High-sulphidation Epithermal Deposits Research Articles

Cu3(As,Sb)S4 minerals from the Baia Mare metallogenic district, Eastern Carpathians, Romania – a case study from the Cisma ore deposit

The occurrence of Cu 3 (As,Sb)S 4 minerals, i.e. the enargite and luzonite-famatinite series, is poorly known in the Neogene Baia Mare metallogenic district, NW Romania, with few records and no analytical data. This study provides the first EPMA and XRD data on enargite/luzonite from the Baia Mare metallogenic district, i.e. the Cisma ore deposit from the Baiuţ metallogenic field, and the Herja ore deposit. Vein-type ore bodies bearing Cu 3 (As,Sb)S 4 minerals from Cisma are hosted by Paleocene–Eocene flysch successions, while the host rock of the enargite-bearing ore from Herja is unknown. The enargite and luzonite-famatinite series are known as “exotic” in low sulphidation epithermal ore deposits, “typical” in high sulphidation epithermal deposits in association with tennantite, and “sometimes common” in small amounts in intermediate sulphidation epithermal ore deposits. The occurrence of enargite/luzonite in the Baia Mare district suggests the need to update the metallogenic interpretation for several ore deposits, partly at the district scale in relation to other ore deposit/mineralogy features, e.g. vuggy silica, hypogene argillic alteration.

What is Down Plunge of the Dobroyde Hill High-Sulphidation Epithermal Deposit, near Junee, NSW? An Emerging Carbonate-Base-Metal-Gold Epithermal System

Discovered by Getty Oil in 1983, the outcropping Dobroyde Hill 77k Oz gold high-sulphidation epithermal prospect is now known to transition into a carbonate base-metal epithermal gold system down plunge. The project is 50km north of Wagga Wagga and 10km north of Junee which is the far southern toe of the Junee-Narromine Volcanic Belt. It is hosted within a 4km long package of middle Ordovician andesitic to basaltic sub-marine volcanic rocks and early Silurian plagioclase and hornblende phyric shoshonitic andesite sub-marine breccias (Crawford 2014, 2017). Revival of the prospect was driven by field mapping armed with a TerraSpec Halo, ground magnetics and 3D IP and magnetic inversions (by Chris Moore, Moore Geophysics). This lead to the recognition of a 3m x 2km surficial dickite ± pyrophyllite ± kaolinite ± illite ± silica ± pyrite alteration in shoshonitic andesite breccias. Follow up drilling which targeted a zone 700m down plunge of the Dobroyde Hill resource was completed in partnership with the NSW Government through the New Frontiers cooperative drill funding scheme. This hole, NDD005, intersected a far larger system than expected, with 280m of intense dickite + pyrophyllite + kaolinite + pyrite ± illite ± silica alteration then 200m of unmistakable carbonate-base-metal-gold epithermal style alteration as presented in Corbett & Leach (1998). Evolution Mining’s giant Cowal Gold Mine (~8M Oz Resource) 120km north of Dobroyde is also characterised as carbonate-base-metal-gold epithermal. Both mineralising styles show a genetic link by overprinting relationships and both having associated broad low-grade gold and silver mineralisation with individual 1m intervals to 9.5g/t gold. NDD005 is interpreted to have stepped over the top of the main epithermal gold zone, hence the target remains open to further exploration. Additionally, this 700m down plunge gap between the new and historic drilling is completely untested and if mineralised could extend the strike length of the deposit to one kilometre.

Spatial distribution and variation of ore body, alteration and ore-forming fluid of the giant Zijinshan epithermal Cu-Au deposit, SE China: implication for mineral exploration

The Zijinshan high-sulphidation Cu-Au deposit located in the west Fujian Province is the largest active gold mine in China with total production of gold over 300 tonnes to date. The high-sulphidation ore bodies are characterized by the upper supergene Au and lower hypogene Cu zones bounded roughly by the paleowater table near 600 m elevation. Au-bearing goethite and limonite are the main ore minerals in the supergene zone whereas the hypogene Cu mineralization is dominated by covellite and anilite (or digenite) with minor enargite. The expected porphyry mineralization that coupled with Zijinshan is so far not exposed even to a depth of 1500 m from surface. However, the Luoboling Cu-Mo deposit which occurs only 3 km NE to Zijinshan is identified as a typical porphyry deposit. Current debate centered on whether the Zijinshan high-sulphidation epithermal deposit and the adjacent Luoboling porphyry deposit are genetically linked or discrete hydrothermal systems. Recent deep exploration at Zijinshan revealed an extension trend of Cu ore bodies and quartz-alunite alteration zone toward SE below 0 m elevation. This contradicts previous considered NE trend above 0 m elevation. In this study, fluid inclusion mapping based on rigorous strategy and sample selection was carried out on 20 sample suites scattered over the whole high-sulphidation Cu-Au ore bodies from −450 to 800 m elevation. Aqueous fluid inclusions of unambiguous primary origin in syn-ore miarolitic quartz, coarse-grained alunite crystals and quartz overgrowth zones were measured. Microthermometry results show the average T h and salinity of each sample suite ranging from 205 to 304°C and 2.4 to 5.5 wt% NaCl equiv., respectively. Both fluid temperature and salinity exhibit negative correlation with increasing elevation, indicating convective cooling by mixing and circulating of meteoric water has involved. The good consistence of mineralization distribution pattern and fluid evolution trend implies that the southeastward extending ore bodies probably reflect the fluid pathway which is connected to the deeper porphyry mineralization and their magmatic source. Considering the temperature condition (370–400°C) required for brittle-plastic transition which commonly characterizes porphyry deposits, the potential porphyry mineralization coupled to Zijinshan should have occurred at greater depth under higher temperature. Combined with recent exploration progress and the fluid evolution pattern, we therefore proposed that the Luoboling porphyry Cu-Mo deposit and the Zijinshan high-sulphidation Cu-Au deposit are likely discrete hydrothermal systems, and future porphyry deposit exploration at the deep SE segment of Zijinshan is recommended.

Hyperspectral cathodoluminescence study of indium-bearing sphalerite from the Mt Carlton high-sulphidation epithermal deposit, Queensland, Australia

Sphalerite is the most important host mineral for the recovery of indium. New techniques to study the presence and distribution of this critical metal in sphalerite can therefore be of interest for scientific and technological purposes alike. In this study we use the emerging tool of hyperspectral cathodoluminescence (CL) mapping, combined with X-ray element mapping and spot analyses, to characterise the composition and CL properties of indium-bearing sphalerite from the Mt Carlton high-sulphidation epithermal deposit (NE Queensland, Australia). Mt Carlton sphalerite contains highly elevated concentrations of indium (up to 19.59 wt%) occurring within similar to 1 mu m thick colloform bands, which show an average composition of (Zn0.63Cu0.20In0.15Ga0.01) S-0.96. Indium, Cu and Ga are interpreted to have been incorporated via the coupled substitution 2Zn(2+) Cu+(In, Ga) (3+). Hyperspectral CL mapping reveals a high-intensity CL emission directly related to In-Cu-(Ga)-rich sphalerite, centred at wavelengths between similar to 500 and similar to 600 nm. The CL peak is shifted to the higher-wavelength positions when the proportion of In relative to Cu increases. Our study shows that hyperspectral CL mapping is a powerful and efficient technique to study the distribution of In in sphalerite.

Release of toxic elements from rocks and mine wastes at the Furtei gold mine (Sardinia, Italy)

The Furtei gold mine in Sardinia (Italy) exploits a volcanic-hosted high-sulphidation epithermal deposit. Large amounts of materials derived from exploitation are present in open pits, waste rock dumps and cyanidation tailings impoundment. Mineralized rocks in outcrops and waste dumps contain significant amounts of sulphides (mainly pyrite and enargite). These materials have a high potential for acid drainage generation and release of toxic elements (notably Cu and As, but also Al, Ni, Co and Cd) as pointed out by laboratory leaching tests and in agreement with chemical composition of waters draining the mining area, that show pH as low as 2, up to 180 mg/L Cu, up to 5 mg/L As, and up to 788 mg/L Al. On the other hand, leaching solutions and waters interacting with mineral assemblages of the propylitic alteration zone (mainly composed of chlorite, quartz, and calcite, with relic magmatic plagioclase) show higher pH, and lower metal loads. Leachates from cyanidation tailings show variable pH (between 6.2 and 9.7, depending on sulphide content in tailings); cyanide concentration varies between 110 µg/L and about 3 mg/L, whereas contents of toxic elements in leachates are, with the exception of Hg, within the limits of Italian regulations for non-dangerous industrial wastes. Reclamation plans provide for confinement of tailings within specific repositories. This measure should effectively reduce the environmental impact of these materials. Reclamation plans should also include an adequate management of other high-sulphide wastes.

Enargite oxidation: A review

Enargite, Cu 3AsS 4, is common in some deposit types, e.g. porphyry systems and high sulphidation epithermal deposits. It is of environmental concern as a potential source of arsenic. In this communication, we review the current knowledge of enargite oxidation, based on the existing literature and our own original data. Explicit descriptions of enargite oxidation in natural environments are scarce. The most common oxidized alteration mineral of enargite is probably scorodite, FeAsO 4.2H 2O, with iron provided most likely by pyrite, a phase almost ubiquitously associated with enargite. Other secondary minerals after enargite include arsenates such as chenevixite, Cu 2Fe 2(AsO 4) 2(OH) 4.H 2O, and ceruleite, Cu 2Al 7(AsO 4) 4.11.5H 2O, and sulphates such as brochantite, Cu 4(SO 4)(OH) 6, and posnjakite, Cu 4(SO 4)(OH) 6·H 2O. Detailed studies of enargite field alteration at Furtei, Sardinia, suggest that most alteration occurs through dissolution, as testified by the appearance of etch pits at the surface of enargite crystals. However, apparent replacement by scorodite and cuprian melanterite was observed. Bulk oxidation of enargite in air is a very slow process. However, X-ray photoelectron spectroscopy (XPS) reveals subtle surface changes. From synchrotron-based XPS it was suggested that surface As atoms react very fast, presumably by forming bonds with oxygen. Conventional XPS shows the formation, on aged samples, of a nanometer-size alteration layer with an appreciably distinct composition with respect to the bulk. Mechanical activation considerably increases enargite reactivity. In laboratory experiments at acidic to neutral pH, enargite oxidation/dissolution is slow, although it is accelerated by the presence of ferric iron and/or bacteria such as Acidithiobacillus ferrooxidans and Sulfolobus BC. In the presence of sulphuric acid and ferric iron, the reaction involves dissolution of Cu and formation of native sulphur, subsequently partly oxidized to sulphate. At alkaline pH, the reactivity of enargite is apparently slightly greater. XPS spectra of surfaces conditioned at pH 11 have been interpreted as evidence of formation of a number of surface species, including cupric oxide and arsenic oxide. Treatment with hypochlorite solutions at pH 12.5 quickly produces a coating of cupric oxide. Electrochemical oxidation of enargite typically involves low current densities, confirming that the oxidation process is slow. Important surface changes occur only at high applied potentials, e.g. + 0.74 V vs. SHE. It is confirmed that, at acidic pH, the dominant process is Cu dissolution, accompanied (at + 0.56 V vs. SHE, pH = 1) by formation of native sulphur. At alkaline pH, a number of surface products have been suggested, including copper and arsenic oxides, and copper arsenates. XPS studies of the reacted surfaces demonstrate the evolution of Cu from the monovalent to the divalent state, the formation of As–O bonds, and the oxidation of sulphur to polysulphide, sulphite and eventually sulphate. In most natural and quasi-natural (mining) situations, it is expected that enargite reactivity will be slow. Moreover, it is likely that the release of arsenic will be further slowed down by at least temporary trapping in secondary phases. Therefore, an adequate management of exposed surfaces and wastes should minimize the environmental impact of enargite-bearing deposits. In spite of an increasing body of data, there are several gaps in our knowledge of enargite oxidation. The exact nature of most mechanisms and products remains poorly constrained, and there is a lack of quantitative data on the dependence on parameters such as pH and dissolved oxygen.

Environmental impact from supergene alteration and exploitation of a high sulphidation epithermal type mineralisation (Kirki, NE Greece)

The base-metal sector of the high-sulphidation epithermal deposit of Kirki (St Philippe) was occasionally mined during the period 1973–1998, without strict environmental management. The mineralisation shows a significant content of polluting elements. Mineralogical investigations indicate a very low degree of sulphide alteration. Sphalerite and galena concentrates were produced by conventional flotation techniques. Mine excavation wastes and flotation tailings were dumped and exposed to weathering. Waters draining downstream of the mine waste dump are near neutral. Acid mine waters forming a pit lake are highly polluted. Evapoconcentration of the pit lake waters leads to the generation of a toxic mineral precipitate on the open pit floor, comprising soluble secondary salts. The paper concludes with a consideration of the remedial measures required.

Ordovician Intrusive‐related Gold‐Copper Mineralization in West‐Central New South Wales, Australia

AbstractThree major types of Ordovician intrusive‐related gold‐copper deposits are recognized in central‐west New South Wales, Australia: porphyry, skarn and high sulphidation epithermal deposits. These deposits are mainly distributed within two Ordovician volcano‐intrusive belts of the Lachlan Fold Belt: the Orange‐Wellington Belt and the Parkes‐Narromine Belt. Available isotopic age data suggest that mineralization of the three types of deposits is essentially coeval with the Ordovician intrusive rocks (480–430 Ma).Porphyry gold‐copper deposits can be further divided into two groups. The first group is associated with monzonite showing shoshonitic features, represented by Cadia and Goonumbla. The second group is associated with diorite and dacite, including the Copper Hill and Cargo gold‐copper deposits. Gold skarn is associated with Late Ordovician (430–439 Ma) monzonitic intrusive complexes in the Junction Reefs area (Sheahan‐Grants, Frenchmans, and Cornishmens), Endeavour 6, 7 and 44, Big and Little Cadia. The epithermal gold deposits with high sulphidation including Gidginbung (Temora) and Peak Hill mainly occur within Ordovician andesite and volcaniclastic rocks, and are associated with advanced argillic alteration. Available isotopic age data indicate that both alteration and mineralization of the porphyry, skarn and epithermal gold‐copper deposits are broadly coeval with the Late Ordovician shoshonitic magmatism, which is thought to result from the melting of sub‐continental lithosphere caused by Palaeozoic subduction events.The Ordovician intrusive‐related gold‐copper deposits are restricted to two longitudinal parallel volcano‐intrusive belts, rarely extending outside them. Diagonal intra‐belt trends of mineralization are common, particularly at the intersections of longitudinal and transverse (oblique) fault/fracture zones based on the authors' review of available geological data. The locations of these gold‐copper deposits are obviously influenced by transverse (oblique) fault/fracture zones that are oriented northwest, eastwest and northeast. The conjunctions of these fault/fractures zones are thought to be zones of structural weakness, and appear to be the favourable locus for the Ordovician intrusive‐related gold‐copper deposits. Differences in structural patterns, intrusive, wall rock types, and depths of ore formation may contribute to the differences among the deposits.