Altered Volcanic Rocks Research Articles

Statistical Evaluation of Classification Diagrams for Altered Igneous Rocks

The International Union of Geological Sciences (IUGS) has proposed recommendations for the classification of relatively fresh volcanic rocks, but with no specific instructions for altered volcanic rocks, other than discouraging the use of the total alkalis versus silica diagram. The Nb/Y-Zr/TiO2 diagram has been in use for the classification of altered rocks now for over 30 years. Recently (during 2007) another diagram (Co-Th) has been proposed to replace this old diagram, particularly for altered arc rocks. Using an extensive database of all kinds of relatively fresh rocks from four tectonic settings (island arc, continental rift, ocean island, and mid-ocean ridge), as well as from three settings excluding island arc, we carried out an objective evaluation of the old Nb/Y-Zr/TiO2 diagram for rock classification. Similarly, for the evaluation of the new Co-Th diagram, an extensive database of similar rocks from island arcs, the Andean active continental margin, continental rifts, ocean islands, and the Mexican Volcanic Belt, was used. Statistical parameters of correct classification or success rate and minimum misclassification defined in this work, respectively, were used to evaluate these diagrams. Our results of the quantification of these parameters showed that none of these diagrams seems to work precisely for the classification of fresh rocks. It is therefore difficult to imagine that they would work well for the classification of altered rocks. Thus, there is an urgent need to apply correct statistical methodology for handling compositional data in proposing new classification diagrams that could provide classification and nomenclature to altered volcanic rocks fully consistent with the IUGS nomenclature for fresh rocks.

Effect of High Aluminum Concentration in Water Resources on Human Health, Case Study: Biga Peninsula, Northwest Part of Turkey

Widespread and intense zones of silicified, propylitic, and argillic alteration exist as outcrop around the Biga Peninsula, NW Turkey. Most of the springs in the study area surface out from these altered volcanic rocks. The concentrations of aluminum (Al) in these springs ranged from 13.17 to 15.70 ppm in this region. These high levels of Al were found to exceed the maximum allowable limits (0.2 ppm) depicted in national and international standards of drinking water quality. Therefore, the effect of high Al in water resources on human health was evaluated in this research. A total of 273 people aged above 18 years and living in the Kirazli region (whose water supply is from springs emerging from these alteration zones) and in the Ciplak-Halileli region (whose water supply is provided from an alluvium aquifer) were selected as the research group. For this group, a questionnaire was completed that contained questions on descriptive characteristics of humans and a Mini-Mental State Examination (MMSE) was administered by the authors using the face-to-face interview technique. A neurological examination was then performed by the neurology specialist as a second-stage investigation. Finally, 10 ml of venous blood samples were obtained from these people as a third-stage analysis to determine the serum Al levels together with vitamin B(12), folic acid, and thyroid-stimulating hormone parameters. The result typically revealed that the MMSE score was less in 31.9% and there was no statistically significant difference between the two regions. However, the result also showed that neuropathy in the history (including a careful past medical history) was significantly higher in the Kirazli region.

Progressive Formation of Halloysite From the Hydrothermal Alteration of Biotite and the Formation Mechanisms of Anatase in Altered Volcanic Rocks From Limnos Island, Northeast Aegean Sea, Greece

AbstractOccurrences of halloysite-rich material in altered volcanic rocks, principally trachyandesites, dacites, and tuffs, extend over an area of ∼1 km2 in the southwestern part of Limnos, Island, northeast Aegean Sea, Greece. The present study was designed to investigate the alteration processes which acted on the biotite in these volcanic rocks, to describe in detail the mechanism of formation of the halloysite, and to specify the mechanisms of formation of anatase during the alteration processes. Samples were examined using polarized-light microscopy, X-ray powder diffraction, scanning electron microscopy, scanning electron microscopy-energy dispersive spectroscopy, and Fourier-transform-Raman techniques. The extensive alteration of the parent rocks, triggered by the circulation of hydrothermal fluids through faults and fractures, resulted in the alteration of biotite to halloysite. Six stages of alteration were recognized. Nanoparticles of halloysite were initially formed on the mica layers, which progressively grew through short-tubular to well formed tubular halloysite, with increasing alteration. In the most altered samples, laths and interconnected laths with the composition (Al3.96Fe0.04)Si4O10(OH)8, were the dominant halloysite morphologies. Anatase was encountered as an alteration product of both ilmenite and biotite. Ilmenite was altered to anatase and Fe oxides. The altered ilmenite crystals constrained most of the newly formed anatase within the space occupied previously by ilmenite, leading to the formation of skeletal anatase. The layered structure of the micas was the main factor governing the morphology of newly formed anatase developed outside ilmenite margins in the form of layers parallel to those of mica. An unusual ring-like structure of anatase was thought to be the result of the uncommon alteration of inner parts of mica folia to tubular halloysite oriented perpendicular to the mica layers. The detachment of the halloysite tubes by circulating hydrothermal fluids was considered to be the reason for the creation of holes which were subsequently surrounded by the anatase ring forms.

Mineralogy and geochemical affinities of bentonites from Kapıkaya (Eskişehir, western Turkey)

AbstractThere are numerous bentonite deposits, formed by the alteration of volcanic rocks, in the Kapıkaya area (Eskişehir, western Turkey). These deposits can be classified into three groups according to their stratigraphical levels. X-ray diffraction (XRD), scanning electron microscope (SEM), major, rare-earth and trace-element analyses of bentonites and their parent rocks from the Kapıkaya area were used to evaluate the mineralogical and geochemical properties of bentonites and their parental affinities. Mineral assemblages resulting from bentonite deposits consist mostly of clay minerals, gypsum, cristobalite/opal-CT, quartz, feldspar, calcite and dolomite. The clay minerals are represented mainly by dioctahedral smectite and lesser amounts of illite and chlorite. The enrichment and depletion of the elements indicates open-system alteration conditions. The enrichments in MgO, Fe2O3, TiO2, Co, Pb, Zn, and Ni are related to the precipitation of hydrothermal solutions channelled throughout ultramafic sources. The main differences in mineralogy and geochemistry of bentonites from the Kapıkaya area are in the smectite composition and the contents of major, rare-earth and other trace elements. The data obtained show that the types of parent rock the influenced the mineralogical and geochemical compositions of the bentonites.

Pb IN THE PACMANUS SEA-FLOOR HYDROTHERMAL SYSTEM, EASTERN MANUS BASIN: NUMERICAL MODELING OF A MAGMATIC VERSUS LEACHED ORIGIN

Mass balance calculations of metal leaching and deposition in the PACMANUS submarine hydrothermal vent field in the eastern Manus basin in the Bismarck Sea off the east coast of Papua New Guinea indicate that simple leaching of the underlying volcanic pile by hydrothermal fluids cannot account for the Pb found in associated sulfide deposits and Pb-enriched altered volcanic rocks. With 100 percent leaching, transport, and precipitation efficiencies, 1.69 km3 of volcanic rock would need to be leached to account for Pb. Applying realistic leaching efficiencies of 50 to 60 percent and transport-precipitation efficiencies of 10 to 20 percent, 14 to 34 km3 of volcanic rock would need to be leached, assuming a static reaction zone. The estimated available volume of leachable rock is 0.144 km3, much less than required. Applying a dynamic reaction zone model, 98 to 235 volumes of fresh, leachable rock (1 volume = 0.144 km3) would need to be exposed to hydrothermal fluids. This translates into a 588 to 1,410 m migration of the reaction zone. A model incorporating such values would not be tenable. Previously published compositions of melt inclusions in volcanic rocks from PACMANUS indicate an aver-age of 199 ppm Cu, 68 ppm Zn and 5.2 ppm Pb in the melt and 1.3 to 1.7 wt percent of degassed H2O. These data suggest that 37.7 million tonnes (Mt) of magmatic fluid containing 7.2 wt percent Cu and 2.3 wt percent Zn could be released from a 1 km3 body of magma. Using the same metal transfer efficiency of 80 percent, the degassed fluid would have a maximum concentration of 0.17 wt percent Pb, which translates to 58,000 t of Pb from the melt. For the highest known fluid discharge rate of 92,000 g/s and over a 100,000-year period, a concentration of only 12 ppm Pb in the original magmatic component could account for all the Pb in the PACMANUS deposits, assuming that 1 percent of the mixed end-member fluid is of magmatic origin. A wide range of durations of hydrothermal activity and discharge rates, assuming a maximum 0.17 wt percent Pb concentration in the degassed magmatic fluid, can account for the total Pb in the deposit.

Chemical compositions of altered volcanic rocks in the western Pacific seamount, Uyeda Ridge

Chemical compositions of altered volcanic rocks in the western Pacific seamount, Uyeda Ridge

Origin of Salts and Brine Evolution of Bolivian and Chilean Salars

Central Andes in Bolivia and northern Chile contain numerous internal drainage basins occupied by saline lakes and salt crusts (salars). Salts in inflow waters stem from two origins: alteration of volcanic rocks, which produces dilute waters, and brine recycling, which leads to brackish waters. Chilean alteration waters are three times more concentrated in average than Bolivian waters, which is related to a higher sulfur content in Chilean volcanoes. Brackish inflows stem from brines which leak out from present salars and mix with dilute groundwater. Most of the incoming salts are recycled salts. The cycling process is likely to have begun when ancient salars were buried by volcanic eruptions. Three major brine groups are found in Andean salars: alkaline, sulfate-rich, and calcium-rich brines. Evaporation modeling of inflows shows good agreement between predicted and observed brines in Chile. Alkaline salars are completely lacking in Chile, which is accounted for by higher sulfate and lower alkalinity of inflow waters, in turn related to the suspected higher sulfur content in Chilean volcanic rocks. Six Bolivian salars are alkaline, a lower number than that predicted by evaporative modeling. Deposition on the drainage basin of eolian sulfur eroded from native deposits shifts the initial alkaline evolution to sulfate brines. The occurrence of calcium-rich brines in Andean salars is not compatible with volcanic drainage basins, which can only produce alkaline or sulfate-rich weathering waters. The discrepancy is likely due to recycled calcic brines from ancient salars in sedimentary basins, now buried below volcanic formations. Calcic salars are not in equilibrium with their volcanic environment and may slowly change with time to sulfate-rich salars.

Formation of alteration zones and kaolin genesis, Limnos Island, northeast Aegean Sea, Greece

AbstractKaolin deposits extending over an area of ~10 km2in the western and southern parts of Limnos Island, northeast Aegean Sea, Greece, were studied. The kaolin deposits are alteration products of volcanic rocks, mainly trachytes, trachyandesites, andesites and dacites. Study of the collected samples was carried out using X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), energy-dispersive scanning electron microscopy (SEM-EDS), Fourier transform Raman spectroscopy (FT-Raman), Fourier transform infrared (FTIR) techniques and inductively-coupled plasma (ICP) bulk rock chemical analyses for major, trace and rare earth elements. The extensive alteration of the parent rocks resulted from the circulation of hydrothermal fluids through faults and fractures. The development of the various assemblages depends not only on the temperature and composition of the hydrothermal fluids but also on the distance of the rock from the fault or the channel of the ascending hydrothermal fluids.Kaolinite, dickite, halloysite, illite, smectite and mixed-layer illite-smectite and jarosite were detected in the altered volcanic rocks forming locally various alteration zones. Smectite-rich zones and illite-rich zones are relatively infrequent. In the halloysite-rich zones, the kaolinization of feldspars is accomplished in four stages. The kaolinizaton of feldspars in the kaolinite-dickite-rich zones follows five discrete stages. In the less altered rocks, thin layers of kaolinite are formed on the surface of feldspars. With increasing kaolinization, kaolinite is developed on the surface of feldspars forming extended parallel booklets of newly formed kaolinite. In the third stage, feldspar crystals are partially altered to kaolinite booklets. As kaolinization advances, kaolinite becomes well formed and, in the most altered rocks, feldspars are partially or completely altered to dickite books, depending on the temperature of the hydrothermal fluids.

Exploration guidelines for copper-rich iron oxide–copper–gold deposits in the Mantoverde area, northern Chile: the integration of host-rock molar element ratios and oxygen isotope compositions

The Mantoverde area of the III Región of northern Chile hosts numerous copper(–gold) deposits and prospects assigned to the iron oxide–copper–gold (IOCG) clan. These range from sulphide-poor magnetite–apatite–actinolite bodies (e.g. Carmen) to chalcopyrite-rich, hematite-cemented breccias and veins (e.g. Mantoverde, Cerro Negro and Palmira). The most important Cu-rich deposits are associated with the main or, more commonly, subsidiary structures of the plate-boundary-parallel Atacama Fault System, and are hosted by Middle to Upper Jurassic andesites and Lower Cretaceous dioritic to quartz dioritic plutons. The rocks in the wider Mantoverde area exhibit a complex evolution involving early sub-seafloor albitization (stage R1), subsequent, very low-grade diastathermal metamorphism (R2), and more local stage I K and Fe metasomatism and stage II hydrolysis, all preceding the emplacement of sulphide-bearing hematitic breccias and veins (i.e. ore-stage III). We develop a lithogeochemical exploration protocol specifically for Cu-rich IOCG-type deposits hosted by calc-alkaline volcano-plutonic terrains, based on the integration of whole-rock molar element ratios and oxygen isotope chemistry. Zirconium, the most conserved (least mobile) element in the volcanic and plutonic rocks of the area, was used as a common denominator for molar element ratios. The molar ratios Na/Zr, K/Zr and Al/Zr indicate that the host rocks proximal to the major deposits were affected by sodium depletion and potassium enrichment. The molar ratio (14Ca+19Na−14CO 2 )/(6Si+Al+2Fe+2Mg), recalculated to a percentage scale, constitutes a modified alteration index which can be used to quantify the degree of hydrothermal alteration in the host rocks. However, although this index differentiates barren from potentially prospective sectors (i.e. strongly altered host rocks), it does not identify the hydrolytically altered rocks that are closely associated with economic copper mineralization. The δ 18 O values of igneous rocks in the Mantoverde area vary widely from +4.2 to +14.1‰. Integrating the modified lithogeochemical alteration index with whole-rock δ 18 O values differentiates the least altered volcanic and plutonic rocks (i.e. alteration index <30%), with δ 18 O values close to +7‰, from moderately altered volcanic rocks (alteration index of 30 to 65%) having higher δ 18 O values of +10 to +13.2‰ due to enrichment in 18 O during regional metasomatism hydrothermal alteration sub-seafloor related to albitization and metamorphism. A third group comprises strongly K-feldspathized, Fe-metasomatized and chloritized rocks localized in Cu-mineralized centres which have alteration indexes exceeding 65% and δ 18 O values of +4.2 to +14.1‰. The δ 18 O values of paragenetically related minerals indicate that barren, K- and Fe-metasomatized host rocks equilibrated with magmatically derived fluids at temperatures exceeding 420°C. Although no recognizable alteration haloes are associated with the ensuing lower-temperature Cu-(Au) ore stage (III), the genetically related hydrolytic alteration (II) which preceded it generated distinctive alteration indices and Na/Al and K/Al ratios, as well as δ 18 O values averaging +10‰, with chlorite O and H isotope compositions indicating equilibration with non-magmatic fluids. We propose, in combination, that these parameters can be used to vector onto Cu-rich IOCG-type mineralization in Andean, calc-alkaline terrains.

Mineralogy and Isotope Geochemistry of Active Submarine Hydrothermal Field at Suiyo Seamount, Izu–Bonin Arc, West Pacific Ocean

AbstractThis report presents mineralogical, geochemical and isotopic data on samples obtained using the Benthic Multi‐coring System (BMS) to drill a submarine hydrothermal deposit developed in a caldera on the summit of the Suiyo Seamount in the Izu–Bonin Island Arc, south of Japan. This deposit is regarded as the first example of Kuroko‐type sulfide mineralization on a volcano at the volcanic front of an island arc. The mineralization and hydrothermal alteration below the 300 × 150‐m area of active venting was investigated to depths of 2–9 m below the sea floor. Drilling beneath the area of active venting recovered a sequence of altered volcanic rocks (dacite lavas, pyroclastic rocks of dacite–rhyolite compositions and pumice) associated with sulfide veining and patches/veins of anhydrite. No massive sulfide was found, however, and the subsea‐floor mineralization to 10 m depth is dominated by anhydrite and clay minerals with some sulfides. Sulfide‐bearing samples contained high Au (up to 42 ppm), Ag (up to 263 ppm), As (up to 1550 ppm), Hg (up to 55 ppm), Sb (up to 772 ppm), and Se (up to 24 ppm). Electron probe microanalyzer indicated that realgar, orpiment, and mimetite were major As‐bearing minerals. The sulfides were also characterized by high Zn (>10%) compared to Cu (<6.3%) and Pb (<0.6%). The δ202Hg/198Hg, δ202Hg/199Hg and δ202Hg/200Hg of the sulfide‐bearing dacite samples and a sulfide chimney decreased with increasing Hg/Zn concentration ratio. The variation of the δ202Hg/198Hg ranged from −2.8 to +0.5‰ to relative to S‐HG02027. The large range of these δ202Hg/198Hg was greater than might be expected for such a heavy element and may be due to a predominance of kinetic effects. The variation of δ202Hg/198Hg of sulfide‐bearing dacite samples suggested that light Hg isotope in the vapor mixed with oxygenated seawater near sea floor during mineralization. Lead isotope ratios of the sulfide were very similar to those of the dacite lava, suggesting that lead is of magmatic origin. The 87Sr/86Sr ratio (0.70872) of anhydrite was different from that of the dacite lava, and suggests an Sr derivation predominantly from seawater. Hydrothermal alteration of the dacite in the Suiyo hydrothermal field was characterized by Fe‐sulfides, anhydrite, barite, montmorillonite, chlorite/montmorillonite mixed‐layer minerals, mica, and chlorite with little or no feldspar or cristobalite. Hydrothermal clay minerals changed with depth from montmorillonite to chlorite/montmorillonite mixed‐layer minerals to chlorite and mica. Hydrogen isotope ratios of chlorite/montmorillonite and mixed‐layer, mica‐chlorite composites obtained below the active venting sites ranged from −49 to −24‰, suggesting seawater as the dominant fluid causing alteration. Oxygen isotope ratios of anhydrite ranged from 9.2 to 10.4‰ and anhydrite formation temperatures were calculated to be 188–207°C. Oxygen isotope ratios ranged from +5.2 to +9.2‰ for montmorillonite, +3.2 to +4.5‰ for chlorite/montmorillonite mixed‐layer minerals, and +2.8 to +3.8‰ in mixtures of chlorite and mica. The formation temperatures of montmorillonite and of the chlorite–mica mixture were 160–250°C and 230–270°C, respectively. The isotope temperatures for clay minerals (220–270°C) and anhydrite (188°C) were significantly lower than the borehole temperature (308.3°C) measured just after the drilling, suggesting that temperature at this site is now higher than when clay minerals and anhydrite were formed.

Variations in Chemical Composition of Clay Minerals and Magnetic Susceptibility of Hydrothermally Altered Rocks in the Hishikari Epithermal Gold Deposit, SW Kyushu, Japan

AbstractHydrothermal alteration, involving chiefly chlorite and illite, is extensively distributed within host rocks of the Pleistocene Hishikari Lower Andesites (HLA) and the Cretaceous Shimanto Supergroup (SSG) in the underground mining area of the Hishikari epithermal gold deposit, Kagoshima, Japan. Approximately 60% of the mineable auriferous quartz‐adularia veins in the Honko vein system occur in sedimentary rocks of the SSG, whereas all the veins of the Yamada vein system occur in volcanic rocks of the HLA. Variations in the abundance and chemical composition of hydrothermal minerals and magnetic susceptibility of the hydrothermally altered rocks of the HLA and SSG were analyzed. In volcanic rocks of the HLA, hydrothermal minerals such as quartz, chlorite, adularia, illite, and pyrite replaced primary minerals. The amount of hydrothermal minerals in the volcanic rocks including chlorite, adularia, illite, and pyrite as well as the altered and/or replaced pyroxenes and plagioclase phenocrysts increases toward the veins in the Honko vein system. The vein‐centered variation in mineral assemblage is pronounced within up to 25 m from the veins in the peripheral area of the Honko vein system, whereas it is not as apparent in the Yamada vein system. The hydrothermal minerals in sandstone of the SSG occur mainly as seams less than a few millimeters thick and are sporadically observed in halos along the veins and/or the seams. The alteration halos in sandstone of the SSG are restricted to within 1 m of the veins. In the peripheral area of the Honko vein system, chlorite in volcanic rocks is characterized by increasing in Al in its tetrahedral layer and the Fe/Fe + Mg ratio toward the veins, while illite in volcanic rocks has relatively low K and a restricted range of Fe/Fe + Mg ratios. Temperature estimates derived from chlorite geothermometry rise toward the veins within the volcanic rocks. The magnetic susceptibility of tuff breccia of the HLA varies from 21 to less than 0.01 × 10−3 SI within a span of 40 m from the veins and has significant variation relative to that of andesite (27–0.06 × 10−3 SI). The variation peripheral to the Honko vein system correlates with an increase in the abundance of hematite pseudomorphs after magnetite, the percentage of adularia and chlorite with high Fe/Fe + Mg ratios, and the degree of plagioclase alteration with decreasing distance to the veins. In contrast, sedimentary rocks of the SSG maintain a consistent magnetic susceptibility across the alteration zone, within a narrow range from 0.3 to 0.2 × 10−3 SI. Magnetic susceptibility of volcanic rocks of the HLA, especially tuff breccia, could serve as an effective exploration tool for identifying altered volcanic rocks.

Sulfur isotope evidence for microbial sulfate reduction in altered oceanic basalts at ODP Site 801

The subsurface biosphere in the basaltic ocean crust is potentially of major importance in affecting chemical exchange between the ocean and lithosphere. Alteration of the oceanic crust commonly yields secondary pyrite that is depleted in 34S relative to igneous sulfides. Although these 34S depleted sulfur isotope ratios may point to signatures of biological fractionation, previous interpretations of sulfur isotope fractionation in altered volcanic rocks have relied on abiotic fractionation processes between intermediate sulfur species formed during basalt alteration. Here, we report results for multiple S-isotope (32S, 33S, 34S) compositions of altered basalts at ODP Site 801 in the western Pacific and provide evidence for microbial sulfate reduction within the volcanic oceanic crust. In-situ ion-microprobe analyses of secondary pyrite in basement rocks show a large range of δ34S values, between −45‰ and 1‰, whereas bulk rock δ34S analyses yield a more restricted range of −15.8 to 0.9‰. These low and variable δ34S values, together with bulk rock S concentrations ranging from 0.02% up to 1.28% are consistent with loss of magmatic primary mono-sulfide and addition of secondary sulfide via microbial sulfate reduction. High-precision multiple sulfur-isotope (32S/33S/34S) analyses suggest that secondary sulfides exhibit mass-dependent equilibrium fractionation relative to seawater sulfate in both δ33S and δ34S values. These relationships are explained by bacterial sulfate reduction proceeding at very low metabolic rates. The determination of the S-isotope composition of bulk altered oceanic crust demonstrates that S-based metabolic activity of subsurface life in oceanic basalt is widespread, and can affect the global S budget at the crust–seawater interface.

Indikasi mineralisasi epitermal emas bersulfi da rendah, di Wilayah Kecamatan Bonjol, Kabupaten Pasaman, Sumatera Barat

http://dx.doi.org/10.17014/ijog.vol2no1.20075Bonjol gold prospect, known as Old Dutch Gold mine, consists of several ore bodies (Malintang, Balimbing, Lubang Sempit, Lubang Belanda and Lubang Perak). The deposit hosts within the altered volcanic rocks known as Gunung Amas Formation of Early Miocene age (9.3 ± 0.4 - 11.9 ±1.0 Ma). This formation consists of various rock types such as rhyolitic tuff, volcanic breccia, dacitic tuffs and rhyolites. These rocks are moderate to strongly alter. Mineralogy of the deposit consists of gold and silver with minor pyrite, sphalerite and galena. Besides this, hematite, jarosite and manganese are also present as supergene minerals. Ore minerals are found within quartz veins ranging from few centimetres to tens of metres thick. The veins are characterized by crustiform, comb, vuggy, botroyidal, layering and bladed. Quartz is a dominant mineral as hydrothermal alteration in addition to illite, dickite, monmorillonite, kaolinite, chlorite, smectite, natrolite, nontronite, calcite, halloysite, palygorskite, muscovite, sepiolite, analcime, heulandite, clino-chlor, zircon, zoisite, laumontite, alunite, biotite and erionite. The presence of these secondary minerals could be classifi ed into prophylitic, argillic and advanced argillic types. Analytical result of gold–bearing quartz vein indicates higher content of gold (0.3%) and silver (400 ppm). In contrast, the content of sulphide minerals (Cu, Pb, and Zn) is very low (< 100 ppm). Combined geology, mineralogy, textures and alteration minerals, it is concluded that gold deposit in the area shows an indication of a low sulphidation epithermal type within Gunung Amas Formation.

Hydrothermal system of the Papandayan Volcano, West Java, Indonesia and its geochemistry evolution of thermal water after the November 2002 eruption

http://dx.doi.org/10.17014/ijog.vol2no1.20072Papandayan is a strato volcano situated in West Java, Indonesia. After the last magmatic eruptionin 1772, only few phreatic explosions have been occurring. At the present time, the activity is centeredin the northeast crater manifested by the presence of fumaroles and hot springs. In November 2002an explosive eruption occurred and ejected ash and altered rocks. Study of the altered rocks revealedthat an advanced argillic alteration took place in the hydrothermal system by an interaction betweenacid fl uids and rocks. Four zones of alteration have been formed as a limited extension along faults oracross permeable structures at different levels beneath the active crater of the volcano.Two types of acid fl uids are distinguished in the crater of the Papandayan Volcano: (1) acidsulphate-chloride water with pH values between 1.6 and 4.6, and (2) acid sulphate water with pHvalues between 1.2 and 2.5. The samples collected after the eruption revealed an increase in the SO4/Cl and Mg / Cl ratios. This evolution is likely explained by an increase in the neutralization of acidfl uids which tends to show that water-rock interactions were more signifi cant after the eruption. Thechanges in chemistry observed in 2003 were the consequence of the opening of new fractures whereunaltered or less altered volcanic rocks were in contact with the ascending acid water. The high δ34Svalues (9-17‰) observed in the acid sulphate-chloride water before the November 2002 eruptionsuggest that dissolved sulphates were mainly formed by the disproportionation of magmatic SO2. Onthe other hand, the low δ34S values (-0.3-7 ‰) observed in acid sulphate-chloride water sampled afterthe eruption suggest that the origin of dissolved sulphates for these waters is the surfi cial oxidation ofhydrogen sulphide.

Low-sulfidation type Au–Ag mineralization at Bergama, Izmir, Turkey

Bergama, the center of Bergama County, is located in western Turkey and includes the villages of Ovacik, Narlica and Saganci. The Ovacik epithermal gold–silver deposit is located in the Western Anatolian Volcanic and Extensional Province, adjacent to the ENE-trending Bergama graben, some 100 km north of the city of Izmir. Gold of economic grades at the Ovacik deposit (reserves 4.19 Mt at 7.6 g/t) occurs in epithermal quartz veins which display low-temperature epithermal textures, including crustiform banding, quartz pseudomorphs after bladed calcite, and multiphase hydrothermal breccias. Alteration minerals at both Ovacik and Narlica are dominated by smectite, mixed-layer illite/smectite, chalcedonic quartz and adularia, whereas major kaolinite and minor mixed-layer smectite/illite (> 13.4 Å) occur at Saganci. The total sulfide content at Ovacik is low (< 2%) and is dominated by pyrite with traces of electrum, chalcopyrite, arsenopyrite, acanthite, tetrahedrite, pyrargyrite, stibnite, galena, chalcocite, bornite, covellite and sphalerite, occurring mainly within breccia clasts. Pyrite and marcasite appear to be the most common opaque minerals at Narlica and form dark sulfide-rich bands along with traces of electrum, native silver and chalcopyrite; pyrite is the only sulfide identified at Saganci. 40Ar/ 39Ar dating of adularia from gold-bearing quartz veins indicates an age of mineralization of about 18.2 ± 0.2 Ma. Fluid inclusion studies at Ovacik reveal that main-stage quartz contains predominantly liquid-rich inclusions with homogenization temperatures ( T h) ranging from 150 to 305 °C, with the majority of T h varying between 165 to 205 °C: ice-melting temperatures ( T m) ranging from − 0.4 to − 1.2 °C (salinity < 2 wt.% NaCl equiv.) are dominant. Higher T h (220 to 248 °C) at the Narlica deposit may be attributed to the deeper level of exposure. Geochemical variations in altered wall rocks at Ovacik and Narlica are generally characterized by two-fold enrichments in K, Rb, Cs and 25 to 93% depletions in Sr, Ca, Mg, Na and even higher depletions (96 to 99% decrease) in the quartz–adularia vein zone. Lanthanum, Ce, Pr, Hf, Zr, Sm, Eu, Gd, Tb and Ho also exhibit up to 50% depletions in the wall rock and even more so (up to 90% depletion for La and Nd) in the vein structure. Wall rock enrichment in Au, Ag, As, Hg and Sc is by factors of 60, 150, 88, 8 and 3, respectively. The ranges of REE in both the altered volcanic rocks and quartz–adularia veins are wide and may reflect either significant mobilization of REE during alteration and mineralization or their dilution by the metasomatic processes from fresh volcanic rocks through montmorillonite–illite–adularia-altered wallrock to quartz–adularia veins. High Rb/Sr ratios in adularia–illite-altered areas are closely related to the presence of K-rich alteration minerals. Low Rb/Sr ratios and corresponding low K values at Saganci are due to acid leaching of volcanic rocks. Positive correlation coefficients between Au and Ag, Pb, Zn, Cd, Cu and Sb in epithermal quartz veins, all of which are > 0.51, are significant. Silver and Sb are remarkably enriched at higher levels of the deposit. No correlation occurs between Au and As or Ag and As. The Ovacik quartz in Au–Ag-bearing veins has δ 18O values ranging from + 9.5 to + 15.7‰, whereas δD values of fluid inclusions in quartz range from − 89 to − 125‰. However, the Narlica quartz in Au–Ag bearing veins returned δ 18O values ranging from + 5.9‰ to + 8.3, whereas the δD values of fluid inclusion in quartz range from − 82 to − 99‰. δ 18O results indicate that ore-forming hydrothermal fluids at Ovacik and Narlica had δ 18O H2O values ranging from − 2.9 to + 3.5‰ (average: − 0.24‰) and from − 2.96 to − 0.6 (average − 1.6), respectively. They are thus 18O-enriched in comparison with present-day meteoric and hydrothermal meteoric waters (− 5.4‰). δ 18O, δ 18O H2O and δD values suggest that mineralizing solutions were a mixture of meteoric and magmatic waters. The δ 34S Σsulfide data at Ovacik and Narlica range from − 2.1 to 5.3‰ (average: + 1.7) and from − 4.6 to + 2.7‰ (average: − 0.36), respectively. These δ 34S Σsulfide values are consistent with a magmatic source for S.

The Nature of Zeolite Occurrences in Deposits of the Olduvai Basin, Northern Tanzania

AbstractThe Plio-Pleistocene deposits of the Olduvai Basin in northern Tanzania consist of a sequence of lacustrine and fluvial sediments. They contain various amounts of zeolite minerals, the formation of which is related to an interaction of volcanic material or detrital clays with saline alkaline lake water and groundwater. Petrographic characteristics of zeolite occurrences provide information about their conditions of formation. They were studied for all four main stratigraphical units that are recognized at Olduvai (Beds I to IV), sampled in the southeastern part of the basin. In the lake-margin deposits of Bed I and the lower part of Bed II, chabazite is the dominant zeolite mineral accompanied by phillipsite and minor amounts of erionite and clinoptilolite. Chabazite commonly occurs as part of altered volcanic rock fragments, characterized by partial or complete dissolution of volcanic glass and the formation of chabazite inside vesicles, following the development of thin smectite coatings. It also formed within the sediment matrix, requiring extended periods of impregnation of the deposits by saline alkaline solutions. Chabazite also occurs extensively as coatings and infillings of pores, developed during periods of subaerial exposure which were characterized by high groundwater levels. Phillipsite formed at a later stage, from more evolved solutions, with higher K/Na ratios than during chabazite formation. The fluvial deposits of Bed IV, Bed III and the upper part of Bed II have a high analcime content. They also contain various amounts of chabazite, phillipsite and natrolite. All zeolite minerals mainly occur in pores. The predominance of analcime indicates a higher salinity and alkalinity than during the preceding period with sedimentation and diagenesis in a lake margin environment. Early development of zeolite occurrences, shortly after the deposits became exposed during breaks in sedimentation, is recorded for some intervals, where zeolites are covered by illuvial clay coatings or by sparitic carbonate cement. In most intervals, however, zeolites mainly formed at a later stage.

Tracing fluid pathways in fossil hydrothermal systems with near-infrared spectroscopy

Research Article| July 01, 2005 Tracing fluid pathways in fossil hydrothermal systems with near-infrared spectroscopy Frank J.A. van Ruitenbeek; Frank J.A. van Ruitenbeek 1Department of Earth Systems Analysis, International Institute of Geo-information Science and Earth Observation (ITC), PO Box 6, 7500 AA, Enschede, the Netherlands Search for other works by this author on: GSW Google Scholar Thomas Cudahy; Thomas Cudahy 2Division of Exploration and Mining, Commonwealth Scientific and Industrial Research Organisation (CSIRO), PO Box 1130, Perth, WA 6151, Australia Search for other works by this author on: GSW Google Scholar Martin Hale; Martin Hale 3Department of Earth Systems Analysis, International Institute of Geo-information Science and Earth Observation (ITC), PO Box 6, 7500 AA, Enschede, the Netherlands Search for other works by this author on: GSW Google Scholar Freek D. van der Meer Freek D. van der Meer 3Department of Earth Systems Analysis, International Institute of Geo-information Science and Earth Observation (ITC), PO Box 6, 7500 AA, Enschede, the Netherlands Search for other works by this author on: GSW Google Scholar Geology (2005) 33 (7): 597–600. https://doi.org/10.1130/G21375.1 Article history received: 30 Nov 2004 rev-recd: 04 Mar 2005 accepted: 10 Mar 2005 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Frank J.A. van Ruitenbeek, Thomas Cudahy, Martin Hale, Freek D. van der Meer; Tracing fluid pathways in fossil hydrothermal systems with near-infrared spectroscopy. Geology 2005;; 33 (7): 597–600. doi: https://doi.org/10.1130/G21375.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We present a new method to detect and reconstruct fluid pathways in fossil hydrothermal systems that is based on systematic study of white micas in hydrothermally altered rock, using near-infrared spectroscopy. This method, developed in an Early Archean volcanic sequence in the Panorama district in Western Australia, uses new near-infrared spectroscopic data and published geological and geochemical data of fossil submarine hydrothermal systems in the area. Analysis of new near-infrared spectroscopic data revealed that the abundance of white mica and its Al content in altered volcanic rock vary systematically along fossil fluid pathways, from zones of low-temperature recharge to zones of high-temperature discharge, as a function of hydrothermal fluid chemistry, temperature of alteration, coexisting minerals, and composition of volcanic host rock. The abundance of white mica relative to that of chlorite and the Al content of white micas can be used to discriminate among the hydrothermal alteration facies along these fluid pathways, which permits detection and reconstruction of fossil fluid pathways. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Supergene destruction of a hydrothermal replacement alunite deposit at Big Rock Candy Mountain, Utah: mineralogy, spectroscopic remote sensing, stable-isotope, and argon-age evidences

Big Rock Candy Mountain is a prominent center of variegated altered volcanic rocks in west-central Utah. It consists of the eroded remnants of a hypogene alunite deposit that, at ∼21 Ma, replaced intermediate-composition lava flows. The alunite formed in steam-heated conditions above the upwelling limb of a convection cell that was one of at least six spaced at 3- to 4-km intervals around the margin of a monzonite stock. Big Rock Candy Mountain is horizontally zoned outward from an alunite core to respective kaolinite, dickite, and propylite envelopes. The altered rocks are also vertically zoned from a lower pyrite–propylite assemblage upward through assemblages successively dominated by hypogene alunite, jarosite, and hematite, to a flooded silica cap. This hydrothermal assemblage is undergoing natural destruction in a steep canyon downcut by the Sevier River in Marysvale Canyon. Integrated geological, mineralogical, spectroscopic remote sensing using AVIRIS data, Ar radiometric, and stable isotopic studies trace the hypogene origin and supergene destruction of the deposit and permit distinction of primary (hydrothermal) and secondary (weathering) processes. This destruction has led to the formation of widespread supergene gypsum in cross-cutting fractures and as surficial crusts, and to natrojarosite, that gives the mountain its buff coloration along ridges facing the canyon. A small spring, Lemonade Spring, with a pH of 2.6 and containing Ca, Mg, Si, Al, Fe, Mn, Cl, and SO 4, also occurs near the bottom of the canyon. The 40Ar/ 39Ar age (21.32±0.07 Ma) of the alunite is similar to that for other replacement alunites at Marysvale. However, the age spectrum contains evidence of a 6.6-Ma thermal event that can be related to the tectonic activity responsible for the uplift that led to the downcutting of Big Rock Candy Mountain by the Sevier River. This ∼6.6 Ma event also is present in the age spectrum of supergene natrojarosite forming today, and probably dates the beginning of supergene alteration at Big Rock Candy Mountain. The δ 34S value (11.9‰) of alunite is similar to those for replacement alunite from other deposits in the Marysvale volcanic field. The δ 34S values of natrojarosite (0.7‰ to −1.2‰) are similar to those for aqueous sulfate in Lemonade Spring, but are larger than those in pyrite (0.4‰ to −4.7‰). The δ 34S and δ 18O SO 4 values of gypsum show an excellent correlation, with values ranging from 15.2‰ to −5.2‰ and 7‰ to −8.2‰, respectively. The stable-isotope data indicate that the aqueous sulfate for gypsum is a mixture derived from the dissolution of hypogene gypsum and alunite, and from the supergene oxidation of pyrite. The aqueous sulfate for the natrojarosite, however, is derived largely from the supergene oxidation of pyrite, with a minor contribution from the dissolution of alunite and gypsum. The exceptional detailed spectral mapping capabilities of AVIRIS led to the recognition of a small amount of jarosite that is probably the top of the steam-heated system that produced the primary hypogene alteration at Big Rock Candy Mountain.

Evaluation of Early Archean Volcaniclastic and Volcanic Flow Rocks as Possible Sites for Carbonaceous Fossil Microbes

Sedimentary rocks have traditionally been the focus of the search for Archean microfossils; the Earth's oldest fossil bacteria are associated with carbonaceous matter in sedimentary cherts in greenstone belts in the eastern Pilbara block of Western Australia and Barberton greenstone belt of South Africa. Reports of possible fossils in a martian meteorite composed of igneous rock and the discovery of modern bacteria associated with basalts have stimulated a new look at Archean volcanic rocks as possible sites for fossil microbes. This study examines silicified volcaniclastic rocks, near-surface altered volcanic flow rocks, and associated stromatolite- like structures from the Archean Barberton greenstone belt to evaluate their potential for the preservation of carbonaceous fossils. Detrital carbonaceous particles are widely admixed with current-deposited debris. Carbonaceous matter is also present in altered volcanic flow rocks as sparse particles in silica veins that appear to be fed by overlying carbonaceous chert layers. Neither microfossils nor mat-like material was identified in the altered volcanic rocks or adjacent stromatolite-like structures. Ancient volcanic flow and volcaniclastic rocks are not promising sites for carbonaceous fossil preservation.

Origin of secondary sulfate minerals on active andesitic stratovolcanoes

Sulfate minerals in altered rocks on the upper flanks and summits of active andesitic stratovolcanoes result from multiple processes. The origin of these sulfates at five active volcanoes, Citlaltépetl (Mexico), and Mount Adams, Hood, Rainier, and Shasta (Cascade Range, USA), was investigated using field observations, petrography, mineralogy, chemical modeling, and stable-isotope data. The four general groups of sulfate minerals identified are: (1) alunite group, (2) jarosite group, (3) readily soluble Fe- and Al-hydroxysulfates, and (4) simple alkaline-earth sulfates such as anhydrite, gypsum, and barite. Generalized assemblages of spatially associated secondary minerals were recognized: (1) alunite+silica±pyrite±kaolinite±gypsum±sulfur, (2) jarosite+alunite+silica; (3) jarosite+smectite+silica±pyrite, (4) Fe- and Al-hydroxysulfates+silica, and (5) simple sulfates+silica±Al-hydroxysulfates±alunite. Isotopic data verify that all sulfate and sulfide minerals and their associated alteration assemblages result largely from the introduction of sulfur -bearing magmatic gases into meteoric water in the upper levels of the volcanoes. The sulfur and oxygen isotopic data for all minerals indicate the general mixing of aqueous sulfate derived from deep (largely disproportionation of SO 2 in magmatic vapor) and shallow (oxidation of pyrite or H 2S) sources. The hydrogen and oxygen isotopic data of alunite indicate the mixing of magmatic and meteoric fluids. Some alunite-group minerals, along with kaolinite, formed from sulfuric acid created by the disproportionation of SO 2 in a condensing magmatic vapor. Such alunite, observed only in those volcanoes whose interiors are exposed by erosion or edifice collapse, may have δ 34S values that reflect equilibrium (350±50 °C) between aqueous sulfate and H 2S. Alunite with δ 34S values indicating disequilibrium between parent aqueous sulfate and H 2S may form from aqueous sulfate created in higher level low-temperature environments in which SO 2 is scrubbed out by groundwater or where H 2S is oxidized. Jarosite-group minerals associated with smectite in only slightly altered volcanic rock are formed largely from aqueous sulfate derived from supergene oxidation of hydrothermal pyrite above the water table. Soluble Al- and Fe-hydroxysulfates form in low-pH surface environments, especially around fumaroles, and from the oxidation of hydrothermal pyrite. Anhydrite/gypsum, often associated with native sulfur and occasionally with small amounts of barite, also commonly form around fumaroles. Some occurrences of anhydrite/gypsum may be secondary, derived from the dissolution and reprecipitation of soluble sulfate. Edifice collapse may also reveal deep veins of anhydrite/gypsum±barite that formed from the mixing of saline fluids with magmatic sulfate and dilute meteoric water. Alteration along structures associated with both hydrothermal and supergene sulfates, as well as the position of paleo-water tables, may be important factors in edifice collapse and resulting debris flows at some volcanoes.