Deformation and mineralisation in the Scotty Creek Basin,Agnew district,Eastern Goldfields,Western Australia: evidence for D1- and D3-related gold mineralisation

Gold deposits in the Agnew district display markedly different structural styles. The Waroonga and Songvang deposits are hosted in layer-parallel extensional shears formed under highly ductile conditions. In contrast, the New Holland–Genesis deposits are shallow-dipping quartz-filled brittle fractures and breccia zones that cut across the tightly folded bedding and formed during east–west compression. It is difficult to attribute their formation to a single compressive event. The Waroonga and Songvang deposits formed during D₁ extension, uplift and exhumation of the Agnew granitic complex and formation of the Scotty Creek Basin at ca 2670–2660?Ma. The New Holland–Genesis deposits formed during east–west D₃ compression at about ca 2650–2630?Ma. An S₁ foliation wraps around the Agnew granitic complex and L₁ stretching lineations form a radial pattern around the granite, consistent with formation during D₁ uplift of the composite granite body. Uplift and erosion of granite bodies in the surrounding area provide a source for the granite clasts in the upper parts of the Scotty Creek Basin. As clasts in the basin are undeformed, no significant deformation occurred prior to the uplift and erosion of the source granites in this area. Syn-tectonic emplacement of the Lawlers Tonalite during formation of the Scotty Creek Basin at ca 2665?Ma may have provided a good heat/fluid source for the mineralising systems during the first gold event. The distribution of the large deposits along the western edge of the Agnew granitic complex indicates that the extensional shear along the granite contact is a first-order control on gold deposition by providing a conduit for rising hydrothermal fluids. The northerly trend of high-grade shoots in the Waroonga deposit coincides with early north-trending growth faults, which are also likely fluid conduits.     

Muscovite dehydration melting: Reaction mechanisms,microstructures, and implications for anatexis

Dehydration melting of muscovite in metasedimentary sequences is the initially dominant mechanism of granitic melt generation in orogenic hinterlands. In dry (vapour-absent) crust, muscovite reacts with quartz to produce K-feldspar, sillimanite, and monzogranitic melt. When water vapour is present in excess, sillimanite and melt are the primary products of muscovite breakdown, and any K-feldspar produced is due to melt crystallization. Here we document the reaction mechanisms that control nucleation and growth of K-feldspar, sillimanite, and silicate melt in the metamorphic core of the Himalaya, and outline the microstructural criteria used to distinguish peritectic K-feldspar from K-feldspar grains formed during melt crystallization. We have characterized four stages of microstructural evolution in selected psammitic and pelitic samples from the Langtang and Everest regions: (a) K-feldspar nucleates epitaxially on plagioclase while intergrowths of fibrolitic sillimanite and the remaining hydrous melt components replace muscovite. (b) In quartzofeldspathic domains, K-feldspar replaces plagioclase by K⁺–Na⁺ cation exchange, while melt and intergrowths of sillimanite+quartz form in the aluminous domains. (c) At 7–8 vol.% melt generation, the system evolves from a closed to open system and all phases coarsen by up to two orders of magnitude, resulting in large K-feldspar porphyroblasts. (d) Preferential crystallization of residual melt on K-feldspar porphyroblasts and coarsened quartz forms an augen gneiss texture with a monzogranitic-tonalitic matrix that contains intergrowths of sillimanite+tourmaline+muscovite+apatite. Initial poikiloblasts of peritectic K-feldspar trap fine-grained inclusions of quartz and biotite by replacement growth of matrix plagioclase. During subsequent coarsening, peritectic K-feldspar grains overgrow and trap fabric-aligned biotite, resulting in a core to rim coarsening of inclusion size. These microstructural criteria enable a mass balance of peritectic K-feldspar and sillimanite to constrain the amount of free H₂O present during muscovite dehydration. The resulting modal proportion of K-feldspar in the Himalayan metamorphic core requires vapour-absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids.     

Linking biogeochemical processes and historic primary producer communities in a SE USA sinkhole lake from the mid-Holocene to present

Many freshwater resources receive materials from human development causing a decrease in ecological services when compared to pre-disturbance periods. As a result, the understanding of eutrophication and limnological change has increased, but less attention has been given to systems under intense human impact that have not eutrophied so that drivers precluding eutrophication can be documented. The primary objective of this research was to reconstruct allochthonous inputs and in-lake processes for Long Pond, Georgia, USA from the mid Holocene to present and link them to primary producer community changes. Long Pond is a mesotrophic lake located in a highly altered watershed from agricultural and municipal land use and housing developments. A 5 m sediment core was collected from Long Pond, and organic matter, nutrients (C, N, P), metals (Al, Fe, Cu), and photosynthetic pigments were measured. Long Pond existed in three limnological states spanning the past ~6000 years. Prior to modern lacustrine conditions, Long Pond was a wetland/peat system that experienced the highest primary producer abundance recorded in the core. The modern lacustrine state began in the late Holocene and was characterized by increased connectivity with the surrounding watershed and low productivity. Human impacts began around 1900 AD and included high levels of phosphorus and metal deposition but moderate levels of primary producer abundance. As a result, in-lake dynamics are believed to be regulating the trophic status of Long Pond. Low concentrations of available phosphorus in the water column combined with high concentrations of sedimentary phosphorus may imply the binding of phosphorus to the sediments by certain materials such as aluminum and iron. Long Pond serves as an example of the complex in-lake processes that can occur from allochthonous inputs and autochthonous responses in lake systems thus complicating management decisions.     

The effect of landscape and hydrologic variables on the prehistoric Salado: Geoarchaeological investigations in the Tonto Basin,Arizona

Geoarchaeological investigations in the Tonto Basin, Arizona provide insights into the Classic period Salado occupation from A.D. 1150 to 1450. Geomorphic reconstructions show that the Salado pursued irrigation agriculture on an intermediate Holocene terrace that was protected from flooding, while permanent settlements were located on stable Pleistocene landforms overlooking the field areas. Carrying-capacity-based calculations show that the population during the Classic period numbered in the low thousands. The development and subsequent cultural changes during the Classic period are tied to streamflow variations and cultural factors. The final abandonment of the Tonto Basin around A.D. 1450 is not the result of a major landscape change, but instead is linked to high frequency hydrologic variations that influenced food production coupled with cultural factors. © 1998 John Wiley & Sons, Inc.     

A Paddock to reef monitoring and modelling framework for the Great Barrier Reef: Paddock and catchment component

Targets for improvements in water quality entering the Great Barrier Reef (GBR) have been set through the Reef Water Quality Protection Plan (Reef Plan). To measure and report on progress towards the targets set a program has been established that combines monitoring and modelling at paddock through to catchment and reef scales; the Paddock to Reef Integrated Monitoring, Modelling and Reporting Program (Paddock to Reef Program). This program aims to provide evidence of links between land management activities, water quality and reef health. Five lines of evidence are used: the effectiveness of management practices to improve water quality; the prevalence of management practice adoption and change in catchment indicators; long-term monitoring of catchment water quality; paddock & catchment modelling to provide a relative assessment of progress towards meeting targets; and finally marine monitoring of GBR water quality and reef ecosystem health. This paper outlines the first four lines of evidence.     

Quantifying Barrovian metamorphism in the Danba Structural Culmination of eastern Tibet

The Danba Structural Culmination is a tectonic window into the late Triassic to early Jurassic Songpan‐Garzê Fold Belt of eastern Tibet, which exposes an oblique section through a complete Barrovian‐type metamorphic sequence. Systematic analysis of a suite of metapelites from this locality has enabled a general study of Barrovian metamorphism, and provided new insights into the early thermotectonic history of the Tibetan plateau. The suite was used to create a detailed petrographic framework, from which four samples ranging from staurolite to sillimanite grade were selected for thermobarometry and geochronology. Pseudosection analysis was applied to calculate P–T path segments and determine peak conditions between staurolite grade at ～5.2 kbar and 580 °C and sillimanite grade at ～6.0 kbar and 670 °C. In situ U–Pb monazite geochronology reveals that staurolite‐grade conditions were reached at 191.5 ± 2.4 Ma, kyanite‐grade conditions were attained at 184.2 ± 1.5 Ma, and sillimanite‐grade conditions continued until 179.4 ± 1.6 Ma. Integration of the results has provided constraints on the evolution of metamorphism in the region, including a partial reconstruction of the regional metamorphic field gradient. Several key features of Barrovian metamorphism are documented, including nested P–T paths and a polychronic field gradient. In addition, several atypical features are noted, such as P–T path segments having similar slopes to the metamorphic field gradient, and T_max and P_max being reached simultaneously in some samples. These features are attributed to the effects of slow tectonic burial, which allows for thermal relaxation during compression. While nested, clockwise P–T–t loops provide a useful framework for Barrovian metamorphism, this study shows that the effects of slow burial can telescope this model in P–T space. Finally, the study demonstrates that eastern Tibet experienced a significant phase of crustal thickening during the Mesozoic, reinforcing the notion that the plateau may have a long history of uplift and growth.     

Phase equilibria modelling of retrograde amphibole and clinozoisite in mafic eclogite from the Tso Morari massif,northwest India: constraining the P–T–M(H2O) conditions of exhumation

Phase equilibria modelling of post‐peak metamorphic mineral assemblages in (ultra)high‐P mafic eclogite from the Tso Morari massif, Ladakh Himalaya, northwest India, has provided new insights into the potential behaviour and source of metamorphic fluid during exhumation, and constrained the P–T conditions of hydration. A series of P–M(H₂O) pseudosections constructed in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (NCKFMASHTO) system show that a number of petrographically distinct hydration episodes occurred during exhumation from peak P–T conditions (~640 °C, 27–28 kbar), resulting in the formation of abundant compositionally zoned amphibole and minor clinozoisite poikiloblasts at the expense of a peak assemblage dominated by garnet and omphacite. Initial hydration is interpreted to have occurred as a result of the destabilization of talc following isothermal decompression to ~23 kbar, which led to the formation of barroisite–winchite amphibole core domains. An episode of fluid infiltration from an external source at ~19 kbar, with or without syn‐decompressional cooling to ~560 °C, resulted in further barroisitic–winchitic amphibole growth, followed by the formation of clinozoisite poikiloblasts. Continued buoyancy‐driven exhumation to the base of the lower crust is constrained to have taken place with no additional fluid input. A final hydration event is characterized by the formation of magnesiohornblende rims on the barroisite–winchite cores, with the former interpreted to have formed during later prograde overprinting in the middle crust associated with the final stages of exhumation. Notably, the vast majority of externally sourced H₂O, comprising just over half of the current bulk rock fluid content, was added during this later hydration event. In a middle crustal setting, this is interpreted as the result of devolatilization reactions occurring in migmatitic host orthogneiss and/or metasedimentary units, or following the crystallization of partial melt.     

Quantification of the breakage of lump materials during handling operations

The strength of raw ironmaking materials is usually determined by tumble and shatter tests. Although an average result for a bulk sample is determined, the strength index is empirical and its numerical value cannot be used to predict the extent of breakage for a particular material handling circuit. A new experimental procedure has been developed to measure the strength based on the extent of breakage of agglomerates repeatedly dropped from a predetermined height. The drop procedure combined with impact and breakage theories has been used to quantify the effect of particle size and drop height on the extent of breakage of lump materials such as iron ore sinter, coke, coal and iron ore. Results from a plant trial indicated that the drop procedure could also be used to examine the stabilization effect of brittle material due to repeated dropping during handling.     

ISO-SWS Spectra of OH/IR Stars

We present the spectra of 5 OH/IR stars observed with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). The spectra are dominated by dust features, both in the amorphous and crystalline state and illustrate the influence of the mass loss rate on the observed spectral features. This revised version was published online in August 2006 with corrections to the Cover Date.