Felsic Volcanic Units Research Articles

Oxygen isotope evidence for magmatic variability and multiple alteration events in the Proterozoic St. Francois Mountains, Missouri

The St. Francois Mountains contain at least two Mesoproterozoic caldera complexes, the Butler Hill and the Taum Sauk calderas, each with diameters greater than 20 km. Igneous zircons from intrusive and extrusive units within the calderas have a range of δ 18O(zircon) from 5.8 to 8.2‰ V-SMOW. These zircons are similar to other Proterozoic zircons from igneous rocks around the world and suggest a magma source from partial melting of older crust with variable mantle component. There is no significant difference in the magmatic oxygen isotope ratios between the Taum Sauk and Butler Hill calderas. Values of Δ 18O(quartz–zircon) = 2.1–2.4‰ indicating that most quartz from samples in the two calderas still preserves near-magmatic oxygen isotope ratios. The average δ 18O(quartz) from the two calderas is 8.6 ± 2.6‰. In contrast, felsic volcanic units within the Taum Sauk caldera exposed at Johnson Shut-Ins suggest interaction with water at low temperature resulting in δ 18O(quartz) as high as 16.7‰. Quartz phenocrysts within a single sample from Johnson Shut-Ins range from 8.7 up to 16.7‰. The isotope heterogeneity, coupled with evidence of healed microcracks in cathodoluminescence images, suggests low temperature hydrothermal alteration affected the volcanics during or shortly after eruption. After caldera collapse the hydrothermally altered rocks were partially re-melted and incorporated into the magma chamber as evidenced by the slight increase in δ 18O(zircon) of the resurgent dome intrusions. In contrast, the Butler Hill caldera preserves evidence of high temperature hydrothermally altered country rock assimilated into the Stono Mountain granitic ring intrusion based on low δ 18O quartz values and visible zoning in CL. The two calderas experienced different alteration histories suggesting the lack of any homogenizing regional fluid influx.

Late Mesozoic silicic magmatism of the North Chukotka area (NE Russia): Age, magma sources, and geodynamic implications

The Cretaceous Okhotsk–Chukotka volcanic belt (OCVB) is one of the largest subduction-related volcanic provinces of the Earth. It is thought to be related with the subduction of paleo-Pacific plates under the collage of terranes of NE Asia accreted during Jurassic and Early Cretaceous time. The OCVB comprises a remarkably high portion of silicic rocks, up to 80–90% in some segments. Within the Central Chukotka segment of the belt, volcanic sequences reveal a rather uniform 40Ar/ 39Ar and U–Pb isotopic age of near 89–87 Ma. But the felsic volcanic unit with coeval subsurface intrusives named the ‘Berlozhya magmatic assemblage’ (BMA) yields much older zircon U–Pb ages (146.0 ± 2.4 for rhyolitic tuff and 145.5 ± 1.8 Ma for related granodiorite). The relationship of the BMA with the active boundary between the Chukotka–Arctic Alaska microcontinent and the Anyui–Angayucham oceanic basin is suggested. BMA rocks seem to be undeformed, and their age puts constraints on the timing of main compressional events in the North Chukotka area. We present analytical data for both BMA and OCVB silicic magmatic rocks, including the first isotopic compositions of Sr, Nd, Pb, and Hf for the 1500 km long northern part of the OCVB. We infer that felsic magmas of both BMA and OCVB were produced by the melting of the continental crust, without a significant direct contribution from mantle sources. However, the crustal protolith could contain some ancient island arc complexes, which affected the chemical and isotopic composition of magmatic derivatives. The BMA exhibits a relatively uniform isotopic composition ( 87Sr/ 86Sr i = 0.7057 to 0.7070, ε Nd i = − 0.51 to − 0.12, ε Hf i = 4.41 to 4.92, 206Pb/ 204Pb i = 18.50 to 18.57, and 208Pb/ 204Pb i = 38.23 to 38.31), and its chemical diversity likely results from crystal fractionation. The protolith of OCVB rhyolites has more variable isotopic characteristics ( 87Sr/ 86Sr i = 0.7032 to 0.7082, ε Nd i = − 4.06 to − 2.84, ε Hf i = − 1.56 to 3.77, 206Pb/ 204Pb i = 18.56 to 18.81, and 208Pb/ 204Pb i = 38.21 to 38.63), and reveals at least three end-members. Each of them is distinct from the source of the Berlozhya assemblage, thus suggestive of compositional layering of the crust. Nd and Hf model ages of all studied rocks correspond to Neoproterozoic. The estimated Nd model age of the OCVB protolith is slightly older than that for the BMA, but both fall into the Neoproterozoic interval (1000 to 800 Ma).

Geologic, Geochemical, and Geochronological Constraints on the Genesis of Gold Mineralization at Poplar Mountain, Western New Brunswick, Canada

The Poplar Mountain gold occurrence in western New Brunswick is hosted in the Poplar Mountain volcanic complex (PMVC), which is located along the southern segment of the regional Woodstock fault zone. The PMVC consists of three principal units including, in ascending order, a porphyritic felsic volcanic unit, a volcaniclastic unit, and a mafic volcanic unit. U-Pb dating of zircon indicates that the age of the volcanic rocks is younger than 459 ± 3 Ma, and 40Ar/39Ar dating of mineralization-associated sericite indicates that the age of mineralization is 411 ± 3.7 Ma. Gold-mineralized zones occur in all the lithologic units but mainly in the porphyritic felsic volcanic unit. Gold zones are not controlled by individual faults but are characterized by high-density brittle fracturing, thin carbonate-quartz veining, and intensive ankerite-sericite alteration, superimposed on an earlier chlorite-cal- cite-quartz alteration. Gold is associated with arsenopyrite, which mostly occurs as disseminations in the host rocks, and to a lesser extent with quartz-carbonate-sericite veins and quartz-cemented breccias. Geochemical data indicate that mineralization-associated alteration is characterized by enrichment of K, Rb, Cs, Ca, Sr, Ba, As, Sb, W, C, and S. Fluid inclusion data indicate that the mineralizing fluids are composed of H 2O, salts, and CO2 with variable amounts of N2 and CH4; salinities range from 2.3 to 10.6 eq. wt.% NaCl, but mainly from 2.3 to 5.6 eq. wt.% NaCl, and homogenization temperatures lie mainly between 220° and 270°C. Fluid pressures at the site of mineralization are estimated to have been 770 to 1200 bars, corresponding to a depth of 2.9 to 4.6 km at lithostat- ic pressure. The δ18OSMOW and δ13CPDB values of mineralization-related ankerite range from +14.5‰ to +16.5‰, and -6.8‰ to -8.3‰, respectively. The δ18O values of the ore-forming fluids are estimated to be 6.4‰ to 8.3‰ at a temperature of 250°C. These carbon and oxygen isotope data fall in the field of magmatic fluids and in part the field of metamorphic fluids. Considering the geological setting of the region, abundant granitic intrusions, the similarity between the age of mineralization (411 ± 3.7 Ma) and the nearby Pokiok batholith (402-415 Ma), and the enrichment of granophile elements in mineralization-related alteration, the ore-forming fluids were probably derived from a granitic intrusion underneath the PMVC. These fluids were focused along structures related to the Woodstock fault. The Poplar Mountain gold occurrence might be compared to some granitic intrusion-related gold systems based on the geochemical data presented herein. © 2008 Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved.

Constraining the age of the Iporanga Formation with SHRIMP U-Pb zircon: Implications for possible Ediacaran glaciation in the Ribeira Belt, SE Brazil

The Ribeira belt in SE Brazil is a Neoproterozoic to Early Palaeozoic orogen, whose architecture and history is not yet fully understood. The depositional age of many of the sedimentary sequences in the Ribeira Belt remains unconstrained, and with debate concerning their depositional environment and tectonic setting. In this paper we present SHRIMP zircon U/Pb age constraints for one such problematic unit in the Ribeira Belt – the Iporanga Formation – and discuss the significance of this age with regards to the timing of Neoproterozoic glacial events in southeast Brazil. Using a felsic volcanic unit immediately under the Iporanga Formation and granite cobbles from breccias in its basal parts a reconnaissance SHRIMP U/Pb zircon maximum depositional age of 580 Ma is assigned for the base of this unit. This age is marginally younger than the 625–605 Ma ages for intrusions into the Lajeado and Ribeira subgroups, with which the Iporanga Formation is in tectonic contact. This indicates that the Lajeado and Ribeira subgroups are not stratigraphically equivalent to the Iporanga Formation, as thought previously by some workers. The maximum depositional age of 580 Ma also places a maximum time constraint on the tectonic juxtaposition of the Iporanga Formation with other supracrustal units, and on the greenschist facies metamorphism and isoclinal folding that affected it. The potential glacial origin for the Iporanga Formation, if correct, would place it in the late Ediacaran — provisionally equivalent to the Gaskiers glaciation.

Aeromagnetic mapping and reconnaissance geochemistry of the Early Cretaceous Henties Bay-Outjo dike swarm, Etendeka Igneous Province, Namibia

An interpretation of high-resolution aeromagnetic data, backed up by Landsat ETM+ images and field observations, reveals a major NE-trending regional dike swarm in west-central Namibia which we name the Henties Bay-Outjo dike swarm (HOD). The HOD is some 100 km wide and extends at least 500 km from the continental margin, thus ranking among the regionally important dike swarms on the South Atlantic margins. Field relations and radiometric dates indicate Early Cretaceous emplacement ages for the dikes, contemporary with Etendeka Group flood basalts and with the Damaraland intrusive complexes that occur in the same area. The orientation and distribution of dikes within the HOD suggest a strong influence by Damara Belt structures within the first 100 km from the coast. Farther inland, the dikes are more discordant to the Damara Belt and finally the swarm leaves the Damara Belt entirely and crosses into the Angola craton, where dikes fan out to the north and extend for at least another 200 km. Geochemical analysis of about 100 dikes distributed throughout the HOD reveals a compositional spectrum ranging from basalt to rhyolite, with the dominant composition being tholeiitic, low-Ti basalt. The basaltic dikes show some compositional diversity, but most can be assigned to known compositional subtypes of the Etendeka Group and are thus likely to represent feeder dikes to now-eroded lava fields. The silicic dikes have compositional variations (metaluminous to peraluminous, 64–76 wt% SiO 2) matching the range found in the Early Cretaceous Damaraland intrusive complexes, and they only marginally overlap with felsic volcanic units of the Etendeka. These dikes are probably related to the silicic magma systems of the Damaraland complexes. We interpret the HOD as the failed arm of a triple junction centered at the shelf edge off Walvis Bay. Late Cretaceous magmatism in Namibia is plume-related, but we believe the triple junction did not result from domal uplift above a plume. The triple junction coincides with the intersection of three Pan-African orogenic belts: the inland Damara Belt and the coast-parallel Gariep/Dom Feliciano Belt and Kaoko/Ribeira Belt. Mesozoic opening of the South Atlantic propagated northward from the Cape, and when rifting reached the inherited Proterozoic triple junction, extension and magmatism affected all three belts initially but the Damara Belt became inactive shortly thereafter and continental separation followed the coast-parallel belts.

Stratigraphy of the tectonically imbricated lithological succession of the Neves Corvo mine area, Iberian Pyrite Belt, Portugal

Biostratigraphic research, based on palynomorphs and ammonoids, of the tectonically imbricated lithological succession of the Neves Corvo mine, in the Portuguese part of the Iberian Pyrite Belt, has yielded ages for all formerly recognised lithostratigraphic units. These can be assembled in three main lithological sequences: (1) detrital sandy/shale substrate (Phyllite-Quartzite Formation) of late Famennian age; (2) Volcano-Sedimentary Complex, divided into a lower and an upper suite, in which one basic, three dolerite sills and four felsic volcanic units and a mineralised package of massive sulphides are identified with ages which range from the late Famennian to the late Visean; (3) flysch succession (Mertola Formation) composed of shale and greywacke dated as late Visean to early Serpukhovian. Precise biostratigraphic dating of the sedimentrary host rocks of massive sulphide mineralisation constrains the age of the latter to the late Strunian (354.8–354.0 Ma). Three stratigraphic hiatuses, corresponding to early/middle Strunian, Tournaisian and early Visean respectively and a south-westward progressive unconformity were also recognised. Sequences 1 and 2 are related to extensional episodes while sequence 3 marks the beginning of compressive tectonic inversion which gave rise to south-westward flysch progradation in close relation to a foreland basin development. Our results lead to the reinterpretation of the tectonic structure of the Neves Corvo mine, with implications for the interpretation of the regional basin dynamics and metal exploration.

Stratigraphy, distribution and geochemistry of widespread felsic volcanic units in the Mesoproterozoic Gawler Range Volcanics, South Australia

Three widespread felsic volcanic units, the Eucarro Rhyolite, Pondanna Dacite Member and Moonaree Dacite Member, have been distinguished in the Mesoproterozoic Gawler Range Volcanics. These three units are the largest in the Gawler Range Volcanics, each in excess of 500 km3. Each unit is ∼300 m thick and includes a black, formerly glassy base, a granophyric columnar‐jointed interior, and an amygdaloidal outer margin. The units are very gently dipping and locally separated by thin (<20 m) lenses of either ignimbrite (Mt Double Ignimbrite), tuffaceous sandstone or faults. The youngest unit, the Moonaree Dacite Member, covers a central area with a diameter greater than 80 km. The southern two units have east‐west extents in the order of 180 km, but are much less extensive from south to north (5–60 km). All three units are dominated by euhedral phenocrysts and are relatively crystal rich. Both the Eucarro Rhyolite and Moonaree Dacite Member contain clasts of basement granitoid and other lithologies and are locally heterogeneous in texture and composition. Some granitoid clasts have disintegrated, liberating feldspar and quartz crystals into the surrounding host. These liberated crystals cause textural variations, but can be identified on the basis of shape (amoeboid or skeletal) and/or size (megacrysts). Textural and lithofacies characteristics are consistent with the interpretation that these units are lavas; the strongly elongate distribution and wide extent of the Eucarro Rhyolite and Pondanna Dacite Member could indicate that vents were aligned along an extensive east‐west‐trending fissure system. Stratigraphic nomenclature has been revised to better reflect the presence of the three emplacement units. The oldest unit, the Eucarro Rhyolite, is dominated by plagioclase‐phyric rhyolite that locally includes granitoid clasts and megacrysts. Along the northern margin, the rhyolite is amygdaloidal and has mingled with a quartz‐rich rhyolite (Paney Rhyolite Member). The Eucarro Rhyolite and Paney Rhyolite Member replace the formerly defined ‘Eucarro Dacite’, ‘Nonning Rhyodacite’, ‘Yannabie Rhyodacite’ and ‘Paney Rhyolite’. The two younger units, Pondanna Dacite Member and Moonaree Dacite Member, are compositionally and spatially distinct, newly defined members of the Yardea Dacite.

U–Pb single zircon geochronology of granitoids in the Vumba granite–greenstone terrain (NE Botswana):: Implications for the evolution of the Archaean Zimbabwe craton

U–Pb SHRIMP single zircon crystallization ages were determined on representative samples of various granitoids exposed in the Vumba granite–greenstone terrain in NE Botswana. Based on structural and petrographic features, these granitoids were previously divided into five chronological groups: G1 (oldest) up to G5 (youngest). This chronology is not confirmed by U–Pb zircon dating, except for G5, which is the youngest granite. Our data show that igneous crystallization ages in the Vumba granite–greenstone terrain fall in the range 2696±4 and 2647±4 Ma. This time span is similar to the range of U–Pb zircon ages of 2710±19–2646±3 Ma for the Matsitama granite–greenstone terrain in NE Botswana. The oldest zircon age (2733±5 Ma) in the Vumba granite–greenstone terrain was obtained on a mafic xenolith from G2 gneiss/migmatite. This xenolith is inferred to represent a fragment of the Lower mafic–ultramafic volcanic unit of the Vumba greenstone belt. The minimum estimate of the protolith age of this gneiss is 2690±4 Ma and is within error of the age of the G1 tonalitic gneiss (2686±6 Ma). A crystallization age of 2696±4 Ma is yielded by the G4 dioritic granitoid, which was previously assumed to be part of the ca. 2.56 Ga post-kinematic granitoids. Field relationships and textural features suggest that this granitoid represent a subvolcanic intrusion probably coeval to the Upper intermediate to felsic volcanic units of the Vumba greenstone belt. A comparison of crystallization ages suggests that the Neoarchaean granitoids from NE Botswana might be correlated to the Sesombi granitoids and Upper Bulawayan volcanic sequence of the Zimbabwe craton. The G5 granitoids in the Vumba granite–greenstone terrain were emplaced at the same time as the Razi suite granitoids in Zimbabwe. The discrete shear zones documented in the Vumba granite–greenstone terrain developed between ca. 2686±6 and 2647±4 Ma. They are correlated to the D1-accretion-related deformation event in the Zimbabwe craton.

Miocene Landscape Evolution and Geomorphologic Controls on Epithermal Processes in the El Indio-Pascua Au-Ag-Cu Belt, Chile and Argentina

The world-class breccia-, stockwork-, and vein-hosted epithermal Au-Ag-Cu deposits of the central Andean El Indio-Pascua belt, latitudes 29°20' to 30°30' S, were emplaced between 6.2 and 9.4 Ma, immediately beneath the shoulders of an actively uplifting north-south tectonic block along the axis of the Cordillera Principal. The deposits of the Pascua-Lama, Veladero, El Indio, and Tambo districts are hosted by largely felsic volcanic units of late Oligocene-Early Miocene and Late Paleozoic age, but were associated with small hypabyssal bodies of dacitic porphyry, the Pascua Formation. There is no evidence that the epithermal centers were overlain by substantial volcanic edifices. The preglacial Miocene landscape, previously undocumented, incorporates three planar erosional landforms of regional extent: (1) the ca. 15- to 17-Ma Frontera-Deidad Surface; (2) the ca. 12.5- to 14-Ma Azufreras-Torta Surface; and (3) the ca. 6- to 10-Ma Los Rios Surface. These pediplains are each vertically separated by 200 to 400 m and, by analogy with the enormous erosion surfaces of the Atacama desert of northern Chile and southern Peru, are considered to have formed in a semi-arid climate in direct response to uplift events. Although the Azufreras-Torta Surface immediately overlies most of the major hydrothermal systems, and hence influenced their evolution, ore formation was contemporaneous with the development of the younger Los Rios pediplain, and was almost entirely focused around the upper extremities of stage III valley pediments. In contrast, precursor high-sulfidation hydrothermal alteration zones in the main mineralized districts, ranging in age from 10.0 to 13.6 Ma and associated with Vacas Heladas Formation dacitic magmatism, developed before incision of the Los Rios pediments and are barren. The areal and temporal relationships between economic epithermal centers and the heads of Upper Miocene valley pediments provide, at the least, clear, empirical exploration guidelines for the El Indio-Pascua belt. However, it is further proposed that changes in the surficial hydrodynamic environment, including the rapid lowering of the water table, increased lateral ground-water flow, and fluid boiling and mixing, all favoring ore deposition, may have been directly induced by pediment incision in this and other semi-arid cordilleran environments in which epithermal mineralization was not associated with significant volcanic edifices.

Geochemistry of late Mesoproterozoic volcanism in southwestern Scandinavia: implications for Sveconorwegian/Grenvillian plate tectonic models

Abstract: The last magmatic stage before the Sveconorwegian orogeny in the Baltic Shield is represented by bimodal, c. 1.16 Ga volcanics from the Bandak Group in southern Norway and largely coeval basalts from the Dal Group in SW Sweden. In both areas, the volcanic rocks are intercalated with sedimentary units and the onset of basin development is marked by deposition of clastic sediments. In the lower part of the Bandak Group, the Morgedal basalts are relatively evolved and geochemical signatures suggest assimilation of lower crustal components. In contrast, the subsequent Gjuve basalts are more primitive and record assimilation of upper crustal rocks similar to exposed basement rocks. In the Bandak Group, the chemistry of the basalts changes within the stratigraphy, such that the older Morgedal basalts (MgO = 5.2–8.8 wt%, initial ϵ Nd 2.81–3.96, Zr/Y = 4.0–5.1, Zr/Nb = 34–50, La/Nb = 2.4–3.7) are more fractionated and have more enriched Nd isotope ratios compared to the Gjuve basalts (MgO = 6.4–12.3 wt%, initial ϵ Nd 4.05–4.97, Zr/Y = 3.4–4.7, Zr/Nb = 20–30, La/Nb = 1.5–2.4). This suggests that there earliest basalts are more contaminated, and it appears that the composition of the crustal component changed with time. A thin felsic volcanic unit, the Dalen Formation, separates the two basalt sequences. It represents melting of upper crustal rocks, triggered by injection of mafic magmas into crustal magma chambers. In the Dal Group, basalts form a relatively thin unit (&lt; 500 m) where flows with high MgO values (&gt; 9%) were least affected by assimilation of crustal components. Whole-rock geochemistry and Nd isotope ratios for the Bandak and Dal basalts indicate shallow melting of sub-continental lithospheric mantle in response to extension in a continental back-arc setting, related to subduction along the western margin of Baltica. These constraints envision a similar tectonic evolution with that of eastern Laurentia, which support models of a pre-Grenvillian supercontinent with a long-lived, active margin that reached western Baltica.

Thrust-related accretion of an Archaean greenstone belt in the Midlands of Zimbabwe

Detailed structural data from the Midlands greenstone belt show that, in contrast to pre-existing models, the evolution of the greenstone belt involved an early episode of thin-skinned thrusting affecting separate lithological domains. These display different sedimentary and structural-metamorphic histories prior to their juxtaposition across steep, west-directed thrust zones. The domains include 3.5Ga old gneiss, 2.67–2.88Ga, mafic–felsic volcanic units and early-syn-tectonic, clastic sedimentary sequences. Concomitant with thrusting along domain boundaries, upright folding and ‘minor’ shear accommodated strain within domains.The early thin-skinned, and later, steeper, thrusting events can be interpreted as progressive stages in an accretionary and crustal thickening sequence, possibly associated with under-plating or low-angle subduction. The clastic sedimentary sequences contain intraformational clasts and show coarsening upward cycles below major early thrust horizons, and may have formed on alluvial fans developing in front of west-moving nappes. Strike-slip motion on the main shear zones in the Midlands greenstone belt only occurred late in the tectonic history and there is no evidence that this resulted in large displacements. A shift of σ1, from E–W to N–S during the late stages of Archaean deformation of the greenstone belt can be attributed to extensional collapse following E–W shortening and crustal thickening of the Zimbabwe craton.

Global comparisons of volcanic-associated massive sulphide districts

Abstract Although volcanic-associated massive sulphide (VMS) deposits have been studied extensively, the geodynamic processes that control their genesis, location and timing remain poorly understood. Comparisons among major VMS districts, based on the same criteria, have been commenced in order to ascertain which are the key geological events that result in high-value deposits. The initial phase of this global project elicited information in a common format and brought together research teams to assess the critical factors and identify questions requiring further research. Some general conclusions have emerged. (1) All major VMS districts relate to major crustal extension resulting in graben subsidence, local or widespread deep marine conditions, and injection of mantle-derived mafic magma into the crust, commonly near convergent plate margins in a general back-arc setting. (2) Most of the world-class VMS districts have significant volumes of felsic volcanic rocks and are attributed to extension associated with evolved island arcs, island arcs with continental basement, continental margins, or thickened oceanic crust. (3) They occur in a part of the extensional province where peak extension was dramatic but short-lived (failed rifts). In almost all VMS districts, the time span for development of the major ore deposits is less than a few million years, regardless of the time span of the enclosing volcanic succession. (4) All of the major VMS districts show a coincidence of felsic and mafic volcanic rocks in the stratigraphic intervals that host the major ore deposits. However, it is not possible to generalize that specific magma compositions or affinities are preferentially related to major VMS deposits world-wide. (5) The main VMS ores are concentrated near the top of the major syn-rift felsic volcanic unit. They are commonly followed by a significant change in the pattern, composition and intensity of volcanism and sedimentation. (6) Most major VMS deposits are associated with proximal (near-vent) rhyolitic facies associations. In each district, deposits are often preferentially associated with a late stage in the evolution of a particular style of rhyolite volcano. (7) The chemistry of the footwall rocks appears to be the biggest control on the mineralogy of the ore deposits, although there may be some contribution from magmatic fluids. (8) Exhalites mark the ore horizon in some districts, but there is uncertainty about how to distinguish exhalites related to VMS from other exhalites and altered, bedded, fine grained tuffaceous rocks. (9) Most VMS districts have suffered fold-thrust belt type deformation, because they formed in short-lived extensional basins near plate margins, which become inverted and deformed during inevitable basin closure. (10) The specific timing and volcanic setting of many VMS deposits, suggest that either the felsic magmatic-hydrothermal cycle creates and focuses an important part of the ore solution, or that specific types of volcanism control when and where a metal-bearing geothermal solution can be focused and expelled to the sea floor, or both. This and other questions remain to be addressed in the next phase of the project. This will include in-depth accounts of VMS deposits and their regional setting and will focus on an integrated multi-disciplinary approach to determine how mineralisation, volcanic evolution and extensional tectonic evolution are interrelated in a number of world-class VMS districts.

Origin of a carbonate-hosted Fe-Mn-(Ba-As-Pb-Sb-W) deposit of Långban-type in central Sweden

The Sjogruvan deposit is one of the Langban-type Fe-Mn oxide deposits hosted by marble interbeds within Svecofennian metavolcanic rocks in the Bergslagen region, central Sweden. Mineralogical and geochemical studies have been carried out to clarify the premetamorphic origin of this type of deposit, which is set apart from most other Mn mineralizations by a significant enrichment in Ba, As, Sb, Pb, W and Be contained by various oxyminerals. The principal ore types at Sjogruvan are (1) hematite+quartz±magnetite, (2) hausmannite+calcite+tephroite and (3) braunite+celsian+phlogopite. The Mn ores are compositionally akin to modern Mn deposits formed by submarine hydrothermal processes (with a high Mn/Fe ratio and low contents of Co, Ni, Th, U and REE) and likely owe their existence to similar mechanisms of formation. Pb isotope data indicate that the metal source and timing of deposition is similar to the major stratabound base-metal and iron deposits in Bergslagen. All the key elements have been leached from the local felsic volcanic units and were deposited on the sea floor; the excellent Mn-Fe separation occurred in an Eh-pH gradient that essentially corresponded to the mixing zone of hydrothermal solutions and seawater. The braunite ore is chemically distinct from the hausmannite ore, with a high concentration of refractory elements (Al, Ti, Zr) and a positive Ce anomaly, which indicate a detrital/hydrogenetic contribution to its protolith. Carbon isotope (δ13C) values around 0‰ (relative PDB) suggest that carbonates in the deposit formed directly from seawater.

A Geological Interpretation of Observed Electrical Structures in the Regolith: Lawlers, Western Australia

SALTMAP airborne electromagnetic (AEM) data for the Lawlers District, W.A., show that areas of complex regolith cover are characterised by marked variations in electrical conductivity. Their interpretation against ground electromagnetic, petrophysical and drill hole data indicates that conductive zones lie within the regolith, usually between a thin, relatively resistive, surface layer and a resistive basement. The conductive layer is commonly associated with the saprolite and to a lesser extent with alluvium in palaeovalleys and drainage sumps. Interpretation of the AEM with aeromagnetic and field data indicates that there is a strong lithodependence in the observed conductivity structure. Structure is also important. One-dimensional layered earth inversions and conductivity depth sections showed that the conductivity and thickness of regolith materials (predominantly saprolite) varied with lithology. For example, the felsic volcanics located in the NE margin of the survey area are characterised by a thick, poorly conductive saprolite. This contrasts with a thinner, more conductive saprolite developed over adjacent mafic lithologies. Somewhat surprising were the electrical characteristics of the ultramafic units, which appeared to be similar to those of the felsic volcanic units, suggesting thick, relatively resistive, materials. This behaviour is at odds with that reported in other studies concerning the electrical properties of weathered ultramafics in other environments. The Lawlers study suggests that differences in the electrical properties of in-situ regolith materials can be attributed to the complex interplay between the manner in which particular lithologies weather, the character of the resulting regolith in terms of porosity and permeability, and to variations in the soluble salt content and quantity of the saturant waters. The "EM response map" of the Lawlers area is further complicated by transported cover which appears to impose a cross cutting conductivity structure over that of the underlying saprolites.

Partially melted lithic megablocks in the Yardea Dacite, Gawler Range Volcanics, Australia: implications for eruption and emplacement mechanisms

Lithic megablocks ranging from 200 m) felsic volcanic unit in the Mesoproterozoic Gawler Range Volcanic Province (GRV) of South Australia. Throughout its vast extent, the Yardea Dacite shows typical lava-like features, in that it is massive, columnar jointed and evenly porphyritic with 30-40% crystals in a spherulitic and granophyric groundmass. In addition, flow banding is present at many locations. The megablocks are abundant at two sites 50 km apart, but isolated megablocks and smaller (<6 cm) lithic clasts are also scattered throughout the unit. At both sites the megablocks are matrix supported, non-graded, randomly oriented and show no evidence of being confined to a particular stratigraphic level in the dacite. The most abundant and largest megablocks are granitoids derived from older basement and from early-crystallised plutons of the Hiltaba Suite, which is broadly coeval and comagmatic with the GRV. The granitoid megablocks have been partially melted, most likely prior to eruption when resident in the thermal aureole of the Yardea Dacite magma chamber. The lithic megablock occurrences are unlike coarse pyroclastic breccias but are similar in distribution and abundance to xenoliths in lavas, consistent with the lava-like character of the host dacite. Using reasonable estimates of megablock density, magma density and magma viscosity, we show that the rise rate of the dacitic magma exceeded the settling velocity of the megablocks, implying that they could have been entrained and erupted effusively. All but the largest and least-melted megablocks would have remained suspended or else settled very slowly in the dacitic lava during outflow. The rapid rate of magma withdrawal required to produce such an extensive felsic sheet could have also triggered disintegration of the thermally stressed wallrock surrounding the magma chamber, dislodging megablocks that were later entrained and effusively erupted.

片貝地域のグリーンタフ火山岩貯留層に認められる熱水変質と二次孔隙の性質

The Katakai gas field produces gas from the Green Tuff volcanic rock reservoirs deeper than 4000m. The “Green Tuff” volcanic rocks of Miocene age are submarine felsic volcanic units and have undergone an extensive hydrothermal alteration which has improved the pore properties of the volcanic rock reservoirs. The timing of hydrothermal alteration in the volcanic rock reservoirs is a key issue in the argument on the formation of effective reservoirs. Two scenarios can be considered: the formation of the reservoirs might have occurred after deep burial or soon after the submarine volcanism. Based on the results of isotopic, mineralogical and fluid inclusion studies, the latter scenario seems to be more rational, which can explain the hydrothermal alteration process.The alteration zoning of clay minerals and carbonates revealed two upflow zones of hydrothermal fluids. Basaltic highs in the lower half of the Green Tuff unit correspond to the presumed upflow zones. The microthermometry data of fluid inclusions show the mixing of high temperature brines with moderate temperature waters of very low salinity. It is presumed that the latter waters are of catagenesis origin in contrast to the former of modified sea water origin. Catagenesis waters are likely to have been expelled from overlying mudstones, which is conformable with the occurrence of oil inclusions in quartz amygdales.It is concluded that the hydrothermal activity followed the felsic submarine volcanism and heated overlying mudstones to commence to generate petroleum in a shorter period of time than the maturation of organic matter during burial.

Chronology of the Mount Magnet granite-greenstone terrain, Yilgarn Craton, Western Australia: implications for field based predictions of the relative timing of granitoid emplacement

In this paper we report ion-probe UPb ages for zircons from thirteen granitoids and five supracrustal rocks from the Mount Magnet area of the Archaean Murchison Province of Western Australia. The oldest granitoids in the area are two porphyry dykes and three small, apparrently underformed bodies that intrude the greenstones. The porphyries have ages of 2715 and 2752 Ma and the undeformed granitoids range in age between 2696 and 2716 Ma. The external monzogranites that surround the greenstone belt are larger than the internal granitoids and show variable evidence of deformation from a foliated texture to recrystallization at grain boundaries. With the exception of a gneissic sample, which has an age of 2710 Ma, they are younger than the undeformed granites with ages between 2640 and 2694 Ma. A late leucocratic vein in one sample is ∼ 20 Ma younger than the mesocratic foliated rock it cuts. The young age for the external monzogranite is unexpected and it implies that the supracrustal rocks away from the margins of the Mount Magnet greenstone belt, and the small granitoids that intrude them, were cold and rigid at the time of emplacement of the monzogranites so that they behaved as a coherent block. We suggest that the stress associated with the deformation even that produced foliation, gneissic textures and recrystallization in the external granitoids was dissipated at the margins of the greenstone belts, so that the cores of these belts were little affected by this event. The age difference of ∼ 20 Ma between the melanocratic and leucocratic phases in the same rock suggest slow cooling and crystallization in the lower or middle crust, followed by tectonic emplacement into the upper crust. The Mount Magnet granitoids contain abundant xenocrystic zircons that show three pronounced peaks in their age distribution: a broad peak between 2910 and 2950 Ma and sharp peaks at 2800 Ma and 2730 Ma. These ages correlate with eruption of volcanic rocks in the Murchison. We suggest that the thermal events that produced these periods of volcanism were also recorded in the lower crust which later melted to produce the Mount Magnet granitoids. Two felsic volcanics from near the top of the stratigraphy yield ages of 2704 and 2727 Ma, and a chert, the Warramboo-Galtee Moore Chert of the Windaning Formation, an age of 2798 Ma, not 3.0 Ga as had previously been thought. The 2727 Ma felsic volcanic unit unconformably overlies the Warramboo-Galtee Moore Chert and is in turn overlain by the Wattle Creek Basalt of the Mount Farmer Group. This suggests that the Mount Farmer Group has an age of ∼ 2.7 Ga and that 2.7 Ga mafic volcanism is present in the Murchison. The younger external monzogranites, which make up the bulk of the granitoids in the Mount Magnet region, may have derived their heat from the mantle plume head that is believed to be responsible for basaltic and komatiitic volcanism of the Norseman-Wiluna greenstone belt. However, the smaller internal granitoids are older than the plume-related volcanic activity of the eastern Yilgarn and require an additional heat source to have affected the northern part of the Yilgarn Craton.

Rare earth element geochemistry of the metamorphosed volcanogenic massive sulfide deposits of the Manitouwadge mining camp, Superior Province, Canada; a potential exploration tool?

Trace and rare earth element (BEE) geochemistry indicates that rocks in the Manitouwadge mining camp were derived from three major lithological units: mafic volcanic rocks, acid volcanic tuffs, and dacites. The Geco, Willroy, Willecho, and Big Nama Creek Cu-Zn-Ag deposits are contained within the 2720 Ma acid volcanic tuffs. Our study suggests that the thickness of the felsic volcanic unit (acid tuffs and volcaniclastics) may be significantly underestimated in the area. Medium- to high-grade metamorphism (upper amphibolite, lower granulite) gave rise to specific alteration assemblages which crystallized during several episodes of deformation and plutonism. Because major element geochemistry of the rocks was modified during ore- forming processes and later by several episodes of metamorphism and tectonism, the present alteration minerals merely reflect the cumulative effect(s) of these episodes and are often misleading in the identification of the protoliths. Thus, due to extensive alteration, the protoliths of various lithological units can be positively identified only from trace and rare earth element geochemistry. Extensive (up to 97%) light REE depletion was observed in the felsic volcanic rocks around the Geco, Willroy, and Willecho deposits. The positive correlation between BEE mobility and mineralization suggests that REE geochemistry can be successfully used in the exploration for volcanogenic massive sulfide deposits around the Manitouwadge mining camp. The absence of a well-defined discordant alteration pipe at Geco and localized enrichment in Fe, Al, + or - K at the Willroy, Willecho, Big Nama Creek, and Geco deposits implies that they may be highly metamorphosed equivalents of Mattabi-type deposits where the original iron-rich carbonates have recrystallized to calc-silicate or amphibole-rich assemblages during regional metamorphism. Localized enrichment in Fe, Al, and K in the felsic tuffs could be used to identify possible alteration pipes or semicomformable alteration zones associated with mineralization.

Stratigraphic and tectonic setting of early Proterozoic volcanogenic massive sulfide deposits, Flin Flon, Manitoba

Volcanogenic massive sulfide deposits in the Flin Flon metavolcanic belt occur in an Early Proterozoic island-arc setting that includes arc tholelite, back arc-ocean floor, and shoshonitic assemblages. Near the town of Flin Flon, Manitoba, these volcanic assemblages are structurally juxtaposed in a tectonic collage. Nearly all of the volcanogenie massive sulfide deposits at Flin Flon are associated with the arc tholeiite assemblage and occur in complex stratigraphic sequences which represent volcanic constructs and associated intravolcanic basins within the former magmatic arc. Back-arc or ocean-floor basalts in the Flin Flon belt host only a few, small, Cu-rich massive sulfide deposits, and consequently, have a lower exploration potential than the arc tholeiite assemblage.Within the arc tholeiite assemblage, volcanogenie massive sulfide deposits occur in association with felsic volcanic units, at major stratigraphic and compositional breaks in the volcanic sequence. The breaks can be recognized by contrasting major element, trace element, and isotopic characteristics of the underlying and overlying mafic rocks. Most volcanogenie massive sulfide deposits are underlain by coarse, mafic, intermediate or felsic volcaniclastic rocks. The best-documented examples (including the Flin Flon, Cuprus, and White Lake deposits discussed in this paper) are clearly localized in basinal structures that vary widely in size.The Flin Flon deposit (62.4 Mt) occurs in a proximal volcanic environment. It lies on the upper flank of a shallow subaqueous basalt volcano, possibly in a fault-bounded depression. The massive sulfides are emplaced within a rhyolite flow complex and are underlain by a disconformable chloritic hydrothermal alteration zone developed in footwall basalt breccia. The massive sulfides were overlain by a thick (>3 km) subaqueous basalt-basaltic andesite succession compositionally unrelated to basalts in the stratigraphic footwall.The Cuprus and White Lake deposits (total 1.3 Mt) occur in a 5.5-km-thick sequence dominated by more than 3 km of basaltic andesitc interpreted as a subaqueous shield volcano. This volcano contains a collapse caldera within which an intracaldera subaqueous rhyolite flow and coeval felsic and intermediate basin-filling volcaniclastic rocks were deposited. Massive sulfides accumulated in subbasins within the caldera, hosted by a thin unit of graphitic mudstone and chert. Overlying formations are laterally extensive (unlike the wedgeshaped caldera-filling deposits) and were deposited in a large, deep-water basin; these rocks are chemically and isotopically unrelated to formations underlying the massive sulfide deposit.

UPb zircon geochronology of Archaean felsic units in the Marble Bar region, Pilbara Craton, Western Australia

Archaean supracrustal rocks that are well exposed in the Marble Bar region, Pilbara Craton, have been assigned to the Warrawoona Group and younger sequences. The predominantly volcanic Warrawoona Group, previously dated at 3300 to 3500 Ma, is largely basaltic with locally intercalated thick felsic volcanic units. The supracrustal rocks have been folded and are distributed around the margins of large, roughly circular to ovoid, “granitic-gneissic” batholiths. A UPb zircon geochronology study was undertaken to obtain precise age constraints for some of the ore deposits in the area, especially the Big Stubby, North Pole (barite), and Miralga Creek (ZnPbCuAu) deposits, in support of efforts to improve Archaean lead isotope models. The results also help significantly in interpreting stratigraphic relationships and crustal evolution in the area. The new conventional zircon UPb data indicate that most of the Warrawoona Group (from the Duffer Formation upwards) was deposited over a period of ∼20 Ma, from ∼3470 Ma to 3450 Ma. The age of the Duffer Formation is closely constrained by results of 3471±5 Ma and 3465±3 Ma, and the age of the upper Salgash Subgroup appears to be established at 3454±1 Ma. The Panorama Formation at the southern margin of the North Pole Dome is 3458±1.9 Ma; unless the Duffer Formation was deposited over a period of at least 10 Ma, this casts doubt on a previously suggested Panorama-Duffer correlation. The age of the lower part of the North Pole succession is greater than ∼3458 Ma, consistent with the correlation of the basal greenstones at North Pole with the Talga Talga Subgroup. The North Pole chert-barite unit, which is well known for its preservation of Early Archean stromatolites, microfossils, and evaporite sediments, is clearly older than 3458 Ma. The 3325 Ma age now established for the Wyman Formation shows that this unit must be excluded from the Warrawoona Group. The age of the Talga Talga Subgroup remains uncertain; a felsic schist in the Mount Ada Basalt, which has been dated at 3449±3 Ma, is interpreted to be a sill, probably related to the 3450 Ma granitoid bodies. An age > 3724 Ma for a zircon xenocryst in the Panorama Formation is the oldest obtained so far for zircon from the Pilbara granite-greenstone terrane, and indicates the existence of pre-Pilbara Supergroup sialic basement. Ages of 3465±3 Ma and 3449±2 Ma applied to previous Pb isotope data for the Big Stubby and Miralga Creek deposits, respectively, in combination with comparable data for other syngenetic Archaean sulphide deposits, yield a general single-stage Pb evolution model with the parameters T 0=4560 Ma, A 0=9.0818, B 0=9.9002 and C 0=29.343. This gives model ages in accord with known constraints for many Archean deposits. A model lead age of 3490 Ma for the North Pole deposit, which is older than 3457 Ma, may be too great, but attempts to directly date this deposit have not yet succeeded. There are striking parallels between the Warrawoona Group ages reported here and those recently obtained for the Barberton region, Kaapvaal Craton, South Africa. The Talga Talga Subgroup and Lower Onverwacht (Tjakastad Subgroup) sequences have approximately equivalent constraints, and an age for the upper Salgash Subgroup agrees closely with those for the Hoogenoeg Formation of the upper Onverwacht Group (Geluk Subgroup).