Hydrothermal Alteration Processes Research Articles

OPAQUE MINERAL CONTENT OF DUTLUCA VOLCANICS (BURHANİYE - BALIKESİR): THE EFFECT OF HYDROTHERMAL ALTERATION ON THESE MINERALS

Dutluca volcanics, which are known as Hallaclar Formation in regional scale in the study area (Kurshens- ky, 1976), are composed of hydrothermally altered andesite and basaltic andesite. In these rocks, sulfidic minerals such as pyrite, enargite and chalcosine, and oxide and hydroxide minerals such as magnetite, hematite and goethite were detected as opaque minerals. The presence of enargite in opaque mineral para- genesis, and the changes observed in structures and textures of opaque and silicate minerals indicate that examined volcanics have been altered by highly sulfidic hydrothermal solutions. During the hydrothermal alteration process, which indicates at least in two phases, a diffuse pyritization rich in H S in reducing conditions and enargite mineral, which is known as pathfinder minerals in such processes, formed in the first phase. Later on; the extensive martitization developed in oxidizing conditions.

In-situoxygen isotope records of crustal self-cannibalization selectively captured by zircon crystals from high-δ26Mg granitoids

Zircon oxygen-hafnium and biotite magnesium isotopic compositions of Late Cretaceous to Oligocene (88–27 Ma) granitoids from the Gyeongsang continental arc, southeastern Korea, collectively provide a new insight into shallow magma petrogenesis. Zircon crystals extracted from eight calc-alkaline or alkaline plutons commonly contain xenocrystic cores and typically show magmatic cathodoluminescence zonings and sharp Raman spectra. Most zircons have positive eHf values plotting below the mid-ocean-ridge basalt evolution path, indicating that the arc magmatism was dominated by the recycling of juvenile materials. Zircon δ18O values shifted upward or downward from the mantle range attest to the O isotopic exchange of protoliths with surface waters at various temperature conditions. Small (<1‰) but discernable core-to-rim decreases of δ18O values are recognized selectively in magmatic zircon grains from four plutons possessing biotite δ26Mg values (–0.07‰ to +2.32‰) distinctly higher than the mantle range. Such a concomitant O-Mg isotopic variation provides compelling evidence for a series of self-induced hydrothermal alteration and assimilation processes (i.e., crustal self-cannibalization). This study shows that the supracrustal input to magmas in a young and juvenile orogen can be traced effectively by the combination of stable isotope records from the plutonic root.

Geochemical bias in drill cutting samples versus drill core samples returned from the Reykjanes Geothermal System, Iceland

The wholerock major and trace element composition of drill cutting samples are compared to drill core samples from adjacent depths in the seawater recharged Reykjanes geothermal system in Iceland. The first appearance of alteration minerals and lithologies in drill cutting samples is a useful tool for interpreting broad subsurface characteristics. However, use of drill cutting samples for determining igneous affinity and elemental exchanges during hydrothermal alteration is problematic. Samples recovered from immediately above and below the cored intervals in wells RN-17B and RN-30 demonstrate that drill-cutting samples are biased towards preservation of least altered primary igneous minerals and more resistant alteration minerals, including albite, quartz, and epidote, with preferential loss of finer-grained and less resistant minerals including chlorite and actinolite. This selective recovery obscures elemental exchanges resulting from hydrothermal alteration processes. For some elements, compositional variations (enrichments and depletions) measured from 9.5m of core exceeds that observed in ∼3000m of cutting analyses. Concentration ratios of hydrothermally immobile elements including Zr, Nb, V, Y, HREE, Hf, Ta and Th in deep (>2245m) spot drill core samples record bimodal, trace element-enriched and trace element-depleted precursor compositions similar to subaerial Reykjanes Peninsula basalts. The same elements in nearly 3000m of drill cutting samples from well RN-17 overwhelmingly reflect the more common trace element-enriched igneous precursor, demonstrating that mixing of drill cutting samples obscures details of their igneous affinity. A new and different drill rig was used to deepen well RN-17 below 2266m in a sidetrack hole (RN-17ST), which resulted in a change in drilling conditions, accompanied with an increased well deviation angle from ∼0° to ∼4°. Wholerock geochemical results for drill cutting samples from RN-17ST are homogenous for virtually every element; suggesting the change in drilling conditions resulted in extreme mixing of the drill cuttings. Anomalously high concentrations of Cu, Ni, Cr and Ta in some drill cutting samples likely reflects contamination of drill cutting samples by metal alloys used in drill bits and drill collars or more resistant spinel and sulfide phases.

Investigation of physical deterioration of Malayer granitic rocks using a new weathering coefficient (Kr4)

The main aim of this research is to find the most appropriate weathering indices among geomechanical, chemical and petrographical indices to characterize Malayer granitic rocks. In the study area, physical weathering and hydrothermal alteration are predominant degradation phenomena. Therefore, it is necessary to distinguish between the effect of these two phenomena and chemical weathering on mineralogy as well as on geomechanical rock properties. The most important evidences of hydrothermal alteration are the selective sericitization of plagioclase and the occurrence of prehnite. Also, statistical analyses, including simple and multiple linear regressions, were applied to identify those indices that are most influenced on the progression of weathering. Based on the simple regression analysis, petrographical and chemical indices do not show any good trend by increasing weathering degrees (R 2 < 34). Although, There are some exceptions in petrographical and chemical indices such as microfracture index (MI: R 2 = 0.68), unsound minerals (UM: R 2 = 0.67), micropetrographic index (Ip: R 2 = 0.59), and Loss on ignition (LOI: R 2 = 0.75) which are sensitive to cold climate and subtle geochemical changes due to hydrothermal alterations. Also, all geomechanical parameters are good quantitative indicators for describing the degree of weathering (R 2 > 0.72). Finally, a new quantitative weathering index (Kr4) has been developed regarding quick absorption index, P-wave velocity (Vp), point load index (Is) and total porosity (n). All the mentioned parameters can be quickly and inexpensively determined by portable laboratory instruments. This new index (Kr4) has strongly correlated with weathering degrees (R 2 = 0.96). Based on some results, it seems that Kr4 could be more suitable and credible to numerically evaluate the physical weathering accompanied by hydrothermal alteration processes of the studied rocks.

Mineral chemistry and geochemical behavior of hydrothermal alterations associated with mafic intrusive-related Au deposits at the Atud area, Central Eastern Desert, Egypt

Vein-type gold deposits in the Atud area are related to the metagabbro–diorite complex that occurred in Gabal Atud in the Central Eastern Desert of Egypt. This gold mineralization is located within quartz veins and intense hydrothermal alteration haloes along the NW–SE brittle–ductile shear zone, as well as along the contacts between them. By using the mass balance calculations, this work is to determine the mass/volume gains and losses of the chemical components during the hydrothermal alteration processes in the studied deposits. In addition, we report new data on the mineral chemistry of the alteration minerals to define the condition of the gold deposition and the mineralizing fluid based on the convenient geothermometers. Two generations of quartz veins include the mineralized grayish-to-white old vein (trending NW–SE), and the younger, non-mineralized milky white vein (trending NE–SW). The ore minerals associated with gold are essentially arsenopyrite and pyrite, with chalcopyrite, sphalerite, enargite, and goethite forming during three phases of mineralization; first, second (main ore), and third (supergene) phases. Three main hydrothermal alteration zones of mineral assemblages were identified (zones 1–3), placed around mineralized and non-mineralized quartz veins in the underground levels. The concentrations of Au, Ag, and Cu are different from zone to zone having 25–790ppb, 0.7–69.6ppm, and 6–93.8ppm; 48.6–176.1ppb, 0.9–12.3ppm, and 39.6–118.2ppm; and 53.9–155.4ppb, 0.7–3.4ppm, and 0.2–79ppm for zones 1, 2, and 3, respectively.The mass balance calculations and isocon diagrams (calculated using the GEOISO-Windows program) revealed the gold to be highly associated with the main mineralized zone as well as sericitization/kaolinitization and muscovitization in zone 1 more than in zones 2 and 3. The sericite had a higher muscovite component in all analyzed flakes (average XMs=0.89), with 0.10%–0.55% phengite content in wall rocks and 0.13%–0.29% phengite content in mineralized quartz veins. Wall rocks had higher calcite (CaCO3) contents and lower MgCO3 and FeCO3 contents than the quartz veins. The chlorite flakes in the altered wall rocks were composed of pycnochlorite and ripidolite, with estimated formation temperatures of 289–295°C and 301–312°C, respectively. Albite has higher albite content (95.08%–99.20%) which occurs with chlorite in zone 3.

Weathering profiles in granitoid rocks of the Sila Massif uplands, Calabria, southern Italy: New insights into their formation processes and rates

In this paper we characterized several weathering profiles developed on granitoid rocks in the Sila Massif upland (Calabria, southern Italy), integrating detailed macro- and micromorphological observations with physico-mechanical field tests and petrographic, mineralogical and geochemical analyses. We focused our attention on the main weathering and pedogenetic processes, trying to understand apparent discrepancies between weathering grade classes based on field description and geomechanical properties, and two common weathering indices, such as the micropetrographic index (Ip) and the chemical index of alteration (CIA). Our results showed that sericite on plagioclase and biotite chloritization, that represent inherited features formed during late-stage hydrothermal alteration of granitoid rocks, may cause an overestimation of the real degree of weathering of primary mineral grains under meteoric conditions, especially in lower weathering grade classes. Moreover, the frequent identification of Fe–Mn oxides and clay coatings of illuvial origin (rather than or in addition to those formed in situ), both at the macro- and microscale, may also explain an overestimation of the weathering degree with respect to field-based classifications. Finally, some apparent inconsistencies between field geomechanical responses and chemical weathering were interpreted as related to physical weathering processes (cryoclastism and thermoclastism), that lead to rock breakdown even when chemical weathering is not well developed. Hence, our study showed that particular caution is needed for evaluating weathering grades, because traditional field and geochemical-petrographic tools may be biased by inherited hydrothermal alteration, physical weathering and illuvial processes. On the basis of chronological constraints to soil formation obtained from a 42ka-old volcanic input (mixed to granite parent materials) detected in the soil cover of the Sila Massif upland, a first attempt to estimate soil formation rates was achieved for different depths of corresponding weathering profile zones. Soil formation rates ranged from 0.01–0.07mm a−1 for A and Bw horizons (weathering class VI) to 0.04–0.36mm a−1 for the underlying saprolite (C and Cr layers; class V). By comparing these results with the corresponding erosion rates available in the literature for the study area, that range from <0.01–0.05 to 0.10–0.21mm a−1, we suggest that the upland landscape of the Sila Massif is close to steady-state conditions between weathering and erosive processes.

Origin and evolution of geothermal fluids from Las Tres Vírgenes and Cerro Prieto fields, Mexico – Co-genetic volcanic activity and paleoclimatic constraints

Major and trace elements, noble gases, and stable (δD, δ18O) and cosmogenic (3H, 14C) isotopes were measured from geothermal fluids in two adjacent geothermal areas in NW-Mexico, Las Tres Vírgenes (LTV) and Cerro Prieto (CP). The goal is to trace the origin of reservoir fluids and to place paleoclimate and structural-volcanic constraints in the region. Measured 3He/4He (R) ratios normalized to the atmospheric value (Ra = 1.386 × 10−6) vary between 2.73 and 4.77 and are compatible with mixing between a mantle component varying between 42 and 77% of mantle helium and a crustal, radiogenic He component with contributions varying between 23% and 58%. Apparent U–Th/4He ages for CP fluids (0.7–7 Ma) suggest the presence of a sustained 4He flux from a granitic basement or from mixing with connate brines, deposited during the Colorado River delta formation (1.5–3 Ma). Radiogenic in situ4He production age modeling at LTV, combined with the presence of radiogenic carbon (1.89 ± 0.11 pmC – 35.61 ± 0.28 pmC) and the absence of tritium strongly suggest the Quaternary infiltration of meteoric water into the LTV geothermal reservoir, ranging between 4 and 31 ka BP. The present geochemical heterogeneity of LTV fluids can be reconstructed by mixing Late Pleistocene – Early Holocene meteoric water (58–75%) with a fossil seawater component (25–42%), as evidenced by Br/Cl and stable isotope trends. CP geothermal water is composed of infiltrated Colorado River water with a minor impact by halite dissolution, whereas a vapor-dominated sample is composed of Colorado River water and vapor from deeper levels. δD values for the LTV meteoric end-member, which are 20‰–44‰ depleted with respect to present-day precipitation, as well as calculated annual paleotemperatures 6.9–13.6 °C lower than present average temperatures in Baja California point to the presence of humid and cooler climatic conditions in the Baja California peninsula during the final stage of the Last Glacial Pluvial period. Quaternary recharge of the LTV geothermal reservoir is related to elevated precipitation rates during cooler-humid climate intervals in the Late Pleistocene and Early Holocene. The probable replacement of connate water or pore fluids by infiltrating surface water might have been triggered by enhanced fracture and fault permeability through contemporaneous tectonic–volcanic activity in the Las Tres Vírgenes region. Fast hydrothermal alteration processes caused a secondary, positive δ18O-shift from 4‰ to 6‰ for LTV and from 2‰ to 4‰ for CP geothermal fluids since the Late Glacial infiltration.

Gamma ray spectrometry for recognition of hydrothermal alteration zones related to a low sulfidation epithermal gold mineralization (eastern Pontides, NE Türkiye)

This study presents an interpretation of radiospectrometric and magnetic data of Arzular mineralization site, which is one of the best examples for epithermal gold deposits located in the southern zone of the Eastern Pontides (NE Türkiye). Potassium is generally the most useful pathfinder element for gold mineralization zones because of its high level in altered rock surrounding the deposits. Where gold is hosted within quartz veins, typically the vein is low in the radioelements, but the hydrothermally altered host rocks will usually have a distinct radioelement signature useful for exploration. In this study, magnetic, susceptibility and radiospectrometric survey data radiometric signatures associated with the host rocks favorable for the mineralization, enhancing techniques such as the ratio maps as well as potassium (%K), equivalent thorium (eTh ppm) and equivalent uranium (eU ppm) maps were utilized. Our analysis showed that the gold mineralization associated with the alteration is significantly related to increase in potassium, due to adularia, a low T K-feldspar, and decreases in uranium and thorium due to the hydrothermal alteration and magmatic intrusion processes during the regional tectonic activities.

Spatial distribution and compositional variation of APS minerals related to uranium deposits in the Kiggavik-Andrew Lake structural trend, Nunavut, Canada

The Kiggavik-Andrew Lake structural trend consists of four mineralized zones, partially outcropping, lying 2 km south of the erosional contact with the unmetamorphosed sandstone and basal conglomerates of the Paleoproterozoic Thelon Formation. The mineralization is controlled by a major E-W fault system associated with illite and sudoite alteration halos developed in the Archean metagraywackes of the Woodburn Lake Group. Aluminum phosphate sulfate (APS) minerals from the alunite group crystallized in association with the clay minerals in the basement alteration halo as well as in the overlying sandstones, which underwent mostly diagenesis. APS minerals are Sr- and S-rich (svanbergite end-member) in the sedimentary cover overlying the unconformity, whereas they are light rare earth elements (LREE)-rich (florencite end-member) in the altered basement rocks below the unconformity. The geochemical signature of each group of APS minerals together with the petrography indicates three distinct generations of APS minerals related to the following: (1) paleoweathering of continental surfaces prior to the basin occurrence, (2) diagenetic processes during the burial history of the lower unit of the Thelon sandstones, and (3) hydrothermal alteration processes which accompanied the uranium deposition in the basement rocks and partially overlap the sedimentary-diagenetic mineral parageneses. In addition, the association of a first generation of APS minerals with both detrital cerium oxide and aluminum oxy-hydroxide highlights the fact that a part of the detrital material of the basal Thelon Formation originated from eroded paleolaterite (allochthonous regolith). The primary rare earth element (REE)-bearing minerals (e.g., monazite, REE carbonates, and allanite) of the host rocks were characterized to identify the potential sources of REE. The REE chemical composition highlights a local re-incorporation of the REE released from the alteration processes in the APS minerals of hydrothermal origin. The distinctive geochemical signatures between diagenetic (or sedimentary) and hydrothermal APS minerals suggest a different source material in the Thelon basin than in the Athabasca basin.

Biogeochemical characteristics of Rosa canina grown in hydrothermally contaminated soils of the Gümüşhane Province, Northeast Turkey.

Kırkpavli alteration area (Gümüşhane, Northeast Turkey) is contaminated by heavy metals such as Cd, Pb, As, Cu and Zn. The quantity of accumulation of heavy metal trace elements and macroelements in 32 leaves of Rosa canina of the Kırkpavli alteration area has been studied within the scope of geochemical studies. Element contents of samples were assessed using various parameters including descriptive statistics, factor analysis, correlation coefficients and bioaccumulation factor. Concentrations were detected in the acceptable range for Mo, Cu, Pb, Ni, As, Cd, Sb, P, Ti, Na, Se and Sn. Concentrations of Co, Mn, Ba and Hg were detected close to the acceptable values, whereas Zn, Fe, Sr, V, Ca, Cr, Mg, B, Al, K, W, Sc, Cs and Rb concentrations were detected above the acceptable values. Principal component analysis was used to identify the elements that have a close relationship with each other and/or similar origins. It has been concluded that Zn, Cu, As and Mo content of the plant were related to hydrothermal alteration process and they behaved together, whereas Mn and Fe were especially products of weathering conditions, also behaved together. In terms of macroelements, Ca, Mg and Na had similar behaviour, while P and K had the same correlation.

Regional-scale pressure shadow-controlled mineralization in the Príncipe Orogenic Gold Deposit, Central Brazil

The Príncipe Gold Deposit, situated in the Natividade Gold District, resulted from establishment of the regional Transbrasiliano Strike–Slip System during the late Neoproterozoic Brasiliano Event. The host rocks, however, were formed in the Paleoproterozoic (with Sm–Nd model ages from 2250 to 2500Ma), therefore the Príncipe Gold Deposit is epigenetic. The local lithological association is a tonalitic–granodioritic suite formed in an island arc environment that later experienced continental collision tectonics, leading to the intrusion of the peraluminous Príncipe Granite (with an age of 2169±42Ma by zircon U–Pb). During the Neoproterozoic, these units underwent deformational greenschist facies metamorphism and hydrothermal alteration, forming metamorphic tectonites inside the Príncipe Granite and hydrothermally altered rocks at the northern termination of the granite, where the Príncipe Gold Deposit is located.A strong structural control is identified in the deposit, with N10°W–N20°E-trending brittle–ductile shear zones that are visible at all scales, from thin sections to regional satellite images. These shear zones played an important role in the formation of the deposit, acting as pathways that promoted a channelized fluid flow to metal precipitation sites. Hydrothermal alteration processes of different grades occur along these structures, mainly characterized by saussuritization, biotitization, chloritization, sericitization, carbonatization, silicification and sulfidation reactions, producing a neoformed mineral assemblage. The Príncipe Granite acted as a regional-scale porphyroclast for migration of the hydrothermal solutions as well as deformation, forming two pressure shadows that acted as structural traps for the ore. As stress release zones, these structures concentrated hydrothermal solutions during seismic fault movement events under a transpressive regime. They also changed the physicochemical conditions in the fluid by abruptly reducing the pressure and increasing the fluid–rock interaction rate, producing a structural framework characterized by a pervasively interconnected system of discontinuous and anastomosed quartz–carbonate–sulfide–gold-veins.Fieldwork, petrographic analysis and mineral chemistry characterization suggest that the fluid had an aqueous–carbonic nature and low salinity values. Thermometric studies suggest that gold mineralization occurred between 250°C and 400°C. Stable isotopes of S, C and O indicate a metamorphic source for the fluid. Whole rock geochemistry and microprobe studies highlight hydrothermal alteration processes involving Fe, Ca, Mg, K, Na, Au and Ag metasomatism.The Príncipe Gold Deposit belongs to the orogenic class, but it is distinguished from other orogenic-type occurrences in the Natividade Gold District and from most orogenic deposits of the world for not being located along a shear zone. Instead, it is situated within a regional-scale auriferous pressure shadow that was brought about by a short-lived pressure drop during the Brasiliano Cycle.

Altered granitic rocks, Nusab El Balgum Area, Southwestern Desert, Egypt: Mineralogical and geochemical aspects of REEs

Gabel Nusab El Balgum is located in the northern part of Bir Safsaf Area Southwestern Desert, Egypt (about 600km southwest of Aswan City) and covers an area of about 40km2. A few workers have investigated the Nusab El Balgum Area from the mineralogical and geochemical point of view. The area consists mainly of Devonian sandstone, Permo–Triassic volcanic rocks, altered granitic rocks and post-granitic dykes. The altered granitic rocks (silicified and kaolinized) have undergone several hydrothermal alteration processes and were subjected to different types of alteration (hematitization, carbonatization, kaolinization, sericitization and silicification) and are characterized by extreme enrichment in rare metals (such as Zr, Nb, Y and REEs).Bastnaesite, parisite, fergusonite, columbite, hydrothermal zircon, monazite and xenotime are common in the silicified and kaolinized granites. The silicified granite commonly contains more rare metals than the kaolinized granite. The REEs in Nusab El Balgum granites were introduced by hydrothermal fluids during alteration and occur in carbonate minerals such as bastnaesite (LREE) (CO3) F and parisite Ca (LREE) 2(CO3)3F2.The REE chondrite-normalized patterns of altered granites show an M-type tetrad effect and indicate higher LREEs, negative Ce anomaly (due to alteration by hydrothermal fluids) and negative Eu anomaly.

Composite volcanoes in the south-eastern part of İzmir–Balıkesir Transfer Zone, Western Anatolia, Turkey

During the Early–Middle Miocene (Western Anatolia) several volcanic fields occur along a NE–SW-trending shear zone, known as İzmir–Balıkesir Transfer Zone. This is a deformed crustal-scale sinistral strike-slip fault zone crossing the Bornova flysch and extending along the NW-boundary of the Menderes Massif by accommodating the differential deformation between the Cycladic and Menderes core complexes within the Aegean extensional system. Here we discuss the volcanic activity in Yamanlar and Yuntdağı fields that is closely related to the extensional tectonics of the İzmir–Balıkesir Transfer Zone and in the same time with the episodic core complex denudation of the Menderes Massif.This study documents two composite volcanoes (Yamanlar and Yuntdağı), whose present vent area is strongly eroded and cut by a variety of strike-slip and normal fault systems, the transcurrent NW–SE being the dominant one. The erosional remnants of the vent areas, resembling a shallow crater intrusive complex, illustrate the presence of numerous dykes or variably sized neck-like intrusions and lava flows, typically associated with hydrothermal alteration processes (propylitic and argillic). Such vent areas were observed in both the examined volcanic fields, having ~6km in diameter and being much more eroded toward the south, along the NW–SE fault system. Lava flows and lava domes are sometimes associated with proximal block and ash flow deposits. In the cone-building association part, besides lava flows and remnants of lava domes, rare block and ash and pumice-rich pyroclastic flow deposits, as well as a series of debris-flow deposits, have been observed.The rocks display a porphyritic texture and contain various proportions of plagioclase, clinopyroxene, orthopyroxene, amphibole, rare biotite and corroded quartz. The examined rocks fall at the limit between calc-alkaline to alkaline field, and plot predominantly in high-K andesite and dacite fields and one is rhyolite. The trace element distribution suggests fractional crystallization processes and mixing in upper crustal magma chambers and suggests a metasomatized lithospheric mantle/lower crust source. This preliminary volcanological–petrological and geochronological base study allowed documenting the Yamanlar and Yuntdağı as composite volcanoes generated during post-collisional Early–Middle Miocene transtensional tectonic movements.

Disseminated sulphides in basalts from the northern Central Indian Ridge: implications on late-stage hydrothermal activity

This study examined the mineralogy and mineral chemistry of disseminated sulphides (mainly chalcopyrite-pyrite) in partly altered basalts from the northern Central Indian Ridge, Indian Ocean in order to understand the role of hydrothermal alterations and infer possible sulphide formation history. Pyrite and chalcopyrite are dominant sulphide minerals and generally associated with the oxide phases including magnetite and often ilmenite. Close association of sulphide and oxide minerals suggests that they are paragenetically related. Sulphides also occur as late impregnated veins cutting through the basaltic hosts. The chemical compositions of pyrite (avg. Fe 46.3 wt%, S 53.7 wt%) and chalcopyrite (avg. Cu 34.4 wt%, Fe 30.7 wt%, S 34.7 wt%) are almost uniform, while the secondary ilmenite often shows MnO enrichment (up to 3.0–3.4 wt%). The associated altered minerals typically resemble the greenschist facies mineral assemblages—e.g. chlorite±epidote. Evidence of albitisation and silicification suggests low-temperature hydrothermal alteration processes. This is supported by the bulk Au content (up to 60 ppb) of host-altered basalts with pyrite mineralisation. Au is usually associated with late-stage pyrites and thus related with low-temperature hydrothermal activity. Close to the dredge location, tectonic activity around the Vityaz megamullion might have promoted hydrothermal circulation and subsequent alteration of the mineral constituents in basalts, eventually inducing the formation of late-stage disseminated sulphide minerals in these rocks.

Mapping of Seafloor Hydrothermally Altered Rocks Using Geophysical Methods: Marsili and Palinuro Seamounts, Southern Tyrrhenian Sea

Research Article| December 01, 2014 Mapping of Seafloor Hydrothermally Altered Rocks Using Geophysical Methods: Marsili and Palinuro Seamounts, Southern Tyrrhenian Sea Marco Ligi; Marco Ligi † 1Istituto di Scienze Marine, CNR, Via Gobetti 101, 40129, Bologna, Italy †Corresponding author: e-mail, marco.ligi@bo.ismar.cnr.it Search for other works by this author on: GSW Google Scholar Luca Cocchi; Luca Cocchi 2Istituto Nazionale di Geofisica e Vulcanologia, Roma 2, Via Pezzino Basso 2, 19025, Fezzano (La Spezia), Italy Search for other works by this author on: GSW Google Scholar Giovanni Bortoluzzi; Giovanni Bortoluzzi 1Istituto di Scienze Marine, CNR, Via Gobetti 101, 40129, Bologna, Italy Search for other works by this author on: GSW Google Scholar Filippo D’Oriano; Filippo D’Oriano 1Istituto di Scienze Marine, CNR, Via Gobetti 101, 40129, Bologna, Italy Search for other works by this author on: GSW Google Scholar Filippo Muccini; Filippo Muccini 2Istituto Nazionale di Geofisica e Vulcanologia, Roma 2, Via Pezzino Basso 2, 19025, Fezzano (La Spezia), Italy Search for other works by this author on: GSW Google Scholar Fabio Caratori Tontini; Fabio Caratori Tontini 3GNS Science, Ocean Exploration Section, 1 Fairway Dr., Avalon, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar Cornel E. J. de Ronde; Cornel E. J. de Ronde 3GNS Science, Ocean Exploration Section, 1 Fairway Dr., Avalon, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar Cosmo Carmisciano Cosmo Carmisciano 2Istituto Nazionale di Geofisica e Vulcanologia, Roma 2, Via Pezzino Basso 2, 19025, Fezzano (La Spezia), Italy Search for other works by this author on: GSW Google Scholar Economic Geology (2014) 109 (8): 2103–2117. https://doi.org/10.2113/econgeo.109.8.2103 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Marco Ligi, Luca Cocchi, Giovanni Bortoluzzi, Filippo D’Oriano, Filippo Muccini, Fabio Caratori Tontini, Cornel E. J. de Ronde, Cosmo Carmisciano; Mapping of Seafloor Hydrothermally Altered Rocks Using Geophysical Methods: Marsili and Palinuro Seamounts, Southern Tyrrhenian Sea. Economic Geology 2014;; 109 (8): 2103–2117. doi: https://doi.org/10.2113/econgeo.109.8.2103 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 SocietyEconomic Geology Search Advanced Search Abstract Hydrothermal alteration processes involve mineralogical, chemical, and textural changes as a result of hot aqueous fluid-rock interaction under evolving boundary conditions. These changes affect the physico-chemical properties of the rocks, enabling high-resolution geophysical prospecting to be an important tool in the detection of seafloor hydrothermal alteration. Here we present the results of recent geophysical investigations of the Marsili and Palinuro volcanic complexes, southern Tyrrhenian Sea, during the 2010 TIR10 and 2011 MAVA2011 cruises by the R/V Urania. The new dataset includes a dense grid of multibeam bathymetry; seafloor reflectivity, magnetic and gravity lines; and high-resolution single (CHIRP) and multichannel seismic profiles. The surveys were focused on areas known to host intense hydrothermal alteration in order to provide a more detailed description of the Marsili and Palinuro hydrothermal systems. Ground-truthing was based on earlier discoveries of hydrothermal vents and their associated deposits, and on direct observations made by ROV dives. High-resolution morpho-bathymetry, sonar reflectivity, rock magnetization, and density distribution together enabled us to assess the extent of seafloor hydrothermal alteration and its relationship to local volcanic and tectonic structures. Hydrothermal alteration associated with the Marsili seamount is largely distributed along primary volcano-tectonic structures at the ridge crest. By contrast, at Palinuro hydrothermal alteration is mostly associated with secondary volcanic structures such as collapsed calderas and volcanism reactivation along ring faults. In particular, evidence for intense hydrothermal activity occurs at Palinuro where volcanotectonic features interact with regional tectonic structures. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Distribution of buried hydrothermal alteration deduced from high‐resolution magnetic surveys in Yellowstone National Park

AbstractYellowstone National Park (YNP) displays numerous and extensive hydrothermal features. Although hydrothermal alteration in YNP has been extensively studied, the volume, geometry, and type of rock alteration at depth remain poorly constrained. In this study, we use high‐resolution airborne and ground magnetic surveys and measurements of remanent and induced magnetization of field and drill core samples to provide constraints on the geometry of hydrothermal alteration within the subsurface of three thermal areas in YNP (Firehole River, Smoke Jumper Hot Springs, and Norris Geyser Basin). We observe that hydrothermal zones from both liquid‐ and vapor‐dominated systems coincide with magnetic lows observed in aeromagnetic surveys and with a decrease of the amplitude of short‐wavelength anomalies seen in ground magnetic surveys. This suggests a strong demagnetization of both the shallow and deep substratum within these areas associated with the removal of magnetic minerals by hydrothermal alteration processes. Such demagnetization is confirmed by measurements of rock samples from hydrothermal areas which display significantly decreased total magnetization. A pronounced negative anomaly is observed over the Lone Star Geyser and suggests a significant demagnetization of the substratum associated with areas displaying large‐scale fluid flow. The ground and airborne magnetic surveys are used to evaluate the distribution of magnetization in the subsurface. This study shows that significant demagnetization occurs over a thickness of at least a few hundred meters in hydrothermal areas at YNP and that the maximum degree or maximum thickness of demagnetization correlates closely with the location of hydrothermal activity and mapped alteration.

MINERALOGY AND PETROGRAPHY OF PHOSPHATE MINERAL ASSOCIATIONS FROM THE JOCÃO PEGMATITE, MINAS GERAIS, BRAZIL

The Jocao pegmatite (or Cigana) is located in the well-known Eastern Brazilian Pegmatitic Province (EBPP), Minas Gerais, Brazil. According to their macroscopic textures, three different kinds of phosphate mineral masses were collected from the dumps of the pegmatite: association I is composed of dendritic triphylite, forming intergrowths with silicate minerals (spessartine garnet or albite); association II forms blocky nodules of triphylite-ferrisicklerite-heterosite; and association III shows exsolution lamellae of ferrisicklerite-heterosite in massive beusite. The primary textures and the Fe/(Fe+Mn) ratios of primary triphylite make it possible to establish the crystallization sequence of the primary phases; the petrogenesis of primary intergrowths between garnet and triphylite of association I is also discussed. In the three associations, these primary minerals are hydrothermally altered and secondary species are produced. In association I, the first hydrothermal alteration event was a weakly oxidizing hydroxylation stage, during which triphylite was only replaced by hureaulite (rarely associated with barbosalite) along its cleavage planes. The final stage affecting association I corresponds to meteoric processes during which ludlamite and then vivianite progressively replaced triphylite. Association II evolved under more oxidizing conditions and the first alteration stage corresponds to the progressive oxidation of triphylite accompanied by Li-leaching, leading to ferrisicklerite and heterosite. The second hydrothermal stage corresponds to a hydroxylation event, and the secondary species depend on the phosphate mineral that they replace: triphylite is only altered to Fe 2+ -Mn 2+ -bearing hydrated species (colorless hureaulite); ferrisicklerite is altered to Fe 2+ -, Mn 2+ -, and Fe 3+ -bearing phosphate minerals such as jahnsite s.l. , frondelite s.l., and orange hureaulite; heterosite is replaced by Fe 3+ -bearing species such as ferristrunzite. Consequently, it appears that the secondary phosphate minerals, which crystallize during this second hydrothermal stage, strongly depend on the cations which are locally available in the sample zone. The final stage forms the meteoric species: leucophosphite and phosphosiderite directly replace heterosite, whereas vivianite, a ferrous meteoric species, only appears in the triphylite core. In association III, exsolution lamellae are formed at the expense of a high-temperature homogenous Ca-Li-bearing graftonite-beusite-like phase; when the temperature decreased, Li migrated into triphylite and Ca to the larger M1 site of beusite. During the high-temperature hydrothermal alteration processes, triphylite transforms into ferrisicklerite and heterosite, like in association II; however, beusite is not affected by any transformation process at that stage. During the low temperature hydroxylation stage, ferrisicklerite from the core remains almost unaltered, while beusite is replaced by an intimate mixture of pleochroic Ca-rich hureaulite and tavorite. After this hydroxylation stage, the meteoric stage is characterized by an increase in Ca 2+ and H 2 O activities, responsible for the replacement of hureaulite and tavorite by mitridatite-robertsite. At the nodule border, beusite and heterosite are completely replaced by an intimate mixture mainly composed of frondelite and robertsite-mitridatite. Finally, this study shows that the small phosphate nodules from Jocao are more affected by oxidation than large nodules, thus indicating that the diffusion kinetics of hydrothermal fluids is relatively low in these phosphate nodules. As a consequence, large phosphate nodules show a typical zoning with triphylite (core)–ferrisicklerite-heterosite (rim); this zoning, which preserves the crystal structure of the phosphate minerals, was achieved during the high temperature hydrothermal transformations. The phosphate nodules consequently appear as a relatively closed system compared to the silicate matrix.

Magnetic properties related to hydrothermal alteration processes at the Escondida porphyry copper deposit, northern Chile

Fluid–rock interaction related to the circulation of hydrothermal fluids can strongly modify the physicochemical properties of wall rocks in porphyry Cu deposits. These processes can also produce compositional and textural changes in ferromagnetic minerals, which can be quantified using magnetic methods. In the Escondida porphyry Cu deposit of northern Chile, each hydrothermally altered lithology is characterized by a discrete assemblage of Fe–Ti oxide minerals. These minerals have distinctive bulk magnetic susceptibility (K bulk), temperature-dependent magnetic susceptibility, and magnetic hysteresis parameters. Selectively altered rocks (i.e., potassic and chloritic alteration types) exhibit the highest K bulk values (>3.93 × 10−3 SI units), and their hysteresis parameters indicate multidomain magnetic mineral behavior. This suggests that these rocks are composed of the coarsest magnetic grain sizes within the deposit. Optical analyses and susceptibility–temperature curves confirm that the magnetic signals in selectively altered rocks are mainly carried by secondary magnetite. In contrast, pervasively altered rocks (i.e., quartz-sericite and argillic alteration types) exhibit low K bulk values (<1.93 × 10−4 SI units) and contain smaller pseudo-single domain magnetic grain assemblages. This is consistent with the destruction and/or reduction in size of magnetite under acidic conditions. The results therefore demonstrate a genetic relationship between the hydrothermal alteration processes, Fe–Ti oxide minerals, and magnetic properties of the wall rock in the Escondida deposit. These magnetic methods can be considered a sensitive and efficient petrophysical tool for the identification and semi-quantification of alteration assemblages, and facilitating the recognition and mapping of discrete hydrothermal zones during exploration and operation of porphyry Cu deposits.

Precious and base metal geochemistry and mineralogy of the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, northern Bushveld Complex, South Africa: implications for a multistage emplacement

The Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member within the northern limb of the Bushveld Complex is a mineralized, layered package of mafic cumulates developed to the south of the town of Mokopane, at a similar stratigraphic position to the Platreef. The concentration of platinum-group elements (PGE) in base metal sulfides (BMS) has been determined by laser ablation inductively coupled plasma–mass spectrometry. These data, coupled with whole-rock PGE concentrations and a detailed account of the platinum-group mineralogy (PGM), provide an insight into the distribution of PGE and chalcophile elements within the GNPA member, during both primary magmatic and secondary hydrothermal alteration processes. Within the most unaltered sulfides (containing pyrrhotite, pentlandite, and chalcopyrite only), the majority of IPGE, Rh, and some Pd occur in solid solution within pyrrhotite and pentlandite, with an associated Pt–As and Pd–Bi–Te dominated PGM assemblage. These observations in conjunction with the presence of good correlations between all bulk PGE and base metals throughout the GNPA member indicate the presence and subsequent fractionation of a single PGE-rich sulfide liquid, which has not been significantly altered. In places, the primary sulfides have been replaced to varying degrees by a low-temperature assemblage of pyrite, millerite, and chalcopyrite. These sulfides are associated with a PGM assemblage characterized by the presence of Pd antimonides and Pd arsenides, which are indicative of hydrothermal assemblages. The presence of appreciable quantities of IPGE, Pd and Rh within pyrite, and, to a lesser, extent millerite suggests these phases directly inherited PGE contents from the pyrrhotite and pentlandite that they replaced. The replacement of both the sulfides and PGM occurred in situ, thus preserving the originally strong spatial association between PGM and BMS, but altering the mineralogy. Precious metal geochemistry indicates that fluid redistribution of PGE is minimal with only Pd, Au, and Cu being partially remobilized and decoupled from BMS. This is also indicated by the lower concentrations of Pd evident in both pyrite and millerite compared with the pentlandite being replaced. The observations that the GNPA member was mineralized prior to intrusion of the Main Zone and that there was no local footwall control over the development of sulfide mineralization are inconsistent with genetic models involving the in situ development of a sulfide liquid through either depletion of an overlying magma column or in situ contamination of crustal S. We therefore believe that our observations are more compatible with a multistage emplacement model, where preformed PGE-rich sulfides were emplaced into the GNPA member. Such a model explains the development and distribution of a single sulfide liquid throughout the entire 400–800 m thick succession. It is therefore envisaged that the GNPA member formed in a similar manner to its nearest analogue the Platreef. Notable differences however in PGE tenors indicate that the ore-forming process may have differed slightly within the staging chambers that supplied the Platreef and GNPA member.

Episyenite formation in the Toki granite, central Japan

Episyenite is a quartz-depleted vuggy rock resulting from hydrothermal alteration of granitic rocks. This is the first report of its existence in an island arc, which is identified in a deep drill core of the Toki Cretaceous granite distributed in central Japan. In order to understand the petrographical features of the episyenite, neutron porosity measurement, geochemical analysis, microscopic observation, and X-ray computed tomography scanning were carried out. The results show remarkably high porosity (35.4 %) due to interconnecting vugs and the removal of quartz, plagioclase, and biotite. The Rb–Sr isotopic results and the paragenetic sequence of secondary minerals in the vugs suggest that the hydrothermal alteration process can be divided into an episyenitization stage and a later hydrothermal stage. At the episyenitization stage (70.6 ± 3.1 Ma) ca. 6 million years after the emplacement of the unaltered granite (76.3 ± 1.5 Ma), dissolution of quartz, biotite, and plagioclase occurred and was followed by the precipitation of albite, vermicular chlorite, and platy calcite. The episyenitization is considered as a local alteration of the Toki granite in an isotopically closed system. At the later hydrothermal stage, illite and secondary quartz precipitated from circulating meteoric-derived water in the dissolution vugs. Superimposing alteration at the later hydrothermal stage is limited, which results in the preservation of the episyenite in an almost primitive condition.