Intermediate Argillic Alteration Research Articles

Deep epithermal Au mineralization at Tuanjiegou, northeast China

To better understand the physicochemical features of deeply mineralized intervals of low-sulfidation (LS) Au deposits, the Early Cretaceous Tuanjiegou Au deposit in northeastern China was characterized as it comprises alteration and mineralization styles that reflect deeper portions of the mineralizing system. From shallow to deeper portions of the Tuanjiegou deposit, the mineralization is zoned from sulfide-poor quartz bands through hydrothermal breccia to deep pyrite–marcasite-rich veinlets that occur along hydrofractures within a granodiorite porphyry stock. The zoned alteration halo is centred on the ore zones, and comprises an inner phyllic alteration zone (illite + quartz + muscovite) and an outer intermediate argillic alteration zone (illite ± smectite), with propylitic alteration being limited. Given that apatite fission track ages are extremely sensitive to the ambient temperatures of 60–120 °C, the overlap in the closure dates of magmatic apatite (108.1 ± 3.8 Ma) and zircon (108.2 ± 1.2 Ma) from the granodiorite porphyry is indicative of shallow emplacement of the granodiorite porphyry, likely at depths slightly >0.7 km. A similar paleodepth (900 ± 188 m) was estimated from homogenization temperatures (291 ± 14 °C) of boiling fluids. Preservation of the deep portions of the Tuanjiegou ore system may imply that the productive ore intervals of poorly explored LS epithermal deposits globally are underestimated. Given the evidence for a magmatic origin of the ore-forming fluids at Tuanjiegou, we highlighted that ore-forming fluids are released initially from a water-saturated, oxidizing felsic melt under low-pressure (∼4 km) conditions.

Late-Stage Low-Temperature Hydrothermal Alteration Overprint at the East Zone in the Red Chris Porphyry Cu-Au Deposit, Northwestern British Columbia, Canada

Abstract High- and intermediate-temperature alteration assemblages at the East zone in the Red Chris porphyry Cu-Au deposit, northwestern British Columbia, Canada, are varyingly overprinted by a lower-temperature intermediate argillic alteration assemblage composed of illite-kaolinite-hematite-carbonate. The intermediate argillic assemblage extensively overprinted the upper 600 m of the porphyry deposit and is present discontinuously to depths of 1,500 m below the premining surface. Kaolinite is dominant in shallow levels and gradually diminishes with depth, replaced by illite as the dominant clay mineral. Hematite replaced hydrothermal and igneous magnetite, but the intensity diminishes with depth. Mixtures of ankerite, dolomite, siderite, and calcite replaced mafic silicates and formed veins. Oxygen and hydrogen isotopes confirm a magmatic fluid source for the potassic assemblages preserved at depth as well as for the overprinted phyllic assemblage in the upper part and flanks of the East zone. In contrast, the superposed intermediate argillic assemblages formed by a mixture of magmatic and meteoric fluids. Sulfide minerals and sulfur isotopes retain zonal patterns for porphyry Cu deposits and appear largely unaffected by the overprinted intermediate argillic assemblage. Carbon and oxygen isotopes in carbonate vary with depth that may reflect a thermal gradient as a rising fluid cooled. The intermediate argillic assemblage is spatially associated with and overprinted on as yet undated late monzodioritic dikes—the youngest phase in the host Late Triassic Red stock. The relative age relationships and stable isotopic geochemistry indicate the intermediate argillic alteration assemblage represents the flux of magmatic-derived hydrothermal fluid that mixed with external fluid and thus represents either the last fluid pulse in the porphyry Cu deposit or a younger, temporally distinct hydrothermal fluid.

Exploration Implications of Multiple Formation Environments of Advanced Argillic Minerals

Abstract Advanced argillic minerals, as defined, include alunite and anhydrite, aluminosilicates (kaolinite, halloysite, dickite, pyrophyllite, andalusite, zunyite, and topaz), and diaspore. One or more of these minerals form in five distinctly different geologic environments of hydrolytic alteration, with pH 4–5 to <1, most at depths <500 m. (1) Where an intrusion-related hydrothermal system, typical of that associated with porphyry Cu ± Au deposits, evolves to white-mica stability, continued ascent and cooling of the white-mica–stable liquid results in pyrophyllite (± diaspore) becoming stable near the base of the lithocap. (2) A well-understood hypogene environment of formation is vapor condensation near volcanic vents, where magmatic SO2 and HCl condense into local groundwater to produce H2SO4 and HCl-rich solutions with a pH of 1–1.5. Close to isochemical dissolution of the host rock occurs because of the high solubility of Al and Fe hydroxides at pH <2, except for the SiO2 component, which remains as a siliceous residue because of the relatively low solubility of SiO2. This residual quartz, commonly with a vuggy texture, is largely barren of metals because of the low metal content in high-temperature but low-pressure volcanic vapor. Rock dissolution causes the pH of the acidic solution to increase, such that alunite and kaolinite (or dickite or pyrophyllite at higher temperatures) become stable, forming a halo to the residual quartz. This initially barren residual quartz, which forms a lithocap horizon where permeable lithologic units are intersected by the feeder structure, may become mineralized if a subsequent white-mica–stable liquid ascends to this level and precipitates copper and gold. (3) Boiling of a hydrothermal liquid generates vapor with CO2 and H2S. Where the vapor condenses above the water table, atmospheric O2 in the vadose (unsaturated) zone causes oxidation of H2S to sulfuric acid, forming a steam-heated acid-sulfate solution with pH of 2–3. In this environment, kaolinite and alunite form in horizons above the water table at <100°C. Silica derived within the vadose zone will precipitate as amorphous silica at the water table, as the condensate follows the hydraulic gradient, causing opal replacement above and at the aquifer. (4) By contrast, where condensation of this vapor occurs below the water table, the CO2 in solution forms carbonic acid (H2CO3), leading to a pH of 4–5. This marginal carapace of condensate, with temperatures up to 150°–170°C, commonly acts as a diluent of the ascending parental NaCl liquid. This steam-heated liquid forms intermediate argillic alteration of clays, kaolinite, and Fe-Mn carbonates; this kaolinite, which can be present at depths of several hundreds of meters, can potentially be mistaken as having been caused by a steam-heated acid-sulfate or supergene overprint. (5) The final setting is supergene, caused by posthydrothermal weathering and oxidation of mainly pyrite, locally creating pH <1 liquid because of high concentrations of H2SO4 within the vadose zone and forming kaolinite, alunite, and Fe oxyhydroxides. This genetic framework of formation environments of advanced (and intermediate) argillic alteration provides the basis to interpret alteration mineralogy, in combination with alteration textures and morphology plus zonation, including the overprint of one alteration style on another. This framework can be used to help focus exploration for and assessment of hydrothermal ore deposits, including epithermal, porphyry, and volcanic-hosted massive sulfide.

Ore characterization of LSE gold deposit in “X” Pit, Toka Tindung Project, North Sulawesi

Toka Tindung Project is a low sulphidation epithermal gold deposit, which is located in North Sulawesi, Indonesia. To be able to maximize the production yield of the Toka Tin-dung Project, it is important to understand the characteristics of the ore along with its mineral associations. This paper discusses the ore characterization and its mineral associations in “X” pit using thin section analysis to define the rock characteristics and hydrothermal alteration, X-Ray Diffraction (XRD) to identify the hydrothermal alteration minerals, and ore microscopy to identify the type of ore minerals. The study area is occupied by three alteration zones, namely silicification, advanced argillic and intermediate argillic alteration. The silicification is characterized by quartz with accessory minerals such as kaolinite, smectite, and celadonite. The advanced argillic alteration is typified by kaolinite, with minor quartz. The intermediate argillic alteration is characterized by clay minerals such as illite, mixed illite/smectite, smectite, and minor quartz minerals. Mineralization in “X” pit is in the form of sheeted vein type quartz with colloform texture, and the ore minerals are found as sulfides such as pyrite and sphalerite.

La Caridad Vieja: Vestiges of a removed HS-epithermal system in the La caridad porphyry Cu–Mo system, northeastern sonora, Mexico, based on XRD, SEM, SWIR spectral analysis and U–Pb zircon dating

The present work contributes with new data to better understand the geology and spatial distribution of the alteration and mineralization events, as well as new U–Pb zircon ages, that allow elucidating the magmatic history of the La Caridad Vieja mineral deposit, which is part of the porphyry Cu–Mo system from La Caridad mine, northwest Mexico. Mineralization is hosted in a sequence of volcanic rocks from the Late Cretaceous-Paleocene Tarahumara Formation and is associated with the emplacement of quartz monzonite stocks, which intruded to volcanic rocks. Detailed studies based on petrography, XRD, SEM and SWIR, indicate that the La Caridad Vieja is characterized by a zone of phyllic alteration dominated by qz-ser in the deepest parts, and it is associated with molybdenite + chalcopyrite + pyrite + galena + sphalerite, with values ranging from 0.1 to 0.9% Cu and <0.02% Mo. Above the phyllic alteration zone, there is an intermediate argillic alteration zone (quartz + kaolinite + halloysite + smectite + illite), mainly with pyrite. In specific areas, advanced argillic alteration is observed (alunite + pyrophyllite + dickite + kaolinite + halloysite) accompanied by pyrite + enargite + famantinite + barite. The new U–Pb radiometric ages in zircons indicate an age of 54.71 ± 0.39 Ma for the upper part of the Tarahumara Formation, while a sample of the mineralizing stock yielded 51.95 ± 0.48 Ma. The results of this investigation suggest that La Caridad Vieja corresponds to a portion of the upper part of the Cu–Mo porphyry of La Caridad, which was downthrown by the La Caridad fault, during the Basin and Range extensional event in the Miocene. The reconstruction of the geological model suggests that La Caridad Vieja corresponds to the transitional zone between the Cu–Mo porphyry and an epithermal high sulfidation mineralization zone, which was eroded. Prior to the dislocation caused by the La Caridad fault, the eroded area was covered by a ferruginous conglomerate overlain by rhyolite flows dated at ~24 Ma. On the other hand, geochronological data suggests that the mineralizing event at La Caridad Vieja, although it is contemporaneous with the porphyry system of the La Caridad mine, it continued in this area at least until ~52 Ma.

Geological and mineralogical constraints on the genesis of the Bilihe gold deposit in Inner Mongolia, China

The Bilihe gold deposit is a rare example that economical gold mineralization occurred in unidirectional solidification textures (UST) quartz at the cupola of middle Permian granodioritic intrusion. However, there is a key dispute about whether gold precipitated from low-diffusion silicate melt or high-activity fluid phase. In this paper, based on in-situ trace elements analyzing, SEM-CL mapping, and infra-red and Raman spectroscopy, the magmatic fluids are supposed to play a critical role in the gold mineralization by metal extraction from below magma at magmatic-hydrothermal transition stage. At Bilihe, the UST quartz represents the most fertile unit. Statistic results show the quartz–K-feldspar (KV) and gray banded quartz veins (GBV) also host about 22–27% of total reserves. A decreasing trend in Au/Ag, and Au/S, but elevated Au/Cu ratios of gold compositions indicate a relatively continuous fluid evolution in the Bilihe system. Petrography and spectroscopy studies show that the Bilihe UST quartz can be divided into two different domains, gold-rich brownish and barren colorless zones. The former can contain more water, occurring as tiny fluid inclusion or as structural OH– in muscovite, corresponding to sector CL zones (UQ1) or euhedral oscillatory bands (UQ2). The latter usually contain melt inclusions and correspond to obscure CL-textures (UQ3). Besides, the LA-ICP-MS trace elements show that there is a significant decreasing trends on fluid-mobile elements (e.g., K, Fe, B, and Li) from UQ1 over UQ2 to UQ3. In particular, the trace elements in UQ3 show greater affinity with magmatic quartz phenocrysts but with higher Ge, B, and Li contents, and higher Ge/Ti and Ge/Al, but lower Li/Ti ratios, indicating a highly evolved and fluid-rich environment. Based on previous model, we consider that the Bilihe UST quartz grew in the variable growth mediums, namely fluctuant alternation between fluid-dominant pocket (for UQ1 + UQ2) and silicate melt phase (for UQ3) near the melt-fluid interface at the apex of magma body. The relatively H2O-poor nature of parent magma is an important factor to cause the specific crystal growth environment. Mineral thermobarometers demonstrate that the parent magma from deep reservoir has experienced a significant ascent or decompression (from ~ 3–5 kbar to ~ 0.5–1.8 kbar). In addition, high fluxing volatiles, such as B, F and P, effectively protract magma crystallization to form highly evolved melt, and finally resulting in water saturation at shallow level. Thus, gold saturation might reach in earlier exsolving low-salinity fluids at low pressure (as low as 0.5 kbar) and high temperature (~660–700 °C) condition. Then voluminous fluids get release accompanying with hydrofracturing, and some gold sequentially precipitate in the KV with fluids cooling (<550 °C) and later in the GBV with further cooling and flashing during transition from lithostatic to hydrostatic loads. With increasing sulfur fugacity by fluid condensation and further cooling in fluid temperature, secondary gold can precipitate with possible Cu-(Fe) or As-Sb sulfides associated with pervasive intermediate argillic alteration, which intensively overprinted on earlier potassic alteration zone.

Magmatic-Hydrothermal Gold Mineralization at the Lone Tree Mine, Battle Mountain District, Nevada

Abstract The Lone Tree deposit is located in the northern Battle Mountain mining district, Nevada. Prior to mine closure in 2006, Santa Fe Pacific Gold and Newmont produced 4.2 Moz of gold at an average grade of 2.06 g/t at Lone Tree, primarily from the N-S– to NNW-SSE–striking Wayne zone. The ore is located between the Roberts Mountain and Golconda thrusts in siliciclastic rocks of the Ordovician Valmy Formation and in the Pennsylvanian-Permian Battle Mountain and Edna Mountain Formations, and above the Golconda thrust in siliciclastic and carbonate rocks of the Mississippian to Permian Havallah sequence. Ore is also hosted by rhyolitic dikes that were emplaced at 40.95 ± 0.06 Ma based on zircon U-Pb chemical abrasion-thermal ionization mass spectrometry. The gold is associated with sericitic and argillic alteration of the siliciclastic rocks and dikes and with decarbonatization and Fe carbonate alteration of the carbonate-bearing units, as well as in Fe-As sulfide and finegrained quartz alteration of all rock types. Oxidation affects 30 to 45% of the deposit, penetrating into the stratigraphy along numerous steeply dipping north-south, east-west, and north-northeast–south-southwest structures. Gold is positively correlated with Ag, As, Hg, and Sb. The highest Au grades occur in quartz-sulfide ore hosted in siliciclastic and carbonate sedimentary rocks and rhyolitic intrusions. In this ore style, fine-grained quartz and sericite are intergrown with disseminated sulfide minerals (quartz-sericite-pyrite alteration), constituting cores of weakly mineralized pyrite or marcasite, which are surrounded by fuzzy arsenopyrite rims that contain up to ~2,000 ppm Au. Low gold grades occur in late-stage banded pyrite breccias consisting of a finely zoned Au-poor pyrite matrix surrounding jigsaw-fit clasts of quartz-, illite-, barite-, and adularia-altered siliciclastic rock. The timing of main-stage mineralization is bracketed between the emplacement of the dikes and an adularia 40Ar/39Ar age of 40.14 ± 0.74 Ma. Sericite intergrown with arsenopyrite-rimmed pyrite in phenocrysts of the rhyolite dikes gave δ18O values of 1.6 to 9.5‰ and δD values of –105 to –145‰. For temperatures of 300 ± 100°C, the calculated fluid isotopic compositions are consistent with felsic magmatic water and minor modifications by mixing with meteoric water and exchange with wall rocks. In the silica-sulfide ore, in situ isotopic laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) analyses of pyrite cores yielded δ34S values ranging from 3.4 to 7.7‰, with average values of 5.6‰ in the felsic dikes, 4.5‰ in the siliciclastic rocks, and 5.3‰ in the carbonate rocks. These values match conventional pyrite δ34S data reported for Eocene porphyry systems elsewhere in the district. Nanoscale secondary ion mass spectrometry analyses show that gold and associated trace elements occur in submicron-scale zones within arsenopyrite rims on pyrite. The average δ34S values of the arsenopyrite rims are 5.3 to 6.5‰ heavier than the pyrite cores, indicating cooling and an increasing H2S/SO2 ratio. The highest grades resulted from episodic pulses of a gold-rich fluid that was partly derived from, or exchanged with, the sedimentary host rocks. In situ LA-MC-ICP-MS δ34S values for the late-stage banded pyrite breccia become progressively lighter from veinlet margin to center, reaching a low of –32‰. These veinlets indicate a shift from main-stage quartz-sericite-pyrite and intermediate argillic alteration to more neutral pH and oxidizing conditions during late-stage mineralization, indicating either increasing interaction between the fluid and sedimentary sulfur sources in the host-rock package or bacterial sulfate reduction and supergene sulfide precipitation.

Influence of alteration on the mechanical behaviour and failure mode of andesite: implications for shallow seismicity and volcano monitoring

Volcanoes often host hydrothermal systems that alter the host rock. To understand the influence of alteration on mechanical behaviour of edifice-forming rock, we performed a series of triaxial deformation experiments on variably altered andesite from Mt. Ruapehu (New Zealand) under constant effective pressure. Under the imposed conditions, andesite with intermediate argillic alteration deforms in a brittle manner forming fractures. By contrast, andesite with advanced argillic alteration deforms in a ductile manner, with sample failure driven by distributed cataclastic pore collapse. We consider this the result of an increase in porosity and clay content with increasing alteration. Ancillary experiments highlight that the brittle-ductile transition occurs at lower effective pressure (i.e. at shallower depths) in andesites with advanced argillic alteration relative to unaltered andesites of comparable porosity. We conclude that advanced argillic alteration can create an anomalous shallow ductile zone, which has important implications for fluid flow and pre-eruptive seismicity.

Application of WorldView-3 imagery and ASTER TIR data to map alteration minerals associated with the Rodalquilar gold deposits, southeast Spain

The Rodalquilar epithermal quartz-alunite gold deposits that occur within the Rodalquilar caldera complex in southeast Spain, are associated with a pronounced hydrothermal alteration of the country rocks. The hydrothermal alteration zones that are exposed on the surface consist of the vuggy silica zone, the advanced argillic alteration zone, the intermediate argillic alteration zone, the propylitic alteration zone, and a second stage supergene acid sulfate alteration. High spatial resolution multispectral imagery recorded by the WorldView-3 satellite was used in this study to map the spatial distribution of the main alteration minerals in the Rodalquilar caldera complex. Thermal infrared (TIR) data of the ASTER satellite were used to detect the quartz-rich zones. The analysis of the Rodalquilar WorldView-3 data was based on the Adaptive Coherence Estimator (ACE), a partial unmixing algorithm. The ACE processing accurately mapped the spatial distribution of alunite, kaolinite, illite and goethite. Alunite is abundant in the vuggy silica and advanced argillic alteration zones, and in the second stage supergene acid sulfate alteration. Kaolinite is predominant in the intermediate argillic alteration zone. Illite is abundant in the outer parts of the intermediate argillic alteration zone. Goethite image maps gossans that mainly occur in the vuggy silica and advanced argillic alteration zones, and in the areas characterized by the second stage supergene acid sulfate alteration. The detection of quartz-rich zones from the ASTER TIR data complemented the WorldView-3 mapping results. The study shows the efficiency of high spatial resolution multispectral remote sensing imagery recorded by the WorldView-3 satellite for district-level mineral exploration studies.

Hydrothermal alteration products in the vicinity of the Ahırözü kaolin deposits, Mihalıççık-Eskişehir,Turkey

ABSTRACTThe mineralogy, texture and composition of rocks associated with the kaolin deposits in the Ahırözü-Hamidiye-Üçbaşlı area, SE of Mihalıççık, Eskişehir, Turkey, were investigated. In the study area, Triassic, blue-green schists and serpentinized ultramafic rocks are exposed. Kaolinization occurs at the contact between a Triassic granitic intrusion and metamorphic rocks. Textural and mineralogical characteristics were identified by X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) analyses. The effects of hydrothermal alteration on the whole-rock chemical composition were determined by inductively coupled plasma mass spectrometry (ICP-MS) analysis. Homogenization temperature (Th, °C), salinity, (wt.% NaCl) and melting temperature (Tm, °C) were detected by fluid-inclusion analyses.The argillic alteration zone (Zone A) includes kaolinite, smectite, natroalunite and accessory pyrite, hematite and goethite. This paragenesis corresponds to a low–intermediate argillic alteration caused by fluids having neutral to acidic pH, at a temperature of &lt;200°C. The propylitic alteration zone (Zone B) is characterized by epidote, chlorite, illite and pyrite and was caused by fluids with a neutral pH and a temperature of &gt;250°C. The rocks in Zone C are mainly silicified and represent sinter formation in the study area. The rocks in the kaolin deposit are depleted in Rb, Pb and Y and enriched in Cs, U and Sr, and represent the products of epithermal hydrothermal alteration of granitic intrusions emplaced in a volcanic arc.

Petrography and Mineral Chemistry of Magmatic and Hydrothermal Biotite in Porphyry Copper-Gold Deposits: A Tool for Understanding Mineralizing Fluid Compositional Changes During Alteration Processes

DOI: 10.17014/ijog.5.1.47-64This study aims to understand the petrography and chemistry of both magmatic and hydrothermal biotites in porphyry copper-gold deposits, and to evaluate the fluid compositional changes during alteration processes. A total of 206 biotite grains from selected rock samples taken from the Batu Hijau porphyry Cu-Au deposit was analyzed. Detailed petrography and biotite chemistry analysis were performed on thin sections and polished thin sections, respectively, representing various rocks and alteration types. A JEOL JXA-8900R electron microprobe analyzer (EMPA) was used for the chemistry analysis. The biotite is texturally divided into magmatic and hydrothermal types. Ti, Fe, and F contents can be used to distinguish the two biotite types chemically. Some oxide and halogen contents of biotite from various rocks and alteration types demonstrate a systematic variation in chemical composition. Biotite halogen chemistry shows a systematic increase in log (XCl/XOH) and decrease in log (XF/XOH) values from biotite (potassic) through chlorite-sericite (intermediate argillic) to actinolite (inner propylitic) zones. The y-intercepts on the log (XCl/XOH) vs. XMg and log (XF/XOH) vs. XFe plotted for biotite from potassic and intermediate argillic zones are similar or slightly different. In contrast, the y-intercepts on the log (XCl/XOH) vs. XMg and log (XF/XOH) vs. XFe plotted for biotite from inner propylitic zone display different values in comparison to the two alteration zones. Halogen (F,Cl) fugacity ratios in biotite show a similar pattern: in the potassic and intermediate argillic zones they show little variation, whereas in the inner propylitic zone they are distinctly different. These features suggest the hydrothermal fluid composition remained fairly constant in the inner part of the deposit during the potassic and intermediate argillic alteration events, but changed significantly towards the outer part affected by inner propylitic alteration. High halogen content, particularly Cl, in hydrothermal biotite may portray that copper and gold were transported in mineralizing fluids in the form of chloride complexes CuCl2- and AuCl2-, respectively.

Characteristics and 40Ar/39Ar Geochronology of the Erdenet Cu-Mo Deposit, Mongolia

The Early to Middle Triassic Erdenet porphyry Cu-Mo deposit, in northern Mongolia, developed in a continent-continent arc collision zone, within the Central Asian orogenic belt. The porphyry system is related to multiple intrusions of crystal-crowded biotite granodiorite porphyry, which formed a composite stock about 900 m in diameter, with multiple porphyritic microgranodiorite dikes. Wall rocks are Late Permian to Early Triassic, medium-grained granodiorite, with similar whole-rock geochemistry, mineralogy, and composition to the granodiorite porphyry. Whole-rock analysis of the granodiorite porphyry and wall rocks shows that these rocks cannot be discriminated, but both have depleted middle heavy rare earth elements and Y, typical of fertile porphyry magmatic suites. At the current pit level (1,250 m elev), early porphyry-style quartz veins (A and B type) are locally infilled by pyrite-chalcopyrite, with subordinate bornite, but most of the chalcopyrite occurs in D veins that constitute more than 50% of the Cu grade (~0.5 wt % Cu). The 0.3 wt % Cu shell resembles a molar tooth, enveloping the granodiorite porphyry, with deeper roots extending down the wal-rock contacts. Molybdenite occurs in monomineralic veins, and in finely laminated to massive quartz-molybdenite veins. The most important alteration is quartz-muscovite, which occurs as relatively coarse (100–500 μ m) alteration selvages (1–5 cm) that envelop D veins. The D veins cut illite ± kaolinite-smectite (or intermediate argillic) alteration. Intermediate argillic alteration, together with abundant pink anhydrite (commonly hydrated to gypsum), extends from at least 1,300- to 900-m elevation in the deepest drill holes, and has overprinted early potassic alteration, or relatively unaltered red granodiorite. Meter-wide zones of kaolinite cut the anhydrite-gypsum at all levels. There is an abrupt transition outward from the intermediate argillic alteration to chlorite-epidote (propylitic) alteration, at 50 to 200 m from the granodiorite porphyry contact, although D veins (and chalcopyrite) extend outward to the propylitic zone. The Erdenet porphyry system, was overprinted by advanced argillic alteration, which outcrops 2 km northwest of the pit, and forms a lithocap that extends over 10 × 2.5 km. It is characterized by residual quartz, andalusite, Na-Ca and K-alunite, diaspore, pyrophyllite, zunyite, topaz, dickite, and kaolinite. The upper part of the porphyry Cu-Mo deposit (removed by mining), comprised a bornite-chalcocite enriched zone up to 300 m thick with an average grade of 0.7 wt % Cu and up to 5 wt % Cu locally. Based on hypogene bornite-chalcocite mineral textures and high-sulfidation state mineralogy, the enriched zone is inferred to be of hypogene origin, but modified by supergene processes. Consequently, it may be related to formation of the lithocap. Previous Re-Os dates of 240.4 and 240.7 ± 0.8 Ma for molybdenite in quartz veins are comparable to new 40 Ar/ 39 Ar dates of 239.7 ± 1.6 and 240 ± 2 Ma for muscovite that envelops D veins. One 40 Ar/ 39 Ar date on K-alunite from the lithocap of 223.5 ± 1.9 Ma suggests that it may be about 16 m.y. younger than Erdenet, but this result needs to be verified by further dating.

Mineralogical and Geochemical Control of Altered Andesitic Tuff upon Debris Slide Occurences at Pelangan Area, Southern Mountain of Lombok Island, Indonesia

Debris slides were recently found in the Pelangan area at Southern Mountain of Lombok Island, Indonesia. Pelangan is well known as the gold mineralization and hydrothermal alteration area. This study is aimed to identify the mineralogy and geochemistry of altered andesitic tuff that controlled slope instability and induced debris slides. For this purpose, it is necessary to prepare the field observation and laboratory analyses. Landslide inventory reveals that the Pelangan debris slides mostly occur in altered andesitic tuff. Based on the outcrop observations in the field, andesitic tuff found around the Pelangan debris slides have been altered in general. The strong intensity of alteration developed by hydrothermal alteration in this study area produces large amount of clay minerals especially montmorillonite, kaolinite, and illite. The abundance of those clay minerals reflect the intermediate argillic alteration. Montmorillonite is a type of clay mineral that easily swells at wet condition and easily shrinkages at dry condition. Swelling of clay mineral destroys intersheet and interlayer bonds, and reduces shear strength. The presence of clay minerals in the altered andesitic tuff of intermediate argillic zone can be considered as one of the factors that induced to the Pelangan debris slides. Further studies on geotechnical and slope stability analysis of the landslide area are crucial to be done for better understanding of the characteristics of the altered rocks inducing hazardous landslides.

Reflectance spectroscopy and alteration assemblages at the Leven Star gold deposit, Victoria, Australia

Hylogger-2 reflectance spectroscopy data from two drill holes at the Leven Star gold deposit, provides an insight into the hydrothermal alteration associated with vein sets and faults that host gold mineralisation. The metasedimentary host rock is a biotite-rich hornfels, which overprints a regional greenschist facies metamorphism. Gold mineralisation is associated with quartz (±carbonate) stockwork-breccia zones. HyLogging short-wave infrared reflectance spectra detected carbonates, chlorites, epidote (propylitic: carbonatisation, chloritisation, epidote), muscovite (phyllic/sericitic) and kaolinite, illite, ammonium alunite, montmorillonite and dickite (intermediate argillic alteration) assemblages that were confirmed using petrographic observations, drill core and geochemical data. The gold occurs within a sulfidation halo with carbonate phases (ankerite–siderite–dolomite) in structurally controlled veins and faults, which also host a late Sb–As–Bi–Cu–W assemblage, attributed to an intrusion-related style of mineralisation superimposed on a hydrothermal orogenic gold event.

A study of the hydrothermal alteration in Paleoproterozoic volcanic centers, São Félix do Xingu region, Amazonian Craton, Brazil, using short-wave infrared spectroscopy

Hypogene hydrothermal minerals have been identified by short-wave infrared spectroscopy in hydrothermally altered rocks from the Sobreiro and Santa Rosa formations, which belong to a Paleoproterozoic volcano–plutonic system in Amazonian craton. Three clay minerals are spectrally recognized: montmorillonite, kaolinite, and illite. The integration of these data with those available in the literature, including gold occurrences, suggests that those rocks are hydrothermal products of both volcanic thermal sources and later crustal intrusions, as evidenced by variable styles of propylitic, sericitic, potassic, and intermediate argillic alteration. The influence of meteoric fluids is emphasized. This low cost exploratory technique, which can be applied to hand samples, seems to be promising in the separation of hydrothermally altered volcano–plutonic centers in regions submitted to severe weathering conditions, in addition to aid elaborating models for prospecting mineral deposits.

High-Sulfidation Epithermal Pyrite-Hosted Au (Ag-Cu) Ore Formation by Condensed Magmatic Vapors on Sangihe Island, Indonesia

Although gold in high-sulfidation epithermal deposits generally occurs as the native metal or electrum, in some deposits, a significant proportion of the gold is hosted in pyrite. Here we use a combination of petrography, whole-rock geochemistry, pyrite chemistry, crystallography, and phase stability relationships to determine how gold was transported and incorporated into pyrite in two relatively young high-sulfidation epithermal deposits, where the gold occurs almost exclusively in solid solution or as nanoparticles in pyrite. The genetically related Bawone and Binebase Au (Cu-Ag) deposits, located 1 km apart on the volcanic island of Sangihe, northeastern Indonesia, are hosted by andesitic volcaniclastic rocks that were altered to a proximal advanced argillic association of quartz + pyrite (py I) + pyrophyllite + natroalunite + alunite + dickite + kaolinite and a more distal intermediate argillic association of quartz + pyrite (py I) + kaolinite + dickite + illite. The economic mineralization takes the form of multiple generations of auriferous pyrite, the first of which, pyrite I (py I), developed during advanced argillic alteration. Mass balance calculations show that all elements were mobile with the exception of Nb, Ti, some rare earth elements, and possibly Al. The highest gold concentration is in pyrite II (py II), which occurs in veins that cut pyrite I. This drusy variety of pyrite is characterized by complex growth and sector zoning, and contains as much as 6.0 wt % Cu. The elevated Cu concentrations correlate positively with Au and As concentrations, whereas the Ag concentration correlates strongly with Au but not Cu. Later barite-enargite mineralization exploited py II veins and vugs, and significant concentrations of Ag and Au are hosted by enargite, although the Au concentration in enargite is lower than in py II or py I. A model is presented in which the fluid responsible for advanced argillic and intermediate argillic alteration and associated stage 1 gold mineralization was a condensed magmatic vapor derived from an oxidized magma. The gold and other metals were transported as hydrated species that ascended through the volcanic pile via fractures and zones of enhanced permeability to a depth between 900 and 1300 m, where the vapor condensed at a temperature between 250 and 340°C to form an acidic liquid with a pH of ~2.5; f o2 ranged up to four log units above the hematite-magnetite buffer. Interaction of this liquid with the host andesites caused advanced argillic and intermediate argillic alteration, including sulfidation of mafic minerals to form py I. During crystallization of py I, Au, Cu and Ag were adsorbed onto the surface of the pyrite and deposited as nanoparticles, or were incorporated in the pyrite structure. Adsorption of Au, Cu, and Ag from the condensed vapor reached a peak during the crystallization of vein-hosted py II, and the uptake of Ag and minor Au continued during later crystallization of enargite. From the distribution of metals among growth and sector zones in py II, incorporation of gold and other metals appears to have been maximized when physicochemical conditions were relatively stable. This is in contrast to the requirement for native gold precipitation, namely that physicochemical gradients be steep to ensure supersaturation of gold in the ore fluid.

Stable isotope record of hydrothermal sulfate, sulfide and silicate minerals in the Darreh-Zar porphyry copper deposit in Kerman, southeastern Iran: Implications for petrogenesis and exploration

The Darreh-Zar porphyry copper deposit is found together with other porphyries such as Sar-Cheshmeh and Sungun in the Cenozoic age Urumieh–Dokhtar magmatic belt that is related to the subduction of the Arabian plate beneath Central Iranian microcontinent. The deposit is associated with Miocene granodiorite porphyry intrusions into Eocene basaltic volcanic rocks. The principal hydrothermal facies of the area are a core zone of potassic alteration enclosed by a peripheral zone of propylitic alteration. Chlorite-sericite, sericitic, advanced argillic and intermediate argillic alteration zones cut the upper part of the potassic zone and are developed near the interface between the potassic and propylitic zones. Chalcopyrite, pyrite or local bornite, and associated anhydrite are the chief hypogene sulfide minerals, and constitute <0.1 to 0.8wt.% Cu ores. A ~50-m-thick supergene zone containing 0.8 to 1.5wt.% Cu, characterized by covellite and chalcocite, underlies an oxidized and partially leached ~20-m-thick zone containing chrysocolla, malachite and azurite.The hydrogen isotopic composition of biotite from Darreh-Zar ranges from δD of −89 to −101‰, and the calculated aqueous fluids (δD of −66 to −78‰) in equilibrium with biotite are consistent with a large component of magmatic fluid. Epidote varies from δD of −56 to −53‰, and yields calculated fluids responsible for the propylitic alteration that range from −22 to −19‰. Oxygen isotopic data for the studied quartz, biotite, anhydrite and epidote vary between δ18O of 0.8 to 9.0‰. The calculated δ18O values for aqueous fluids in equilibrium with the biotite, quartz (from barren type and A veins), anhydrite (from B veins) and epidote are 7.8±0.4, 5.1±0.6, 2.6±0.5 and −1.1±0.1‰, respectively. This marked decrease likely reflects a change from magmatic-derived fluids in central potassic zone to meteoric or sedimentary-derived fluids in the outer propylitic zone. Sulfur isotope compositions are 11.8 to 14.0‰, for anhydrite (n=3), 2.4‰ for molybdenite (n=1), 1.7 to 3.9‰ for pyrite (n=17), and 1.6 to 2.9‰ for chalcopyrite (n=2). The sulfide data, alone, suggest a conventionally ‘magmatic’ value of about 1.6 to 2.9‰ for Darreh-Zar sulfur. However, the fairly oxidized granitic parental magma shows relatively heavy bulk sulfur (δ34SΣS≈+5‰) and sulfate–sulfide sulfur isotopic fractionation is consistent with an approach to isotopic equilibrium at calculated temperatures of 520±50°C for most of the coexisting anhydrite–pyrite pairs (n=4). An exploration implication is that the identification of isotopically high sulfur isotopic compositions of gypsum from near-surface samples determines a hydrothermal system that contains deeper hypogene anhydrite, a common indicator of large porphyry Cu-Mo deposits. In contrast, isotopically low sulfur isotopic values similar to and derived from weathering of pyrite and other sulfides are common in many hydrothermal environments.

Evidence of Ediacaran glaciation in southernmost Brazil through magmatic to meteoric fluid circulation in the porphyry–epithermal Au–Cu deposits of Lavras do Sul

The Lavras do Sul Au–Cu mining district, southern Brazil, hosts an Ediacaran plutonic–volcanic association with hydrothermal alteration. This association is interpreted as a ca. 600–580 Ma tilted porphyry–epithermal system, where the main ore occurs in phyllic and intermediate argillic halos around quartz veins. In this paper we used detailed field mapping, fluid inclusion microthermometry, oxygen and hydrogen isotope measurements of whole-rocks, feldspars, clay minerals and quartz from propylitic, phyllic and intermediate argillic alteration to reconstruct the hydrothermal fluid temperatures and compositions. The results showed that the deeper zones of the hydrothermal system were predominantly influenced by magmatic fluid and its shallower zones by meteoric water with very negative δ 18O compositions. Our data support a model of mixing between magmatic-derived fluids with meteoric waters of glacial origin. Based on stable isotope data we propose indirect evidence of high latitude position for the study area during Brasiliano/Pan-African convergence of the Kalahari and Rio de La Plata cratons.

Geological setting, alteration, and fluid inclusion characteristics of Zaglic and Safikhanloo epithermal gold prospects, NW Iran

AbstractThe Zaglic and Safikhanloo epithermal gold prospects are located in the Arasbaran zone, to the west of the Cenozoic Alborz-Azarbaijan magmatic belt in NW Iran. Mineralization is mainly restricted to quartz and quartz -carbonate veins and veinlets. Pyrite is the main sulphide, associated with subordinate chalcopyrite and bornite. Gold occurs as microscopic and submicroscopic grains in quartz and pyrite.The country rocks are Tertiary intermediate–mafic volcanic and volcaniclastic rocks of andesite to trachy-andesite composition intruded by a composite granitic to syenitic pluton. They are medium- to high-K, calc-alkaline and alkaline rocks and display fractionated REE (rare earth element) patterns, with light rare earth elements (LREE) significantly enriched relative to the heavy rare elements (HREE). On primitive mantle normalized plots, they display depletions in Nb, Ti and P, and enrichments in Pb, which are common characteristics of arc-related magmas worldwide. Hydrothermal alteration minerals developed in the wall rocks include quartz, calcite, pyrite, kaolinite, montmorillonite, illite, chlorite, and epidote. Minor alunite occurs in Safikhanloo. Gold is locally enriched in the altered rocks immediate to the veins.The ore-stage quartz from both prospects is dominated by liquid-rich fluid inclusions; vapour-rich inclusions are rare. The homogenization temperature varies between 170–230 and 170–330 °C and salinity varies between 1.4 to 9.5 and &lt;1 to 6.7 wt% NaCl equivalent, for Safikhanloo and Zaglic, respectively. The occurrence of hydrothermal breccias, bladed calcite, adularia, and rare coexisting vapour- and liquid-dominant inclusions suggest that boiling occurred in the course of the evolution of the ore fluids. The large variations in Th and the salinity values can be explained by boiling and/or mixing.Lack of sulphate minerals in the veins suggests that sulphides and gold precipitated from a reduced, H2S-dominant fluid. Calculated δ34S values for the ore fluid vary between −4.6 and −9.3‰. Sulphur could have been derived directly from magmatic sources, or leached from the volcanic and plutonic country rocks. Ore formation in Zaglic and Safikhanloo occurred in response to mixing, boiling, and interactions with wall rocks. Considering the intermediate-argillic alteration, the low contents of base metal sulphides, and the overall low salinities, the Zaglic and Safikhanloo can be classified as low-sulphidation epithermal systems.

Intermediate sulfidation epithermal gold-base metal deposits in Tertiary subaerial volcanic rocks, Sahinli/Tespih Dere (Lapseki/Western Turkey)

Gold in the Sahinli and Tespih Dere intermediate sulfidation gold-base metal deposits in Western Turkey occurs in relatively deep epithermal quartz veins along with base metal minerals which have epithermal textures, including plumose quartz, vug infills, comb and cockade textures and matrix-supported milled breccias. The total sulfide content of the veins in the area is variable ranging from < 1% to 60% and is dominated by pyrite, galena, sphalerite and chalcopyrite. Sphalerite is Fe-poor (0.6 to 1.4 mol% FeS). Minor amounts of Ag-rich tetrahedrite are present. Primary hydrothermal alteration minerals include illite/muscovite, mixed-layer illite/smectite (11.6 Å) and clinochlore towards the east and, alunite, dickite/nacrite and pyrophyllite towards the west at Sahinli; major illite/muscovite and dickite occur at Tespih Dere and Sarioluk, respectively. Fluid inclusions in main-stage quartz at Sahinli are only liquid-rich, with homogenization temperatures ranging from 220 to 322 °C and the majority of T h values between 250 and 300 °C. Salinity ranges from 4.3 to 6.9 wt.% NaCl equiv. First ice-melting temperatures ( T mf) between −24.5 and −19.0 °C indicate that the fluids were dominated by NaCl – H 2O during mineralization. The relatively higher average T h at the Tespih Dere deposit (295 °C) is attributed to a relatively deeper level of exposure. Calculated δ 18O values indicate that ore-forming hydrothermal fluids in the study area had δ 18O H2O ranging from + 1.1 to + 9.7‰ (average = 3.8‰), strongly 18O-enriched compared with present-day hydrothermal meteoric water in the area ( δ 18O = −8.5‰). δD values of fluid inclusions in quartz range from −58 to −93‰ and δD values of clay minerals and alunite from −40 to −119‰. δD values from intermediate argillic alteration (average = −68‰) in the study area are very similar to δD values of the present-day local geothermal system (average δD = −54‰) whereas δD values from advanced-argillic alteration (average δD = −33‰) are very different from the present-day local geothermal system. The δ 34S values in samples from the Sahinli and Tespih Dere deposits average −2.9‰ for pyrite; −3.3‰ for chalcopyrite; −5.4‰ for sphalerite and −7.6‰ for galena. These data are consistent with derivation of the sulfur from either igneous rocks or possibly from local wallrock.