Volcanic Facies Research Articles

Ni-Cu-Co-rich hydrothermal manganese mineralization in the Wallis and Futuna back-arc environment (SW Pacific)

The Wallis and Futuna back-arc system is a complex area composed of at least two active oceanic spreading centers (the Futuna and Alofi spreading centers) and young volcanic zones characterized by diffuse magmatism locally affected by the Samoan hotspot. This geological setting is favorable to the establishment of hydrothermal systems, in the form of either high-temperature (HT) hydrothermal venting or low-temperature (LT) diffuse flow. During the 2010 Futuna cruise aboard the R/V L'Atalante, three remarkable inactive LT Fe-Si-Mn deposits were discovered (Utu Uli, Anakele and Utu Sega). Some of the Mn-rich precipitates discovered exhibit the highest base metals concentrations so far recorded in ferromanganese rocks, including in the well-documented hydrogenetic crusts and polymetallic nodules. The deposits lie on top of volcanoes and formed in close association with the volcanic facies. The manganese mineralization occurs in the form of massive layered crusts and Mn-rich cements within strongly altered basaltic pyroclastic rocks, brecciated lavas and, more rarely, in sediments. Field observations and mineralogical and chemical studies support a hydrothermal origin for the mineralization and show that nickel, cobalt and copper enrichments are controlled by the precipitation of 7Å and 10Å manganates. The conventional geochemical classifications (e.g. Bonatti et al., 1972) used to decipher the origin of Mn mineralization cannot be used for this new type of deposit and new robust discrimination diagrams need to be established. We suggest that the unusual enrichment of metals recorded in our samples is due to (i) a lack of precipitation of high-temperature massive sulfides at depths that would have retained metals (e.g. Cu, Ni, Co); (ii) isolation of the hydrothermal system, thereby avoiding Ni, Co and Cu losses in the water column; and (iii) the ability of birnessite and buserite/todorokite to scavenge Co, Ni, and Cu from aqueous fluids. The Utu Uli and Anakele deposits share certain characteristics with the active hydrothermal system at Loihi seamount (e.g. the depth of mineralization, relationships with pyroclastic volcanoes, and the influence of a mantle plume source) and thus might represent late-stage products of this specific type of hydrothermal activity. Elsewhere, the Co-rich mineralization of the Calatrava volcanic field (CVF) in Spain may be a potential analog of the Utu Sega deposit. The Mn-(Co) deposits of the CVF formed in close proximity to Pliocene volcanic rocks. Metals were transported by epithermal hydrothermal solutions with high fO2 and cobalt was scavenged by Mn oxides. Together with the well-documented stratabound Mn deposits (González et al., 2016; Hein et al., 2008; Hein et al., 1996), the Mn deposits discovered in the Wallis and Futuna back-arc provide crucial insights into LT hydrothermal activity in the deep ocean. The metal-rich character of this LT hydrothermal activity may be of major importance for future research on the net flux of hydrothermally derived metals (e.g. Ni, Co, Cu) to the open ocean.

Hydrothermalism Associeted To Ediacarian Volcanism Of Ait Sawn And Tissouktai (Eastern Anti- Atlas, Morocco)

In the South East of Ouarzazate (south west of the Saghro massif, Anti Atlas) outcrop the geological formations of Ait Sawn and Tissouktai. These, attributed to the Ediacaran (or terminal Neoproterozoic), consist of basic volcanic facies, intermediate and acid intercalated with pyroclastics and epiclastic levels. This set is traversed locally by acidic veins. The rocks studied suffered intense weathering processes marked by the development of secondary minerals which are also formed in the hydrothermal veins and at the expense of the primary magmatic minerals and mesostasis. The main paragenesis of mineralisation consists of hematite, malachite, chalcocite, pyrite, barite, epidote, quartz, chlorite, calcite, albite, sericite and iron oxides. Paleogeography sector is governed by inherited active faults of the Pan African base. Magmatism studied seems to be guided by these tectonic lineaments during the terminal Proterozoic extension. The reactivation of these tectonic structures, after the Cambrian, and probably during the Variscan and / or Atlasic, is certain. Field investigations, supplemented by microscopic and geochemical study, indicate that much of the mineralization associated with volcanic Southwest Massif Saghro, is related to a hydrothermalisme contemporary with the stage of fault reactivation.

The geology of offshore drilling through basalt sequences: Understanding operational complications to improve efficiency

Offshore drilling in basins with a significant basaltic component poses a number of complications to drilling operations. These include slow and variable rates of penetration (ROP), drilling fluid losses, drill bit or string sticking, overpressured formations, wellbore spalling and collapse. In this contribution we investigate features of large igneous province (LIP) volcanic facies that may directly contribute to drilling complications. Combining borehole and field based examples we identify highly vesicular brecciated lava flow tops and hydro-volcanic facies as a major potential cause of catastrophic drilling fluid losses. The risk for drilling losses is generally higher in the upper parts of thick volcanic successions where hydrothermal secondary mineralization and alteration may be less pervasive. We demonstrate how these facies may be identified early by integrating real time mud logging data with characteristic drill cuttings and LWD (logging while drilling) wireline data as it becomes available. We highlight the potential of basaltic formations to maintain porosity and permeability to greater depths than is typical for sediments due to the high compressive strength of crystalline basalt frameworks. The heterogeneous sub surface distribution of key volcanic facies commonly includes intimate mixing and layering of hard and dense with soft and weak volcanic rock which may promote complex in-situ stresses to develop during burial compaction. Where this occurs, wellbore stability predictions such as the fracture and wellbore collapse gradients for volcanic sequences may deviate from those expected in basin equivalent sedimentary sequences. Examples of swelling volcanic clay horizons, unstable volcaniclastic sequences, and intrusion related ledge effects are used to characterize the main causes of drill bit and string sticking. A greater understanding of drilling complications encountered within prospective areas containing volcanic sequences is needed if better pre-drilling risk assessment, cost prediction, real-time identification and mitigation is to be achieved.

Reconstruction of Pliocene-Pleistocene sediment sources and weathering intensity in the paleo-life rich Olduvai and Laetoli basins of northern Tanzania using major and trace element geochemistry and Sr isotopic data

Major and trace elements and Sr isotopic compositions were analyzed for samples from the Pliocene-Pleistocene to Recent Olduvai and Laetoli basins of northern Tanzania with the aim of constraining their provenance and paleo-climatic conditions. The Olduvai samples are characterised by La/Cr ratios of 0.69–3.73, Th/Sc = 1.12–24.6 and 87Sr/86Sr = 0.70448–0.70563; features which suggest the volcanic facies of the nearby Olmoti as their possible source. The relatively lower values for the Laetoli basin (Th/Sc = 0.4–0.9 and 87Sr/86Sr = 0.70412–0.70508) coupled with Zr/Nb, Nb/Ta ans Zr/Hf ratios are suggestive of a mafic protolith inferred to be the Sadiman lavas. The calculated Chemical Index of Alteration (CIA) values for the Laetoli samples are 58–78% (mean = 65%) indicating higher weathering intensities of the source compared to 45% in the Olduvai samples. The calculated Mean Annual Precipitation (MAP) values are higher at Laetoli (mean = 950 ± 181 mm/yr) relative to Olduvai (mean = 694 ± 181 mm/yr) consistent with increased rainfall in the Pliocene during the deposition of Laetoli strata when compared to the Pleistocene times when the Olduvai sediments were deposited.The CIA and MAP patterns observed at Laetoli and Olduvai can be related to temporal changes in weather conditions during the Pliocene and Pleistocene, respectively. The Lower CIA and MAP values at Olduvai imply the prevalence of arid to semi-arid climatic conditions during the Pleistocene whereas the higher values at Laetoli correspond with wetter conditions during the Pliocene times. This could have the potential for improved eutrophication at Laetoli. The climatic inferences drawn from CIA and MAP data may point to a more favourable habitat for life proliferation in the wetter environment of Laetoli compared to the drier Olduvai. This inference is consistent with archaeological evidence which indicates a greater abundance of hominin fossils including the well preserved footprints in Laetoli.

Characterization of the paleocrusts of weathered Carboniferous volcanics from the Junggar Basin, western China: Significance as gas reservoirs

Volcanic reservoirs comprise reservoir spaces made up of pores and cracks, with secondary solution pores and tectonic-induced cracks being the most dominant. Various volcanic lithologies and lithofacies show differences in porosity and permeability because resistance to weathering differs among volcanic rock types. This study shows that among volcaniclastic rocks, volcanic breccia shows higher porosity and permeability as well as poor pore radius, sorting, and connectivity compared with volcanic tuff. Basalt and monzonitic porphyry show higher porosity and permeability compared with other volcanic lava and subvolcanic rocks. The level of porosity of different facies is in the order of effusive facies > explosive facies > subvolcanic rock facies > volcanic sedimentary facies > eruptive facies. Reservoirs show relatively high porosity and permeability near the top surface of the weathered crust. Porosity and permeability generally decrease with an increase in the distance from the Carboniferous top weathered crust. This condition indicates that weathering controls the properties of volcanic reservoirs. The distribution pattern of porosity and permeability in the weathered crust of volcanic facies differs. Strong weathering contributes to a decrease in porosity and permeability at the top part of the weathered crust. Various weathering effects on volcanic rocks control porosity and permeability vertically, and the gas accumulates mainly in the range of 0 m–200 m from the top of the weathered crust. Gas accumulation in subvolcanic and volcaniclastic rocks is better than that in volcanic lava. The log curve in geophysical exploration can help determine the vertical structure of weathered crust. The strong weathering belt shows high gamma, low electrical resistivity, low density, high neutron porosity, and high interval transit time.

Sedimentology, eruptive mechanism and facies architecture of basaltic scoria cones from the Auckland Volcanic Field (New Zealand)

Scoria cones are a common type of basaltic to andesitic small-volume volcanoes (e.g. 10−1–10−5km3) that results from gas-bubble driven explosive eruptive styles. Although they are small in volume, they can produce complex eruptions, involving multiple eruptive styles. Eight scoria cones from the Quaternary Auckland Volcanic Field in New Zealand were selected to define the eruptive style variability from their volcanic facies architecture. The reconstruction of their eruptive and pyroclastic transport mechanisms was established on the basis of study of their volcanic sedimentology, stratigraphy, and measurement of their pyroclast density, porosity, Scanning Electron Microscopy, 2D particle morphology analysis and Visible and Near Visible Infrared Spectroscopy. Collection of these data allowed defining three end-member types of scoria cones inferred to be constructed from lava-fountaining, transitional fountaining and Strombolian type, and explosive Strombolian type. Using the physical and field-based characteristics of scoriaceous samples a simple generalised facies model of basaltic scoria cones for the AVF is developed that can be extended to other scoria cones elsewhere. The typical AVF scoria cone has an initial phreatomagmatic phases that might reduce the volume of magma available for subsequent scoria cone forming eruptions. This inferred to have the main reason to have decreased cone volumes recognised from Auckland in comparison to other volcanic fields evolved dominantly in dry eruptive condition (e.g. no external water influence). It suggests that such subtle eruptive style variations through a scoria cone evolution need to be integrated into the hazard assessment of a potentially active volcanic field such as that in Auckland.

Extensive soft-sediment deformation and peperite formation at the base of a rhyolite lava: Owyhee Mountains, SW Idaho, USA

In the Northern Owyhee Mountains (SW Idaho), a >200-m-thick flow of the Miocene Jump Creek Rhyolite was erupted on to a sequence of tuffs, lapilli tuffs, breccias and lacustrine siltstones of the Sucker Creek Formation. The rhyolite lava flowed over steep palaeotopography, resulting in the forceful emplacement of lava into poorly consolidated sediments. The lava invaded this sequence, liquefying and mobilising the sediment, propagating sediment subvertically in large metre-scale fluidal diapirs and sediment injectites. The heat and the overlying pressure of the thick Jump Creek Rhyolite extensively liquefied and mobilised the sediment resulting in the homogenization of the Sucker Creek Formation units, and the formation of metre-scale loading structures (simple and pendulous load casts, detached pseudonodules). Density contrasts between the semi-molten rhyolite and liquefied sediment produced highly fluidal Rayleigh-Taylor structures. Local fluidisation formed peperite at the margins of the lava and elutriation structures in the disrupted sediment. The result is a 30–40-m zone beneath the rhyolite lava of extremely deformed stratigraphy. Brittle failure and folding is recorded in more consolidated sediments, indicating a differential response to loading due to the consolidation state of the sediments. The lava-sediment interaction is interpreted as being a function of (1) the poorly consolidated nature of the sediments, (2) the thickness and heat retention of the rhyolite lava, (3) the density contrast between the lava and the sediment and (4) the forceful emplacement of the lava. This study demonstrates how large lava bodies have the potential to extensively disrupt sediments and form significant lateral and vertical discontinuities that complicate volcanic facies architecture.

Volcanic and sedimentary facies of the Huka Group arc-basin sequence, Wairakei–Tauhara Geothermal Field, New Zealand

ABSTRACTDetailed facies analyses of intra-arc basin stratigraphy provide spatial and temporal insights into the nature and timing of past volcanic, sedimentary and tectonic events. Rift and caldera basins hosting lakes in the central Taupo Volcanic Zone (TVZ) continental arc preserve a sequence of locally and regionally sourced volcanic deposits and interbedded lacustrine sediments. A series of new continuous core samples from Wairakei–Tauhara Geothermal Fields in the TVZ intersecting thick Huka Group stratigraphy has provided a unique opportunity to study c. 300 ka of local arc volcanism, sedimentary processes and ancient surface environments. Facies analysis is utilised on 11 cored wells from across Wairakei–Tauhara, including a newly defined Huka Group Reference Section well (WKM15). Interpreted facies identify that the Huka Group commenced with a large ignimbrite (Waiora Ignimbrite Member) emplaced onto faulted relief covered by a pre-existing ancient lake, Lake Huka. Graded volcaniclastic facies, lavas and sediments comprising the Waiora Volcaniclastics Member then accumulated in the basins over the next c. 100 ka. A spatially variable switch from volcaniclastic (Waiora Volcaniclastics Member) to lacustrine siltstones and sandstones (Huka Falls Formation) deposition then followed in Lake Huka. Excluding an intra-lake phreatomagmatic eruption (Middle Huka Falls Formation), these lacustrine sediments reflect a c. 200 ka local pyroclastic hiatus. Late Huka Group deposition became gradually more fluvial as rivers prograded marginal areas of the lake until its destruction by the 25.4 ka Oruanui eruption. The drill core samples provide new complete and detailed insights into processes and environments associated with the Huka Group. These enhance our understanding of the geological history at Wairakei–Tauhara Geothermal Fields and contribute to our knowledge of the ancient processes that occurred during evolution of the TVZ.

A drilling data-constrained seismic mapping method for intermediate-mafic volcanic facies

Abstract Based on 112.5 km 2 of 3-D seismic data and data of 8 prospecting wells drilled volcanic rocks in the 3 rd member of the Paleogene Shahejie Formation in Hongxing area of the Eastern Sag of the Liaohe Depression, Bohai Bay Basin, three levels of volcanic interfaces (stage→edifice→lithofacies) of the intermediate-mafic volcanic formation are identified to reveal favorable prospecting facies through comprehensive studies on geology, well logging and seismic data in single well and multiple wells following the seismic volcano stratigraphy principle. According to stage interfaces, three volcanic stages were identified in the 3 rd member of Shahejie Formation. One or more volcanic edifice-seismic facies were identified in each volcanic stage and volcanic facies-seismic facies were identified in each volcanic edifice-seismic facies. Based on single well points, we described volcanic edifices on well-tie seismic sections; identified volcanic bodies by extracting coherent seismic attribute (superimposed volcanic edifices) taking the volcanic stages as the units; then identified volcanic edifices and volcanic lithofacies by extracting waveform classification properties. Volcanic facies mapping were completed by constituting the relationship between the volcanic facies and the seismic facies in drilling wells, seismic cross sections and mappings. There are two types of plane volcanic facies sequences in the intermediate-mafic volcanic facies of this study area: volcanic conduit facies→extrusive facies (→explosive facies)→effusive facies→volcanic sedimentary facies, volcanic conduit facies (→explosive facies)→effusive facies→volcanic sedimentary facies. Among them, the near crater assemblage (volcanic conduit, extrusive and explosive facies) has better hydrocarbon shows and is the most favorable target of hydrocarbon exploration.

Reconstruction and evolution of Archean intracaldera facies: the Rouyn–Pelletier Caldera Complex of the Blake River Group, Abitibi greenstone belt, Canada

Subvertically- to vertically-dipping Archean strata provide an excellent opportunity to study synvolcanic structures and internal organization of subaqueous volcanic complexes. The Abitibi greenstone belt in Quebec, Canada, hosts a number of these volcanic complexes and specifically the unique Blake River Megacaldera Complex. The Blake River Megacaldera Complex is composed of (i) an initial shield phase known as the Misema Caldera and (ii) two graben-type calderas known as the New Senator and Noranda calderas. The southern portion of the New Senator Caldera, the focus of this study, is of particular interest as the structure hosts the 54 Mt Horne Au-rich volcanogenic massive sulfide deposit. Detailed facies mapping, coupled with geochemical and geochronological analysis at multiple outcrop localities throughout the city of Rouyn-Noranda, Quebec, show that the stratigraphy of the New Senator Caldera is dominated by subaqueous effusive mafic volcanic facies with local felsic effusive and intrusive deposits. With the incorporation of structural data, we have used synvolcanic faults and dyke complexes to divide these facies into specific blocks within this region, here renamed the Rouyn–Pelletier sector. This study focuses on the facies and characteristics of the Pelletier, Senator, and Glenwood blocks and identifies the Évain, Stadacona, and Chadbourne blocks. Geochronological analysis reveals that facies of the Glenwood Block are approximately the same age as those of the felsic flows of the previously identified Horne Block. Together, all extrusive and intrusive volcanic facies from the Stadacona unit north to the Horne Creek fault compose a large caldera complex known as the Rouyn–Pelletier Caldera Complex.

Hydrothermal processes related to Triassic and Jurassic submarine basaltic complexes in northeastern Hungary and in the Dinarides and Hellenides

Comparative studies on hydrothermal alteration of submarine peperitic basalt occurrences related to the Triassic early rifting of the Neotethys were carried out in various parts of the Dinarides and Hellenides. The study areas included the displaced fragments of the Dinarides in the Darno Unit, NE Hungary, the Kalnik Mts in Croatia and the Vares-Smreka area in Bosnia and Herzegovina. In the Hellenides, similar environments were studied in the Stragopetra Mts., Greece. Jurassic pillow basalts formed in a back-arc-basin of the Neotethys were also studied (in the Szarvaskő Unit, NE Hungary, which also represents a displaced unit of Dinaridic origin). Within the submarine basaltic lava flows, six volcanic facies were distinguished. The hydrothermal alteration was characterized according to those facies. The first process was the albitization of the rock-forming plagioclase at ~300°C temperature in all localities. During the higher temperature stage of the subsequent cooling, chloritization in the ground mass is typical for all types of basalts, however chlorite and rarely quartz formed in the fractures and amygdales of the Triassic basalts, while chlorite, quartz and prehnite precipitated in the fractures of the Jurassic rocks. At lower temperatures of this cooling-related process, calcite is a common mineral filling up the larger amygdales, jig-saw type fractures and other open spaces, but some epidote, pumpellyite, prehnite and laumontite also occur in the Triassic basalts. The late stage alteration (happened at the lowest temperature) is characterized by argillitization at every locality. The observed hydrothermal alteration patterns also show slight differences according to the volcanic facies as a function of the distal/proximal setting in relation to the eruptive centers and the presence/absence of water-saturated and unconsolidated carbonate or siliciclastic sediments at the time of the emplacement of lava flows. The study revealed that the most important factors influencing mineralogy and zoning of hydrothermal alteration in these short living local hydrothermal systems are the rapid cooling of the hydrothermal fluid, the dominance of the not much evolved seawater as the source of hydrothermal fluid and the local, i.e. effective water/rock ratio, determined by the degree of fracturing in the rock. The mineralogical-textural peculiarities of the highly localized hydrothermal fluid/rock interaction in the studied submarine sea-mount type volcanoes are clearly different from the products of the large-scaled hydrothermal processes occurring at mid-oceanic ridges. Recognition of these differences is important in the evaluation of ore potential of Neotethyan realm or other areas with occurrences of submarine basaltic units

Early Permian volcano-sedimentary successions, Beishan, NW China: Peperites demonstrate an evolving rift basin

The Lower Permian volcano-sedimentary Zhesi Group has been investigated in the Hongliuhe and Liuyuan areas in Beishan, China, which is significant for the reconstruction of Late Paleozoic evolution in the southern part of the Central Asian Orogenic Belt. A variety of volcanic facies were distinguished in the Upper Zhesi Group: pillow basalt with interstitial limestone, thin-interbedded limestone and basalt, closely packed pillows, pillow-fragmented hyaloclastite breccia, and peperite. Laser 40Ar/39Ar whole-rock dating of the basalt yielding an age of 277±11Ma, as well as Early Permian brachiopod fossils in the limestone interbedded with the basalt, indicate that basalt was erupted in the Early Permian. The identification of the peperite and other facies originating from magma-sediment mingling reveals that the basaltic lava flows were derived from autochthonous basaltic magmatism and formed as part of the Lower Permian succession. The peperite also indicates that these subaqueous basaltic lava flows are not dismembered ophiolitic components, but formed in an autochthonous extensional setting in the Early Permian. The clastic rocks in the Lower Zhesi Group underlying the basaltic flows and peperites in the Hongliuhe and Liuyuan areas show a general fining-upwards sequence, indicating that they were deposited in a progressively deepening basin overlying the Devonian Hongliuhe suture zone. Subaqueous volcanism in a rift basin or basins, accompanied by coeval deposition of carbonate sediment and mud, built up the peperite-bearing volcanogenic-sedimentary successions. From among the various tectonic hypotheses for the Beishan region, this study demonstrates that by Early Permian the region was developing post-collisional rift basins.

Frontier exploration and the North Atlantic Igneous Province: new insights from a 2.6 km offshore volcanic sequence in the NE Faroe–Shetland Basin

The Lagavulin exploration well 217/15-1Z penetrated a c . 2.6 km thick volcanic sequence dominated by extrusive basaltic rocks spanning the Paleocene–Eocene boundary in the NE Faroe–Shetland Basin. The well comprises one of the thickest drilled sequences through the North Atlantic Igneous Province. Integrated analysis of drill cuttings and wireline-log data reveals key volcanic lithofacies: (1) tabular lava flows; (2) compound lava flows; (3) hyaloclastite; (4) volcaniclastic rocks. The volcanic facies reveal two major sub-aqueous to subaerial sequences consistent with lava delta progradation. These sequences are separated by a volcanic hiatus represented by extensive reddened soils, which preceded the re-submergence of the area. Emergence followed by submergence of the first lava delta is interpreted to record an intra-T40 transient uplift event near the Paleocene–Eocene boundary. Basalts from the lower c . 1.3 km have low TiO 2 (<1.5 wt%) and low Zr/Y (2–3), with olivine-phyric picrites towards the base (Mg# 70–82; olivine Fo 85–91 ). The hiatus correlates precisely with a change to high-TiO 2 (2.5–3.2 wt%), high-Zr/Y (>4) compositions, which dominate the upper sequence. The associated change in lava geochemistry, transient uplift and volcanic hiatus appears consistent with a transient pulse of hot buoyant plume material passing beneath the area. Supplementary material: All raw geochemical data and supplementary analyses are available at http://www.geolsoc.org.uk/SUP18888 .

Monogenetic volcanism: personal views and discussion

Monogenetic volcanism produces small-volume volcanoes with a wide range of eruptive styles, lithological features and geomorphic architectures. They are classified as spatter cones, scoria (or cinder) cones, tuff rings, maars (maar–diatremes) and tuff cones based on the magma/water ratio, dominant eruption styles and their typical surface morphotypes. The common interplay between internal, such as the physical–chemical characteristics of magma, and external parameters, such as groundwater flow, substrate characteristics or topography, plays an important role in creating small-volume volcanoes with diverse architectures, which can give the impression of complexity and of similarities to large-volume polygenetic volcanoes. In spite of this volcanic facies complexity, we defend the term “monogenetic volcano” and highlight the term’s value, especially to express volcano morphotypes. This study defines a monogenetic volcano, a volcanic edifice with a small cumulative volume (typically ≤1 km3) that has been built up by one continuous, or many discontinuous, small eruptions fed from one or multiple magma batches. This definition provides a reasonable explanation of the recently recognized chemical diversities of this type of volcanism.

New data on the geology of the Lozovoe zone (Upper Triassic–Middle Jurassic) of the Crimean Mountains

A new scheme of the stratigraphy and correlation of the Upper Triassic–Middle Jurassic Lozovoe zone of the Crimean Mountains based on spore–pollen data, the mineral composition of clastic and clay rocks, climate and volcanic events, facies and thickness analysis, and a general paleogeographic analysis is described. It is shown that the Lozovsky zone is not a tectonic melange. The complex structural–facies pattern is explained by consedimental tectonics, the presence of at least two olistostrome complexes in the sequence, and subsequent diverse deformations.

A 3-D Geological and Structural Synthesis of the Leinster Area of the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia, with Special Reference to the Volcanological Setting of Komatiite-Associated Nickel Sulfide Deposits

Research Article| March 01, 2015 A 3-D Geological and Structural Synthesis of the Leinster Area of the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia, with Special Reference to the Volcanological Setting of Komatiite-Associated Nickel Sulfide Deposits Caroline S. Perring Caroline S. Perring † BHP Billiton Nickel West, 125 St George’s Terrace, Perth, WA 6000, Australia †E-mail, Caroline.Perring@bhpbilliton.com Search for other works by this author on: GSW Google Scholar Economic Geology (2015) 110 (2): 469–503. https://doi.org/10.2113/econgeo.110.2.469 Article history received: 06 Mar 2014 accepted: 04 Aug 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Caroline S. Perring; A 3-D Geological and Structural Synthesis of the Leinster Area of the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia, with Special Reference to the Volcanological Setting of Komatiite-Associated Nickel Sulfide Deposits. Economic Geology 2015;; 110 (2): 469–503. doi: https://doi.org/10.2113/econgeo.110.2.469 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The Leinster area is situated toward the southern end of the Agnew-Wiluna Belt in the Yilgarn craton of Western Australia and hosts the giant Perseverance komatiite-associated nickel sulfide deposit. At in excess of 1.2 Mt contained Ni, this is the biggest type 1 (basal sulfide-rich accumulation) deposit in the world, and it is associated with a number of smaller deposits that extend over a strike length of 20 km adjacent to the Perseverance fault on the eastern margin of the belt. In broad terms, the greenstone sequence comprises an eastern, rift-proximal komatiite-dacite sequence and a western and more marginal basalt-dominated sequence with minor komatiite.The komatiites display a wide range of compositions and textures, from highly magnesian, olivine meso- to adcumulate rocks composed of olivine with forsterite contents of up to 94.5% to sequences of thin, spinifex-textured komatiite flow units. This compositional range indicates considerable variation in volcanic facies, from rift-proximal to distal. Loci of focused flow (termed pathways), indicated by thickened cord-like bodies of more olivine rich rock, occur at all scales and are intimately associated with the Ni sulfide mineralization. The pathways are linked by thinner sequences of lower-MgO rocks, either layered olivine orthocumulate rocks or sequences of thin spinifex-textured flow units. Some of the lower-MgO komatiite sequences appear to emanate from the tops of pathways, in the manner of breakout flows in tube-fed basaltic terrains. Variably sulfidic black shales and cherts and barren exhalative iron sulfide horizons are intimately interlayered with the komatiitic rocks.The overprinting effects of regional deformation and metamorphism, spanning at least 80 m.y., are considerable. At least eight local deformation events have been recognized by previous workers. These have been used to constrain the 3-D model and can be linked to craton-wide structural schemes. Some of the major faults are clearly accretionary and reflect reactivation of underlying basin-forming structures. Early north-over-south thrusting was followed by E-W–directed basin closure and the development of NNW-trending open regional folds. Extension accompanied batholithic granitoid intrusion and reactivated rift-parallel NNW-trending faults, producing the major strike-slip fault systems that divide the greenstone succession into a series of structural and stratigraphic domains. The ensuing orogenic collapse resulted in overturning of the stratigraphy to the east. Subsequent brittle deformation events have produced fault sets in a variety of orientations that can be related to ongoing shortening with a variable principal stress orientation.There is a complex interplay of structural and volcanological controls on the formation and subsequent deformation of the Ni sulfide deposits in this ancient terrane. Three-dimensional modeling has furthered a detailed understanding of the geologic and structural evolution of the belt, allowed the identification of komatiite pathway positions and their plunges, and provided quantitative data on the magnitude and direction of fault offsets of different generations. The modeling exercise has shown that it is possible to strip away the effects of overprinting metamorphism and deformation to reveal some of the primary controls on komatiite volcanism and the location of Ni sulfide deposits, even in those portions of greenstone belts that are highly deformed and have undergone amphibolite facies metamorphism. This is illustrated with case studies of the Perseverance, Venus, Rocky’s Reward, Harmony, and Sir Lancelot deposits.Three-dimensional modeling has the capacity to integrate geologic, geochemical, and geophysical datasets in a way that fully utilizes the results of past exploration programs and produces a premium geologic product that can be used to guide further exploration. The recent discovery of the Venus deposit is, in part, a tribute to the power of this approach. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Architecture of the Neoarchaean Jaguar VHMS deposit, Western Australia: Implications for prospectivity and the presence of depositional breaks

The duration and nature of breaks in Archaean volcanosedimentary packages, recently re-evaluated in the Abitibi Greenstone Belt, has been assessed at Jaguar, an Archaean Cu–Zn-rich volcanic-hosted massive sulphide (VHMS) deposit in the Teutonic Bore volcanic complex (TBVC), the Eastern Goldfields of the Yilgarn Craton, Western Australia. The deposit is hosted by coherent volcanic facies and associated volcaniclastic facies, with minor non-volcanic facies.The hypothesis that VHMS ore lies on the intersection of volcanic centres and depositional breaks is found to be valid for the Jaguar deposit, with the site being an active volcanic complex at footwall time, with further dome development close to the final ore location at Mineralised Package time, and then reoccurring volcanic complexes (andesitic and basaltic) at several points in the hangingwall succession broadly tied into the ore position.At Jaguar the entire stratigraphic sequence faces west, with no major structural complications, making it ideal for stratigraphic analysis. The footwall evidences prolific volcanism with no intervening sedimentation. The Mineralised Package (MP) consists of intercalated dacite, breccia, conglomerate, sandstone and mudstone. The footwall volcanic rocks and the MP Dacite appear to be differentiation products from the same magma chamber. A major volcanic geochemical boundary is identified between the footwall/MP (tholeiitic to transitional affinities), and the immediate hangingwall succession (calc-alkaline). The hangingwall package includes four distinct volcanic units intercalated with mass flow deposits and graphitic argillites.VHMS mineralisation occurred across a distinct change in the geochemistry of erupted volcanic facies at a time of increased volcaniclastic sedimentation and extension. This break is comparable to Member level hiatuses seen in the Abitibi GB, i.e. the most brief of breaks, because there is no silicification of sedimentary unit-tops by silicic seawater exposure, and there is minimal development of black shales. A third time-sensitive criteria was the development of peperite on the base of Hanging Wall (HW) andesite lavas. The MP closely resembles breaks within interpreted deeper water domains of the Abitibi GB, for instance, the Deloro-Tisdale assemblage interface south of Timmins.The recognition of the change in the rate of emplacement of volcanic units and particularly the change in chemistry of volcanic rocks, are critical exploration lithologic guides to use in the absence of chemical sediment deposits (exhalites). Although crustal extension and basin subsidence were factors in the shaping of the succession, their effects were too subtle to use as an exploration tool. A corollary of the recognition of a Member-level VHMS-bearing interval in the Teutonic Bore volcanic complex is that, like the Kidd-Munro assemblage stratigraphy (Abitibi GB), where several ore-bearing depositional breaks are known below the great Kidd Creek deposit, there is scope for the discovery of further (potentially larger) deposits on higher Formation or Group-level breaks in the TBVC.

车排子地区石炭系火山岩地震相特征

车排子地区石炭系火山岩储集空间类型主要受岩相控制，不同的岩相控制了不同的孔缝组合类型，通过火山岩地震相特征的研究，可以精细识别火山岩岩相，从而对火山机构进行精细描述。通过对工区钻井地震相特征的剖析，总结工区石炭系高产井特征如下：高产井以近口亚相为主；火山岩产状与地层界面构成“反屋脊”模式；多期次火山作用特征明显；风化壳储层发育；有利的储盖组合。 The volcanic reservoir space in carboniferous of Chepaizi area is mainly controlled by facies, and different facies have different types of cracks and pores. By the study of the characteristics of seismic facies of volcanic rocks, it can fine identify the volcanic facies, thus fine describing volcanic bodies. By the analysis of seismic facies of drilling in the research area, we summarized that the high-yield wells in carboniferous are characterized as follows: high-yield wells subfacies are mainly near the crater; the occurrence of the volcanic and the interface of the formation constitute a “reversed roof” mode; the multiperiodic volcanism characteristics are obvious; weathering crust reservoir is developed; and the reservoir and caprock have a favorable combination.

The mode of emplacement of Neogene flood basalts in eastern Iceland: Facies architecture and structure of simple aphyric basalt groups

Simple flows (tabular) in the Neogene flood basalt sections of Iceland are described and their mode of emplacement assessed. The flows belong to three aphyric basalt groups: the Kumlafell group, the Hólmatindur group and the Hjálmadalur group. The groups can be traced over 50km and originate in the Breiðdalur–Thingmuli volcanic zone. The groups have flow fields that display mixed volcanic facies architecture and can be classified after dominating type morphology. The Kumlafell and the Hólmatindur groups have predominantly simple flows of pāhoehoe and rubbly pāhoehoe morphologies with minor compound or lobate pāhoehoe flows. The Hjálmadalur group has simple flows of rubbly pāhoehoe, but also includes minor compound or lobate flows of rubble and 'a'ā. Simple flows are most common in the distal and medial areas from the vents, while more lobate flows in proximal areas. The simple flows are formed by extensive sheet lobes that are several kilometers long with plane-parallel contacts, some reaching thicknesses of ~40m (aspect ratios <0.01). They have overlapping contacts and are free of tubes and inflation structures. Their internal structure consists generally of a simple upper vesicular crust, a dense core and a thin basal vesicular zone. The brecciated flow-top is formed by clinker and crustal rubble, the clinker often welded or agglutinated. The simple flows erupted from seemingly short-lived fissures and have the characteristics of cooling-limited flows. We estimate the effusion rates to be ~105m3/s for the simple flows of the Kumlafell and Hólmatindur groups and ~104m3/s for the Hjálmadalur group. The longest flows advanced 15–20km from the fissures, with lava streams of fast propagating flows inducing tearing and brecciation of the chilled crust. Compound or lobate areas appear to reflect areas of low effusion rates or the interaction of the lava with topographic barriers or wetlands, resulting in chaotic flowage. Slowing lobes with brecciated flow-tops developed into 'a'ā flows. The groups interdigitated with lava groups from the Reyðarfjörður volcanic zone to the east, and the exhumed Breiðdalur–Thingmuli volcanic zone which appear to have formed as a flank lineament parallel the main rift zone. Flood basalt volcanism in flank areas may support a mantle anomaly more pronounced and/or perhaps more widespread in the Neogene of Iceland than today. Eruptions of simple flows have not been observed in modern times and are significant for models of crustal accretion in Iceland and other Large Igneous Provinces.

Formation mechanism of in-situ volcanic reservoirs in eastern China: A case study from Xushen gasfield in Songliao Basin

A large number of in-situ volcanic reservoirs have been discovered from the Meso-Cenozoic rift basin group in eastern China. Based on drilling results in combination with geological and geophysical analysis, a case study from the Early Cretaceous Xujiaweizi fault-depression shows that the formation mechanism of in-situ volcanic reservoirs is characterized by “fault-controlled body, body-controlled facies, facies-controlled reservoir, and reservoir-controlled accumulation”. In other words, deep faults control the volcanic eruption type, volcanic body, and gas reservoir distribution; the volcanic body determines the spatial distribution of volcanic facies and volcanic gas reservoir size; the volcanic facies control reservoir physical properties and effective thickness of gas formation; the volcanic reservoir properties control gas reservoir type and gas productivity. The result is useful to guiding the discovery of in-situ volcanic gas reservoirs in faulted basins in both theory and practice.