Main Ore Research Articles

Magmatic Evolution of Granodiorite Intrusions at the El Salvador Porphyry Copper Deposit, Chile, Based on Trace Element Composition and U/Pb Age of Zircons

Uranium-lead ages and trace element compositions of zircon from a series of shallow porphyry intrusions document the temporal, chemical, and thermal magmatic evolution of magmatic-hydrothermal porphyry Cu (Mo-Au) ores in the El Salvador district, Chile. Zircons ( n = 240) from 15 Eocene age diorite, granodiorite, and granite porphyry intrusions were analyzed by SHRIMP-RG ion microprobe. The weighted means of 207 Pb-corrected 206 Pb/ 238 U zircon ages span 3 m.y. from about 44 to 41 Ma, with peak magmatic flux at 44 to 43 Ma. The granodiorite porphyries at the Turquoise Gulch copper deposit record waning stages of magmatism at 42.5 to 42.0 Ma and were followed by postmineral latite dikes at about 41.6 Ma. Porphyry copper ores formed contemporaneously with porphyry intrusion centers that progressed temporally from north to south, from the small deposits at Cerro Pelado (~44.2 Ma), Old Camp (~43.6 Ma), and at M Gulch-Copper Hill (~43.5–43.1 Ma) to the main ore deposit at Turquoise Gulch (~42 Ma). The Eocene porphyry intrusions contain a few Mesozoic ( n = 9) inherited zircons and numerous ( n ≥19) antecrystic zircons about 1 to 2 m.y. older than the host intrusion that provide evidence of extensive Eocene magmatic recycling. The Ti-in-zircon geothermometer provides estimates of 890° to 620°C for zircon crystallization and records both core to rim cooling and locally high-temperature rim overgrowths. Most zircon in ore-related K, L, and R porphyries yields near-solidus temperatures of 750° to 650°C and crystallized from compositionally diverse granodiorite porphyries that are a product of crystal fractionation of hornblende, apatite, and titanite with lesser crustal contamination and mixing with high-temperature deep-sourced mafic magma. During a 3-m.y. period, porphyry intrusions tapped an evolving granodioritic magma chamber that was periodically heated, locally remelted, and mixed with mafic magma during recharge events but cooled between recharge events to evolve ore fluids. Europium anomalies (chondrite-normalized Eu N /Eu N * ) in zircons become more pronounced with increased Hf content and cooling but display two distinct evolutionary paths: Eu N /Eu N * of early quartz porphyry evolves from 0.8 to 0.3, whereas the late synmineralization porphyries evolve from 0.8 to 0.65. The Eu N /Eu N * ratio of zircon reflects the Eu 3 +/Eu 2 + ratio of the melt, and therefore the granodiorite porphyries at Turquoise Gulch were the most strongly oxidized of the El Salvador magmas. The strongly oxidized trend porphyry magmas at Turquoise Gulch are apparently directly linked to magmatic degassing at ~700°C to produce large amounts of ore-forming copper, sulfur, and chlorine-enriched magmatic-hydrothermal aqueous fluids.

No genetic link between Late Cretaceous felsic dikes and Carlin-type Au deposits in the Youjiang basin, Southwest China

The Youjiang basin in southwest China is distinguished by preserving a number of Carlin-type gold deposits. Tens of felsic dikes occur in this region, and some are present within the ore fields. A genetic link between these felsic dikes and ore-formation has previously been proposed, but detailed field investigation shows that the dikes crosscut the main ore body, and clearly postdate gold mineralization. Three felsic dikes in the Youjiang basin have been dated, and SIMS zircon U-Pb dating results show that the Liaotun and Xiabaha dikes were emplaced at 97.2±1.1Ma (MSWD=2.9) and 95.4±2.4Ma (MSWD=0.56), respectively, broadly consistent with a LA-ICP-MS zircon U-Pb age for the Bama dike (99.4±0.37Ma, MSWD=1.4). Combined with published dates of the other felsic dikes in this region, it is suggested that the felsic magmatic event in the Youjiang basin occurred between ∼100 and 95Ma. Although a wide range of ages for gold mineralization (275–46Ma) has been reported by a variety of methods, the best available ages suggest a timing between 235 and 193 (Direct Re-Os dating on Fe-sulfides; Chen et al., 2015, Re–Os isochron ages for arsenopyrite from Carlin-like gold deposits in the Yunnan–Guizhou–Guangxi “golden triangle”, southwestern China), significantly older than the emplacement age of the felsic dikes. Taken together, we conclude that there is no genetic link between these Late Cretaceous felsic dikes and Carlin-type Au mineralization in the Youjiang basin.

Evidence for Two Stages of Mineralization in West Africa’s Largest Gold Deposit: Obuasi, Ghana

The supergiant Obuasi gold deposit is the largest deposit in the Paleoproterozoic Birimian terranes of West Africa with 62 Moz of gold (past production + resources). The deposit is hosted in the Paleoproterozoic Kumasi Group sedimentary rocks composed of carbonaceous phyllites, slates, psammites, and volcaniclastic rocks intruded by different generations of felsic dikes and granites. A three-stage deformation history is defined for the district. The D1Ob stage is weakly recorded in the sedimentary rocks as a layer-parallel fabric and indicates that bedding parallel shearing occurred during the early stage of deformation at Obuasi. The D2Ob is the main deformation stage affecting the Obuasi district and corresponds to a NW-SE shortening. Tight to isoclinal folding, as well as intense subhorizontal stretching, occurred during D2Ob, parallel with the plane of a pervasive NE-striking subvertical foliation (S2Ob). Finally, a N-S shortening event (D3Ob) refolded previously formed structures and formed a distinct ENE-striking, variably dipping S3Ob cleavage that is domainal in nature throughout the deposit. Two economic styles of mineralization occur at Obuasi and contribute equally to the gold budget. These are (1) gold-bearing sulfides, dominantly arsenopyrite, mainly disseminated in metasedimentary rocks and (2) native gold hosted in quartz veins that are as much as 25 m wide. Microstructural evidence, such as strain shadows surrounding gold-bearing arsenopyrite parallel with S2Ob, but folded by S3Ob, indicates that the sulfides were formed during D2Ob. Concentrations of as much as 700 ppm Au are present in the epitaxial growth zones of the arsenopyrite grains. Although the large mineralized quartz veins are boudinaged and refolded (indicating their formation during D2Ob), field and microanalytical observations demonstrate that the gold in the veins is hosted in microcracks controlled by D3Ob, where the S3Ob cleavage crosscuts the quartz veins in the main ore zones. Thus, these observations constitute the first evidence for multiple stages of gold deposition at the Obuasi deposit. Futhermore, three-dimensional modeling of stratigraphy, structure, and gold orebodies highlights three major controls on oreshoot location, which are (1) contacts between volcaniclastic units and pre-D1 felsic dikes, (2) fault intersections, and (3) F3Ob fold hinges. The maximum age for the older disseminated gold event is given by the age of the granites at 2105 ± 2 Ma, which is within error of hydrothermal rutile in the granites of 2098 ± 7 Ma; the absolute age of the younger gold event is not known.

Influences of Hydraulic Fracturing on Fluid Flow and Mineralization at the Vein-Type Tungsten Deposits in Southern China

Wolframite is the main ore mineral at the vein-type tungsten deposits in the Nanling Range, which is a world-class tungsten province. It is disputed how wolframite is precipitated at these deposits and no one has yet studied the links of the mechanical processes to fluid flow and mineralization. Finite element-based numerical experiments are used to investigate the influences of a hydraulic fracturing process on fluid flow and solubility of CO2and quartz. The fluids are aqueous NaCl solutions and fluid pressure is the only variable controlling solubility of CO2and quartz in the numerical experiments. Significant fluctuations of fluid pressure and high-velocity hydrothermal pulse are found once rock is fractured by high-pressure fluids. The fluid pressure drop induced by hydraulic fracturing could cause a 9% decrease of quartz solubility. This amount of quartz deposition may not cause a significant decrease in rock permeability. The fluid pressure decrease after hydraulic fracturing also reduces solubility of CO2by 36% and increases pH. Because an increase in pH would cause a major decrease in solubility of tungsten, the fluid pressure drop accompanying a hydraulic fracturing process facilitates wolframite precipitation. Our numerical experiments provide insight into the mechanisms precipitating wolframite at the tungsten deposits in the Nanling Range as well as other metals whose solubility is strongly dependent on pH.

Geochemical, microthermometric, and isotopic constraints on the origin of fluorite deposits in central Anatolia, Turkey

We investigate rare earth element geochemistry, microthermometric characteristics, and radiogenic isotope systematics of fluorites and stable isotope compositions of gangue minerals from several fluorite deposits in central Turkey. In the deposits, fluorite is the main ore mineral and it is accompanied by quartz, calcite, and minor pyrite and barite. Veins are represented by three different fluorite types based on their color. Total REY contents of fluorites are highly variable, ranging from 24 to 693 ppm. LREE concentrations of fluorites of all colors are similar but medium and heavy REE abundances of green fluorites are nearly an order of magnitude greater than in both host rocks and purple and yellow fluorites, indicating multiple sources for crystallization. REEs show significant fractionation and purple fluorites with relatively low HREE contents were likely precipitated at an earlier stage. As crystallization continued, green fluorites were nucleated because of ion exchange of LREEs with the host rock/minerals. Fluid inclusions yielded a wide range of homogenization temperatures from 86 °C to 292 °C and salinities from 0 to 20 wt.% NaCl equiv. The 87Sr/86Sr ratios of fluorites, varying from 0.707627 to 0.709380, overlap with the range of host rocks. 143Nd/144Nd values suggest two populations: purple fluorites with less radiogenic and green fluorites with more radiogenic Nd isotope ratios. The Sr-Nd isotope systematics of Bayindir fluorites are consistent with that of the Bayindir syenite, indicating that hydrothermal solutions progressively reacted with the host rock until equilibrium was established. δ18O values of quartz are slightly higher than the magmatic range. δ13C and δ18O of calcites fall into the range of marine carbonates. δ34S values of barites indicate derivation from diverse reservoirs changing from marine to terrestrial sources. In contrast, sulfur in pyrites points to a magmatic origin. Therefore, is it suggested that magmatic fluids to some extent contributed to the precipitation of fluorite veins.

Perfection Of Methods Of Mathematical Analysis For Increasing The Completeness Of Subsoil Development

The economy of Russia is based around the mineral-raw material complex to the highest degree. The mining industry is a prioritized and important area. Given the high competitiveness of businesses in this sector, increasing the efficiency of completed work and manufactured products will become a central issue. Improvement of planning and management in this sector should be based on multivariant study and the optimization of planning decisions, the appraisal of their immediate and long-term results, taking the dynamic of economic development into account. All of this requires the use of economic mathematic models and methodsApplying an economic-mathematic model to determine optimal ore mine production capacity, we receive a figure of 4,712,000 tons. The production capacity of the Uchalinsky ore mine is 1560 thousand tons, and the Uzelginsky ore mine - 3650 thousand. Conducting a corresponding analysis of the production of OAO “Uchalinsky Gok”, an optimal production plan was received: the optimal production of copper - 77961,4 rubles; the optimal production of zinc - 17975.66 rubles. The residual production volume of the two main ore mines of OAO “UGOK” is 160 million tons of ore.

A comparison of Jiaojia- and Linglong-type gold deposit ore-forming fluids: Do they differ?

The Jiaodong peninsula contains the most important concentration of gold deposits in China, which can be divided into Jiaojia-type and Linglong-type deposits based on mineralization style. The former is characterized by disseminated- and stockwork-style mineralization hosted in first-order regional faults, with relatively larger tonnages and lower gold grades. The latter is characterized by massive auriferous quartz veins commonly hosted in subsidiary second- or third-order faults, with smaller tonnage but higher grade orebodies. Despite these differences, both groups of deposits have the same alteration assemblages, mineral paragenesis, element concentrations, and ore-forming ages.The mainly Jiaojia-type Luoshan gold deposit and the mainly Linglong-type Fushan gold deposit are characterized by H-O-S-Pb isotope data that indicate the ore-forming fluids have a dominantly metamorphic source. The fluids were derived during the Yanshanian orogenic event, and were most likely associated with dehydration and decarbonization processes near the top of the subducting paleo-Pacific plate. The Linglong-type ores have relatively lighter calculated δ18O compositions (−3.9 to −2.3‰) than the Jiaojia-type ores (0.3–8.0‰), possibly because of a greater degree of mixing with meteoric water. Petrographic, cathodoluminescence, microthermometric, and laser Raman spectroscopic analyses of fluid-inclusion assemblages in quartz from the two types of ores indicate fluids were similar, in both cases characterized by medium–high homogenization temperatures (211–393°C), significant CO2 (∼15%mol), minor CH4 (⩽18% in the carbonic phase), and low salinity (⩽11.2wt% NaCl eq.). The Linglong-type ores, however, have a wider range of CO2 and CH4 concentration and salinity than the Jiaojia-type ores. Fluid immiscibility, occurred in main ore stage of both ore types, with the trapping conditions of 77–185MPa and 284–328°C, although the unmixing is more intense and widespread in the Linglong-type ores. Both fluid-wallrock interaction and fluid immiscibility are important gold-deposition processes in the two types, but immiscibility is more important in the Linglong-type ores and that has led to the typical higher gold grade.In general, there is little geochemical differences between the ore-forming fluids for Jiaojia- and Linglong-type gold deposits. Both Jiaojia- and Linglong-type ores can exist in a single deposit and form in the same metallogenic event. The Linglong-type ores developed as more massive veins, because of their location in zones of more extensive extension and they lack significant post-ore cataclastic deformation.

Nature and origin of dolomitization associated with sulphide mineralization: new insights from the Tappehsorkh Zn‐Pb (‐Ag‐Ba) deposit, Irankuh Mining District, Iran

Several types of dolomitization are observed in the Lower Cretaceous carbonate sequence of the Tappehsorkh deposit, Irankuh Mining District, Iran: 1) regional and extensive early diagenetic dolomitization (D1), and 2) localized hydrothermal dolomitization (D2 and D3). D1 regional dolomite, which partly to completely replace limestone, is characterized by fine‐grained euhedral to subhedral dolomite rhombs. There is no evidence for Zn‐Pb sulphide mineralization associated with this type of dolomite. Medium to coarse‐grained D2 and D3 hydrothermal dolomites occur along the syn‐sedimentary Gushfil‐Baghabrisham normal fault, about 40 m above the Lower Cretaceous sequence within black siltstone, dolostone, and crystal lithic tuff and lava rocks. Hydrothermal dolomite cross‐cuts and brecciates the host rocks. This dolomite was replaced by quartz and sulphide minerals of the main ore stage. D1 regional dolomite has average values of ‐7.37‰ and 2.20‰ for δ18O and δ13C, respectively. The δ13C values of this dolomite fall well within the range of the Lower Cretaceous carbonates. D2 hydrothermal dolomite has average δ18O and δ13C values of ‐7.92‰ and 2.93‰, respectively. The respective δ18O and δ13C values of D3 hydrothermal dolomite are ‐12.27‰ and 2.39‰. The δ18O values of D2 and D3 dolomites are more negative than those of D1 regional dolomite; this can be due to their hydrothermal origin. Fluid inclusion studies on D3 dolomite and quartz show temperatures of 170‐260 °C. The concentrations of Fe and Mn in D1 regional dolomite are very similar to those of limestone, suggesting that they likely precipitated from the same fluid (seawater), whereas those of D2 and D3 dolomites are relatively high, in agreement with the hydrothermal origin for these dolomites. Precipitation of gypsum, which is ubiquitous in the study area, could have lowered the Ca/Mg ratio of seawater. While the extensive regional dolomitization formed from low‐temperature (evolved) seawater during the early diagenesis, D2 and D3 dolomites associated with the main stage of sulphide mineralization were formed by high‐temperature hydrothermal fluids moving along the Gushfil‐Baghabrisham Fault. Copyright © 2016 John Wiley & Sons, Ltd.

The Iron Age copper industrial complex: A preliminary study of the role of ground stone tools at Khirbat en-Nahas, Jordan

The first industrial revolution in the southern Levant crystallized during the Iron Age when copper production reached scales never before seen in this part of the Middle East. Ever since copper ore was first smelted during the Chalcolithic period, the Arabah valley, and its widespread distribution of copper mineralization, was the main source for copper ore in the region. The main ore deposits are located in Timna (Israel) in the southern part of the valley, and some 105 km to the north, in the Faynan region (Jordan). Faynan is the largest copper ore resource zone in the southern Levant. Excavations at the Iron Age Faynan site of Khirbat en-Nahas and the recent final publication of that project have revealed peaks in industrial-scale production during the 10th and 9th centuries BCE. However, the role of ground stone tools in the Iron Age copper industry in Faynan has not been systematically presented. This paper presents a preliminary study of the ground stone assemblage from one excavation season at Khirbat en-Nahas, thereby highlighting the great potential for ground stone tools research at the site. Using the chaîne opératoire method of technological study, this paper takes a quantitative approach to the typological, material, and spatial distribution of ground stone artefacts at Khirbat en-Nahas to understand their role in copper production. Ethnoarchaeological study of hereditary bronze casting workshops in southern India provides a compelling model of how ground stone tools played a critical role in one of the most important dimensions of metal production in all periods - recycling - in an Iron Age copper factory.

Petrogenesis of granitoids in the Dewulu skarn copper deposit: implications for the evolution of the Paleotethys ocean and mineralization in Western Qinling, China

The Tethyan tectonic domain hosts numerous world-class mineral deposits. Among these, the Dewulu skarn copper deposit in Western Qinling, China belongs to the Paleotethys ore belt. The skarn and orebodies here occur as stratoids or lenses at the contact between the Triassic Dewulu intrusive complex and Permian marine clastic and carbonates. Alteration minerals include prograde skarns (garnet, diopside, wollastonite), plagioclase, hornblende, actinolite, tremolite, epidote, chlorite, calcite, quartz and sericite. The main ore types include early disseminated skarn-type replacement orebodies and late-stage quartz-sulfide veins. Chalcopyrite is the major ore mineral, along with pyrite, bornite and sphalerite. The Dewulu intrusive complex comprises quartz diorite, quartz diorite porphyry and dioritic mafic microgranular enclaves (MME). The MMEs are spheroidal in shape, and have igneous mineral assemblages, acicular apatites, complex oscillatory zoned plagioclase and quartz megacrysts surrounded by mafic minerals. The MMEs are metaluminous and calc-alkaline to high-K calc-alkaline, and possess relatively high Ni, Cr and MgO contents and Mg# values. They display sub-parallel patterns in trace element spider diagrams and rare earth element (REE) plots. They are also characterized by the enrichment of Rb, U and Th, depletion of Ba, Sr, Nb and Ta and negative Eu anomaly. Zircon LA-ICP-MS U–Pb dating of the dioritic MME yields an age of 247.0±2.2Ma, coeval with the host quartz diorite, quartz diorite porphyry and ore-related sericite 40Ar/39Ar plateau ages within analytical uncertainties. Oxygen fugacity estimated from trace element compositions of zircons from the dioritic MME shows FMQ±3.3. The zircons have negative εHf(t) values in a range of −8.0 to −3.3, corresponding to two-stage model ages ranging from 1.48 to 1.78Ga. The integrated data from petrology, geochronology and bulk geochemistry suggest that the Early Triassic granitoids associated with Cu skarn mineralization at Dewulu were products of arc magmatism and involved magma mixing in an active continental margin setting. The magma was sourced through partial melting of enriched sub-continental lithospheric mantle that had been previously modified by slab-derived melt during the continuous northward subduction of the Paleotethys oceanic slab.

Copper mining and smelting technology in the northern Lowveld, South Africa, ca. 1000 CE to ca. 1880 CE

We report chemical, petrographic and metallographic studies of copper ores and slags recovered during sporadic surface surveys and excavations over the past fifty years in the Phalaborwa and Murchison Range areas of the northern Lowveld of South Africa. The copper slags around Phalaborwa have unusual mineral assemblages, attributable to the unique geochemistry of the main ore body, the Phalaborwa Complex, where copper minerals were mined from a carbonatite composed of magnetite, calcite and apatite. Strongly reducing conditions had to be avoided to minimise contamination of the copper with iron and phosphorus. As the copper ores contain almost no silicates, silica/alumina flux was added to produce slag. The Precambrian zinc-copper ores of the Murchison Range were also smelted, but during smelting any zinc that was not volatilised was taken up by minerals in the slag, so brass was not produced.

Characterization and origin of low-T willemite (Zn2SiO4) mineralization: the case of the Bou Arhous deposit (High Atlas, Morocco)

Willemite (Zn2SiO4) usually reported in hypogene non-sulfide deposits is described as the main ore mineral in the carbonate-hosted Bou Arhous zinc deposit. This deposit is located in the High Atlas intracontinental range that formed during the Tertiary. Based on a set of microscopic observations, it was possible to establish that willemite replaces primary sphalerite. On the basis of cathodoluminescence imaging, three successive generations of willemite are distinguished, with evidence of dissolution–reprecipitation processes. Willemite is also variably enriched in Ge (up to 1000 ppm), while Ge contents lower than 100 ppm are reported in the primary sulfide minerals. Depending on the willemite generation, this substitution was positively or negatively correlated to the Zn-Pb substitution. According to the nature of zoning (sector versus oscillatory), the incorporation of Ge was either controlled by crystallographic factors or by the nature of the mineralizing fluids. Willemite is associated with other oxidation-related mineral species, like cerussite (PbCO3) but is not in isotopic equilibrium and therefore not considered to be cogenetic. Oxygen isotope compositions support the formation of willemite at temperatures below 130 °C, from mixed meteoric and deeper, hydrothermal fluids. The formation of the High Atlas Belt during the Tertiary has contributed to the exhumation of the sulfide minerals and the development of vertical conduits for percolation of meteoric water and ascending hydrothermal fluids. In addition to a local contribution of silicate minerals of the host limestone, hydrothermal fluids probably transported Si and Ge that are incorporated in willemite.

Genesis of the Late Jurassic Shizitou Mo deposit, South China: Evidences from fluid inclusion, H[sbnd]O isotope and Re[sbnd]Os geochronology

The Shizitou deposit is a middle-sized Mo deposit in the Qin-Hang metallogenic belt, South China. Three stages of mineralization are recognized at the Shizitou, including the early stage characterized by barren quartz veins with potassic alteration, followed by the main ore stage represented by quartz-molybdenite veins with strong phyllic alteration, and the late stage featured by quartz-pyrite veins with weak propylitization. Two-phase liquid-rich aqueous (type I), two-phase gas-rich aqueous (type II), and halite-bearing (type III) inclusions are present in the main ore stage veins, but only type I inclusions occur in the early and late stages of veins. Type I inclusions in the early stage veins show homogenization temperatures (Th) of 291–376°C, with salinities of 2.2–9.5wt.% NaCl equivalent. Type II and III inclusions in the main ore stage veins display Th of 266–351°C and 245–336°C, and salinities of 0.6–3.5 and 28.0–36.2wt.% NaCl equivalent, respectively. Type I inclusions in the late stage quartz-pyrite veins have Th of 209–288°C and salinities of 0.5–5.0wt.% NaCl equivalent. Fluid boiling took place in the main ore stage and promoted the quick precipitation of molybdenite from ore-forming solutions. Raman analysis of fluid inclusions implies that mineralizing fluids contain little CO2 and N2. Hydrogen and oxygen isotopes indicate that initial ore-forming fluids were magmatic in origin and shifted gradually to meteoric regime. ReOs dating of molybdenite gives a 187Re/187Os isochron age of 162.3±2.4Ma, in response to a localized extensional setting resulting from the far-field stresses of the initial subduction of the Paleo-Pacific plate beneath the margins of the South China Block.

Polymetallic sulfide ores hosted in Late Permian carbonate at the Alanish locality, northern Iraq: petrography and mineral chemistry

Polymetallic sulfide ores (Zn, Pb, Fe, Cu, Ag, and Cd) found in the Alanish locality of northern Iraq are hosted by dolostone in the Late Permian Chia Zairi Formation. The Alanish locality is one of several Zn–Pb deposits that are widespread in northern Iraq, situated along the northern passive margin of the Arabian plate. This paper describes the ore deposit classification, mineral chemistry, and paragenetic sequence of the area and proposes an ore formation model. We report the presence of acanthite and greenockite for the first time in Iraq. A brine solution derived from the sedimentary basin formed the primary sulfide ore minerals (sphalerite, galena, acanthite, pyrite, chalcopyrite, greenockite, and marcasite). The pre-tectonic mineralization is characterized by replacement textures including (1) high-Fe, low-Zn, dark-colored, coarse-grained sphalerite; (2) deformed anisotropic coarse-grained galena; and, (3) idiomorphic cubes of crushed pyrite. Conversely, the post-tectonic mineralization is characterized by open-space filling textures, including (1) low-Fe, high-Zn, light-colored, fine aggregated sphalerite; (2) fine-grained galena; and, (3) the existence of acanthite and marcasite. Although galena is an Ag carrier, both mineralization phases contained non-argentiferous galena. Non-sulfides (smithsonite, cerussite, and goethite) have replaced older sulfides in many areas due to supergene process. Gangue minerals present are dolomite, calcite, barite, and siderite. Open spaces and cavity filling of small paleo-karsts, replacement, veins, and veinlets are common features of the ore body. Metals were sourced from brines generated in the sedimentary basin, whereas sulfur was derived from nearby evaporates. Sediment compaction and tectonic activity, probably during Late Cretaceous, were the driving forces that squeezed and moved ore-bearing fluids derived from the sedimentary basin. Multiple stages of ore-bearing fluids were epigenetically intruded into the Late Paleozoic dolostone, forming an epigenetic strata-bound Mississippi Valley-type deposit precipitated under a temperature of 120 °C, as indicated by the cadmium fractionation in sphalerite and galena. Dolomitization and tectonic activity provided the necessary permeability for accumulating ores. The main ore body is directly connected to a fault plane and to adjacent dolostone that is frequently fractured and brecciated.

Molybdenite Re–Os, zircon U–Pb dating and Lu–Hf isotopic analysis of the Xiaerchulu Au deposit, Inner Mongolia Province, China

The Xiaerchulu Au deposit, located in the Southern Orogenic Belt (SOB) of Western Inner Mongolia (WIM), is hosted in an Early Permian (271–261Ma) volcanic-plutonic sequence. Mineralization took place in silicified biotite granites or along the contact zone between the Neoproterozoic Baiyinbaolage Group and the biotite granite. In order to constrain the timing of the Xiaerchulu mineralization and discuss the petrogenesis of the hosting granites, molybdenite Re–Os, and zircon U–Pb and, Lu–Hf, and REE, geochemical, and Sr–Nd isotopic studies were completed in this study. We measured Re–Os isotopes of six molybdenite samples from the main ore body, which yielded a weighted average model age of 261.7±1.5Ma with a MSWD of 0.55, indicating that the time of mineralization was at ca. 262Ma. High precision U-Pb dating for the studied granites yields Permian 206Pb/238U ages ranging from 271 to 269Ma. These age data confirm that both the intrusion and related mineralization were initiated in Early Permian period. These granites are strongly peraluminous with A/CNK=1.11–1.12, high SiO2–K2O contents, as well as containing biotite and muscovite, indicating a petrogenesis of typical S-type granites, the above consideration is also consistent with the result of discrimination diagrams. The Re contents of molybdenite, εNd(t), and zircon εHf(t), as well as the 176Hf/177Hf values of the granites, fall into the ranges from 1.153 to 2.740μg/g, −11.1 to −9.3, −8.8 to −0.9, and 0.282358 to 0.282688, respectively. All of this evidence suggests that the metals were derived from a predominantly crustal source, the granites originated from crust in an extensional setting, and the rejuvenation of the continent may have play an important role during the ore-forming processes of the Early Permian epoch.

Isotopic characteristics of gold deposits in the Yangshan Gold Belt, West Qinling, central China: Implications for fluid and metal sources and ore genesis

The six main Triassic gold deposits of the Yangshan gold belt are hosted by Devonian metasedimentary rocks and Triassic granitic dikes. It has been suggested by previous workers that they are best classified as either Carlin-like or orogenic gold deposit types. Sulfide minerals spatially associated with disseminated and quartz vein-hosted gold mineralization at Yangshan were deposited in five stages that include syn-metamorphic, early ore, main ore, late ore, and post-ore stages. Based on the relationships defined by ore-mineral assemblages and paragenesis, H, O, C, S, Pb, and Sr isotopic data were obtained from sulfides, quartz, and ore host rocks. The δ18O values of quartz from gold-bearing veins range from 15.9‰ to 21.5‰, and the calculated δ18OH2O values of ore-forming fluids vary from 6.4‰ to 11.8‰, which are consistent with those from typical orogenic gold deposits. The δDH2O values of fluid-inclusions extracted from the quartz vary from −82‰ to −56‰, which are in accord with the δDH2O values of most orogenic gold deposits. Interpretation of the δ13CCO2 values (−4‰ to −2.5‰) in fluid inclusion, shows that marine carbonate is the dominant source for the carbon in the ore-forming fluids, with a minor contribution from magmatic sources. The δ34S values of hydrothermal pyrite, arsenopyrite, and stibnite range from −6.6‰ to 3‰, which suggests sulfur is derived from a deep source. The Pb isotope values for hydrothermal sulfides from both the granitic dike-hosted ores and phyllite-hosted ores overlap the fields for their respective wall rocks, which suggests the source of lead is either the local host rocks, or ore-related fluids that may have pervasively penetrated the host rocks. The age-corrected (87Sr/86Sr)i values of pyrite are 0.70627 to 0.71304, and values for arsenopyrite are 0.71258 to 0.71294, showing that the strontium in the ore fluid could have been derived from the granitic dikes and the regionally extensive Mesoproterozoic to Neoproterozoic Bikou Group. The concordance of δ34S, Pb and (87Sr/86Sr)i values between those for hydrothermal sulfides and country rocks suggests that these crustal rocks contributed the gold that was subsequently concentrated in the ore deposits. The syn-metamorphic stage and early ore stage fluids were likely derived from the country rocks during prograde metamorphism at depth. The oxygen isotope compositions indicate relatively widespread interaction of such fluids with δ18O-rich country rocks during the main ore stage. Compared with most orogenic gold deposits and Carlin-type gold deposits, the gold deposits in the Yangshan gold belt exhibit isotopic characteristics that are most like those of orogenic gold deposits.

Petrogenesis and Ni–Cu sulphide potential of mafic–ultramafic rocks in the Mesoproterozoic Fraser Zone within the Albany–Fraser Orogen, Western Australia

The Albany Fraser Orogen is located along the southern and southeastern margins of the Archean Yilgarn Craton. The orogen formed during reworking of the Yilgarn Craton, along with variable additions of juvenile mantle material, from at least 1810Ma to 1140Ma. The Fraser Zone is a 425km long and 50km wide geophysically distinct belt near the northwestern edge of the orogen, hosting abundant sills of predominantly metagabbroic non-cumulate rocks, but including larger cumulate bodies, all emplaced at c. 1300Ma. The gabbroic rocks are interpreted to have crystallised from a basaltic magma that had ∼8.8% MgO, 185ppm Ni, 51ppm Cu, and extremely low contents of platinum-group elements (PGE, <1ppb). Levels of high field-strength elements (HFSE) in the least enriched rocks indicate that the magma was derived from a mantle source more depleted than a MORB source. Isotope and trace element systematics suggest that the magma was contaminated (εNd 0 to −2 throughout, La/Nb around 3) with small (<10%) amounts of crust before and during ascent and emplacement. Larger bodies of cumulate rocks show evidence for additional contamination, at the emplacement level, with country-rock metasedimentary rocks or their anatectic melts. The area has been the focus of considerable exploration for Ni–Cu sulphides following the discovery of the Nova deposit in 2012 in an intrusion consisting of olivine gabbronoritic, noritic and peridotitic cumulates, interlayered with metasedimentary rocks belonging to the Snowys Dam Formation of the Arid Basin. Disseminated sulphides from a drillcore intersecting the structurally upper portion of the intrusion, above the main ore zone, have tenors of ∼3–6.3% Ni, 1.8–6% Cu and mostly <500ppb PGE, suggesting derivation from magma with the same composition as the regional Fraser Zone metagabbroic sills, at R factors of ∼1500. However, the Nova rocks tend to have higher εSr (38–52) and more variable δ34S (−2 to +4) than the regional metagabbros (εSr 17–32, δ34S around 0), consistent with the geochemical evidence for enhanced crustal assimilation of the metasedimentary country-rock in a relatively large magma staging chamber from which pulses of sulphide bearing, crystal-charged magmas were emplaced at slightly different crustal levels. Preliminary investigations suggest that the critical factors determining whether or not Fraser Zone mafic magmas are mineralised probably relate to local geodynamic conditions that allow large magma chambers to endure long enough to sequester country-rock sulphur.

Geological characteristics and origin of the Hadamengou gold deposit in Inner Mongolia, China

The Hadamengou gold deposit is located in the western segment of the northern margin of the North China Craton (NCC). It is hosted by Archean metamorphic rocks of the Wulashan Group. The main ore types include gold-bearing quartz vein type, gold-bearing quartz-potassic feldspar vein type, and gold-bearing altered rock type. Gold mineralization is closely related to K-feldspathization. Hydrogen and oxygen isotope data indicate that ore-forming fluids were dominated by magmatic water mixed with minor meteoric water. Sulfur and lead isotope data indicate that metallogenic materials were mainly supplied by the magmatic and Archean Wulashan Group. The gold mineralization was mainly formed during the Early Indosinian tectonic movement, which drove ore-forming fluids to the favorable depositional environment. The northern margin of the NCC is a prospective area for gold exploration. Gold deposits hosted by or related to alkaline intrusions have become one of the most important mineral exploration targets in northern China.

Fluid inclusion and H–O–S–Pb isotopic evidence for the Dongxiang Manto-type copper deposit, South China

The Dongxiang copper deposit is located in northeastern Jiangxi Province, South China. Ore bodies in the Zishan Formation of this province are primarily stratoid and lentiform and show a close spatial association with Jurassic intrusive rocks. The current debates generally concentrate on whether the Dongxiang copper deposit is a Carboniferous sedimentary deposit or, alternatively, a Jurassic porphyry–skarn deposit. Three stages of mineralization can be observed in this deposit: the pre-ore, main ore, and post-ore stages. The massive sulfide ore bodies are identified as the main ore stage, whereas the pyrite-quartz veins and barren quartz veins are identified as pre-ore and post-ore stages, respectively. Three fluid inclusion types were recognized: two-phase vapor-rich fluid inclusions (type 1), two-phase liquid-rich fluid inclusions (type 2), and halite-bearing fluid inclusions (type 3). All three of these types of inclusions are present in the main copper mineralization stage, but only type 2 inclusions occur in both the pre-ore and post-ore stages. Type 2 inclusions in the pre-ore stage display homogenization temperatures (Th) of 349–392°C, with salinities of 1.4–8.1wt.% NaCl equivalent. In the main ore stage, types 1 and 3 inclusions show similar Th at 289–346°C and 273–348°C but contrasting salinities of 0.4–4.5 and 29.4–41.9wt.% NaCl equivalent, respectively. In the post-ore stage, type 2 inclusions have Th of 144–228°C and salinities of 0.4–6.0wt.% NaCl equivalent. The coexisting types 1 and 3 inclusions in the main ore stage have similar Th ranges but contrasting salinities, which indicates that fluid boiling occurred. The determined hydrogen and oxygen isotopic values for these fluids (δD from −42‰ to −69‰; δ18OH2O 5.04‰ to 9.01‰), based on analysis of fluid inclusions and host quartz, imply that the inferred ore fluids were principally derived from magmatic water. The lead isotopic compositions of sulfides from the ores and plagioclases from the adjacent porphyry indicate that they share a common origin. The sulfides from the ores show δ34S values of 0.3–3.2‰, also suggests a magmatic source. As the fluid inclusions and isotopic compositions strongly suggest a magmatic origin, this argues against a SEDEX origin for this deposit. In addition, the presence of stratoid and lentiform ore bodies and the absence of skarn alteration minerals suggest that the Dongxiang copper deposit is best described as a Manto-type rather than a porphyry–skarn deposit.

Review paper: Exploration geophysics for intrusion‐hosted rare metals

ABSTRACTIgneous intrusions, notably carbonatitic–alkalic intrusions, peralkaline intrusions, and pegmatites, represent significant sources of rare‐earth metals. Geophysical exploration for and of such intrusions has met with considerable success. Examples of the application of the gravity, magnetic, and radiometric methods in the search for rare metals are presented and described. Ground gravity surveys defining small positive gravity anomalies helped outline the shape and depth of the Nechalacho (formerly Lake) deposit within the Blatchford Lake alkaline complex, Northwest Territories, and of spodumene‐rich mineralization associated with the Tanco deposit, Manitoba, within the hosting Tanco pegmatite. Based on density considerations, the bastnaesite‐bearing main ore body within the Mountain Pass carbonatite, California, should produce a gravity high similar in amplitude to those associated with the Nechalacho and Tanco deposits. Gravity also has utility in modelling hosting carbonatite intrusions, such as the Mount Weld intrusion, Western Australia, and Elk Creek intrusion, Nebraska.The magnetic method is probably the most successful geophysical technique for locating carbonatitic–alkalic host intrusions, which are typically characterized by intense positive, circular to sub‐circular, crescentic, or annular anomalies. Intrusions found by this technique include the Mount Weld carbonatite and the Misery Lake alkali complex, Quebec. Two potential carbonatitic–alkalic intrusions are proposed in the Grenville Province of Eastern Quebec, where application of an automatic technique to locate circular magnetic anomalies identified several examples. Two in particular displayed strong similarities in magnetic pattern to anomalies accompanying known carbonatitic or alkalic intrusions hosting rare‐metal mineralization and are proposed to have a similar origin.Discovery of carbonatitic–alkalic hosts of rare metals has also been achieved by the radiometric method. The Thor Lake group of rare‐earth metal deposits, which includes the Nechalacho deposit, were found by follow‐up investigations of strong equivalent thorium and uranium peaks defined by an airborne survey. Prominent linear radiometric anomalies associated with glacial till in the Canadian Shield have provided vectors based on ice flow directions to source intrusions. The Allan Lake carbonatite in the Grenville Province of Ontario is one such intrusion found by this method. Although not discovered by its radiometric characteristics, the Strange Lake alkali intrusion on the Quebec–Labrador border is associated with prominent linear thorium and uranium anomalies extending at least 50 km down ice from the intrusion. Radiometric exploration of rare metals hosted by pegmatites is evaluated through examination of radiometric signatures of peraluminous pegmatitic granites in the area of the Tanco pegmatite.