摘要:

Traces of galena have been found within amygdales of the Kylena Basalt, third unit of the Fortescue Group which is, by definition, the oldest sequence within the Proterozoic Hamersley Basin of the NW of Western Australia. Mutually-supporting Pb-isotope data from two laboratories yield a model age 2.76 ± 0.03 Ga, in good agreement with Rb-Sr and U-Pb datings of stratigraphically equivalent material. The low indicated source U/Pb is in accord with the observed basalt association, and contrasts with some other, fluorite-associated, samples of comparable model age, for which a ‘granite’ association is postulated. Lead ratios from a stratiform deposit within the Whim Creek Group, defined as of Late Archaean age, yield a model age 2.95 ± 0.01 Ga, in accord with a published Rb-Sr mineral age. Hence the base of the Fortescue Group, generally regarded as initiating the Early Proterozoic Era in Australia, cannot be younger than 2.8 Ga. This date has implications for future definitions of the Archaean-Proterozoic boundary.

ISSN:0812-0099    

DOI:10.1080/14400958408527928

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Two small dykes consisting of a quartz-topaz-loellingite rock type have recently been discovered within the aplitic phase of the Pilot Range granite, near Eldorado, in NE Victoria. Minor biotite, muscovite, chlorite, kaolinite, anatase and pharmacosiderite are associated. Apart from the loellingite, the dykes are similar mineralogically to the ‘topazites’ from New England, NSW. These were considered to be magmatic in origin, based on field relationships and high homogenization temperatures for fluid inclusions in topaz (Eadington & Nashar 1978,Contrib. Mineral. Petrol.67, 433–438). Although experimental evidence on F-enriched ‘granitic’ systems is inconclusive, the emplacement of the Eldorado topazite dykes most likely involved both magmatic and hydrothermal components operating essentially simultaneously. The topazite melt represented a F-rich residual granitic magma, from which aqueous alkali halide-rich solutions separated during high level intrusion. Separation of these aqueous solutions was responsible for miarolitic cavities into which topaz crystals grew. F-OH equilibration calculations for coexisting topaz-biotite pairs suggest the minerals equilibrated in the presence of hydrothermal solutions of variable composition (in terms of the HF/H2O fugacity ratio), at temperatures around 550°C. Alteration of topaz to muscovite, the precipitation of loellingite and the formation of clay and fluorite in the cavities occurred at progressively lower temperatures. The widespread alluvial topaz in the Beechworth-Eldorado area may be derived from similar quartz-topaz dykes.

ISSN:0812-0099    

DOI:10.1080/14400958408527929

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

A preliminary evaluation of the extent of racemization of a range of amino acids contained in molluscs from fossiliferous marine sediments associated with calcretes has been made for the ultimate purpose of establishing a chronology of calcrete formation in southern Australia. Amino acids detected in molluscs from Holocene and Late Pleistocene marine sediments intersected in trenches cut into alluvial terraces in the valley of the Hindmarsh River at Victor Harbor exhibit distinctd/lratio differences, as expected from the geological and palaeontological assessment. In addition, the rates of racemization, which are known to vary from amino acid to amino acid, appear to have followed the general order described in the literature. Based on comparitived/lratio data for radiocarbon-dated molluscs from a stranded Holocene marine deposit exposed in an excavation 8 km ESE of Robe, an age of approximately 4000 yearsbpis estimated for the Holocene marine incursion into the Hindmarsh River Valley. The incursion deposited sediments equated palaeontologically with the St Kilda Formation which post-dates the major development of calcretes in South Australia. At this stage we have insufficient amino acid racemization data to speculate on the absolute age of the Late Pleistocene marine incursion into the valley and resulting sediments equated with the Glanville Formation, which commonly in southern Australia exhibits an indurated carbonate crust or is interbedded with calcretes. Further work on the extent of amino acid racemization in molluscs from the Glanville Formation, and from the Early Pleistocene marine deposits (Point Ellen Formation and Burnham Limestone) which predate major calcrete formation in southern South Australia, is in progress.

ISSN:0812-0099    

DOI:10.1080/14400958408527930

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Heavy minerals are mined from two Cainozoic fossil strandlines in the southern Perth Basin. These arcuate, sub-parallel shorelines are situated on the Swan Coastal Plain, and are remote from the modern coast. The Capel shoreline is located 7 km inland and the Yoganup shoreline, 15 km inland, is adjacent to the arcuate-northwest facing Whicher Scarp, at the southern boundary of the plain. In the Yoganup shoreline, high-energy beach environments have concentrated heavy minerals by reworking sand from the Lower Cretaceous Leederville Formation. Heavy minerals were deposited in thin, shallowing-upward sequences along wave-dominated, eroding shorelines cut into pre-Cainozoic sediments.

ISSN:0812-0099    

DOI:10.1080/14400958408527931

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Some 526 gravity measurements were made to an accuracy of 1 mgal in order to further delineate the main gravity features of the Sydney Basin. Corrections for an Airy-type isostatic model and for the effect of the basin sediments are applied to a profile between Bathurst and Mona Vale. An asymmetric gravity high of 44 mgal amplitude, centred about 10 km E of the Lapstone Monocline, emerges as the major feature. This is interpreted to indicate the presence of a mafic body, of density 2.9 t m-3and a maximum thickness of 12 km, underlying the basin within the upper crust. The base of the body has an easterly dip of 53° on the western side, representing a major basement fault beneath the Lapstone Monocline-Kurrajong Fault System. The westerly dips on the eastern flank range from 21° to 33° and these may indicate a series of step faults. The large size of the source implies a significant crustal extension preceding the development of the Sydney Basin. Although the basement faults did not propagate upwards into the Permo-Triassic strata, the western fault controlled sedimentation and is expressed as a warp — the Lapstone Monocline.

ISSN:0812-0099    

DOI:10.1080/14400958408527932

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Although most quartz-bearing gold nuggets have been shed from quartz veins, supergene processes commonly upgrade the quality (fineness) of their gold. Similarly, some quartz-free nuggets can be shown to have been shed from ultramafic schists or carbonates where they were precipitated from hydrothermal fluids. On the other hand, other quartz-free nuggets of superior fineness (e.g. Au 96.5, Ag 3.5, wt.%) enclose pisolitic laterite concretions or other soil detritus. Earthy haematitic inclusions near the edge of some nuggets have expanded during hydration and burst open the otherwise solid gold. In several localities where nuggets of this type are found there is no obvious vein or lode source of the accreted gold. In many of these the source of the migrating gold which is liberated by weathering is presumably finely divided ‘invisible’ gold (atomic or colloidal), or gold-bearing sulphides or arsenides which are disseminated throughout the country rocks. At many Australian localities almost pure supergene gold is found in weathered gossanous zones but gives way beneath the water table to primary sulphide, sulph-arsenide or carbonate veins containing gold alloyed with much silver and/or some copper. Supergene gold has been found in the following forms: (1) octahedra of a very pure gold (0.5 mm diameter) set on felted surfaces of Mn oxide, or in puggy clays; (2) filamentous and arborescent forms in several types of decomposed rock; (3) dendritic or paint gold on cracks in weathered rock, clay and laterite; (4) gold impregnations of fossil wood and coaly matter; (5) some gold nuggets of mammillary form. Variations in the non-gold components of solid gold are thought to reflect the chemical properties of gold-bearing non-hydrothermal solutions.

ISSN:0812-0099    

DOI:10.1080/14400958408527933

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The Hole 16 deposit is a small unexposed F-Sn-W exoskarn deposit with underlying associated endoskarn, greisenized granite and largely ungreisenized Carboniferous ‘Elizabeth Creek’ granite. Sphalerite geobarometry indicates that this was a high level intrusion. The skarn formed above a granite cusp and is mantled by pure marble. Assemblages and textures representing successive stages of skarn genesis are: (1) massive andradite, (2) massive Sn-rich garnet + magnetite ± clinopyroxene ± fluorite, (3) ‘wrigglite’ which refers to a characteristic fine-grained contorted rhythmically layered texture, consisting of alternating layers of magnetite and fluoro-vesuvianite + fluorite, or cuspidine ± fluorite and magnetite ± Zn-Fe spinel, (4) massive fluoro-vesuvianite + fluorite, (5) Fe-F-amphibole + Fe-F-biotite + Fe-F-phyllosilicate + calcite ± scheelite, (6) chlorite ± hematite ± clay minerals. Assemblages (5) and (6) are found in very minor amounts throughout the skarn. Stages (1)–(4) also correspond to the skarn types extending upwards from the granite contact. Growth zoned garnets are andradite-rich (And100to And60Gross40) with up to 2.46 wt% SnO (0.81 average), vesuvianite is always relatively Fe-rich (Fe90Mg10to Fe40Mg60), pyroxene is Mg-rich (Di81He13to Di92He8) and powellite-rich scheelite (Sch77Pow23) occurs mainly near the outer of the skarn. Tin occurs within the exoskarn mainly near the granite contact in solid solution in garnet, as minute (0.5μm long) crystals of cassiterite in magnetite and rarely as large cassiterite crystals (⩽ 1 mm) associated with retrograde amphibole alteration. The granite is a typical ‘A-type’ or tin granite, having very high Ga/Al, high SiO2and high total alkalis. In the drill core it is variable in appearance, consisting of aplitic, pegmatitic and even-grained areas. The endoskarn consists of fluorite-calcite-quartz-white mica, and the greisen is dominantly white mica. On the basis of a mass balance calculation using bulk chemical analyses, it is suggested that about 10 volumes of granite would have to be altered to greisen and endoskarn to produce the volume of skarn observed, and significant Fe and F would need to be added to the system as well. The data presented here, with fluid inclusion data presented elsewhere, suggest the system was a largely closed circulation system, with FeCl2and CaF2derived from the greisenization processes (FeCl2.xH2O and CaF2daughter crystals occur in quartz fluid inclusions in the granite) whereas CaCl2and CO2were derived from the skarn-forming process. Temperatures were largely magmatic throughout (≈ 600°C) and very limited meteoric water was involved.

ISSN:0812-0099    

DOI:10.1080/14400958408527934

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Coralloidal and flowstone speleothems occur in two caves in the Stanthorpe Granite at Girraween National Park, south-east Queensland. The cave coral is found on the cave roofs and consists of banded opal-A with small mineral inclusions derived from the overlying granite. Coral from one cave contains a small component of allophane. The flowstone occurs on the floor of one cave, and is composed of thinly banded allophane, which has an unusual composition characterised by high levels of aluminium, phosphate and organic carbon. The allophane is precipitating only from water that has seeped through humus and soil. This is the first positive identification of allophane in speleothems, and also provides convincing evidence that allophane can precipitate directly from solution. (Most allophane forms by alteration of vitric parent materials in soils.)

ISSN:0812-0099    

DOI:10.1080/14400958408527935

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

ISSN:0812-0099    

DOI:10.1080/14400958408527927

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor