Deep-seated Source Research Articles

Analytic Signal and Euler Depth Interpretation of Magnetic Anomalies: Applicability to the Beatrice Greenstone Belt

We apply the Analytic Signal and Euler depth filtering techniques on magnetic data to identify a magnetic causative body location-depth relationship, two parameters of importance in both geophysical exploration and ore body modelling. We identify a dipping magnetic contact from the interpreted Euler depth anomalies, showing a good agreement with both the Total Field Magnetic (TFM) map and the Analytic Signal (AS) map. The Euler depth anomalies correlate well with the locations and edges of shallow causative bodies. The deeper Euler interpreted sources explain the magnetic high on the regional aeromagnetic map which is coincident with neither geological contacts nor the more recent dolerite intrusions. This suggests that the magnetic highs on the regional aeromagnetic map are due to deep seated sources, otherwise invisible on the regional geological map. The results show the usefulness and relevancy of these two filters not only in interpreting routine TFM data from the study area, but up to a regional scale. While the aeromagnetic data shows that the magnetisation pattern is predominantly divorced from the geological map, the ground magnetic data interpretation points to a more recent magnetisation of the belt, enabling conclusions to be drawn about the geological history and structural geology otherwise not evident on the geological map.

The origin and hydrothermal mobilization of carbonaceous matter associated with Paleoproterozoic orogenic-type gold deposits of West Africa

The chemical and physical properties of carbonaceous matter (CM) were studied in Paleoproterozoic metasediment-hosted, orogenic-type gold deposits in Burkina Faso (the Inata deposit), in Mali (the Syama deposit), and in Ghana (the Obuashi and Bogoso deposits). Two types of CM occur in all the studied deposits: metamorphosed and hydrothermal. Metamorphosed CM prevails in all the deposits. Hydrothermal CM occurs in small veinlets in hydrothermally altered rocks and in quartz veins or forms irregular accumulations parallel or sub-parallel to C-type cleavage within the shear zones. The origin of hydrothermal CM, which occurs in paragenesis with Au-bearing arsenopyrite or pyrite, seems to have been due to supersaturation of hydrothermal fluids with carbon at the deeper or middle crustal levels. The isotopic composition of carbon in bulk CM (−33.1 to −26.2‰, VPDB) indicates its biogenic origin. The isotopic composition of carbon in hydrothermal carbonates ranges from −14.5 to −4.4‰ (VPDB), which suggest mixing of carbon derived from a deep-seated source with carbon derived from an organic source. The interaction of hydrothermal fluids with metamorphosed CM could be one of the causes of the reduction of hydrothermal fluids and formation of the respective mineralization. The optical properties and Raman spectra of the metamorphosed CM particles in the individual studied mineral deposits differ considerably. The temperatures calculated on the basis of the Raman spectra of metamorphosed CM vary between 280 and 440°C, depending on the thermometer used, and correspond to temperatures of metamorphism of upper sub-grenschist and greenschist facies. The temperatures calculated for hydrothermal CM at the individual deposits, are only slightly lower compared to the metamorphosed CM at the same deposits, which indicates approximately the same temperature of the metamorphic and hydrothermal processes.

Sulfur-containing particles emitted by concealed sulfide ore deposits: an unknown source of sulfur-containing particles in the atmosphere

Abstract. Sources of sulfur dioxide, sulfates, and organic sulfur compounds, such as fossil fuels, volcanic eruptions, and animal feeding operations, have attracted considerable attention. In this study, we collected particles carried by geogas flows ascending through soil, geogas flows above the soil that had passed through the soil, and geogas flows ascending through deep faults of concealed sulfide ore deposits, and analysed them using transmission electron microscopy. Numerous crystalline and amorphous sulfur-containing particles or particle aggregations were found in the ascending geogas flows. In addition to S, the particles contained O, Ca, K, Mg, Fe, Na, Pb, Hg, Cu, Zn, As, Ti, Sr, Ba, Si, etc. Such particles are usually a few to several hundred nanometres in diameter with either regular or irregular morphology. The sulfur-containing particles originated from deep-seated weathering or faulting products of concealed sulfide ore deposits. The particles suspended in the ascending geogas flow migrated through faults from deep-seated sources to the atmosphere. This is a previously unknown source of the atmospheric particles. This paper reports, for the first time, the emission of sulfur-containing particles into the atmosphere from concealed sulfide ore deposits. The climatic and ecological influences of these sulfur-containing particles and particle aggregations should be assessed.

Deep-seated hydrocarbons in the seep “Brecciated Limestones” of the Maiella area (Adriatic foreland basin): Evaporitic sealing and oil re-mobilization effects linked to the drawdown of the Messinian Salinity Crisis

The 13C-depleted “Brecciated Limestones” record a hydrocarbon seep event that occurred between 5.56 and 5.532 Ma in the Maiella area. They unconformably overlie the “Primary Lower Gypsum” and crop out as carbonate buildups and authigenic patches fed by fluid migration pathways filled with flow-mobilized pelites. Tar occurs as solid bitumen shows, corresponding to pore-filling tar in the microfacies, and as a distinctive brown facies, resulting in finely impregnating patterns in thin section. Geochemical rock characterization confirmed that the measured Total Organic Carbon, with values up to 11.07%, mostly consists of migrated hydrocarbons in the carbonate buildups and the authigenic patches, while the flow-mobilized pelites are devoid of oil traces. Bitumen characterization showed an overall homogeneity among the samples, established by their similarities in the tricyclic fraction and their uniform bitumen δ13C signals (−26.3/−28.66‰ PDB-1). Gas chromatography–mass spectrometry revealed five distinctive markers indicative of an oil that originated and migrated from a carbonate source rock: high C29/C30 hopane ratio; low Ts/Tm ratio; abundant C24 tetracyclic terpane; absence of diasteranes; and occurrence of methylsteranes. Considering the regional source rock setting, the carbonate source rock might presumably be Upper Triassic/Lower Liassic in age. We favor a conceptual model that envisages a step-wise migration of hydrocarbons, which originated from a deep-seated source and migrated through a hydrofractured reservoir (Bolognano Fm.). The Primary Lower Gypsum of the Messinian Salinity Crisis provided an efficient seal until the major evaporative drawdown of the Mediterranean Sea at ∼5.55 Ma triggered a renewed hydrocarbons migration, which occurred first with a vigorous gaseous release and then with a seeping oil flow. Oil impregnated the early cemented “Brecciated Limestones” and partly contributed to their precipitation, whereas a self-clogging effect supposedly prevented oil impregnation in correspondence with the feeder channels formed during the first gaseous migration phase.

Peculiarities of the geomagnetic field, revealed by gradient magnetic measurements at stratospheric altitudes

The results of balloon gradient magnetic surveys in the stratosphere using a long (6 km) vertical oriented measuring base for studying the magnetic field, which is generated by deep-seated sources, are considered. The errors in the current analytical model of the geomagnetic field were analyzed. The device designed to measure the geomagnetic field vertical gradients on a drifting balloon is described. The data of the balloon gradient magnetic surveys, performed at altitudes about 30 km, are analyzed. A new method for separating magnetic anomalies from the data of surveys along extensive profiles is considered. It was shown that the gradients of the magnetic sources located near the bottom of the Earth’s crust are reliably measured by a measuring base with a length of 6 km. An independent new method for determining the errors of global analytical models of the main Earth’s magnetic field (EMF), which are used to separate magnetic anomalies from data of surveys along extensive routes, was justified. The cause of unsatisfactory results of previous magnetic field convertions from the Earth’s surface upward, which is explained by defects of used ground data, was studied. It was shown that the EMF EMM/720 model unsatisfactorily represents an anomalous EMF at an altitude of 30 km. It has been concluded that balloon gradient magnetic surveys is of prime importance in studying and modeling the anomalous magnetic field structure in near-Earth space. A model of EMF, developed up to 720 spherical harmonics, should be constructed based on satellite and balloon gradient magnetic data.

Sedimentogenesis in the Mesozoic-Cenozoic continental rifts of Central Asia

An analysis of continental sedimentogenesis in Mesozoic rifts of the Transbaikal region and Mongolia, as well as in the Cenozoic Baikal rift zone, revealed a succession of volcanic-sedimentary formations, which constitute the sedimentary cover. The peculiar sedimentation features reflect exogenic processes that are characteristic of rift structures and the influence of deep-seated (exogenic and catagenic) sources. It is proposed to define a specific, rift, type of sedimentogenesis.

The gravimetric picture of magmatic and hydrothermal sources driving hybrid unrest on Tenerife in 2004/5

We present results from the inversion of gravity changes observed at the central volcanic complex (CVC) of Tenerife, Canary Islands, between May 2004 and July 2005. Marking a period of elevated activity and a reawakening of the volcanic system, the data depict spatial and temporal variations in the sub-surface processes that defined this period of unrest at the Pico Viejo (PV)–Pico Teide (PT) complex, after the last volcanic eruption on Tenerife in 1909. An initial non-linear inversion, based on 3D line segments approximation, yielded three line segments at depths between 1kma.s.l. and 2km b.s.l. Our interpretation of the initial inversion results is that the line segments represent apparent composite sources, a superposition of deep and shallow seated sources. We therefore decomposed the gravity changes into shallow and deep parts (fields) using a procedure based on triple harmonic continuation. The shallow and deep fields could then be inverted separately, using the same inversion methodology. The deep field constrains two connected line segments at the depth of about 6km b.s.l., in the center of the NW seismogenic zone of VT event swarm of the seismic unrest, that we interpret as magma input. The inversion of the shallow field images three weak line segments that are all situated at very shallow, near-surface depths. We interpret the weak segments as hydrothermal sources potentially excited by the deeper magma injection. Our results indicate no significant input into the shallow phonolitic plumbing system of the PV–PT complex, but rather a deeper-seated rejuvenation of the mafic feeder reservoir. The emerging picture from our analysis is that the 2004/5 unrest on Tenerife was of a hybrid nature due to the combination of a deep magma injection (failed eruption?) coupled with fluid migration to shallow depths. The identified causative link between deep and shallow unrest sources indicates the presence of permeable pathways for shallow fluid migration at the CVC.

Deep structure of the North Vent Area, Great Tolbachik Fissure Eruption of 1975–1976, Kamchatka: Evidence from low-frequency microseismic sounding

Studies were conducted to improve our knowledge of the deep structure of the magmatic system and the plumbing system for the North Vent, Great Tolbachik Fissure Eruption of 1975–1976 based on recordings of background microseismic emission by broadband digital instruments along two parallel lines running through eruptive centers of various ages across the main magma-conducting fault. The method of low-frequency microseismic sounding was used for constructing deep sections down to a depth of 20 km, showing the shear-velocity distributions along these lines. Elements of the magmatic system were revealed beneath both vents in the form of low-velocity anomalies. We identified regions of magma chambers at different depths together with the channelways that connect these. It was found that magma might come to shallow chambers from different deep-seated sources along spatially isolated magma conduits, which is one of the possible causes of the variation in the basalt composition during the eruptions. For the zone of areal volcanism we are the first to demonstrate a change in magma-conducting conduits in the transition from the crystalline basement to the volcanogenic sedimentary rock sequence, with subvertical channels being replaced by inclined forms. It was shown that the elements of the magmatic system beneath both eruptive centers studied here are similar. It is hypothesized that there is a regularity in the configuration of plumbing systems in the middle part of the Tolbachik regional zone of areal volcanism.

Carbon isotopes and geochemical processes in CO2-rich cold mineral water, N-Portugal

This paper summarizes a new outlook on the conceptual model of Melgaco–Messegaes CO2-rich cold (≈18 °C) mineral water systems, issuing in N of Portugal, based on their isotopic (2H, 3H, 13C, 14C and 18O) and geochemical features. Stable isotopes indicate the meteoric origin of these CO2-rich mineral waters. Based on the isotopic fractionation with the altitude, a recharge altitude between 513 up to 740 m a.s.l. was estimated, corroborating the tritium results. The lowest 3H content (0 TU) is found in the groundwater samples with the highest mineralization. The mineral waters circulation are mainly related to a granitic and granodioritic environment inducing two different groundwater types (Ca/Na–HCO3 and Na/Ca–HCO3), indicating different underground flow paths. Calcium dissolution is controlled by hydrolysis of rock-matrix silicate minerals (e.g. Ca-plagioclases) and not associated to anthropogenic sources. The shallow dilute groundwaters exhibit signatures of anthropogenic origins (e.g. NO3) and higher Na/Ca ratios. The stable isotopes together with the geochemistry provided no indication of mixing between the regional shallow cold dilute groundwater and mineral water systems. The heavy isotopic signatures identified in the δ13C data (δ13C = 4.7 ‰, performed on the total dissolved inorganic carbon (TDIC) of CO2-rich mineral waters) could be derived from a deep-seated (upper mantle) source or associated to methanogenesis (CH4 source). The negligible 14C content (≈2 pmC) determined in the TDIC of the mineral waters, corroborates the hypothesis of a mantle-derived carbon source to the mineral groundwater systems or dissolution of carbonate layers at depth.

Geochronology of igneous rocks at and near to the Nezhdaninka gold deposit, Yakutia, Russia: U-Pb, Rb-Sr, and Sm-Nd isotopic data

The intrusive rocks associated with the large Nezhdaninka gold deposit (Au > 470 t) hosted in the Permian carbonaceous terrigenous sequence have been dated on zircon and rock-forming minerals with precision U-Pb (ID-TIMS) and Rb-Sr methods. The lamprophyre of the dike complex that occurs in the ore field and spatially is related to gold mineralization has concordant U-Pb zircon age (121 ± 1 Ma) and the same isochron Rb-Sr age (121.0 ± 2.8 Ma). The concordant U-Pb zircon age of granodiorite that dominates in the Kurum pluton is 94 ± 1 Ma, whereas the Rb-Sr isochron age of various intrusive rocks from this pluton is 1–4 Ma younger. This difference is caused by long-term cooling of the Kurum pluton and later closure of Rb-Sr isotopic system of biotite (300–350°C) and other rock-forming minerals as compared with U-Pb isotopic system of zircon (∼ 900°C). The Rb-Sr age of quartz diorite from the Gel’dy group of stocks (92.6 ± 0.8 Ma) coincides within uncertainty limits with the age of the Kurum pluton. Thus, the rocks pertaining to two epochs of magmatic activity, which developed in the South Verkhoyansk Foldbelt and divided by a time span of 25–28 Ma, are documented in the Nezhdaninka ore field. Taking into account that the age of gold mineralization is no less than 120 Ma, the data obtained allow us to specify the previously proposed formation model of the Nezhdaninka deposit. These data give grounds to rule out the Late Cretaceous Kurum pluton and the Gel’dy group of stocks from constituents of the ore-magmatic system, and to suggest that an Early Cretaceous deep-seated magma source existed beneath the deposit. Along with host terrigenous rocks, this magma source participated in the supply of matter to the hydrothermal system. The Nd, Sr, and Pb isotopic systematics of igneous rocks and ore mineralization in the Nezhdaninka ore field show that the Early and Late Cretaceous magma sources were formed in the Precambrian crust dated at ∼1.8 Ga.

Geology and geochemistry of the granitic rocks and associated dykes, East Gabal Nuqra, South Eastern Desert, Egypt

A crescent-shape granitic stock and associated dykes is located to the East Gabal Nuqra at the extreme western part of Wadi Natash, South Eastern Desert, Egypt. The examined granites are classified as alkali-feldspar granites and mainly consist of quartz, potash feldspars, plagioclases, and aegirine-augite. Xenotime, zircon, apatite and allanite are accessories representing the source of Y, U, Th and REEs in these rocks. These granites are characterized by high K2O, Na2O and Zn contents and Rb/Sr ratio. Also, they are highly enriched in high field strength elements (HFSE), especially Zr (1529×10−6), Nb (100×10−6), Hf (91×10−6) and Y (624×10−6) and light rare-earth elements (LREE, 141×10−6) concentrations and strongly depleted in Ca, Mg, Sr and Eu contents. These features suggest that they are similar to A-type granites (type-2). The rhyolite dykes and granites have similar geochemical characteristics whereas the chondrite-normalized REE patterns show a LREE enriched feature with strong negative Eu-anomaly, whereas the REE pattern of trachydacites show slightly fractionated pattern with no Eu-anomaly. It is suggested that the trachydacites were generated by small degree of partial-melting deep-seated basic source. Such liquid, when subjected to fractional crystallization involving separation of plagioclases as residue, generated the alkali-feldspar granites. And further fractional crystallization gave rise to the alkali rhyolites. The igneous rock suite originated from metaluminous to alkaline trachytic magma, and was developed in a within-plate tectonic environment. The extension caused by NW-SE right-lateral shear in area led to the emplacement of the alkali-feldspar granites. The later extrusion of the alkali rhyolite and trachydacite dykes was due to cauldron subsidence.

Elemental and Isotopic Evidence for Granitoid Genesis From Deep-Seated Sources in the Coast Mountains Batholith, British Columbia

Major element, trace element and Nd^Sr isotopic data are presented for 82 plutonic rocks from the southern Coast Mountains Batholith (CMB) in British Columbia, Canada, ranging in emplacement age from 210 to 50 Ma. The rocks are part of a large composite magmatic arc batholith, which the major element data show to be of calc-alkaline affinity. The majority of CMB samples lack the depletion in Eu that would be consistent with equilibration of magmas and plagioclase-bearing crystalline residues or fractionates, suggesting that equilibration took place deeper than the pressure limit of plagioclase stability at 35^40 km depth. The CMB samples show a wide variation in the slope of normalized rare earth element (REE) patterns, with chondrite-normalized La/Yb ratios above 10 being mostly confined to periods of high magmatic flux in the arc at 160^140, 120^80, and 60^50 Ma. The clearest relationships between major and trace elements are negative correlations between SiO2 and each of Sc, Y, and the heavier REE Gd to Lu. Nd and Sr isotopes mostly document juvenile origins for the granitoids, but show variations to higher Sr/Sr and lower eNd during high-flux periods.The results are interpreted to indicate a deep origin for most CMB magmas, below 40 km where mafic to intermediate rock assemblages previously added to the arc crust by mantle melting were transformed to an (amphibole-bearing)eclogite facies cumulate or restite, such that melting residues consisted mainly of two pyroxenes, garnet and variable proportions of amphibole. Thickened orogenic crust, for which there is clear geological evidence during the period 100^80 Ma, promoted this process. During high-flux periods, larger amounts of older rocks, mostly mafic rocks and some metasediments added to the base of the arc during orogenic shortening, became involved in magma genesis.

Microbial processes and the origin of the Úrkút manganese deposit, Hungary

The Transdanubian Range (Hungary, ALCAPA Unit) preserves a series of black shale-hosted Mn-carbonate deposits and cherty, Fe-rich Mn-oxide mineralized rocks associated with varicolored metalliferous claystones. Coccoid clumps (spherules), oval, tubular, and filamentous morphologies were observed by petrography, SEM-EDS, and TEM studies and are interpreted to be mineralized cellular materials. Local selective enrichment of bioessential elements (Mn, Fe, S, As, P, Mg, Ba, Sr, Co, Ce) occurs, and together with low δ13C values of the Mn carbonates also supports microbial mediated reactions. The results strongly suggest that the formation of Mn–Fe and Si minerals was associated with microbial metabolic processes. The role of aerobic chemolithoautotroph bacteria was essential in sequestering metal ions (Mn2+, Fe2+) from solution, which were deposited in the sediment pile and serve as a paleoenvironmental indicator of oxic conditions. These deposits are examples of, and therefore provide important criteria for identifying, non-sulphidic, oxic, microbial mineral processes. We provide a new genetic model for giant black shale-hosted Mn-carbonate deposits that involves episodic aerobic microbial processes.Although a part, if not the whole of the black shale-hosted Mn-carbonate deposits is of biogenetic-bacterial sedimentary origin, a hydrothermal/exhalative source of metals may have contributed to the formation of the deposits. An Fe–Mn-oxide chimney system is proposed to be a proximal facies to geofluid vents that occurred along fracture systems, which may have provided metals from deep-seated sources.

Combined Nd and Hf isotope evidence for deep-seated source of Isua lavas

We present the first combined 146,147Sm– 142,143Nd and 176Lu– 176Hf study of mafic rocks (amphibolites) from the western part of the Isua Supracrustal Belt (ISB, SW Greenland). Sm–Nd and Lu–Hf whole-rock isochrons yield identical ages within error: 3.72 ± 0.08 and 3.67 ± 0.07 Ga, respectively. 142Nd measurements confirm and extend the database of 142Nd excesses in Isua samples (μ 142Nd = 7–16 ppm relative to the terrestrial Nd standard), indicating that early-differentiated reservoirs escaped complete homogenization by mantle convection until the Archean. Combined 146,147Sm– 142,143Nd systematics suggest differentiation of the Isua amphibolite reservoir at maximum ages of ~ 4.53, ~ 4.47, and ~ 4.32 Ga. These ages are relative to, respectively, ordinary-chondritic, enstatite-chondritic, and higher-than-chondritic mantle compositions. The intercept of the Sm–Nd whole-rock isochron is in accordance with 142Nd results and consistent with a superchondritic initial 143Nd/ 144Nd ratio (ε 143Nd 3.7Ga = + 1.41 ± 0.98). In contrast, the corresponding initial ε 176Hf 3.7Ga = − 1.41 ± 0.57 is subchondritic. Since Lu/Hf and Sm/Nd fractionate similarly during mantle processes, the Sm–Nd and Lu–Hf isotope systems display incongruent relative parent–daughter behavior in the source of Isua amphibolites. Based on high-pressure and -temperature phase partition coefficients, we propose a model that satisfies 147Sm– 143Nd, 176Lu– 176Hf, and 142Nd results, as well as trace element characteristics. A deep-seated source composed largely of magnesium perovskite (98% MgPv) and containing 2% calcium perovskite satisfactorily explains the Nd and Hf isotopic discordance observed for Isua amphibolites. The negative HFSE anomalies characterizing Isua basalts likewise could have been inherited from such an early (4.53–4.32 Ga) deep mantle cumulate. Since the ~ 4 Ga old Acasta Gneisses (Northwest Territories, Canada) have radiogenic 176Hf/ 177Hf (Scherer et al., 2010), dissociation of the Lu–Hf and Sm–Nd isotope systems may not be a ubiquitous feature of the Hadean mantle, but rather reflects that a deep-seated source was involved in the formation of ISB lavas.

Origin of the sequential Shirakawa ignimbrite magmas from the Aizu caldera cluster, northeast Japan: Evidence for renewal of magma system involving a crustal hot zone

The Shirakawa ignimbrites were generated by caldera-forming eruptions in the Aizu caldera cluster, northeast Japan, during the Early Pleistocene. They have a total volume of > 300 km 3 dense rock equivalent and consist of the Kumado ignimbrite (1.4 Ma) from Ono caldera, the Ashino ignimbrite (1.3 Ma) and the Nagurasawa ignimbrite (1.2 Ma) from Tonohetsuri caldera, the Nishigo ignimbrite (1.1 Ma) from Narioka caldera and the Ten-ei ignimbrite (1.0 Ma) from an unknown, concealed caldera. These calderas overlap each other within a 20 × 20 km area in the eastern part of the cluster. The Shirakawa ignimbrites are relatively small in the eruption rate and have sufficient interval times that are longer than the residence times for their magma volume. The individual ignimbrites are composed of 20–40% phenocrysts set in high-silica rhyolite glass and they lack vertical chemical zoning. The SiO 2 content of the Kumado, Ashino, Nagurasawa, Nishigo and Ten-ei ignimbrites are 68–70 wt.%, 67–70 wt.%, 68–69 wt.%, 72–74 wt.% and 69–70 wt.%, respectively. They plot within the medium-K field, but show distinct trends for most major elements at the same SiO 2 ranges. Although they all have similar arc-related trace-elements patterns with Nb, Ta and Ti negative anomalies, high LREEs and flat HREEs, they differ in their Zr/Nb, Ba/Th and K/La ratios. Also, isotopic ratios of these pumices from each unit are distinct; 87Sr/ 86Sr varies from 0.7044 to 0.7047 and 143Nd/ 144Nd from 0.51271 to 0.51278. These results imply that the ignimbrite sources were renewed with every caldera-forming eruption. Seismic tomography and pre-eruption uplifting of the caldera cluster suggest that a substantial amount of mantle-derived magma was stored within the lower crust and contributed to the evolution of a crustal hot zone. Observed geochemical features also suggest that silicic magmas were generated by the melting of variably assimilated mafic intrusions. To explain the renewal of the Shirakawa magma system, melted materials must have been completely evacuated from a deep-seated source zone by each caldera-forming eruption and different mafic rocks subsequently subsided into the hot zone as raw materials for the next eruption.

Jurassic volcanic and sedimentary rocks of the La Silla and Todos Santos Formations, Chiapas: Record of Nazas arc magmatism and rift-basin formation prior to opening of the Gulf of Mexico

Stratigraphic relationships, detrital zircon provenance, U-Pb and 40 Ar/ 39 Ar geochronology, and trace element geochemistry in volcanic and sedimentary rocks of the Sierra homocline of central Chiapas near La Angostura reservoir in Mexico document an extensive pulse of Early–Middle Jurassic arc magmatism in rocks that overlie and intrude the Permian–Triassic Chiapas massif. Upper Jurassic rift-basin strata unconformably overlie the volcanic rocks and the massif. A Pliensbachian U-Pb (zircon) SHRIMP (sensitive high-resolution ion microprobe) age from porphyritic andesite (191.0 ± 3.0 Ma), Early to Middle Jurassic 40 Ar/ 39 Ar dates from andesitic dikes, U-Pb grain ages of detrital zircons in overlying strata (196–161 Ma), and previously reported K-Ar dates indicate that subduction-related magmatism occurred in the western portion of the Maya block from Early to latest Middle Jurassic time. We assign the volcanic rocks to the La Silla Formation, which correlates with the informal Pueblo Viejo andesite of the Cintalapa and Uzpanapa regions to the northwest. La Silla magmatism predates opening of the Gulf of Mexico Basin. The Todos Santos Formation, which overlies La Silla Formation, was deposited in extensional basins during the early stages of gulf opening. We recognize a lower El Diamante Member of the Todos Santos, consisting of red fluvial sandstone, mudstone, and minor conglomerate containing primarily volcanic-lithic detritus; this member is characterized by a nearly unimodal Jurassic detrital zircon age population that indicates a Callovian or younger depositional age. Volcanic activity continued into the upper part of the El Diamante Member, but with a more mafic character. We also recognize an upper member, which we term the Jerico Member. This member is characterized by thickly bedded, coarse-grained pebbly arkose intercalated with several thick intervals (tens of meters) of conglomerate and pebbly sandstone. Sandstone petrology indicates a source in the granitic rocks of the Chiapas massif, with a tendency to show deep-seated sources and a diverse zircon population in the upper part of the section. The upper Todos Santos Formation in the study area is gradational into the overlying San Ricardo Formation (Kimmeridgian–Tithonian). The La Silla Formation was deposited in volcanic-complex environments, with a clear lack of differentiated volcanic rocks. Fluvial strata of the El Diamante Member were deposited in a mud-rich sinuous river system. The Jerico Member was deposited in large, sand-rich fluvial systems, which probably represent deposits of rift-axis trunk streams; conglomerate facies were deposited in adjacent and interfingering alluvial fan systems. We suggest that the stratigraphic record of the western Maya block records a transition from volcanic arc to intra-arc basin and subsequently to rift basin during Pliensbachian to Oxfordian time.

The 390 Ma high-T metamorphic event in the Chinese Altai: A consequence of ridge-subduction?

High-grade rocks occur in the Chinese Altai, but the timing of metamorphism is poorly constrained, which hinders our understanding of the thermo-tectonic history of the region. Representative high-grade samples from the sillimanite zone extending from Hanas to Fuyun were selected for zircon U-Pb dating and temperature estimation. LA-ICP-MS analyses of zircon overgrowth rims and recrystallized domains give consistent ages of ∼390 Ma, which is interpreted to record a regional metamorphic event. Temperature (T) estimations using the amphibole-plagioclase-quartz (Amp-Pl-Qtz) and garnet-biotite (GB) geothermometers give relatively high temperatures ranging from 650 to 700 °C. The zircon metamorphic rims yield temperature estimates of ∼720 °C by using the Ti-in-zircon thermometer. These data suggest that a high-temperature metamorphic event took place in the Chinese Altai in the Middle Devonian, and may imply a tectonic environment involving an unusually elevated heat flux from a deep-seated source. Our data support possible ridge subduction around 390 Ma that caused upwelling of the hot asthenosphere and triggered the high-T metamorphism. This model can also account for coeval volcanic activity with a range of geochemical characteristics, diverse mafic intrusions and extensive hydrothermal mineralization in the Altai orogen.

Multiple sources for mineralizing fluids in the Charmitan gold(-tungsten) mineralization (Uzbekistan)

Mineral assemblages present within the Charmitan gold(-tungsten) quartz-vein mineralization have been investigated for their cathodoluminescence behaviour, chemical composition and noble gas isotope systematics. This inventory of methods allows for the first time a systematic reconstruction of the paragenetic relationships of quartz, scheelite, sulphides and native gold within the gold mineralization at Charmitan and provides the basis to utilise noble gas data in the discussion of sources and evolution of ore-forming fluids. The vein quartz is classified into four generations based on microscopic and cathodoluminescence investigations. Quartz I shows intense brittle deformation as associated scheelite I. Undeformed scheelite II overgrows scheelite I and has lower light rare earth element and higher intermediate rare earth element contents as well as higher strontium concentrations. Scheelite II is associated with the economic gold mineralization and formed during re-crystallisation and re-precipitation of material which was partly re-mobilised from early scheelite I during infiltration of gold-bearing fluids. Early stage native gold inclusions are often associated with stage 2 sulphides, scheelite II and bismuth tellurides and contain Ag (3.6–24.4 wt.%), Hg (≤1.0 wt.%) and Bi (≤0.2 wt.%). Later stage electrum grains occur in association with stage 3 sulphides and sulphosalts and contain Hg (<0.8 wt.%) and elevated Sb concentrations (up to 3.0 wt.%). Noble gas isotope data (3He/4He: 0.2-0.4 Ra) for hydrothermal ore fluids trapped in the gold-related sulphides and sulphosalts (stage 2 pyrite and arsenopyrite; stage 3 pyrite, sphalerite, galena and lead sulphosalts) suggest that diverse fluid sources were involved in the formation of the Charmitan gold deposit. These data are indicative of a small, but significant input of fluids from external, deep-seated (mantle and possibly lower crust) sources. A decrease in the input of mantle helium and an increasing role of crustal helium from early to later stages of the mineralization is suggested by the measured 3He/4He and 40Ar*/4He ratios. Sulphides from ore veins in meta-sedimentary rocks contain higher portions of meteoric fluids than those in intrusive rock types as indicated by their lower 3He/36Ar ratios. The 3He/36Ar ratios in the meta-sedimentary rocks agree well with ratios typical of gold mineralizations in the Tien Shan gold province completely hosted by meta-sedimentary sequences, indicating intense fluid-wall rock interaction.

The dynamics of deep magmas

This study attempts to substantiate a model of magma ascent from deep-seated sources driven by the density difference between magmas and wall rocks in a plastic solid medium via hydraulic fracturing (magma-driven fracturing). The difference between magma and wall rock densities causes overpressure development in the head of a magma column. With increasing height of the column or decreasing magma density, the overpressure can build significantly, which is valid for the case of continuous magma conduit from a deepseated source. The depth of mantle chambers, their vertical extent, and magma densities seem to be the key factors in the formation of volcanoes and intracrustal intrusions. Intermittent (or peripheral) magma chambers under volcanoes and crustal-mantle intrusive bodies may be formed at strength barriers, in the zone of elastoplastic transition and at the mantle-crust boundary.

The hydrogeochemistry of thermal springs on Mutnovskii Volcano, southern Kamchatka

This paper reports results from a hydrogeochemical study of thermal springs on Mutnovskii Volcano. The solutions of boiling pots and the pore water of thermal localities in the Donnoe Field were found to have an anomalous composition over a wide range of chemical elements, including rare earth elements (REE) and platinum-group elements (PGEs). The solutions of different springs exhibit considerable variations in their major and minor element compositions, indicating differences in the generation and upward transport of solutions. Physicochemical modeling showed that the mud pots studied could have been formed when the separate was ascending along an open fissure channel from the secondary zone of mixed boiling fluids. The high concentrations of titanium, vanadium, chromium, nickel, cobalt, REEs, and PGMs suggest a deep-seated source for these. The chemical forms of the elements in the solutions (primarily free aquaions) and the acidic environment favor high mobility of these elements. Deposition and concentration of these elements under near-surface conditions is unlikely.