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The Akcal epithermal Au–Ag deposit, located on the Biga Peninsula, southwestern Turkey, is estimated to contain 32.5 million tonnes of mineralization at an average grade of 1.03 g t−1 Au, equivalent to 1.2 million ounces of gold. Epigenetic mineralization is hosted by the lower Miocene Sapci volcanic suite, a calc-alkaline subduction-related volcanic arc consisting mainly of andesitic and basaltic andesitic rocks, while the coeval Soma Formation is composed of sandstone, claystone, conglomerate, marl-limestone, siltstone and tuffite. Near the base of the volcanic pile, the Karakaya Complex is represented by limestone blocks of various sizes (from a few millimetres to several kilometres) within the Sapci volcanic suite. Short-wave infrared and X-ray diffraction analyses have shown that kaolinite, illite–smectite, montmorillonite, illite, beidellite, gypsum, vermiculite, quartz and chlorite are the most common alteration minerals at Akcal. The alteration mineralogy indicates that the temperature of hydrothermal fluids broadly varied from 150° to 230°C, with a mildly acidic pH range of 5.0–6.5; these conditions accompanied by a lack of alunite suggest low-sulfidation epithermal conditions for the formation of the Akcal epithermal Au–Ag deposit. Petrographic examination and microanalysis by scanning electron microscopy indicate that mineralization at Akcal consists of pyrite and arsenopyrite with lesser hematite, ilmenite, rutile and apatite. The occurrence of pyrite and hematite on cleavage planes of hydrothermal biotite is evidence of replacement of an earlier potassic alteration phase. Further petrographic examination of the basal limestone reveals replacement by sulfides displaying complex colloform intergrowths of pyrite and marcasite. The lack of skarn alteration and marble supports a carbonate-replacement origin for sulfides occurring in the basal limestone and is likely related to epithermal mineralization at Akcal. Geochemical results show that an Au–Ag–As–Sb ± Pb ± Zn signature is dominant across the volcanic-hosted epithermal mineralization and carbonate-replacement occurrences in the basal limestone; the lack of Cu mineralization (<92 ppm) is typical of low-sulfidation epithermal systems. Strong positive correlations between Au and Ag (r′ = 0.77), As (0.80) and Sb (0.86) indicate that gold is likely hosted as a sub-microscopic phase within pyrite, arsenian pyrite and arsenopyrite; visible gold (or free gold) and (or) electrum were not identified at Akcal. A model of fluid mixing involving shallow circulating meteoric waters with deeper magmatic-hydrothermal fluids is proposed for the genesis of gold mineralization at Akcal; a positive Au correlation with Mo (r′ = 0.74) and Mo concentrations of up to 582 ppm support the involvement of magmatic-hydrothermal fluids in the epithermal system. Boiling as a mechanism for gold precipitation is unlikely given palaeodepths of greater than 250 m (3.0–5.0 MPa) and model temperatures of 150° to 230°C. Mineralogical and textural evidence of boiling, such as adularia, bladed calcite and lattice-texture quartz, have not been observed at Akcal. However, the presence of a silica-rich blanket with chalcedony (at the Akcal surface) and having formed near the palaeo water table may indicate some involvement of CO2-rich steam-heated waters, which could suggest some influence from boiling at depth, although evidence of boiling has not been confirmed in samples from Akcal. Precipitation of gold during the fluid mixing process would have occurred through a shift to higher fO2 conditions and destabilization Au–thiosulfide complexes. This shift towards higher fO2 is supported by the occurrence of gypsum and hematite with gold mineralization at depth. [ABSTRACT FROM AUTHOR]

Pyrite is the most common sulfide mineral in hydrothermal ore-forming systems. The ubiquity and abundance of pyrite, combined with its ability to record and preserve a history of fluid evolution in crustal environments, make it an ideal mineral for studying the genesis of hydrothermal ore deposits, including those that host critical metals. However, with the exception of boiling, few studies have been able to directly link changes in pyrite chemistry to the processes responsible for bonanza-style gold mineralization. Here, we report the results of high-resolution secondary-ion mass spectrometry and electron microprobe analyses conducted on pyrite from the Brucejack epithermal gold deposit, British Columbia. Our δ34S and trace element results reveal that the Brucejack hydrothermal system experienced abrupt fluctuations in fluid chemistry, which preceded and ultimately coincided with the onset of ultra-high-grade mineralization. We argue that these fluctuations, which include the occurrence of extraordinarily negative δ34S values (e.g., -36.1%) in zones of auriferous, arsenian pyrite, followed by sharp increases of δ34S values in syn-electrum zones of nonarsenian pyrite, were caused by vigorous, fault valve-induced episodic boiling (flashing) and subsequent inundation of the hydrothermal system by seawater. We conclude that the influx of seawater was the essential step to forming bonanza-grade electrum mineralization by triggering, through the addition of cationic flocculants and cooling, the aggregation of colloidal gold suspensions. Moreover, our study demonstrates the efficacy of employing high-resolution, in situ analytical techniques to map out individual ore-forming events in a hydrothermal system. [ABSTRACT FROM AUTHOR]

The tri-smectite(S)–corrensite(Co)–chlorite(C) series minerals from two epithermal deposits show a discontinuous stepwise sequence of different mixed-layering of chlorite and smectite layers, and there are differences in the mode of occurrence of the two deposits. The Al/Si ratios and Fe/(Fe + Mg) ratios of the S–Co–C minerals vary closely related to mixed-layering and mode of occurrence. The S–Co–C minerals as a product of direct precipitation from ascending hydrothermal solutions may reflect fluid chemistry that originated in water–rock interaction at deeper strata. The differences in mixed-layering of the S–Co–C series minerals may be related to different thermal and redox conditions affected by fluid mixing and boiling, and to kinetic factor such as time length of hydrothermal activity that affected vein formation. The corrensite and Co–C minerals as a product of hydrothermal alteration involving dissolution, re-precipitation, and crystallization, may undergo smectite-to-chlorite transformation in epithermal systems. The transformation and distribution of corrensite and Co–C minerals in the host rocks around vein areas may have been controlled by thermal conditions related to fluid mixing and water/rock ratios. In addition, the corrensite and Co–C minerals with high Fe/(Fe + Mg) ratios may be affected by the host rock. The factors influencing the conversion of the S–Co–C series minerals may be similar in both ore veins and host rocks. However, it is emphasized that fluid/rock ratios may be a major factor influencing the conversion of the S–Co–S series minerals in host rocks. Additionally, the time length of hydrothermal activity that affected vein formation may be an important factor influencing the conversion of the S–Co–C series minerals in ore veins. [ABSTRACT FROM AUTHOR]

Aqueous complexation has long been considered the only viable means of transporting gold to depositional sites in hydrothermal ore-forming systems. A major weakness of this hypothesis is that it cannot readily explain the formation of ultrahigh-grade gold veins. This is a consequence of the relatively low gold concentrations typical of ore fluids (tens of parts per billion [ppb]) and the fact that these "bonanza" veins can contain weight-percent levels of gold in some epithermal and orogenic deposits. Here, we present direct evidence for a hypothesis that could explain these veins, namely, the transport of the gold as colloidal particles and their flocculation in nanoscale calcite veinlets. These gold-bearing nanoveinlets bear a remarkable resemblance to centimeter-scale ore veins in many hydrothermal gold deposits and give unique insight into the scale invariability of colloidal flocculation in forming hyperenriched gold deposits. Using this evidence, we propose a model for the development of bonanza gold veins in high-grade deposits. We argue that gold transport in these systems is largely mechanical and is the result of exceptionally high degrees of supersaturation that preclude precipitation of gold crystals and instead lead to the formation of colloidal particles, which flocculate and form much larger masses. These flocculated masses aggregate locally, where they are seismically pumped into fractures to locally form veins composed largely of gold. This model explains how bonanza veins may form from fluids containing ppb concentrations of gold and does not require prior encapsulation of colloidal gold particles in silica gel, as proposed by previous studies. [ABSTRACT FROM AUTHOR]

• The Torud–Chah Shirin (TCS) in northern Iran is defined by an NE alignment of Tertiary Ag/Au and base metal-rich epithermal systems. • Intermediate- to high-sulfidation epithermal deposits are present as vein-type mineralization and hosted by the Eocene-Oligocene igneous rocks. • The magmatism mainly occurred sporadically from Eocene to Oligocene with two major episodes between early to middle Eocene (EME) and early to late Oligocene (ELO). The Torud–Chah Shirin (TCS) ore district in northern Iran is defined by an NE alignment of Tertiary Ag-Au and base metal-rich epithermal systems, and it is part of the eastern Alborz orogenic belt of Iran. Intermediate- to high-sulfidation mineralization occurs as veins hosted by the Eocene−Oligocene volcanic, subvolcanic and volcaniclastic rocks. The TCS district is characterized by three fault system populations including ∼70°, ∼270°, and ∼340°−trending faults, and detailed structural mapping show that overall strike of the TCS vein system is 320°–340° but varies from ∼290° to ∼350°. The ∼N70°−trending faults are parallel to the Anjilow and Torud regional faults in the TCS ore district. Green- to grey-schist and metamorphosed dolomite and limestone are the oldest units (Ordovician–Silurian) in the TCS. Sedimentary rocks were initiated by limestone, dolomite and green shale in the Cretaceous and continued with conglomerate into the Paleocene (Fajan Formation). Magmatism mainly occurred sporadically from the Eocene to Oligocene (ca. 55–24 Ma), with two major episodes between early to middle Eocene (ca. 55 and 37 Ma, EME) and early to late Oligocene (ca. 34 and 24 Ma, ELO). Whole rock geochemical data of EME and ELO rocks of the TCS district shows a range from basalts to rhyolites with low-K calc-alkaline and shoshonitic affinity. Their rare earth elements (REEs) and high field strength elements (HFSE) signatures indicate the occurrence of a supra-subduction zone magmatism and all rocks have been sourced from the same parent melt. Samples from ELO display higher alkali contents compare with EME but have a similar trace element characteristics. Hydrothermal alteration is pervasive in volcanic and subvolcanic rocks but is mainly localized near the veins and ore zones. Alteration assemblages include quartz-sericite (±pyrite ± adularia), quartz-illite (kaolinite ± pyrite), carbonate-quartz and chlorite + calcite + epidote (propylitic). Homogenization temperatures of fluid inclusions in the epithermal prospects is in range of 125–375 °C, and ore-forming fluids were mainly of magmatic-hydrothermal origin (e.g., Gandy and Abolhassani prospects), with some contributions from meteoric water. The epithermal prospects have more or less similar salinities, ranging from 2 to 18 wt% NaCl equiv., with a distinct cluster between 6 and 15 wt% NaCl equiv. Our shreds of evidence suggest that the EME and ELO calc-alkaline volcanic activity in TCS occurs occurred in tension fractures related to orthogonal plate convergence that postdates the NE-trending strike-slip regional faults, and plays an important role in the development of the epithermal prospects. Simultaneous with, or soon after crustal heating related to the magmatism, strike-slip faults movement may also have been critical in the ore-forming process, leading to trans-tension of local compressive forces, enhancement of crust-scale permeability, and promotion of mixing of ore-forming fluids. [ABSTRACT FROM AUTHOR]

Zones of advanced argillic alteration with general low permeability (i.e., lithocaps) are common place in the shallow parts of porphyry and epithermal ore deposits and active geothermal systems. The study of structural control on alteration distributions is of paramount importance for exploitation purposes as it really influences the caprock efficiency. We present the results of a structural-mineralogical study carried out in the lithocap of the Allumiere-Tolfa epithermal system (Northern Apennines). We characterized the composition, textures and physical properties (i.e., in-situ permeability; relative rock strength) of alteration facies. We then integrated field structural analysis with analysis of a virtual outcrop model to reconstruct the geometry of principal fluid-corridors. It resulted that advanced argillic alteration was promoted by circulation of highly reactive fluid(s) along a complex network of NE- and NW-striking faults and fractures dissecting the acidic volcanic dome. Such structures likely developed in response to a local disturbance of the regional stress field due to the extrusion of the Tolfa dome, which controlled the syn-extensional mineralization of the Allumiere-Tolfa area. • Circulation of hydrothermal fluids is enhanced by doming-related faults. • Allumiere deposit is synextensional to the Tyrrenian back-arc basin opening. • Hydrolitic alteration promotes the formation of lithostructural domains. [ABSTRACT FROM AUTHOR]

Abstract: 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 SiO2) 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 (SO2, H2S, 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. [Copyright &y& Elsevier]

Provides information on a study of the kinetics of ore-forming fluids in epithermal systems. Properties and chemical kinetics of ore-forming fluids; Experimental approach and results; Conclusion.