西澳Darling Range地区铝土矿特征及成矿控制因素

我国铝土矿资源相对匮乏,但需求量大。澳大利亚铝土矿资源丰富,是我国铝土矿进口主要国家之一。Darling Range地区为澳大利亚铝土矿主产地之一,了解该区的铝土矿成矿特征将有助于我国企业走出去在该地区或相似地区找矿提供依据。Darling Range位于西澳大利亚的伊尔岗克拉通,该区铝土矿以红土型为主,铝土矿的显著特点是品位较低,铁、硅含量高,但对铝土矿冶炼有害的可溶性硅含量低,资源潜力巨大。铝土矿产于红土层中,厚度受后期剥蚀作用控制。红土剖面从上到下依次分为覆盖层、硬壳、碎屑层和底部粘土层。覆盖层以砂砾为主,具有经济意义的铝土矿主要集中在碎屑层和硬壳中,底部粘土层则阻止铝土矿继续向下运移。铝土矿中的矿石矿物主要是三水铝石,也有少量一水软铝石,主要由花岗岩或基性岩(如辉绿岩、绿岩)中的长石经过风化淋滤形成。Darling Range地区铝土矿根据基岩的种类可分为花岗岩类铝土矿和镁铁质类铝土矿,分别以Jarrahdale铝土矿矿床和Mount Saddleback铝土矿矿床为代表。铝土矿的成矿主要受气候、母岩、地貌、排水和植被等因素控制。气候主要通过控制气温和降水影响红土化作用的进行;母岩通过自身的物理化学性质和矿物组成控制铝土矿的形成;地貌和排水影响铝土矿的风化速度;植被的作用既保证了土壤风化淋滤所需的渗透性,又降低了外部因素对土壤的侵蚀速率。在各种成矿因素的综合作用下,经过漫长的时间演化最终形成了红土型铝土矿。     

西澳Darling Range地区红土型铝土矿地球化学特征及源区研究

Darling Range位于西澳大利亚地区,红土型铝土矿资源丰富。本文对Darling Range地区红土型铝土矿及相关岩石开展了元素地球化学分析,并对铝土矿中的碎屑锆石和其下伏的花岗岩开展了LA-ICP-MS锆石U-Pb测年,目的是探讨西澳地区红土型铝土矿的成矿作用及其源区。结果表明,西澳地区红土型铝土矿的主量元素以Al2O3、Fe2O3、SiO2和TiO2为主,Al2O3与SiO2呈显著的负相关关系,与TiO2则呈现显著的正相关关系,表明铝土矿的形成过程是一个去Si富Al、Ti的过程。铝土矿微量元素富集Th、U、Nb、Ta、Zr、Hf、Ti,相对亏损Ba、K、Sr、P、Sm;稀土总量较低,为1.36×10~(-6)~65.58×10~(-6),稀土元素球粒陨石标准化分布曲线略微向右倾斜,富集轻稀土。铝土矿碎屑锆石U-Pb年龄分布于1167Ma和1267Ma与2539~2696Ma(16颗)两个年龄段,分析锆石来源前者可能来自Albany-Fraser造山带;后者加权平均年龄为2579±15Ma(MSWD=1.16),与研究区本次获得的太古宙花岗岩加权平均年龄2585±12Ma(MSWD=0.83)在误差范围内一致,表明铝土矿中的锆石除了来自本区太古宙花岗岩,还有少量的锆石来自Albany-Fraser造山带。结合元素地球化学钛率(Al2O3/TiO2)、lgCr-lgNi、微量元素比值、稀土元素配分模式对铝土矿源区进行示踪表明,Darling Range地区的太古宙花岗岩为红土型铝土矿的主要物质来源。     

Mapping bedrock lithologies through in situ regolith using retained element ratios: a case study from the Agnew-Lawlers area,Western Australia

Large, high-quality multi-element geochemical datasets are becoming widely available in the exploration industry, and afford excellent opportunities to investigate geochemical processes. A dataset of over 2500 analyses of unweathered and variably weathered mafic and ultramafic rocks for over 50 elements has been collected by Gold Fields Ltd. in the auriferous Agnew-Lawlers area of the eastern Yilgarn Craton of Western Australia. This dataset is used to investigate changes in element abundances and inter-element ratios through varying degrees and styles of weathering in an area of thick regolith characterised by deep in situ weathering. Systematic interrogation of the data, using lithostratigraphic controls derived from regional mapping and geophysics, reveals that a suite of elements, including Ti, Al, Zr, Th, La, Sc and Nb, and to a lesser extent Cr and Ni, behave as essentially immobile components during saprolite formation. In some cases diagnostic element ratios persist into siliceous duricrust. Ratios of these elements are used as reliable discriminants of bedrock type, and delineate features such as cryptic layering within fractionated sills and subtle geochemical variants in a sequence of tholeiitic and komatiitic basalts. Mapping on the basis of discriminant element ratios greatly extends previous trace-element ratio-based schemes for rock type discrimination. The potential to determine several of these elements with adequate precision and accuracy using portable XRF technology opens a potentially useful technique for rapid geochemical bedrock mapping in residual terrains.     

湖北蛇屋山金矿床含金碳酸盐岩风化成矿过程

为阐述含金碳酸盐岩的风化成土过程,以湖北蛇屋山红土型金矿床为例,系统开展了元素地球化学和矿物学研究.研究发现,从风化壳腐泥层→杂色粘土带,碱金属、FeO、MnO、有机碳(Morg)、稀土元素(REE)和高场强元素(HFSE)等元素,以及钾长石、斜长石和黄铁矿等矿物的含量逐渐降低.质量平衡计算表明,风化过程中Si、Fe、...     

黔中地区蔡家坝铝土矿床富锂铝土岩地球化学特征及沉积环境研究

贵州清镇蔡家坝铝土矿床是近年来在黔中铝土矿成矿区内新发现的铝土矿床之一,铝土矿体呈层状及似层状产于下石炭统九架炉组地层中。本研究通过对蔡家坝铝土矿床中以铝土岩为主的含铝岩系钻孔岩心剖面开展详细的矿物学和地球化学研究,发现含铝岩系中的铝土岩明显具有Li超富集的现象。研究结果表明,含铝岩系的所有样品中均富含高浓度的Li（210~2480）×10-6;其中,10件铝土岩的Li含量为（210~2480）×10-6（平均1198.9 ×10-6）,3件铝土矿石的Li含量为（412~1440）×10-6（平均980.7×10-6）,表现出明显超富集的特征。光学显微镜和XRD分析显示,富锂铝土岩主要由伊利石、高岭石和绿泥石组成,以及少量的硬水铝石、勃姆石、黄铁矿、锐钛矿等。结合矿物学和地球化学证据,研究区铝土岩中的Li主要赋存在高岭石和伊利石等粘土矿物中,当铝土岩中Al2O3含量介于30%~40%之间以及Al2O3/SiO2比值介于1~1.5时,铝土岩中的Li最为富集。元素地球化学分析表明,研究区内的富锂铝土岩形成于温暖潮湿和相对氧化的陆相淡水环境。     

Aluminosilicate cementation of saprolites,grits and silcretes in Western Australia

Amorphous to poorly crystalline aluminosilicates have been found as cementing agents within saprolites, hardpans and silcretes, particularly in granitic terrains in the Yilgarn Block, Western Australia. The cements range mineralogically from siliceous allophane to a kaolinite‐opal‐CT assemblage probably derived from the allophane. The allophane is non‐crystalline, with no distinctive X‐ray or electron diffraction patterns, and has close optical similarities to opaline silica. It is characterised by Al/Si ratios of 0.35–1.0, with small amounts of adsorbed iron and titanium. It occurs as colloform void and vein fillings, and permeating the kaolinitic matrix of saprolites. The kaolinite and opal‐CT have a similar occurrence, with the kaolinite oriented parallel to colloform and related banding. The aluminosilicate cements are found in saprolite, usually in the upper zones, and in overlying grits and silcretes. At Gabbin, in the central Yilgarn, the grits are up to 10 m thick and have aluminosilicate as the only cement. Although very hard when in situ, the grits tend to disaggregate on exposure and rarely, if ever, crop out. In profiles exposed in breakaways of the Barr‐Smith Range in the northern Yilgarn, the saprolite is overlain by silcretes with a quartz‐anatase‐zircon assemblage as cementing matrix, in turn overlain by alumino‐silicate‐cemented grits. The contact is gradational, and most silcretes and grits contain both types of cement, with the aluminosilicate increasing in abundance upwards. Kaolinitic spherules in sand plains are possibly derived from similar aluminosilicates.     

河南大冶矿区晚石炭世岩相古地理特征及铝土矿找矿方向

河南大冶铝土矿区晚石炭世早期处于古岛高地之间的滨岸潟湖沉积环境。中奥陶统以后,经历了160Ma风化剥蚀,在碳酸盐岩的侵蚀面上发育了钙红土铁、铝、硅风化壳。晚石炭世本溪期,海侵作用使矿区所在区域变为滨岸潟湖沉积环境,沉积相出现滨湖相、浅滩相、浅湖相、浅海相、滨岸沼泽相等5种;晚石炭世晚期形成海陆交互相的复理石沉积建造,将早期形成的铝土矿完整保存下来。今后铝土矿的找矿工作应重点放在浅滩-浅湖岩相带、含铝岩系相序发育相对完全的地段、含铝岩系厚度大且稳定的区段,并注意含铝岩系的表生富集作用。     

Paleomagnetic age of ferruginous weathering beneath the Hamersley Surface,Pilbara, Western Australia,and the Cenozoic apparent polar wander path

During the Mesozoic and Paleogene, the Precambrian rocks in the Pilbara, Western Australia, underwent erosion and deep weathering that produced an undulating landform now represented by the duricrusted and partly eroded Hamersley Surface. A reddened, ferruginous weathering zone occurs immediately beneath this duricrusted surface. Oriented block samples of ferruginised strata of the Neoarchean–early Paleoproterozoic Hamersley Group exposed within approximately 15 m below the duricrust were collected at 20 sites in roadcuts along the Great Northern Highway between Munjina and Newman and exposures along the adjoining Karijini Drive. Stepwise thermal demagnetisation of cored specimens revealed a stable, high-temperature (680°C) component carried by hematite, with a mean direction (n = 55 specimens) of declination D = 182.0°, inclination I = 52.9° (α₉₅ = 3.6°), indicating a pole position at latitude λ_p = 77.6°S, longitude ?_p = 113.2°E (A₉₅ = 4.3°) and a paleolatitude λ = 33.5 +3.6/–3.3°S. Both normal and reversed polarities are present, indicating that the remanent magnetism was acquired over an interval of at least two polarity chrons (say 10⁵–10⁶ years). Chi-square tests on the determined pole position and three different sets of Cenozoic poles, namely those for dated volcanic rocks in eastern Australia supplemented by poles for Australian Cenozoic weathering horizons, and inferred poles from Pacific Ocean and Indian Ocean hotspot analyses and North American Cenozoic poles rotated to Australian coordinates, yielded a mean age of ca 24 ± 3 Ma, i.e. late Oligocene to early Miocene, interpreted as the time of formation of hematite in the sampled ferruginous zone. The ferruginous weathering occurred under globally warm conditions and was followed during the early to middle Miocene climatic optimum by the deposition of channel iron deposits, which incorporated detrital hematitic material derived from erosion of the ferruginous weathering zone beneath the Hamersley Surface.     

Proterozoic biotite Rb–Sr dates in the northwestern part of the Yilgarn Craton,Western Australia

Rb–Sr dating of biotite in the northwestern corner of the Yilgarn Craton identified four areas with distinctive age ranges. Biotite in the northwestern area, which includes the Narryer Terrane and part of the Murchison Terrane, yields reset Rb–Sr dates of ca 1650 Ma. In the western area, along the margin of the craton, biotite has been reset to 629 Ma. Eastward of these areas, mainly in the Murchison Terrane, the modal biotite date is near 2450 Ma, though because of a skewed distribution the mean date is closer to 2300 Ma. Dates in a transition zone between the western and eastern areas range broadly between 2000 and 1000 Ma, averaging about 1775 Ma. The western area and the transition zone are continuous with analogous areas south of the limits of the present study. The 1650 Ma dates in the northwestern area are probably related to plutonic and tectonic activity of similar age in the Gascoyne Province to the north. They may represent cooling after thermal resetting during tectonic loading by southward thrust‐stacking of slices of Narryer Terrane and allochthonous Palaeoproterozoic volcanic arc and backarc rocks during the Capricorn Orogeny. This episode of crustal shortening resulted from the collision of the Yilgarn and Pilbara Cratons to form the West Australian Craton. The dates reflect cooling associated with subsequent erosion‐induced rebound. The 2450 Ma biotite dates of the eastern area are similar to biotite dates found over most of the Yilgarn Craton and represent a background upon which the later dates have been superimposed. The origin of dates in the western area is unknown but may be related to an associated dolerite dyke swarm or to possible thrusting from the west. There is some evidence of minor later intrusion of felsic hypabyssal rock between 2000 and 2200 Ma and localised shearing in the Narryer area at about 1350 to 1400 Ma. One small area near Yalgoo with biotite Rb–Sr dates near 2200 Ma may be cogenetic with the Muggamurra Swarm of dolerite dykes.     

SHRIMP U–Pb zircon ages of tuffaceous mudrocks in the Brockman Iron Formation of the Hamersley Range,Western Australia

Tuffaceous mudrocks are common in the banded iron‐formations (BIF) of the Brockman Iron Formation. These tuffaceous mudrocks are either stilpnomelane‐rich or siliceous. Their compositions reflect bimodal volcanic activity in the vicinity of the Hamersley BIF depositional site. They also contain complex zircon populations that record resedimentation, syndepositional volcanism and post‐depositional isotopic disturbance. The best estimates of depositional age are obtained from siliceous tuffaceous mudrocks in the Joffre Member that contain 2459 ± 3 Ma and 2454 ± 3 Ma zircon populations most likely derived from felsic volcanism coeval with BIF deposition. These dates constrain the sedimentation rates for the ～370 m‐thick Joffre Member BIF to >15 m per million years. Siliceous tuffaceous mudrocks are not present in the underlying ～120 m‐thick Dales Gorge Member and it is uncertain whether previously reported ages of ca 2479–2470 Ma for this unit reflect detrital/xenocrystic or syndepositional zircon populations in resedimented stilpnomelane‐rich tuffaceous mudrocks. The increased abundance of tuffaceous mudrocks in the Joffre Member suggests that a pulse of enhanced igneous and hydrothermal activity accompanied deposition of the bulk of the Brockman Iron Formation BIF after ca 2460 Ma. This preceded and culminated in the emplacement of the 2449 ± 3 Ma large igneous province represented by BIF and igneous rocks of the Weeli Wolli Formation and Woongarra Rhyolite.     

重庆吴家湾铝土矿矿床地质特征及控矿因素探讨

重庆吴家湾铝土矿产于中二叠统梁山组(P₂l)之下,呈假整合覆于中志留统韩家店组粉砂质页岩或中石炭统黄龙组灰岩之上。矿体产状与地层产状基本致,铝土矿呈土状、豆(鲕状)、致密块状、砾屑状。一水硬铝石为主要矿石矿物。控制铝土矿形成的主要因素为古环境、地形地貌、构造位置和地层。     

云南会泽大黑山铝土矿地球化学特征及其物源分析

大黑山铝土矿赋存于上二叠统宣威组底部,下伏地层为峨眉山玄武岩。矿石结构主要以泥晶结构为主,具有少量鲕状、粒屑结构。矿石构造以致密块状构造为主,局部呈层状、似层状。铝土矿石中Al₂O₃与Fe₂O₃、SiO₂呈现较好的负相关关系,TiO₂与Zr、Hf、Nb、Ta相关性较好,Zr-Hf、Nb-Ta的相关性拟合度很高。铝土矿与峨眉山玄武岩样品具有相似的稀土元素配分模式。综合稳定元素相关性、微量元素及稀土元素标准化图解、lgNi和lgCr二元图解等分析方法对大黑山铝土矿的成矿物质来源进行探讨,研究结果表明铝土矿的成矿物质来源主要来自峨眉山玄武岩。     

Spectral characteristics of the Gawler Range Volcanics in drill core Myall Creek RC1

Traditional core-logging methods in conjunction with spectral scanning techniques have been used to log volcanic successions of the lower Gawler Range Volcanics. The open-file drill core Myall Creek RC 1 was re-logged and scanned using HyLogger? core scanning technologies as a part of the Geological Survey of South Australia's Southern Gawler Rangers mapping program and the Mineral System Drilling Program. Myall Creek RC 1 is one of the key stratigraphic drill cores for the region, owing to the intersection of a large section of the Gawler Range Volcanics. Spectral characterisation of the Eucarro Rhyolite revealed differential weathering of plagioclase phenocrysts, while high-resolution imagery and spectral results were used to log new basaltic flows and small flow features in the Roopena Basalt. The composition and distribution of feldspars in the unnamed lower Gawler Range Volcanic units were used to aid traditional logging of visually similar lithologies. A spectral scalar, the felsic–mafic index, was used to identify unusual features in the unnamed sequence and was found to identify iron oxides as either fracture coatings or finely disseminated in the matrix of the sample. Iron oxides were also used to identify features within lithological units, which were difficult to discern visually, especially the layers in the deepest layered ignimbrite at the end of the drill core.     

云南个旧锡铜多金属矿区云母~(40)Ar/~(39)Ar年龄及其地质意义

云南个旧是全球最大的锡铜多金属矿区,主要成矿作用是与燕山期花岗岩密切相关的岩浆-热液体系。矿区内铜矿的主要矿床类型为变玄武岩型层状铜矿和接触带型铜矿。赋存于花岗岩体的凹陷部位,接触带型铜矿体和氧化型矿体的精确年龄尚未有报道。以老厂矿田内与铜矿体同期的等粒花岗岩脉中的黑云母和与氧化矿同时形成的白云母作为研究对象,利用常规~(40)Ar/~(39)Ar同位素定年方法,获得了黑云母和白云母的~(40)Ar/~(39)Ar坪年龄分别为82.47±0.49Ma和76.17±0.42Ma,相应的正等时线年龄为82.38±0.48Ma和76.07±0.66Ma,反等时线年龄为82.38±0.49Ma和76.07±0.73Ma。结合野外地质接触关系和矿区内其他年代学结果认为,黑云母的~(40)Ar/~(39)Ar年龄82.38±0.48Ma可以代表接触带型铜矿体的形成年龄,也揭示了新山花岗岩体形成后的快速冷却作用过程;白云母的~(40)Ar/~(39)Ar年龄76.07±0.73Ma指示了氧化型矿体的形成年龄,也记录了矿区内与甲介山同期的南北向断裂的晚期活动时限。该年龄与个旧锡铜多金属矿床的成矿时代一致。     

Significance of weathering and regolith/landscape evolution for mineral exploration in the NE Albany-Fraser Orogen,Western Australia

The Albany-Fraser Orogen (AFO), southeast Western Australia, is an underexplored, deeply weathered regolith-dominated terrain that has undergone complex weathering associated with various superimposed climatic events. For effective geochemical exploration in the AFO, integrating landscape evolution with mineralogical and geochemical variations of regolith and bedrock provides fundamental understanding of mechanical and hydromorphic dispersion of ore and pathfinder elements associated with the different weathering processes.In the Neale tenement, northeast of the AFO, a residual weathering profile that is 20-55 m thick was developed under warm and humid climatic conditions over undulating Proterozoic sheared granitoids, gneisses, schists and Au-bearing mafic rocks. From the base, the typical weathering profile consists of saprock, lower ferruginous saprolite, upper kaolinitic saprolite and discontinuous silcrete duricrust or its laterally coeval lateritic residuum. These types of duricrusts change laterally into areas of poorly-cemented kaolinitic grits or loose lateritic pisoliths and nodules.Lateritic residuum probably formed on remnant plateaus and was transported mechanically under arid climatic conditions over short distances, filling valleys to the southeast. Erosion of lateritic residuum exposes the underlying saprolite and, together with dilution by aeolian sands, constitutes the transported overburden (2-25 m thick). The reworked lateritic materials cover the preserved silcrete duricrusts in valleys. The lower ferruginous saprolite and lateritic residuum are well developed over mafic and sulphide-bearing bedrocks, where weathering of ferromagnesian minerals and sulphides led to enrichment of Fe, Cu, Ni, Cr, Co, V and Zn in these units. Kaolinitic saprolite and the overlying pedogenic silcrete are best developed over alkali granites and quartzofeldspathic gneisses, which are barren in Au and transition elements, and enriched in silica, alumina, rare earth and high field strength elements.A residual Au anomaly is formed in the lower ferruginous saprolite above a Au -bearing mafic intrusion at the Hercules prospect, south of the Neale tenement, without any expression in the overlying soil (< 20 cm). Conversely, a Au anomaly is recorded in the transported cover, particularly in the uppermost 3 m at the Atlantis prospect, 5 km southwest of the Hercules prospect. No anomalies have been detected in soils using five different size fractions (> 2,000 μm, 2,000-250 μm, 250-53 μm, 53-2 μm and < 2 μm). Therefore, soil cannot be efficiently applied as a reliable sampling medium to target mineralization at the Neale tenement. This is because mechanical weathering was interrupted by seasonal periods of intensive leaching under the present-day surface conditions and/or dilution by recently deposited aeolian sediments which obscure any signature of a potential Au anomaly in soils. Therefore, surface soil sampling should extend deeper than 20 cm to avoid dilution by aeolian sands and seasonal leaching processes. Regolith mapping and the distinction between the residual and transported weathering products are extremely significant to follow the distal or proximal mineralization.     

西澳大利亚伊尔岗克拉通铁矿床研究进展

伊尔岗克拉通位于澳大利亚西南部,是地球上最古老的克拉通之一。该克拉通内产出的铁矿床均与条带状含铁建造（BIF）有关,可分为2种类型：①深成—表生矿床;②表生—富集矿床,主要分布在尤恩米（Youanmi）地体中。深成—表生型铁矿床具有相似的变形历史、镁铁质火成岩围岩、深成热液蚀变事件和高品位的铁矿石类型。深成热液蚀变包括早期碳酸盐-磁铁矿蚀变、中期形成磁铁矿矿石、晚期碳酸盐-赤铁矿蚀变,但是这些矿床在岩相、变质程度、矿物学和地球化学方面都存在差异,目前还没有统一的成因模型。表生—富集型铁矿床可能是通过表生淋滤BIF中的硅质条带形成的,但不含硅质条带的BIF的出现,说明没有对硅质条带的选择性表生溶解也可以形成高品位矿体。     

西澳大利亚伊尔岗克拉通铁矿床研究进展

伊尔岗克拉通位于澳大利亚西南部,是地球上最古老的克拉通之一。该克拉通内产出的铁矿床均与条带状含铁建造(BIF)有关,可分为2种类型:1深成—表生矿床;2表生—富集矿床,主要分布在尤恩米(Youanmi)地体中。深成—表生型铁矿床具有相似的变形历史、镁铁质火成岩围岩、深成热液蚀变事件和高品位的铁矿石类型。深成热液蚀变包括早期碳酸盐-磁铁矿蚀变、中期形成磁铁矿矿石、晚期碳酸盐-赤铁矿蚀变,但是这些矿床在岩相、变质程度、矿物学和地球化学方面都存在差异,目前还没有统一的成因模型。表生—富集型铁矿床可能是通过表生淋滤BIF中的硅质条带形成的,但不含硅质条带的BIF的出现,说明没有对硅质条带的选择性表生溶解也可以形成高品位矿体。     

The age and origin of younger granitic plutons of the Shaw Batholith in the Archaean Pilbara Block,Western Australia

The whole-rock Pb-Pb method has been used to date four of the younger, mainly adamellite, late-tectonic plutonic phases within the ca. 3.5 Ga Shaw Batholith of the Archaean east Pilbara Block. Three suites give ages within error of 2966 Ma (Porphyritic Granites at 2948±50 Ma, Leuco-adamellites at 2943±46 Ma and Garden Creek Adamellite at 3007±48 Ma). The post-tectonic Cooglegong Adamellite gives an age of 2847±34 Ma. The Sm-Nd model isotopic systematics of all four suites indicate derivation from crust ranging between ca. 3200 and 3600 Ma in age. The sources for these four younger plutonic phases were heterogeneous and, although exhibiting some isotopic characteristics of the older (3.5–3.3 Ga) calc-alkali plutonic suites, were more depleted in the LIL elements Rb, U and Th. In addition, the Garden Creek Adamellite and the Cooglegong Adamellite lack the very fractionated and HREE-depleted REE patterns characteristic of both the older calc-alkali plutonic rocks and the Porphyritic Granites and Leuco-adamellites. The crust underlying the Shaw Batholith at ca. 2950 Ma must have been both markedly heterogeneous and variably depleted, a conclusion consistent with the complex tectonic and plutonic evolution of this region.     

Hydrothermal and resedimented origins of the precursor sediments to banded iron formation: sedimentological evidence from the Early Palaeoproterozoic Brockman Supersequence of Western Australia

The Early Palaeoproterozoic Brockman Supersequence comprises banded iron formation (BIF), bedded chert, limestone, mudrock, sandstone, breccia, tuffaceous mudstone, ashfall tuff and, in sections not reported here, basalt and rhyolite. Density current rhythms are preserved in sandstones, mudrocks, tuffaceous mudstones and limestones. Relics of similar rhythms in BIF imply that its precursor sediments were also deposited by density currents. Hemipelagic deposits are siliciclastic or mixed siliciclastic–volcaniclastic mudstones. Bedded chert, chert nodules and the chert matrix of BIF preserve evidence for formation by diagenetic replacement. For bedded chert (and chert nodules), silica replacement occurred before compaction close to or at the sediment–water interface, indicating that it is siliceous hardground. The chert matrix of BIF formed during compaction but before burial metamorphism. Original sediments were resedimented from two sources: (1) limestone, mudrock, sandstone, breccia and tuffaceous mudstone from a shelf; and (2) BIF from within the basin realm. Shelf sediments were resedimented to basin-floor fans during third-order lowstands. The precursor sediments to BIF are interpreted to have been granular hydrothermal muds, composed of iron-rich smectite and particles of iron oxyhydroxide and siderite that were deposited on the flanks of submarine volcanoes and resedimented by density currents. Resedimentation occurred by either bottom currents or gravity-driven turbidity currents, and the resulting sediment bodies may have been contourite drifts. The concept that BIF records high-frequency alternating precipitation from ambient sea water of iron minerals and silica is negated by this study. Instead, it is postulated that the precursor sediments to BIF originated in much the same way as modern Red Sea hydrothermal iron oxide deposits, implying that at least the particles of iron oxyhydroxide originated from the oxidation of vent fluids by sea water. Several orders of cyclicity in basin filling establish a relationship between rising to high sea levels, episodic sea-floor hydrothermal activity and BIF that is reminiscent of the link between eustacy and spreading-ridge pulses.     

贵州福泉高洞铝土矿床成矿地质地球化学特征

贵州福泉高洞铝土矿床是近年在贵州新的铝土矿成矿区探明的中型矿床之一,其含矿岩系为下石炭统九架炉组,铝土矿呈层状、似层状产于上泥盆统高坡场组碳酸盐岩古侵蚀面之上,与上覆中二叠统梁山组和下伏上泥盆统高坡场组均呈假整合接触,矿床成因类型为古风化壳沉积型。通过对含矿岩系中敏感元素Ga、Ba、Sr、Th含量及B/Ga、Rb/K、V/Zr比值特征分析,认为铝土矿主要形成于陆相沉积环境;δCe、Ni/Co、Th/U、Sr/Cu、V/Cr特征揭示铝土矿成岩成矿过程主要为氧化环境。含矿岩系中Al与Zr、Hf、Nb、Ta、Th、U等呈正相关性反映出铝土矿成矿过程中具同迁移、显著富集特征;含矿岩系中微量元素含量与下伏高坡场组碳酸盐岩相比,总体成强烈富集态势。与黎彤值对比,含矿岩系中高温成矿元素(W、Sn、Mo、Bi)、高场强元素(Nb、Ta、Th、U、Hf、Zr)、粘土矿物易吸附元素(Li、V)及细碎屑岩中高地球化学背景元素(Sb)富集强度高。LREE与HREE分馏作用明显,推测为次生风化淋滤过程中离子半径较小的HREE向下渗滤迁移的速率大于离子半径较大的LREE所致。