High-sulfidation Epithermal Systems Research Articles

Mineralogy and a New Fertility Index of Alunite in the Fanshan Lithocap, Luzong Basin, Anhui Province, China

Alunite is used as a representative mineral for indicating deposits in lithocaps, and lithocaps are generally related to the porphyry–(high-sulfidation) epithermal mineralization system. The study of alunite is of theoretical and exploration significance for prospecting potential underlying porphyry and epithermal deposits. Studies on alunite geochemistry have made breakthroughs, but there is little research on alunite mineralogy, for example, using scanning electron microscopy (SEM) images, differential thermal analysis (DTA), and Fourier-transform infrared spectroscopy (FT-IR). This study mainly focuses on alunite micromorphological characteristics, weight loss changes with temperature, and ionic group structure, aiming to identify the relationship between these features and indications for prospecting. The Fanshan lithocap is located in the northwest part of the Luzong basin, Anhui province of China, and it can potentially be used for exploring porphyry and epithermal deposits. Fanshan alunite is formed in two stages with three types of alunite. IA alunite is formed in the early hydrothermal stage and replaces felsic minerals in the Zhuanqiao Formation, IB alunite is formed in the later hydrothermal stage and fills in open spaces with bladed particles, and II alunite is the product of pyrite oxidation and reaction with other minerals in the supergene stage. Alunite electron microprobe data and energy-dispersive spectroscopy data further confirm temperature decreases with hydrothermal evolution, and the presence of a high-sulfidation epithermal system in the Luzong basin. Aside from the forming environment, SWIR, and geochemistry of alunite, there are other indication indexes; for example, the larger peak values at 3480 cm−1 and smaller peak values at 1080 cm−1 in FT-IR spectra and the deeper exothermic valleys at 750 °C and steeper weight loss slopes in the DTA curve suggest a favorable formation environment for alunite and provide valuable indications for deposit exploration and assessments of mineralization potential.

Geology and exploration indications of the lithocap in Qianpu area, Luzong Basin, Anhui

The lithocap is a large-scale alteration including silicification, advanced argillic and argillic that developed near the surface by hydrothermal activities. It is helpful for exploring high-sulfidation epithermal deposits and porphyry deposits in its root and deep parts, respectively, and is used as a new prospecting indicator. There are many minerals in the lithocap and it is difficult to identify them; alunite is the most representative mineral and one of the new minerals used to guide exploration; it plays an important role in assessing the metallogenic potential and guiding ore prospecting. The Luzong basin is dominated by iron oxide apatite deposits (IOAs). In recent years, many researchers have been searching for porphyry copper–gold deposits and epithermal deposits as studies on alunite and lithocaps in the basin have gradually been undertaken. In field explorations and investigations of the Luzong basin, the author found that in Qianpu area of the central basin, where kaolinite deposits were mainly explored in the past, Well-occurring alunite minerals and advanced argillic alterations have developed. A typical lithocap developed in Qianpu area after a series work of geology and mineralogy, such as alteration mineral species, alteration types, and alteration zoning, which were formed by the hydrothermal alteration of the volcanic rocks of the Zhuanqiao Formation. The minerals in this lithocap are mainly quartz, alunite (including aluminum-phosphate-sulfate, APS), pyrite, zunyite, dickite, pyrophyllite, kaolinite, and a small amount of illite, which are present sequentially from the center to the peripheral quartz-alunite-pyrite alteration, alunite-dickite alteration, dickite-pyrophyllite alteration, alunite-kaolinite-illite alteration, and alunite veins superimposed on quartz-alunite-pyrite alteration and alunite-dickite alteration. Three types of alunite were produced in large quantities and were formed in three kinds of environments. The I-type alunite, which was formed by hydrothermal metasomatism alteration in the early stage and is densely disseminated distribution, coexisting with quartz and pyrite, is a magmatic hydrothermal alunite. Relatively pure alunite veins (II-alunite) of varying thicknesses formed in the open space in the mid-hydrothermal period in a magmatic steam environment. The fine-grained III-alunite that coexists with kaolinite formed in the late hydrothermal period and in a steam-heated environment. The alunite produced by various origins represents multiphase hydrothermal activity in the Yanshanian period in the Qianpu area; the Na content decreased, the K and Pb content increased, and the K/Na molar ratios ranged from 2.82 to 6.58 (EPMA) and 1.59 to 3.93 (LA-ICP MS) from the early to middle and late hydrothermal stages, indicating that the formation temperatures were more than 200 °C and gradually decreased with hydrothermal evolution. There is a high-sulfidation epithermal system in Qianpu area. In combination with the specific environmental significance of zunyite indicating a high temperature and closeness to the fluid channel, as well as the characteristics and genesis of the Huangzhuyuan silver polymetallic deposit near Qianpu, it is inferred that the Qianpu-Huangzhuyuan area has prospecting potential for porphyry-epithermal deposits.

KARAKTERISTIK MINERALOGI PADA VEIN TEMBAGA KLUWIH, PACITAN, JAWA TIMUR

The southern mountains of Java are a path of copper-gold mineralization that has proven to have high economic value. Pacitan is part of the geological system and mineralization of the southern mountains, which are part of the Sunda Arc magmatism. The research area is located in the copper mining location in Kluwih village, Tulakan, Pacitan, East Java. The presence of copper mineralization in the form of massive sulfide/sulfosalt veins in the Kluwih area is evidence of an epithermal mineralization system which is rare and has not been studied intensively. This study intends to carry out mineralogical tests both physically and chemically for the characteristics and mineralogical composition of vein in the Kluwih area. The research was conducted using geological mapping methods in mine tunnels, rock sampling, laboratory analysis using petrographic methods, ore microscopy, rock geochemistry using ICP OES/MS – XRF – AAS, mineralogy using SWIR and XRD spectral analysis. The geomorphology of the Kluwih area and its surroundings consists of the morphology of alluvial plains and intrusive hills. Lithology is composed of volcanic–subvolcanic rocks in the form of porphyry dacite, dacite, dacite breccia, and alluvium. Mineralization forms in structural openings that cut and alter dacite rocks and dacite breccias. The alteration that develops in the Kluwih area, especially in the area around the mine (tunnel) is dominated by kaolinite-illite, pyrophyllite-little-sericite, chloritization, and mineralization found in the form of veins that have massive sulphide composition with the dominant internal texture being breccia. The texture or structure of the mineralization is in the form of brecciation (hydrothermal breccia), sulfide-pyrophyllite-kaolinite, quartz- sulfide/sulfosalt, and massif sulfide/sulfosalt. The results showed that the ore had a density of 4.65 gr/cm3 while the highly argillic altered side rocks had a density of 2.54 gr/cm3. In the ore, visible massive sulfide/sulfosalt minerals consist of mosaic bonds and intergrowth between similar or different minerals. The detected ore minerals are pyrite, enargite, covellite, chalcopyrite, and bornite. Enargite and pyrite are the dominant metallic minerals found in ores. Enargite crystals appear to have a size of 56 - 160µm, pyrite accumulates in anhedral crystal forms with a size of 120 - 900µm and inclusions in enargite measuring 50 - 140µm, along with covellite-chalcopyrite ± bornite minerals with a fine size of 5 - 20µm. The geochemistry of the ore is seen as sulfide filling, as evidenced by the SiO2 value of only 4.5% followed by metal enrichment in the form of 27.6% Fe, 12.6% Cu, 4.15% As, and 46.2% S. Alteration minerals which is present is probably a mica group such as phengite with a marked presence of elements of 0.7% MgO and 1.9% Al2O3. The geochemistry of the side rock is seen to have a high SiO2 value of 71.7%, followed by a high alkaline element forming alteration minerals such as pyrophyllite with an Al2O3 value of 15.3% with a low total Ca-Na-K value of 0.25%, and there is residual TiO2 by 0.38%. Terraspectral analysis and XRD confirmed that the main peaks were pyrophilite-phengite, followed by the minerals enargite and pyrite. The Kluwih vein is a copper vein associated with advanced argillic alteration, as part of a rich high sulfidation epithermal system with Cu-Au-As-Sb mineralization.

New perspective Reza gold deposit (Gedabek ore district, Lesser Caucasus, Azerbaijan)


 
 
 The article describes Reza gold deposit of Ugur exploration area located in Geda- bek Ore District of the Lesser Caucasus in NW of Azerbaijan. It is established that main mineralization in the Reza gold deposit consists of hematite-barite-quartz-kaoline veins- veinlets and breccia, pyrite stock-stockverk and quartz-sulfide veins. On the main orebody surface center secondary quartzites with vein-veinlets barite-hematite mineralization have occured over which remain accumulations of hydrous ferric oxides cementing breccias of quartz and secondary quartzites. “Reddish mass” is also observed in erosion parts, being an oxidation product of stock and stockverk limonite-hematite ores. Representing typical gossans, these accumulations by the data of trenches for thickness about 5-10 m contain gold 0.3-3.5 ppm and silver 1.0-45.0 ppm. There are three zones of gold mineralization within the Reza gold deposit: oxide mineralization; transition zone mineralization; sulfide mineralization. The oxide gold mineraliza- tion consists of clay-gravel weathering crust of kaolinite type. The gold-bearing mineralization has been oxidized to a depth of ap- proximately 50-100 meters. Typically, the gold mineralization is coarser and a minor increase in gold grade occurs within the oxides compared to the original rocks. The nugget effect increase in the gold grade of the oxides does not exceed approximately 10%. Deposit alteration signature has characteristics which suggest the current outcrop level may be near the top of a mineralized, gold-bearing high sulfidation epithermal (HSE) system. The gold mineralization at the deposit is interpreted as forming in shallow high sulfidation epith- ermal systems. The mineralization has been noted in well-confined hydrothermal breccia and associated with pyrite stock-stockwork. The majority of the deposit material and current estimates are formed within the barite-hematite-quartz-kaoline mineralization in the secondary quartzite rocks. The main brecciation and stockwork are hosted within secondary quartzite, sometime massive silicified andesite porphyritic rocks.
 
 

Implementation of geochemical modeling in post-mining land uses, the case of the abandoned open pit lake of the Kirki high sulfidation epithermal system, Thrace, NE Greece

Open pit exploitation of the Kirki high sulfidation epithermal deposit (Thrace, NE Greece) resulted in the formation of an acidic pit lake by infilling of the open cast by rain and draining waters after mine closure. Three case-specific PHREEQC geochemical models were developed including “Direct Precipitation”, “Evaporation”, and “Mixing with Rain water” to simulate the environmental conditions through the year, and to investigate the behavior of the acidic and rich in heavy metals waters of the pit lake. Scanning Electron Microscopy examination revealed the predominance of jarosite species and goethite in the mineral precipitate, followed by lower amounts of hematite, schwertmannite and anglesite. According to PHREEQC modeling, jarosite species formation is favored during both dry and wet periods, while the presence of schwertmannite, goethite, hematite and anglesite depicts the fluctuation of the physicochemical conditions through the year. These phases play a key role in controlling the As and Pb dissolved concentrations, mainly through adsorption and co-precipitation; whereas, the soluble metals Zn, Cd and Mn remain dissolved. The current status of the Kirki open pit system is the most stable for the environment; therefore, a case-specific remediation plan focused on both the protection of the environment and the preservation of the Kirki open pit is proposed. The socio-economic benefits of such an approach could include increased tourist numbers due to case-oriented tourism, enhancement of local economy, education; whereas, this approach is also cost effective relative to other proposed remediation measures.

Identification of Potential Distribution of Gold Mineralization Zone of High Sulfidation Epithermal System using Time-Domain Induced Polarization (TDIP) and Magnetic Method at “N” Mountain Qrea

The high gold commodity as well as the Pongkor production field that has existed since decades ago made it necessary to find a new potential zone for gold mining production. To find out the zone of gold mineralization and its continuity, geophysical studies then conducted by using the methods of Time-Domain Induced Polarization (TDIP) and magnetic. TDIP data consisting of resistivity and chargeability data are interpreted to determine alteration zones and subsurface mineralization that are correlated with magnetic data that has been inversed in 3D to obtain susceptibility parameters. Based on the value of resistivity in the study area, the argillic alteration zones are thought to has a value of < 50 Ohm.m. Silicification alteration zones supposedly have the value > 50 Ohm.m. The value of the chargeability at the mineralized zone indicated has a value of 100-810 msec. Also, study in the area of the mineralized zone have a susceptibility value in the range of -0.073 to -0.021 cgs. From that parameter of the mineralized zone, it is assumed that the mineralization zone is in the eastern part of the study area and oriented North-South direction with the orientation of the main fault.

New logical operator algorithms for mapping of hydrothermally altered rocks using ASTER data: A case study from central Turkey

This study uses logical operator algorithms along with Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) spectral reflectance and emissivity data to map hydrothermally altered igneous rocks associated with mineralization around the towns of Hamurcu and Başdere in central Anatolia, Turkey. In addition to band ratios used in previous studies, the logical operator algorithms developed in this work perform multiple mineral indices and directed principal components (DPC) analysis, thereby providing better constraints on the spatial extent of hydrothermally altered rocks around the Hamurcu and Başdere areas. We designed the unique mineral indices and DPC analysis based on the effective use of ASTER bands corresponding to diagnostic spectral absorption features of alteration minerals. The calculation of threshold values of logical operator algorithms is based on a two-stepped statistical approach. The novel logical operator algorithms successfully mapped hydrothermal alteration zones in the study area that include mineral assemblages, such as kaolinite, alunite, sericite, smectite, and ferric iron oxides together with various forms of silica. The Hamurcu area contains altered rocks that display a characteristic alteration pattern of a high-sulfidation epithermal system. On the other hand, alteration minerals are primarily located along the faults in the southern part of the Başdere area. The results of field investigations in conjunction with laboratory spectral reflectance measurements, microscopic examination of thin sections and X-ray diffraction analysis of the samples are quite consistent with ASTER mineral maps derived from the logical operator algorithms from this work. As a result, the new logical operator algorithms implemented to ASTER data of the study area prove to be a powerful tool for generating preliminary, quick, and reliable alteration mineral maps at low cost in similar terrains of different geographic locations.

Gold ± Copper Endowment and Deposit Diversity in the Western Tethyan Magmatic Belt, Southeast Europe: Implications for Exploration

Abstract Major Au and Cu deposits in the Western Tethyan magmatic belt formed during two main periods of Cretaceous and Cenozoic magmatism. The Cretaceous deposits are dominantly Cu-Au porphyry, high-sulfidation epithermal, and volcanic massive sulfide deposits, whereas in the Cenozoic Cu is significant only in porphyry systems. However, the Cenozoic contains approximately three times greater total Au endowment (for Au deposits &gt;0.5 million ounces), and also has a greater deposit diversity, including porphyry Au-Cu and Au-only deposits, high-, intermediate-, and low-sulfidation epithermal Au systems, and Au-rich carbonate replacement and sediment-hosted styles. The differences in endowment and deposit styles likely reflect regional-scale tectono-magmatic processes as well as local preservation and emplacement levels. The Cu ± Au endowment of the Cretaceous is consistent with typical subduction-related arc environments and generation of calc-alkaline porphyry to high-sulfidation epithermal systems, whereas Au enrichment related to Cenozoic magmatism appears to be related to high-K calc-alkalic to shoshonitic compositions. In many of the Au-rich Cenozoic magmatic belts, there is geochemical evidence for sourcing subcontinental lithospheric mantle that was previously enriched by Cretaceous subduction-related metasomatism. Additional differences in Au endowment may reflect the preservation of shallow-level systems in the Cenozoic, particularly for the Au-rich Miocene porphyry deposits such as Kışladağ and Bierly Vrch and the Apuseni porphyry Au-Cu deposits. However, in both the Cretaceous and Cenozoic, crustal exposure levels vary across the belt and cannot explain all the differences in Cu and Au endowment. A compilation of exploration discovery methods highlights the importance of historic workings in addition to geochemistry and geology as an initial vector, whereas geophysics has had limited involvement in direct discovery, primarily due to its limited application historically. Geologic models for well-understood systems such as porphyry and proximal epithermal systems provide excellent guides for explorers; however, more distal deposits such as Au-rich carbonate replacement deposits and deposits with poorly constrained models such as sedimentary rock-hosted and intermediate-sulfidation deposits are more challenging for exploration.

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.

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 &lt; −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.

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.

Injection-Driven Swarm Seismicity and Permeability Enhancement: Implications for the Dynamics of Hydrothermal Ore Systems in High Fluid-Flux, Overpressured Faulting Regimes—An Invited Paper

Many types of hydrothermal ore deposits form at overpressured conditions during high fluid flux through fault zones in the continental seismogenic regime. These include many orogenic gold deposits, some types of Fe oxide Cu-Au systems, and a variety of intrusion-related deposits, including some high-sulfidation epithermal systems. The internal structures of these deposits indicate formation while faults were active, in a regime involving episodic slip. Partial to complete sealing of fault zones by precipitation of hydrothermal minerals occurs between slip episodes. Such ore deposits occur typically within low displacement faults, or networks of such faults. Lodes seldom occupy areas much greater than 1 km 2 of fault surfaces, and net slip accumulated during ore formation is typically less than 150 m. Fluid injection experiments in low-permeability rocks, and seismicity styles in hydrothermally active settings, provide new insights about the dynamics of ore formation in high fluid-flux faults. In particular, they indicate that swarm seismicity is the characteristic response to injection of large volumes of overpressured fluids into intrinsically low-permeability rock. Injection-driven swarm seismicity and related permeability enhancement involves repeated sequences of thousands of ruptures, mostly with moment magnitude M w in the range −2 w 2 ; maximum cumulative slip is usually less than a few centimeters. Diffusion-like migration of a seismicity front away from the injection source at rates up to hundreds of meters per day is a key characteristic of injection-driven seismicity, and correlates with migration of a fluid pressure front along activated faults. By analogy with injection experiments and natural injection-driven swarm seismicity, the formation of fault-related ore deposits in overpressured, high fluid-flux regimes is interpreted to involve swarm seismicity, rather than mainshock-aftershock sequences. Recurrence intervals of swarms in contemporary injection-driven swarm sequences indicate that the total slip associated with formation of fault-hosted lodes can accumulate in periods as short as 104 to 105 years, during thousands of swarm episodes, and commonly involve >10 6 ruptures with −2 w 4 to 10 5 m 3 of fluid at injection rates of at least several tens of L.s −1 . Up to tens of kg Au are deposited per swarm. Cascades of thousands of microseismic ruptures and associated permeability enhancement, coseismic dilatancy, and fluid pressure changes during individual injection-driven swarm sequences create very dynamic hydrologic and reaction regimes. Ore formation involving transitory bursts of rapid flow, abrupt fluctuations in flow rates and fluid pressures, and possibly also fluid temperatures, is likely to promote severe chemical disequilibrium. This has implications for controls on ore deposition and grade distribution in many fault-related ore systems.

HIGH SULFIDATION EPITHERMAL MINERALIZATION AND ORE MINERAL ASSEMBLAGES OF CIJULANG PROSPECT, WEST JAVA, INDONESIA

Cijulang is a high-sulfidation epithermal system hosted in the calc-alkaline rocks of andesite lava and lapilli tuff. Mineralization in the prospect is characterized by pyrite-enargite-gold and associated acid sulfate alteration. Studies on ore and gangue mineral assemblages and their mutual textural relationships were carried out in order to explore the paragenetic sequence of mineralization. Hypogene mineralization primarily occurs in the silicic core and the advanced argillic zone in the form of massive replacement, fracture-filling veinlets, vug-filling, patches and dissemination. Mineralization is apparently controlled by both lithology and structures. Common ore minerals include pyrite, enargite, luzonite, tennantite, chalcopyrite, covellite, galena, emplectite and Te-bearing minerals. The paragenetic study indicates that the epithermal prospect evolved from an early stage of intense acid leaching resulting in the formation of vuggy silica and advantage argillic mineral assemblage which was followed by the sulfides deposition. Two metal stages were identified during ore deposition: an early Fe-As-S stage and the later Cu-Fe-As-S stage. The former stage is char- acterized high-sulfidation state sulfides such as enargite/ luzonite+covellite whereas a later stage of Cu- Fe-As–S is represented by intermediate sulfidation state sulfides assemblage of tennantite+chalcopyrite. Gold is probably introduced in the early stage within the ore system and more abundant in the late stage. Keywords: Cijulang, high-sulfidation, acid sulfate, mineralization, enargite, paragenetic, metal stages

Constraints on Hf and Zr mobility in high-sulfidation epithermal systems: formation of kosnarite, KZr2(PO4)3, in the Chaquicocha gold deposit, Yanacocha district, Peru

We report the first occurrence of Hf-rich kosnarite [K(Hf,Zr)2(PO4)3], space group R-3c, Z = 6, in the giant Chaquicocha high-sulfidation epithermal gold deposit in the Yanacocha mining district, Peru. Kosnarite crystals are small (<100 μm) and occur in 2–3-mm-thick veins that cut intensively silicified rocks. The paragenesis includes a first stage of As-free pyrite and quartz (plus gratonite and rutile), followed by trace metal-rich pyrite [(Fe,As,Pb,Au)S2] and secondary Fe sulfates. Kosnarite is associated with quartz and is clearly late within the paragenetic sequence. Electron microprobe analyses (EMPA) of kosnarite show relatively high concentrations of HfO2 and Rb2O (7.61 and 1.05 wt.%, respectively). The re-calculated chemical formulas of kosnarite vary from KΣ1.00(Zr1.93Na0.01Hf0.01Mn0.01)Σ1.96(P3.04O4)Σ3 to (K0.92Rb0.05Na0.03)Σ1.00(Zr1.81Hf0.19)Σ2.00 [(P2.98Si0.02As0.01)Σ3.01O4]Σ3, where Hf and Rb are most likely incorporated according to a coupled substitution of Hf4+ + Rb+ ⇔ Zr4+ + K+. Back-scattered electron (BSE) images and elemental mapping of kosnarite reveal that Hf and Rb are enriched in 2–10-μm-wide oscillatory and/or sector zones. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observations of such zones reveal a pattern of alternating, 5–50-nm-thick, Hf-rich and Zr-rich nanozones. These high-resolution observations indicate that the incorporation of Hf does not appear to cause significant distortion in the kosnarite structure. Semiquantitative TEM-energy-dispersive X-ray spectrometry (EDS) analyses of the nano-layers show up to 22 wt.% of HfO2, which corresponds to 31 mol% of the hypothetical, KHf2(PO4)3, end-member. The presence of kosnarite in the advanced argillic alteration zone at Yanacocha is indicative of Hf and Zr mobility under highly acidic conditions and points towards an unforeseen role of phosphates as sinks of Zr and Hf in high-sulfidation epithermal environments. Finally, potentially new geochronological applications of highly insoluble vein kosnarite, including Rb-Sr dating, may provide further age constraints in pervasively altered areas where other isotopic systems might have been reset.

Unveiling the hydrothermal mineralogy of the Chapi Chiara gold prospect, Peru, through reflectance spectroscopy, geochemical and petrographic data

Southern Peru contains important epithermal Au–Ag (±base metals) deposits, such as Canahuire, Tucari, Santa Rosa, Caylloma, Shila and Paula. The Chapi Chiara gold prospect is located in this region and is part of a paleo-stratovolcano of the Upper Miocene–Pliocene. The hydrothermal alteration of the prospect was characterized based on spectroradiometric data, geochemistry and petrography. The mineralogical data, interpreted based on reflectance spectroscopy, were spatialized using the sequential indicator simulation technique for producing probabilistic maps of alteration. The inner part of the paleo-stratovolcano (SW sector) is marked by three main cores of advanced argillic alteration (AAA) (quartz–alunite supergroup minerals–kaolinite–dickite±topaz±pyrophyllite±diaspore) associated with topographic highs. The AAA1 core is surrounded by argillic alteration (quartz–illite–paragonitic illite–smectite±pyrite) and propylitic alteration (quartz–plagioclase–chlorite–calcite–epidote–smectite±kaolinite±pyrite±chalcopyrite±magnetite). The central sector of the prospect, situated in the NE flank of the paleo-stratovolcano, is characterized by hydrothermal breccias structured towards N65E. The main mineral phases comprise quartz and abundant pyrite, sometimes with traces of As. Anomalous geochemical values of Ag, As, Bi, Hg, Se, Sb and Te coincide with high gold contents in this sector of the prospect. Jarosite and goethite are evidence of a subsequent supergene event. Based on the mineralogical characterization, we conclude the existence of a high sulfidation epithermal system in Chapi Chiara. Hypogene minerals of higher temperature in the SW sector of the prospect, such as diaspore, pyrophyllite and topaz in the AAA zone, and epidote in the propylitic alteration zone, can reveal that the system is currently in a relatively deep erosion level, suggesting its proximity in relation to the interface between a deep epithermal system and a mesothermal system.

Geology of the Cerro Quema Au-Cu deposit (Azuero Peninsula, Panama)

The Cerro Quema district, located on the Azuero Peninsula, Panama, is part of a large regional hydrothermal system controlled by regional faults striking broadly E-W, developed within the Rio Quema Formation. This formation is composed of volcanic, sedimentary and volcano-sedimentary rocks indicating a submarine depositional environment, corresponding to the fore-arc basin of a Cretaceous–Paleogene volcanic arc. The structures observed in the area and their tectono-stratigraphic relationship with the surrounding formations suggest a compressive and/or transpressive tectonic regime, at least during Late Cretaceous–Oligocene times. The igneous rocks of the Rio Quema Formation plot within the calc-alkaline field with trace and rare earth element (REE) patterns of volcanic arc affinity. This volcanic arc developed on the Caribbean large igneous province during subduction of the Farallon Plate. Mineralization consists of disseminations of pyrite and enargite as well as a stockwork of pyrite and barite with minor sphalerite, galena and chalcopyrite, hosted by a subaqueous dacitic lava dome of the Rio Quema Formation. Gold is present as submicroscopic grains and associated with pyrite as invisible gold. A hydrothermal alteration pattern with a core of advanced argillic alteration (vuggy silica with alunite, dickite, pyrite and enargite) and an outer zone of argillic alteration (kaolinite, smectite and illite) has been observed. Supergene oxidation overprinted the hydrothermal alteration resulting in a thick cap of residual silica and iron oxides. The ore minerals, the alteration pattern and the tectono-volcanic environment of Cerro Quema are consistent with a high sulfidation epithermal system developed in the Azuero peninsula during pre-Oligocene times

UNCLOAKING INVISIBLE GOLD: USE OF NANOSIMS TO EVALUATE GOLD, TRACE ELEMENTS, AND SULFUR ISOTOPES IN PYRITE FROM CARLIN-TYPE GOLD DEPOSITS

One-hundred-nm-resolution secondary ionizing mass spectrometry (nanoSIMS) was used to determine the distribution of Au, Cu, As, Sb, and S, and stable isotopes of S (34S/32S) in gold-bearing pyrite from two refractory Carlin-type gold deposits: West Banshee, northern Carlin Trend, and Turquoise Ridge, Getchell Trend, located in northern Nevada. NanoSIMS maps reveal that gold occurs in two discrete episodes in each deposit. Elevated gold concentrations correlate with elevated concentrations of As, Sb, Cu ± Te, and lower 34S/32S ratios, compared to periods when gold was not deposited. Precipitation of elevated gold and trace elements at West Banshee was followed by precipitation of pyrite with lower gold concentrations, whereas at Turquoise Ridge precipitation of trace element-rich pyrite was followed by precipitation of late ore-stage minerals. These NanoSIMS results are consistent with formation of the deposits by the episodic incursion of gold-rich fluids into hydrothermal systems otherwise dominated by gold-poor fluids. Such gold-rich fluid pulses may be related to incursions of magmatic fluid, which have been shown to have high concentrations of gold, arsenic, and copper in porphyry and high-sulfidation epithermal systems.

Late Cretaceous porphyry Cu and epithermal Cu–Au association in the Southern Panagyurishte District, Bulgaria: the paired Vlaykov Vruh and Elshitsa deposits

Vlaykov Vruh–Elshitsa represents the best example of paired porphyry Cu and epithermal Cu–Au deposits within the Late Cretaceous Apuseni–Banat–Timok–Srednogorie magmatic and metallogenic belt of Eastern Europe. The two deposits are part of the NW trending Panagyurishte magmato-tectonic corridor of central Bulgaria. The deposits were formed along the SW flank of the Elshitsa volcano-intrusive complex and are spatially associated with N110-120-trending hypabyssal and subvolcanic bodies of granodioritic composition. At Elshitsa, more than ten lenticular to columnar massive ore bodies are discordant with respect to the host rock and are structurally controlled. A particular feature of the mineralization is the overprinting of an early stage high-sulfidation mineral assemblage (pyrite ± enargite ± covellite ± goldfieldite) by an intermediate-sulfidation paragenesis with a characteristic Cu–Bi–Te–Pb–Zn signature forming the main economic parts of the ore bodies. The two stages of mineralization produced two compositionally different types of ores—massive pyrite and copper–pyrite bodies. Vlaykov Vruh shares features with typical porphyry Cu systems. Their common geological and structural setting, ore-forming processes, and paragenesis, as well as the observed alteration and geochemical lateral and vertical zonation, allow us to interpret the Elshitsa and Vlaykov Vruh deposits as the deep part of a high-sulfidation epithermal system and its spatially and genetically related porphyry Cu counterpart, respectively. The magmatic–hydrothermal system at Vlaykov Vruh–Elshitsa produced much smaller deposits than similar complexes in the northern part of the Panagyurishte district (Chelopech, Elatsite, Assarel). Magma chemistry and isotopic signature are some of the main differences between the northern and southern parts of the district. Major and trace element geochemistry of the Elshitsa magmatic complex are indicative for the medium- to high-K calc-alkaline character of the magmas. 87Sr/86Sr(i) ratios of igneous rocks in the range of 0.70464 to 0.70612 and 143Nd/144Nd(i) ratios in the range of 0.51241 to 0.51255 indicate mixed crustal–mantle components of the magmas dominated by mantellic signatures. The epsilon Hf composition of magmatic zircons (+6.2 to +9.6) also suggests mixed mantellic–crustal sources of the magmas. However, Pb isotopic signatures of whole rocks (206Pb/204Pb = 18.13–18.64, 207Pb/204Pb = 15.58–15.64, and 208Pb/204Pb = 37.69–38.56) along with common inheritance component detected in magmatic zircons also imply assimilation processes of pre-Variscan and Variscan basement at various scales. U–Pb zircon and rutile dating allowed determination of the timing of porphyry ore formation at Vlaykov Vruh (85.6 ± 0.9 Ma), which immediately followed the crystallization of the subvolcanic dacitic bodies at Elshitsa (86.11 ± 0.23 Ma) and the Elshitsa granite (86.62 ± 0.02 Ma). Strontium isotope analyses of hydrothermal sulfates and carbonates (87Sr/86Sr = 0.70581–0.70729) suggest large-scale interaction between mineralizing fluids and basement lithologies at Elshitsa–Vlaykov Vruh. Lead isotope compositions of hydrothermal sulfides (206Pb/204Pb = 18.432–18.534, 207Pb/204Pb = 15.608–15.647, and 208Pb/204Pb = 37.497–38.630) allow attribution of ore-formation in the porphyry and epithermal deposits in the Southern Panagyurishte district to a single metallogenic event with a common source of metals.

Fluids in early stage hydrothermal alteration of high-sulfidation epithermal systems: A view from the Vulcano active hydrothermal system (Aeolian Island, Italy)

High-sulfidation (HS) epithermal systems have elements in common with passively degassing volcanoes associated with high T, acid fumarole fields or acid crater lakes. They are considered to form in two stages, the first of which involves advanced argillic alteration resulting from intense, strongly acidic fluid–rock interaction. The La Fossa hydrothermal system (Vulcano Island) represents a classic example of such an active HS system and can be considered as a modern analogue of this early stage of alteration, resulting in a core of intense silicic (90–95% pure SiO 2) alteration surrounded by alunitic alteration zones. This paper focuses on a geochemical and stable isotope study of the surficial alteration facies of Vulcano – particularly the horizon characterized by strong silicic alteration – and on deep seated xenoliths ejected during the last eruption of La Fossa volcano (1888–90) that can be considered as representative of fragments of the deep conduit system of La Fossa volcano. Using directly measured temperatures at the sites of sampling, we have calculated fluid composition in isotopic equilibrium with the alteration products. The large range of measured silica δ 18O (12.3 to 29‰) reflects the wide range of formation temperatures (80–240 °C). The fluid compositions calculated for intense silicic alteration vary from − 0.9 to + 6.5‰. These are significantly heavier than local meteoric water (− 6‰), and are consistent with derivation from the condensation of high-temperature fumarolic gases, dominated by magmatic fluids and rich in acid gases (SO 2, H 2S, HCl, HF), into shallow groundwaters of meteoric origin, with dynamically variable ratios of fumarolic steam/meteoric water. The calculated δ 18O and δD of water in equilibrium with alunite also suggest the mixing of magmatic and meteoric waters for the fluids involved in the genesis of advanced argillic alteration facies. The calculated δ 18O of water in equilibrium with hedenbergitic clinopyroxene, found in a veinlet in a metasomatized xenolith is + 8.9‰. This value cannot reasonably result from water–rock interaction between the host volcanic rocks and surface water. Instead, it most likely represents a fluid (brine) exsolved from magma, which was responsible for high temperature metasomatism in the deep conduit system.