Epithermal Systems Research Articles

Rare aluminium phosphates and sulphates (APS) and clay mineral assemblages in silicified hydraulic breccia hosted by a Permian granite (Velence Mts., Hungary) as indicators of a high sulfidation type epithermal system

Uncommon micrometer-sized aluminium phosphates and sulphates of the alunite supergroup (APS minerals) associated with clay minerals were found in a polymictic Permian granite hosted hydrothermal breccia in the Velence Mts. (Hungary). The mineral assemblage was studied with TEM, XRD, EMPA and FTIR with the aim of revealing the formation conditions and the genetic link to the hydrothermal processes that affected the granite host. Two breccia types were identified in the granite. In a polymictic, strongly silicified breccia type zoned plumbogummite was found in association with rolled tube-shaped halloysite and kaolinite. Calcium, barium and lead zonation of the plumbogummite and the Cl and S contents of the halloysite indicate the hypogene origin of this assemblage. Kaolinite, halloysite and plumbogummite form under acidic conditions at temperatures ≤150 °C in hydrothermal systems. APS minerals belonging to the woodhouseite solid solution series (s.s.s.) were detected in the matrix of argillic polymictic breccia beside the alunite in a kaolinite/dickite, illite rich matrix. Alunite and woodhouseite form under acidic conditions in hydrothermal systems. Kaolinite/dickite and illite also suggest acidic conditions and temperatures of ≥240 °C. The close vicinity of the studied breccias to hydrothermal centres hosted by Palaeogene andesitic volcanic series, the strike of the studied breccia veins as well as the similarities in the mineral assemblages imply that the brecciation is spatially and genetically related to the high-sulfidation epithermal systems of the Palaeogene magmatic-hydrothermal activity. In our model, strongly acidic and hot hydrothermal fluids entered into the granite along E-W trending faults that resulted in silicification, brecciation and the formation of APS minerals in association with clay mineral assemblages.

Distribution of alteration, mineralization and fluid inclusion features in porphyry – high sulfidation epithermal systems: The Chodarchay example, NW Iran

The Chodarchay Cu-Au deposit in northwestern Iran represents an example of the transition from the deep porphyry mineralization stage overprinted by the shallow high-sulfidation epithermal stage in porphyry – high sulfidation epithermal systems. The mineralization at Chodarchay mostly occurred within the Eocene volcanic and volcaniclastic units; some parts are hosted within an Oligocene quartz monzonite intrusion. The porphyry mineralization is mainly associated with a potassic zone in the deepest part of the stock. The potassic alteration is followed by late alteration types of phyllic, argillic and propylitic. The advanced argillic alteration near the surface postdates other alteration types. Based on the phase contents, six types of fluid inclusions are recognized. At depth, the early generation of halite-bearing inclusions that in some places contain opaque daughter minerals indicates a high-density, high-salinity mineralizing fluid. Usual trails of only vapor-rich inclusions in the shallow parts, suggest intense boiling at (or below) this level. Coexisting vapor-rich and halite-bearing inclusions, as well as vapor-rich and type II fluid inclusion assemblages, indicate fluid immiscibility. The distribution of alteration, mineralization, and fluid inclusion features at Chodarchay provides an example for systematic variations from depth to surface in porphyry–epithermal systems.

Geophysical properties of an epithermal Au-Ag deposit in British Columbia, Canada

The Newton Au-Ag deposit is an intermediate sulfidation state epithermal system in British Columbia, Canada. Multiple types of geophysical data are interpreted and evaluated with drillcore petrophysical, geochemical, and geologic observations to better understand the geophysical signature of the Newton epithermal system. Airborne [Formula: see text]-ray data sets indicate elevated emission counts of K, eTh, and eU over the Newton epithermal system, which are caused by hydrothermal alteration. Drillcore [Formula: see text]-ray measurements also indicate high potassium concentrations related to the K-rich phyllosilicates in the form of argillic and quartz-sericite alteration assemblages. Magnetization vector inversion (MVI) is used to recover an unconstrained 3D magnetization vector model of the system on regional and deposit scales. The regional MVI has resolved a deep concentric-shaped low magnetic zone that is interpreted as a porphyry system beneath the epithermal deposit. At the deposit scale, 3D direct current (DC) resistivity and induced polarization (IP) inversion, and unconstrained MVI revealed finer details of the epithermal system architecture. Cooperative DC/IP and magnetic inversion, at the deposit scale, constrained the magnetic susceptibility model and recovered a more precise susceptibility image of the epithermal system that is well-matched with borehole geology. The integrated geophysical interpretation helped to resolve several 3D latent geologic features in places without direct access to drillcore samples. We identified four petrophysical domains based on the three cooperatively inverted physical properties, including electrical resistivity, IP chargeability, and magnetic susceptibility. The combined geophysical models differentiated porphyritic intrusions (chargeability/susceptibility lows), disseminated sulfides (resistivity lows and chargeability highs), a Cu-rich zone in mafic volcanic rocks (susceptibility/chargeability highs and resistivity lows), and an Au-Ag-Cu-rich zone with silicification in felsic volcanic rocks (chargeability/susceptibility lows and resistivity highs). These petrophysical domains also provide useful exploration vectors for identification of similar epithermal systems.

Characteristics and variations of sinters in the Coromandel Volcanic Zone: application to epithermal exploration

ABSTRACTThe Hauraki Goldfield of the Coromandel Volcanic Zone, North Island, New Zealand, hosts up to 19 known siliceous hot spring deposits, or sinters, some directly associated with Au–Ag ore bodies, in a Miocene–Pliocene, sub-aerial, calc-alkaline volcanic arc overlying Jurassic meta-sedimentary basement. Sinters are surface expressions of predominately adularia-sericite epithermal systems, potentially linked to Au–Ag mineralised conduits at depth. They provide a paleosurface marker to determine the level of erosion, and help determine zones of thermal fluid upflow and/or lateral outflow. Mapped sinters of the Coromandel Volcanic Zone are mainly affiliated with rhyolite domes, occur along structural corridors of the volcanic arc, and contain characteristic textures, mineralogy and pathfinder elements typical of near-neutral pH alkali chloride fluid discharge derived from convectively circulating magmatic fluids diluted by meteoric waters. Textures and trace element compositions of the Coromandel sinters are similar to those of the Taupo Volcanic Zone, New Zealand, and the Jurassic Au–Ag mining districts of the Deseado Massif, Argentine Patagonia. Thus, detailed mapping, textural analysis and trace-element studies demonstrate the prospecting potential of siliceous hot spring deposits for locating ore deposits, and in particular to help define proximal to distal positions relative to vent zones at the paleo-surface.

Geochemistry, fluid inclusions and sulfur isotopes of the Govin epithermal Cu-Au mineralization, Kerman province, SE Iran

Govin area is located southeast of the Urmieh-Dokhtar volcano- plutonic arc, which formed as the result of subduction of the Neo-tethyan oceanic crust beneath the central Iran continent. The arc hosts world-class Sar-Cheshmeh Cu porphyry deposit and many other Cu, Cu-Mo, Cu-Au porphyry, and Cu-Zn-Pb-Ag-Au skarn and epithermal deposits. The rock units in the Govin area include Eocene volcaniclastic rocks that intruded by post-Miocene hypabyssal quartz-monzonite, grano-diorite, diorite and quartz-diorite stocks. The stream sediment geochemical surveys display Cu, Au, Mo, Bi, Sb, Pb and As anomalies. The lithogeochemical prospecting in anomaly checking stage identified quartz veins with considerable abundances of Cu (2.98%) and Au (2.41 ppm). The ore-bearing quartz veins classify into quartz + iron oxides ± gold and quartz + sulfide ± gold. The dominant structural trend of vein systems is 60°–70° (NE-SW), with dips from 75° NW to vertical that cut altered andesite, porphyritic andesite, basaltic andesite and tuff. Mineral associations are quartz, adularia, chalcopyrite, pyrite, bornite, galena, sphalerite, as well as chalcocite, covellite, malachite, iron oxides and hydroxides in oxidized zones. Hydrothermal alteration is generally well-developed surrounding the ore-bearing systems. The main types of alterations in the area include propylitization, phyllic, and argillic associated with silicification. Fluid inclusions studies on quartz grains separated from mineralized veins exhibits the melting temperature of ice (Tm ice) values range from −12 to −1 °C corresponding to salinities of 2.5 to 37.04 wt% NaCl equiv. The superfluity LV fluid inclusions have salinity of 2.47 to 15.96 wt% NaCl equiv. The homogenization temperature of LV, VL and LVS fluid inclusions range from 146 to 366 °C, 405 to 425 °C and 236 to 270 °C, respectively. The fluid inclusions observations reveal that both dilution and boiling of fluids were important in the deposition of metals in the Govin. The δ34SCDT of pyrite and chalcopyrite grains associated with mineralized quartz veins range from −2.13‰ to −8.54‰ that indicate sulfur originated from host volcanic-sedimentary series in the area. The Cu-Au bearing quartz veins in the Govin area are classified as low-sulfidation epithermal system, located in closed association with the Sar-Cheshmeh and Darreh-Zar porphyry copper deposits.

Alteration Mapping for Porphyry Copper Exploration Using ASTER and QuickBird Multispectral Images, Sonajeel Prospect, NW Iran

The Sonajeel prospect is located in the Arasbaran belt which is one of the significant copper mineralization belts in NW Iran. There are mostly Eocene volcano-sedimentary rocks and Post Eocene intrusive units which are the source for mineralization in the area. ASTER multispectral images were used for delineation of alteration zones as a key feature of porphyry copper mineralization. Due to the need for geometric correction and ortho-rectification of the ASTER images, a high resolution QuickBird image with pixel width of about 60 cm (in PAN) was employed as a reference image in order to boost the rectification process. Ortho-rectification has been done by using digital elevation model which is created by topographic map in scale 1:1000. Potassic alteration as one of the essential alteration types in porphyry copper deposits, distributed mostly in the north of the Sonajeel prospect that determined by Thermal infrared bands processing. Phyllic and argillic alteration zones detected by creating relative absorption band-depth grids which are comparable to field observations. Moreover, silica rich areas which are remnants of hydrothermal circulation and form at the top of porphyry copper systems were detected for recognition of epithermal deposits (with 1 km distance from Sonajeel porphyry system). Finally, Remote Sensing results were compared by field evidences especially for determination of an epithermal system. Most parts of the alteration zones were observed of the surface confirmed with the remote sensing alteration (in average about 75% matched fittingly), displayed concentrations anomalous in the NE and NW parts of the studied area.

Studies of Metallic and Trace Minerals of the Tiegelongnan Cu‐Au Deposit, Central Tibet, China

Electron probe micro‐analysis (EPMA) and scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS) have been used to investigate the principal ore minerals and coexisting metallic mineral inclusions in polished thin sections from the Tiegelongnan deposit, which consists of a high‐sulfidation epithermal system (HSES) and a porphyry system (PS). Molybdenite, chalcopyrite, bornite, tennantite, enargite, digenite, anilite, covellite, and tetrahedrite have been identified by EPMA. Intergrowth, cross‐cutting and replacement relationships between the metallic minerals suggest that molybdenite formed first (stage 1), followed by chalcopyrite ± bornite ± hematite (stage 2), then bornite ± Cu‐sulfides ± Cu‐Fe‐sulfoarsenides (stage 3), and lastly Cu‐Fe‐ sulfoarsenides ± Cu‐ sulfides (stage 4). Pyrite is developed throughout all the stages. Droplet‐like inclusions of Au‐Te minerals commonly occur in tennantite but not in the other major sulfides (molybdenite, chalcopyrite and bornite), implying that tennantite is the most important Au telluride carrier. The pervasive binary equilibrium phases of calaverite and altaite constrain fTe2 in the range from ∼−6.5 to ∼−8 and fS2 < −11. The intergrowth of bornite and chalcopyrite and the conversion from bornite to digenite suggest fluctuated and relatively low precipitation temperature conditions in the HSES relative to the PS. Contrastingly, the dominance of chalcopyrite in the PS, with minor bornite, suggests relatively high temperature conditions. These new results are important for further understanding the mineral formation processes superimposed by HSES and PS systems.

Petrogenesis and tectonic setting of Early Cretaceous granodioritic porphyry from the giant Rongna porphyry Cu deposit, central Tibet

The Rongna Cu-(Au) deposit is a recently discovered giant low-grade composite porphyry-high sulfidation epithermal system in the Duolong ore district of central Tibet. We present zircon U-Pb ages, bulk-rock geochemical and Sr-Nd isotope data, as well as zircon Hf-O isotope data to constrain the petrogenesis of the Rongna granodioritic porphyry, on which the ore system is centered. Secondary ion mass spectrometry (SIMS) zircon U-Pb data for two samples yielded ages of 118.7 ± 0.9 Ma and 117.1 ± 0.9 Ma, respectively. The granodioritic porphyry is of high-K calc-alkaline and magnetite-series affinity, with enrichment of light rare earth elements (LREEs) and large-ion lithophile elements (e.g., Rb, Pb, and K), and depletion of high-field strength elements (e.g., Ta, Nb, and Ti), typical of the geochemical signature of arc magmas. The Rongna granodioritic porphyry has initial Sr isotopic ratios of 0.7060–0.7099, and εNd(t) values of −5.7 to −1.7 with TDM (Nd) of 983–1900 Ma. The zircons have relatively narrow Hf-O isotopic compositions (εHf(t) = +3.30– +6.43; δ18O = +5.94– +6.96‰), indicating the involvement of old crustal materials. Our data suggest that the Rongna granodioritic porphyry likely evolved by fractional crystallization from a hybrid magma of mantle-derived mafic melts and lower crustal felsic melts during a period of magmatic flare-up following flat-slab subduction.

The Geochemical Zoning of the Peschanka Porphyry–Epithermal System in the West of the Chukchi Peninsula

The results of chemical analyses and the data on core samples for 266 drill holes were used to identify the geochemical zoning of the Peschanka porphyry–epithermal system. A 3D model of geochemical zoning was developed using factor analysis and GIS technologies to represent the spatial distribution of elemental associations and mineral assemblages. The lateral and longitudinal zoning patterns are described. The AgPbZn: CuAuMo ratio is suggested as an indicator of vertical zoning that allows to evaluate the erosion level of porphyry copper stockworks.

Tectonics and Geology of Porphyry Cu-Au Deposits along the Eastern Sunda Magmatic Arc, Indonesia

The discovery of the world-class Cu-Au deposit at Tumpangpitu, East Java (1.9 Gt @ 0.45% Cu, 0.45 g/t Au), has reinforced the eastern Sunda arc as a significant metallogenic belt that is highly promising for the discovery of major porphyry deposits. The arc hosts three premier porphyry Cu-Au deposits at Batu Hijau, Elang, and Tumpangpitu (>300 t Au and >5 Mt Cu). These giant porphyry deposits are confined to the eastern segment of the eastern Sunda arc (eastern Java to Sumbawa), constructed on island-arc crust where the Roo Rise is being subducted. By contrast, major epithermal (low-sulfidation dominant) deposits associated with poorly endowed porphyry prospects occur along the western segment of the arc (western Java), developed on thick continental crust on the southern margin of Sundaland associated with subduction of typically thin Indian oceanic crust. Porphyry Cu-Au deposits of the eastern Sunda arc are spatially associated with small, nested, dioritic to tonalitic intrusive complexes with low-K calc-alkaline to weakly alkaline signatures. Hypogene alteration and mineralization developed during three main temporally and spatially overlapping events, termed early, transitional, and late. The hypogene Cu-Au mineralized zone of the giant deposits forms a bell-shaped body of 0.3% Cu shell greater than 1 km in both diameter and vertical extent that is centered on multiphase, elongate, pencil-like tonalite intrusions, 200 to 1,200 m in diameter and >2 km vertical extent. Large (>20 km 2 ) lithocaps associated with high-sulfidation epithermal systems and postporphyry mineralization diatreme breccias characterize productive porphyry districts at Batu Hijau, Elang, and Tumpangpitu. The known porphyry deposits with metal content exceeding 300 t Au and >5 Mt Cu all formed later than 5 Ma, suggesting an important change in the metallogeny of the arc at this time. These giant porphyry deposits formed relatively rapidly, typically over a period of 30 to 860 k.y. during the Pliocene-Pleistocene, between 5 and 2.5 Ma. Porphyry deposits along the eastern Sunda arc display characteristic geologic features similar to porphyry deposits elsewhere. Significant differences include low-K systems, dominance of hydrothermal biotite over K-feldspar, early intense actinolite alteration associated with ore, widespread shreddy chlorite after secondary biotite, narrow quartz-sericite-pyrite alteration zones, lack of development of chalcopyrite veins, large overlying lithocaps, and abundant chalcocite-bornite-covellite veinlets associated with late overprinting epithermal systems. Hydrothermal oligoclase is also more abundant than K-feldspar in the central biotite-magnetite zones.

Mineral associations and their distribution in hydrothermal alteration zones of the Chelopech high-sulphidation deposit, Bulgaria

This study is focused on the mineral assemblages developed during hydrothermal alteration in the host rocks of the Chelopech deposit and aims to reveal some characteristics of ore-forming fluids. Three well-distinguished and simultaneously formed hydrothermal alteration zones, characterized by specific mineral associations, occur in the area: advanced argillic, sericitic and propylitic. The presence of silica residue localities in the advanced argillic zone marks the major channels of hydrothermal fluids and indicates participation of extremely acid fluids. The existence of alunite and zunyite in the alteration indicates development of acid-sulphate hydrothermal environment, with presence of F in the fluid, which is considered to be an important feature of ore-bearing high-sulphidation epithermal systems. Dickite in association with alunite in the upper levels of the deposit suggests temperature of formation 200–250 °C. The highest temperature in depth is limited up to 375 °C, considering the occurrence of zunyite, pyrophyllite and dickite.

Geology and characteristics of low sulphidation epithermal vein in Senepo area, East Java

Senepo area is one of several epithermal mineralization prospects in Southern Mountains of Java Island. This study is aimed to investigate the geological framework and hydrothermal alteration of the prospect as well as to characterize the low sulphidation (LS) epithermal vein including vein textures, ore mineral, ore chemistry and hydrothermal fluid that are responsible for the formation of the quartz vein. Detailed field mapping was completed to understand the geological framework and hydrothermal alteration zones. Petrographic, ore microscopic, XRD and AAS analyses were applied to characterize the quartz vein. The study area is occupied by pebbly sandstone, andesite breccia, andesite units and alluvial deposit. Ore mineralization is mainly hosted by andesite of Oligo-Miocene of Watupatok Formation. The host-rock has been suffered by overprinted hydrothermal alteration and weathering. The epithermal veins are found as N-S-trending quartz veins with a thickness of 1-2 meters. These veins show massive texture, crustiform, cockade, druzy, comb, and disseminated sulphides, which is included in Crystalline Quartz Zonation (X). Optical microscopy and XRD analysis indicate that these veins contain several ore minerals such as chalcopyrite, sphalerite, galena, pyrite, hematite, covellite and malachite, associated with gangue mineral including quartz, felspar, chlorite and clay minerals. AAS analysis displays variable chemical composition of precious and base metals such as Au (17-37 ppb), Ag (8.1-32.6 ppm), Cu (122.1-81,450 ppm), Pb (35.4-883.6 ppm) and Zn (18.3-2,628 ppm). Gold and silver are relatively low compared to the base metals. Fluid inclusion micro thermometric analysis indicates that ore mineralization originated at temperatures of 187-263 o C and salinity of 0.18-0.53 % wt. NaCl eq. This salinity corresponds with a minimum hydrostatic pressure of about 10-55.1 bars. Based on those characteristic data, mineralized quartz veins in the study area are classified into a base metal horizon of LS epithermal system.

Wavelength feature mapping as a proxy to mineral chemistry for investigating geologic systems: An example from the Rodalquilar epithermal system

The final product of a geologic remote sensing data analysis using multi spectral and hyperspectral images is a mineral (abundance) map. Multispectral data, such as ASTER, Landsat, SPOT, Sentinel-2, typically allow to determine qualitative estimates of what minerals are in a pixel, while hyperspectral data allow to quantify this. As input to most image classification or spectral processing approach, endmembers are required. An alternative approach to classification is to derive absorption feature characteristics such as the wavelength position of the deepest absorption, depth of the absorption and symmetry of the absorption feature from hyperspectral data. Two approaches are presented, tested and compared in this paper: the ‘Wavelength Mapper’ and the ‘QuanTools’. Although these algorithms use a different mathematical solution to derive absorption feature wavelength and depth, and use different image post-processing, the results are consistent, comparable and reproducible. The wavelength images can be directly linked to mineral type and abundance, but more importantly also to mineral chemical composition and subtle changes thereof. This in turn allows to interpret hyperspectral data in terms of mineral chemistry changes which is a proxy to pressure-temperature of formation of minerals. We show the case of the Rodalquilar epithermal system of the southern Spanish Gabo de Gata volcanic area using HyMAP airborne hyperspectral images.

Mineralogy, structural control and age of the Incachule Sb epithermal veins, the Cerro Aguas Calientes collapse caldera, Central Puna

The Incachule Sb epithermal veins is located near to the N-E rim of the Cerro Aguas Calientes collapse caldera (17.5–10.8 Ma), in the geologic province of Puna, Salta- Argentina. It is hosted in Miocene felsic volcanic rocks with continental arc signature. The district includes twelve vein systems with mineralization of Sb occurring in hydrothermal breccias and stockwork. The veins are composed of quartz-sulfide with pyrite, stibnite and arsenopyrite. All around the veins, wall rocks are variably altered to clay minerals and sulfates in an area of around 2.5 km wide by more than 7 km long. The hydrothermal alterations recognized are: silicic, phyllic and argillic.The veins are characterized by high contents of Sb, As, and Tl and intermediate contents of Pb-Zn-Cu, and traces of Ag and Au. Homogenization and ice-melting temperatures of fluid inclusions vary from 125 °C to 189 °C and −2.4 °C to −0.8 °C. The isotopic data indicated a range of δ34S −3.04‰ to +0.72‰ consistent with a magmatic source for sulfur.We present the firsts K-Ar ages for hydrothermal illite/smectite mixed layers (I/SR1, 60% illite layers) and illite that constrain the age of the ore deposit (8.5–6.7 ± 0.2 Ma).The data shown here, let characterized the Incachule district as a shallow low sulfidation epithermal system hosted in a collapse caldera. Our data also indicate that mineralization is structurally controlled by a fault system related to the 10.3 Ma collapse of Aguas Calientes caldera. The interpreted local stress field is consistent with the regional one.

A study by electron microscopy of gold and associated minerals from Round Mountain, Nevada

Details regarding the mineralization patterns of gold in epithermal systems are poorly understood. A refined understanding of gold microtextures, the interface between gold and associated minerals, and the role that nanoparticles play in gold mineralization could provide insight into the details of gold growth and may contain indicators of gold ore concentration mechanisms. Furthermore, a refined understanding of the interface also may explain variation in cyanide leaching extraction efficiency and may enable enhancement of recovery methods. Macrocrystalline gold samples from Round Mountain, Nevada were analyzed using field emission scanning electron microscopy and transmission electron microscopy. This study suggests that gold nanoparticles are common in Round Mountain gold and associated mineral phases, and may play an important role in the formation of macrocrystalline gold in this deposit. Microtopographic evidence indicates that the two dimensional nucleation and growth mechanism is dominant and nanotextural evidence suggests that nanoparticulate gold was the first stage of growth in the formation of these macrocrystalline samples that grew rapidly at high degrees of supersaturation. Results suggest that the interface between gold and quartz and other related minerals is far more complex than previously thought, and that textures present at the interface and in the bulk gold can help explain mineralization history and can also have implications for gold recovery efficiency for macrocrystalline bearing ore.

Smoking gun for thallium geochemistry in volcanic arcs: Nataliyamalikite, TlI, a new thallium mineral from an active fumarole at Avacha Volcano, Kamchatka Peninsula, Russia

This paper describes the new mineral nataliyamalikite, the orthorhombic form of thallium iodide (TlI), from high-temperature fumaroles from the Avacha volcano, Kamchatka Peninsula, Russia. We also present some chemical analyses showing extreme enrichment of Tl in the volcanic gases at the Avacha volcano, and a review of thallium geochemistry that highlights the fascinating processes that led to the formation of nataliyamalikite. Nataliyamalikite occurs as pseudo-cubic nanocrystals (≤0.5 μm) within vacuoles in an As-(Te)-rich amorphous sulfur matrix and rarely as irregularly shaped aggregates up to ~50 μm in diameter within the amorphous sulfur matrix. Associated minerals include an unidentified Tl-As-S mineral, barite, and rare inclusions of a Re-Cu-bearing phase. The mean empirical composition based on four EDS analyses is Tl 1.00 (I 0.95 Br 0.03 Cl 0.02 ), corresponding to the ideal formula TlI. Nataliyamalikite crystallizes in the orthorhombic system, space group Cmcm , which is consistent with the low-temperature ( Pm 3 m ) of the high-temperature polymorph, although most analyzed grains display the orthorhombic symmetry. Single-crystal X-ray studies of material extracted by the focused ion beam-scanning electron microscopy (FIB-SEM) technique, and carried out on the MX2 macromolecular beamline of the Australian Synchrotron, determined the following cell dimensions: a = 4.5670(9), b = 12.803(3), c = 5.202(1) A, V = 304.2(1) A 3 , and Z = 4. The six strongest calculated X-ray reflections and their relative intensities are: 3.31 (100), 2.674 (73), 3.20 (43), 2.601 (28), 2.019 (21), and 2.284 A (19). The combination of EBSD analysis (providing an efficient test of the crystallinity and crystal symmetry of a population of micrometer-sized grains) and synchrotron single-crystal X-ray micro-diffraction (beam size ~7.5 μm) on micro-aggregates extracted using FIB-SEM opens the way to the characterization of challenging specimen—in this case, the sulfur matrix is highly beam sensitive, and the nataliyamalikite grains could not be isolated using optical microscopy. The high-temperature (>600 °C) sulfidic (~1.2 wt% S) vapors at Avacha are extremely enriched in thallium; with 34 ppm, they contain an order of magnitude more Tl than the richest volcanic gases analyzed to date and ~100× more Tl than most metal-rich fumarolitic fluids associated with volcanic arcs. The formation of nataliyamalikite illustrates the complex processes that control thallium geochemistry in magmatic arc systems. Thallium minerals have now been reported in andesitic (Avacha), basaltic (Tolbachik, Kamchatka), as well as rhyolitic (Vulcano, Eolian Islands, Italy) volcanoes. Ultimately, these thallium minerals result from the transfer of thallium from subducted sediments to volcanic gases in arc volcanoes. We suggest that the extremely thallium-enriched vapors from which nataliyamalikite formed result from complex and transient interactions between Tl-rich sulfosalt melts and magmatic vapors, a process that may be important in controlling metal distribution in boiling epithermal systems.

Effect of S on the aqueous and gaseous transport of Cu in porphyry and epithermal systems: Constraints from in situ XAS measurements up to 600 °C and 300 bars

The effect of S on the solubility of Cu in high-temperature fluids at conditions relative to porphyry and epithermal ore deposit formation has been investigated conducting in situ X-ray absorption (XAS) and Raman spectroscopy from 325 to 600°C at 300bars. The experimental results identify the Cl or S-complexes responsible for Cu transport in high-temperature fluids and provide information on transport in porphyry/epithermal systems and the effect of fluid composition on the transfer of Cu from the porphyry to the epithermal environment.In S-free HCl solutions, Cu solubility is about 10 times higher in high-density (ρ>0.6g·cm−3) versus low-density (ρ<0.3g·cm−3) fluids. During the transition from high- to low-density, Cu speciation evolves from CuCl2− complexes to CuCl(H2O) or CuCl(HCl), depending on HCl concentrations. In sulfur-only solutions Cu solubility is extremely low. The addition of S to Cl-bearing fluids results in a drop of Cu solubility in high- and low-density fluids. Solubilities drop from weight percent levels (in Cl-only solutions) to hundreds of ppm in high-density fluids (containing sulfur). In low-density fluids, Cu concentrations similarly drop, from hundreds of ppm to below detection. The low Cu concentrations in the presence of sulfur preclude the characterization of Cu complexes, except in high-density Cl+S fluids, where CuCl2− is found to remain the dominant specie even for Cl concentrations <1wt%.Overall, these results suggest that the presence of S of mixed oxidation state (sulfide S2−, sulfite S4+ and sulfate S6+) limits transport of Cu in porphyry and epithermal systems and may even trigger Cu precipitation. Furthermore, these experiments provide evidence that the presence of the S3− ion does not increase Cu solubility in fluids. Thus, if present in such environments, S3− ion could fractionate Cu from Au in porphyry-epithermal environments.

Hydrothermal alteration and fluid pH in alkaline-hosted epithermal systems

Epithermal gold mineralisation is found in a wide compositional range of host lithologies, but despite the diversity the alteration mineral assemblages are often similar between deposits. Notable exceptions are those gold deposits hosted in alkaline host rocks. Alkaline-hosted epithermal deposits are rare, but important, as they include some of the world’s largest known epithermal deposits by contained metal (e.g. Ladolam, Cripple Creek, Porgera). As well as the exceptional gold contents, the alkaline-hosted systems tend to exhibit different alteration mineral assemblages, with less quartz and widespread silicification than sub-alkaline-hosted equivalents, and greater enrichments in tellurium, and a scarcity of acid alteration (advanced argillic) types. In this study, geochemical modelling is used to demonstrate that 300°C hydrothermal fluids in equilibrium with alkali, silica-undersaturated host rocks at low water/rock ratios reach significantly higher pH than equivalents in sub-alkaline lithologies. A maximum, near-neutral pH (5.5–6) is buffered by reactions involving quartz in silica-saturated alkaline and calc-alkaline lithologies. In silica-undersaturated, alkaline host rocks, quartz is exhausted by progressive water-rock interaction, and pH increases to 7 and above. Both tellurium and gold solubility are favoured by neutral to high fluid pH, and thus there is a clear mechanism within these hydrothermal systems that can lead to effective transport and concentration to produce gold telluride ore deposits in alkaline igneous hosts. This modelling demonstrates that alkaline rocks can still be altered to advanced argillic assemblages; the paucity of this alteration type in alkaline hosts instead points to NaCl≫HCl in magmatic volatile phases at the initiation of hydrothermal alteration.

Hyperspectral remote sensing applied to mineral exploration in southern Peru: A multiple data integration approach in the Chapi Chiara gold prospect

Remote sensing is a strategic key tool for mineral exploration, due to its capacity of detecting hydrothermal alteration minerals or alteration mineral zones associated with different types of mineralization systems. A case study of an epithermal system located in southern Peru is presented, aimed at the characterization of mineral assemblies for discriminating potential high sulfidation epithermal targets, using hyperspectral imagery integrated with petrography, XRD and magnetic data. HyMap images were processed using the Mixture Tuned Matched Filtering (MTMF) technique for producing alteration map in the Chapi Chiara epithermal gold prospect. Extensive areas marked by advanced argillic alteration (alunite-kaolinite-dickite±topaz) were mapped in detail, as well as limited argillic (illite-smectite) and propylitic (chlorite spectral domain) alteration. The magmatic-hydrothermal processes responsible for the formation of hypogene minerals were also related to the destruction of ferrimagnetic minerals (e.g., magnetite) of host rocks such as andesite, and the remobilization/formation of paramagnetic Fe-Ti oxides (e.g., rutile, anatase). The large alteration zones of advanced argillic alteration are controlled by structures related to a regional NW-SE trend, and also by local NE-SW and ENE-WSW ones.

The Kassiteres porphyry – epithermal system, NE Greece: evidence for rapid exhumation leading to sub-economic mineralisation

Porphyry – epithermal systems host >75% of the world’s known Cu reserves along with considerable volumes of precious metal mineralisation. Consequently, a considerable body of work has studied the ...