Characteristics Of Hydrothermal Alteration In Cijulang Area, West Java, Indonesia
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Abstract:
Characterization of hydrothermal alteration in theCijulang area (West Java, Indonesia) was carriedout using shortwave infrared spectroscopy. Hydrothermal alteration in the Cijulang area occurs in the calc-alkaline volcanic and volcaniclastic rocks. Shortwave infrared spectroscopic measurements of reflectance for altered rocks and minerals were carried out by ASD-FieldSpec and the laboratory spectra acquired were then analysed with “The Spectral Geologist” software program. Shortwave infrared spectroscopy is capable of detecting most finegrained alteration minerals from different hydrothermal alteration zones. Characteristic alteration minerals identified from the SWIR technique include pyrophyllite, alunite, kaolinite, dickite, illite, montmorillonite, polygorskite, gypsum, epidote, paragonite, and muscovite. Most of the spectra show mixture ofalteration minerals and only a few display pure spectra of single mineral. The crystallinity of kaolinite from the samples was also determined from the reflectance spectra and show moderately to high crystallinity. Alteration system of the Cijulang prospectis similar to others documented high-sulfidation epithermal deposits, such as Rodalquilar (Spain), Summitville (Colorado), and Lepanto (Philippines). A characteristic alteration sequence and zonation of advanced argillic, argillic and propylitic alterationoutward from the silica core has resulted from the progressive cooling and neutralization of hot acidic magmatic fluid with the host rocks.Keywords: Cijulang, High-sulfidation, Alteration minerals, Shortwave Infrared SpectroscopyKeywords:
Alunite
Argillic alteration
Pyrophyllite
Hypogene
Breccia
Argillic alteration
Hypogene
Alunite
Sericite
Authigenic
Marcasite
Sulfide Minerals
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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.
Alunite
Argillic alteration
Hypogene
Overprinting
Fumarole
Paragenesis
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Hydrothermal mineral deposits are the primary source of many mineral commodities of global importance. Since hydrothermal alteration minerals associated with the formation of these mineral deposits are active in the visible and infrared range, the analysis of spectral absorption features can be used to identify the mineralogy associated with different alteration events. Some hydrothermal events are responsible for the occurrence of mineral commodities, while other events create hydrothermal alteration unrelated to the introduction of base and precious metals. Therefore, it is crucial to develop a mineral exploration strategy to rapidly identify and map the indicator minerals linked to a mineralising event. The separation of minerals of different alteration events which are spectrally active in the same overlapping range of the spectrum, is the challenge addressed in this study. High spatial resolution airborne and laboratory-based hyperspectral images are combined to detect and visualise textures of muscovite replacing pyrophyllite in the shortwave infrared (SWIR) imaging spectroscopy survey over the Buckskin Range, the volcanic-hosted lithocap part of the Yerington porphyry district, Nevada (USA). Spectral wavelength maps in different SWIR ranges are used to map the hydrothermal alteration mineralogy at both laboratory (26 µm) and airborne (1 m) scales. The airborne spectral data define outward zoning from alunite ± pyrophyllite to muscovite characterized by variable wavelength positions of its Al-OH absorption feature. The wavelength range of 1650–1850 nm is used to differentiate zones of pyrophyllite predominance over alunite within the inner domain. The laboratory data improves the characterisation of the hydrothermal alteration mineralogy, which includes alunite, pyrophyllite, muscovite, dickite, chlorite, topaz and zunyite. The textural relationship of muscovite replacing pyrophyllite is addressed through the development of a novel spectral index, the pyrophyllite-muscovite index (PMI). The characterisation of the intergrowths of pyrophyllite and muscovite at the laboratory scale is based on two aspects: (1) the definition of pervasive versus veinlet-controlled textures and (2) a subtle shift detection in the wavelength position of the Al-OH absorption feature of muscovite from 2189 to 2195 nm. The combination of the spatial patterns with the textural relationship of the pyrophyllite-muscovite association allows the identification of areas which contain the muscovite replacement of pyrophyllite. The recognition of a late muscovite replacement of pyrophyllite suggests that advanced argillic alteration reflecting intense acid leaching is followed by late near-neutral pH magmatic-hydrothermal fluids, adding K+ and potentially other alkali elements and metals in the epithermal environment. As a result of this study, we document the hydrothermal muscovite-pyrophyllite intergrowth relationships in the study area, thus contributing to an improved understanding of the lithocap epithermal system and a better assessment of its exploration potential for Au, Ag and Cu mineralisation.
Alunite
Pyrophyllite
Muscovite
Hypogene
Argillic alteration
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Woodhouseite and svaubergite, previously documented from roughly two dozen localities, have been found with advanced argillic alteration in three hydrothermal ore deposits. These aluminum phosphate-sllfate (APS) minerals are isostructural with alunite (R3m), with the generalized formula RAl3(POdl +r(SO/1-r(OH)6-r. (HzO)r, where R is Ca or Sr, and x is less than 0.5. At Summitville, Colorado, an epithermal gold*opper deposit, svanbergite occurs with hypogene kaolinite and alunite in the upper portions of the deposit, and woodhouseite is observed at deeper levels, where pyrophyllite is locally abundant. In the porphyry-copper deposit at La Granja @eru), woodhouseite occurs witl pyrophyllite and appears to have replaced apatite. The porphyry-copper deposit at La Escondida (Chile) contains woodhouseite-svanbergite solid solutions, some hypogene, others supergene, as judged from textual criteria. Alunite specimens from Summitville, La Escondida, and several other localities investigated in this study contain some APS component, lvith up to 2.41 wt. q0 P2O5 and l.l2wt.tlo SrO, reflecting tie coupled substitution K+ + SO42- = (Ca,Sr)z+ + POo3- io gt. t*t r*ture. The APS phases are considered to form by replacement of apatite in the acidic, sulfate-rich environment ttrat characterizes advauccd argillic alteration. Their occurrence probably has been overlooked in loany areas showing such alteration of apatite-bearing host rocks.
Alunite
Argillic alteration
Hypogene
Pyrophyllite
Supergene (geology)
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Citations (137)
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.
Alunite
Argillic alteration
Pyrophyllite
Hypogene
Anhydrite
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Citations (68)
The lithocap is a large-scale alteration including silicification, advanced argillic and argillic that developed near the surface by hydrothermal activities. It is helpful for exploring high-sulfidation epithermal deposits and porphyry deposits in its root and deep parts, respectively, and is used as a new prospecting indicator. There are many minerals in the lithocap and it is difficult to identify them; alunite is the most representative mineral and one of the new minerals used to guide exploration; it plays an important role in assessing the metallogenic potential and guiding ore prospecting. The Luzong basin is dominated by iron oxide apatite deposits (IOAs). In recent years, many researchers have been searching for porphyry copper–gold deposits and epithermal deposits as studies on alunite and lithocaps in the basin have gradually been undertaken. In field explorations and investigations of the Luzong basin, the author found that in Qianpu area of the central basin, where kaolinite deposits were mainly explored in the past, Well-occurring alunite minerals and advanced argillic alterations have developed. A typical lithocap developed in Qianpu area after a series work of geology and mineralogy, such as alteration mineral species, alteration types, and alteration zoning, which were formed by the hydrothermal alteration of the volcanic rocks of the Zhuanqiao Formation. The minerals in this lithocap are mainly quartz, alunite (including aluminum-phosphate-sulfate, APS), pyrite, zunyite, dickite, pyrophyllite, kaolinite, and a small amount of illite, which are present sequentially from the center to the peripheral quartz-alunite-pyrite alteration, alunite-dickite alteration, dickite-pyrophyllite alteration, alunite-kaolinite-illite alteration, and alunite veins superimposed on quartz-alunite-pyrite alteration and alunite-dickite alteration. Three types of alunite were produced in large quantities and were formed in three kinds of environments. The I-type alunite, which was formed by hydrothermal metasomatism alteration in the early stage and is densely disseminated distribution, coexisting with quartz and pyrite, is a magmatic hydrothermal alunite. Relatively pure alunite veins (II-alunite) of varying thicknesses formed in the open space in the mid-hydrothermal period in a magmatic steam environment. The fine-grained III-alunite that coexists with kaolinite formed in the late hydrothermal period and in a steam-heated environment. The alunite produced by various origins represents multiphase hydrothermal activity in the Yanshanian period in the Qianpu area; the Na content decreased, the K and Pb content increased, and the K/Na molar ratios ranged from 2.82 to 6.58 (EPMA) and 1.59 to 3.93 (LA-ICP MS) from the early to middle and late hydrothermal stages, indicating that the formation temperatures were more than 200 °C and gradually decreased with hydrothermal evolution. There is a high-sulfidation epithermal system in Qianpu area. In combination with the specific environmental significance of zunyite indicating a high temperature and closeness to the fluid channel, as well as the characteristics and genesis of the Huangzhuyuan silver polymetallic deposit near Qianpu, it is inferred that the Qianpu-Huangzhuyuan area has prospecting potential for porphyry-epithermal deposits.
Alunite
Argillic alteration
Pyrophyllite
Prospecting
Jarosite
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