黄铁矿原位LA-ICP-MS微量元素分析在金矿床中应用

运用LA-ICP-MS对金属硫化物、氧化物进行原位微区微量元素分析具有灵敏度高、检出限低、多元素同时检测的优势, 应用前景广阔。本文综述了黄铁矿LA-ICP-MS原位微量元素分析在金矿研究方面的应用, 指出在详细的岩相学观察基础上, 将LA-ICP-MS分析与电子探针背散射分析、X射线面扫描分析、原位S/Fe同位素分析技术等连用, 可用于判断黄铁矿中微量元素的赋存状态, 约束变质变形过程中微量元素的活化-迁移行为, 反演成矿流体性质及演化规律, 示踪成矿物质来源, 精细刻画成矿过程, 有效约束矿床成因。     

The Granites gold deposits, Northern Territory, Australia: evidence for an early syn-tectonic ore genesis

The ore deposits of The Granites goldfield are shear-hosted within Palaeoproterozoic amphibolite facies metasedimentary rocks in the Tanami Region, Northern Territory, Australia. The ore bodies are located within a 5- to 35-m thick sequence of steeply dipping unit of metamorphosed iron-rich metasedimentary rocks. Deformation at The Granites was complex and is characterized by five successive deformation phases (D_1–5). Shear veins (central and oblique) are the dominant type of vein geometry, with minor development of extensional veins and reverse-fault related veins. Four generations of syn-tectonic veins, corresponding to D₁, D₃, D₄, and D₅, have been recognized and are comprised of quartz, quartz-carbonate, calc-silicate, and calcite. In addition, two generations of disseminated sulfide–arsenide mineralization, dominated by pyrrhotite, arsenopyrite, and loellingite, with minor pyrite, chalcopyrite and rare marcasite, formed syn-D₁ and syn- to post-D₃. Textural and structural evidence indicates deposition of gold was contemporaneous with the syn-D₁ veins and sulfide–arsenide mineralization. Four hydrothermal phases are proposed for the formation of the veins and disseminated sulfide–arsenide assemblages. The first phase (D₁) was responsible for transport and deposition of the majority of the gold. Minor remobilization and deposition of gold occurred during the D₃ and D₄ phases. Little is known about the nature of the D₁ ore fluid, although a relatively low sulfur content is indicated by the assemblage pyrrhotite–arsenopyrite–loellingite+rare pyrite. The growth of amphibolite facies metamorphic minerals andalusite and almandine garnet during D₁ indicates a high temperature for the fluid. The D₃ hydrothermal phase coincided with peak metamorphism. D₄ fluids were hypersaline, high temperature, CO₂-poor, and H₂S-poor. Editorial handling: L. Meinert     

Evaluation of lead isotopic methods for uranium exploration, Koongarra Area, Northern Territory, Australia

Analysis of radioactive (²¹⁰Pb) and stable lead isotopes in near-surface samples has been tested as a method of uranium exploration in the Pine Creek Geosyncline, Northern Territory, Australia. The lead isotopes were extracted from the samples by a mild leaching agent and were measured by alpha spectrometry for ²¹⁰Pb and by mass spectrometry for stable lead isotopes. The results are compared with those obtained by conventional methods utilizing measurements of radioactivity and radon (Track Etch) in situ and ²²⁶Ra, ²²⁸Ra and U contents of soils. The major problems addressed were whether the lead isotopic methods are more sensitive than the conventional methods and whether they can discriminate “real” anomalies from the common barren anomalies found in black soils and swamps which contain radium in excess of the uranium present.Four test areas, representing a range of exploration problems, were chosen in the vicinity of the Koongarra uranium deposits and 25 samples from each area were analyzed. Most samples have more ²²⁶Ra than uranium. Radium analyses of several water samples show the source of this radium to be non-uraniferous rocks within the Kombolgie sandstone. The results for soil ²²⁶Ra, radon, scintillometry and ²¹⁹Pb were generally closely correlated, and as a result, the ²¹⁰Pb method was not considered to have any advantages over the conventional methods.At the Koongarra X prospect, which has a weak surface expression, the ratio gave the strongest indication of the underlying uranium mineralization with an anomaly to background ratio of 12.5. However, this ratio is correlated with uranium content and does not offer any particular advantages over uranium analyses alone. More subtle indications of uranium mineralization were found by relating the radiogenic lead (²⁰⁶Pb) and the thorium-derived lead (²⁰⁸Pb) to the common lead content (²⁰⁴Pb). A plot of versus (horizontal axis) is linear for country rock samples, irrespective of the amount of more recently introduced ²²⁶Ra. Samples above uranium mineralization lie off this trend, along a line of near-zero slope. By the use of this plot, indications were found of the Koongarra No. 2 orebody, which is concealed by about 40 m of barren overburden; none of the other techniques detected this mineralization.     

Lithogeochemical exploration for shale-hosted lead-zinc deposits: Case studies from the McArthur River and Mt Isa Districts,northern Australia

The large lead-zinc deposits of Middle Proterozoic age in northern Australia occur in sediments interpreted as having been deposited under saline, shallow-emergent conditions. Significant alkaline diagenesis is evident from the presence of abundant K-feldspar and locally albite. Alkali element compositions clearly distinguish the sediments of the mineralised shallow-emergent locations both from those of more permanent water cover (lacustrine or lagoonal), and from those close to basin margins. The key variables for identifying the geochemical contrasts are the levels of K, Rb and Na, and the ratios of K : Al and K : Rb. These geochemical parameters can be used in conjunction with the levels of the target and indicator elements to detect whether mineralisation may be present in a favourable geological environment. Two case studies of the application of whole rock geochemistry in base metal exploration are given for the Glyde River Sub-basin (McArthur Basin) and the Western Succession, Mt Isa Block.     

A 1642 Ma age for the Fraynes Formation,Birrindudu Basin,confirms correlation with the economically significant Barney Creek Formation,McArthur Basin,Northern Territory

Abstract

An interpreted CA–IDTIMS age of 1642.2?±?3.9?Ma for a volcanogenic tuffaceous siltstone from the previously undated Fraynes Formation of the Birrindudu Basin in the northwestern Northern Territory enables a rigorous chronostratigraphic correlation to be made with the economically important Barney Creek Formation of the southern McArthur Basin. This result supports previous interpretations that these geographically widely separated formations are probably linked in the subsurface. It also establishes the stratigraphic interval encompassing the Fraynes and Barney Creek formations as a potential target for greenfields base metals and petroleum exploration programs across the greater McArthur Basin.

• KEY POINTS

• A new interpreted CA–IDTIMS age provides a chronostratigraphic link between the Fraynes Formation of the Birrindudu Basin and the economically important Barney Creek Formation of the southern McArthur Basin.

• The Fraynes and Barney Creek formations are probably linked in the subsurface.

• This stratigraphic level is a potential target for greenfields base metals and petroleum exploration across the greater McArthur Basin.

     

Stratigraphic resolution of a multiphase intracratonic basin system: The McArthur Basin,northern Australia

The Broadmere Structure is a concealed complex inversion structure that developed in a well preserved part of the Batten Trough in the southern McArthur Basin of northern Australia. The structure formed in response to a number of major tectonic events over a period of at least 600 million years. Evaluation of more than 300 km of seismic data gathered across the structure has provided significant insight into the evolution of the basin architecture which, in turn, has provided a framework for understanding the spatial and temporal distribution of mineralisation. The Palaeoproterozoic McArthur Basin, a polyphase basin with a long and complex evolutionary history, was initiated some time after 1.8 Ga as a sag basin following mantle activity associated with supercontinent assembly. This shallow sag then began to accumulate a relatively mature clastic sedimentary succession with interspersed basic volcanics. Subsequent extension of the weakened crust, beginning at approximately 1730 Ma, led to the development of a series of normal faults and half grabens within zones (e.g. Batten Trough) defined laterally by major strike slip fault complexes. The half grabens were filled initially by volcaniclastic sediments with associated bimodal volcanics that were later followed by mixed carbonate and clastic sediments exceeding 8 km in thickness. A major change in basinal and regional dynamics between 1575 and 1500 Ma (Isan Orogeny) led to the inversion of the thickened succession overlying the half grabens and extensive erosion of earlier depositional units, followed by deposition of a further 3 km of shallow marine clastic sediments. The analysis of the Broadmere Structure has provided a simple framework in which to evaluate prospectivity. The primary northwest striking half grabens formed localised anoxic, sediment starved depocentres in which organic rich sediments that potentially host mineralisation accumulated. The distribution of subsequent ore bodies is a function of the timing of fluid movement and the nature of fluid pathways, as determined by the evolving basin architecture. Fluids moved laterally through the permeable channels into the Batten Trough in two stages. Spatial data show that Pb and Zn occurrences lie close to the bounding faults, while Cu occurrences are distributed across the width of the Batten Trough, supporting a more or less syngenetic origin for the former and a much later, post inversion, epigenetic origin, for the latter. Mineralisation thus occurred in association with inversion structures formed above the primary half grabens that were generally concealed by later sedimentation. Because the half grabens occur at regularly spaced intervals along the Batten Trough there may well be more mineral occurrences similar to the McArthur River deposit. Clearly, these conclusions must be integrated into any exploration strategy.     

C and ESR dating of calcrete and gypcrete cores from the Amadeus Basin, Northern Territory, Australia

More than forty calcrete and gypcrete samples from bore holes in the Curtin Springs area of the Amadeus Basin, Central Australia, have been dated by means of the ¹⁴C and ESR techniques. The results indicate that the application of multiple dating techniques to samples from calcrete and gypcrete deposits enables age limits to be expanded and the number of datable material types to be increased with respect to Quaternary studies. The ability of the ¹⁴C and ESR techniques to determine the approximate age of calcrete and gypcrete deposits has been demonstrated.     

Metal source of gold deposits in the Jiaoxibei area,Eastern China: Insights from trace element analysis of gold and pyrite

The metal source of gold deposits in the Jiaoxibei area, eastern China, has been investigated by many researchers, but no consensus has been reached so far. In this study, three typical gold deposits, the Xinli, Jiaojia, and Dayingezhuang deposits, were selected for trace element analysis of gold and pyrite to constrain the metal source. Pyrite from these three deposits has similar morphological and compositional characteristics, and can be divided into three types: Py1 with euhedral to subhedral textures, Py2 with subhedral to anhedral textures with micro-fractures, and Py3 with subhedral to anhedral textures and intergrowing polymetallic sulfides. Among them, Py2 and Py3 were formed in the main ore-forming stage and they are the dominant host minerals of visible gold. In these deposits, visible gold occurs mainly in micro-fractures or as inclusions in Py2 and Py3. Most of the pyrite has extremely low concentration of invisible gold, indicating that visible gold in the Jiaoxibei district is not a product of the remobilization of invisible gold from earlier pyrite. Both Py2 and Py3 are characterized by low Co concentration of <100 ppm and Co/Ni ratio of <1, which are similar to those of pyrite in sedimentary rocks. Therefore, ore-forming metals of these gold deposits in the Jiaoxibei district may originate mainly from a sediment-related metal source.     

Daly Basin,Northern Territory: lithostratigraphic revision resolves context of incongruous Ordovician fossils

A local succession of interbedded dolostone, limestone and glauconitic sandstone in the central Daly Basin of the Northern Territory, dated as Early Ordovician, has for many years appeared incongruous in terms of lithology and age relative to mapped formations of the Cambrian Daly River Group. Geological mapping and stratigraphic drilling have now shown that this interval, recently named the Florina Formation and described here, unconformably overlies the karstified surface of the uppermost formation of the Daly River Group, the Oolloo Dolostone. It is the youngest formation of the Daly Basin succession, but due to the lengthy hiatus between it and the underlying units, it is not included within the Daly River Group. It comprises three intervals of carbonate rocks alternating with thicker intervals of siliciclastic rocks. The latter are viewed as forming under dominantly moderate energy, shallow marine conditions with sediments derived from a distant terrigenous source. The carbonate rocks were dominantly subtidal, too far offshore to receive significant amounts of terrigenous material but shallow enough to be above storm wave base. The Oolloo Dolostone is formally divided here into two members, the lower Briggs Member and the upper King Member. Both consist largely of dolostone, but the Briggs Member is typically well bedded, contains ooids and has a minor component of quartz sand. It accumulated mainly as ooid shoals seaward of tidal flats. The overlying King Member is massive to coarsely bedded with only traces of terrigenous sediment and was deposited in deeper water seaward of the Briggs Member.     

LA-ICP-MS trace element analysis of pyrite from the Xiaoqinling gold district, China: Implications for ore genesis

The Xiaoqinling district, the second largest gold producing district in China, is located on the southern margin of the North China Craton. It consists of three ore belts, namely, the northern ore belt, the middle ore belt and the southern ore belt. Pyrite from the Dahu gold deposit in the northern ore belt and Wenyu and Yinxin gold deposits in the southern ore belt were investigated using a combination of ore microscopy and in-situ laser-ablation inductively-coupled plasma-mass spectrometry (LA-ICP-MS). A range of trace elements was analyzed, including Au, Te, Ag, Pb, Bi, Cu, Co, Ni, Zn, Mo, Hg, As and Si. The results show that there are no systematic differences between the trace element compositions of pyrite in the three deposits from different ore belts. In general, Au concentrations in pyrite are low (from < 0.01 ppm to 2.2 ppm) but Ni concentrations are rather high (up to 8425 ppm). A four-stage mineralization process is indicated by microscopic and field observations and this can be related to the systematic trace element differences between distinct generations of pyrite. Stage I precedes the main gold mineralization stage; pyrite of this stage has the lowest Au concentrations. Stages II and III contributed most of the gold to the ore-forming system. The corresponding pyrite yielded the highest concentrations of Au and Ni. Our microscopic observations suggest that pyrite in the main gold mineralization stage precipitated simultaneously with molybdenite that has been previously dated as Indosinian (~ 218 Ma by Re–Os molybdenite dating), indicating the Indosinian as the main gold mineralization stage. The Indosinian mineralization age and the geological and geochemical features of these gold deposits (e.g., low salinity, CO₂-rich ore fluids; spatial association with large-scale compressional structures of the Qinling orogen; δ¹⁸O and δD data suggestive of mixing between metamorphic and meteoric waters; δ³⁴S and Pb-isotopic data that point to a mixed crustal-mantle source) all point to typical orogenic-type gold deposits. High Ni concentrations (up to 8425 ppm) of pyrite possibly linked to deep-seated mafic/ultramafic metamorphic rocks provide further evidence on the orogenic gold deposit affinity, but against the model of a granitic derivation of the mineralizing fluid as previously suggested by some workers. Generally low Au concentration in pyrite is also consistent with those from worldwide orogenic gold deposits. Therefore, the gold mineralization in the Xiaoqinling district is described as orogenic type, and is probably related to Indosinian collision between the North China Craton and the Yangtze Craton.     

A case study of the application of a microcomputer-based favorability evaluation system: Uranium exploration in the Alligator Rivers Uranium Field, Northern Territory, Australia

We have attempted to develop an expert system to evaluate the favorability of uranium exploration projects using the Data-directed Numerical Method (DdM). We developed the system for the Macintosh personal computer, so as to facilitate the iterative processes of trial and error inherent in the DdM. In a detailed case study, we use the above system to evaluate the favorability of the Alligator Rivers Uranium Field, Northern Territory, Australia. The favorability of the target area is evaluated by considering several exploration parameters related to the geological factors controlling ore formation. The selection of appropriate exploration parameters is important to the success of the method. We then selected 15 model cells to validate each exploration parameter and also to design the exploration model by the application of the chi-square test. The result of the favorability evaluation, as displayed by the developed system, indicates that our designed exploration model is acceptable. By employing the above processes we conclude that the system using the DdM is applicable as an exploration tool and is useful in establishing an exploration model.     

A detailed fluid inclusion study in silicified breccias from the Kombolgie sandstones (Northern Territory, Australia): inferences for the genesis of middle-Proterozoic unconformity-type uranium deposits

The relative chronology and detailed chemistry of paleofluids circulating at the base of the Kombolgie Sub-basin were investigated in the East Alligator River district (Northern Territory, Australia), where world-class unconformity-type uranium deposits are located. The chemistry of fluid inclusions was determined using in-situ analysis (Raman microprobe and laser-induced breakdown spectroscopy [LIBS]) and by observing the melting sequences by microthermometry. This study revealed the occurrence of three distinct fluids: (i) a sodium-rich brine that corresponds to a diagenetic fluid percolating at the bottom of the Kombolgie sandstones at a temperature close to 150±15 °C; (ii) a calcium-rich brine, probably corresponding to a residual brine in evaporitic environment that has evolved by fluid–rock interactions with the basement lithologies; and (iii) a low salinity fluid, heated in the basement, injected into the base of the sandstone cover. H₂ and O₂ and/or traces of CH₄ were detected in the vapor phase of some fluid inclusions, especially in the low salinity ones in quartz breccia samples taken above mineralized areas. Hydraulic brecciation of the sandstone was associated with a pressure decrease favoring fluid mixing and the subsequent cementation of breccias. According to the fluid inclusion study and other geologic constrains, the minimum thickness of the Sub-Kombolgie Basin is estimated at 4 km. Drusy quartz breccias with evidence of fluid mixing are quite common at the base of the Kombolgie Basin, but not necessarily linked to U-mineralization. However, it is proposed that the presence of gases such as H₂ and O₂ in fluid inclusions, which results from water radiolysis, constitutes an indicator of gas linked to significant U concentrations deeper in the basement rocks.     

Trace element chemistry of pyrite: A useful guide to the occurrence of sulfide base metal mineralization

Trace element contents for pyrite from a range of sulfide mineral occurrences in the Kangiara region, eastern Australia, illustrate two main groups of pyrite. The first group, with higher Ag, Cu, Pb and Mo contents, corresponds to samples from sulfide base metal deposits and the second group, with higher Mn, Ti and Ni contents, contains samples from skarn mineralization, volcanic rocks and quartz veins. The model proposed for the development of pyrite in the Kangiara region is that the first group was formed from base metal-bearing solutions, while the second group reflects diagenetic pyrite and metamorphic pyrite. Thus, the pyrite trace element chemistry may provide a means of distinguishing types of mineral occurrences, in particular, those containing significant base metal mineralization.     

Assessment of recharge to groundwater systems in the arid southwestern part of Northern Territory, Australia, using chlorine-36

 The sustainability of community water supplies drawn from shallow aquifers in the arid southwest of the Northern Territory has been evaluated using the radioactive isotope chlorine-36 (³⁶Cl). These aquifers include fractured sandstones of the Ngalia Basin, fractured metamorphic rocks and Cainozoic sands and gravels. ³⁶Cl/Cl ratios for these shallow, regional groundwaters exhibit a bimodal distribution with peaks at 205 (±7) and 170 (±7)×10^–15. The higher ratio probably represents modern (Holocene) recharge, diluted with windblown salts from local playa lakes, and occurs mostly around the margin of the basin. The lower ratio corresponds to a ³⁶Cl "age", or mean residence time, of 80–100 ka, implying that the last major recharge occurred during the last interglacial interval (Oxygen Isotope Stage 5). These values are mainly observed in the interior of the Ngalia Basin. Lower values of the ³⁶Cl/Cl ratio measured near playa lakes are affected by addition of chloride from remobilised salts. Finite carbon-14 (¹⁴C) data for the groundwaters are at variance with the ³⁶Cl results, but a depth profile suggests low recharge, allowing diffusion of recent atmospheric carbon to the water table. The ³⁶Cl results have important implications for groundwater management in this region, with substantial recharge only occurring during favourable, wet, interglacial climatic regimes; most community water supplies are dependent on these "old" waters. Received, September 1997 · Revised, August 1998, March 1999 · Accepted, March 1999     

Geochemical assessment of an analogue site for an engineered landform at Ranger Uranium Mine, Northern Territory, Australia

 An essential element in identifying sites as analogues for the long-term development of soils and vegetation on engineered landforms of the rehabilitated Ranger Uranium Mine, Northern Territory, Australia, is the need to match rocks. Comparison of the geochemistry of rocks from the waste rock dump of Ranger Uranium Mine and the potential analogue site of Tin Camp Creek area, Western Arnhem Land, indicates that there are several sites along Tin Camp Creek that may be used as analogue sites. Detailed comparisons between the Ranger and Tin Camp Creek lithologies have been undertaken using a variety of techniques, including cluster analysis. It is demonstrated that at least 70% of the rocks being mined at Ranger have analogues in the Tin Camp Creek area. Received: 2 October 1996 · Accepted: 4 November 1997     

Partial least squares-discriminant analysis of trace element compositions of magnetite from various VMS deposit subtypes: Application to mineral exploration

The petrography and mineral chemistry of magnetite from fifteen volcanogenic massive sulfide (VMS) deposits in Canada, and the Lasail VMS deposit in Oman, as well as from two VMS-associated banded iron formations (BIF), Austin Brook (New Brunswick, Canada) and Izok Lake (Nunavut, Canada), were investigated using optical microscopy, electron probe micro-analyzer (EPMA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The method of robust estimation for compositional data (robCompositions) was applied to investigate geochemical censored data. Among thirty-seven elements analyzed by EPMA and/or LA-ICP-MS in magnetite from the studied deposits/bedrock lithologies, only the results for Si, Ca, Zr, Al, Mg, Ti, Zn, Co and Ni contain < 40% censored values, and thus could be imputed using robCompositions. Imputed censored data were transformed using centered log-ratios to overcome the closure effect on compositional data. Transformed data were classified by partial least squares-discriminant analysis (PLS-DA) to identify different compositional characteristics of magnetite from VMS deposits and BIFs. The integration of petrography and mineral chemistry identifies three types of magnetite in VMS settings: magmatic, hydrothermal, and metamorphic. Magmatic magnetite in VMS deposit host bedrocks is characterized by ilmenite exsolution and may be overprinted by metamorphism. Some VMS deposits contain hydrothermal magnetite, which is intergrown with sulfides, and shows a metamorphic overprint as it is partly replaced by common metamorphic minerals including chlorite, sericite, anthophyllite, and/or actinolite, whereas the majority of the deposits are characterized by metamorphic magnetite formed by replacing pre-existing sulfides and/or silicates, and is intergrown with metamorphic minerals. Among VMS deposits of the Noranda mining district, the West Ansil deposit is characterized by hydrothermal-metamorphic magnetite zoned by inclusion-free cores and Si- and Mg-rich rims. Magnetite from the studied VMS-associated BIFs is also metamorphic in origin. Aluminum, Ti and Zn contents of magnetite can separate BIF from the other mineralized and un-mineralized bedrock lithologies in the studied VMS settings.PLS-DA shows that variable compositions of magnetite slightly discriminate different studied deposits/bedrock lithologies. The geochemical observations suggest that the variation in magnetite chemistry from different VMS settings might be sourced from differences in: 1) the composition and temperature of parental magmas or hydrothermal fluids, 2) the composition of host bedrocks, 3) the composition of co-forming minerals, and 4) oxygen fugacity. PLS-DA distinguishes magnetite compositions from the studied VMS deposits and BIFs from that of the other ore deposit types including Ni–Cu, porphyry Cu-Mo-Au, iron oxide-copper- gold, iron oxide-apatite, and the Bayan Obo REE-Fe-Nb deposit. Magnetite from the VMS settings on average contains lower concentrations of Si, Zr, Al, Mg, Ti, Zn, Co and Ni relative to that from the other mineral deposit types. PLS-DA of magnetite data from VMS deposits and BIFs of the Bathurst mining camp as well as PLS-DA of magnetite compositions from various mineral deposit types yield discrimination models for application to mineral exploration for VMS deposits using indicator minerals in Quaternary lithified sedimentary rocks.     

Geology of the Ranger Uranium Mine, Northern Territory, Australia: structural constraints on the timing of uranium emplacement

The geology of the No 1 and 3 pits at the Ranger Mine in the Pine Creek Inlier (PCI) of Australia is dominated by Palaeoproterozoic volcanic, carbonate and sedimentary sequences that unconformably overlie Archaean granitic gneiss of the Nanambu Complex (2470±50 Ma). These sequences are folded, faulted and sheared, and crosscut by east-trending granite (sensu stricto) dykes and pegmatite veins, and gently dipping N–NE trending mafic dykes of the Oenpelli Dolerite (1690 Ma). Regional metamorphism is to greenschist facies and contact metamorphism is to hornblende-hornfels facies.The rocks of the Ranger Mine have been subjected to at least two phases of ductile–brittle deformation (D2–D3) and one phase of brittle deformation (D4). These events were preceded by regional diastathermal or extension-related metamorphism (D1) and the development of an ubiquitous bedding-parallel cleavage (S1).D2 resulted in the development of NNE–NNW trending mesoscopic folds (F2) and a network of thrusts and dextral reverse shears. The modelled palaeo-stress directions for the emplacement of pegmatite veins suggests that they formed early in D2. D3 resulted in the development of WNW–NW trending mesoscopic folds (F3), a weakly defined axial planar cleavage (S3) and sinistral reactivation of D2 shears. D2–D3 are correlated with deformation during the Maud Creek Event of the Top End Orogeny (1870–1780 Ma), while the emplacement of granite dykes and pegmatite veins is correlated with emplacement of regional granites at 1870–1860 Ma.D4 is associated with brittle deformation and resulted in the development of normal faults and fault breccias during a period of east–west extension. This event is correlated with regional east–west extension during deposition of Palaeo- to Mesoproterozoic platform sequences.The sequence of tectonic events established in this study indicates that uranium-bearing ore shoots in the Ranger No 1 and 3 pits formed during extension in D4, and after emplacement of the Oenpelli Dolerite at 1690 Ma. However, the currently accepted 1737±20 U–Pb Ma age places the mineralising event at time of regional post-orogenic erosion, after the Top End Orogeny and before emplacement of the Oenpelli Dolerite and extension in D4. The U–Pb age is not consistent with Sm–Nd ages for primary uranium mineralisation at Nabarlek and Jabiluka at 1650 Ma [Econ. Geol. 84 (1989) 64] and does not concur with currently accepted regional tectonic data of Johnston [Johnston, J.D., 1984. Structural evolution of the Pine Creek Inlier and mineralisation therein, Northern Territory, Australia. Unpublished PhD Thesis, Monash University, Australia], Needham et al. [Precambrian Res. 40/41 (1988) 543] and others. Consequently, the absolute age of uranium mineralisation at the Ranger Mine is open.     

Pathways and distances of fluid flow during low-grade metamorphism: evidence from pyrite deposits of the Cameros Basin, Spain

New geochemical and sulphur isotopic data are presented for a number of pyrite deposits from the Late Jurassic–Early Cretaceous Cameros Basin, Spain. The deposits were formed at, or close to, the peak of metamorphism and are always related to sandstone units in the mainly metapelite sequence. Iron remained immobile and conservative, pyrite iron being derived by sulphidation of chlorite in the host metapelites. Reduced sulphur, however, was supplied from two external sources: thermochemical reduction of sulphate and release of S during metamorphism of sedimentary sulphides. These sources provided isotopically heavy and light S, respectively, with variation in pyrite isotopic composition between different deposits resulting from differences in their relative importance at each site. During metamorphism, the sandstone units acted as aquifers, carrying the sulphidic pore waters to locations where permeability provided by syn-depositional fractures on a scale of 0.5–5  m allowed its interaction with the metapelites. Transport distances for sulphide during metamorphism were of the order of hundreds of metres.