澳大利亚西部哈默斯利铁成矿省BIF富铁矿的成矿特征与控矿因素

澳大利亚西部哈默斯利铁成矿省含有世界级高品位的赤铁矿体。主要铁矿床包括芒特维尔贝克、汤姆普莱斯山、帕拉伯杜等,它们均产于元古宙早期布罗克曼BIF型含铁建造中。高品住铁矿体的空间分布明显受到元古宙区域隆起和拉张环境下形成的古老正断层系统的控制。该成矿省高品位铁矿层的形成可分为3个阶段：第1阶段为深层阶段,该阶段硅从含铁建造中淋滤出来,留下薄层状富含铁氧化物、碳酸盐岩、硅酸镁和磷灰石的残余物;第2阶段为深部大气水氧化阶段,该阶段含铁建造的磁铁矿-菱镁矿组合被氧化为赤铁矿-铁白云石,并以发育假象赤铁矿为特征;第3阶段为浅层风化作用。通过对成矿特征和成矿模式的总结,认为成矿时代、断层、褶皱等构造特征及流体和表生风化作用是富铁矿床形成的主要控矿因素。     

Pyrite paragenesis and multiple sulfur isotope distribution in late Archean and early Paleoproterozoic Hamersley Basin sediments

The sulfur isotope record in late Archean and early Paleoproterozoic rocks is of considerable importance because it provides evidence for changes in early Earth atmospheric oxygen levels and potentially constrains the origin and relative impact of various microbial metabolisms during the transition from an anoxic to oxic atmosphere. Mass independently fractionated (MIF) sulfur isotopes reveal late Archean and early Paleoproterozoic sulfur sources in different pyrite morphologies in Western Australia's Hamersley Basin. Multiple sulfur isotope values in late Archean pyrite vary according to morphology. Fine grained pyrite has positive sulfur MIF, indicating a reduced elemental sulfur source, whereas pyrite nodules have negative sulfur MIF, potentially derived from soluble sulfate via microbial sulfate reduction. The Hamersley Basin δ³⁴S–Δ³³S record suggests that the extent of oxygenation of the surface ocean fluctuated through the Late Archean from at least 2.6 Ga, more than 150 million yr before the Great Oxidation Event. In the early Paleoproterozoic, there is less distinction between pyrite morphologies with respect to sulfur isotope fractionation, and pyrite from the Brockman Iron Formation trends toward modern sulfur isotope values. An important exception to this is the strong negative MIF recorded in layer parallel pyrite in Paleoproterozoic carbonate facies iron formation. This may suggest that deeper water hydrothermal environments remained anoxic while shallower water environments became more oxidised by the early Paleoproterozoic. The results of the current study indicate that sulfide paragenesis is highly significant when investigating Archean and early Paleoproterozoic multiple sulfur isotope compositions and sulfur sources.     

Neoarchaean impact spherule layers in the Fortescue and Hamersley Groups,Western Australia: stratigraphic and depositional implications of re-correlation

Previously, two layers containing impact melt spherules, the Wittenoom spherule layer and the Carawine spherule layer, exposed in the main outcrop area and Oakover River area, respectively, of the Neoarchaean?–?Palaeoproterozoic Hamersley Basin of Western Australia, were correlated. Subsequent discovery and study of the Jeerinah spherule layer in the main outcrop area, as well as a new Carawine spherule layer exposure now suggest that the Carawine and Jeerinah spherule layers are correlates. The previous Wittenoom?–?Carawine correlation was based on the presence of spherules and sedimentological consistency: both layers comprise sediment gravity flows, and the Wittenoom spherule layer was interpreted as the downflow equivalent of the Carawine layer. However, the Jeerinah spherule layer also consists of sediment gravity flows, which could be related to the Carawine layer. Since all three layers reflect events triggered by oceanic impacts, these similarities are not surprising, but they do eliminate sedimentology as a correlation tool. However, two compositional trends suggest that the Carawine and Jeerinah layers are correlates: (i) the textures of their spherules are very similar and are distinctly different from the Wittenoom layer; and (ii) only the Carawine and Jeerinah layers contain irregular impact melt particles. The latter observation is strong evidence as irregular particles are unknown in any other early Precambrian spherule layers in Western Australia. While triggered by the same impact, it is unlikely that the Carawine and Jeerinah spherule layers were deposited by the same sediment gravity flows, as they contain very different intraclast populations.     

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.     

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.     

澳洲铁矿床研究现状及存在的问题??

澳大利亚是世界上铁矿石资源最为丰富的国家之一,其铁矿床主要产在西澳皮尔巴拉地区,有三种类型,分别是:①赋存在条带状含铁建造(BIF)中的层状铁矿床(BID),②产在古河道中的河道型铁矿床(CID),和③主要由BID受侵蚀崩塌或冲积形成的碎屑型铁矿床(DID),以前两种类型为主。BID型铁矿通常品位高,规模大,是本区最为重要的矿床类型,其矿床成因尚存在争论,主要有三种观点,分别是表生—变质模式、同造山的热液模式和深成—表生模式。CID型铁矿由于其规模较大和容易开采,因此在西澳的铁矿石开采中占有很重要的地位,矿石以球粒状构造和富含铁化的木屑为主要特点。关于CID型矿床的成因,争议较大,观点甚多。一些学者认为CID型矿床的形成受特定条件(包括气候、地表风化和地质背景)的控制;而有些学者则认为CID型矿床形成于一个富含有机酸的饱和地下水的加积河道内,与铁的原位溶解和再沉淀有关。矿化发生在古地下水—大气界面,因此受地下水位的控制。由于对铁矿的矿床成因没有形成统一的认识,因此对指导找矿产生了较大影响。     

Opaque mineralogy and magnetic properties of selected banded iron‐formations,Hamersley Basin,Western Australia

The oxide mineralogy and rock magnetic properties of unmineralised banded iron‐formations in selected portions of four drillholes in the Hamersley Basin, Western Australia are reviewed. In all four drillholes, petrographic studies indicate that primary euhedral to subhedral hematite is partially replaced by magnetite as a result of subsolidus reduction. All drillholes show partial recrystallisation of the secondary magnetite, suggesting that early subsolidus reduction was probably a regional event occurring during prograde metamorphism. Incomplete replacement of primary hematite by magnetite within and between sedimentary band structures indicates that equilibration in the magnetite stability field was not reached even at the mesoband scale. Subsequent subsolidus oxidation of magnetite and the formation of a second‐generation hematite are documented in only two of the drillholes. Goethite‐filled veins and thick selvages of goethite around some veins reflect movement of circulating oxidising fluids. The absence of goethite and second‐generation hematite in two of the drillholes indicates that subsolidus oxidation is not a regional event, but very much localised. Rapid changes in down‐hole susceptibility measurements correlate directly with detailed petrographic results as susceptibility readings change with the hematite/magnetite ratio on a mesoband scale. Acquisition of the main remanence correlates with the formation of hematite as the primary oxide phase followed by partial replacement by magnetite as a result of subsolidus reduction, supporting regional models requiring pre‐folding remanence. The strong orientation of the primary hematite parent parallel to band structures in the banded iron‐formations has influenced the direction of crystallisation remanent magnetisation during subsolidus reduction to the magnetite daughter. The strong planar alignment has also produced a planar magnetic fabric and marked anisotropy of magnetic susceptibility. A natural remanent magnetisation overprint and reduction in anisotropy of magnetic susceptibility are only recorded in samples that have undergone subsolidus oxidation and the recognition of localised post‐metamorphic oxidation overprinting can also explain ore deposit models requiring post‐folding remanence. The relative timing of and between oxidising fluid events is not known, but both petrographic and rock magnetic evidence to date suggests that there was at least one and probably two post‐folding oxidising events in the area of study.     

Inliers of banded iron‐formation in the Proterozoic Wyloo Group sediments near Paraburdoo,Western Australia

Two inliers with a total outcrop length of 3000 m and a maximum width of 200 m, consisting of a sedimentary klippe (olistolith) and an olistostrome (both composed of banded iron‐formation and shale belonging to the Hamersley Group) occur within the Mininer Turbidite Member of the Wyloo Group, south of Paraburdoo, W.A., 2500 m from the top of the Hamersley Group proper. The olistostrome is a typical debris slide produced by slumping of unconsolidated material. The klippe was rafted into position as a solid block by a turbidity current.

The pattern of mineralisation within the banded iron‐formation part of the klippe, which is identified as being from the Brockman Iron Formation, together with evidence from the basal conglomerate of the Wyloo Group, shows that the formation of the Hamersley iron ore deposits commenced prior to the deposition of the Wyloo Group sediments.     

Genesis of the channel iron deposits (CID) of the Pilbara region,Western Australia

Miocene fluvial goethite/hematite channel iron deposits (CID) are part of the Cenozoic Detritals 2 (CzD2), of the Western Australian Pilbara region. They range from gravelly mudstones through granular rocks to intraformational pebble, cobble and rare boulder conglomerates, as infill in numerous meandering palaeochannels in a mature surface that includes Precambrian granitoids, volcanics, metasediments, BIF and ferruginous Palaeogene valley fill. In the Hamersley Province of the Pilbara, the consolidated fine gravels and subordinate interbedded conglomerates, with their leached equivalents, are a major source of export iron ore. This granular ore typically comprises pedogenically derived pelletoids comprising hematite nuclei and goethite cortices (ooids and lesser pisoids), with abundant coarser goethitised wood/charcoal fragments and goethitic peloids, minor clay, and generally minimal porous goethitic matrix, with late-stage episodic solution and partial infill by secondary goethite, silica and siderite (now oxidised) in places. Clay horizons and non-ore polymictic basal and marginal conglomerates are also present. The accretionary pedogenic pelletoids were mostly derived from stripping of a mature ferruginous but apparently well-vegetated surface, developed in the Early to Middle Miocene on a wide variety of susceptible rock types including BIF, basic intrusives and sediments. This deep ferruginisation effectively destroyed most remnants of the original rock textures producing a unique surface, very different to those that produced the underlying CzD1 (Palaeogene) and the overlying CzD3 (Pliocene – Quaternary). The peloids were derived both intraformationally from fragmentation and reworking of desiccated goethite-rich muds, and from the regolith. Tiny wood/charcoal fragments replaced in soil by goethite, and dehydrated to hematite, formed nuclei for many pelletoids. Additionally, abundant small (≤10 mm) fragments of wood/charcoal, now goethite, were probably replaced in situ within the consolidating CID. This profusion of fossil wood, both as pelletoid nuclei and as discrete fragments, suggests major episodic wild fires in heavily vegetated catchments, a point supported by the abundance of kenomagnetite – maghemite developed from goethite in the pelletoids, but less commonly in the peloids. The matrix to the heterogeneous colluvial and intraformational components is essentially goethite, primarily derived from modified chemically precipitated iron hydroxyoxides, resulting from leaching of iron-rich soils in an organic environment, together with goethitic soil-derived alluvial material. Major variations in the granular ore CID after deposition have resulted from intermittent groundwater flow in the channels causing dissolution and reprecipitation of goethite and silica, particularly in the basal CID zones, with surface weathering of eroded exposures playing a role in masking some of these effects. However, significant variations in rock types in both the general CID and the granular ore CID have also resulted from the effects of varied provenance.     

Coupled asteroid impacts and banded iron-formations,Fortescue and Hamersley Groups,Pilbara, Western Australia

Asteroid impact spherule layers and tsunami deposits underlying banded iron-formations in the Fortescue and Hamersley Groups have been further investigated to test their potential stratigraphic relationships. This work has included new observations related to the ca 2.63 Ga Jeerinah Impact Layer (JIL) and impact spherules associated with the 4th Shale-Macroband of the Dales Gorge Iron Member (DGS4) of the Brockman Iron Formation. A unit of impact spherules (microkrystite) correlated with the ca 2.63 Ga JIL is observed within a >100 m-thick fragmental-intraclast breccia pile in drill cores near Roy Hill. The sequence represents significant thickening of the impact/tsunami unit relative to the JIL type section at Hesta, as well as relative to the 20–30 m-thick ca 2.63 Ga Carawine Dolomite spherule-bearing mega-breccia. The ca 2.48 Ga-old Dales Gorge Member of the Brockman Iron Formation is underlain by an ?0.5 m-thick rip-up clast breccia located at the top of the ca 2.50 Ga Mt McRae Shale, and is interpreted as a tsunami deposit. We suggest that the presence of impact ejecta and tsunami units stratigraphically beneath a number of banded iron-formations, and units of ferruginous shale in the Pilbara and South Africa may result from a genetic relationship. For example, it could be that under Archean atmospheric conditions, mafic volcanism triggered by large asteroid impacts enriched the oceans in soluble FeO. If so, seasonal microbial and/or photolytic oxidation to ferric oxide could have caused precipitation of Fe₂O₃ and silica. In view of the possible occurrence of depositional gaps and paraconformities between impact ejecta units and overlying ferruginous sediments, these relationships require further testing by isotopic age studies.