Intense Hydrothermal Alteration Research Articles

Investigation of the Extent of Hydrothermal Activities of the Sikidang Crater, Dieng Volcanic Complex (Central Java), Using Geomagnetic Method

Abstract The Sikidang Crater (± 2000 m asl) is one of the active craters in the Dieng Volcanic Complex (DVC). It is situated on the southeastern flank of Mt. Pangonan. The crater is significant for development of both tourism and energy sectors in Dieng. Thermal manifestations inside the crater which consist of boiling mud pools and pots, hot springs, fumaroles, gas vents, steaming and altered ground make the place a popular tourist attraction. They also serve as windows to the shallow subsurface processes and help better understand the dynamics of the Dieng geothermal system. However, the land use of the crater and its vicinity disregards the fact that the actual extent of the hydrothermal activity of the crater has not been figured out, and that the foci of the thermal fluid discharges in the crater often shift. Some agriculture plots, housing areas, as well as tourism and geothermal infrastructures are located within or near the morphological boundary of the crater. The inherent hydrothermal hazard of the crater therefore, must be well mitigated to minimize the negative impacts to the people, as well as their assets. Our research aims to delineate the lateral and vertical extent of the hydrothermal activity of the crater by geomagnetic method. Geomagnetic anomaly in geothermal field occurs due to the difference in the intensity of hydrothermal alteration of the rocks. The equipment used was a pair of Proton Precision Magnetometers GEOTRON G5. Our result shows that low geomagnetic anomaly (-2200 to -400 nT) correlated with demagnetized, hydrothermally altered andesitic rocks covers an area of ≥ 1.15 km2, i.e., more than seven times larger than the area outlined by morphological boundary of the crater. The anomalous zone extends N/NE - S/SW from beyond the areas of inter-volcanic plain between Pangonan and Igir Binem in the north, to the areas beyond Sikunang and Kendil cones in the south. Below the Sikidang mud pool the demagnetized rock mass reaches a maximum thickness of 2000 m. The supporting data, i.e., the fluctuations of the gas discharge, and the seismicity of the DVC indicate that the recent dynamics of the Sikidang Crater is mostly controlled by the internal processes within the crater.

Mineralizations of Nb-Ta-Rb-Zr and rare-earth elements in Boziguoer, South Tianshan, NW China: Geochronology and geochemistry of monazite and bastnäsite

The Boziguoer alkaline intrusion is located in the South Tianshan Orogenic Belt on the northern margin of the Tarim Craton. The intrusion is entirely mineralized and forms a super-large rare metal (RM) and rare-earth element (REE) deposit dominated by Nb-Ta-Rb, accompanied by Zr-REE. The primary RM minerals include pyrochlore, astrophyllite, and zircon, while the main REE minerals consist of fluocerite, monazite, xenotime, and bastnäsite, which are commonly present as granular aggregates or singularly filling the interstices between gangue minerals. Through a combination of geochronological and geochemical analyses of different types of monazite and bastnäsite in the mineralized alkaline rocks, this study elucidates the role of magmatic-hydrothermal evolution on the Boziguoer RM-REE mineralization and reconstructs the geochronological framework of alkalic magmatic-hydrothermal evolution and mineralization processes, as well as establishing mechanisms responsible for RM-REE enrichment. Petrographic observations and back-scattered electron (BSE) imaging revealed several types of monazite and bastnäsite with different characteristics as follows: (1) type Ia monazite (Mnz-Ia) experienced intense hydrothermal alteration, forming residual cores of monazite mantled by apatite and allanite coronas; (2) type Ib monazite (Mnz-Ib) was partially eroded, forming concentric zoning patterns with a core of monazite, a mantle of apatite, and an outer rim of allanite coronas; (3) type II monazite (Mnz-II) is slightly modified, and commonly associated with fluorite; (4) bastnäsite and fluocerite exhibit a core-rim structure, with the core of fluocerite being brighter than the rim of bastnäsite in BSE images. According to the paragenetic relationships and compositional variations, all two types of monazite are of primary magmatic origin, while bastnäsite is of hydrothermal origin. In addition, principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) results indicate that Mnz-Ia and Mnz-Ib belong to the same group, different from Mnz-II. The latter has higher (Ce/Gd)N and (Th/U)N ratios, indicating relatively low-temperature crystallization of Mnz-II at late stages. In-situ LA-ICP-MS U-Th-Pb dating of monazite and bastnäsite show that Mnz-Ia and Mnz-Ib have ages of ca. 290 Ma, consistent with the zircon age of the intrusion, whereas the Mnz-II and bastnäsite have younger ages of ca. 280 Ma, representing a post-magmatic hydrothermal mineralization event at Boziguoer. These new ages indicate that both magmatic and subsequent hydrothermal processes played critical roles in the RM-REE mineralization in this deposit. The latest findings also support the model that the formation of the alkaline belt where the Boziguoer ore-bearing intrusion was temporally and spatially linked to the Tarim Large Igneous Province that is genetically related to a mantle plume.

Zircon as a Monitoring Tool for the Magmatic–Hydrothermal Process in the Granitic Bedrock of Shitouping Ion-Adsorption Heavy Rare Earth Element Deposit, South China

The Shitouping pluton in Jiangxi Province, southern China, hosts an ion-adsorption heavy rare earth element (HREE) deposit identified by a recent geological survey. This study reveals the HREE pre-enrichment mechanism during the magmatic–hydrothermal process of granitic bedrock based on the comprehensive study of zircon structure and composition. Zircon from the Shitouping pluton, composed of syenogranite and monzogranite, can be categorized into three types based on structure and compositions. The Type-1 zircons, the predominate type in monzogranite, are early magmatic zircons with prismatic crystals and bright oscillatory zoning in CL images. In contrast, the late magmatic-hydrothermal zircons (Type-2 and Type-3) mainly occur in the syenogranite. The Type-2 zircons occur as dark CL images and euhedral crystals crystallized during the late magmatic stage. The Type-3 zircons with irregular zoning and abundant mineral inclusions in BSE images are possibly formed via intense hydrothermal alteration during the hydrothermal stage. The increase in Y/Ho ratios from Type-1 to Type-3 zircon indicates that the Shitouping syenogranites underwent magmatic to hydrothermal evolution. Compared with Type-1 and Type-2 zircons, Type-3 zircons exhibit the highest concentrations of F and HREEs. The significant increase in HREE concentrations both in zircons and bulk-rock composition of syenogranite can be attributed to the introduction of HREE-rich fluids during magma evolution. Therefore, we propose that the increase in HREE contents in zircon reflect the exsolution of HREE-rich fluids during a late stage in the magma evolution, which is an important factor controlling HREE enrichment in Shitouping syenogranites and furthermore in the generation of ion-adsorption HREE deposits.

Spectral characterisation of hydrothermal alteration associated with sediment-hosted Cu–Ag mineralisation in the central European Kupferschiefer

Abstract. The analysis of hydrothermal alteration in exploration drill cores allows for fluid–rock interaction processes to be traced, for fluid flow paths to be identified, and thus for vectors in mineral systems to be determined. Hyperspectral imaging techniques are increasingly being employed to fill the scale gap between lab-based petrographic or geochemical analyses and the typical size of exploration targets. Hyperspectral imaging permits the rapid, cost-efficient, and continuous characterisation of alteration mineralogy and texture along entire drill cores, with a spatial sampling of a few millimetres. In this contribution, we present the results of an exploratory study on three mineralised drill cores from the Spremberg–Graustein Kupferschiefer-type Cu–Ag deposit in the Lusatia region of Germany. We demonstrate that hyperspectral imaging is well-suited to recognising and tracking the effects of hydrothermal alteration associated with strata-bound hydrothermal mineralisation. Micro X-ray fluorescence spectrometry was used to corroborate the alteration mineral assemblages identified in hyperspectral data acquired in the visible, near- (400 to 970 nm), shortwave (970 to 2500 nm), mid-wave (2700 to 5300 nm), and longwave infrared (7700 to 12 300 nm). We identified two main shortcomings of the technique, namely the overlapping of some mineral features (e.g. carbonate and illite absorption in the shortwave infrared) and the darkness of the organic-matter-rich dolostones and shales that results in low reflectance. Nevertheless, spectral features associated with iron oxide, kaolinite, sulfate, and carbonates were successfully identified and mapped. We identified different markers of hydrothermal alteration spatially associated with or stratigraphically adjacent to Cu–Ag mineralisation. Importantly, we can clearly distinguish two mineralogically distinct styles of alteration (hematite and ferroan carbonate) that bracket high-grade Cu–Ag mineralisation. Intensive hydrothermal alteration is characterised by the occurrence of well-crystallised kaolinite in the sandstone units immediately below the Kupferschiefer horizon sensu stricto. Proximal Fe-carbonate and kaolinite alteration have not previously been documented for the high-grade Cu–Ag deposits of the central European Kupferschiefer, whereas hematite alteration is well-known in Kupferschiefer-type ore deposits. The latter marks the flow path of oxidising, metal-bearing hydrothermal fluids towards the site of hydrothermal sulfide mineralisation. In contrast, ferroan carbonate alteration in carbonate rocks located above the main mineralised zone is interpreted as a mark of hydrothermal fluid discharge from the mineralising system. Although this study is limited to a small number of drill cores, our results suggest that hyperspectral imaging techniques may be used to identify vectors towards high-grade Cu–Ag mineralisation in Kupferschiefer-type mineral systems.

Hydrothermal Alteration and Element Migration Patterns in the Zhangjiawu Uranium Deposit, Northern Zhejiang Province, China

The Zhangjiawu uranium deposit is the largest volcanic rock-type uranium deposit in the northern Zhejiang Province, China. The deposit has developed hydrothermal alteration, with obvious alternating zoning phenomena from the mineralized center to the fresh surrounding rocks. Based on detailed field and petrographic observations of typical ore bodies, the uranium mineralized section of the Zhangjiawu uranium deposit was divided into mineralized central, ore-side alteration, near-ore alteration and far-ore alteration zones, whose hydrothermal alteration intensity decreases sequentially. Using the standardized Isocon diagram method, the results show that CaO, MgO, Na2O, P2O5, LOI, Zn, Co, Cu, Pb and heavy rare earth elements (HREE) show gain during uranium mineralization, while K2O, Cs, Rb and Tl show loss, which is consistent with the development of hematite mineralization, sodic feldsparization and carbonation in the mine area. A negative correlation of MnO and MgO in the alteration zone indicates a possible elemental convective equilibrium migration mode in the ore zone. The migration mechanism of the elements indicates that the migration of P2O5 and HREE indicates the deep source nature of the mineralizing fluids, and that alteration can lead to a reduction in the activity of uranyl complexes, the formation of a reducing environment and a neutral to weak alkaline environment favorable for uranium precipitation, which can lead to uranium precipitation and enrichment of mineralization. The study of hydrothermal alteration and elemental migration patterns of the deposit provides supporting evidence for a better understanding of the process of uranium mineralization in Zhangjiawu, and also provides a basis for the next step of mineral search and exploration.

Trace-element signatures of pegmatite and post-pegmatite minerals from the Govedarnika Pb-Zn deposit, Laki district, Central Rhodopes

The current study presents mineralogical and geochemical characteristics of pegmatite bodies from the Govedarnika Pb-Zn deposit, situated in Laki district, South Bulgaria. The studied pegmatites are composed mainly of K-feldspar, plagioclase, quartz and accessories of titanite, apatite, allanite and zircon. Intensive hydrothermal alteration resulted in specific mineral assemblage of chlorite, hematite, epidote and carbonates. According to the mineral relationships, it preceded or coincided with the time of base metal ore formation and affected both pegmatites and host metamorphic rocks. Both pegmatite accessories and post-pegmatite hydrothermal silicates often show enhanced contents of some strategic elements such as Sr, Li, V, Ga, Ni and REE. Pilot U-Pb geochronology of titanite from the studied pegmatites yielded a lower intercept age of 39.9±2.3 Ma.

Zircon texture and composition fingerprint HREE enrichment in muscovite granite bedrock of the Dabu ion-adsorption REE deposit, South China

More than 90% of the global heavy rare earth elements (HREE) supply is currently from the ion-adsorption REE deposits in South China. However, the mechanism of HREE enrichment in granite bedrock of REE mineralized weathering crusts remains poorly understood. In this study, we present a comprehensive investigation on zircon texture and composition as a proxy to unravel the magmatic-hydrothermal processes which resulted in HREE enrichment of the Dabu muscovite granites. Our results show that zircon in the Dabu muscovite granites can be classified into three types based on morphology, internal structure and chemical compositions. The early-magmatic zircon grains (ZrnI) are prismatic crystals with bright oscillatory zoning, and crystallized early in the granitic magma. The late-magmatic zircon grains (Zrn-II) occur as murky euhedral or subhedral crystals or as overgrowth on ZrnI, and crystallized from a volatile-enriched residual melt in the late magmatic stage. The hydrothermal zircon (Zrn-III) is anhedral without oscillatory zoning, possibly formed via intense hydrothermal alteration of Zrn-II in the hydrothermal stage. Raman spectra show a decrease in degree of crystallinity from Zrn-I to Zrn-III. In-situ compositional and isotopic analyses suggest that zircon geochemistry is a good proxy for the magmatic-hydrothermal processes of the host granite. Zircon UPb dating yields similar ages of 155.3 ± 1.4 Ma (2σ) and 154.1 ± 1.0 Ma (2σ) for Zrn-I and Zrn-II, respectively. The decrease of Zr/Hf from Zrn-I to Zrn-III indicates that the Dabu muscovite granites underwent magmatic to hydrothermal evolution. Furthermore, the decreasing δ18O values of Zrn-I to Zrn-III suggest the increasing involvement of meteoric water in the fluids. Significantly, compared with Zrn-I and Zrn-II, Zrn-III has the highest U, Hf and REE, in particular HREE. The LREE/HREE ratios decrease with the increase of REE contents and the decrease of Zr/Hf ratios from early to late zircon of the granite bedrock of ion-adsorption HREE deposits, which is significantly different from those of carbonatite-alkaline related REE deposits. The significant increase of HREE concentrations in early to late zircon should be ascribed to the introduction of HREE-rich fluids during magma evolution. Therefore, we propose that the HREE-rich fluids from magma exsolution metasomatized the granites to achieve further enrichment of HREE, which is an important prerequisite for the formation of ion-adsorption HREE deposits. Accordingly, we argue that zircon composition can be used as a chemical indicator for the REE behaviors in magmatic-hydrothermal systems that are vital to the formation of REE deposits.

Niobium, Critical Metal, and Progeny of the Mantle

Abstract Niobium is a critical metal in high demand because of technological advances and the supply risk created by the fact that over 90% of its production is by a single country (Brazil). In this paper, we review the geology of the deposits that are currently being mined and other potentially economic deposits as well as develop models for their genesis. With the exception of the Lovozero deposit (Russia), which is hosted by a layered silica-undersaturated alkaline igneous complex, all the deposits that are currently being mined for niobium are hosted by carbonatites, and most of the deposits with economic potential are also hosted by these rocks. Niobium owes its concentration in carbonatites and alkaline silicate rocks to its highly incompatible nature and the small degree of partial melting of the mantle required to generate the corresponding magmas. The primary control on the concentration of niobium to economic levels in alkaline silicate magmas is fractional crystallization, partly prior to but mainly after emplacement. In the case of silica-undersaturated magmas, the final residue saturates in minerals like eudialyte and loparite to form niobium-rich horizons in the layered complexes that crystallize from these magmas. The final residue, in the case of silica-saturated magmas, crystallizes the pegmatites that are the hosts to the economic niobium mineralization, which commonly takes the form of pyrochlore. In contrast, carbonatitic magmas undergo little to no fractional crystallization prior to emplacement. Moreover, fractional crystallization on emplacement has minimal impact on the concentration of niobium to economic levels. Instead, we propose that the metasomatic interaction of the carbonatitic magmas with their hosts to form rocks like phlogopitite (glimmerite) consumes much of the magma, leaving behind a phoscoritic residue from which pyrochlore crystallizes in amounts sufficient to form economic deposits. Although many niobium deposits display evidence of intense hydrothermal alteration, during which there can be major changes in the niobium mineralogy, the extremely low solubility of niobium in aqueous fluids at elevated temperature precludes significant mobilization and, thus, enrichment of the metal by hydrothermal fluids. However, weathering of carbonatite-hosted niobium deposits leads to supergene enrichment (due largely to the dissolution of the carbonate minerals) that can double the niobium grade and make subeconomic deposits economic. Pyrochlore is the principal niobium mineral in these laterite-hosted deposits, although its composition differs considerably from that in the primary mineralization. This paper evaluates the processes that appear to be responsible for the genesis of niobium ores and provides a framework that we hope will guide future in-depth studies of niobium deposits and lead to more effective strategies for their successful exploration and exploitation.

New insights into the petrogenesis of rhyolites and evolution of volcanic‐hosted massive sulphide mineralization from shallow borehole core, Betul Belt, Central India

Shallow borehole drilling has been conducted at the Sarni area of Betul Belt, Central India to understand the subsurface geology and nature of mineralization. The predominant lithounits are ferruginous quartzites, felsic volcanic rocks, and with minor intrusions of amphibolites. The encountered felsic volcanic rocks are rhyolites varying from pink (rich in K‐feldspar) to grey (rich in Na‐feldspar and amphiboles). The rhyolitic rocks have undergone intense hydrothermal alteration. The alteration zones are defined by the presence of biotite‐chlorite‐garnet‐gahnite; biotite‐sericite‐chlorite‐anthophyllite and anthophyllite‐actinolite‐tremolite‐carbonate mineral assemblages. Sulphide pits, highly leached zones with occasional bluish tinges, limonitic patches are present in the felsic volcanic rocks, followed by semi‐massive to disseminated sulphide mineralization indicative of Kuroku‐ type Zn–Cu–Pb volcanic‐hosted Massive Sulphide (VHMS) mineralization. Scanning electron microscope and energy‐dispersive x‐ray spectroscopy studies documented sphalerite, pyrite, chalcophyrite, arsenopyrite, pyrrhotite, and minor Au–Ag phases. The Sarni rhyolites exhibit chalcopyrite disease in sphalerite, replacement, and overgrowth ore‐textures. The high HFSE, low Zr/Y ratio, pronounced negative Eu anomalies characterize the felsic volcanic rocks as FIIIb type with high zircon saturation temperatures of 856–931°C. These rhyolites are formed at a back‐arc rift setting in an ensialic island‐arc tectonic regime. The synvolcanic structures facilitated the magma ascent to the surface. The hydrothermal circulation due to adequate heat flow in the back‐arc rift environment, in an episodic multicyclic process provided sufficient time for accumulation of sulphides from the seafloor, which was later deposited to form a VHMS mineralization. We also report tungsten (W) mineralization in the quartz veins of these felsic volcanic rocks. It is suggested that the felsic magma has interacted with metasedimentary sequences and assimilated W‐ore bearing fluids, and during remobilization in the later stages of tectonic activity enabled the deposition of tungsten in these quartz veins.

Hallaçlar Volkanitleri’nde (Bayramiç, Çanakkale) Stratejik Element Zenginleşmeleri

In this study, Hallaçlar Volcanites located in Bayramiç-Çanakkale region were evaluated in terms of strategic element contents. Mineralogical-petrographical and geochemical investigations were carried out on the samples which were taken from the outcrops with intense hydrothermal alteration. The stratigraphy of the study area consists of the Sütüven Formation (Kazdağı Metamorfites), Çetmi Ophiolitic Melange, Evciler Pluton and Hallaçlar Volcanites. The intensity of hydrothermal alterations gradually increases in Hallaçlar Volcanites towards to the ophiolitic rocks and the Evciler Pluton. As a result of the geochemical analyzes carried out, it was observed that the yttrium (Y), zirconium (Zr), thorium (Th), tin (Sn) and antimony (Sb) elements were enriched in the Hallaçlar volcanites. It can be concluded that these elements were enriched in the volcanites with the hydrothermal solutions leaching from the elder metamorphic rocks.

Hydrothermal alteration, lithogeochemical marker units and vectors towards mineralisation at the Svärdsjö Zn-Pb-Cu deposit, Bergslagen, Sweden

ABSTRACT The Svärdsjö Zn-Pb-Cu deposit is situated in the heavily mineralised Bergslagen lithotectonic unit of the Fennoscandian shield, south-central Sweden. Intense hydrothermal alteration followed by a strong overprint by deformation and metamorphism during the Svecokarelian orogeny complicate interpretation of the local geology. Integration of whole-rock lithogeochemical and petrographic methods has allowed the mainly dacitic volcanic host succession and effects of ore-related hydrothermal alteration to be characterised. Mineralisation is hosted by 2–15 m thick, commonly skarn-altered dolomitic marble interbeds. Zones of strong–intense hydrothermal chlorite-sericite alteration envelop the marble units, recording mass gains of Fe and Mg, as well as Na depletion. Minerals such as cordierite, anthophyllite and sillimanite formed in these rocks during regional metamorphism. Mineralisation via sub-seafloor replacement is suggested for the deposit based on alteration zoning and the irregular, stratabound, marble-hosted style of sulphide lenses. It is inferred that mineralisation formed via neutralisation of hot, acidic metalliferous fluids. Geochemically and lithologically distinct units adjacent to the mineralised zones can serve as marker units to aid further exploration in the area. Mass change calculations reveal that Fe and Mg enrichment, as well as Na depletion exhibit detectable changes extending up to 100 m from the mineralised lenses, providing exploration vectors.

Geochemical, geochronological and isotopic studies of the Taishanmiao batholith and the Zhuyuangou Mo deposit it hosted, Qinling, China

The Qinling Orogen is well known for its huge Mo resource. The porphyry Mo deposits are associated with small granite porphyries, albeit great batholiths accommodate limited Mo mineralization. In this study, detailed geochemical, isotopic, and geochronological studies were carried out to constrain the petrogenesis of the Taishanmiao batholith and time framework of magmatism and associated Mo mineralization at Zhuyuangou. Zircon geochronology reveals gradually decreasing emplacement ages, from 124.8 ± 1.0 Ma for the medium- to coarse-grained syenogranite, through 122.8 ± 1.4 Ma for fine- to medium-grained syenogranite, to 115.1 ± 0.95 Ma for porphyritic syenogranite. The last two phases hosted the Zhuyuangou Mo mineralization with a molybdenite Re-Os isochron age of 117.7 ± 1.0 Ma. Major and trace element features support highly fractionated I-type affinity for Taishanmiao. Zircon Lu-Hf isotopic studies yield εHf(t) values of −19.6 − -8, and two-stage Hf model ages (TDM2) of 1679–2408 Ma. It thus supports ancient continental crust, especially Taihua Supergroup and Xiong'er Group, as the primary source. Compared to the source rock, the low Mo concentrations of the batholith are attributed to the massive fluid flux that contributes to the formation of the Zhuyuangou and Donggou deposits. Small porphyries can effectively concentrate exsolved fluids and metal compared with great batholith. Their shallow emplacement depths result in greater temperature/pressure/composition gradient with the wall rocks, higher cooling rate, more fractures or even breccia, and fluid boiling and mixing, facilitating intensive hydrothermal alteration and mineralization.

The thermal properties of hydrothermally altered andesites from La Soufrière de Guadeloupe (Eastern Caribbean)

The heat flux of an active volcano provides crucial information on volcanic unrest. The hydrothermal activity often responsible for volcanic unrest can be accompanied by an increase in the extent and intensity of hydrothermal alteration, which could influence the thermal properties of the volcanic edifice. Therefore, an understanding of the influence of alteration on the thermal properties of rocks is required to better interpret volcano heat flux data. We provide laboratory measurements of thermal conductivity, thermal diffusivity, and specific heat capacity for variably altered (intermediate to advanced argillic alteration) andesites from La Soufrière de Guadeloupe (Eastern Caribbean). We complement these data with previously published data for altered basaltic-andesites from Merapi (Indonesia) and new data for altered rhyodacites from Chaos Crags (USA). Our data show that thermal conductivity and thermal diffusivity decrease as a function of increasing porosity, whereas the specific heat capacity does not change systematically. Thermal conductivity decreases as a function of alteration (the percentage of secondary minerals) for the rocks from La Soufrière and Merapi (from ~1.6 to ~0.6 W·m−1·K−1 as alteration increases from ~1.5 to >75 wt%), but increases for the rocks from Chaos Crags (from ~1.1 to ~1.5 W·m−1·K−1 as alteration increases from ~6 to ~15 wt%). Although the thermal diffusivity of the rocks from Chaos Crags increases from ~0.65 to ~0.75–0.95 mm2·s−1 as alteration increases from ~6 to ~15 wt%, the thermal diffusivity of the rocks from La Soufrière and Merapi does not appear to be greatly influenced by alteration. The specific heat capacity is not significantly affected by alteration, although there is a slight trend of increasing specific heat capacity with alteration for the rocks from La Soufrière. We conclude that the decrease in thermal conductivity as a function of alteration in the rocks from La Soufrière and Merapi is the result of the low conductivity of the secondary mineral assemblage, and that a combination of the high thermal conductivity of cristobalite and the reduction in porosity as a result of the void-filling mineral precipitation can explain the increase in thermal conductivity in the rocks from Chaos Crags. Calculations show that an increase in alteration of a dome or edifice can result in decreases and increases in heat flow density, depending on the type of alteration. Therefore, alteration-induced changes in the thermal properties of dome or edifice rocks should be considered when interpreting volcano heat flux data. We conclude that it is important not only to monitor the extent and evolution of alteration at active volcanoes, but also the spatial distribution of alteration type.

Anion Composition of Apatite in the Au-Cu Epithermal Deposit of Palai-Islica (Almería, SE Spain) as an Indicator of Hydrothermal Alteration

The Palai-Islica deposit (Almería, SE Spain) is an Au-Cu epithermal deposit hosted in Neogene calc-alkaline andesites and dacites from the Cabo de Gata-Cartagena volcanic belt in the Betic Cordillera. Major element compositions of apatite from Palai-Islica orebody and related hydrothermally altered and unaltered volcanic rock from the region hosting the deposit were obtained to clarify the processes involved in their formation. Apatite in the host volcanic rocks is rich in chlorapatite and hydroxylapatite components (50–57% and 24–36%) and poor in fluorapatite components (12–21%), indicating assimilation processes of cortical Cl-rich material in the magmatic evolution. Apatite in the orebody sometimes has corrosion textures and is mostly fluorapatite (94–100%). Apatite from the hydrothermally altered host rock of the orebody systematically bears signs of corrosion and has variable and intermediate fluorapatite (19–100%), chlorapatite (1–50%), and hydroxylapatite (0–47%) components. The style of zonation and the composition are related to the proximity to the orebody. These features can be interpreted as the result of hydrothermal modification of high Cl, OH-rich volcanic apatites into F-rich apatites. The enrichment of F is related to the intensity of hydrothermal alteration and could therefore constitute a geochemical index of alteration and of mineralization processes.

Metamictization and fluid-driven alteration triggering massive HFSE and REE mobilization from zircon and titanite: Direct evidence from EMPA imaging and LA-ICP-MS analyses

The major and trace element composition of the main HFSE-bearing phases from highly evolved miaskitic syenites has been investigated to assess their stability, an important control on HFSE retention and mobility during post-magmatic hydrothermal activity. The major HFSE carrier, abundant zircon (up to 5 vol%), has undergone intensive metamictization, caused by high initial U and Th along with the presence of early magmatic U-thorite inclusions. Subsequent to metamictization, zircon experienced pervasive fluid-driven alteration, as evidenced by strong depletion in Zr and Si, enrichment in non-formula elements (Ca, Fe, Al, P), deficient EMPA analytical totals, high porosity, and low BSE-intensities of strongly altered crystal interiors. Element maps suggest that extensive HFSE remobilization from altered zircon was controlled by interaction with oxidized aqueous solutions circulating through abundant microfractures that formed by volume expansion due to zircon metamictization. Enhanced HFSE mobility in hydrothermal fluids is reflected by the presence of a hydrated amorphous Zr-Th-U-Si-phase, filling microveinlets intimately associated with metamict zircons. In addition to HFSE remobilization from altered zircon, titanite released a significant amount of REE during interaction with the hydrothermal fluids that concurrently caused a fractionation of Eu in titanite, producing increased EuN/EuN* (4.16–22.2) in altered crystals. Extensively altered titanite grains with elevated EuN/EuN* are considerably depleted in ∑REE + Y compared to unaffected crystals, showing that degree of positive Eu anomalies, relating to post-magmatic metasomatic overprint, appears to be a sensitive indicator for an intensity of hydrothermal alteration and even the total amount of released incompatible elements. Pronounced LREE enrichment accompanied by HREE depletion in the strongly altered crystals indicates greater LREE retention in titanite and higher HREE mobility in the hydrothermal system probably caused by complexing with fluoride ligands. Fluorine was most likely supplied through amphibole and biotite chloritization, as suggested by notably low F in the mafic silicates along with the formation of secondary F-rich titanite at the expense of altered biotite. Moreover, the dissolution of zircon and titanite and resistance of apatite to pervasive alteration indicates that remobilization of HFSE and REE from these phases occurred under alkaline conditions (pH 8–12), presumably controlled by potassium-dominated complexes transporting HFSE along with REE over long distances, as indicated by a close spatial linkage of syenite outcrops with U-mineralization. Our results clearly demonstrate: (1) extensive chemical dissolution of zircon controlled by its self-induced radiation damage; (2) high solubility of titanite and metamict zircon under alkaline conditions; (3) enhanced mobility and long-distance transport of HFSE and REE in alkali- and F-rich hydrothermal systems.

Airborne Geophysical Imaging of Weak Zones on Iliamna Volcano, Alaska: Implications for Slope Stability

AbstractWater‐saturated, hydrothermally altered rocks reduce the strength of volcanic edifices and increase the potential for sector collapses and far‐traveled mass flows of unconsolidated debris. Iliamna Volcano is an andesitic stratovolcano located on the western side of the Cook Inlet, ∼225 km southwest of Anchorage and is a source of repeated avalanches. The widespread snow and ice cover on Iliamna Volcano make surface alteration difficult to identify. However, intense hydrothermal alteration significantly reduces both the electrical resistivity and magnetization of volcanic rock and can therefore be identified with airborne geophysical measurements. We use airborne electromagnetic and magnetic data to map snow and ice thickness and identify underlying alteration zones at Iliamna Volcano, Alaska. Resistivities were calculated to an average depth of >300 m, and a 3‐D susceptibility model extends from the surface to the base of the volcano, about 3,000 m below the summit. Geophysical models image low resistivity (<30 ohm‐m) and low susceptibilities near the summit of Iliamna and below its older vent complex, with the low susceptibilities indicating alteration up to ∼800 m in thickness. Thin conductors (∼50–100 m thick) on the edifice slopes coincide with recorded locations of repeated debris avalanches over the past ∼60 years and are attributed to saturated zones at high elevation. Three‐dimensional slope stability models based upon the geophysically constrained alteration distribution suggest the edifice of Iliamna is unstable and could lead to collapse scars ∼400 m deep near the current and former vent complexes.

Petrophysical and mechanical rock property database of the Los Humeros and Acoculco geothermal fields (Mexico)

Abstract. Petrophysical and mechanical rock properties are key parameters for the characterization of the deep subsurface in different disciplines such as geothermal heat extraction, petroleum reservoir engineering or mining. They are commonly used for the interpretation of geophysical data and the parameterization of numerical models and thus are the basis for economic reservoir assessment. However, detailed information regarding petrophysical and mechanical rock properties for each relevant target horizon is often scarce, inconsistent or distributed over multiple publications. Therefore, subsurface models are often populated with generalized or assumed values resulting in high uncertainties. Furthermore, diagenetic, metamorphic and hydrothermal processes significantly affect the physiochemical and mechanical properties often leading to high geological variability. A sound understanding of the controlling factors is needed to identify statistical and causal relationships between the properties as a basis for a profound reservoir assessment and modeling. Within the scope of the GEMex project (EU H2020, grant agreement no. 727550), which aims to develop new transferable exploration and exploitation approaches for enhanced and super-hot unconventional geothermal systems, a new workflow was applied to overcome the gap of knowledge of the reservoir properties. Two caldera complexes located in the northeastern Trans-Mexican Volcanic Belt – the Acoculco and Los Humeros caldera – were selected as demonstration sites. The workflow starts with outcrop analog and reservoir core sample studies in order to define and characterize the properties of all key units from the basement to the cap rock as well as their mineralogy and geochemistry. This allows the identification of geological heterogeneities on different scales (outcrop analysis, representative rock samples, thin sections and chemical analysis) enabling a profound reservoir property prediction. More than 300 rock samples were taken from representative outcrops inside the Los Humeros and Acoculco calderas and the surrounding areas and from exhumed “fossil systems” in Las Minas and Zacatlán. Additionally, 66 core samples from 16 wells of the Los Humeros geothermal field and 8 core samples from well EAC1 of the Acoculco geothermal field were collected. Samples were analyzed for particle and bulk density, porosity, permeability, thermal conductivity, thermal diffusivity, and heat capacity, as well as ultrasonic wave velocities, magnetic susceptibility and electric resistivity. Afterwards, destructive rock mechanical tests (point load tests, uniaxial and triaxial tests) were conducted to determine tensile strength, uniaxial compressive strength, Young's modulus, Poisson's ratio, the bulk modulus, the shear modulus, fracture toughness, cohesion and the friction angle. In addition, X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses were performed on 137 samples to provide information about the mineral assemblage, bulk geochemistry and the intensity of hydrothermal alteration. An extensive rock property database was created (Weydt et al., 2020; https://doi.org/10.25534/tudatalib-201.10), comprising 34 parameters determined on more than 2160 plugs. More than 31 000 data entries were compiled covering volcanic, sedimentary, metamorphic and igneous rocks from different ages (Jurassic to Holocene), thus facilitating a wide field of applications regarding resource assessment, modeling and statistical analyses.

Magmatic-hydrothermal processes and controls on rare-metal enrichment of the Baerzhe peralkaline granitic pluton, inner Mongolia, northeastern China

Although it is well known that many peralkaline granite plutons show enrichment of the rare earth elements (REE) and other high field strength elements (HFSE), the magmatic-hydrothermal processes that concentrate these elements are still poorly understood. The Early Cretaceous Baerzhe pluton in northeastern China provides an ideal example of super-enrichment in REE, Zr, Nb and Be in peralkaline granites. Three kinds of granites are identified, namely, the barren, the moderately-mineralized and highly-mineralized granites. The barren granite includes hypersolvus porphyritic arfvedsonite granite, transsolvus fine-grained granite, as well as transsolvus pegmatite and aplite; the moderately-mineralized granite includes transsolvus spherulitic granite and coarse-grained granite; and the highly-mineralized granite is subsovlus and contains spherulitic granite and medium-grained granite. Replacements of arfvedsonite by aegirine and alkaline feldspar by albite occurred in the moderately-mineralized granite, whereas an extensive alteration assemblage of Fe-Ti oxides and albite laths developed in the highly-mineralized granite. The barren porphyritic arfvedsonite granite samples show moderate SiO2 (63.20–68.00 wt%), high total alkali (Na2O + K2O = 9.94–10.23 wt%), and variable and high total FeO contents (7.56–13.45 wt%). The barren fine-grained granite, as well as moderately-mineralized spherulitic granite and coarse-grained granite, are characterized by high SiO2 (73.21–75.92 wt%), high total FeO (1.04–5.44 wt%) and total alkali (Na2O + K2O = 8.72–9.85 wt%) concentrations with A/CNK = 0.80–0.99 and NK/A = 1.00–1.24, indicative of peralkaline affinity. In comparison, the highly-mineralized spherulitic granite and medium-grained granite have more variable SiO2 (72.10–79.50 wt%) and lower total alkali (K2O + Na2O = 4.40–8.70 wt%) contents. All these samples have positive and uniform εNd(t) varying from + 2.25 to + 3.28, along with restricted 206Pb/204Pb (18.107–18.464) and 207Pb/204Pb ratios (15.497–15.549), showing that they were probably derived from a common juvenile melt. When compared with the barren fine-grained granite, the spherulitic granite contains more microcline and albite but less perthite, suggesting the spherulitic granite formed in a more evolved magma. The amounts of spherulites are in direct proportion to the intensity of hydrothermal alteration and mineralization. It is suggested that the spherulite structure was a product of low nucleation rates due to the high volatile content of the late-stage evolved magma. Extensive magma differentiation leads to the exsolution of F- and Cl-rich hydrothermal fluids which are enriched in large amount of REE and HFSE elements. Furthermore, intense fluid-rock interaction occurred in the most advantage stage of magmatic-hydrothermal evolution and led to extensive alteration as well as the formation of associated REE-Zr-Nb-Be mineralization in the large Baerzhe deposit.

Unravelling the P-T-t history of three high-grade metamorphic events in the Epupa Complex, NW Namibia: Implications for the Paleoproterozoic to Mesoproterozoic evolution of the Congo Craton

The Epupa Complex of northern Namibia constitutes the south-western margin of the Archean Congo Craton of central Africa. We present new petrological and geochronological data for metasedimentary migmatites that decode the poorly-known Paleoproterozoic to Mesoproterozoic evolution of this remote part of the craton. Detrital magmatic zircons with concordant 207Pb/206Pb ages between 1898 and 1774 Ma are interpreted to demonstrate the formation of the metasedimentary protoliths through reworking of Paleoproterozoic granitoids of a magmatic arc in a back-arc basin. SHRIMP U-Pb zircon analyses of metamorphic rims around the detrital grains linked with constraints from pseudosection modelling revealed three distinct high-grade metamorphic events in the migmatites. Late Paleoproterozoic regional HT-LP metamorphism between 1740 and 1720 Ma constitutes the oldest event and only affected the rocks of the northernmost part of the Epupa Complex (Eyao Unit). Heating to mid-amphibolite facies peak P-T conditions of 720 °C and 4 kbar caused the partial replacement of early biotite-sillimanite by garnet-cordierite assemblages during melt-producing reactions in metapelites. The near-isobaric heating-cooling P-T paths and the high geothermal gradient (60 °C/km) are consistent with HP-LP metamorphism in a back-arc basin setting with up-rising basic melt as heat source. Early Mesoproterozoic (1530 Ma) HT-LP metamorphism is recorded by rare Mg-rich Opx-Crd rocks that are restricted to the westernmost part of the Eyao Unit. Near-isobaric P-T paths culminate at granulite facies peak-conditions of 830 °C and 2 kbar. This local HP-LP event is interpreted to record contact metamorphism related to the coeval emplacement of early-Mesoproterozoic granitoids or the emplacement of intimately associated gabbros. Metapelitic Grt-Bt-Sil migmatites record the third high-grade metamorphic event that was active at ca. 1330 Ma and is restricted to the southern part of the Epupa Complex (Orue Unit). Sillimanite pseudomorphs after kyanite and late cordierite coronas around garnet indicate a clockwise P-T path during regional upper-amphibolite facies metamorphism that peaks at lower crustal conditions of 770 °C and 7.5 kbar. The clockwise P-T path is interpreted to reflect crustal thickening through magmatic accretion of basic melts that produced the coeval Kunene Intrusive Complex, the largest massif-type anorthosite in the world. The metamorphic events detected in the migmatites record a change of the geotectonic position of the Epupa Complex from a convergent continental margin during the late Paleoproterozoic to an intracratonic position in the Mesoproterozoic. The migmatites of the Eyao Unit were affected by an intense hydrothermal alteration during the Pan-African orogeny (ca. 520 Ma) that also caused the partial re-setting of the U-Pb zircon isotope system.

PETROLOGICAL CHARACTERISTICS OF CLASTIC SEDIMENTARY ROCKS FROM THE ST. BARBARA ORE MINE IN RUDE NEAR SAMOBOR

In the scope of this research, the petrographic and geochemical analyses of clastic sedimentary rocks from the St. Barbara, copper and iron ore mine in Rude near Samobor, have been made, aiming to classify and determine the provenance and environment of formation of clastic sedimentary rocks, as well as the influence of hydrothermal fluids on their properties. Eight samples were collected in total from different locations in the mine. Six of those samples have been studied in detail. The results showed structural and geochemical variability and uneven hydrothermal alteration intensity in the samples. Based on petrographic analyses, three rocks are classified as sublithoarenite, quartz arenite and quartz greywacke. Three of the samples are classified as ore breccia, ore sandstone, and ore silt-sandstone due to the pronounced hydrothermal alterations and ore mineralisation. Comprehensively, petrographic and geochemical analyses indicate that the source rocks of the investigated clastic sedimentary rocks were felsic (La/Sc = 0.9-6.2; Th/Sc = 1.3-2.4), most probably sedimentary, possibly resedimented rocks. The extent of source rock weathering was very high (Chemical Index of Alteration CIA = 52.1 - 81.3 %) and the geotectonic position of the sedimentary basin was very likely to be at the passive continental margin.