean = –68‰, n = 13) fluid that corresponds to the main magmatic-hydrothermal stage of the deposits, and the second episode involved a low-temperature (down to 90 ℃), low-salinity (down to 1% NaCl equiv.), low-δ18O (–8.8‰ to –5.3‰), and low-δ2H (–83‰ to –60‰, mean = –73‰, n = 6) fluid that represents an accessary mineralization stage. Variations in fluid inclusion types and compositions indicate that the processes of fluid mixing, boiling, cooling, and degassing played important roles in mineralization, and that magmatic fluid became less important than meteoric water along during the formation of ore-free veins. LA-ICPMS U–Pb dating reveals totally different age populations for these veins: zircons from the ore-bearing quartz veins yield older and more variable ages (pre-Devonian: >400 Ma, n = 68) that are similar to that of the local basement/sedimentary rocks, whereas zircons from the ore-free quartz veins yield younger and less variable ages (Triassic: 250–200 Ma, n = 31; and Jurassic: 170–140 Ma, n = 13) that are identical to those of the host granite zircons. In addition, the ore-free veins show lower temperatures, salinities, and total REEs (as recorded by quartz) but greater alteration (as reflected by CL and trace element features of hydrothermally-altered zircons) and wider vein size than the ore-bearing veins. We propose a “mineral–water separation” model to explain the formation of the double-vein structures. In this model, ore-bearing veins and ore-free veins formed in succession during faulting events: ore-bearing veins formed first in the smaller fractures and were the dominant sites of mineralization, whereas the ore-free veins served as dissipating passageways for residual water and silica in larger fractures. We suggest that the mineral-water separation model has good application prospects in mineral exploration, and that zircons can be applied as an indicative mineral to differentiate the mineralization potential of different types of quartz veins.

Topological characterization of a polyphased gold-bearing vein network

It is challenging to evaluate the connectivity of metal-bearing hydrothermal vein systems, i.e. the degree to which fractures or veins are connected within a network. Moreover, many ancient ore-bearing vein networks have been deformed and metamorphosed, which hinders the reconstruction and quantification of their primary characteristics. Here, we explore the reliability and limitations of the network distribution and topologic (classification of vein terminations into I, Y and X nodes) approaches to study deformed and metamorphosed mineral deposits. We focus on the Cheechoo gold deposit located in the Eeyou Istchee James Bay area of Quebec, Canada (Archean Superior Province). This gold deposit shows a complex hydrothermal and magmatic polyphased gold-bearing vein system hosted in a granodioritic-tonalitic intrusion. The Cheechoo deposit is composed of quartz-feldspar-diopside (Qtz-Fsp-Di) and quartz (Qtz) veins that underwent ductile deformation and amphibolite facies metamorphism.The high-grade metamorphism led to the modification of vein shape, relative position and spacing, making these parameters unreliable for the characterization of the Cheechoo vein system and similar metamorphosed networks. However, the number of veins per length unit and topology are reliable tools to characterize such networks. In the gold-bearing Cheechoo system, the gold mineralization appears to be mainly controlled by (i) the number of Qtz and Qtz-Fsp-Di veins per length unit and by (ii) the connectivity of these networks as shown by the increase of both parameters with the gold grade.Furthermore, we introduce the Y/I ratio as a simple topological parameter to quantify the connectivity of two dimensional (I-Y dominated) systems. This parameter is positively correlated with the number of connections per branch (CB) used to quantify network connectivity, and ultimately represent the development of a fault or fracture zone.

Mineralogy, alteration, and sulfur isotope geochemistry of the Zehabad intermediate-sulfidation epithermal deposit, NW Iran

The Zehabad Pb-Zn-Au-Ag (Cu) deposit lies in the Alborz magmatic arc of northwestern Iran. Ore-bearing breccia veins hosted by Eocene tuffs emplaced along the 80?130° trending fault and fracture zone. Mineralization occurs in the contact of the late Eocene igneous bodies and the Eocene volcanic and volcanosedimentary Karaj Formation. Mineralization formed in five stages: 1) disseminated framboidal pyrite and minor chalcopyrite and sphalerite; 2) quartz veins containing chalcopyrite, bornite, pyrite, and sphalerite; 3) deposition of specularite and gold grains hosted in quartz veins that crosscut chalcopyrite; 4) the main stage of mineralization that contains galena, sphalerite, tennantite-tetrahedrite, pyrite, sulfosalts, and gold; 5) barren quartz-calcite veins with sulfide mineral fragments of earlier stages. The hydrothermal alteration from closest to the veins outwards includes: a) silicification; b) phyllic with quartz, pyrite, sericite, and calcite; c) argillic with illite, kaolinite, and montmorillonite; d) propylitic containing epidote, calcite, chlorite, and sericite and; e) carbonatization that crosscuts all previous alteration types. Quartz and calcite are the most important gangue minerals at the deposit and show a close relationship with mineralization. Sulfur isotope compositions (0.8‰ to -10.1‰) suggest that the ore-forming fluids derived from magmatic sources with a temperature range of 276-288 °C. According to the field (macroscopic), microscopic, alteration, and sulfur isotope studies, the Zehabad base and precious metal mineralization is considered an intermediate-sulfidation epithermal deposit.