Crustal Reservoirs Research Articles

Two stage of gold-bearing sulfide ores of early Proterozoic gabbroids in the north Ladoga area

In a study based on Pb isotopes, it has been shown that the formation of gold-sulfide mineralization in the Early Proterozoic gabbros of the Northern Ladoga region is associated with different tectonic phases. This conclusion is based on a comparative study of isotopic data for feldspars and lead sulfides in two Early Proterozoic intrusions; Velimäki and Alattu-Päkylä. The investigated intrusions have similar geological settings at the junction of the Svecofennian accretionary complex and the Karelian craton. Both massifs are characterized by manifestations of noble metal sulfide mineralization associated with zones of local shear deformations. The formation of the studied ore massifs is attributed to the Svecofennian and Caledonian tectonic stages. Feldspars of Velimäki intrusion have parameters and model age of Pb significantly older than U-Pb age of zircon (1.9 Ga), although they correspond to the Svecofennian time of magmatic crystallization of gabbro and clinopyroxenites, while sulfide pyrite-pyrrhotite mineralization is associated with gold in these rocks with the Caledonian stage of fluid-thermal processing of the Early Proterozoic magmatic mineral paracenteses. Feldspars and sulfides of the Alattu-Päkylä intrusion have similar lead isotopic parameters and Pb model age (~ 2 Ga), also somewhat older than the U-Pb age of zircon, but at the same time indicating the formation of rock-forming and ore sulfide associations with gold during the Svecofennian (Early Proterozoic) plutonic and tectonic-thermal events. The source of lead sulfides of the Velimäki massif is characterized by the parameters of the upper crust with high µ2 = 238U / 204Pb, while the µ2 parameter of the minerals of the Alattu-Päkylä massif is slightly lower, which indicates the participation of the mantle-lower crustal reservoir material in the formation of magma.

New Ti-in-quartz diffusivities reconcile natural Ti zoning with time scales and temperatures of upper crustal magma reservoirs: REPLY

New Ti-in-quartz diffusivities reconcile natural Ti zoning with time scales and temperatures of upper crustal magma reservoirs: REPLY

At the crossroads of the Lesser Caucasus and the Eastern Pontides: Late Cretaceous to early Eocene magmatic and geodynamic evolution of the Bolnisi district, Georgia

The Bolnisi district is a distinct tectonic zone of the Lesser Caucasus, which is considered to represent the eastern extremity of the Turkish Eastern Pontides. Late Cretaceous, low-K, calc-alkaline to high-K rhyolite of the Mashavera and Gasandami Suites is the predominant rock type of the district, and is accompanied by subsidiary dacite, and rare high-alumina basalt and trachyandesite of the Tandzia Suite. The Mashavera and Gasandami rhyolite and dacite have yielded U-Pb LA-ICP-MS and TIMS zircon ages between 87.14 ± 0.16 and 81.64 ± 0.94 Ma, which are in line with the Coniacan-Santonian ages of radiolarian fauna of the Mashavera Suite. The felsic rocks of the Mashavera and Gasandami Suites were deposited during a ~6.6 m.y.-long silicic magmatic flare-up event, which together with the Tandzia Suite mafic rocks, documents Late Cretaceous bimodal magmatism in an extensional tectonic setting. Trace element data indicate that high Y-Zr, low- to high-silica rhyolite and dacite, and low Y-Zr high-silica rhyolite have been erupted, respectively, from coeval deep and shallow crustal reservoirs. The rocks of the bimodal magmatic event are overlain by high-K volcanic rocks of the Campanian Shorsholeti Suite, which have been erupted during slab roll-back and steepening, from magmas produced by deep melting of a metasomatised mantle. Eocene postcollisional felsic intrusions crosscut the Late Cretaceous rock.The Coniacian to early Campanian bimodal magmatism, and the subsequent high-K magmatism of the Bolnisi district are contemporaneous and share geochemical characteristics with the Late Cretaceous magmatism of the Eastern Pontides. It documents the existence of a Late Cretaceous regional silicic magmatic province, and subsequent high-K magmatism during slab steepening. This regional magmatic evolution coincided with the opening of the Black Sea and the Adjara-Trialeti basins. This evolution was coeval with the wanning stages of northern Neotethyan subduction, after a ~40 m.y.-long magmatic lull along the southern Eurasian convergent margin. Early Eocene adakite-like magmatism affected both the Bolnisi district and the Eastern Pontides, demonstrating a common postcollisional magmatic evolution.

Exploration domains and technological breakthrough directions of natural gas in SINOPEC exploratory areas, Sichuan Basin, China

Natural gas exploration in SINOPEC's exploratory areas in the Sichuan Basin has achieved rapid development. Since discovering the Xinchang gas field, natural gas exploration has expanded gradually, with both reserves and production growing fast. The deepening understanding of geological theory and crucial technology's progress is the main driving force for the natural gas industry's significant development. Considering the rich natural gas resources, various types of promising targets, exploration degree of 16% of conventional natural gas, and the initial stage of shale gas exploration, the HCZ model predicted natural gas exploration. Additionally, it indicated that the shale gas in the SINOPEC exploratory areas in the Sichuan Basin would consistently have rapid development with proven reserves increasing by (160–200) × 109m3/a. In the future, the development strategy is to vigorously develop shale gas, steadily promote marine conventional natural gas, and continually research continental tight sandstone gas. The following tasks need to be strengthened for efficient exploration of the three resource types: prediction of deep and ultra-deep reef-shoal reservoirs, weathered crust reservoirs, and technological breakthrough of efficient drilling and fracturing test of ultra-deep wells; the technological breakthrough of geological-logging-seismic integrated sweets pot prediction of tight sandstones and low-cost multi-layer fracturing of vertical wells; the technological breakthrough of increasing-yield and reducing-cost drilling, completion, and fracturing of deep marine shale gas; resources potential evaluation and adaptive exploration and development technologies preparation of continental and transitional shale gas.

Tracking metal evolution in arc magmas: Insights from the active volcano of La Fossa, Italy

The mineralization potential of arc magmas depends, among other factors, on the timing of sulfide melt saturation relative to magma differentiation and to exsolution of a magmatic fluid phase. In fossil mineralized or barren systems, understanding the evolution of metals along the magma differentiation path is often hindered by late magmatic processes and hydrothermal alteration. To better understand the process of metal evolution “caught in the act” in crustal reservoirs, we analyzed magmatic sulfides and melt inclusions found within eruptive products from the active arc volcano, La Fossa (Vulcano Island, Italy), for the basalt to rhyolite compositional spectrum. We found that, in case of sulfide-undersaturated and volatile-rich arc basalts, metals are scarcely subtracted by degassing during ascent to shallow crustal reservoirs and reach the highest abundances in intermediate magmas (250 ppm Cu). At sulfide saturation the sulfide melt has 34–66 wt% Cu, leading to a dramatic decrease in chalcophile metals dissolved in the silicate melt. After fractionation of only 0.2–0.3 wt% of sulfide in the solid assemblage, the exsolved sulfide is a monosulfide solid solution (pyrrhotite) containing <3 wt% Cu. Metals that do not partition in sulfides (Pb, Zn) increase their concentrations during magmatic evolution until they are sequestered by a Cl-rich aqueous fluid phase exsolved at the rhyolitic stage. The absolute and Cu-normalized concentrations of metals in sulfide inclusions are similar to sulfide accessories in magmatic rocks associated with world-class porphyry Cu systems. Our results demonstrate that the mechanisms governing metal evolution inferred for the magmatic stage in porphyry Cu environments can be also tracked at an active arc volcano, using eruptive products as snapshots of the magmatic evolution. Arc volcanoes can thus be viewed as ideal active analogues when studying these crucial processes for the formation of porphyry Cu deposits.

Evidence of Rapid Phenocryst Growth of Olivine During Ascent in Basalts From the Big Pine Volcanic Field: Application of Olivine‐Melt Thermometry and Hygrometry at the Liquidus

AbstractThe Quaternary Big Pine (BP) volcanic field in eastern California is notable for the occurrence of mantle xenoliths in several flows. This points to rapid ascent of basalt through the crust and precludes prolonged storage in a crustal reservoir. In this study, the hypothesis of phenocryst growth during ascent is tested for several basalts (13–7 wt% MgO) and shown to be viable. Phenocrysts of olivine and clinopyroxene frequently display diffusion‐limited growth textures, and clinopyroxene compositions are consistent with polybaric crystallization. When the most Mg‐rich olivine in each sample is paired with the whole‐rock composition, resulting (olivine‐melt) values (0.31–0.36) match those calculated from literature models (0.32–0.36). Application of a Mg‐ and a Ni‐based olivine‐melt thermometer from the literature, both calibrated on the same experimental data set, leads to two sets of temperatures that vary linearly with whole‐rock MgO wt%. Because the Ni thermometer is independent of water content, it provides the actual temperature at the onset of olivine crystallization (1247–1097°C), whereas the Mg thermometer gives the temperature under anhydrous conditions and thus allows ΔT (=TMg − TNi = depression of liquidus due to water) to be obtained. The average ΔT for all samples is ~59°C, which is consistent with analyzed water contents of 1.5–3.0 wt% in olivine‐hosted melt inclusions from the literature. Because the application of olivine‐melt thermometry/hygrometry at the liquidus only requires microprobe analyses of olivine combined with whole‐rock compositions, it can be used to obtain large global data sets of the temperature and water contents of basalts from different tectonic settings.

Zircon U–Pb-Hf isotope data in eclogite and metagabbro from southern Sweden reveal a common long-lived evolution and enriched source

ABSTRACT Several orogenies have shaped the bedrock of southern Sweden. While mafic intrusions represent significant sources of information for reconstructing geodynamics and crustal evolution, the characterization of the various generations of such intrusions in Sweden remains limited. We report in situ zircon U–Pb ages and Hf isotope data from a Fe-Ti eclogite and a coronitic metagabbro from the Eastern Segment in southern Sweden. Crystallisation ages at 1683 ± 17 Ma of the eclogite suggest affiliation with the surrounding 1730–1660 Ma Transscandinavian Igneous Belt intrusions that dominate the Eastern Segment. Secondary zircon growth and Pb-loss in the eclogite sample at 1459 ± 44 Ma and the crystallisation of the metagabbro at 1431 ± 26 Ma overlap and are related to magmatic activity during the Hallandian orogeny. Zircon Hf isotope signatures with chondritic and sub-chondritic values at ~1683 Ma and ~1431 Ma, respectively, correspond to an enriched (or mildly depleted) source in line with a “Mixed Svecofennian Crustal Reservoir”. These isotope signatures are more enriched than those in the surrounding gneisses. Zircon isotope data from the herein analysed zircon grains indicate that the eclogite and metagabbro had an enriched mafic source in the mid to lower crust, or within the subcontinental lithospheric mantle below Fennoscandia.

The Long-Term Life-Cycle of Nevado de Toluca Volcano (Mexico): Insights Into the Origin of Petrologic Modes

The petrologic diversity of volcanic rocks reflects the dynamics of magma reservoirs and the temporal evolution of magma chemistry can provide valuable information for hazard assessment. While some stratovolcanoes monotonously produce intermediate magmas (55–68 wt% SiO2), dominantly erupted magma types (e.g., basaltic andesite, andesite or dacite) frequently differ even between neighboring volcanoes. If such differences arise due to thermal maturation processes over time or are predetermined by other properties of magmatic systems remains poorly understood. This study helps to elucidate the underlying factors modulating the chemistry of the magma preferentially erupted by Nevado de Toluca volcano in Central Mexico. We present a new dataset of bulk-rock and mineral chemistry spanning the entire 1.5 Million years of the volcanos’ eruptive history. The results reveal that Nevado de Toluca dacites and minor andesite originate in a stable configuration of pre-eruptive processes and plumbing system architecture by hybridization between an upper crustal silicic mush and deeper sourced basaltic andesite magmas. Yet, a subtle trend toward increasing silica content with time (2 wt% in 1.5 Ma) and episodicity in magma hybridization conditions are observed. We use thermal simulations of pulsed magma injection to probe the controlling variables on the temporal variation and compositional mode of magma geochemistry. The results show that the subtle temporal trend toward increasing bulk-rock SiO2 content is plausibly explained by slightly dropping recharge rates and continued upper crustal reservoir growth. Our modeling also shows that the dominant composition of eruptible magmas (“petrologic mode”) can shift as a function of magma flux, extrusive:intrusive ratio and temperature of the recharge magma. A comparison of SiO2 whole rock distributions for monotonous Mexican stratovolcanoes and their peripheral cones shows that their petrologic modes vary in concert, indicating that the recharge magma chemistry or temperature is a major control on the preferentially erupted magma composition for these volcanoes.

Large Silicic Eruptions, Episodic Recharge, and the Transcrustal Magmatic System

AbstractLarge silicic eruptions result from rapid evacuation of large, upper crustal reservoirs of silicic magmas. These silicic magmas are thought to be replenished by melt extracted from underlying crystal‐rich source mushes, but the timescales and mechanisms of such melt extraction are unclear. Geochemical observations suggest that the replenishing melt is often more primitive than the eruptives and must thus cool and crystallize to generate the highly silicic magmas that eventually lead to large eruptions. Motivated by these observations, we use thermal models to explore the replenishment conditions capable of building an eruptible silicic reservoir to generate a large eruption. Results show that the minimum melt replenishment rate required for a silicic reservoir to start growing increases with the effective thermal diffusivity of the overlying crust and decreases with the depth of the reservoir. For an eruptible reservoir at 6 km depth to grow, the initial replenishment rate needs to be greater than 2 × 10−9 m/s. High replenishment rates are required to provide enough advected heat to counterbalance rapid heat loss and consequent freezing that would prevent the eruptible reservoir from growing. However, these high initial replenishment rates must then subside over time for the eruptible reservoir to cool, crystallize, and evolve to highly silicic melts. Thermal histories of some natural systems suggest assembly of large eruptible reservoirs in &lt;20 kyr. The rapid replenishment followed by its decay suggests that replenishment was triggered by a pronounced but ephemeral increase in the porosity and permeability of the underlying crystal‐rich source mush, allowing for rapid melt expulsion. We speculate that this perturbation may be driven by the sudden incursion of deep‐seated, hotter magmas into the base of the crystal‐rich source mush.

Correlation between tectonic CO2 Earth degassing and seismicity is revealed by a 10-year record in the Apennines, Italy.

Deep CO2 emissions characterize many nonvolcanic, seismically active regions worldwide, and the involvement of deep CO2 in the earthquake cycle is now generally recognized. However, no long-time records of such emissions have been published, and the temporal relations between earthquake occurrence and tectonic CO2 release remain enigmatic. Here, we report a 10-year record (2009-2018) of tectonic CO2 flux in the Apennines (Italy) during intense seismicity. The gas emission correlates with the evolution of the seismic sequences: Peaks in the deep CO2 flux are observed in periods of high seismicity and decays as the energy and number of earthquakes decrease. We propose that the evolution of seismicity is modulated by the ascent of CO2 accumulated in crustal reservoirs and originating from the melting of subducted carbonates. This large-scale, continuous process of CO2 production favors the formation of overpressurized CO2-rich reservoirs potentially able to trigger earthquakes at crustal depth.

Tectonomagmatic evolution of a Jurassic Cordilleran flare-up along the Korean Peninsula: Geochronological and geochemical constraints from granitoid rocks

This study used new and published U-Pb geochronological, chemical, and Sr-Nd-Hf-O isotopic data (n > 2500) from Jurassic granite-granodiorite-diorite-monzonite-gabbro plutons in the southern part of the Korean Peninsula to assess the spatiotemporal evolution of a flare-up magmatism, its tectonic connection, and specific contributions of mantle and crustal reservoirs to the magmas generated. After a ~15 m.y. magmatic gap in the Late Triassic, calc-alkaline granitoids intruded into the outboard Yeongnam Massif, then magmatic activity migrated systematically toward the inboard Gyeonggi Massif. The early phase of the Jurassic magmatism is represented by relatively sodic plutons showing distinctly primitive isotopic signatures. The crustal signature of the plutons became increasingly prominent with decreasing age. Voluminous inboard plutons in the Gyeonggi Massif and the intervening Okcheon Belt are dominated by Middle Jurassic peraluminous granites that show isotopic compositions conspicuously shifted toward old crustal values. The Nd-Hf isotopic compositions of the inboard plutons are distinctly less radiogenic than those of Jurassic plutons in Southwest Japan and southeastern China, which corroborates the North China affinity of the Yeongnam and Gyeonggi massifs. The geochronological and geochemical data compiled in this study suggest a tectonomagmatic model consisting sequentially of (1) an extension-dominated arc system in the margin of the Yeongnam Massif (ca. 200–190 Ma); (2) low-angle subduction and the development of an advancing arc system (ca. 190–180 Ma); (3) continued low-angle subduction, extensive underthrusting of fertile crustal materials to the arc root, and consequent magmatic flare-up (ca. 180–170 Ma); and (4) flat subduction and the development of the Honam Shear Zone (ca. 170–160 Ma). The subsequent magmatic lull and previous dating results for synkinematic rocks and minerals indicate that the compressional arc system was maintained until the Early Cretaceous.

Structural division of granite weathering crusts and effective reservoir evaluation in the western segment of the northern belt of Dongying Sag, Bohai Bay Basin, NE China

With improvements in exploration, increasing numbers of large and medium-sized granite reservoirs have been discovered in many parts of the world. However, there is no unified agreement about the structural division of granite weathering crusts and the characteristics of favorable oil and gas reservoirs have not been clarified. Based on cores and thin section observations, outcrop surveys, conventional logging data, image logging analysis and seismic interpretations, our study analyzes the vertical structural characteristics of granite weathering crusts and the characteristics of reservoirs in the western segment of the northern belt of Dongying Sag, Bohai Bay Basin, NE China. The results indicate that the vertical structure of granite weathering crusts in the study area is a double layer composed of a top eluvium or dissolution layer (completely weathered layer) and a bottom disintegration layer (semiweathered layer) and the dissolution and eluvium layers are the products of different evolution stages on top of the disintegration layer. The types of effective reservoirs can be divided into three categories: type I (dissolution layer), with a porosity of 2–8% (averaging 4%) and permeability of 0.1–25 md (averaging 0.5 md), which are mainly distributed in the middle and bottom parts of the structure and yield high oil and gas productivity; type II (eluvium), with a porosity of 3–5% (averaging 4%) and permeability of 0.5–5 md (averaging 1 md), which are mainly distributed in the gentle slope area of the uplift and have poor oil- and gas-bearing properties; and type III (disintegration layer), with a porosity of 1–3% (averaging 2%) and permeability of 0.1–0.5 md (averaging 0.11 md), which are widely distributed and found in the high and middle to low parts of the structure and present oil spots in the cores. The lateral distribution of effective reservoirs in the study area can be divided into the top section of the uplift, the edge section of the uplift and the bottom section of the slope. The top mainly develops a disintegration layer, the edge mainly develops eluvium, and the bottom of the slope develops a dissolution layer. The dissolution layer in the middle and bottom sections of the structure is the most favorable reservoir development zone. The dynamic genetic pattern of the double layer structure of granite weathering crust and the distribution characteristics of reservoir differences proposed in this paper have important theoretical and application value for determining the geological genesis and performing oil and gas exploration of granite weathering crust reservoirs in other rift basins.

Low-temperature silica-rich gold mineralization in mafic VMS systems: evidence from the Troodos ophiolite, Cyprus

The Troodos ophiolite Cyprus hosts the on-land analogue for actively forming mafic, Cu-rich, or Cyprus-type VMS deposits. In addition to high-temperature (> 350 °C) sulfide mound-related mineralization, other fossil seafloor mineralizing systems are known to operate, including those characterized by an enrichment in Au and abundant silicification. In this study the mineralogy and geochemistry of four Au and silica-rich localities in Troodos are considered, and these include Kokkinovounaros, Mathiatis South, Touronjia, and Alpen Rose. We present whole rock geochemical and mineralogical data characterizing the distribution of Au in the hydrothermal systems of Troodos. Samples from silica-rich localities have two distinct sample mineralogies: supergene samples that contain predominantly goethite, hematite, and jarosite and hypogene samples that contain quartz, amorphous silica and minor hematite. Hypogene samples from Mathiatis South and Kokkinovounaros are enriched in Au with a median concentration of 1.5 ppm relative to supergene samples that contain 0.1 ppm (n = 107). This indicates that Au enrichment occurred on the seafloor and is not solely related to supergene weathering of silica-rich mineralization. We suggest that silica-rich Au mineralization in Troodos formed during the migration of newly formed crust off-axis or as white smokers proximal to known VMS deposits. In silica-rich mineralization that is located distally from known massive sulfide deposits, Au was probably remobilized from shallow crustal reservoirs during the low-temperature fluid flow (100–300 °C) in the lower extrusive sequence as the crust cooled and migrated off-axis. Based on modern seafloor analogues, we propose a revised model for the Troodos hydrothermal system that explains the distribution of Au in silica-rich mineralization that formed off-axis and postdates VMS formation and in zones proximal to VMS deposits that formed simultaneously as white smoker-type vents. We suggest that silica-rich mineralized zones formed an intermediary between high-temperature (> 350 °C) on-axis VMS deposits and the low-temperature silicification of umbers (< 100 °C).

The partitioning behavior of trace elements in subduction zones: Advances and prospects

The partitioning behavior of trace elements is of key importance for understanding the geochemical process and material cycle mechanism in subduction zones. This paper focuses on the advances and prospects on the studies of trace element partitioning in subduction zones from the following four aspects. (1) The properties of fluids derived from subducting slabs and their ability in element transport. How slab-derived solute-rich fluids and supercritical fluids are formed and what the roles and key control factors of these fluids are in transferring of elements (especially the high field strength elements) from slab to wedge are discussed. We point out that the detailed investigations of supercritical fluids may provide a new perspective for the element migration mechanism, material cycle process, arc magma genesis and so on. (2) The behavior of transition elements during mantle wedge melting. The behavior of the first row transition elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) in the mantle partial melting process is compatible or incompatible, depending on residual mineral assemblage and physicochemical conditions. The partitioning behavior of the elements such as Sc, Ti, Co, Ni and Zn whose valence states do not change in the melting process mainly depends on the residual mineral assemblage and temperature, whereas the partitioning behavior of the multivalent elements such as V and Fe is also the function of oxygen fugacity (fO2) in addition to mineral assemblage and temperature. Therefore, the partitioning behavior of transition elements has important applications in tracing lithologic inhomogeneity and fO2 of the mantle wedges. (3) The specificity of element partitioning behavior during arc magma evolution. Garnet has Dy/Yb partitioning behavior different from amphibole, and rutile has Nb/Ta partitioning behavior different from amphibole. Dy/Yb and Nb/Ta partitioning differences for these minerals enable to distinguish the specific evolution process of arc magmas. The Dy/Yb and Nb/Ta generally decrease with the increase of SiO2 in arc magmas, indicating that amphibole fractionation should be the most important during arc magma differentiation. (4) The behavior of sulfur and chalcophile elements and porphyry metallogeny. In subduction zones, the behavior of chalcophile elements such as Cu and Au is controlled by sulfide and fluid. Therefore, the stability of sulfide, the time at which the fluid exsolves from the melt relative to sulfide saturation, the fluid/sulfide mass ratio and fluid/melt Cu and Au partition coefficients in intermediate-felsic magma- H2O systems are especially important in understanding Cu and Au enrichment in magma-hydrothermal processes. Intermediate-felsic magmas mainly originate from the differentiation of arc magmas at lower crustal reservoirs, and thus the fluid exsolution from the lower crustal reservoirs and the fluid/melt and fluid/sulfide partition coefficients of Cu and Au should be the keys to understanding quantitatively how Cu and Au are migrated from the deep crust to the shallow site of mineralization.

Magma diversity reflects recharge regime and thermal structure of the crust

The chemistry of magmas erupted by volcanoes is a message from deep within the Earth’s crust, which if decrypted, can provide essential information on magmatic processes occurring at inaccessible depths. While some volcanoes are prone to erupt magmas of a wide compositional variety, others sample rather monotonous chemistries through time. Whether such differences are a consequence of physical filtering or reflect intrinsic properties of different magmatic systems remains unclear. Here we show, using thermal and petrological modelling, that magma flux and the thermal structure of the crust modulate diversity and temporal evolution of magma chemistry in mid to deep crustal reservoirs. Our analysis shows that constant rates of magma input leads to extractable magma compositions that tend to evolve from felsic to more mafic in time. Low magma injection rates into hot or deep crust produces less chemical variability of extractable magma compared to the injection of large batches in colder or shallower crust. Our calculations predict a correlation between magma fluxes and compositional diversity that resembles trends observed in volcanic deposits. Our approach allows retrieval of quantitative information about magma input and the thermal architecture of magmatic systems from the chemical diversity and temporal evolution of volcanic products.

Magmatic evolution of the mineralized Štiavnica volcano (Central Slovakia): Evidence from thermobarometry, melt inclusions, and sulfide inclusions

The factors that control the fertility (e.g. capacity to form an ore deposits) of a magmatic system are still a matter of debate, requiring more studies on the evolution of ore-related magmas. This study presents a detailed reconstruction of the magmatic system forming the Štiavnica volcano, the biggest mineralized center of the Western Carpathians (Central Slovakia). The Štiavnica volcano represents an interesting study object because the magmatic system was sulfide-saturated during its entire evolution, and porphyry systems with contrasting Cu/Au ratio are present in the same volcano. This situation allows to study the impact of magmatic sulfide saturation on the Cu/Au ratio of porphyry deposits. Pressure, temperature, and the compositional evolution of pre-, syn-, and post-mineralization magmas were reconstructed based on the chemical composition of phenocrysts and hosted silicate melt inclusions and sulfide inclusions. Dominantly andesitic magmas forming the volcanic and intrusive rocks of the volcano were sourced from an upper crustal (~1 to ~3 kbar) reservoir in which most phenocrysts grew from a sulfide-saturated dacitic to rhyolitic melt. This upper crustal magmatic reservoir was active during >3 My, and most ore-deposits, including Cu-Au porphyries, formed during cooling periods where the residual melt reached fluid saturation. In contrast, the Au-only porphyry deposit formed after shallow emplacement (<1 km) of a hot magma that was produced by mixing between a magma derived from a lower crustal (>8 kbar) reservoir and one from the upper crustal reservoir. Inclusions of monosulfide solid solution were found in phenocrysts crystallizing over a large range of temperature and pressure in ore-related and barren magmas suggesting that sulfide saturation doesn't play a critical role on the fertility of the magmatic system. Finally, this study reveals that composition of the magmatic sulfides does not play a direct role in the Cu/Au ratio of the formed porphyry deposits.

Tectonic Controls on Global Variations of Large-Magnitude Explosive Eruptions in Volcanic Arcs

Linking tectonic setting to eruptive activity in volcanic arcs provides a framework to understand processes that control the production, accumulation and eruption of magma on Earth. We use the Holocene eruptive records of 162 volcanoes, which are selected based on an assessment of recording biases, to calculate the probability of recording large eruptions (between Magnitudes 4 and 7). We quantify regional variability in the sizes of volcanic eruptions and compare it with subduction parameters influencing the generation, transport and storage of magma. Given the tectonic setting of a subduction zone is multidimensional (e.g., age, speed, obliquity of the subducting plate) we use a graphical model to explore the strength of probabilistic relationships between tectonic and volcanic variables. The variable that exhibits the strongest probabilistic relationship with eruption size is convergence obliquity, with larger eruptions favored in settings where convergence is normal. Normal convergence favors the storage and accumulation of larger volumes of magma, whereas oblique convergence favors the transport and eruption of smaller volumes of magma. In low-obliquity arcs where magma storage is promoted, the subduction of older slabs results in higher mantle productivity, which thermally favors the accumulation of eruptible magma and larger eruptions on average. However, the highest mantle productivity also results in more frequent magma injection and pressurization of crustal reservoirs. Consequently, arcs with moderate slab ages and low obliquity produce the highest proportion of larger eruptions. In high-obliquity arcs mantle productivity does not dominantly control eruption sizes. Instead, thinner crust facilitates frequent transport of magma to the surface, resulting in smaller eruptions. For the largest eruptions on Earth (e.g., Magnitude 8), however, accumulation of eruptible magma will be dominantly controlled by thermomechanical modification of the crust and not the frequency of magma intrusion. Despite the importance of convergence obliquity, our results show that variability in the sizes of volcanic eruptions is controlled by complex relationships with other parameters including slab age and crustal thickness. By using a graphical model, we have been able to explore complex volcano-tectonic relationships. We suggest a similar approach could be extremely valuable for exploring other complex multidimensional datasets within the Earth Sciences.

Ore Geology, Fluid Inclusions, and (H-O-S-Pb) Isotope Geochemistry of the Sediment-Hosted Antimony Mineralization, Lyhamyar Sb Deposit, Southern Shan Plateau, Eastern Myanmar: Implications for Ore Genesis

The Lyhamyar deposit is a large Sb deposit in the Southern Shan Plateau, Eastern Myanmar. The deposit is located in the Early Silurian Linwe Formation, occurring as syntectonic quartz-stibnite veins. The ore body forms an irregular staircase shape, probably related to steep faulting. Based on the mineral assemblages and cross-cutting relationships, the deposit shows two mineralization stages: (1) the pre-ore sedimentary and diagenetic stage, and (2) the main-ore hydrothermal ore-forming stage (including stages I, II, and III), i.e., (i) early-ore stage (stage I) Quartz-Stibnite, (ii) late-ore stage (stage II) Quartz-calcite-Stibnite ± Pyrite, and (iii) post-ore stage (stage III) carbonate. The ore-forming fluid homogenization temperatures from the study of primary fluid inclusions in quartz and calcite indicate that the ore-forming fluid was of a low temperature (143.8–260.4 °C) and moderate to high-salinity (2.9–20.9 wt. % NaCl equivalent). Hydrogen and oxygen isotopes suggest that the ore-forming fluids of the Lyhamyar deposit were derived from circulating meteoric water mixed with magmatic fluids that underwent isotopic exchange with the surrounding rocks. Sulfur in Lyhamyar was dominated by thermochemical sulfate reduction (TSR) with dominant magmatic source sulfur. The lead isotope compositions of the stibnite indicate that the lead from the ore-forming metals was from the upper crustal lead reservoir and orogenic lead reservoir. On the basis of the integrated geological setting, ore geology, fluid inclusions, (H-O-S-Pb) isotope data, and previous literature, we propose a new ore-deposit model for the Lyhamyar Sb deposit: It was involved in an early deposition of pyrite in sedimentary and diagenetic stages and later Sb mineralization by mixing of circulating meteoric water with ascending magmatic fluids during the hydrothermal mineralization stage.

Geology, fluid inclusions, C–O–S–Pb isotopes and genesis of the Ahangaran Pb-Ag (Zn) deposit, Malayer-Esfahan Metallogenic Province, western Iran

The Malayer-Esfahan Metallogenic Province (MEMP) of western Iran hosts many Pb-Zn deposits and occurrences, largely hosted within a sequence of clastic detrital and carbonate host rocks. A well-known example is Ahangaran which is composed of several hypogene orebodies with estimated resources of ~1.5 Mt sulfide ore grading 4 wt% Pb, 0.5 wt% Zn and 200 g/t Ag, plus a supergene iron-bearing oxyhydroxide ore with 1 Mt reserve and grading 30–35 wt% FeO. The sulfide orebodies are primarily hosted in Lower Cretaceous dolostone and sandstone along the marginal-southern part of the NW–SE trending Kolehbid Syncline. Galena and pyrite, together with chalcopyrite and sphalerite in lesser amounts, comprise the primary sulfide ores. Minor sulfosalt minerals including bournonite, tetrahedrite, freibergite, famatinite and meneghinite have also been identified by electron probe microanalysis. Three paragenetic stages are recognized: (1) pre-sulfide stage (oxide stage); (2) main sulfide stage (including three sulfide sub-stages); and (3) supergene stage. Fluid inclusion studies conducted on pre- and main-sulfide stages identified two end-member fluids: a hot (~290 °C) and saline fluid (~20 wt% NaCl eq.) and a cooler (<150 °C), lower salinity fluid (~7 wt% NaCl eq.). C–O–S isotope studies suggest that these fluids originated as an 18O-enriched, heated basinal brine that carried Pb, Ag, and Ba, and a low salinity, SO42−- and H2S-bearing fluid derived from Mesozoic seawater. Sulfide required for ore formation was generated by thermochemical sulfate reduction and the sulfur of sulfate minerals was derived directly from dissolved SO42−. The Pb isotope compositions are homogeneous with 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios ranging from 18.458 to 18.501, 15.623 to 15.708 and 38.512 to 38.715, respectively. These ratios are indicative of a continental crustal reservoir as the source of lead and presumably other metals. Taken together, our observations suggest that the Ahangaran sediment-hosted Pb-Ag (Zn) deposit is an epigenetic, structurally-controlled and strata-bound deposit with fluids and metals derived from the Paleozoic basement rocks. This model for formation of the Ahangaran deposit is different to magmatic-hydrothermal, sedimentary-exhalative or Mississippi Valley-types, and to some extent is similar to Irish-type deposits.

The Paleoproterozoic Kandalaksha-Kolvitsa Gabbro-Anorthosite Complex (Fennoscandian Shield): New U–Pb, Sm–Nd, and Nd–Sr (ID-TIMS) Isotope Data on the Age of Formation, Metamorphism, and Geochemical Features of Zircon (LA-ICP-MS)

The paper provides new U–Pb, Sm–Nd, and Nd–Sr isotope-geochronological data on rocks of the Paleoproterozoic Kandalaksha-Kolvitsa gabbro-anorthosite complex. Rare earth element (REE) contents in zircons from basic rock varieties of the Kandalaksha-Kolvitsa area were analyzed in situ using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Plots of REE distribution were constructed, confirming the magmatic origin of zircon. Temperatures of zircon crystallization were estimated using a Ti-in-zircon geochronometer. The U–Pb method with a 205Pb artificial tracer was first applied to date single zircon grains (2448 ± 5 Ma) from metagabbro of the Kolvitsa massif. The U–Pb analysis of zircon from anorthosites of the Kandalaksha massif dated the early stage of the granulite metamorphism at 2230 ± 10 Ma. The Sm–Nd isotope age was estimated on metamorphic minerals (apatite, garnet, sulfides) and whole rock at 1985 ± 17 Ma (granulite metamorphism) for the Kolvitsa massif and at 1887 ± 37 Ma (high-temperature metasomatic transformations) and 1692 ± 71 Ma (regional fluid reworking) for the Kandalaksha massif. The Sm–Nd model age of metagabbro was 3.3 Ga with a negative value of εNd = 4.6, which corresponds with either processes of crustal contamination or primary enriched mantle reservoir of primary magmas.