Downdip Development of the Ni-Cu-PGE-Bearing Mafic to Ultramafic Uitkomst Complex, Mpumalanga Province, South Africa

The about 2055-Ma-old mafic to ultramafic Uitkomst Complex in the Mpumalanga Province of South Africa hosts the low-grade-large-tonnage Ni-Cu-PGE deposit, Nkomati. The complex is regarded to represent a satellite to the Bushveld Complex and a feeder to an eroded magmatic reservoir in the southeast. Aeromagnetic surveys and previous drilling indicated an overall northwestern-downdip extension of the complex, but the question is to what extent and in which expression can the complete intrusion be found under cover in the northwest? Answering this, a mineralogical, geochemical and geochronological investigation of a borehole intersection of the whole complex at Little Mamre was carried out, using petrography, XRF, EPMA and LA-ICP-MS U–Pb analyses of zircons for age determination. Although the total thickness of the rock units is larger than to the southeast, emplacement, litho- and mineral chemistry trends, expression of alteration mineralogy and style of sulphide mineralisation are similar. The amount of sulphide mineralisation is on average less than in the southeast. The upper ultramafic unit contains, more frequently, pegmatoidal sections, and the Chromitiferous Harzburgite unit has less massive chromitite layers than the southeastern parts of the complex, whereas the gabbro(-norite) units contain more interstitial liquid with late-stage minerals. The findings confirm that the anvil-shaped intrusion in cross section continues with increased thickness towards northwest at a shallow dip; although approaching the deeper part of the igneous reservoir, mineral compositions are partially more evolved. The overall mineralogical consistency downdip supports a situation of multiple magma replenishment along a flat-lying, northwest–southeast trending conduit, resulting in an evolved cumulus mineral assemblage in the upper part.

Establishing genetic relationships between the Takidani pluton and two large silicic eruptions in the Northern Japan Alps

The Takidani pluton (1.1-1.6 Ma) represents a shallow magmatic reservoir at the base of an exhumed caldera floor. The deposits of two large caldera-forming eruptions including the Nyukawa Pyroclastic Flow Deposit (1.76 Ma; crystal-rich dacite) and the Chayano Tuff and Ebisutoge Pyroclastic Deposits (1.75 Ma; a sequence of crystal-poor rhyolite) are distributed concentrically around the pluton. We use major and trace element chemistry of whole-rock, glass and minerals to show (1) that the crystal-rich dacite (>400 km3 DRE; dense rock equivalent) is the erupted portion of a shallow mush zone constituting the Takidani pluton and (2) that the crystal-poor rhyolite (>100 km3 DRE) was extracted from a deeper part of this vertically extended magmatic plumbing system. Whole-rock geochemistry indicates that the Nyukawa and Takidani compositions were produced dominantly through crystal fractionation of amphibole, pyroxene and plagioclase in the mid-to-lower crust and subsequently emplaced in the upper crust prior to eruption and solidification, respectively. The crystal-poor Chayano-Ebisutoge rhyolite (>100 km3 DRE) is compositionally distinct from the Nyukawa and Takidani magmas and its generation is associated with a substantial contribution of crustal melts. Yet, plagioclase and orthopyroxene textures and chemistry provide strong evidence that the ascending rhyolite percolated through the upper Takidani-Nyukawa mush zone prior to eruption. Overgrowth of “rhyolitic plagioclase” on “xenocrystic dacitic plagioclase” typical of the Takidani-Nyukawa magmas indicates that the extraction and accumulation of the rhyolitic melts could have occurred in less than 10 kyr (i.e. time between eruptions) prior to eruption providing maximum timescales for pre-eruption storage. Overall, our findings show a progressive growth and thermal maturation of a vertically extended magmatic plumbing system over hundreds of thousands of years and imply that large volcanic eruptions can occur in relatively short succession without dramatic changes in the plumbing system, thus, complicating the identification of signs of an impending large eruption.

REE Minerals as Geochemical Proxies of Late-Tertiary Alkalic Silicate ± Carbonatite Intrusions Beneath Carpathian Back-Arc Basin

The accessory mineral assemblage (AMA) of igneous cumulate xenoliths in volcanoclastic deposits and lava flows in the Carpathian back-arc basin testifies to the composition of intrusive complexes sampled by Upper Miocene-Pliocene basalt volcanoes. The magmatic reservoir beneath Pinciná maar is composed of gabbro, moderately alkalic to alkali-calcic syenite, and calcic orthopyroxene granite (pincinite). The intrusive complex beneath the wider area around Fiľakovo and Hajnáčka maars contains mafic cumulates, alkalic syenite, carbonatite, and calc-alkalic granite. Both reservoirs originated during the basaltic magma underplating, differentiation, and interaction with the surrounding mantle and crust. The AMA of syenites is characterized by yttrialite-Y, britholite-Y, britholite-Ce, chevkinite-Ce, monazite-Ce, and rhabdophane(?). Baddeleyite and REE-zirconolite are typical of alkalic syenite associated with carbonatite. Pyrochlore, columbite-Mn, and Ca-niobates occur in calc-alkalic granites with strong peralkalic affinity. Nb-rutile, niobian ilmenite, and fergusonite-Y are crystallized from mildly alkalic syenite and calc-alkalic granite. Zircons with increased Hf/Zr and Th/U ratios occur in all felsic-to-intermediate rock-types. If rock fragments are absent in the volcanic ejecta, the composition of the sub-volcanic reservoir can be reconstructed from the specific AMA and zircon xenocrysts–xenolith relics disintegrated during the basaltic magma fragmentation and explosion.

Resurgent uplift at large calderas and relationship to caldera-forming faults and the magma reservoir: new insights from the Neapolitan Yellow Tuff caldera (Italy)

<p>Resurgence uplift is the rising of the caldera floor, mainly due to pressure or volume changes in the magma reservoir. Identifying resurgence structures and understanding their relationship to the magmatic reservoir is challenging. We investigate the resurgence structures of the Neapolitan Yellow Tuff (NYT) caldera (Italy) by integrating bathymetric data, high-resolution seismic profiles and Differential Synthetic-Aperture Radar Interferometry data. Our results show that the resurgent area is manifested as 1) a central dome constituted by two main blocks bounded by NNE-SSW trending faults, 2) an apical graben developed on top of the most uplifted block, 3) a peripheral zone including several uplifted and tilted blocks, bounded by inward-dipping faults. The onset of the uplift of the central dome occurred through re-activation, in reverse motion, of normal faults formed during the caldera collapse, and located in the peripheral zone. During periods of unrests, the blocks of the central dome move independently at different velocities, and the peripheral zone accommodates the deformation. The restless behaviour of the NYT caldera is the result of a shallow magmatic reservoir located at 3.5 ± 0.7 km, and characterised by a width that roughly corresponds to the extension of the overlaying resurgent area. Defining the caldera-forming fault system and identifying the area involved by the resurgence is crucial for estimating depth and width of the magma reservoir, and predicting the caldera behaviour during periods of unrest by localising possible vents and sectors that will deform. This knowledge contributes to the evaluation of the volcanic hazard.</p>

Anatomy of a volcanic island inferred from a multiphysics approach

<p>Phreatic and phreatomagmatic eruptions are difficult to predict with accuracy on volcanoes due to complex interactions at depth between heat, water, and magmatic fluids. To better understand such multifaceted interactions, we present here a multidisciplinary geophysical approach performed on Miyakejima, a 10-km wide stratovolcano in the Izu Bonin arc. Its plumbing system was highlighted by combining four geophysical methods: magnetotellurics, seismicity (hypocenters), self-potential, and thermal image (remote sensing). We thus propose the first large-scale interpretation of the volcanic structure in terms of rock properties, temperature, fluid content, and fluid flow. Our findings indicate that hot volatiles released from a <em>deep</em> magmatic reservoir (> 350°C, 2.5–4.5 km depth) rise through a narrow permeable path, interact with the conductive hydrothermal system beneath the 2000 A.D. caldera (<250°C, 0–2 km depth). This mixture of fluid is finally released in the fumarolic area in the southern part of the caldera at 181°C. This combined approach allow us to: 1) delineate the water table of the volcano (300–700 m depth), 2) determine the general fluid flow circulation beneath the island, 3) characterize seismic signatures of long-period and volcano-tectonic events, and 4) elucidate the origin of the high water content of fumaroles developed since the last eruption in A.D. 2000.</p>

A review of the geology and origin of CO2 in mineral water springs in east Belgium

Naturally CO2-rich mineral water springs (pouhons) in east Belgium occur in the context of the Rhenohercynian domain of the Variscan fold-and-thrust belt, mostly within the Cambro-Ordovician Stavelot-Venn Massif. The origin of the CO2 is still unclear, although different hypotheses exist. In this review study, we show pouhon waters are of the calcium bicarbonate type (~310 mg/l HCO3- on average), with notable Fe (~15 mg/l) and some Ca (~43 mg/l). Pouhon waters are primarily meteoric waters, as evidenced by H and O isotopic signature. The δ13C of CO2 varies from -7.8 to +0.8‰ and contains up to ~15% He from magmatic origin, reflecting a combination of carbonate rocks and mantle as CO2 sources at depth. Dinantian and Middle Devonian carbonates at 2–6 km depth could be potential sources, with CO2 generated by dissolution. However, carbonates below the Stavelot-Venn Massif are only predicted by structural models that assume in-sequence thrusting, not by the more generally accepted out-of-sequence thrust models. The mantle CO2 might originate from degassing of the Eifel magmatic plume or an unknown shallower magmatic reservoir. Deep rooted faults are thought to act as preferential pathways. Overall low temperatures of pouhons (~10 °C) and short estimated residence times (up to 60 years) suggest magmatic CO2 is transported upwards to meet infiltrating groundwater at shallower depths, with partial to full isotopic exchange with carbonate rocks along its path, resulting in mixed magmatic-carbonate signature. Although the precise role and interaction of the involved subsurface processes remains debatable, this review study provides a baseline for future investigations.

Geology and Genesis of the Tuwu Porphyry Cu Deposit, Xinjiang, Northwest China

The Tuwu porphyry Cu deposit is located in the northern segment of the Jueluotage metallogenic belt in Eastern Tianshan, on the southern margin of the Central Asian orogenic belt, Xinjiang, northwest China. Tuwu is hosted by diorite porphyry and tonalite porphyry intrusions, which intruded volcanic rocks of the Carboniferous Qi’eshan Group. Four stages (I-IV) of hydrothermal activity have been identified. Chalcopyrite is the dominant ore mineral and mainly occurs in vein stages II (quartz-chalcopyrite-pyrite ± sericite ± bornite ± enargite veins with phyllic halos) and III (quartz-molybdenite-chalcopyrite ± pyrite ± chlorite ± epidote veins). Re-Os dating of molybdenite samples yielded an isochron age of 335.6 ± 4.1 Ma (2σ, mean square of weighted deviates = 0.15, n = 8). Silicon, oxygen, and carbon stable isotope compositions of quartz and calcite provide evidence for predominantly magmatic contributions with a late meteoric water component at Tuwu. Chalcopyrite samples from stages I and III record a narrow range of bulk δ34S values between –3.9 and 0.4‰, whereas pyrite samples from stages I to IV show decreasing δ34S values from 1.7 to 0.2‰. Chalcopyrite has 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios similar to those of porphyry intrusions, and Pb isotope data from sulfide samples display a positive trend transecting the growth curves of crustal lead. The ore-forming components (metals and sulfur) were sourced from a mantle-derived magmatic reservoir with some upper crustal components in a subduction-related arc setting. Plagioclase compositions in the porphyry intrusions are consistent with magmatic H2O contents of ~7 wt %. Copper sulfides in the high-grade phyllic alteration zones at Tuwu are characterized by elevated δ65Cu values consistent with deposits from oxidized and hydrous magmatic-hydrothermal fluids, whereas lower δ65Cu values and low copper grades correspond to the potassic alteration zones. Recognition of copper isotope zonation patterns at Tuwu has potential applications in the exploration of porphyry Cu deposits.

The architecture of an intrusion in magmatic mush

<p class="western" align="justify"><span lang="en-US">Magmatic reservoirs located in the upper crust have been shown to result from the repeated intrusions of new magmas, and spend most of their dwelling time as cristal-rich mush. Despite advances in our understanding of the physical processes that may occur in a magma reservoir, the architecture of the intrusion into a mush remains poorly constrained. The geometry of such intrusions, however, may greatly affect the thermal and compositional evolution of the magmatic reservoir. We performed numerical simulations coupling computational fluid dynamics with the discrete element method to identify the geometry and emplacement dynamics of an intrusion in a mush, and the relevant physical parameters controlling it. Our results show that the intrusion geometry is to first-order controlled by the density contrast between the melt phases of the intruded and resident materials rather than the bulk density contrast as usually considered. When the melt phase of the intruded materials is denser than that of the host, the intrusion ponds at the base of the mush and is emplaced as a horizontal layer. However, when the intruded melt is lighter, the intrusion rises through the mush forming a Rayleigh–Taylor instability. In the absence of density contrast between the two melts, the intrusion fluidizes the host crystal network and slowly ascends through the mush. The presence of a viscosity contrast between the intruded and resident materials as well as the intrusion injection velocity were found to have less of an influence on the final geometry and intrusion dynamics in mush. In addition, we analyzed the eruptive sequence of well documented eruptions involving an intrusion as a trigger, and found good agreement with our modeling results. This study sheds light on the importance of explicitly considering granular mechanisms and the relative motion between the crystals and the melt phase when studying the physical processes of magmas and mush.</span></p>

Significant changes in the magma dynamics of Stromboli steady-state volcano recorded by clinopyroxene crystals.

<p>Steady-state volcanic activity implies equilibrium between the rate of magma replenishment and eruption of compositionally homogeneous magmas, lasting for tens to thousands of years in an open conduit system. The Present-day activity of Stromboli volcano (Aeolian Islands, Southern Italy) has long been recognised as typical of a steady-state volcano, with a shallow magmatic reservoir (highly porphyritic or hp-magma) continuously refilled by more mafic magma (with low phenocryst content or lp-magma) at a constant rate and accompanied by mixing, crystallisation and eruption. The lp-magma is erupted only during more violent explosive events (paroxysms), which usually occur at intervals of a few years. However, the two most recent paroxysms occurred at very short timescales on 3 July and 28 August 2019 offering the unique opportunity of obtaining crucial information on the current magma dynamics of Stromboli.</p><p>Albeit the plumbing system shows such uniformity, clinopyroxene phenocrysts exhibit marked chemical heterogeneities and complex textures caused by continuous lp-hp magma mixing as well as antecryst recycling from different mush portions. The compositional zoning in clinopyroxene provides essential information on pre-eruptive magma dynamics, indicating multi-stage crystallization across the lp-hp-reservoirs, where diopsidic compositions are markers of more primitive, high-T magmas injecting into shallow, low-T domains of the plumbing system. By comparing clinopyroxene texture, chemistry and residence times from the Present-day eruptions with the previous Post-Pizzo activity, we conclude that a distinct phase in the life of Stromboli volcano commenced after the violent 2003 paroxysm. Our observations suggest there are more efficient mechanisms of mush disruption and cannibalization, in which old diopsidic antecrysts are continuously remobilized and transported by the lp-magmas permeating the mush. The disappearance of diopsidic recharge bands within augitic overgrowths indicates that over time, magmatic injections feeding the persistent Present-day activity are more intensively mixed and homogenized prior to eruption.</p>

Unrest episodes at Alcedo and Cerro Azul (Galapagos) revealed by InSAR data and geodetic modelling

<p>Western Galápagos calderas experienced repeated eruptive and non-eruptive unrest in the last decades, only partially studied. Here we investigated, using the Synthetic Aperture Radar Interferometry (InSAR) and geodetic modelling, the eruptive and the non-eruptive unrest episodes occurred in two of the less studied calderas of the western Galápagos: Alcedo and Cerro Azul. Alcedo underwent repeated non-eruptive unrest from 2007 to 2011, while Cerro Azul experienced an unrest, from 2007 to 2008, culminated in two eruptive phases from May 29th to June 11th 2008. Results highlight how Alcedo experienced two episodes of uplift due to new magma injections in its shallow magma reservoir, separated by an episode with a limited lateral propagation of magma, probably interrupted for the lack of new magma supply in the magma reservoir. Results also hint to a possible relationship between these short-term unrest episodes and the longer-term process of resurgence at Alcedo. As for Cerro Azul, we overcame unwrapping errors affecting some of the InSAR data of Cerro Azul by proposing a new method, based on the wrapped phase differences among nearby pixels, to invert the wrapped phase directly. Our results highlight how the eruption was preceded by long-term pre-eruptive inflation (October 2007 – April 2008). During the first eruptive phase, most of the magma responsible for the inflation fed the lateral propagation of a radial dike, which caused a first deflation of the magmatic reservoir. During the second eruptive phase, the further lateral propagation of the dike fed a radial eruptive fissure at the base of the edifice, causing further deflation of the magmatic reservoir. From the first to the second eruptive phase, the radial dike changed its strike propagating towards a topographic low between Cerro Azul and Sierra Negra. An increase in magma supply from the reservoir to the dike promoted the further lateral propagation of the dike, confirming the importance of a continuous supply of magma in the propagation of a dike. </p>