Textures and U-W-Sn-Mo signatures in hematite from the Olympic Dam Cu-U-Au-Ag deposit,South Australia: Defining the archetype for IOCG deposits

The textural characteristics and trace element geochemistry of hematite with U-W-Sn-Mo signatures from the Cu-U-Au-Ag orebody at Olympic Dam, South Australia, are documented. Olympic Dam is the archetype for iron-oxide copper–gold (IOCG) deposits where hematite is by far the most abundant mineral in the orebody. The deposit is located within hematite-bearing breccias (>5% Fe) hosted by the ∼1.6 Ga Roxby Downs Granite (RDG). Although such breccias are mostly derived from RDG, they also include volcanic clasts and sedimentary rocks. Samples cover the ∼6 km strike length and ∼2 km vertical extent of mineralisation, including hematite from the aforementioned lithologies. Hematite with U-W-Sn-Mo (‘granitophile’ elements) signatures is recognised throughout all lithologies and parts of the deposit. Hematite enriched in granitophile elements is represented by a variety of textures, of which zoned hematite, defined by oscillatory zonation patterns, is the most prominent and can be tied to the age of the RDG, and thus initiation of the IOCG system as confirmed by published U-Pb geochronology. Other categories of hematite with granitophile signatures include hematite resulting from replacement of pre-existing minerals (e.g., carbonates and feldspars), as well as replacement of previous oscillatory-zoned hematites. Matrix and vacuole filling hematite from volcanoclastic-dominated intervals also carry ‘granitophile’ signatures. In addition, some colloform types which likely post-date primary IOCG mineralisation are also rich in ‘granitophile’ elements. Trace element mapping and spot analysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) defines complex trace element signatures of hematite, which, in addition to the ‘granitophile’ elements, also comprise rare earth elements, high field strength elements, chalcogens and transition metals.The distinct geochemical signature, characterised by enrichment in the ‘granitophile’ elements (up to wt% levels of U and W within individual zones, and up to thousands of ppm Mo and Sn) prevails throughout the hematite in the deposit irrespective of textures. Iron-oxides have been repeatedly formed, reworked and overprinted by subsequent cycles of brecciation, fluid-mineral reaction, remobilization, element redistribution and recrystallisation. Coupled dissolution-replacement reactions are discussed as having played a major role in the modification of textural and geochemical patterns in hematite, but also allow for widespread preservation of primary geochemical signatures. Despite its simple chemistry, the crystal-structural modularity of hematite can adapt and retain evolving fluid signatures. The reported trace element signatures are fully concordant with conceptual frameworks for the genesis of IOCG systems, and may be an inherent, albeit hitherto under-reported characteristic of other IOCG systems. Hematite is probably by far the most important W-, Sn- and Mo-bearing phase in the deposit by mass.     

Ore minerals down to the nanoscale: Cu-(Fe)-sulphides from the iron oxide copper gold deposit at Olympic Dam,South Australia

Cu-Fe-sulphide mineral assemblages from the Olympic Dam (OD) Fe-oxide Cu-U-Au-Ag deposit, South Australia, are studied down to the nanoscale to explore the potential these minerals have for understanding genetic processes such as primary deposit zonation. Cu-Fe-sulphide pairs: ‘brown’ bornite associated with chalcopyrite (bornite-chalcopyrite zone); and symplectites of ‘purple’ bornite with species from the chalcocite group, Cu_2 − xS (bornite-chalcocite zone), co-define an upwards and inwards deposit-scale zonation at OD. In the bornite-chalcocite zone, there is also an increase in the proportion of chalcocite relative to bornite within the symplectites towards upper levels. In this case, two-phase Cu_2 − xS assemblages are also present, as anisotropic, hexagonal chalcocite (CcH) with lamellar exsolutions of digenite, distinguishable at the μm-scale. Using compositional data (electron microprobe) combined with Transmission Electron Microscopy (TEM) study of foils prepared in–situ via Focused Ion Beam (FIB)-SEM, we show that Cu-Fe-sulphides from different ore zones feature nanoscale intergrowths, lattice defects, superstructure domains (na) and antiphase boundary domains (APBs) that can be interpreted as due to exsolution, coarsening and phase transformation during cooling from high-T solid solutions in the system Cu-Fe-S and sub-systems according to published phase diagrams. ‘Brown’ bornite [(Cu + Fe)/S > 5] contains pervasive lamellae of chalcopyrite which extend down to the nanoscale; such specimens appear homogeneous at the μm-scale. ‘Purple bornite’ [(Cu + Fe)/S < 5] in high-bornite symplectites is associated with chalcocite that shows APBs with 6a digenite and low-T chalcocite. Comparable APBs are also found in the ‘chalcocite’ zone with apparent homogeneity at the μm-scale. Both bornites contain exsolutions of djurleite. Systematic variation of Me/S and Cu/Fe in the two types of bornite points, however, to distinct origins from different bornite solid-solutions in the system Cu-Fe-S. Both show 2a and 4a intermediate superstructures. High-order superstructures (6a and incommensurate na) are restricted to the ‘purple’ bornite whereas the 2a4a low-T superstructure is found in both cases. Me/S ratios in the chalcocite group are variable; lower ratios (down to 1.8; digenite) are more common in chalcocite from symplectites with ‘purple’ bornite. Me/S can be as low as 1.4 where associated with ‘blue’ varieties (‘blaubleibender covellin’) of replacement origin. The two-phase Cu_2 − xS associations contain hexagonal chalcocite (Me/S = 1.95), lamellae of Cu-rich digenite (Me/S = 1.92), and anilite (Cu₇S₄) as nm-scale lamellae. Digenite shows 3a and 6a superstructures and CcH shows transition to pseudo-orthorhombic chalcocite. The presence of superstructures, high-T species and APBs is evidence for Cu-(Fe)-sulphide formation from high-T solid solutions at T > 300 °C (high-T phases, Cu-poor digenite), followed by cooling along distinct paths down to < 120 °C (APBs). The scenario of ‘exsolution from primary solid-solution’, corroborated by the consistency in phase relations within each zone across different scales of observation from deposit scale to nanoscale, backs up a model of primary hypogene ore precipitation rather than replacement, and accounts for the observed vertical zoning at OD. The FIB-TEM approach here is readily applicable to other deposits and shows that nanoscale observations are a valuable, although often overlooked, source of information to constrain ore genesis.     

The mineral chemistry,near-infrared,and mid-infrared reflectance spectroscopy of phengite from the Olympic Dam IOCG deposit,South Australia

Phengite is the main potassic dioctahedral mica identified at the Olympic Dam iron oxide–copper–gold (IOCG) deposit, South Australia, where its mineral chemistry is quite variable. These differences can be explained by contrasting degrees of hydrothermal alteration. In the heavily-sericitized, ore-bearing rocks, the phengites display a lower-Si content, a higher-Al content, and a lower Mg-number than the phengites from the weakly-sericitized alteration halo that surrounds the deposit. Variations are also observed in the near- and mid-infrared reflectance spectra collected from phengite-bearing rocks. In the near-infrared, high-Al phengite produces a spectral absorption feature at 2.206 μm, and this feature is displaced to 2.213 μm for low-Al phengite. In the mid-infrared, high-Al phengite produces a strong reflectance peak at 9.59 μm, whereas this peak is observed at 9.57 μm in the spectra from low-Al phengite. Additional peaks were also identified at 10.98, 12.22, and 13.33 μm. These were most intense in the spectra from high-Al phengite. A drill core profile was produced using the results of the spectral analysis that shows the change in phengite mineral chemistry and phengite abundance as a function of depth. In general, near- and mid-infrared reflectance spectroscopy can be used to characterize the aluminum content of potassic dioctahedral micas like phengite, and this information can be used to infer the degree of sericitic alteration that has occurred as a result of hydrothermal fluid flow.     

论安徽省姑山铁矿床的热液成矿特征

姑山铁矿床产在燕山期辉石闪长岩与中三叠统黄马青组砂页岩的接触带上。矿石具有典型的斑状、球颗状和骨架状结构。镜下研究表明,这些结构不是矿浆结晶的产物,而是热液成因微晶赤铁矿在后期地质过程中发生变晶生长所致。矿床中的蚀变和矿化自下而上呈现规律性的分带。碳酸盐化是特征性的蚀变类型,其形成与成矿密切有关,并可作为一种近矿找矿标志。在热液成矿过程中,辉石闪长岩中的铁很可能以羰基络合物的形式发生活化转移。在热液演化过程中由于氧逸度的升高和碱度、压力的降低,羰基络合物发生分解而使铁在接触带附近沉淀成矿。     

A sulphur isotope study of the Broken Hill deposit,New South Wales,Australia

Sulphur isotopic compositions of sulphides within garnet-rich rocks and high-grade ore from the Broken Hill deposit, New South Wales, Australia, have been determined and show a range of values of –3.3 to +6.7 per mil. Thermochemical considerations, including the spread of values of ³⁴S, suggest that the deposit was derived from a mixed source of sulphur in which seawater, reduced by inorganic processes, mixed with magmatic sulphur or that sulphate from contemporaneous seawater was reduced biogenically at low temperatures. Thermochemical considerations also suggest that pyrrhotite formed by desulphidation of pyrite so that the original Fe-S-O assemblage was pyrite ± magnetite.³⁴S measurements show a broad range which is considered to be due to isotopic reequilibration during retrograde metamorphism and analytical and sampling technique. These data should not be used to indicate original temperatures of deposition or metamorphic temperatures associated with the various metamorphic events.     

The pilot knob magnetite deposit in the Proterozoic St. Francois Mountains Terrane,southeast Missouri,USA: A magmatic and hydrothermal replacement iron deposit

The Pilot Knob magnetite deposit is located in southeast Missouri within the 1380–1480 Ma St. Francois Mountains terrane rhyolitic/trachytic volcanic rocks. The deposit is tabular, dips about 45°, and is sill-like in nature, being approximately parallel to the bedding in the host tuffs. The deposit was uncovered by erosion and exposed to weathering in the late Proterozoic, and is overlain in angular unconformity by the Cambrian Lamotte Sandstone. This Proterozoic weathering cycle apparently had little effect on the deposit with only the updip edge being converted to hematite. Textural and mineralogical features of the deposit suggest a combined magmatic and hydrothermal replacement origin. The magnetite-rich ores that make up the bulk of the deposit are interpreted as having crystallized from an iron-rich magma, and a surrounding envelope of lower- to moderate-grade ores where magnetite has clearly replaced the tuffaceous host rocks are interpreted as hydrothermal in origin. After the development of the higher-grade magnetite ores and the envelope of lower- to moderate-grade ores, late hydrothermal minerals were deposited as cross-cutting veins and breccia fill. The two most abundant minerals in the higher-grade portions of the deposit are magnetite and albitic plagioclase, and petrologically the higher-grade ores could be described as a magnetite sodic syenite. The most abundant gangue mineral within the lower-grade impregnated envelope of ores around the higher-grade ores is K-feldspar, apparently relict from the rhyolites/trachytes. Thin lenses within the higher-grade ores contain calcite as a matrix mineral to the magnetite and are considered to indicate carbonatitic affinity. Rare earth elements are elevated in one of five whole rock chemical analyses of the ores and the REE-bearing mineral ferriallanite has been identified. Minor portions of the deposit, below the Proterozoic weathering cap, consist of high-grade hematite ores having equilibrium textures. At depth, the deposit is intruded by the Shepherd Mountain gabbro, a 120 m thick, near-horizontal dike, which resulted in minor contact metamorphism of the ore.     

An intrusion-related origin for Cu–Au mineralization in iron oxide–copper–gold (IOCG) provinces

Major Cu–Au deposits of iron oxide–copper–gold (IOCG) style are temporally associated with oxidized, potassic granitoids similar to those linked to major porphyry Cu–Au deposits. Stable and radiogenic isotope evidence indicates fluids and ore components were likely sourced from the intrusions. IOCG deposits form over a range of crustal levels because CO₂-rich fluids separate from the magmas at higher pressures than in CO₂-poor systems, thereby, promoting partitioning of H₂O, Cl and metals to the fluid phase. At deep levels, the magma–fluid system cannot generate sufficient mechanical energy to fracture the host rocks as in porphyry systems and the IOCG deposits therefore form in a variety of fault-related structural traps where the magmatic fluids may mix with other fluids to promote ore formation. At shallow levels, the IOCG deposits form breccia and fracture-hosted mineralization styles similar to the hydrothermal intrusive breccias and sulphide vein systems that characterize many porphyry Cu–Au deposits. The fluids associated with IOCG deposits are typically H₂O–CO₂–salt fluids that evolve by unmixing of the carbonic phase and by mixing with fluids from other sources. In contrast, fluids in porphyry systems typically evolve by boiling of moderate salinity fluid to produce high salinity brine and a vapor phase commonly with input of externally derived fluids. These different fluid compositions and mechanisms of evolution lead to different alteration types and parageneses in porphyry and IOCG deposits. Porphyry Cu–Au deposits typically evolve through potassic, sericitic and (intermediate and/or advanced) argillic stages, while IOCG deposits typically evolve through sodic(–calcic), potassic and carbonate-rich stages, and at deeper levels, generally lack sericitic and argillic alteration. The common association of porphyry and IOCG Cu–Au deposits with potassic, oxidized intermediate to felsic granitoids, together with their contrasting fluid compositions, alteration styles and parageneses suggest that they should be considered as part of the broad family of intrusion-related systems but that they are typically not directly related to each other.     

A fluid inclusion study of the Tourmaline Hill Granite,Umberatana, South Australia: Implications for hydrothermal activity and wallrock metasomatism

Summary Homogenization temperatures and salinity data are documented for fluid inclusions present in quartz from a Palaeozoic alkaline to peralkaline granite intrusion. A wide range of salinities (3.8 to 60.6 wt% NaCl equivalent) and homogenization temperatures (25.8 to 537 °C) indicates a sequential entrapment of different fluids, whereby the detected salinities decreased as a direct result of decreasing homogenization temperatures. Fluids were saturated with NaCl at 537°C and the development of hydrothermal solutions occurred at 1 100 ± 300 bars. Hydrothermal activity and the release of volatiles, F and B, during granite crystallization resulted in phlogopitization and tourmalinization of intruded metapelites. Volatile degassing, the presence of hypersaline hydrothermal fluids and contamination on the periphery of the intrusives by aluminous sediments led to the transport of K, Rb, Li, Mg, Cs, Be and Ta via halogen complexes from the granite system into the metapelitic aureole.

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Eine Untersuchung von Flüssigkeitseinschlüssen am Tourmaline Hill Granite, Umberatana, Südaustralien: Hinweise auf hydrothermale Aktivität und Nebengesteinsmetasomatose

Zusammenfassung Homogenisationstemperaturen und Salinitätsdaten von Flüssigkeitseinschlüssen in einer paläozoischen alkalinen-peralkalinen Granitintrusion sind dokumentiert. Eine weite Spanne von Salinitätsdaten (3.8 bis 60.6 Gewichts% NaCl äquivalent) und Homogenisationstemperaturen (25.8 bis 537 °C) deutet einen aufeinanderfolgenden Einschluß von fluiden Phasen an, wobei die Salinitäten mit verringerten Homogenisationstemperaturen abnahmen. Die Fluida waren bei 537°C mit NaCl gesättigt; hydrothermale Aktivität und der Verlust von Gasen, F und B, während der Granitkristallisation verursachte Phlogopitisierung und Turmalinisierung der intrudierten Metapelite. Entgasung, hochsaline hydrothermale Lösungen und Kontamination am Rande der Intrusionen durch Al-reiche Sedimente führte zum Transport von K, Rb, Li, Mg, Cs, Be und Ta als Halogen-Komplexe von der Granitintrusion in die metapelitische Kontaktaureole.

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With 4 Figures and one Plate     

Synchronous deformation and hydrothermal activity in the shear zone hosted high-sulphidation Au-Cu deposit at Peak Hill, NSW, Australia

Altered and mineralised rocks at Peak Hill, are confined to a 300–500 m wide, north-south striking, steeply dipping, shear zone that is flanked by the Mingelo Volcanics along its western side, and Cotton Formation siltstones along its eastern side. This shear zone is defined by extensive zones of cataclasite and strongly foliated micaceous schists in marked contrast to the largely undeformed nature of the adjacent rocks. Advanced argillic assemblages (quartz-kaolinite-pyrite ± alunite ± illite) occur throughout the core of the Peak Hill deposit. Propylitic assemblages, including albite, quartz, interlayered chlorite-smectite, illite and ankerite, and a narrow discontinuous zone of argillic (quartz-illite-pyrite) alteration are developed in the Mingelo Volcanics along the western side of the deposit. Propylitic, argillic and advanced argillic assemblages are overprinted by an internally zoned phase of phyllosilicate alteration that grades inwards from a peripheral sericite-clay-chlorite assemblage, through phyllic assemblages (muscovite/illite-pyrite ± paragonite) to a pyrophyllite-pyrite ± diaspore ± andalusite altered core. Au-Cu mineralisation is hosted by barite-pyrite veins that cut the advanced argillic assemblage, but pre-date the phyllosilicate-dominated alteration. Native Au (lacking Ag), calaverite, Te-rich tennantite-tetrahedrite (goldfieldite), chalcopyrite, covellite and chalcocite occur in the barite-pyrite veins. No ore-bearing minerals were detected in any of the alteration assemblages. The total gold content of the Peak Hill deposit is currently 720 K ounces and this includes 100 K ounces of unmined reserves. Within the shear zone phyllosilicate minerals are developed in strain shadows and partly define the stretching lineation associated with dip-slip movement. The zonation within the phyllosilicate assemblages mimics the geometry of bends in the shear zone and minor internal structures. These textures indicate that the phyllosilicate alteration developed synchronous with movement on the shear zone. Earlier advanced argillic alteration and mineralisation are developed in rocks derived from both sides of the shear zone. Hydrothermal activity associated with the earlier advanced argillic alteration was therefore either synchronous with juxtaposition of these distinct rock units, or occurred during a later phase of movement on the shear zone. Cross-cutting fibrous textures in the auriferous barite-pyrite veins indicate that repeated fracturing of the advanced argillic altered rocks accompanied development of successive generations of auriferous veins. Concentrations of auriferous veins are localised in steeply plunging shoots that are oriented parallel to the stretching lineation in the shear zone. These features all indicate movement on the host shear zone accompanied each phase of hydrothermal activity in the Peak Hill deposit. The location, alteration zonation and distribution of mineralised veins within the deposit are intimately controlled by deformation on the host shear zone synchronous with hydrothermal activity. The development of high-sulphidation hydrothermal systems synchronous with deformation along brittle-ductile shear zones is a predictable consequence of intrusive activity during deformation in areas characterised by a high geothermal gradient. The close relationship between tectonism and hydrothermal activity indicates that these deposits are likely to be located in the vicinity of regional-scale shear zones. Deposits are likely to be aligned parallel to the regional-scale structural “grain” and restricted to areas of conspicuous deformation as is the case at Peak Hill (and Temora, NSW). Aluminous alteration zones concentrated in the vicinity of regional-scale structures in the Carolina Slate Belt may be a further example of this style of hydrothermal activity. Received: 30 September 1996 / Accepted: 28 August 1997     

福建省紫金山铜金矿床原生晕地球化学特征及深部找矿前景

通过系统采集紫金山铜金矿床4~11线共41个钻孔的岩矿样,绘制原生晕剖面图,建立垂向元素分带序列、矿体剥蚀程度准则,结合深部指示元素的特征,进行隐伏矿预测。该矿床金矿近矿晕为Au-Ag-Zn1,尾部晕为BiGa-Mo-Sn-Ti-Co-Ni-W1-V;铜矿前缘晕为Hg-Sb-As,近矿晕为Cu-Pb1-Zn2,尾部晕为Be-W2。矿体剥蚀程度评价表明,高硫型铜矿体往深部已尖灭。深部F-Mn-Pb2-Zn3的异常形态和元素组合符合典型斑岩矿床的外带特征,斑岩体延伸至矿床周边;深部的蚀变矿物组合、金属矿物组合、流体包裹体特征等均表明深部可能存在斑岩铜(钼)矿床。     

广东大降坪硫(铅锌)矿床岩浆热液叠加作用——来自闪锌矿Rb-Sr年龄及硫同位素的证据

广东大降坪硫(铅锌)矿床位于与岩浆作用有关的大绀山多金属矿田的中部,主矿体为赋存在震旦纪变质岩中的层状、透镜状黄铁矿矿体,最近在两个不同产状的黄铁矿矿体下部又新发现了脉状及层状铅锌矿体。文章通过对铅锌矿体的年龄及硫同位素研究,探讨其与主矿体的成因关系,获得了脉状铅锌矿体中闪锌矿的Rb-Sr等时线年龄为(88.5±3.9)Ma,即晚白垩世,与上部黄铁矿矿体的年龄(约630 Ma)相差较大,而与整个大绀山多金属矿田的成矿作用时限一致。三种不同产状的矿体硫同位素组成差异明显:层状黄铁矿矿体富集硫的轻同位素(δ34S=-10.90‰~-25.55‰),且与围岩的硫同位素范围一致,说明硫来自生物的细菌还原硫;透镜状黄铁矿矿体δ34S组成范围较宽(-9.38‰~22.69‰),具多源性硫的特征;铅锌矿体的δ34S在-7.1‰~6.4‰之间变化,硫可能来自深源岩浆,并受围岩成分混染。多方面的证据表明,大降坪黄铁矿矿体下部的铅锌矿体形成于晚白垩世的岩浆热液成矿作用,透镜状黄铁矿矿体受到岩浆热液的叠加。     

A hydrothermal origin for the large Xinqiao Cu-S-Fe deposit,Eastern China: Evidence from sulfide geochemistry and sulfur isotopes

The Xinqiao Cu-S-Fe deposit in the Tongling ore district, Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB; Eastern China), is located along the northern margin of the Yangtze Craton. The stratiform- and skarn-type Xinqiao mineralization comprises five stages, namely the early skarn (Stage I, garnet and diopside), late skarn (Stage II, epidote-dominated), iron oxides (Stage III, hematite and magnetite), colloform pyrite (Stage IV) and quartz-sulfides (Stage V). There are three pyrite types at Xinqiao, i.e., colloform (Py1; Stage IV), fine-grained (Py2, from Py1 recrystallization; Stage V) and coarse-grained (Py3; Stage V) pyrites.Scanning Electron Microscope (SEM) imagery for Py1 reveals that they are cubic microcrystalline pyrite aggregates, and the EDS and XRD data indicate that some Py1 contain minor siderite impurities. Electron Microprobe Analysis (EMPA) and LA-ICP-MS geochemical data demonstrate that the three pyrite types have relatively high Fe/S ratios and distinctly high Mn, Cu and As concentrations. Compared to Py2 and Py3, Py1 has higher Pb, Bi and Ag, but lower Co, Ni, Se, Cd, Te and Au. Ratios of Fe/S (0.837 to 0.906), Se/Te (2.39 to 14.50) and Co/Ni (0.67 to 4.67) of the Xinqiao pyrites resemble typical hydrothermal pyrites. δ³⁴S_CDT of Py1 (− 0.6‰ to 2.7‰, average 0.58‰), Py2 (1.8‰ to 2.5‰, average 2.1‰) and Py3 (1.9‰ to 4.4‰, average 3.5‰) are close to those of the Xinqiao skarn-type orebodies (1.3‰ to 4.1‰), but distinct from those of the Upper Carboniferous Huanglong Formation limestone (− 9.5‰ to − 15.4‰), suggesting that the three pyrite types (especially Py1) were genetically linked to the Yanshanian (Jurassic-Cretaceous) magmatic-hydrothermal events, with Py1 probably reflecting rapid crystallization during fluid mixing. We interpret that the Xinqiao stratiform mineralization may have been associated with the Jitou quartz diorite stock, as may be the case also for the skarn-type mineralization hosted in the contact between the Yanshanian Jitou stock and the Lower Permian Qixia Formation limestone. Overall, the Xinqiao Cu-S-Fe mineralization may have been generated by the Jurassic-Cretaceous tectono-thermal event in Eastern China.     

Constraining models of the tectonic setting of the giant Olympic Dam iron oxide–copper–gold deposit, South Australia, using deep seismic reflection data

In the Gawler Craton, the completeness of cover concealing the crystalline basement in the region of the giant Olympic Dam Cu–Au deposit has impeded any sufficient understanding of the crustal architecture and tectonic setting of its IOCG mineral-system. To circumvent this problem, deep seismic reflection data were recently acquired from  250 line-km of two intersecting traverses, centered on the Olympic Dam deposit. The data were recorded to 18 s TWT ( 55 km). The crust consists of Neoproterozoic cover, in places more than 5 km thick, over crystalline basement with the Moho at depths of 13–14 s TWT ( 40–42 km). The Olympic Dam deposit lies on the boundary between two distinct pieces of crust, one interpreted as the Archean–Paleoproterozoic core to the craton, the other as a Meso–Neoproterozoic mobile belt. The host to the deposit, a member of the  1590 Ma Hiltaba Suite of granites, is situated above a zone of reduced impedance contrast in the lower crust, which we interpret to be source-region for its  1000 °C magma. The crystalline basement is dominated by thrusts. This contrasts with widely held models for the tectonic setting of Olympic Dam, which predict extension associated with heat from the mantle producing the high temperatures required to generate the Hiltaba Suite granites implicated in mineralization. We use the seismic data to test four hypotheses for this heat-source: mantle underplating, a mantle-plume, lithospheric extension, and radioactive heating in the lower crust. We reject the first three hypotheses. The data cannot be used to reject or confirm the fourth hypothesis.     

Petrographic and geochemical arguments for hydrothermal formation of the Ouenza siderite deposit (NE Algeria)

The Ouenza siderite deposit is located proximal to evaporitic diapirs of Triassic age. Mineralization occurs mainly in Aptian neritic limestones which host important iron concentrations (120–150 MT) and minor Pb, Zn, Cu, Ba and F occurrences. The iron ore consists of iron carbonate minerals which have been oxidized partially to hematite. Fine-grained ankerite and siderite replace limestones, whereas sparry ankerite and siderite were emplaced in veins. Limited variation in the chemical and isotopic compositions of ankerite and siderite were observed, which indicate that they precipitated from the same fluid. Stable isotope compositions (δ¹⁸O and δ¹³C) of iron carbonates and limestones allow estimation of the isotopic composition of the mineralizing fluid and precipitation temperature: δ¹⁸O = 7.5‰ SMOW, T = 100–120 °C. Later deposition of Pb, Zn, Cu, Ba and F minerals is controlled by fractures oriented NE–SW and SE–NW. Fluid inclusion studies of quartz yield salinities of 18–22 wt.% equivalent NaCl and homogenization temperatures between 150 and 180 °C. These values are similar to those of Mississippi Valley type deposits which are associated with basinal brines. Received: 4 January 1996 / Accepted: 17 July 1996     

豫西矿集区岩浆热液叠加改造型矿床特征

文章对豫西矿集区成矿类型进行了划分,归纳为3大类6个成矿类型.对岩浆热液叠加改造型矿床定义、成矿地质背景、构造转换与成矿进行了阐述,并通过对上宫金矿的矿石矿物共生组合、矿化蚀变特征、同位素地质特征研究,认为岩浆热液叠加改造型矿床赋矿地层古老,经历了长期的构造演化;成矿物质多种来源,具深源特性和造山带物质的痕迹;矿石的物质组成复杂,矿物共生组合种类繁多;成矿围岩遭受了强烈的变质、蚀变作用,蚀变岩石类型较多,矿化多期次叠加;具有多成因性.     

西藏尕尔穷铜金矿床发现罕见金属(Ni-Cr-Fe，Cu-Zn)互化物

班公湖—怒江成矿带是西藏自治区重要的铜金多金属成矿带,尕尔穷矿床是该成矿带中重要的矽卡岩型铜金矿床,笔者通过野外地质调查采样、显微镜鉴定、扫描电镜观察和X射线能谱仪测试分析,对矿石物质组分做了系统的研究.首次在矿石中发现了罕见的Ni-Cr-Fe,Cu-Zn互化物,这也是矽卡岩型矿床中首次发现此类矿物,在国内外尚无报道.这些金属互化物均属自然界罕见的矿物种类,其生成条件独特,是在缺氧、低硫的强还原环境中形成.首次在矽卡岩型矿床中发现与幔源岩浆有着成矿专属性的成矿元素,在成矿理论、成矿物质来源、成矿条件、成矿环境、矿床成因等方面具有重要的科学意义.     

西秦岭谢坑矽卡岩型铜金矿床地质特征与矿区岩浆岩年代学研究

谢坑铜金矿床位于西秦岭造山带西段,区域内主要出露下二叠统大关山群和下三叠统隆务河群,构造和岩浆活动发育.岩石学研究表明,谢坑铜金矿区内出露侵入岩主要为辉长闪长岩和辉长岩,它们是岗察岩体的重要组成部分.在辉长闪长岩与二叠纪灰岩接触带部位发育铜金和铁矿化并形成相关工业矿体.矿石类型主要有块状、脉状和浸染状三类.磁铁矿、磁黄铁矿表现为块状,黄铜矿为脉状和浸染状;脉状和浸染状黄铜矿穿插于块状磁铁矿矿石现象十分普遍,含金碳酸盐石英脉主要沿着矿石裂隙发育.围岩蚀变呈现清晰的蚀变分带现象,自岩体向外依次表现为钾化、青磐岩化和矽卡岩化.其中矽卡岩化主要发育于辉长闪长岩与灰岩接触带部位,与成矿关系密切,磁铁矿矿化主要发育于晚期矽卡岩阶段,磁黄铁矿、黄铁矿、黄铜矿等金属硫化物以及毒砂等矿物主要形成于早期硫化物阶段,金矿化主要发育于石英硫化物阶段.LA-ICP-MS锆石U-Pb测年结果表明,辉长闪长岩和角闪安山岩分别形成于243.8±1.0Ma和242.1±1.2Ma.该成岩、成矿时代与区城构造岩浆、成矿事件相一致.这些结果表明,谢坑铜金矿床为与西秦岭中三叠世孤岩浆作用密切相关的矽卡岩型矿床.