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The Eastern Pontides Orogenic Belt (EPOB), geographically corresponding to the northeastern part of Turkey hosts several different types of mineralizations that are closely related to Late Mesozoic-Early Cenozoic arc magmatism. Of these, Zarani gold mineralization, which is located in the southern part of the EPOB occurs as quartz-carbonate veins/veinlets within the Pulur metamorphic massif of Paleozoic age. Mineral textures indicate low-temperature vein-type mineralization. Gold mainly occurs in quartz and pyrite. The presence of calcite, dolomite, and sericite in the mineralization indicates near neutral pH conditions of ore-forming fluid. Geochemical analyses of chlorites in the quartz-carbonate veins reveal a temperature of ore formation at 140-297 °C, which is compatible with those obtained from fluid inclusions in the mineralized quartz (i.e. 132-226 °C). The average salinity value obtained from fluid inclusions is 4.32% NaCl equiv. O and H isotope values range from -6.03? to +1.47? and from -60? to -119?, respectively, indicating a mixture of magmatic and meteoric fluids. Mixing is also evidenced by the XFe analytical data of hydrothermal chlorites associated with ore-forming gangue minerals. Gold precipitation in Zarani mineralization is caused by boiling. The occurrence of mineralization within quartz-carbonate vein/veinlets in metamorphic host rocks, the mineral paragenesis with near-neutral pH conditions of the ore-forming low salinity fluids and the H-O isotopic data imply that the Zarani gold mineralization is an orogenic type epithermal mineralization that may be related to deeply buried Early Cenozoic felsic intrusions, produced by subduction-induced processes in the southern part of the EPOB. [ABSTRACT FROM AUTHOR]

Myanmar is occupied by the N-wards continuation of the Sunda arc and by the Shan Plateau and its continuation through Yunnan into Tibet. Our new tectonic interpretation of the ophiolite–flysch belts, world-famous jadeite and tin deposits in Myanmar west of the Salween adopts previous proposals that, before 450-km post-early Oligocene dextral displacement along the Sagaing Fault, the ophiolite belt in NE Myanmar continued through the topography that is now located west of the fault in the Indo-Burman Ranges. Differences in cross-section through Mogok and the Shan Scarps are reconciled by the recently proposed emplacement, in our view during Permian time, of the Mogok Metamorphic Group onto the Slate Belt to form Sibumasu. We argue that during Early Jurassic time a Neo-Tethys ophiolite nappe was obducted over turbidites on Sibumasu's passive western margin. Following reversal in tectonic polarity, the remaining Neo-Tethys subducted E-wards generating the 113–128 Ma Mondaung Arc. During ocean closure the Victoria–Katha Block and its Triassic flysch subducted beneath Sibumasu, resulting in jadeite veins in overlying serpentinite that ascended in the subduction zone and were exhumed at Hpakant and Nat Hmaw, bordering the Jade Mines Uplift. Subduction of the Indian Ocean since Albian time generated the Popa–Loimye arc, while extensional faulting led to uplift of the Indo-Burman Ranges and to the formation of the Western Tin Belt granites. Tectonic effects in Myanmar of the India–Asia collision may be confined to the Disang thrust belt in the Naga Hills. [ABSTRACT FROM AUTHOR]

The major target of this research is the classification of pyrite types using fractal and stepwise factor analyses in the Chah Zard ore deposit, Central Iran. The ore occurs within a breccia/vein type and the major ore mineral hosting gold mineralization is pyrite. In this study, data were selected using optical, scanning electron microscopy (SEM) and backscattered electron observations as well as laser ablation-inductively coupled plasma–mass spectrometry (LA-ICP–MS) analysis. Conventional interpretations represent four gold-bearing pyrite types of various textures including fractured and porous Py1, oscillatory-rimmed and simple-zoned Py2, colloform Py3 and inclusion-rich Py4. The stepwise factor process was performed on the centred log ratio (clr) transformed data in two phases and Au was grouped with As, Te, Ni and Co in the second factor from the second stage (F2-2). Also, C-N fractal modelling was performed on the As, Au, Te and F2-2 values, all of which demonstrate multifractal nature. Four populations were separated based on F2-2 values and the C-N log–log plot. The main gold mineralization starts from 32 ppm, 2.8%, 7.94 ppm and 1.26 for Au, As, Te concentrations and F2-2, respectively, based on the C-N fractal modelling. These values are correlated with inclusion-rich Py4 and simple-zoned and oscillatory-rimmed Py2. The results obtained in this study show that fractal interpretation of LA-ICP–MS data by stepwise factor analysis may provide a suitable tool for the recognition of ore mineralization in epithermal gold deposits. [ABSTRACT FROM AUTHOR]

Introduction The Masjed- Daghi gold deposit lies in an area of widespread Cenozoic volcanic and plutonic rocks at the intersection of the Alborz- Azarbaijan and Urumieh- Dokhtar belts. The area was covered by a detailed exploration program, including geological maps at 1:1,000 scales (~8 km²), several hundred meters of trenches and systematic sampling for Au, Ag, Pb, Zn, Cu, As, Hg analysis, and 16 diamond drill holes at a total of 1200 meters (Mohammadi et al, 2005). The vein type gold deposit in Masjed- Daghi is closely associated with a porphyry type Cu-Au deposit. Our study focuses on the gold bearing veins system in an attempt to understand the characteristics of ore fluids and mechanisms of ore formation, and to develop exploration criteria for Masjed Daghi and similar occurrences in Alborz and other Cenozoic magmatic assemblages in Iran. Materials and methods Various rock types, alteration assemblages and mineral parageneses were characterized by transmitting and reflected light microscopy, X-ray diffraction (XRD) and electron microprobe analysis. Microprobe analyses were performed using a JEOL 8600 Superprobe electron microprobe at Saskatchewan University. Operating conditions were an accelerating voltage of 15 kV and a beam current of 50 nA. Representative samples from drill holes were selected for fluid inclusion studies. Fluid inclusion data were obtained using a fluid Inc. adapted USGS gas flow heating and freezing system at the Department of Geological Science at the University of Saskatchewan, Canada. To investigate the source of ore fluids, representative sulfidic samples from drill holes were selected for sulfur isotope studies. Isotopic analyses were performed using a Thermo Finnigan DeltaPlus at the G.G. Hatch Stable Isotope Laboratories, University of Ottawa. The standard error of analyses is less than ±0.1 per mil. Results Auriferous quartz veins in Masjed- Daghi are associated with porphyry style mineralization. Various alteration assemblages including argillic, silicic, potassic, phyllic, and propylitic occur in the district (Emamalipour et al., 2010). The auriferous quartz veins are hosted by silicified and kaolinitized volcanic rocks, dominated by trachyandesite. The mean grade of Cu is 0.15% the gold assay varies from

Volcanic-hosted epithermal Au-Ag quartz vein deposits in the Hauraki Goldfield are investigated using a range of geophysical methods. Tools such as airborne magnetics and ground-based resistivity have focussed on mapping alteration zones, lateral contacts in the volcanic rocks, and prospect scale faults. Reviewing the merits of each geophysical method is valuable as it provides exploration teams with the background material required to apply the most effective tools. The review compiles existing data and summarises the rock properties useful for constraining geophysical models. The Golden Cross mine in the Waitekauri Valley is used to illustrate the strengths and limitations of various techniques. Electrical resistivity is the most valuable rock property to target exploration, but magnetic surveying and gamma-ray spectrometry can reduce the ambiguity between quartz-rich mineralised zones and un-altered volcanic host formations. Density and porosity variations caused by alteration can be mapped with gravity data provided the regional field is defined. [ABSTRACT FROM AUTHOR]

Abstract Sometimes geochemical anomalies linked with buried mineral deposits are too weak to be recognized by conventional methods. In this study, element concentration data from a stream sediment survey were subjected to a combined method of SWPCA (spatially-weighted principal components analysis) and WT (wavelet transformation) to derive a geochemical anomaly model for epithermal Au deposits in the Dashui ore-concentration district. The SWPCA was applied as a data integration method to extract information related to mineralization in the geochemical data. The WT was applied as a powerful tool for recognizing mineralization related anomalies in a complex geochemical field and for enhancing weak anomalies from background. The SWPCA–WT geochemical anomaly model indicated high favorability values for the known mineral occurrences and out-performed PCA–WT and SWPCA models. The SWPCA–WT geochemical model generated in this study provides a robust guide for further gold exploration in the Dashui ore-concentration district. Graphical abstract Unlabelled Image Highlights • A combined method of SWPCA and WT for Au potential mapping. • Wavelet Transformation recognizes ore related anomalies from complex background. • SWPCA out performs PCA with stronger spatial association with known ore occurrence. • The SWPCA–WT model provides a robust guide for Au exploration in the study area. [ABSTRACT FROM AUTHOR]

Whole-rock geochemical and Sr- Nd isotopic data are presented for late Miocene volcanic rocks associated with the Chah Zard epithermal Au- Ag deposit in the Urumieh- Dokhtar Magmatic Arc ( UDMA), Iran, to investigate the magma source, petrogenesis and the geodynamic evolution of the study area. The Chah Zard andesitic to rhyolitic volcanic rocks are characterized by significant Large Ion Lithophile Element ( LILE) and Light Rare Earth Element ( LREE) enrichment coupled with High Field Strength Element ( HFSE) depletion. Our geochemical data indicate an adakitic-like signature for the volcanic rocks (e.g. SiO2 > 62 wt%, Al2 O3 > 15 wt%, MgO < 1.5 wt%, Sr/ Y > 70, La/ Yb > 35, Yb < 1 ppm, and Y < 18 ppm, and no significant Eu anomalies), distinguishing them from the other volcanic rocks of the UDMA. The Chah Zard volcanic rocks have similar Sr and Nd isotopic compositions; the 87 Sr/86 Sr(i) ratios range from 0.704 902 to 0.705 093 and the εNd(i) values are from +2.33 to +2.70. However, the rhyolite porphyry represents the final stage of magmatism in the area and has a relatively high 87 Sr/86 Sr ratio (0.705 811). Our data suggest that the andesitic magmas are from a heterogeneous source and likely to result from partial melting of a metasomatized mantle wedge associated with a mixture of subducted oceanic crust and sediment. These melts subsequently underwent fractional crystallization along with minor amounts of crustal assimilation. Our study is consistent with the model that the volcanic host rocks to epithermal gold mineralization in the UDMA are genetically related to late Miocene Neo- Tethyan slab break-off beneath Central Iran. [ABSTRACT FROM AUTHOR]

http://dx.doi.org/10.17014/ijog.vol2no3.20072Taliwang conservation forest, West Nusa Tenggara, is particularly covered by volcanic rocks consisting of alternated breccia and tuff. The breccia is composed of andesitic – dioritic clasts and matrix of fine-grained to lapilli tuff which had partially been hidrothermally altered and invaded by gold bearing quartz veins. Outcrops of quartz veins are commonly limonitized, trending nearly east-west and north-south with respectively dip of 70o and 85o. Identification of quartz veins originated from people mining’s waste leads to a prediction that there are quartz stockworks beneath the earth surface (50 – 100 m depth) in the conservation forest area, from which quartz veins penetrated the illite-paragonite-calcite-siderite- nacrite altered country rock. Textures of quartz veins were identified such as comb with some coarse- grained euhedral crystals, sugary/saccharoidal/fine grained crystalline quartz and ghost-bladed. Veins and host rocks generally contain disseminated and spotted pyrites. Evaluation of quartz textures, altered rocks analysis (PIMA method), fluid inclusion studies and chemical analysis (AAS method) of selective altered rocks/quartz vein samples exhibits that the alteration and mineralization processes might occur in an epithermal system, connecting with a change of hydrothermal fluids from near neutral into acid conditions at a temperature ranging from 231 to 185oC. Alteration of illite-paragonite-kalsit-siderite is suggested as a result of reaction between host rock and a near neutral fluid, whilst nacrite (kaolin group) or argillic is a result of reaction between host rock and an acid fluid, within a mixing zone of meteoric fluid and condensed acid gas released during boiling process of hydrothermal fluid in the depth. On the basis of salinity ranging from 0.9 to 2.2 equivalent wt.% NaCl of fluid inclusion, it is predicted that the deposition of gold bearing quartz and associated ore minerals takes place in a depth ranging from 293 to 120 m beneath paleosurface.

Mapping hydrothermal alteration minerals using multispectral remote sensing satellite imagery provides vital information for the exploration of porphyry and epithermal ore mineralizations. The Ahar-Arasbaran region, NW Iran, contains a variety of porphyry, skarn and epithermal ore deposits. Gold mineralization occurs in the form of epithermal veins and veinlets, which is associated with hydrothermal alteration zones. Thus, the identification of hydrothermal alteration zones is one of the key indicators for targeting new prospective zones of epithermal gold mineralization in the Ahar-Arasbaran region. In this study, Landsat Enhanced Thematic Mapper+ (Landsat-7 ETM+), Landsat-8 and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) multispectral remote sensing datasets were processed to detect hydrothermal alteration zones associated with epithermal gold mineralization in the Ahar-Arasbaran region. Band ratio techniques and principal component analysis (PCA) were applied on Landsat-7 ETM+ and Landsat-8 data to map hydrothermal alteration zones. Advanced argillic, argillic-phyllic, propylitic and hydrous silica alteration zones were detected and discriminated by implementing band ratio, relative absorption band depth (RBD) and selective PCA to ASTER data. Subsequently, the Bayesian network classifier was used to synthesize the thematic layers of hydrothermal alteration zones. A mineral potential map was generated by the Bayesian network classifier, which shows several new prospective zones of epithermal gold mineralization in the Ahar-Arasbaran region. Besides, comprehensive field surveying and laboratory analysis were conducted to verify the remote sensing results and mineral potential map produced by the Bayesian network classifier. A good rate of agreement with field and laboratory data is achieved for remote sensing results and consequential mineral potential map. It is recommended that the Bayesian network classifier can be broadly used as a valuable model for fusing multi-sensor remote sensing results to generate mineral potential map for reconnaissance stages of epithermal gold exploration in the Ahar-Arasbaran region and other analogous metallogenic provinces around the world.

Gold mineralization at Chah Zard, Iran, is mostly concentrated in breccia and veins, and is closely associated with pyrite. Optical and scanning electron microscopy-backscattered electron observations indicate four different pyrite types, each characterized by different textures: porous and fractured py1, simple-zoned, oscillatory-rimmed, framboidal and fibrous py2, colloform py3, and inclusion-rich py4. Laser ablation ICP–MS analysis and elemental mapping reveal the presence of invisible gold in all pyrite types. The highest concentrations (161–166 ppm Au) are found in py2 and py4, which correlate with the highest As concentrations (73,000–76,000 ppm). In As-poor grains, Au concentrations decrease by about two orders of magnitude. Copper, Pb, Zn, Te, Sb, and Ag occur with invisible gold, suggesting that at least part of the gold occurs in nanoparticles of sulfosalts of these metals and metalloids. Gold distribution patterns suggest that only negligible Au was originally trapped in py1 from the initial ore fluids. However, most, if not all, Au was transported and deposited during subsequent overprinting hydrothermal fluid flow in overgrowth rims around the margins of the py2 and within microfractures of py4 grains. Oscillatory zonation patterns for Co, Ni, Sb, Cu, Pb, and Ag in pyrite reflect fluctuations in the hydrothermal fluid chemistry. The LA-ICP–MS data reveal that Cu, Pb and Ag show systematic variations between different pyrite types. Thus, Cu/Pb and Pb/Ag ratios in pyrite may provide a potentially powerful exploration vector to epithermal gold mineralization at Chah Zard district and elsewhere. [ABSTRACT FROM AUTHOR]

[Display omitted] • Gold precipitation occurred due to destabilization of Au-bisulfide complexes during fluid phase separation. • The Kalchuyeh gold deposit can be considered as a low-sulfidation style epithermal system. • The ore-forming fluids, metals, and sulfur have been made possible due to devolatilization of the volcanic and subvolcanic magmatic rocks. • The Eocene volcanic rocks in the central UDMA have a high exploration potential for targeting epithermal-porphyry deposits in Iran. The Kalchuyeh deposit (∼1 Mt @ 30 g/t Ag and 1.1 g/t Au) is located in the central Urumieh–Dokhtar Magmatic Arc (hereafter UDMA), central Iran. Hosted by Middle Eocene pyroxene andesite lavas, the mineralization occurs as veins and veinlets structurally controlled by NE-trending normal faults. The volcanic host rocks have calc-alkaline compositions with geochemical compositions typical of subduction-related arc magmas, including strong enrichments in LILE (K, Cs, Rb, Sr, and Ba), depletions in HFSE, and distinct negative Ti and Nb anomalies. Three ore-forming stages are recorded based on vein textures and alteration and sulfide mineral assemblages, including early quartz-pyrite-chalcopyrite (stage I), middle quartz-sulfide-gold (stage II), and late quartz-calcite-chlorite veins (stage III). Fluid inclusions in quartz veins are characterized by aqueous LV- and minor VL-types and the absence of any solid mineral- or CO 2 -bearing inclusions. The ranges of homogenization temperature decreased progressively through stages I to III from 235 to 308 °C, to 205–257 °C, and finally to 174–217 °C, respectively, and salinity ranged from 3.3 to 7.6, 5.5–10.7, and 1.3–4.7 wt% NaCl equiv. Fluid boiling during the intermediate temperature stage II caused a loss of H 2 S from the hydrothermal solution, reducing Au(HS)2− solubility and triggering gold precipitation. Fluid hydrogen and oxygen stable isotope values, which were measured from fluid inclusion waters from quartz and calculated for hydrothermal quartz, respectively, ranged from δD H2O = −84 ‰ to −70 ‰ and δ18O H2O = −4.0 ‰ to +1.5 ‰. This shows that the ore-forming fluids had a magmatic source but with late addition of meteoric waters. The δ34S sulfide isotope compositions of pyrite and chalcopyrite from stages I and II range between −1.9 ‰ and +0.7 ‰ (mean = −0.7 ‰) and reveal a magmatic sulfur source. Geochemical data and the regional geological history indicate that Middle Eocene magmatism and coeval mineralization at the Kalchuyeh Au ± Ag deposit occurred during subduction of the NeoTethys oceanic plate beneath the Central Iran continental plate, consistent with most epithermal gold systems in the UDMA. Mineralogical evidence that includes comb textures in quartz, bladed calcite, and low total sulfide and base metal contents of the mineralized veins, indicate a low-sulfidation epithermal deposit classification for the Kalchuyeh deposit. Importantly, the results of this research show that the Eocene volcanic rocks in the central UDMA have a high exploration potential for targeting epithermal-porphyry deposits in this area. [ABSTRACT FROM AUTHOR]

The newly discovered and large ca. 99 Ma Gaosongshan gold deposit is located in the Lesser Hinggan (or Xinggan) Range to the west of the Jiamusi Massif. The deposit is located at the border of the Great Hinggan Range–Mongolian Orogen with the Circum-Pacific tectonic belt and has a resource of ~ 22 t @ 6.3 g/t Au. Gaosongshan is classified as an adularia–sericite epithermal deposit associated with the large tensional Shaqihe Fault and has a low-sulfide content with a high concentration of gold hosted by pure chalcedonic quartz, which makes the deposit both unique and economically important in the area. Early Cretaceous volcanic rocks host the deposit as well as other gold deposits in the region. The mineralization at Gaosongshan includes Au, Ag, Mo, Sb, As, Pb and Hg. Stable isotope studies of fluid inclusions associated with the mineralization suggest that the mineralizing fluid had a meteoric origin (D V-SMOW : − 129 to − 111‰; 18 O fluid : 7.9 to 13.8‰). Fluid inclusion thermography indicates that the mineralized fluid was between 150° and 310 °C and had a low salinity (0.7–3.71 wt.% NaCl), low density (0.48–0.94 g/cm 3 ), shallow metallogenic depth (430–1270 m), and a large amount of reducing gaseous components (CH 4 , C 2 H 6 , CO, N 2 , and CO 2 ). The presence of large quantities of flaky quartz, adularia, gas-rich fluid inclusions, low-sulfide minerals and pure gold-bearing chalcedonic quartz suggests that fluid boiling was the principal mechanism for the gold precipitation. Sulfur isotopic data (δ 34 S: ~ − 2.4 to 2.9‰) indicate a deep magmatic origin for the mineralization, and the Pb isotopes ( 206 Pb/ 204 Pb: 18.14–18.46; 207 Pb/ 204 Pb: 15.51–15.57; 208 Pb/ 204 Pb: 38.01–38.40) also indicate that the metallogenic source of the deposit contained a significant mantle component. The mineralizing fluid interacted with a Late Paleozoic (ca. 260–253 Ma) substrate and ca. 141–91 Ma volcanic host rocks. The geological and geochemical characteristics of the deposit, and the Tuanjiegou, Dongan, Sandaowanzi and other large gold deposits represent an Early Cretaceous (114–80 Ma) epithermal mineralizing events in the Lesser Hinggan Range. The belt is controlled by the volcanic Sunwu–Jiayin Basin formed in a back-arc extensional setting along an active continental margin in East China, and the gold deposits in the belt have the same genesis as porphyritic Au(–Cu) deposits along the continental margin in eastern Jilin and Heilongjiang provinces. It is here proposed that the mineralization in both regions is related to the subduction of the Paleo-Pacific plate. [ABSTRACT FROM AUTHOR]

In this contribution we combine different image processing and pattern recognition methodologies to map the probability of discovering epithermal mineral deposits in the northern part of the Coromandel peninsula, in New Zealand. The objective of this work is to propose a case-study where the substitution of structural geology GIS themes (commonly developed by humans) with products derived by image processing, computer-based, semi-automatic edge detection analyses, is carried out to reduce subjective input in the prospectivity analysis. Semi-automated lineament extraction results introduced in the mineral favourability statistical modelling can more easily reveal unexpected potentially mineralised target domains, being less subjective. We present initial results of this analysis and explain some of the methodologies adopted. Preliminary results suggest that this approach increases significantly the number of geological discontinuities mapped in the region, with the following implications: (1) prospectivity models are more risk-tolerant and result in an increased number of targets; (2) increments in posterior probability affect the statistical validity of the model due to conditional independence violation, requiring careful assessment of probability overestimation; (3) the feature extraction process identifies numerous lineaments that in some instances represent false positives (lineaments determined by a variety of causes, without geological significance); however, we find that Contrast calculations in the Bayesian analysis tend to penalize these evidential themes, because of the higher number of pixels (cells) containing a positive pattern (lineament existence = 1, being positive). This aspect reduces the overall impact of these predictors on the analysis, mitigating the effect of false positives (lower positive weights of evidence). Despite the limitations, results obtained are encouraging with a clearly superior and more detailed mapping of potential structural sites and their relative probabilities of hosting epithermal deposits. [ABSTRACT FROM AUTHOR]

A 2D prospectivity model of epithermal gold mineralisation has been completed over the Taupo Volcanic Zone (TVZ), using the weights of evidence modelling technique. This study was used to restrict a 3D geological interpretation and prospectivity model for the Ohakuri region. The TVZ is commonly thought of as a present-day analogue of the environment in which many epithermal ore deposits, such as in the Hauraki Goldfield, Coromandel Volcanic Zone, are formed. The models utilise compiled digital data including historical exploration data, geological data from the Institute of Geological and Nuclear Sciences Ltd. Quarter Million Mapping Programme, recent Glass Earth geophysics data and historic exploration geochemical data, including rock-chip and stream sediment information. Spatial correlations between known deposits and predictive maps are determined from the available data, which represent each component of the currently accepted mineral system model for epithermal gold. The 2D prospectivity model confirms that the TVZ has potential for gold mineralisation. However, one of the weaknesses of this weights of evidence model is that the studies are carried out in 2D, with an approximation of 3D provided by geophysical and drilling data projected to a 2D plane. Consequently, a 3D prospectivity model was completed over the Ohakuri area, constrained by the results of the 2D model and predictive maps. The 3D model improved the results allowing more effective exploration targeting. However, the study also highlighted the main issues that need to be resolved before 3D prospectivity modelling becomes standard practise in the mineral exploration industry. The study also helped develop a work flow that incorporates preliminary 2D spatial data analysis from the weights of evidence technique to more effectively restrict and develop 3D predictive map interpretation and development. [ABSTRACT FROM AUTHOR]

Prospectivity modelling of epithermal gold mineralization has been completed over the Taupo Volcanic Zone (TVZ), North Island, New Zealand. The TVZ is unique in that it is a present-day analogue of the environment in which many epithermal ore deposits, such as in the adjacent Hauraki Goldfield in the Coromandel Volcanic Zone, are formed. The model utilizes compiled digital data including: historical exploration data; geological data from the Institute of Geological and Nuclear Sciences Quarter Million Mapping Programme over Rotorua, Hawkes Bay and Taranaki; recent Glass Earth Gold Ltd geophysical data; and historic exploration geochemical data from rock-chip and stream sediment samples. The weights of evidence modelling technique was used to determine spatial correlations between known gold prospects and predictive maps, created from the available data, representing each component of the currently accepted mineral system model for epithermal gold. Despite the limited coverage of several data-sets, the resulting prospectivity model over the TVZ confirms potential for gold mineralization. The most prospective areas are those associated with active and inactive hydrothermal systems represented by areas of hydrothermal alteration. The Rotokawa and Waiotapu hydrothermal centres have been identified as particularly prospective, suggesting these systems may represent modern analogues of epithermal gold mineralization. [ABSTRACT FROM AUTHOR]

We studied both colloform-banded macro- and micro-textures as well as the composition of electrum from the epithermal Khan Krum (or Ada Tepe) gold deposit, Bulgaria (5 Mt at 5.1 g/t Au + 2.7 g/t Ag) using optical and electron scanning microscopy, vibration spectroscopy, electron micro-probe analysis, and LA-ICP-MS. The individual textural bands differ in grain size, porosity, quartz-to-adularia ratio, and abundance of electrum, pyrite, and scattered dusty opaque micro-inclusions. The individual macro-bands have formed successively via intermittent fault dilation from episodic hydrothermal pulses, as a result of regional extension and most likely originated from colloidal solutions formed by vigorous boiling during rapid pressure drop and supersaturation with respect to amorphous silica in a closed or quasi-closed hydrothermal system. Electrum is highly concentrated in the finest quartz-adularia colloform micro-bands, which fill joints with widths up to 1-2 mm. We presume that the joints have filled in with more concentrated colloidal solutions than those in the macro-bands, most likely due to extreme boiling of fluids in open or quasi-open hydrothermal system. Electrum in the micro-bands forms transverse dendrite- and chain-like aggregates as well as oval clots along the banding, all of globular morphology. We explain these locations of electrum with reorientation of aggregated electrum globules during plastic deformation of a mixed electrum-silicate gel and in result from the consequent crystallization of silicates. At the same time, electrum is not present in pores and cracks of syneresis, which indicates that it had not been a soluble phase at the time of syneresis, rather colloidal particles. Electrum also forms dense sprinkles of globular morphology deposited in open space on the surfaces of some quartz-adularia bands, due to a condensation of gaseous phase, separated during the boiling of fluids. We found that the electrum-rich quartz-adularia micro-banding is enriched (in descending order) in Bi, Te, Cu, Fe, Pb, Au, As, U, Ba, Zn, Mg, Cr, Al, Tl, Na, K, and Th, and is slightly depleted in Si, Ti, Se, Ag, and Sb in comparison with the electrum-poor macro-bands. Ca displayed equal abundances in both macro- and micro-bands. The highest grades of electrum correspond to the highest abundances of Fe, As, Cu, Pb, Zn, Bi, and Te, which have deposited as sulfides and tellurides, thus revealing the voluminous electrum deposition in response to a significant decrease of sulfur and tellurium activities, and transport of gold and silver in the paleofluids in the form of sulfuric and telluric complexes. The epithermal Khan Krum deposit is Au dominant, Bi and Te rich, thus indicating its relationship to an igneous source. [ABSTRACT FROM AUTHOR]

Abstract: The Arzular mineralization is one of the best examples of epithermal gold deposits in the eastern Pontides orogenic belt. The mineralization is hosted by the subduction-related basaltic andesites and is mainly controlled by E–W and NE–SW trending fracture zones. The main ore minerals are galena, sphalerite, pyrite, chalcopyrite, tetrahedrite and gold. Homogenization temperatures of fluid inclusions are between 130 and 295 °C for quartz and between 90 and 133 °C for sphalerite. Sulphur isotope values obtained from pyrite, galena and sphalerite vary between −1.2‰ and 3‰, indicating that sulphur belongs to magmatic origin and was derived from the Lutetian non-adakitic granitic intrusions in the region. Oxygen isotope values are between 15.0‰ and 16.7‰, and hydrogen isotope values are between −87‰ and −91‰. The sulphur isotope thermometer yielded temperatures in the range of 244–291 °C for the ore formation. Our results support the hypothesis that the Arzular mineralization is a low-sulfidation epithermal gold deposit associated with non-adakitic subduction-related granitic magmas that were generated by slab window-related processes in a south-dipping subduction zone during the Lutetian. [Copyright &y& Elsevier]

Abstract: Detailed hydrothermal alteration investigations, including petrography, infrared reflectance spectroscopy (IRS) and XRD of the low sulfidation epithermal Co–O mine, located in Eastern Mindanao (Philippines) revealed that both distal and intermediate hydrothermal alteration zones contain dominantly illite and chlorite, whereas the proximal alteration zone comprises mainly illite, chalcopyrite and pyrite. The gold-bearing veins and the proximal hydrothermal alteration zone display a distinct absence of K-rich hydrothermal alteration minerals such as K-feldspar (adularia). Gold mineralization in the Co–O mine is controlled by an extensive quartz-breccia vein system, which is characterized by three distinct stages of vein (incl. breccias) formation. Gold is mainly observed in stages 2 and 3 veins. Stage 1 veins appear as fragments in stage 2 veins and display boiling textures such as quartz pseudomophs after bladed calcite. These veins further display colloform to crustiform banding and contain pyrite, chalcopyrite and minor gold located in the colloform bands and between bladed quartz pseudomorphs. Stage 2 veins comprise mostly banded to massive quartz and contains sulfides parallel to bands or disseminated. These veins are fine-grained with mosaic/jigsaw quartz and contain calcite blebs and/or fragments of stage 1 veins. Gold is in textural equilibrium with chalcopyrite, sphalerite, and locally pyrite. Stage 3 veins consist of quartz and carbonate (locally Mn-rich), and display irregular banded and comb textures. In auriferous veins of this stage gold is in textural equilibrium with chalcopyrite and pyrite (with local abundance of sphalerite). Other sulfide minerals observed with gold in stages 2 and 3 are galena, acanthite and locally jalpaite. The XRD and IRS provide inconsistent results regarding the abundance of K-rich clays (e.g., illite) associated with auriferous veins. Illite, with possibly interlayered swelling clays, such as Al-smectite, was identified in auriferous vein stages 2 and 3 using IRS, but could not be confirmed by XRD. Comparative analysis of the results of these techniques with respect to the ordering of micaceous minerals, suggest less ordered white mica proximal to the veins. Vein textures such as banded quartz, the absence of K-feldspar and the abundance of illite (interlayered Al-smectite) suggest relatively low temperatures of formation of the hydrothermal alteration system and point to a potential boiling horizon located deeper or marginal to the currently exploited levels of the Co–O mine. The absence of K-feldspar may also be related to relatively low temperatures of the hydrothermal fluid, the medium potassium-rich magma series of the host rocks, and/or a relatively low oxidation state of the hydrothermal fluid. [Copyright &y& Elsevier]

The breccia-hosted epithermal gold-silver deposit of Chah Zard is located within a high-K, calc-alkaline andesitic to rhyolitic volcanic complex in the central part of the Urumieh-Dokhtar Magmatic Arc (UDMA), west central Iran. The total measured resource for Chah Zard is ∼2.5 million tonnes of ore at 12.7 g/t Ag and 1.7 g/t Au (28.6 t Ag, 3.8 t Au), making it one of the largest epithermal gold deposits in Iran. Magmatic and hydrothermal activity was associated with local extensional tectonics in a strike-slip regime formed in transtensional structures of the Dehshir-Baft strike-slip fault system. The host rocks of the volcanic complex consist of Eocene sedimentary and volcanic rocks covered by Miocene sedimentary rocks. LA-ICP-MS U-Pb zircon geochronology yields a mean age of 6.2 ± 0.2 Ma for magmatic activity at Chah Zard. This age represents the maximum age of mineralization and may indicate a previously unrecognized mineralization event in the UDMA. Breccias and veins formed during and after the waning stages of explosive brecciation events due to shallow emplacement of rhyolite porphyry. Detailed systematic mapping leads to the recognition of three distinct breccia bodies: volcaniclastic breccia with a dominantly clastic matrix; gray polymict breccia with a greater proportion of hydrothermal cement; and mixed monomict to polymict breccia with clay matrix. The polymictic breccias generated bulk-mineable ore, whereas the volcaniclastic breccia is relatively impermeable and largely barren. Precious metals occur with sulfide and sulfosalt minerals as disseminations, as well as in the veins and breccia cements. There is a progression from pyrite-dominated (stage 1) to pyrite-base metal sulfide and sulfosalt-dominated (stages 2 and 3) to base metal sulfide-dominated (stage 4) breccias and veins. Hydrothermal alteration and deposition of gangue minerals progressed from illite-quartz to quartz-adularia, carbonate, and finally gypsum-dominated assemblages. Free gold occurs in stages 2 and 4, principally intergrown with pyrite, quartz, chalcopyrite, galena, sphalerite, and Ag-rich tennantite-tetrahedrite, and also as inclusions in pyrite. High Rb/Sr ratios in ore-grade zones are closely related to sericite and adularia alteration. Positive correlations of Au and Ag with Cu, As, Pb, Zn, Sb, and Cd in epithermal veins and breccias suggest that all these elements are related to the same mineralization event. [ABSTRACT FROM AUTHOR]

Abstract: Mindanao is the second largest island of the Philippines and is located in the southern part of the archipelago. It comprises the suture zone between the Eurasian and the Philippine plate, which is displayed in the Philippine Mobile Belt. Eastern Mindanao is part of the Philippine Mobile Belt and outcropping rocks are mainly Eocene to Pliocene in age related to episodes of arc volcanism alternating with sedimentation. New high-precision elemental analysis of the Oligocene magma series, hosting the Co-O epithermal Au deposit, which represents an arc segment in the central part of Eastern Mindanao, revealed dominantly calc-alkaline rocks ranging in composition between basalt and dacites. Major element trends (MgO vs. TiO2 and Fe2O3) are comparable to other magmas in Central and Eastern Mindanao as well as other SW Pacific Islands such as Borneo. Rare earth and trace element distribution patterns display typical island arc signatures highlighted by the conjunction of LILE-enrichment with troughs at Nb, Ta, and Ti. Ratios of Zr/Nb in basalts vary between 17 and 39, signifying a depleted subarc mantle wedge comparable to the range of MORB, and other Indonesian island arc basalts. In basalts, Nb/Ta and Zr/Sm ratios are 12–37 and 14–27 respectively indicative of deep melts of rutile-eclogite subducted slab, as well as fluids, infiltrating the mantle wedge source of basalts. Moderate large ion lithophile element contents and low Th/La and Th/Ce ratios suggest no significant slab-derived components such as sediment or crustal fragments. The comparatively low Ce and Yb values in basalts, but also andesites and dacites, are consistent with a thin arc crust related to an intraoceanic convergent margin setting. This is further supported by Nb contents in basalts that range between 1 and 3ppm and are within the range of modern oceanic convergent margin basalts. The range of HREE fractionation signifies that basaltic melts separated at deeper levels of the subarc wedge, possibly between the forearc and arc axis, followed by a calc-alkaline convergent margin magma suite involving shallower crustal AFC near the central arc sector. The analysed Oligocene arc segment is related to a potentially steep to intermediate dipping subduction zone in an extensional to neutral geotectonic regime. The large subduction accretion complex of the Philippine Mobile Belt provides an ideal setting for significant metal deposits during its entire evolution. This is evidenced in the Eastern Mindanao Ridge, which hosts substantial porphyry Cu and epithermal Au deposits. [Copyright &y& Elsevier]

Abstract: The Wadi Abu Khushayba area in southwestern Jordan hosts an auriferous, quartz-veined shear zone with features of an epithermal low-sulfidation gold system. The quartz-lode and quartz-carbonate mineralization is characterized by open-space textures, typically with sharp-walled veins and multi-stage brecciation of the late Neoproterozoic (~560Ma) rhyolitic and granophyric host rocks. Silicic alteration by cryptocrystalline quartz is abundant; potassic alteration is expressed by muscovite and K-feldspar blastesis, commonly with dispersed hematite laths. Sulfide content is low. Relict pyrite and chalcopyrite occur in comb and cockade quartz. Gold occurs disseminated in the volcanic and subvolcanic wall rocks and as late vein infill. The coexistence of vapor-rich and liquid-rich fluid inclusions in quartz, quartz textures, and the presence of adularia and calcite in the vein mineralogy suggest boiling concomitant with gold deposition in a structural setting favoring high fluid flow. Fluid inclusions in quartz define a temperature range of 350–380°C and low salinity (~1.5–7wt.% NaC1 equiv.) for the ore system. Pressure is estimated at 200–250bars, indicating a maximum paleo-depth of 2500m (hydrostatic). It appears likely that the Wadi Abu Khushayba system represents the near-surface epithermal expression of a larger unexposed mesothermal system. Such mesothermal systems are typical elsewhere in the Arabian–Nubian Shield, but are not exposed in southwestern Jordan where the present-day erosional level is close to the unconformity of the transgressive Early Cambrian platform sedimentation. [Copyright &y& Elsevier]

Abstract: Late Mesozoic volcanism is widespread throughout NE China. On the basis of lithological associations and spatial relationships, the volcanic rocks in the Lesser Hinggan Range can be divided into two formations, i.e., felsic-dominant Fuminghe Formation and overlying mafic-dominant Ganhe Formation. The Dong''an gold deposit, a typical adularia–sericite epithermal system, is spatially closely associated with rhyolitic porphyry, which is a subvolcanic intrusion of the Fuminghe Formation. Total measured, indicated, and inferred resources for the Dong''an deposit are 70tonnes (2.25Moz) of gold with the grade of 5.04g/t Au, making it one of the largest epithermal gold deposits in China. SHRIMP U–Pb zircon and 40Ar/39Ar geochronology applied to one rhyolitic porphyry sample and sericite separated from auriferous quartz veins of the main mineralization stage were carried out to constrain magmatic and hydrothermal events. The results suggest that the mineralization age of 107.2±0.6Ma overlaps with the age of the rhyolitic porphyry 108.1±2.4Ma. Our new age data indicate that there was a previously unrecognized mineralization event in NE China at 107–108Ma. Systematic geochemical investigations on the volcanic rocks in the Lesser Hinggan Range show that both Fuminghe and Ganhe Formations are characterized by significant large ion lithophile elements (LILE) and light rare earth elements (LREE) enrichment coupled with high field strength elements (HFSE) depletion, but they have distinct Sr and Nd isotopic compositions. The Fuminghe Formation has relative high 87Sr/86Sr ratios of 0.707253 to 0.707373, and negative ε Nd(t) values of −2.78 to −3.05 (t =108Ma), whereas the Ganhe Formation displays slightly lower 87Sr/86Sr range of 0.705434–0.705763 and positive ε Nd(t) values of +0.76 to +1.83. These geochemical data suggest that the rhyolitic magmas of the Fuminghe Formation probably represent the final differentiates of parental andesitic magmas, resulted from the partial melting of mafic lower crust, whereas the volcanic rocks of the Ganhe Formation were produced by fractionation of basaltic magmas generated from partial melting of a mixture of an incompatible element depleted anhydrous lherzolite asthenospheric mantle source and a hydrous enriched lithospheric mantle source in an extensional tectonic setting, in response to upwelling of asthenospheric mantle. The rhyolite porphyries of the Fuminghe Formation are inferred to have supplied heat that drove the convective hydrothermal system at Dong''an deposit, but also provided some of the fluid sources responsible for the development of the Dong''an epithermal system. [Copyright &y& Elsevier]

Abstract: The high-sulfidation system of Cerro Millo is hosted in a Late Miocene andesitic paleo-stratovolcano in the High Andes of southern Peru. Very pronounced advanced-argillic (alunite), silicic, and argillic (kaolinite and smectite) alteration characterizes the central part of the hydrothermal system. Propylitic alteration is developed in a 3 to 4 km wide outer halo. Abundant alunite occurs as hypogene, acicular crystals, and very fine-grained aggregates; the latter formed during near-surface steam-heated overprinting. Hypogene alunite has an Ar–Ar plateau age of 10.8±0.9 Ma (2σ), and is synchronous with the andesitic volcanism (Ar–Ar on biotite: 11.0±0.5 Ma). A second ill-defined alunite age plateau of 8.0±0.9 Ma is probably related to steam-heated overprint and points to major erosion in between both hydrothermal events. Telescoping is also evident by a series of silicified horizons which mark the paleo-groundwater table. These units have elevated mercury, antimony and arsenic levels. Late-phase barite occurs in some structurally controlled advanced-argillic altered envelopes. The hypogene alteration mineralogy points to temperatures at ≤250 °C. Hydraulic fracturing and steam-heated overprinting suggest a shallow boiling environment. Slight gold enrichment is observed in the lowermost exposed parts of the system. [Copyright &y& Elsevier]

The Sibutad gold deposit has gold associated in quartz veins. The most important of these is the Lalab orebody, which contains ore-grade gold, predominantly, in milky quartz veins and veinlets. Here, alteration quartz and fine-grained crystalline clear and milky quartz were formed from hydrothermal fluids in three stages, namely stages I, II and III. Fluid inclusion microthermometry was carried out on stage I milky quartz, stage II fine-grained alteration quartz and stage III milky quartz ± barite veins and veinlets. Homogenization temperatures (TH) are >248°C in stage I, 214–232°C in stage II and 186–239°C in stage III. These fluid inclusions have salinity between 1 and 2 wt% NaCl equivalent. In terms of gold assay, stage I drill-core samples have gold grades 0.53–0.76 g/ton Au, stage II samples have 1.12–3.70 g/ton Au and stage III samples have 9.06–23.88 g/ton Au. This correlation suggests that gold was precipitated from the stage II and III fluids. [ABSTRACT FROM AUTHOR]

There is a great diversity of gold-bearing deposits and mineral manifestations in Cuba, but, there is not yet, among the other aspects of interest, a classification that takes into consideration the kind of deposits and its different styles, the specific feature of the geological areas, the particular characteristics of mineralization, the types of accompanying attention and the geoquemical and mineralogical indicators. In this paper it is proposed a classification for Cuban gold-bearing deposits that takes as reference the classification made by Corbett and Leach (1995) in which criteria of many authors have been integrated. Special attention is devoted to districts, poorly studied and characterized by a particular gold-bearing mineralization, like Santa Clara, Holguín and Sagua-Baracoa districs. They have been developed fundamentally in geological environment characterized by the presence of complexes of ofiolítico association. [ABSTRACT FROM AUTHOR]