Coherent Rhyolite Research Articles

Early Paleozoic long-lived silicic volcanism in the Eastern Puna Magmatic Belt, Argentina

In several locations along the Eastern Puna Magmatic Belt (EPMB) porphyritic rhyolite facies are interbedded with thinly bedded mudstone and fine-grained sandstone deposited during deepening of the marine setting. Previous research mainly focused on the northern part of EPMB, identified four main syn-sedimentary rhyolitic-dacitic volcanic episodes from Early to Late Ordovician, the oldest being Tremadocian (earliest Ordovician). In this contribution, we describe coherent rhyolite facies associated with volcaniclastic breccias in the central part of the EPMB (Cajón, Tuzgle, Concordia and Agua de Castilla creeks), and we present new geochemical data and U/Pb zircon ages on coherent rhyolite facies (Cajón, Huancar and Opla). The studied rhyolites are syn-sedimentary sills with peperitic margins. The coherent rhyolites have SiO2 content of ∼65–71 wt%, peraluminous signature, enrichment in LILE (Cs, Rb, Ba) and marked negative Sr, Ti and P anomalies. The REE patterns normalized to chondrite show a higher content of the light REE and a negative Eu anomaly (Eu/Eu* between 0.44 and 0.52). The integration of the regional data with our results suggests that at least two rhyolitic-dacitic volcanic episodes occurred during the Ordovician evolution of the EPMB: The Early Ordovician (∼485-462 Ma) including Tremadocian and Arenigian facies (Rio Taique, Niño Muerto, Huancar, Opla, northern Pastos Chicos, Agua Cavada, Muñayoc, oldest Quichagua, Cordón de Escaya, Cajón, Tuzgle, Concordia and Agua de Castilla); and the youngest Late Ordovician facies (∼449-443 Ma; Oplita, southern Pastos Chicos, Cordón de Escaya and youngest Quichagua). Hence, sedimentation and silicic volcanism across the Eastern Puna Magmatic Belt spanned at least in two episodes along the Ordovician period with more volume and duration during the Early Ordovician (ca. 20 Ma).

HIGH-PRECISION CA-ID-TIMS AGE CONSTRAINTS ON THE NIBLACK Cu-Zn-Au-Ag DEPOSITS: A NEOPROTEROZOIC VOLCANIC-HOSTED MASSIVE SULFIDE DEPOSIT IN THE NORTH AMERICAN CORDILLERA

Abstract The Neoproterozoic-Cambrian Wales Group and Ordovician-early Silurian Moira Sound unit of Prince of Wales Island, Alaska, USA, host numerous volcanic-hosted massive sulfide (VHMS) deposits and occurrences, including the Niblack VHMS deposits. Previous attempts to determine the age of the felsic volcanic host rocks in the Niblack area have resulted in conflicting results and interpretations. We have utilized chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon geochronology to acquire highly precise crystallization and maximum depositional ages for a total of six samples of felsic volcanic and intrusive rocks from Niblack. This study establishes age constraints for the Niblack felsic succession of (1) crystallization ages of 565.1 ± 0.9 and 564.8 ± 1.0 Ma for coherent rhyolite flows, (2) maximum depositional ages of 565.3 ± 0.9 and 565.2 ± 0.9 Ma for felsic volcaniclastic rocks, (3) a crystallization age of 565.2 ± 0.9 Ma for a quartz-feldspar-phyric subvolcanic sill, and (4) a crystallization age of 564.8 ± 1.0 Ma for a felsic dike that crosscuts the Niblack felsic succession. These results indicate that the ~200-m-thick Niblack felsic succession and VHMS deposits formed during one episode of felsic volcanism at ca. 565.1 ± 0.9 Ma and are thus confirmed as part of the Neoproterozoic Wales Group. Results of this study provide the first chronostratigraphic framework for felsic volcanism associated with VHMS deposit formation at Niblack and have implications for mineral exploration on Prince of Wales Island and elsewhere in the Alexander terrane.

Reconstruction of the effusive and explosive deposits of the Aruri and Salustiano formations in the Tapajós Domain, Southern Amazonian Craton, from field relationship, petrography and geochemistry

Proterozoic Silicic Large Igneous Provinces (SLIP) are part of the geological inventory since they record the planet's thermal evolution. The Amazonian Craton hosted the poorly understood Uatumã SLIP, which formed ca. 1.88 Ga years ago. The so-called Uatumã magmatism represents the volcanic products of SLIP. This magmatism also refers to specific stratigraphic units, such as the Uatumã, Iriri, and Iricoumé groups. The Aruri and Salustiano formations within the Iriri Group represent an important SLIP stratigraphy segment transitioning from effusive to explosive volcanism. However, this interval is still undisclosed and lacks a thorough comparison of the Iriri Group stratigraphy. From field observations, detailed petrographic description, new geochemistry data on the Aruri and Salustiano formations, and a compilation of geochemistry data on the Uatumã SLIP, we present the litho-stratigraphy for the two formations and a comparison to the other levels of the SLIP stratigraphy. Our data show a predominance of effusive over explosive volcanism and a set of intermediate rocks that intrude the sequence. The Aruri Formation contains six lithofacies organized as interbedded facies representing coherent rhyolitic lava flows and volcaniclastic rocks (tuffs), which are intruded by massive dacite dykes. The Salustiano Formation contains three lithofacies that comprehend coherent rhyolite flows and ignimbrites. Whole-rock geochemistry data indicate that the felsic facies rocks represent rhyolites to alkali-rhyolites (>70% SiO2) of the high-K calc-alkaline to shoshonite series with metaluminous to a peraluminous character. The rocks are considered as formed in transitional settings from a mature arc to an extensional regime. The comparison to the remainder of the Iriri Group corroborates an interpretation of their origin as subaerial volcanism with coeval effusive and explosive phases in a caldera-type volcanic system and epiclastic rocks deposited in a distal position concerning the volcanic center. The whole-rock geochemistry is similar to the other levels of the Uatumã SLIP and confirms the large extent of the ca. 1.88 Ga years ago in the Amazonian Craton.

Volcanic reconstruction and geochemistry of the Powderhouse formation in the Paleoproterozoic VMS-hosting Chisel sequence, Snow Lake, Manitoba, Canada

The Powderhouse formation of the Paleoproterozoic Snow Lake arc assemblage comprises the stratigraphic footwall to six volcanogenic massive sulfide (VMS) deposits at Snow Lake, Manitoba, Canada. It is interpreted to be a product of voluminous pyroclastic eruptions and concomitant subsidence followed by a period of relative volcanic quiescence that was dominated by suspension sedimentation, the reworking of previously deposited pyroclastic units by debris flows and bottom currents, and localized emplacement of rhyolite domes. The rhyolite domes are spatially associated with the Chisel, Chisel North, Lost, Ghost, Photo, and Lalor deposits. The Chisel, Lalor, and Lost members compose the Powderhouse formation and are subdivided into 13 lithologically distinct lithofacies, which allows, for the first time, correlation of stratigraphy between the South Chisel basin and Lalor areas, critical in predicting the location of largely stratiform VMS deposits. The Chisel and Lalor members contain lithofacies and bedforms that are characteristic of emplacement by subaqueous pyroclastic mass flows and concomitant subsidence. The Chisel member also contains coarse volcaniclastic breccias emplaced by mass debris flows derived from movement along fault scarps following early pyroclastic eruptions, and during continued subsidence. The Lost member consists of lithofacies deposited by mass flows generated from faults scraps during continued subsidence, but also contains lithofacies reworked by bottom currents, those deposited by suspension sedimentation, and, locally, coherent rhyolite. The Lost member represents a time stratigraphic interval, the “ore interval”, that marks contemporaneous rhyolite dome emplacement, VMS formation, and a hiatus in explosive volcanism.

Detailed mapping and lithogeochemistry of Neoarchean volcanic rocks: Beaulieu volcanic belt, Slave Craton, NWT

Neoarchean volcanic rocks of the Beaulieu River volcanic belt structurally overlie basement rocks of the Sleepy Dragon Complex (ca. 2.85 Ga), approximately 100 km east northeast of Yellowknife. The volcanic belt is comprised of complex lithofacies, including basalt, andesite, rhyolite, and associated volcaniclastic rocks, and hosts the Sunrise volcanogenic massive sulfide deposit. The absolute age of the volcanic strata is not known, nor is the stratigraphy well-defined; therefore, the Beaulieu River volcanic belt cannot be easily correlated to other greenstone belts within the Slave craton. The main objective of this study is to document the litho- and chemo-stratigraphy of the volcanic rocks, and particularly the rhyolite dome, located at the south end of Sunset Lake to reconstruct their volcanic and petrogenetic evolution, and determine their relationship to the volcanic strata that hosts the Sunrise VMS deposit, located ~6km to the north of the study area. Detailed mapping (1:2000) was completed over two field seasons (2018 and 2019) and shows that the volcanic rocks in the south Sunset Lake area comprise a complex stratigraphy consisting of basaltic, andesitic and rhyolitic lithofacies. This includes massive to pillow basalt and andesite, with lesser amounts of massive to in-situ brecciated, weakly quartz-plagioclase porphyritic rhyolite, heterolithic tuff to lapilli- tuff and felsic tuff to tuff breccia. The felsic clasts within the felsic volcaniclastic rocks are similar in composition to the coherent rhyolite. Units have a trace element geochemical signatures that vary from tholeiitic to calc-alkaline, arc-like rocks. Volumetrically, the volcanic strata in the south Sunset Lake area has a significant amount of volcaniclastic rocks, ranging from tuff to tuff breccia units. The volcaniclastic rocks are interpreted to have been deposited by a series of debris flows and eruption-fed density currents. The stratigraphy of the volcanic rocks in south Sunset Lake is very similar to that of the stratigraphy that hosts the Sunrise VMS deposit. Evidence of a vent proximal environment (e.g. rhyolite dome, peperite, syn-volcanic intrusions) and porous, volcanic debris accumulating on the seafloor highlight conditions favourable for volcanogenic massive sulfide-type mineralization in the south Sunset Lake area.

Geochemistry and architecture of the host sequence of the massive sulfides in the northern Iberian Pyrite Belt

The northernmost Iberian Pyrite Belt hosts several small felsic volcanogenic massive sulfide orebodies (<15 Mt), except the Aguas Teñidas and Magdalena each with ca. 45 Mt, which are economically important because of their high base and precious metal grades if compared with the bulk of the IPB. They occur aligned in a E-W belt located just south of the major suture that limits the South Portuguese Zone with the Pulo do Lobo Unit. The mineralization is related to a specific felsic volcanic unit that occurs in different tectonically dismembered units, but has a specific geochemical signature and a characteristic hydrothermal alteration. Six principal tectonostratigraphic units can be differentiated: (1) The Lower Felsic Unit is composed of feldspar-quartz-phyric rhyodacite (crypto)-dome complexes, intruded by sills and dykes of similar composition. Pumice and glass-rich breccia beds occur at the top and on the lateral margins of the dome complexes. A major shear zone separates the Lower Felsic Unit from the overlying Volcano-Sedimentary Unit; (2) The Volcano-Sedimentary Unit consists of vesicular basaltic lava and associated mafic epiclastic sandstone and siltstone, intercalated with thin beds of shale. A major tectonic contact marks the upper boundary; (3) The Middle Felsic Unit represents a series of intercalated coherent rhyolite domes with associated volcaniclastic rocks, and polymictic epiclastic sedimentary rocks. They are cut by numerous volcanic dykes and geochemically equivalent sub-volcanic intrusions; (4) The Sedimentary Unit is a highly deformed package of grey siltstone with interlayered shale and fine-grained epiclastic sedimentary rocks; (5) The Upper Felsic Unit is a package of dacitic to rhyolitic dome complexes with similar characteristics to the Lower Felsic Unit. Both the basal and upper contacts of this unit are narrow and high-strain tectonized zones. (6) The uppermost unit, the Andesite Unit, is comprised of andesitic dome complexes rich in hyaloclastite breccias and andesitic volcaniclastic rocks. Massive sulfides are hosted dominantly by the Lower Felsic Unit with only some orebodies related spatially to the Upper Felsic Unit (Lomero Poyatos, San Telmo and El Carpio).Geochemically, the Volcanic Sedimentary Complex is dominated by andesitic to rhyolitic calc-alkaline magmatic rocks with a significant proportion of dacite and andesite. The Lower Felsic and Upper Felsic Units are geochemically identical and probably represent dismembered parts of a single volcanic felsic complex. If that holds true, massive sulfide deposits of the northern IPB are hosted by volcaniclastic rocks located in the hanging wall of a single unit dominated by dacite-rhyolite dome complexes. This volcanic unit is characterized by Zr contents <200 µg/g (ppm) which are significantly lower than those of the more evolved Middle Felsic Unit (200–750 µg/g).Structural analysis and the geochemistry suggest that the volcanic sequence is tectonically imbricated, with the youngest units located at the lowest structural position. The proposed reconstructed stratigraphic sequence comprises an andesitic footwall, overlain by the felsic complexes of the Middle Felsic and Lower-Upper Felsic Units. Thus, the massive sulfide deposits formed toward the top of the whole sequence, and associated with the youngest felsic rocks having a specific geochemistry.

The Eruption of Submarine Rhyolite Lavas and Domes in the Deep Ocean – Havre 2012, Kermadec Arc

Silicic effusive eruptions in deep submarine environments have not yet been directly observed and very few modern submarine silicic lavas and domes have been described. The eruption of Havre caldera volcano in the Kermadec arc in 2012 provided an outstanding database for research on deep submarine silicic effusive eruptions because it produced 15 rhyolite (70–72 wt.% SiO2) lavas and domes with a total volume of ∼0.21 km3 from 14 separate seafloor vents. Moreover, in 2015, the seafloor products were observed, mapped and sampled in exceptional detail (1-m resolution) using AUV Sentry and ROV Jason2 deployed from R/V Roger Revelle. Vent positions are strongly aligned, defining NW-SE and E-W trends along the southwestern and southern Havre caldera margin, respectively. The alignment of the vents suggests magma ascent along dykes which probably occupy faults related to the caldera margin. Four vents part way up the steeply sloping southwestern caldera wall at 1,200-1,300 m below sea level (bsl) and one on the caldera rim (1,060 m bsl) produced elongate lavas. On the steep caldera wall, the lavas consist of narrow tongues that have triangular cross-section shapes. Two of the narrow-tongue segments are connected to wide lobes on the flat caldera floor at ∼1,500 m bsl. The lavas are characterized by arcuate surface ridges oriented perpendicular to the propagation direction. Eight domes were erupted onto relatively flat sea floor from vents at ∼1,000 m bsl along the southern and southwestern caldera rim. They are characterized by steep margins and gently convex-up upper surfaces. With one exception, the domes have narrow spines and deep clefts above the inferred vent positions. One dome has a relatively smooth upper surface. The lavas and domes all consist of combinations of coherent rhyolite and monomictic rhyolite breccia. Despite eruption from deep-water vents (most > 900 m bsl), the Havre 2012 rhyolite lavas and domes are very similar to subaerial rhyolite lavas and domes in terms of dimensions, volumes, aspect ratio, textures and morphology. They show that lava morphology was strongly controlled by the pre-existing seafloor topography: domes and wide lobes formed where the rhyolite was emplaced onto flat sea floor, whereas narrow tongues formed where the rhyolite was emplaced on the steep slopes of the caldera wall.

Conduit margin heating and deformation during the AD 1886 basaltic Plinian eruption at Tarawera volcano, New Zealand

During explosive eruptions, a suspension of gas and pyroclasts rises rapidly within a conduit. Here, we have analysed textures preserved in the walls of a pyroclastic feeder dyke of the AD 1886 Tarawera basaltic Plinian fissure eruption. The samples examined consist of basaltic ash and scoria plastered onto a conduit wall of a coherent rhyolite dome and a welded rhyolitic dome breccia. We examine the textural evidence for the response of the wall material, built of ∼75 vol.% glass and ∼25 vol.% crystals (pore-free equivalent), to mass movement in the adjacent conduit. In the rhyolitic wall material, we quantify the orientation and aspect ratio of biotite crystals as strain markers of simple shear deformation, and interpret juxtaposed regions of vesiculation and vesicle collapse as evidence of conduit wall heating. Systematic changes occur close to the margin: (1) porosity is highly variable, with areas locally vesiculated or densified, (2) biotite crystals are oriented with their long axis parallel to the margin, (3) the biotites have greater aspect ratios close to the margin and (4) the biotite crystals are fractured. We interpret the biotite phenocryst deformation to result from crystal fracture, rotation and cleavage-parallel bookcase translation. These textural observations are inferred to indicate mechanical coupling between the hot gas-ash jet and the conduit wall and reheating of wall rock rhyolite. We couple these observations with a simple 1D conductive heating model to show what minimum temperature the conduit wall needs to reach in order to achieve a temperature above the glass transition throughout the texturally-defined deformed zone. We propose that conduit wall heating and resulting deformation influences conduit margin outgassing and may enhance the intensity of such large basaltic eruptions.Electronic supplementary materialThe online version of this article (doi:10.1007/s00445-016-1006-7) contains supplementary material, which is available to authorized users.

Reconstructing the geological and structural history of an active geothermal field: A case study from New Zealand

The utilisation of geothermal systems benefits from an understanding of the host-rock geology, locations and controls of permeability pathways, and the nature and timing of magmatic sources providing thermal energy. Kawerau Geothermal Field in the central Taupo Volcanic Zone (TVZ) of New Zealand is currently developed for electricity generation and direct uses of high-temperature steam to ~200MW electrical output. The Kawerau geothermal system is hosted in a sequence of volcanic lithologies (tuffs, lavas and intrusive bodies) and sediments that overlie faulted Mesozoic metasedimentary (greywacke) basement. Identification of lithologies in the volcanic/sedimentary sequence is challenging due to the levels of hydrothermal alteration and lithological similarities. A combination of detailed petrological investigations, consideration of the emplacement processes and greater certainty of crystallisation or eruption ages through U–Pb age determinations on zircons is used to reconstruct the depositional and faulting evolution of the rocks hosting the currently active hydrothermal system. The oldest event inferred is faulting of the greywacke along northwest–southeast orientated, dominantly strike-slip structures to generate half-grabens that were filled with sediments, incorporating two dated ignimbrites (2.38±0.05 and 2.17±0.05Ma). A 1.46±0.01Ma ignimbrite was deposited relatively evenly across the field, implying that any topographic relief was subdued at that time. Subsequent deposition of ignimbrites occurred in episodes around 1.0, 0.55–0.6, and 0.32Ma, interspersed with thin sedimentary sequences that accumulated at average rates of 0.06mmyr−1. Andesite lavas from a buried composite cone occur as a conformable package between units dated at 1.0 and 0.6Ma. Bodies of coherent rhyolite occur at multiple stratigraphic levels: two magma types with associated tuffs were emplaced as domes and sills at 0.36±0.03Ma, and a third type at 0.138±0.007Ma as dikes, and domes that are exposed at surface. The andesitic Putauaki composite cone southwest of the field first erupted around 8ka, but earlier hydrothermal eruption breccias imply that magma was intruded to shallow depths as early as ~16ka.Age data and associated correlations show that post-1.5Ma normal faulting has accompanied episodic subsidence of the Kawerau area, with fault movement focused between northeast–southwest structures (associated with the geometry of the modern TVZ) and the reactivated northwest–southeast structures associated with most displacement in the area prior to 1.5Ma. Contrasts between emplacement of coherent rhyolite as sills at 0.36Ma and a dike at 0.138Ma reflect a shift in orientation of the principal stress axes in response to initiation of the modern TVZ rifting regime. Most volcanic rocks at Kawerau are distally sourced from elsewhere in the TVZ but form local marker horizons that delineate topographic relief within the field, and additionally constrain past subsidence rates. Current rates of subsidence and thermal output at Kawerau are geologically recent features associated with latest Quaternary rifting processes (<~50ka) and emplacement of the magmatic system for Putauaki volcano (~16ka) respectively.

U–Pb dating of zircon in hydrothermally altered rocks of the Kawerau Geothermal Field, Taupo Volcanic Zone, New Zealand

Crystallisation-age spectra have been obtained by SIMS techniques (SHRIMP-RG) on zircons from altered volcanic units penetrated by drillholes at Kawerau Geothermal Field in the central Taupo Volcanic Zone (TVZ), New Zealand. Drillholes penetrate 700–1300m of volcanic rocks and sediments before reaching the basement Mesozoic greywacke. Twenty-seven samples of altered volcanic lithologies and two surficial, fresh rock units have been studied in order to constrain ages of the major stratigraphic units. Within the volcanic/sedimentary pile the oldest in-situ ignimbrites that can be widely correlated have ages of ~1.45Ma. Between them and the basement greywacke is a variable thickness of sediments, mostly greywacke gravels and minor volcaniclastic units, reflecting localised basinal deposition associated with strike–slip faulting. Two ignimbrites within this sequence yield age estimates of c. 2.4 and 2.1Ma, consistent with these being distal southern Coromandel Volcanic Zone deposits, pre-dating TVZ activity. Below the regional marker plane of the 0.32Ma Matahina ignimbrite, three main ignimbrite groups occur, with ages around 1.45Ma, 1.0Ma and 0.6–0.5Ma, which are separated by sediment-dominated intervals and andesite volcanics. All of these ignimbrites represent marker horizons from other volcanic centres and do not reflect the presence of local magmatic heat sources.Numerous bodies of coherent rhyolite, previous labelled as Caxton and Onepu rhyolites, have been intersected at all pre-Matahina ignimbrite levels (including within the basement greywacke) and reflect earlier local magmatic heat sources. Our geochronological data resolve these rock bodies into three packages. The youngest is represented by the surficial rhyodacite Onepu domes, 40Ar/39Ar dated at 0.138±0.007Ma. U–Pb ages on zircons from dome material yield a spectrum that can be matched (consistent with petrography) with two dikes intersected at 880m and 2.67km depth, and with an estimated age of 0.15±0.01Ma (Onepu Formation). The older two packages consist of older crystal rich (~15%) and younger crystal-poor (~5%) rhyolite, here grouped as Caxton Formation and with eruption/intrusion age of 0.36±0.03Ma. The shallowest Caxton rhyolite bodies are interpreted to be domes, whilst deeper intersections are inferred to be sills based on the lateral extents relative to thicknesses.Net subsidence rates inferred from depths to key units do not reflect the present-day situation. Modern rates of subsidence (2±1mm/yr) associated with TVZ rifting processes can have been active for no more than ~50,000years, based on elevation differences of the Matahina ignimbrite top surface. An inferred change in intrusion geometry from sill (Caxton) to dike (Onepu) indicates a change in principal stress orientations reflecting onset of the modern Whakatane Graben. This change is dated at ~0.37Ma in coastal sedimentary sequences 23km to the north of Kawerau, consistent with our age data. Although previously interpreted to be a long-lived system, the modern Kawerau Geothermal Field is a Holocene entity reflecting the rejuvenation of magmatic heat flux associated with Putauaki volcano superimposed on an area of multiple reactivated fault structures, sporadic magmatism and variable rates of subsidence.

Textural analysis of a Late Palaeozoic coherent-pyroclastic rhyolitic dyke system near Burkersdorf (Erzgebirge, Saxony, Germany)

Abstract Located between two large Late Palaeozoic rhyolitic Caldera systems (the Tharandter Wald Caldera (TWC) and the Altenberg–Teplice Caldera (ATC)) in the eastern Erzgebirge region, the extended NE–SW-trending rhyolitic Sayda–Berggießhübel dyke swarm (SBDS) was emplaced into Variscan gneisses. The dykes expose massive coherent rhyolite (CR), and four pyroclastic bodies reveal welded rhyolitic tuffs (WRT). Overlying the CR dykes, the WRT form subcircular topographic elevations of up to 1 km in diameter. These have been interpreted by previous authors as erosional remnants of the ATC ignimbrite sheet. Another hill located on a dyke is built up of a columnar jointed rhyolitic lava dome or laccolith. An abandoned quarry near Mulda exposes sills propagating from a dyke into weathered gneiss. The WRT display a fluidal moderately to mostly vertically dipping welded texture with fiamme-like fragments up to 90 cm long. Strike orientation of the fiamme varies greatly. Gneiss lithic fragments make up to 2 vol.% of the analysed area of rock samples. The WRT are columnar jointed. The columns have a predominantly horizontal–subhorizontal orientation. In outcrop, rock slab and thin section maximum particle size and the relative proportions of fiamme, phenocrysts in fiamme, lithic fragments and of crystal clasts (QZ, KF, PL, BT) in tuff have been measured. The correlation of the data with the regional geology shows that the welded rhyolitic tuff bodies resemble individual vent systems. Compared to the fiamme, the tuff matrix has a higher crystal clast content, indicating that pyroclastic fractionation has taken place. Comparison of granulometric and geochemical SBDS data with ignimbrite samples from TWC and ATC confirms a close relationship between the SBDS and ATC. The SBDS is a possible vent area for the ATC. The textural data, the orientation of both fiamme and cooling columns, and other field relations suggest that the WRT of the SBDS represent welded fall-back tuffs. They formed in vents positioned on an active magmatic dyke system.

Draa Sfar, Morocco: A Visean (331 Ma) pyrrhotite-rich, polymetallic volcanogenic massive sulphide deposit in a Hercynian sediment-dominant terrane

Draa Sfar is a Visean, stratabound, volcanogenic massive sulphide ore deposit hosted by a Hercynian carbonaceous, black shale-rich succession of the Jebilet terrane, Morocco. The ore deposit contains 10 Mt grading 5.3 wt.% Zn, 2 wt.% Pb, and 0.3 wt.% Cu within two main massive sulphides orebodies, Tazakourt (Zn-rich) and Sidi M'Barek (Zn–Cu rich). Pyrrhotite is by far the dominant sulphide (70 to 95% of total sulphides), sphalerite is fairly abundant, chalcopyrite and galena are accessory, pyrite, arsenopyrite and bismuth minerals are rare. Pyrrhotite is monoclinic and mineralogical criteria indicate that it is of primary origin and not formed during metamorphism. Its composition is very homogeneous, close to Fe 7S 8, and its absolute magnetic susceptibility is 2.10 − 3 SI/g. Ar–Ar dating of hydrothermal sericites from a coherent rhyolite flow or dome within the immediate deposit footwall indicates an age of 331.7 ± 7.9 Ma for the Draa Sfar deposit and rhyolite volcanism. The Draa Sfar deposit has undergone a low-grade regional metamorphic event that caused pervasive recrystallization, followed by a ductile–brittle deformation event that has locally imparted a mylonitic texture to the sulphides and, in part, is responsible for the elongated and sheet-like morphology of the sulphide orebodies. Lead isotope data fall into two compositional end-members. The least radiogenic end-member, ( 206Pb/ 204Pb = 18.28), is characteristic of the Tazakourt orebody, whereas the more radiogenic end-member ( 206Pb/ 204Pb ∼ 18.80) is associated with the Sidi M'Barek orebody, giving a mixing trend between the two end-members. Lead isotope compositions at Draa Sfar testify to a significant continental crust source for the base metals, but are different than those of the Hajar and South Iberian Pyrite Belt VMS deposits. The abundance of pyrrhotite versus pyrite in the orebodies is attributed to low fO 2 conditions and neither a high temperature nor a low aH 2S (below 10 − 3 ) is required. The highly anoxic conditions required to stabilize pyrrhotite over pyrite are consistent with formation of the deposit within a restricted, sediment-starved, anoxic basin characterized by the deposition of carbonaceous, pelagic sediments along the flank of a rhyolitic flow-dome complex that was buried by pelitic sediments. Deposition of sulphides likely occurred at and below the seafloor within anoxic and carbonaceous muds. Draa Sfar and other Moroccan volcanogenic massive sulphide deposits occur in an epicontinental volcanic domain within the outer zone of the Hercynian belt and formed within a sedimentary environment that has a high pelagic component. In spite of the diachronous emplacement between the IPB deposits (late Devonian to Visean) and Moroccan deposits (Dinantian), all were formed around 340 ± 10 Ma following a major phase of the Devonian compression.

Facies architecture of a silicic intrusion-dominated volcanic centre at Highway–Reward, Queensland, Australia

The Highway–Reward massive sulphide deposit is hosted by a silicic volcanic succession in the Cambro-Ordovician Seventy Mile Range Group, northeastern Australia. Three principal lithofacies associations have been identified in the host succession: the volcanogenic sedimentary facies association, the primary volcanic facies association and the resedimented syn-eruptive facies association. The volcanogenic sedimentary facies association comprises volcanic and non-volcanic siltstone and sandstone turbidites that indicate submarine settings below storm wave base. Lithofacies of the primary volcanic facies association include coherent rhyolite, rhyodacite and dacite, and associated non-stratified breccia facies (autoclastic breccia and peperite). The resedimented volcaniclastic facies association contains clasts that were initially formed and deposited by volcanic processes, but then redeposited by mass-flow processes. Resedimentation was more or less syn-eruptive so that the deposits are essentially monomictic and clast shapes are unmodified. This facies association includes monomictic rhyolitic to dacitic breccia (resedimented autoclastic facies), siltstone-matrix rhyolitic to dacitic breccia (resedimented intrusive hyaloclastite or resedimented peperite) and graded lithic-crystal-pumice breccia and sandstone (pumiceous and crystal-rich turbidites). The graded lithic-crystal-pumice breccia and sandstone facies is the submarine record of a volcanic centre(s) that is not preserved or is located outside the study area. Pumice, shards, and crystals are pyroclasts that reflect the importance of explosive magmatic and/or phreatomagmatic eruptions and suggest that the source vents were in shallow water or subaerial settings. The lithofacies associations at Highway–Reward collectively define a submarine, shallow-intrusion-dominated volcanic centre. Contact relationships and phenocryst populations indicate the presence of more than 13 distinct porphyritic units with a collective volume of 0.5 km 3. Single porphyritic units vary from <10 to 350 m in thickness and some are less than 200 m in diameter. Ten of the porphyritic units studied in the immediate host sequence to the Highway–Reward deposit are entirely intrusive. Two of the units lack features diagnostic of their emplacement mechanism and could be either lavas and intrusions. Direct evidence for eruption at the seafloor is limited to a single partly extrusive cryptodome. However, distinctive units of resedimented autoclastic breccia indicate the presence nearby of additional lavas and domes. The size and shape of the lavas and intrusions reflect a restricted supply of magma during eruption/intrusion, the style of emplacement, and the subaqueous emplacement environment. Due to rapid quenching and mixing with unconsolidated clastic facies, the sills and cryptodomes did not spread far from their conduits. The shape and distribution of the lavas and intrusions were further influenced by the positions of previously or concurrently emplaced units. Magma preferentially invaded the sediment, avoiding the older units or conforming to their margins. Large intrusions and their dewatered envelope may have formed a barrier to the lateral progression and ascent of subsequent batches of magma.

Contents volume 57, 1993

The Highway–Reward massive sulphide deposit is hosted by a silicic volcanic succession in the Cambro-Ordovician Seventy Mile Range Group, northeastern Australia. Three principal lithofacies associations have been identified in the host succession: the volcanogenic sedimentary facies association, the primary volcanic facies association and the resedimented syn-eruptive facies association. The volcanogenic sedimentary facies association comprises volcanic and non-volcanic siltstone and sandstone turbidites that indicate submarine settings below storm wave base. Lithofacies of the primary volcanic facies association include coherent rhyolite, rhyodacite and dacite, and associated non-stratified breccia facies (autoclastic breccia and peperite). The resedimented volcaniclastic facies association contains clasts that were initially formed and deposited by volcanic processes, but then redeposited by mass-flow processes. Resedimentation was more or less syn-eruptive so that the deposits are essentially monomictic and clast shapes are unmodified. This facies association includes monomictic rhyolitic to dacitic breccia (resedimented autoclastic facies), siltstone-matrix rhyolitic to dacitic breccia (resedimented intrusive hyaloclastite or resedimented peperite) and graded lithic-crystal-pumice breccia and sandstone (pumiceous and crystal-rich turbidites). The graded lithic-crystal-pumice breccia and sandstone facies is the submarine record of a volcanic centre(s) that is not preserved or is located outside the study area. Pumice, shards, and crystals are pyroclasts that reflect the importance of explosive magmatic and/or phreatomagmatic eruptions and suggest that the source vents were in shallow water or subaerial settings.The lithofacies associations at Highway–Reward collectively define a submarine, shallow-intrusion-dominated volcanic centre. Contact relationships and phenocryst populations indicate the presence of more than 13 distinct porphyritic units with a collective volume of 0.5 km3. Single porphyritic units vary from <10 to 350 m in thickness and some are less than 200 m in diameter. Ten of the porphyritic units studied in the immediate host sequence to the Highway–Reward deposit are entirely intrusive. Two of the units lack features diagnostic of their emplacement mechanism and could be either lavas and intrusions. Direct evidence for eruption at the seafloor is limited to a single partly extrusive cryptodome. However, distinctive units of resedimented autoclastic breccia indicate the presence nearby of additional lavas and domes.The size and shape of the lavas and intrusions reflect a restricted supply of magma during eruption/intrusion, the style of emplacement, and the subaqueous emplacement environment. Due to rapid quenching and mixing with unconsolidated clastic facies, the sills and cryptodomes did not spread far from their conduits. The shape and distribution of the lavas and intrusions were further influenced by the positions of previously or concurrently emplaced units. Magma preferentially invaded the sediment, avoiding the older units or conforming to their margins. Large intrusions and their dewatered envelope may have formed a barrier to the lateral progression and ascent of subsequent batches of magma.

Large-scale rhyolite peperites (Jurassic, southern Chile)

Development of a Jurassic volcano-tectonic rift basin in the southern Andes created a setting in which thick, rhyolitic volcaniclastic sequences accumulated in submarine environments and were penetrated by hypabyssal intrusions during or shortly after deposition. In the Ultima Esperanza District of southern Chile, extensive masses of peperite were produced when rhyolite magma underwent quenching, disruption, and commingling with wet, unconsolidated sediments during intrusion at shallow levels beneath the sea floor. The peperite forms discordant intrusive masses with volumes of up to several cubic kilometers, in which large, widely spaced, coherent rhyolite feeder pods are surrounded by, and grade into closely packed and dispersed peperite. Closely packed peperite consists of tightly fitting clasts separated by sediment-filled fractures. In dispersed peperite, the sediment forms a matrix surrounding large masses of fractured rhyolite and smaller more widely separated rhyolite clasts; evidence of in situ quench fragmentation is well preserved on both outcrop and thin-section scales. Thin sections show that clast margins and, in some cases, entire small clasts underwent cooling-contraction granulation, releasing shards of quenched rhyolite and fragments of phenocrysts into the adjacent sediment. Interaction between magma and wet sediment was non-explosive and involved fluidization of the host sediments, creating space for the intruding magma and causing pervasive injection of highly mobile sediment along thermal contraction cracks in quench-fragmented rhyolite. The ability of the magma to undergo complex intermixing with large volumes of sediment, with widespread preservation of in situ fragmentation textures, is interpreted to reflect a relatively low magma viscosity, presumably caused by retention of volatiles in the magma at the ambient pressures involved. Beds of redeposited peperite within the rift-basin fill indicate that some of the intrusive peperite masses reached the sea floor, undergoing slumping and mass flow. The peperites were thus an important local source of coarse-grained debris during the evolution of the basin.