Origin of sandplains in Western Australia: A review of the debate and some recent findings

Views on the origin of sandplains in Western Australia remain controversial with debate focusing around three different models of formation. These are in situ, aeolian and in situ formation with local remobilisation by wind or colluvial transport. The only recent work on the subject to date espouses a dominantly aeolian origin. New work from a detailed study on the Victoria Plateau is described and demonstrates the applicability of utilising a range of evidence in understanding the origin of sandplains in Western Australia. Field investigations show a strong association of sandplain with sandstone and an absence of sand on non‐arenaceous geology in similar and adjacent topographic settings. Grainsize, mineral magnetic analysis and heavy‐mineral spectra show the Victoria Plateau to be a heterogeneous body of sand. These findings coupled with a lack of internal sedimentary structures are not consistent with an aeolian origin for the sandplain. Furthermore, scanning electron microscopy, grainsize and heavy minerals also demonstrate a clear link between bedrock and overlying sandplain. These data support the hypothesis that Western Australian sandplains are mostly the product of in situ weathering. Such findings question whether the origin of sandplains can be satisfactorily deduced without such a range of data.     

Trace‐element signatures of apatites in granitoids from the Mt Isa Inlier,northwestern Queensland

The concentrations of trace elements in apatite from granitoid rocks of the Mt Isa Inlier have been investigated using the laser‐ablation inductively coupled plasma‐mass spectrometry (ICP‐MS) microprobe. The results indicate that the distribution of trace elements (especially rare‐earth elements (REE), Sr, Y, Mn and Th) in apatite strongly reflects the chemical characteristics of the parental rock. The variations in the trace‐element concentrations of apatite are correlated with parameters such as the SiO₂ content, oxidation state of iron, total alkalis and the aluminium saturation index (ASI). The relative enrichment of Y, HREE and Mn and the relative depletion of Sr in the apatites studied reflect the degree of fractionation of the host granite. Apatites from strongly oxidised plutons tend to have higher concentrations of LREE relative to MREE. Manganese concentrations are higher in apatite from reduced granitoids because Mn²⁺substitutes directly for Ca²⁺. The La/Ce ratio of apatite is well‐correlated with the whole‐rock K²O and Na²O contents, as well as with the oxidation state and ASI. Because apatite trace‐element composition reflects the chemistry of the whole rock, it can be a useful indicator mineral for the recognition of mineralised granite suites, where particular mineralisation styles are associated with granitoids that have specific geochemical fingerprints.     

Silurian and Devonian systems

The Ediacara mineral field is situated 30 km W of Beltana on the western margins of the Flinders Ranges, South Australia, and consists of silver‐lead and copper deposits in lower Cambrian carbonate rocks that contain anomalous base‐metal contents throughout the Adelaide Geosyncline. The lower Cambrian rocks, which consist of the basal Parachilna Formation and overlying Ajax Limestone, rest disconformably on the Precambrian, and at Ediacara occupy a shallow N‐S elongate syncline near the hinge zone of the Adelaide Geosyncline. The main primary ore minerals of the silver‐lead mineralization are galena and pyrite, with very minor chalcopyrite and sphalerite, and rare tetrahedrite and pearceite. The gangue consists mainly of silica (both chalcedony and quartz), with minor dolomite and rare barite. The mineralization is stratabound and occurs in conformable zones, the lowest of which commences about 30–50 m above the base of the Cambrian sequence. The host to the silver‐lead mineralization, the Ajax Limestone, can be subdivided into three units which represent a set of lithologies, structures and organic traces indicative of a shallow near‐shore carbonate environment. The silver‐lead mineralization is mainly present in sandy and laminated dolomites which were deposited in an environment ranging from sub‐tidal to bar and channel and tidal flat, respectively. Four types of mineralization have been recognized; disseminated sulphides of syngenetic and/or diagenetic origin and epigenetic concentrations along stylolites, in veins and as breccia fillings. Post‐depositional solution activity has affected a large proportion of the carbonate sequence. The effects of this activity range from stylolites through stylobreccias to solution collapse breccias. The epigenetic concentrations of mineralizations have apparently been formed by the remobilization of the disseminated sulphides during solution activity. The ore and gangue minerals of the epigenetic mineralization display both euhedral forms and distinct colloform banding, and framboidal textures have also been observed in both pyrite and galena. There is evidence of repeated episodic precipitation and no simple paragenetic sequence can be recognized. Fluid inclusions in silica and dolomite associated with the epigenetic mineralization have homogenization temperatures of 159 to 199°C and freezing temperatures that indicate the fluids to be saline brines containing NaCl with CaCl₂ and/or MgCl₂. Sulphur isotope analyses show a range of 8³⁴S values from ‐12.5 to +8.6 per mil, with no evidence of significant differences between the four types of mineralization. The data suggest deposition of the disseminated sulphides as a result of biological reduction of seawater sulphate in a system partially open with respect to sulphate supply. Subsequent remobilization of sulphides apparently involved little or no sulphur isotope fractionation. The Ediacara silver‐lead deposits have many features in common with Mississippi Valley‐type lead‐zinc deposits and appear to have similarities in terms of genesis, in that the epigenetic mineralization has been formed as a result of post‐depositional solution activity during diagenesis in a sedimentary basin. The scale of transport of the metals deposited as the epigenetic mineralization at Ediacara appears, however, to have been very much less than that of the metals in other Mississippi Valley‐type deposits.     

矿物热释氡找矿在某地含铀砂岩中的试验研究

该文探讨了矿物热释氡的氡来源较多的原因是每(μm)~3次生含铀矿物或眇岩中有数量巨大的——m×10~n(n>4,m、n为自然数)个的铀离子或晶胞。本文并介绍了取自4210砂岩铀矿床的坑道、钻孔及其矿区内的河流、陡壁的271个样品矿物热释氡的特征:热释氡晕范围较矿体宽约几十米,在平面、剖面上都有热释氡晕存在,产状与矿体一致,与地层和冲刷面有一定交角。表明热释氧晕受层位、岩相、构造等多种因素控制。这些特点,对指导技矿和分析矿床成因,无疑是有意义的。     

青海湖沉积物中的粘土矿物

本文对青海湖沉积物中的粘土矿物和沉积环境进行了初步研究。沉积物的粒度成分一般以粉砂级为主,湖周沉积物较粗,湖内沉积物较细。湖中粘土矿物以伊利石-绿泥石为主,含少量蒙脱石和高岭石等。沉积物表层未见蒙脱石,粘土中Al₂O₃、K₂O和MgO的相对百分含量的特征与海粘土的化学成分特征相似。     

长江口泥沙的来源分析与数量计算的研究

本文根据地学原理，建立了长江口泥沙的来源分析与数量计算的模式。定量分析了长江口泥沙来源，计算了泥沙的数量。通过验证表明沙量的计算值与验证值两者十分吻合。     

攀西地区铁钛氧化物矿物的显微结构

本文用矿石显微镜及单晶X射线分析方法,研究了攀枝花—西昌地区铁钛氧化物矿物的显微结构共生组合特点、各出溶相间的关系和分布方位以及不同岩石和矿石显微结构共生组合上的差异。业已查明它们受岩浆旋回和岩石类型所控制。本文指出了磁铁矿-钛铁晶石固溶体显微出溶结构可能是拐点分解经调制结构后的终期产物。     

自然金—银类质同象系列矿物晶胞参数变化规律及其表述方程

本文提出了自然金-银类质同象系列矿物的晶胞参数与金、银原子数百分比成二次函数关系,推导出了表示二者之间函数关系的方程式。利用该方程式,可以用金原子数百分比计算出晶胞参数,反之亦然。并且从金、银二者的电负性出发,对a值的变化作了解释。     

纵观近代岩矿测试分析技术

本文就近代岩矿测试分析技术作了较为全面的介绍。文章指出，地矿部从８０年代起，由于引进了大批现代先进仪器设备，使岩矿测试分析技术出现了崭新面貌，形成为多学科相结合的新格局，从而促进地质科学在理论上和实践上均取得显著进步，在某些学科领域已发生了根本性变化，并为地质或其它市场提供了大量信息，为促进科技成果转化为生产力作出了巨大贡献。纵观近代岩矿测试分析技术，文章提出了多机联用、整体分析和拓宽测试分析领域的建议，以更好地为祖国经济主战场服务。