Alteration Mineral Assemblages Research Articles

Modelling wavelength position of absorption feature using covariance weighted least square inversion

Reflectance spectroscopy is a rapid and non-destructive technique for probing the chemical composition of the earth surface. The minerals are characterised by absorption features whose wavelength is function of its crystal chemistry. In the present study, we propose a covariance weighted least square inversion approach to model the wavelength position of the absorption features. The approach takes into consideration the noise in the reflectance spectra and estimates the wavelength position with root mean square value of less than 5 nm for library spectra. The algorithm is then implemented on AVIRIS-NG datasets of Hutti-Maski schist belt to map the spatial variations in wavelength position of absorption features in wavelength range 2290–2360 nm. Sensitivity of model parameters to number of points, effect of number of data points on striping noise and limitations of the proposed algorithm are also discussed. It is observed that the algorithm based wavelength map is smooth and enables identification of gradients in absorption wavelength position. Mapping of such gradients are very important for understanding subtle changes in crystal chemistry due to hydrothermal alteration and possible mineralization. The generated wavelength map can be used in conjunction with alteration mineral assemblage to identify prospective zones for mineral exploration.

Rare Earth Element Mobility and Sr Isotope Systematics of Altered Volcanic Rocks from Seawater-Based and Magmatically Influenced Hydrothermal Systems: Brothers Volcano, Kermadec Arc (IODP Expedition 376)

AbstractIn 2018, International Ocean Discovery Program (IODP) Expedition 376 successfully drilled into seawater-based and magmatically influenced hydrothermal systems within and on the wall of the caldera of Brothers volcano, an active submarine volcano in the Kermadec arc. We report rare earth element (REE) abundances and ratios for altered volcanic material recovered from three holes to assess mobility and partitioning under the different conditions of fluid-rock reactions. We combine these with loss on ignition (LOI) and X-ray diffraction (XRD) data to document the extent of alteration, and with Sr concentration and isotope data to evaluate water/rock (W/RSr) ratios and provide constraints on hydrothermal exchange and conditions of alteration. Rocks recovered from the seawater-based NW Caldera hydrothermal system show variable extents of alteration (LOI values of 1–11 wt %) at low W/RSr ratios (1–14) to chlorite- and quartz-rich alteration mineral assemblages. We attribute the limited changes observed in total REE content (ΣREE) and chondrite-normalized REE patterns to a combination of their incorporation into secondary minerals such as chlorite and smectite and complexation with chloride ligands in the reacting fluids. Rocks recovered from the magmatically influenced Upper Cone hydrothermal field (and from an older interval deep beneath the NW Caldera wall) are more extensively altered (LOI values of 4 to >22 wt %), although at similar low W/RSr ratios (~1–16) to heterogeneous natroalunite- and pyrophyllite-bearing assemblages. The REEs exhibit considerably greater mobility with lower ΣREE contents, particularly in the middle and heavy REEs (LaN/YbN ratios up to 4). We suggest that accommodation in secondary minerals is unlikely to play a role in REE behavior due to their lack of substitution sites for the REEs. We attribute enhanced mobility of all the REEs to increased solubility due to the very low pH, and interpret fractionation of the middle and heavy REEs as likely due to complexation with fluoride, which enhances their solubility relative to the light REEs.

Fluid-rock interactions at shallow depths in subduction zone: Insights from trace elements and B isotopic composition of metabasites from the Mariana forearc

To reveal the spatial variations of slab-derived fluids and to trace the in-situ dehydration in the shallow subduction zone, we investigated the petrography, mineral chemistry, and whole-rock B isotopes of metabasites that were recovered from the Fantangisña and Asùt Tesoru serpentinite mud volcanoes and originated from the shallow subduction channel at the forearc of the Mariana subduction zone. The alteration mineral assemblages in the investigated metabasites suggest zeolite- to prehnite-pumpellyite-facies metamorphism and lawsonite-blueschist facies metamorphism beneath Fantangisña and Asùt Tesoru seamounts, respectively. The fluid mobile elements (e.g., B, As, Sb, Pb) are preserved in the low grade metamorphic phyllosilicate minerals (e.g., glauconite, pumpellyite, celadonite), thus fixing the B concentrations of the subducted oceanic crust during shallow subduction (<18 km). Both B concentrations (16.7 to 43.9 μg/g) and δ 11 B values (−5.0 to +3.2‰) of the investigated metabasites are significantly higher than fresh normal mid-ocean ridge basalts (N-MORB) and ocean island basalts (OIB), and are generally comparable to the uppermost altered oceanic slab. Notably, the recovered metabasites from the Mariana forearc exhibit a decreasing trend in δ 11 B values with increasing distance to the trench from Fantangisña (62 km) through Asùt Tesoru (72 km) to South Chamorro (78 km) Seamounts. This trend together with Rayleigh dehydration modeling indicate that the 11 B-enriched aqueous fluids were released from the subducting slab during prograde metamorphic dehydration. The estimated B isotopic compositions of the slab-derived fluids released at arc magma genesis depths are generally comparable to that of the Mariana arc lavas. However, the fluids released by dehydration of subducted sediments at shallow depths should be characterized by lower δ 11 B values than fluids released from the slab at depths of magma genesis beneath the island arc. Then, the recycling of the hydrated forearc mantle is necessary to explain the high δ 11 B values of the Mariana arc lavas. While, the variable B isotope compositions of Mariana arc lavas should be controlled by the different ratios of sediment/AOC ratios. • The investigated metabasites experienced low-grade metamorphism. • Both B concentrations and δ 11 B values are significantly higher than their protolith. • δ 11 B values exhibit a decreasing trend with increasing distance to the trench axis. • Subducted serpentine is necessary to explain the high δ 11 B values of the arc lavas.

Advances on Exploration Indicators of Mineral VNIR-SWIR Spectroscopy and Chemistry: A Review

Establishing exploration vectors to infer the properties of ore-forming fluids, locate blind ore bodies with the aid of visible to near-infrared (VNIR) and short-wave infrared (SWIR) spectroscopy, and infer the chemistry of minerals, is a new research interest for economic geology. Common alterations and clay minerals, including sericite, chlorite, epidote, alunite, kaolinite, tourmaline, etc., are ideal objects for the study of exploration indicators due to their sensitivity to variations in the nature of hydrothermal fluid. The diagnostic spectral feature and chemistry vary spatially and systematically with physicochemical change. VNIR spectroscopy can characterize the REE-bearing clay minerals directly. Obtaining spectral or chemical parameters with the aid of VNIR-SWIR spectroscopy, electron probe micro-analyzer (EPMA) or laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) can help to establish exploration vectors. This paper systematically summarizes recent advances in mineral exploration indicators (MEIs) of VNIR-SWIR spectroscopy and chemistry, and compares them in different regions or deposits. We found that some MEI spatial variation trends are random, even the same type of deposit can show an opposite trend. The controlling factors that limit the application of the established MEIs are vague. Conducting further research on petrology and mineralogy with the aids of observation under microscopy, X-ray diffraction (XRD), TESCAN Integrated Mineral Analyzer (TIMA), and EPMA are suggested to discover alteration mineral assemblage, alteration stages, and behaviors of “the pathfinder elements” related to mineralization. Based on the above research, the physicochemical properties of ore-forming fluids and their control over MEIs can be inferred. Refining the theoretical basis is critical to understanding and popularization of spectral and chemical MEIs.

Hydrothermal Alteration Within the Brothers Submarine Arc Volcano, Kermadec Arc, New Zealand

Abstract The hydrothermally active Brothers volcano on the Kermadec arc, New Zealand, hosts two geochemically distinct hydrothermal systems within a single caldera. At the NW Caldera, metal-sulfide–rich black smoker spires form on the caldera wall. In contrast, Fe-rich crusts and native sulfur-rich chimneys occur at the resurgent central Upper Cone. Previous studies have revealed that the contrasting styles of hydrothermalism relate to the variable contribution of magmatic volatiles between these sites, with the Upper Cone experiencing relatively higher amounts of magmatic volatile influx. We present results of a study of the hydrothermal alteration within Brothers volcano based on core samples to a depth of 453 meters below sea floor (mbsf) from both the Upper Cone (Site U5128) and NW Caldera sites (Site U1527 and U1530), drilled by the International Ocean Discovery Program. The dacitic to rhyolitic breccias that make up the volcano are variably altered to alteration mineral assemblages consisting of chlorite + quartz, illite + pyrophyllite, natroalunite + pyrophyllite, and smectite-rich assemblages. The distribution and textures of the alteration minerals within and between different sites at Brothers volcano reflect variations in temperature, fluid pH, and fluid flux. We find that natroalunite only occurs at the Upper Cone, while alteration at the NW Caldera is more diverse and is characterized by both chlorite and pyrophyllite-rich alteration, indicating that seawater-derived hydrothermal fluids overprinted earlier magmatic volatile-influenced alteration. Our data indicate that in magmatic volatile-dominated systems, the alteration mineralogy transitions from natroalunite to pyrophyllite-rich with increasing age or maturity. This is accompanied by a distinct change in sample texture from dominantly bleached selvages to a more massive, equigranular texture.

Alteration and mineralization patterns in orogenic gold deposits: Constraints from deposit observation and thermodynamic modeling

Hydrothermal alteration and mineralization types of orogenic gold deposits display many variations depending on formation depths, types of host rock, compositions of ore fluids, spatial positions, fluid/rock ratios, and repetitive modifications during multiple fluid infiltrations in the complicated processes of fluid-rock interaction. Therefore, establishing holistic reference patterns of hydrothermal alteration with associated mineralization for the orogenic gold system is necessary. Based on the observation of typical orogenic gold deposits and thermodynamic simulation of fluid-rock reaction, the general features of proposed patterns include:(1)Orogenic ore fluids are capable of transporting sufficient gold from deep (5 kbar and 600°C) to shallow (1 kbar and 200°C) crustal levels, resulting in alteration and mineralization occurred in a much-varied depth. Only considering the effects of fluid-rock reaction and sulfidation, the formation of pyrrhotite and pyrite at relatively high and low P-T conditions, respectively, is the most important mechanism driving gold precipitation. In addition, the changes of fluid fO2 and pH can also trigger a decrease in the gold solubility in fluids.(2)Multiple waves of fluid infiltration are capable of dissolving the early-formedulfides and gold, and reprecipitate them in the distal alteration zones, which can explain the irregular gold concentration distribution patterns in wide alteration zones. The fluids under higher P-T conditions (5 kbar and 600°C and 3 kbar and 400°C) are more capable of dissolving early-formed sulfides and gold. Additionally, the later infiltrated fluids can transform early-formed pyrrhotite into pyrite.(3)The model of interactions between ore fluids and aluminosilicate rocks at varied P-T conditions indicates that the alteration mineral assemblages in hypozonal deposits (>12 km) mainly include biotite, amphibole, anorthite, K-feldspar, quartz, and pyrrhotite assemblages; that those in mesozonal deposits (6 to 12 km) are characterized by chlorite, calcite, quartz, muscovite, pyrite or pyrrhotite. The epizonal deposits (<6 km) commonly develop more abundant Fe-Mg bearing carbonates (e.g., dolomite, ankerite, and siderite), quartz, muscovite, and pyrite. The compositional variations of ore fluids and host rocks have important controls on the development of iron-oxides and carbonaceous material.(4) The alteration halo is relatively restricted under hypozonal to lower mesozonal conditions. However, broad carbonation develops under upper mesozonal to epizonal conditions, with Fe-Mg bearing carbonates and calcite distributed in proximal and distal zones, respectively.

Identifying and Mapping Alteration Minerals Using HySpex Airborne Hyperspectral Data and Random Forest Algorithm

Airborne hyperspectral remote sensing data provide rapid, non-destructive, and near laboratory quality reflectance spectra for mineral mapping and lithological discrimination, thereby ushering an innovative era of remote sensing. In this study, NEO HySpex cameras, which comprise 504 spectral channels in the spectral ranges of 0.4–1.0 μm and 1.0–2.5 μm, were mounted on a delta wing XT-912 aircraft. The designed flexibility and modular nature of the HySpex aircraft hyperspectral imaging system made it relatively easy to test, transport, install, and remove the system multiple times before the acquisition flights. According to the design fight plan, including the route distance, length, height, and flight speed, we acquired high spectral and spatial resolutions airborne hyperspectral images of Yudai porphyry Cu (Au, Mo) mineralization in Kalatag District, Eastern Tianshan terrane, Northwest China. By comparing the features of the HySpex hyperspectral data and standard spectra data from the United States Geological Survey database, endmember pixels of spectral signatures for most alteration mineral assemblages (goethite, hematite, jarosite, kaolinite, calcite, epidote, and chlorite) were extracted. After a HySpex data processing workflow, the distribution of alteration mineral assemblages (iron oxide/hydroxide, clay, and propylitic alterations) was mapped using the random forest (RF) algorithm. The experiments demonstrated that the workflow for processing data and RF algorithm is feasible and active, and show a good performance in classification accuracy. The overall classification accuracy and Kappa classification of alteration mineral identification were 73.08 and 65.73%, respectively. The main alteration mineral assemblages were primarily distributed around pits and grooves, consistent with field-measured data. Our results confirm that HySpex airborne hyperspectral data have potential application in basic geology survey and mineral exploration, which provide a viable alternative for mineral mapping and identifying lithological units at a high spatial resolution for large areas and inaccessible terrains.

A Review of Research on Grove Mountains CM-Type Chondrites

CM chondrite is the most important carbonaceous chondrite containing abundant Ca, Al-rich inclusions (CAIs) and other interesting objects, which probably experienced early condensation processes in the Solar Nebula environment and later alteration in parent body surroundings. Thus, it is a vital raw material to explore in the formation and evolution of the early Solar System. Grove Mountains (GRV) CM chondrites have been collected from Antarctica by Chinese Antarctic Research Expedition (CARE) for nearly 20 years. In this paper, we review the study of GRV CM chondrites. In total, there are eight CM chondrites named Grove Mountains officially approved by the Meteoritical Society. Petrology and mineral, matrix, CAIs, metal and sulfide in GRV CM chondrites are carefully reviewed. All the meteorites have similar characteristics with a dominant component of matrix. Phyllosilicate minerals generally developed in the matrix. The different altered mineral assemblages, contents and chemical compositions show that these chondrites underwent varying degrees of aqueous alteration, of which GRV 020005 is the most heavily altered CM chondrite. GRV 020025 is the second heaviest of the CM samples with the most extensive studies among these chondrites. It contains abundant CAIs and amoeboid olivine aggregates (AOAs). The modal content is about 1.0 vol% for CAIs. The findings of some new types of CAIs (such as hibonite-rich and spinel-pyroxene inclusions with forsterite-rich accretionary rims), AOAs and a complex, fine-grained P-bearing sulfide phase enrich the study of GRV 020025.

Genesis of orogenic gold systems in the Daduhe belt: Evidence of long-lived fertile mantle lithosphere as a source of diverse metallogeny on the western margin of the Yangtze Craton, China

The poorly documented Daduhe gold belt on the western margin of the Yangtze Craton, an important metallogenic belt in southwest China, is characterized by typical orogenic gold systems against a highly anomalous geodynamic setting. A synthesis of regional and deposit geology, mineralization ages, and S-O isotopic compositions of 33 gold deposits from the Danba-Kangding-Shimian-Mianning area of the belt reveals regional metallogenic patterns, establishes a consistent genetic model, and demonstrates a source equivalent to other diverse mineral systems on the western margin of the Yangtze Craton. The Daduhe orogenic gold deposits are controlled by the lithosphere-scale Xianshuihe and Anninghe strike-slip faults in the northern and southern segments of the belt, respectively. The alteration and ore mineral assemblages of the Daduhe orogenic gold deposits define a wide range of hypozonal, mesozonal, and epizonal sub-types. They formed during two episodes: 1) Early Jurassic gold mineralization related to closure of the Paleo-Tethys Ocean, with a large hypozonal gold deposit on the margin of the Danba dome in the northern segment; 2) Cenozoic gold mineralization associated with continental collision on the Tibetan Plateau, with small mesozonal and epizonal gold deposits in shear zones in migmatite basement and at contacts with cover sequences in both the northern and southern segments. At the belt and deposit scales, the ore-controlling structures for both mineralization episodes are NNE-trending second-order extensional faults or their intersections with the dominant NW-trending compressional faults. The three deposit sub-types have overlapping δ34S of 0 ∼ 10‰, δ18Ofluid of 6 ∼ 11‰, and low He-Ar isotope ratios, all consistent with derivation of ore fluids from metasomatized mantle lithosphere. Importantly, the Daduhe orogenic gold deposits have similar S-O-He-Ar isotope compositions to the giant IOCG, Cu-Ni-PGE, Fe-V-Ti, anomalous porphyry Cu-Mo-Au, and carbonatite-related REE deposits in the Panxi region on the shared margin of the western Yangtze Craton. This suggests that these diverse mineral systems have a common source of ore fluids and metals derived from fertile mantle lithosphere during different geodynamic and metallogenic events. Such a widespread hydrous metal-rich deep source with lithosphere-scale faults as ore fluid pathways, and a depth continuum of orogenic gold deposits in the Daduhe belt suggest a high prospectivity and a higher future gold endowment for the belt. This study adds to an increasing awareness of the diversity of mineral systems on specific craton margins globally.

Characteristics and evolution of the Etili silica sinter epithermal deposits, Çanakkale – Turkey: Relation to alkali chloride vs acid-sulfate fluids

The Oligo-Miocene Etili epithermal deposits are a well-preserved fossil geothermal system in the Çanakkale Region and is one of the largest fossil siliceous hot spring deposits of Turkey. Main E-W and NE-SW trending faults systems occur with minor NW-SE fracture systems perpendicular to the main faults. Silica-rich deposits are commonly observed in or on the calc-alkaline tuffs, ashes and pyroclastic rocks that were the products of the magmatism controlled by the extensional tectonic regime.The Etili epithermal system was examined at two representative locations including the Hamamtepe and Muratlar sites. Morphologies of the proximal apron were defined by lithofacies that included silica infiltrate, spring conduits, nodular and finely laminated geyserite, sinter clast breccia, silicified volcanic rocks, and epithermal veins. Microfossils were detected in mat structures developed in the proximal apron. The silica deposits have low abundances of altered mineral assemblages (e.g., kaolinite, halloysite, and alunite), which are otherwise commonly observed in the region. The origin of the Etili Fossil Silica Sinter Region (EFSSR) was constrained by using geochemical and isotopic data. δ18O and δD isotopic values of kaolinites ranged +9.4/+9.6 ‰; to 84/−74 ‰ respectively. δ18O isotopic values from siliceous and silicified samples ranged from 8.3 to 18.4 ‰, which shows two different formation types for the Etili Fossil Silica Sinter Region. We hypothesized hypogene origins for the kaolins and a mixed hypo/supergene origin for siliceous and silicified samples. The calculated model formation temperature from the δ18O values of the silica and siliceous samples is in the range of 63° to 140 °C. δ34S isotopic values of alunite ranged from −19.6 to +16.6 ‰, which also indicated two different sources that include magmatic hydrothermal and meteoric waters affected by bacterial activity. The timing of acid-sulphate alteration can be grouped into three periods by 40Ar/39Ar dating of alunites. These ages are: (A) 32.4 ± 1.2 to 22.6 ± 0.22 Ma in the Muratlar silica site, (B) 12.3 ± 0.3 to 15.2 ± 0.3 Ma in the northern part of the Hamamtepe site and (C) 5 ± 0.18 to 7 ± 0.3 Ma in the southern part of the Hamamtepe silica site. These consistent findings show that the emplacement periods of the Evciler pluton and the ages of alteration deposits are similar to each other. Silica sinters and geyser mounds represent very late stage near-neutral pH alkali chloride geothermal activities. The epithermal deposits in the EFSSR comprises a complex history of three different hydrothermal processes, each of which have different compositional and temporal emplacement periods related to the Evciler pluton.

Hydrothermal Alteration and Its Superimposed Enrichment for Qianjiadian Tabular-Type Uranium Deposit in Southwestern Songliao Basin

The evolution characteristics of hydrothermal activity and superimposed uranium mineralization in the Qianjiadian ore field in southwestern Songliao Basin are still controversial and lack direct evidence. In this comprehensive study, a detailed identification of dolerite and hydrothermally altered un-mineralized sandstone and sandstone-hosted ore in the Yaojia Formation have been performed through the use of scanning electron microscopy observation, electron probe, carbon-oxygen-sulfur isotope, and fluid inclusion analyses. The results show that the hydrothermal fluid derived from the intermediate-basic magma intrusion is a low-temperature reducing alkaline fluid and rich in CO2, Si, Zr, Ti, Fe, Mg, Mn, and Ca, producing different types of altered mineral assemblages in the rocks, including carbonation, pyritization, sphalerite mineralization, clausthalite mineralization, silicification, and biotitization. Specifically, the carbonate minerals in sandstone are mixed products of deep hydrothermal fluid and meteoric water, with carbon and oxygen isotopes ranging from −5.2‰ to −1.7‰ and −20.4‰ to −11.1‰, respectively. Carbon source of the carbonate minerals in dolerite is mainly inorganic carbon produced at the late stage of intermediate-basic magma evolution, with carbon and oxygen isotopes from −16.1‰ to −7.2‰ and −18.2‰ to −14.5‰, respectively. Various carbonate minerals in the rocks may have been precipitated by the hydrothermal fluid after the magmatic stage, due to the change of its CO2 fugacity, temperature, and cation concentration during the long-term evolution stage. A series of carbonate minerals were generated as calcite, dolomite, ankerite, ferromanganese dolomite, and dawsonite. The precipitation processes and different types of carbonate mineral mixtures identified in this study mainly occur as parallel, gradual transition, interlacing, or inclusion metasomatism in the same vein body, without obvious mineralogical and petrologic characteristics of penetrating relationship. Homogenization temperature of fluid inclusions in calcite is high, in the range of 203–234 °C, with a low salinity of 0.71–4.34% NaCl, and the data range is relatively concentrated. Homogenization temperature of fluid inclusions in ankerite is usually low, ranging from 100 °C to 232 °C, with a high salinity of 4.18–9.98% NaCl. The precipitation processes of carbonate minerals and the results of this study are basically in consistent. Overall, the sandstone-type uranium deposits have a temporal and genetic relationship with hydrothermal activities during Paleogene. (1) Hydrothermal activity was directly involved in uranium mineralization, result in dissolution and reprecipitation of earlier uranium minerals, forming uranium-bearing ankerite and complexes containing uranium, zirconium, silicon, and titanium. (2) Hydrothermal fluid activity provided reducing agent to promote hydrocarbon generation from pyrolysis of carbonaceous fragments and accelerate uranium precipitation rate. (3) Regional water stagnation prolongs reaction time, contributing to huge uranium enrichment. This study provides new petrologic, mineralogical, and geochemical evidence for multi-fluid coupled and superimposed mineralization of sandstone-hosted uranium deposits in the sedimentary basin.

Active Basalt Alteration at Supercritical Conditions in a Seawater‐Recharged Hydrothermal System: IDDP‐2 Drill Hole, Reykjanes, Iceland

AbstractThe 4.5 km deep IDDP‐2 drill hole was drilled at Reykjanes, Iceland, in an active seawater‐recharged hydrothermal system on the landward extension the Mid‐Atlantic Ridge. Drilling targeted a well‐defined hydrothermal up‐flow zone feeding the Reykjanes geothermal reservoir, which produces fluids compositionally equivalent to basalt‐hosted deep sea hydrothermal vents. Spot cores recovered from depths between 3,648 to 4,659 m depths consist of pervasively altered sheeted diabase dikes. The shallowest core has an alteration mineral assemblage similar to the producing geothermal reservoir but is being overprinted by the underlying Hornblende Zone assemblage dominated by labradorite and hornblende. The deepest cored section (4,634–4,659 m) retains a diabasic texture, but all primary minerals are replaced or have changed composition. Plagioclase ranges from An30 to An99 and igneous augite is replaced by intergrown hornblende, clinopyroxene, and orthopyroxene. Hydrothermal biotite and potassium feldspar formed locally due to reaction with a phase‐separated brine, indicated by co‐existing vapor‐rich and salt + vapor‐rich fluid inclusions. Seawater‐derived supercritical hydrothermal fluid entering the bottom of the bore hole actively phase separates due pressure drop controlled by the fluid levels in the drill hole. Felsic veins, present in trace amounts, record a continuous transition from magmatic to hydrothermal conditions, including incipient hydrous melting. The vertical changes observed in mineralogy and mineral chemistry indicate that fluids from the deep high temperature reaction zone can undergo significant cooling and reaction with host rocks along their path to the seafloor.

The role of magmatic fluids in the ~3.48 Ga Dresser Caldera, Pilbara Craton: New insights from the geochemical investigation of hydrothermal alteration

Hydrothermal fluids played a key role in establishing the environmental conditions in which ancient stromatolites grew within the North Pole Chert of the ~3.48 Ga Dresser Formation (Pilbara Craton, Western Australia). However, there has been uncertainty as to the physicochemical conditions of the hydrothermal system in relation to (i) the distribution of hydrothermal alteration, (ii) the relative contribution of seawater and/or magmatic volatiles to the hydrothermal fluids, and (iii) the origin of some of the major elements mobilized in the hydrothermal fluids.This study examines the hydrothermal alteration of the underlying North Star Basalt in order to better understand the nature of the circulating fluids and to determine the processes responsible for the transport and accumulation of metals and metalloids to the near surface environment. Detailed geological mapping reveals a complex distribution of alteration mineral assemblages that is controlled at all stratigraphic depths by the distance to the major fluid pathways that are now represented by hydrothermal silica veins. With increasing distance from the vein margins, alteration assemblages change from argillic (kaolinite–quartz) to phyllic (illite–quartz), and then to propylitic (chlorite–albite–epidote) and actinolitic (actinolite–albite–chlorite–epidote) at more distal positions. Illite Ar–Ar dating of argillic–altered basalt proximal to major hydrothermal veins immediately below the North Pole Chert confirms a syn–depositional hydrothermal origin of alteration, and demonstrates that the mineralogical and chemical features developed through the circulation of hydrothermal fluids were largely preserved after subsequent thermal overprints at 3.25, 3.06, and 2.29 Ga.The spatial distribution of the alteration mineral assemblages indicates that the fluids circulating in the hydrothermal system were highly acidic (pH < 3) for at least some time during the evolution of the Dresser Caldera. Such highly acidic fluid conditions were likely promoted by the input of magmatic volatile phases, such as HCl, SO2, H2S, and F. Bulk geochemical analyses of altered basalts reveal that large amount of metals, including Fe, Mg, Ni, and Zn, were leached from the North Star Basalt during hydrothermal alteration and delivered to the surface. Furthermore, our data indicate that K and Ba were introduced into the hydrothermal system from external reservoir(s). Although the contribution of K–rich seawater cannot be completely discounted, we argue that the bulk of K and Ba was sourced from an underlying magma chamber undergoing fractional crystallization of a melt with TTG–like composition.

Geology, mineralogy, and sulfur isotopes of the Mowana copper deposit, Matsitama Schist Belt, NE Botswana

AbstractThe Mowana hydrothermal Cu deposit is located within the Matsitama–Motloutse Complex in the southwestern part of the Zimbabwe Craton in the northeastern part of Botswana. This study aims to document the characteristics of the mineralization based on geology, quartz textures, ore mineralogy, chlorite geothermometry, and sulfur isotope analyses. The deposit is hosted by the NNE‐striking and nearly vertically dipping (70–80°) Bushman Lineament, within the graphitic schist lenses in the carbonaceous and argillaceous metasedimentary rocks of the Neoarchean to Paleoproterozoic Matsitama Sedimentary Group. The hydrothermal alteration of the host rocks is characterized by silicification, chloritization, epidotization, sericitization, hematite, and calcite alteration. Based on the alteration mineral assemblage, the main mineralization stage is attributed to near neutral pH fluids at temperatures between ~200 and ~340°C. The base metal mineralization of the Mowana deposit was evolved in at least two vein types. The first mineralization type, represented by the quartz+calcite±K‐feldspar veins and breccias is characterized by the precipitation of principal chalcopyrite with pyrite, minor bornite, and trace amounts of galena. The Type 2 veins represented by the quartz+calcite±fluorite veins, host appreciable amounts of galena. The supergene mineralization widely distributed in the shallow levels of the deposit is manifested by the significant presence of chalcocite, bornite, covellite, anglesite, malachite, and hematite. The temperature obtained from the chlorite geothermometry in the Type 1 veins indicate that the mineralization associated with chlorite alteration formed at a temperature ranging from 340 to 400°C. The ore mineral assemblage: pyrite, bornite, and chalcopyrite, paired with the chlorite geothermometry data indicate that the Type 1 veins formed at an intemediate to high sulfidation state. Sulfur isotopic ratios determined on the sulfides indicate the magmatic S and/or leaching of the host metasedimentary rocks and closed system reduction of seawater sulfate as the sources of S.

Origin of the Kyaukmyet Low-Sulfidation Epithermal Gold Prospect, Monywa District, Central Myanmar

The Kyaukmyet prospect is one of the principal epithermal gold prospects in the Monywa District, Central Myanmar; its gold- and base metal-bearing quartz veins contain around 3 g/t gold. Ore minerals are mainly hosted by volcanic and volcaniclastic rocks of the Late Oligocene to Middle Miocene Magyigon Formation. The distribution of magmatic intrusions in the area is controlled by ENE-WSW trending faults; these faults are likely related to ore mineralization. Common ore minerals at the Kyaukmyet prospect include pyrite, sphalerite, galena, chalcopyrite, and electrum. They occur in mineralized crustiform-textured brecciated quartz veins and banded (colloform) and massive quartz veins. Mineralized rock is accompanied by silicification and propylitic and argillic alterations. The alteration mineral assemblages include quartz, adularia, calcite, chlorite, illite/smectite, sericite, and illite. Fluid inclusions in the quartz veins have homogenization temperatures ranging from 148 °C to 304 °C and salinities from 0.35 wt % to 2.75 wt % NaCl equiv. The quartz in the mineralized quartz veins was most likely precipitated at a depth ranges165-256 m below the paleosurface. The precipitation of gold at the Kyaukmyet prospect may have been formed by mixing large amounts of meteoric fluid with small amounts of magmatic fluid. The coexistence of liquid-rich and vapor-rich inclusions and presence of adularia and bladed calcite indicate that fluid boiling is caused the main mechanism of ore formation. The vein textures, ore mineral assemblages, alteration minerals and fluid inclusion data suggest that the Kyaukmyet prospect is a polymetallic low-sulfidation epithermal gold deposit.

Mineralogy of the Svetloye epithermal district, Okhotsk-Chukotka volcanic belt, and its insights for exploration

The Svetloye epithermal district (SED) is located within the Okhotsk-Chukotka volcanic belt (OCVB), 220 km southwest of Okhotsk. The OCVB, which formed during the Cretaceous with ore potential, is a large marginal volcanic rock belt associated with continental subduction. Svetloye occupies two volcanic edifices of different ages – the northwestern volcanic center (including the Emmy deposit) consists of basaltic-andesite of the Khetanian suite (Coniacian-Santonian time (K2), and the southeastern volcanic center (hosts the Elena, Tamara, Lyudmila, and Larisa deposits) comprises the dacite-rhyolite-leucogranite formation of the Urak suite (Campanian-Maastrichtian time (K2). Distinct host rocks and different levels of erosion in the two edifices of Svetloye led to various altered rocks, mineral assemblages, and diverse types of gold mineralization. The alteration halo of the Emmy deposit contains vuggy residual quartz, with Au-Ag-telluride and Au-Ag mineralization plus other tellurium-bearing minerals (goldfieldite, kawazulite, coloradoite, melonite, altaite, tellurobismuthite, tellurantimony, and native tellurium). Altered rocks of the other deposits are characterized by variable halos, including vuggy residual quartz, alunite, dickite, quartz-alunite-dickite, illite-chlorite, illite, illite–smectite, smectite, quartz-carbonate, and quartz-white mica alteration, hypogene and supergene processes. The hydrothermal (hypogene) and supergene ore processes within the SED occurred in 3 stages: leaching and barren quartz-rutile-pyrite zone with halos of quartz-alunite-dickite-pyrite (or quartz-illite-chlorite-calcite-pyrite), then quartz-pyrite with polysulfides and gold, gold-silver telluride, and finally supergene with 'mustard' gold. The alteration halo, epithermal ore bodies as hydrothermal veins and breccias with vug infill, colloform, and crustiform textures, hypogene sulfide assemblage represented by galena, sphalerite, tennantite-tetraedrite, and chalcopyrite of SED are typical of epithermal styles with an high-intermediate sulfidation signature, possibly indicates a porphyry deposit at depth. The lack of high-sulfidation-state sulfides such as hypogene covellite, enargite, famatinite, luzonite in the residual quartz may be due to their erosion at shallower paleodepth, and/or to their supergene oxidation below the present day surface.

Hydrothermal alteration and sealing at Turrialba volcano, Costa Rica, as a mechanism for phreatic eruption triggering

Turrialba is a basaltic to andesitic Holocene stratovolcano that after decades of quiescence re-activated in 1996 and has been highly active ever since. Turrialba is characterized by a highly active magmatic-hydrothermal system, and we propose that hydrothermal sealing and volatile accumulation are the mechanisms responsible for the reactivation and persistent phreatic activity at Turrialba since 2010. Evidence of sealing is found in pyroclastic breccias from phreatic eruptions as high concentrations of hydrothermal minerals coupled with low intrinsic permeability. The suite of volcanic breccias studied herein erupted from the main vent between 2014 and 2019 and has an alteration mineral assemblage of SiO2 polymorphs ± gypsum ± natroalunite ± pyrite. The mineral assemblage is indicative of acid sulphate alteration within the advanced-argillic alteration facies characterized by temperatures of approximately 200–350 °C as indicated by the presence of gypsum and natroalunite, the high temperature endmember of the alunite series. Acid sulphate alteration is the result of extreme base leaching by acidic fluids (pH <4) with a high sulphate content. Measurements of permeability and porosity yielded variable porosity and very low to non-existent permeability in all hydrothermal breccia samples. Back-scatter electron (BSE) images reveal nano-, micro- and macro-scale fracture networks discontinuously filled with hydrothermal gypsum and pyrite which are responsible for diminished permeability, supporting the conclusion that hydrothermal sealing is active at Turrialba. Diminished permeability associated with the formation of a seal inhibits the escape of gases, causing them to accumulate below the seal and pressurize the system. Eventual seal failure due to overpressure results in the frequent phreatic eruptions seen at Turrialba.

Fusion of ASTER satellite imagery, geochemical and geology data for gold prospecting in the Astaneh granite intrusive, West Central Iran

ABSTRACT In this study, ASTER imagery, geochemical, lithological, and structural data are exploited for Mineral Potential Mapping (MPM) of the Astaneh granitic pluton and its surrounding area. The independent component analysis (ICA) and Matched Filtering (MF) techniques are applied to ASTER data for detecting alteration mineral assemblages. Sericitically argillically altered minerals associated with jarosite and chlorite/epidote are mapped using the ICA technique. MF fraction images derived from n-dimensional visualisation (n-DV) tool facilitated detecting goethite, haematite, limonite, muscovite, kaolinite, illite, chlorite and epidote associated with gold occurrences. The distribution of Cu, Pb, Zn and Au is considered for generating geochemical anomaly layers. Strong Cu, Zn and Au anomalies are found to be associated with gold mineralisation. The lithological units hosting gold mineralisation and intersection of NE–SW and NW–SE trending lineaments are also considered. Fuzzy Logic Model (FLM) was used to generate gold prospectivity map for the study area by fusing the alteration, geochemical, geology and structural layers. Several high prospective zones are identified in the central and southeastern part of the study area. A majority of delineated exploration targets are either linked to the plutonic body or its surrounding metamorphic rocks. This study demonstrated a viable approach for future gold prospecting in the study area.

Gold mineralization of the Thone Myae Song area, Sagaing Region, northern Myanmar

• Central Magmatic-Volcanic Belt (CMVB) hosts several Au-Cu ore types and Thone Myae Song deposit. • Formation of CMVB is related to the eastward subduction of Indian Ocean floor during the Cretaceous. • Sericite-carbonate alteration is important as confines to the margins of mineralized veins. • Fluid immiscibility was a major trigger to the precipitation of gold and base metals at EE3 deposit. Several gold deposits are found in the Central Magmatic-Volcanic Belt (CMVB) of Myanmar, which forms a nearly N–S-trending, 1500-km-long arc extending from the Andaman Sea to northern Myanmar. The CMVB consists of Cretaceous granitoids intruded into folded andesites and pillow basalts (Mawgyi Andesite) which rest on cherts, talc-schists, mudstones and phyllites. The EE3 gold deposit is part of gold mineralization in the Thone Myae Song area, located in the Kawlin–Wuntho block in the northern part of the CMVB. The orebodies occur as sulfide-bearing massive quartz veins hosted by Mawgyi Andesite. Based on the mineral assemblages and cross-cutting relationships, three mineralization stages are identified in the EE3 gold deposit: Stage-I quartz-carbonate-sulfide veins; Stage-II quartz-carbonate veins intersecting the Stage-I veins; and Stage-III quartz-carbonate-sulfide veins parallel to the Stage-I veins. The main hydrothermal alteration minerals are sericite, epidote, chlorite and calcite. Three main types of fluid inclusions were distinguished from the mineralized quartz veins hosted by Mawgyi Andesite: liquid-rich aqueous fluid inclusions (Type A), vapor-rich aqueous inclusions (Type B), and H₂O-CO₂-NaCl inclusions (Type C). The latter occurs only in the Stage-I veins. The ore-forming conditions are estimated from fluid inclusions in the Stage-I, Stage-II and Stage-III veins to be at 180 °C and 28 bars, 176 °C and 26 bars, and 158 °C and 21 bars, respectively. The formation pressures of the Stage-I, Stage-II and Stage-III mineralization correspond to a shallow depth of 300 m, 280 m, and 225 m, respectively. Based on the ore and alteration mineral assemblages, and estimated temperature and pressure conditions of the mineralization, the EE3 gold deposit was formed in an epithermal condition.

Exploration radioactive mineralization using mappable data integration approach: example from Wadi Dahab area, Southeastern Sinai, Egypt

Mineral resources exploration and mapping are main reasons for developed countries around the world, especially those experiencing comprehensive sustainable development based on industrial activities such as power generation and urban development. Recent techniques of the geospatial tools such as remote sensing (RS) and Geographical Information Systems (GIS) are used to delineate lithological, structural lineaments, hydrothermal alteration, radiometric, and geochemical element distributions of the mineralized zones. The purpose of this study is to delineate and incorporate the different evidence relating to the possible radioactive mineralization sites, in the Wadi Dahab (WD), Southeastern Sinai, Egypt, to formulate a predictive model to explore radioactive mineralization, in which the weights of integrated evidence modelled for radioactive mineralization are used to define the probable sites of mineralization based on the statistical analyses. Digital image processing and GIS tools are usually applied more for mineral exploration to detect associated altered mineral assemblages and data modeling. Also, Landsat 8 and ASTER space-borne and other ancillary geological mappable data are processed and integrated to map alteration zones and structural lineaments and predict zones of mineralization potential as well as recognize different types of rock units. In the present study, the weighted index overlay of each essential mineral exploration evidence including alteration minerals, geochemical ore elements, radiometric measurements, and lithological and structural data, is integrated into the GIS environment suitable approach. Finally, the obtained constructed model indicates that it is an effective tool for mapping mineral alteration zones, controlled structural lineaments, and the main radioactive-rich potential rock units particularly in the granitic rocks of the WD area, which should be verified by fieldwork and laboratory examination. These validated models of newly mapped mineralized zones in the WD area would assist decision-makers in developing and benefiting from it in any possible planning for sustainable development.