Sulfur isotope distribution in Late Silurian volcanic‐hosted massive sulfide deposits of the Hill End Trough,eastern Lachlan Fold Belt,New South Wales     

P. M. Downes  P. K. Seccombe 《Australian Journal of Earth Sciences》2013,60(1):123-139

Volcanic‐hosted massive sulfide (VHMS) deposits of the eastern Lachlan Fold Belt of New South Wales represent a VHMS district of major importance. Despite the metallogenic importance of this terrane, few data have been published for sulfur isotope distribution in the deposits, with the exception of previously published studies on Captains Flat and Woodlawn (Captains Flat‐Goulburn Trough) and Sunny Corner (Hill End Trough). Here is presented 105 new sulfur isotope analyses and collation of a further 92 analyses from unpublished sources on an additional 12 of the VHMS systems in the Hill End Trough. Measured δ³⁴S values range from ‐7.4% to 38.3%, mainly for massive and stockwork mineralisation. Sulfur isotope signatures for polymetallic sulfide mineralisation from the Lewis Ponds, Mt Bulga, Belara and Accost deposits (group 1) are all very similar and vary from ‐1.7% to 5.9%. Ore‐forming fluids for these deposits were likely to have been reducing, with sulfur derived largely from a magmatic source, either as a direct magmatic contribution accompanying felsic volcanism or indirectly through dissolution and recycling of rock sulfide in host volcanic sequences. Sulfur isotope signatures for sulfide mineralisation from the Calula, Commonwealth, Cordillera and Kempfield deposits, Peelwood mine and Sunny Corner (group 2) are similar and have average δ³⁴S values ranging from 5.4% to 8.1%. These deposits appear to have formed from ore fluids that were more oxidising than group 1 deposits, representing a mixed contribution of sulfur derived from partial reduction of seawater sulfate, in addition to sulfur from other sources. The δ³⁴S values for massive sulfides from the John Fardy deposit are the highest in the present study and have a range of 11.9–14.5%, suggesting a greater component of sulfur of seawater origin compared to other VHMS deposits in the Hill End Trough. For barite the sulfur isotope composition for samples from the Commonwealth, Stringers and Kempfield deposits ranges from 12.6% to 38.3%. More than 75% of barite samples have a sulfur isotope composition between 23.4 and 30.6%, close to the previously published estimates of the composition of seawater sulfate during Late Silurian to earliest Devonian times, providing supporting evidence that these deposits formed concurrently with the Late Silurian volcanic event. Sulfur isotope distribution appears to be independent of the host rock unit, although there appears to be a relation linking the sulfur isotope composition of different deposits to defined centres of felsic volcanism. The Mt Bulga, Lewis Ponds and Accost systems are close to coherent felsic volcanic rocks and/or intrusions and have sulfur isotope signatures with a stronger magmatic affinity than group 2 deposits. By contrast, group 2 deposits (including John Fardy) are characterised by ³⁴S‐enrichment and a lesser magmatic signature, are generally confined to clastic units and reworked volcanogenic sediments with lesser coherent volcanics in the local stratigraphy, and are interpreted to have formed distal from the magmatic source. An exception is the Belara deposit, which is hosted by reworked felsic volcanic rocks and has a more pronounced magmatic sulfur isotope signature.  相似文献   

Cyclic sedimentation in the Permian coal measures of New South Wales     

S. E. Bryan  R. J. Holcombe  C. R. Fielding 《Australian Journal of Earth Sciences》2013,60(2):293-316

Statistical analysis of borehole sections through the Illawarra and Newcastle Coal Measures of the Sydney Basin shows that cyclic sedimentation is present. The composite sequence for the Southern Coalfield (Illawarra Coal Measures) is (in ascending order): sandstone—sandstone/siltstone alternations—shale—coal, whereas that for the Newcastle Coalfield is: shale—sandstone/siltstone alternations—sandstone, often conglomeratic, or conglomerate—sandstone/siltstone alternations—shale —coal.

The environment of deposition is discussed. It is suggested that in the Southern Coalfield cyclicity is due to sedimentational processes inherent in the deltaic and alluvial conditions envisaged during Permian times. Periodic influxes of glacial meltwaters, although not essential, are not ruled out.

In the Newcastle Coalfield, however, the composite sequence does not match easily the ideal cycles expected in deltaic and/or alluvial regimes. Contemporary volcanism and tectonism complicated matters and lack of sedimentological details makes it impossible at present to give preference to any one mechanism of cycle formation.  相似文献   

Spores from a carboniferous section in the hunter valley,New South Wales     

Geoffrey Playford  Robin Helby 《Australian Journal of Earth Sciences》2013,60(1):103-119

In this initial systematic study of Carboniferous spores from New South Wales, Australia, fifteen species (all but one of them new) are formally described and are distributed among eight established genera and two new genera (Rattiganispora, a distally annulate trilete form, and Psomospora, an inaperturate or proximally hilate form). The species were selected as being the most characteristic and distinctive forms found in the Italia Road Formation at its well‐exposed type section in the Hunter Valley, east‐central New South Wales. The formation is a cyclical non‐marine unit, over 300 metres (1,000 ft) thick, consisting of lithic arenites together with carbonaceous shales, claystones, and siltstones; its age is regarded as West‐phalian‐Stephanian. The microfiora is compared with those known from sediments of similar age elsewhere and its place in the Australian Palaeozoic palynostratigraphic record is discussed.

New specific institutions are as follows: Punctatisporites lucidulus, P. sub‐tritus, Verrucosisporites aspratilis, V. italiaensis, Raistrickia accincta, R. radiosa, Reticulatisporites asperidictyus, R. magnidictyus, Foveosporites pellucidus, Rattiganispora apiculata (type species), Kraeuselisporites kuttungensis, Grandispora maculosa, Psomospora detecta (type species), and Wilsonites australiensis.  相似文献   

The Bivalves Megadesmus Sowerby and Astartila Dana from the Permian of Eastern Australia     

Bruce Runnegar 《Australian Journal of Earth Sciences》2013,60(2):227-252

Megadesmus Sowerby 1838 and Astartila Dana 1847 are bivalves from the Australian Permian, which belong to a group that Newell has termed “primitive desmodonts”. Both genera have a single blunt tooth in the right valve and a corresponding socket in the left. The tooth and socket are derived from folds in the valve margin and are not related to the teeth of heterodonts. Differences in shape, size, and pedal musculature separate Astartila from Megadesmus. Cleobis Dana 1847 differs only in having slightly different dentition and a small siphonal gape and is retained as a subgenus of Megadesmus. Astartila (Pleurikodonta) n. subgen. has been proposed for a small Astartila‐like species with well‐developed radial ornament.  相似文献   

Timing of emplacement and deformation of the Tia Granodiorite,southern New England Fold Belt,NSW: Implications for the metamorphic history     

P. H. G. M. Dirks  R. Offler  W. J. Collins 《Australian Journal of Earth Sciences》2013,60(2):103-108

The Tia Granodiorite, a Hillgrove Suite pluton in the southern New England Fold Belt, intruded complexly deformed metasediment and metabasite belonging to the Tia Complex, which at the time of intrusion had already been affected by two deformation events at low‐T moderate‐P metamorphic conditions and two overprinting deformation events at high‐T low‐P metamorphic conditions. Emplacement took place during D₅ thrusting associated with limited uplift as low‐P amphibolite facies metamorphism prevailed. Large‐scale warping during D₆ was followed by a second penetrative thrusting event (D₇) that caused further uplift and was initiated under lower amphibolite facies conditions.

The granodiorite has been dated at ～ 300 Ma using magmatic zircon, an age which is thought to approximate the emplacement age and thus D₅. Biotite grains associated with D₇ uplift yield a Rb/Sr age of 264±1.3 Ma. D₅ and D₇ appear to have formed during one extended high‐T metamorphic event because intervening retrogression is lacking in spite of extensive hydrous fluxing, as indicated by numerous syn‐D₆ quartz veins. This thermal event coincided with the opening of the extensional Permian basins.  相似文献   

Granitic and monzonitic rocks dredged from the southeast Australian continental margin     

T. C. T. Hubble  G. H. Packham  D. A. F. Hendry  I. McDougall 《Australian Journal of Earth Sciences》2013,60(5):619-630

Four Middle Devonian (381 Ma) granodiorite samples have been recovered from two dredge sites approximately 65 km east of Green Cape, New South Wales. The granodiorite samples are similar in age and composition to members of the Moruya Suite and probably form an along‐strike extension of that suite. The location of granodiorite on the southeastern margin requires that a piece of continental lithosphere was located to the present east of the study area in the Devonian. This piece of lithosphere may now be located somewhere on the western Lord Howe Rise.

A sample of Early Cretaceous leuco‐quartz monzodiorite was also recovered from a dredge site approximately 45 km north‐northeast of Dalmeny, New South Wales. It represents a body that was intruded at essentially the same time as, and is inferred to be of similar origin to, the syenite rocks of the nearby Mt Dromedary and Montague Island complexes.  相似文献   

Thick‐skinned folding in the eastern Lachlan Fold Belt,Shoalhaven River Gorge,New South Wales     

C. L. Fergusson 《Australian Journal of Earth Sciences》2013,60(5):677-686

In the Shoalhaven River Gorge, in the eastern Lachlan Fold Belt, the Ordovician quartz‐turbidite succession (Adaminaby Group) is affected by one major phase of deformation with northerly trending, gently plunging, upright, close to tight folds (F₁) characterised by a range in half wavelengths up to 3 km. Several anticlinoria and synclinoria are developed and folds occur in at least four orders; these characteristics are consistent with buckling occurring at several scales and are controlled by the thickness of competent units in the multilayered succession. F₁ folding is thick‐skinned in style with the whole crust probably having been affected by deformation. D₁ occurred during the Silurian to Middle Devonian interval and was associated with crustal thickening and the shallowing of depositional environments over time. Locally, F₁ is overprinted by south‐southeast‐trending, steeply to moderately inclined F₂ that reorients F₁ to recumbent attitudes. D₂ is of Early to Middle Carboniferous age. Both deformations are related to convergence in an intra‐arc to backarc region and occurred inboard of a subduction zone, remnants of which occur in the New England Fold Belt.  相似文献   

Genesis of the Ibero-Armorican arc     

A. Ribeiro  R. Dias  J. BrandÃo Silva 《Geodinamica Acta》2013,26(4):173-184

Abstract

The Ibero-Armorican arc is continuous between Iberia and Armorica; its curvature increased with time due to subduction followed by continental collision; indentation produced left lateral transpression in Iberia and right-lateral transpression in Armorica. It is argued that whereas the antithetic shear is predominant in Iberia, in Armorica a synthetic shear prevailed because the identer rotated anticlockwise between the opposed forelands of the Variscan Fold Belt. It is proposed that the major Rheic ocean, closed by subduction towards the inner part of the arc, solving the space problem of centripetal vergences.  相似文献   

Evolution of pore-fluid pressure during folding and basin contraction in overpressured reservoirs: Insights from the Madison–Phosphoria carbonate formations in the Bighorn Basin (Wyoming,USA)     

《Marine and Petroleum Geology》2014

Reconstructing the evolution of paleofluid (over)pressure in sedimentary basins during deformation is a challenging problem, especially when no hydrocarbon-bearing fluid inclusions are available to provide barometric constraints on the fluid system. This contribution reports the application to a natural case (the Bighorn Basin) of recent methodological advance to access fluid (over)pressure level prevailing in strata during sub-seismic fracture development. The fluid pressure evolution in the Mississippian-Permian Madison–Phosphoria limestone reservoir is tentatively reconstructed from the early Sevier Layer Parallel Shortening to the Laramide folding in two basement-cored folds: the Sheep Mountain Anticline and the Rattlesnake Mountain Anticline. Results point out that supra-hydrostatic pressure values prevail in the limestone reservoir during most of its whole Sevier–Laramide history. The comparison of the reconstructed fluid overpressure values within situ measurements in various overpressure reservoirs in other oil-producing basins highlights that the supra-hydrostatic fluid pressure gradually reaches the lithostatic value during the whole basin contraction and fold development. During most of the LPS history, however, overpressure level can be defined by a mean gradient. Among the factors that control the pressure evolution, the mechanical stratigraphy, the stress regime under which fractures developed and regional fluid flow are likely dominating in the case of the Bighorn Basin, rather than classical factors like disequilibrium compaction or fluid generation during burial. A coeval evolution between fluid overpressure and differential stress build-up is also emphasized. The approach presented in this paper also provides estimates of strata exhumation during folding.  相似文献   

Gas hydrate formation in compressional,extensional and un-faulted structural settings – Examples from New Zealand's Hikurangi margin     

《Marine and Petroleum Geology》2017

We investigate gas hydrate formation processes in compressional, extensional and un-faulted settings on New Zealand's Hikurangi margin using seismic reflection data. The compressional setting is characterized by a prominent subduction wedge thrust fault that terminates beneath the base of gas hydrate stability, as determined from a bottom-simulating reflection (BSR). The thrust is surrounded by steeply dipping strata that cross the BSR at a high angle. Above the BSR, these strata are associated with a high velocity anomaly that is likely indicative of relatively concentrated, and broadly distributed, gas hydrates. The un-faulted setting—sedimentary infill of a slope basin on the landward side of a prominent thrust ridge—is characterized by a strong BSR, a thick underlying free gas zone, and short positive polarity reflection segments that extend upward from the BSR. We interpret the short reflection segments as the manifestation of gas hydrates within relatively coarse-grained sediments. The extensional setting is a localized, shallow response to flexural bending of strata within an anticline. Gas has accumulated beneath the BSR in the apex of folding. A high-velocity zone directly above the BSR is probably mostly lithologically-derived, and only partly related to gas hydrates. Although each setting shows evidence for focused gas migration into the gas hydrate stability zone, we interpret that the compressional tectonic setting is most likely to contain concentrated gas hydrates over a broad region. Indeed, it is the only setting associated with a deep-reaching fault, meaning it is the most likely of the three settings to have thermogenic gas contributing to hydrate formation. Our results highlight the importance of anisotropic permeability in layered sediments and the role this plays in directing sub-surface fluid flow, and ultimately in the distribution of gas hydrate. Each of the three settings we describe would warrant further investigation in any future consideration of gas hydrates as an energy resource on the Hikurangi margin.  相似文献