Monte Ollasteddu, a new gold discovery in the Variscan basement of Sardinia (Italy): first isotopic (40Ar−39Ar, Pb) and fluid inclusion data

The Monte Ollasteddu deposit represents a major gold discovery in the Variscan basement of southeastern Sardinia. Gold occurs in late-Variscan extensional brittle structures hosted by meta-volcanic, and subordinately meta-sedimentary, rocks. The vein mineralogy is dominated by quartz; arsenopyrite is the main sulphide. Reconnaissance ⁴⁰Ar–³⁹Ar dating gives ages around ∼260 Ma on K-feldspar from mineralized veins, whereas metamorphic white mica from the host rock gives ages clustering at ∼307 Ma. The best age estimate for biotite from a nearby leucogranite body is 286.3±2.2 Ma. The Pb isotope signature of ore and gangue minerals is entirely consistent with literature data for Variscan deposits of Sardinia, and for European Variscan gold deposits. Fluid inclusion data point to the presence of both CO₂-bearing and CO₂-free fluids, with homogenization temperatures ranging from 220 to 415°C, with low-to-moderate salinities (0.4–6.2 wt% NaCl equivalent). Monte Ollasteddu shows several features similar to European Variscan gold deposits; however, the age of mineralization might post-date granitoid intrusion by as much as 30 Ma, being instead coeval with very late calc-alkaline basaltic dykes, marking the transition to a post-orogenic, pre-Tethyan geodynamic setting. Electronic Supplementary Material Supplementary material is available for this article at     

Orogenic gold mineralisation hosted by Archaean basement rocks at Sortekap, Kangerlussuaq area, East Greenland

A gold-bearing quartz vein system has been identified in Archaean basement rocks at Sortekap in the Kangerlussuaq region of east Greenland, 35 km north–northeast of the Skaergaard Intrusion. This constitutes the first recorded occurrence of Au mineralisation in the metamorphic basement rocks of east Greenland. The mineralisation can be classified as orogenic style, quartz vein-hosted Au mineralisation. Two vein types have been identified based on their alteration styles and the presence of Au mineralisation. Mineralised type 1 veins occur within sheared supracrustal units and are hosted by garnet-bearing amphibolites, with associated felsic and ultramafic intrusions. Gold is present as native Au and Au-rich electrum together with arsenopyrite and minor pyrite and chalcopyrite in thin alteration selvages in the immediate wall rocks. The alteration assemblage of actinolite-clinozoisite-muscovite-titanite-scheelite-arsenopyrite-pyrite is considered to be a greenschist facies assemblage. The timing of mineralisation is therefore interpreted as being later and separate event to the peak amphibolite facies metamorphism of the host rocks. Type 2 quartz veins are barren of mineralisation, lack significant alteration of the wall rocks and are considered to be later stage. Fluid inclusion microthermometry of the quartz reveals three separate fluids, including a high temperature (T _h ?=?300–350 °C), H₂O–CO₂–CH₄ fluid present only in type 1 veins that in interpreted to be responsible for the main stage of Au deposition and sulphidic wall rock alteration. It is likely that the carbonic fluids were actually trapped at temperatures closer to 400 °C. Two other fluids were identified within both vein types, which comprise low temperature (100–200 °C) brines, with salinities of 13–25 wt%?eq. NaCl and at least one generation of low salinity aqueous fluids. The sources and timings of the secondary fluids are currently equivocal but they may be related to the emplacement of Paleogene mafic intrusions. The identification of this occurrence of orogenic-style Au mineralisation has implications for exploration in the underexplored area of east Greenland between 62 and 69°?N, where other, similar supracrustal units are known to be present.     

Sulfur isotope and fluid inclusion geochemistry of metamorphic Cu–Au vein deposits,central Cobar area,NSW, Australia

Vein-type, structurally controlled Cu–Au mineralisation hosted by turbidites of late Silurian to earliest Devonian age, forms an important resource close to the eastern margin of the Cobar Basin. Here we report 103 new sulfur isotope analyses, together with homogenisation temperatures and salinity data for 545 primary two-phase fluid inclusions for the mineralised zones from the central area of the Cobar mining district. A range in δ³⁴S values from 3.8 to 11.2‰ (average 7.9‰) is present. Sulfur is likely to have been derived from rock sulfur/sulfide in basin and basement rocks, but there may be an additional connate-derived component. Homogenisation temperatures (T_h) for inclusion fluids trapped in quartz and minor calcite veins range from near 150°C to 397°C. Fluid inclusions are characterised by a low CO₂ content and low, but variable salinities (2.1–9.1 wt% NaCl equivalent). Generations of inclusion fluids correspond to six paragenetic stages of vein quartz deposition and recrystallisation at the Chesney mine. Primary fluid inclusions in the first two stages were subjected to re-equilibration resulting from increased confining pressure. Their T_h range (151–317°C) is considered a minimum for the temperature of these fluids. Sulfide and gold deposition at Chesney appears to be related to fluids of moderately high T_h (range 270–397°C) associated with the final paragenetic stage. T_h for the ore-related fluids may be close to the solvus of the H₂O–NaCl–CO₂ system and hence near trapping temperatures. Late-stage entry of a hot, moderately saline ore-forming fluid is implicated as the origin of the Cu–Au mineralisation. However, comparison with geochemical data for the vein-style Zn–Pb–Ag deposits at Cobar demonstrates that differences in metal content for individual zones cannot be attributed to major differences in fluid temperature or salinity. Rather, these differences are probably due to variations in source-rock reservoirs and structural pathways along which the ore-forming fluids moved.     

The massive stibnite veins of the French Palaeozoic basement: a metallogenic marker of Late Variscan brittle extension

The stibnite ore deposits of the French Palaeozoic basement are spatially related to major Late Variscan strike-slip faults. They occur as small discontinuous veins mostly hosted in epizonal or catazonal siliciclastics. Stibnite crystallizes in the final stage of a polymetallic paragenesis from an antimony-bearing solution, which deposits a first discrete Fe-As assemblage at 300–400°C and 0.5–0.8 Kbar. Experimental data have shown that antimony solubility drastically decreases on cooling. Characterization of the Late Variscan antimony-bearing fluids enables us to predict the temperature range - 270/150°C - under which stibnite was precipitated. Since the latter coincides with the fluid inclusion homogenization temperature range, the pressure at which stibnite crystallized can be estimated from vapour pressure data to have been around 0.1 Kbar. Extensional brittle shear zones, developed at the end of the Late Variscan orogeny (probably at the Westphalian/Stephanian boundary), drained the hydrothermal fluids near to the surface (c. 1000 m ?) reaching the critical P-T conditions for stibnite crystallization.     

Petrographic,fluid inclusion and isotopic study of the Dikulushi Cu–Ag deposit,Katanga (D.R.C.): implications for exploration

The Dikulushi Cu–Ag vein-type deposit is located on the Kundelungu Plateau, in the southeastern part of the Democratic Republic of Congo (D.R.C.). The Kundelungu Plateau is situated to the north of the Lufilian Arc that hosts the world-class stratiform Cu–Co deposits of the Central African Copperbelt. A combined petrographic, fluid inclusion and stable isotope study revealed that the mineralisation at Dikulushi developed during two spatially and temporally distinct mineralising episodes. An early Cu–Pb–Zn–Fe mineralisation took place during the Lufilian Orogeny in a zone of crosscutting EW- and NE-oriented faults and consists of a sequence of sulphides that precipitated from moderate-temperature, saline H₂O–NaCl–CaCl₂-rich fluids. These fluids interacted extensively with the country rocks. Sulphur was probably derived from thermochemical reduction of Neoproterozoic seawater sulphate. Undeformed, post-orogenic Cu–Ag mineralisation remobilised the upper part of the Cu–Pb–Zn–Fe mineralisation in an oxidising environment along reactivated and newly formed NE-oriented faults in the eastern part of the deposit. This mineralisation is dominated by massive Ag-rich chalcocite that precipitated from low-temperature H₂O–NaCl–KCl fluids, generated by mixing of moderate- and low-saline fluids. The same evolution in mineralisation assemblages and types of mineralising fluids is observed in three other Cu deposits on the Kundelungu Plateau. Therefore, the recognition of two distinct types of (vein-type) mineralisation in the study area has a profound impact on the exploration in the Kundelungu Plateau region. The identification of a Cu–Ag type mineralisation at the surface could imply the presence of a Cu–Pb–Zn–Fe mineralisation at depth.     

The role of evaporites in the genesis of base metal sulphide mineralisation in the Northern Platform of the Pan-African Damara Belt, Namibia: geochemical and fluid inclusion evidence from carbonate wall rock alteration

The Otavi Mountain Land is a base metal sulphide ore province in northern Namibia where deposits are hosted by platform carbonates of the Otavi Group in a foreland fold-and-thrust belt on the northern edge of the Pan-African Damara Belt. Deposits have been classified as the Berg Aukas- or Tsumeb-types, based on differences in ore association, stratigraphic position and geochemistry of ores and gangue carbonates. Mineralisation at these deposits is accompanied by carbonate alteration in the form of dolomite and calcite veins, carbonate recrystallisation, calcitisation and carbonate silicification. Based on cathodoluminescence imaging, trace and rare earth element (REE), O and C isotope, and fluid inclusion data, a series of carbonate generations, constituting wall rock alteration around the Tsumeb and Kombat (Tsumeb-type) and Berg Aukas (Berg Aukas-type) deposits, was established. Similar data obtained on the recently discovered Khusib Springs deposit indicate a strong affinity to Tsumeb-type deposits. Tsumeb-type deposits are distinguished from Berg Aukas-type deposits by having trace element and REE concentrations that are significantly higher in the alteration products compared to the carbonate host rocks. Only around Tsumeb-type deposits a relative enrichment in light REE is noted for the hydrothermal carbonate generations that are cogenetic with the main stage of mineralisation. Microthermometric results from fluid inclusions in carbonate alteration phases and associated quartz indicate relatively high salinity (17–23 wt% NaCl equivalent) for the main mineralising and subsequent sulphide remobilisation stages at the deposits investigated. Estimated mineralisation temperatures are significantly higher for Tsumeb-type deposits (370–405 °C) with early sulphide remobilisation in Tsumeb at 275 °C, whereas they are lower at Berg Aukas (up to 255 °C). Fluid inclusion leachate analysis suggests that most of the observed salinity can be ascribed to dissolved, predominantly Ca- and Mg-carbonates and chlorides with subordinate NaCl. Na-Cl-Br leachate systematics indicate a derivation of the fluid salinity from the interaction with evaporitic rocks en route. Tsumeb-type mineralisation is interpreted to be derived from fluids expelled during Pan-African orogeny in the more intensely deformed internal zones of the Damara Belt further south. When the high salinity fluids reached the carbonate platform after having scavenged high concentrations of base metals, base metal sulphide precipitation occurred in zones of high porosity, provided by karst features in the carbonate sequence. Results obtained for the Berg Aukas-type deposits emphasise their derivation from basinal brines, similar to Mississippi Valley-type deposits, and confirm that mineralisation of the Berg Aukas- and Tsumeb-types are both spatially and temporally distinct. Received: 5 May 1999 / Accepted: 10 November 1999     

Siderite mineralization of the Gemericum superunit (Western Carpathians, Slovakia): review and a revised genetic model

The Gemericum is a segment of the Variscan orogen subsequently deformed by the Alpine–Carpathian orogeny. The unit contains abundant siderite–sulphide and quartz–antimony veins together with stratabound siderite replacement deposits in limestones and stratiform sulphide mineralization in volcano-sedimentary sequences. The siderite–sulphide veins and siderite replacement deposits of the Gemericum represent one of the largest accumulations of siderite in the world, with about 160 million tonnes of mineable FeCO₃. More than 1200 steeply dipping hydrothermal veins are arranged in a regional tectonic and compositional pattern, reflecting the distribution of regional metamorphic zones. Siderite–sulphide veins are typically contained in low-grade (chlorite zone) sedimentary, volcano-sedimentary or volcanic Lower and Upper Paleozoic rocks. Quartz–antimony veins are hosted by higher-grade units (biotite zone). Siderite–sulphide veins are dominated by early siderite followed by a complex set of stages, including quartz–sulphide (chalcopyrite, tetrahedrite), barite, tourmaline–quartz, and sulphide-remobilization stages. The temporal evolution of these stages is difficult to study because of the widespread and repeated tectonic processes, within-vein replacement and recrystallization. Siderite–sulphide veins show considerable vertical (up to 1200 m) and lateral (up to 15 km) extent, and a thickness typically reaching several metres. Carbonate-replacement siderite deposits of the Gemericum are hosted by a Silurian limestone belt and are similar to stratabound siderite deposits of the Eastern Alps (e.g., Erzberg, Austria).Based on a review of geological, petrological and geochronological data for the Gemericum, and extensive stable and radiogenic isotope data and fluid inclusion data on hydrothermal minerals, the siderite–sulphide veins and siderite replacement deposits are classified as metamorphogenic in a broad sense. The deposits were formed during several stages of regional crustal-scale fluid flow. Isotope (S, C, Sr, Pb) fingerprinting identifies the metamorphosed rock complexes of the Gemericum as a source of most components of hydrothermal fluids. Fluid inclusion and stable isotope data evidence the participation of several contrasting fluid types, and the existence of contrasting P–T conditions during vein evolution. A high-δ¹⁸O, medium- to high-salinity, H₂O-type fluid is the most important component during siderite deposition, whereas H₂O–CO₂-type fluid inclusion containing dense liquid CO₂ and corresponding to minimal pressures between 1 and 3 kbar were found in a younger tourmaline–quartz stage. Younger quartz–ankerite(±siderite)–sulphide stages are characterized by high-salinity (17 to 35 wt.% NaCl equivalent) and low-temperature (T_h=90 to 180 °C) H₂O-type fluids.The vein deposits are interpreted as a result of multistage hydrothermal circulation, with Variscan and Alpine mineralization phases. Based on available indirect data, the most important mineralization phase was related to regional fluid flow during the uplift of a Variscan metamorphic core complex, producing siderite–sulphide (±barite) mineralization, while tourmaline–quartz stage and sulphide remobilization stages are related to Alpine processes. Two phases of vein evolution are evident from two groups of ⁸⁷Sr/⁸⁶Sr isotope ratios of Sr-rich, Rb-poor hydrothermal minerals: 0.71042–0.71541 in older barite and 0.7190–0.7220 in late-stage celestine and strontianite.     

Metallogenesis in the Harlech Dome,North Wales: A fluid inclusion interpretation

A regional fluid inclusion study of Cu-Au (+Zn-Pb) mineralisation in the Harlech Dome area, North Wales, gives support to the concept of two distinct metallogenic episodes. The inclusion assemblages associated with the porphyry copper mineralisation at Coed-y-Brenin are consistent with a genetic model of early potassic-propylitic alteration overprinted by later phyllic alteration. High salinity fluids, normally characteristic of potassic alteration, are confined to the host rock quartz. The meteoric/hydrothermal system is closely linked to the emplacement of late-Cambrian diorites. Integrated fluid inclusion and mineralogical studies of the Gold-belt veins suggest that the mineralising fluids were probably dehydration waters released from weakly metamorphosed Cambrian and perhaps Precambrian sediments during hydraulic fracturing in a tensional zone at the close of the Caledonian orogeny. Localisation of economic concentrations of gold in veins at the level of the Clogau Formation is ascribed to a destabilisation of metal complexes caused by a change in fluid buffering from a pyrite-magnetite assemblage in the Lower Cambrian sediments to a pyrite-pyrrhotite-graphite assemblage in the Upper Cambrian sediments. Veining associated with the Coed-y-Brenin porphyry copper deposit and related breccia pipes can be distinguished from the copper-gold veins of the coextensive Dolgellau Gold-belt by the presence in the former of inclusions notably richer in CO₂. Furthermore the Gold-belt fluids have a distinctive low CO₂/CH₄+N₂+H₂ ratio.     

Ore deposits of the French Massif Central: insight into the metallogenesis of the Variscan collision belt

The French Massif Central (FMC) represents the whole West European Variscan (WEV) belt, in terms of both the geodynamic evolution and the metallic content. Thus, a study of the metallogenic evolution of the FMC may elucidate the conditions that allow the mineralisation of a collision belt, since recent collision belts, e.g. the Himalayas or the Alps show that mineralisation does not necessarily result from the collision process. The Palaeozoic history of the FMC is divided into three geodynamic stages unevenly involved from the metallogenic view point. The Eo-Variscan stage (Cambrian to Silurian) was not important; the Meso-Variscan stage (Devonian-Early Carboniferous) was of limited importance; and most of the mineralisations formed during the Neo-Variscan stage (Late Carboniferous-Early Permian). In addition, some more mineralisation was produced during the Mesozoic because of the thermal reactivation linked with the Alpine orogenies. The Eo-Variscan stage (Cambrian-Silurian) corresponded to the pre-collision history, marked at the WEV belt scale by a fragmentation of the northern Gondwana (immature crust evolved from the Late Proterozoic Cadomian orogeny), up to the break-up of the crust and the formation of oceanic basins (Cambrian-Ordovician), followed by their resorption by subduction during the Silurian. In the FMC, no subduction-related magmatism is known (being rare at the WEV belt scale), and consequently subduction-related mineralisation, e.g. porphyry copper, is unknown in the WEV belt. Although some ophiolitic remnants are known, they never display Cyprus-type VMS deposits, nor massive podiform chromitites. Beside platformal sedimentary deposits on passive margins, the only deposits formed during the Eo-Variscan stage were of the SEDEX type, linked with the early rifting of the Gondwanian crust. The Meso-Variscan stage (Devonian-Early Carboniferous) corresponded to the collision proper, with the formation of crustal-scale nappe structures and the intrusion of collision-related peraluminous granites. Although these granites were enriched in rare metals they did not yield significant hydrothermal mineralisation, due to the great depth of their emplacement, as the similar granites in the Himalayas. However, they were a source of rare metals (in particular, uranium) for later mineralisation events. At the WEV belt scale Devonian distensive events are coeval with the collision. They were recorded by the formation of sedimentary basins of limited time and space extent, corresponding to the splitting of the continental crust (up to formation of oceanic domains in many cases), and were characterised by a bi-modal (“spilite-keratophyre”) volcanism. These basins formed in transtensional (or pull-apart) settings along major strike-slip faults, a peculiarity of the Variscan collision belt (which may conveniently be described as a “strike-slip orogen”). In such basins, many deposits linked with the volcanic thermal energy were formed: SEDEX deposits of the Meggen-type, iron deposits of the Lahn-Dill-type and VMS base metal deposits, the latter being the only ones known in the FMC (Brévenne deposits). The Neo-Variscan stage corresponded to the “hypercollision” and was characterised by a shift from compressional tectonics (late thickening of the crust during the Sudetian event and long-lasting dextral strike-slip tectonics along NW-SE to NE-SW fault zones) towards extensional tectonics (“basin and range” of the Late Stephanian-Early Permian), as well as by high heat flows, recorded by LP-HT metamorphism, extensive granitisation and granulitisation of the lower crust. These characteristics record the development of a lithospheric delamination process. In response to the energetic input released by this process, numerous hydrothermal deposits were formed in the FMC, as well as in the whole WEV belt, during the Neo-Variscan stage. These are mainly: (1) high-temperature granite-centered tungsten deposits, mainly associated with cordierite-bearing high level intrusions of Namurian-Westphalian age; (2) rare metal granites (and the associated hydrothermal tin mineralisations), resulting from fluid-induced low-degree partial melting of the middle crust in relation with the devolatilisation of the granulitised lower crust; (3) shear-zone hosted gold and antimony deposits, related to crustal-scale hydrothermal circulation, triggered by the transition to extensional tectonics at about 300 Ma; and (4) uranium deposition in extensional settings related to the Early Permian distension. The Post-Variscan mineralising events recorded the renewal of thermal flows in the lithosphere linked with early Alpine events (mainly the Trias-Lias distension in the Tethyan realm and the middle Cretaceous opening of the Bay of Biscay in the Pyrenean realm). They resulted in low-enthalpy geothermal systems, leading to a variety of deposits, mainly: (1) F-Ba districts, reworking F and Ba from Late Variscan granites and ignimbrites; (2) a major uranium deposit (Lodève), reworking uranium from the Permian Lodève basin; and (3) Zn-Pb districts of the MVT-type. Finally, the mineralisation of the Variscan collision belt is mainly the consequence of the Neo-Variscan lithospheric delamination process. By contrast, the absence of such a process in collision belts like the Himalayas or the Alps is the key of them being devoid of mineralisation. It appears that the mechanical energy released by the collision itself is not sufficient to mobilise and concentrate the trace elements involved in the metallogenic processes. Received: 1 September 1998 / Accepted: 3 February 1999     

Characteristics of orogenic-gold deposits in the Northcote district,Hodgkinson Province,north Queensland: implications for tectonic evolution

We report results of metallogenic, structural, petrological and fluid-inclusion studies that characterise the nature of gold mineralisation in the Northcote district of the Hodgkinson Goldfield, one of the major goldfields in the Palaeozoic Hodgkinson Province of northeastern Queensland, Australia. Gold – arsenic and gold – antimony – arsenic deposits in the Northcote district are spatially associated with distinctive northwest-trending faults and shear zones suggesting a strong structural control on their development during several phases of deformation in the Late Devonian to Carboniferous. Field evidence, as well as petrographic, scanning electron microscope and fluid-inclusion analysis of mineralised samples, indicates the occurrence of two stages of gold genesis. Gold associated with the earlier of the two mineralising episodes exists mainly in solid solution with acicular arsenopyrite. SEM analysis indicated that minor sphalerite, galena and tetrahedrite formed during this episode of mineralisation. Gold deposited during the second mineralising episode has a strong association with antimony. Fluid-inclusion studies of quartz vein samples record minimum homogenisation temperatures between 108 and 312°C, and salinities of up to 13 wt% NaCl equivalent. Little to no difference in the nature of the fluids is observed between samples related to the two gold-forming stages. The style of gold mineralisation in the Northcote district is compatible with orogenic-gold deposits that form primarily during compressional and transpressional deformation along convergent plate margins.     

Fluid inclusion gas studies,carrock fell tungsten deposit,england: implications for regional exploration

A fluid inclusion investigation of the Carrock Fell tungsten deposit, Northern England, confirms that the quartz-wolframite-scheelite veins associated with the Caledonian Skiddaw Granite are almost exclusively related to an exocontact hydrothermal system developed at the margin of a local cupola. Fluid circulation, as defined by the spatial variation in temperature and H₂O/CO₂ ratios for inclusions in vein quartz, reveals a strong structural control. The zone of maximum flow, which extends 0–400 m out from the granite contact, is characterised by high H₂O/CO₂ ratios and corresponds closely with the known distribution of high-grade oreshoots. Based on the fluid inclusion gas signature for the Carrock Fell deposit, a distinction can be made between potentially tungstaniferous quartz veins and those related to Cu-Pb-Zn deposits in the absence of diagnostic ore minerals. Also, a regional survey of quartz veins in the Lake District suggests that at several localities the fluids have a close affinity with those at Carrock Fell. This is interpreted as the high-level, distal expression of tungsten mineralisation at depth. Evidence for similar mineralisation elsewhere in the British Caledonides favours those granites in the paratectonic zones of Ireland and southern Scotland.     

Structural controls on hydrothermal alteration and gold–antimony mineralisation in the Hillgrove area,NSW, Australia

The Hillgrove gold–antimony deposit is hosted in late Palaeozoic, biotite-grade metasedimentary rocks and Permo-Carboniferous granitoid intrusions of the New England Orogen. Mineralisation occurred at a range of structural levels during rapid uplift in the orogen at 255–245 Ma. Hydrothermal fluids were controlled by extensional faults in a regional-scale sinistral strike-slip fault system. Principal faults in this system were developed in, and possibly evolved from, mylonite zones which were active during Late Permian tectonics. Earliest mineralisation formed scheelite-bearing quartz veins, and these were followed by auriferous arsenopyrite–pyrite–quartz–carbonate veins with minor base metal sulphides. This latter type was accompanied by sericitisation and carbonation of the host rock, with addition of sulphur, arsenic and gold, in zones up to 20 m from veins. Quartz–stibnite veins with electrum, gold, aurostibite, and arsenopyrite form a prominent and economically important hydrothermal type, with little wall-rock alteration but extensive hydrothermal breccia formation and local open-space filling textures. Below a mining depth of 300–500 m, this type passes over a short distance downwards into stibnite-poor gold-bearing veins. Late-stage carbonate–stibnite veins with gold and silver sulphosalts cut all earlier veins, and have open-space filling textures. Aspects of the Hillgrove deposit have similarities to many other orogenic gold deposits in the SW Pacific which have been formed at different structural levels. Hillgrove is distinctive in having evidence for mineralisation at this wide range of structural levels in the one deposit, formed progressively during syn-orogenic uplift. Editorial handling: N. White     

Lithogeochemical localisation of disseminated gold in the White River area, Yukon, Canada

Gold mineralisation in the White River area, 80 km south of the highly productive Klondike alluvial goldfield, is hosted in amphibolite facies gneisses in the same Permian metamorphic pile as the basement for the Klondike goldfield. Hydrothermal fluid which introduced the gold was controlled by fracture systems associated with middle Cretaceous to early Tertiary extensional faults. Gold deposition occurred where highly fractured and chemically reactive rocks allowed intense water–rock interaction and hydrothermal alteration, with only minor development of quartz veins. Felsic gneisses were sericitised with recrystallisation of hematite and minor arsenic mobility, and extensively pyritised zones contain gold and minor arsenic (ca 10 ppm). Graphitic quartzites (up to 5 wt.% carbon) caused chemical reduction of mineralising fluids, with associated recrystallisation of metamorphic minerals (graphite, pyrrhotite, pyrite, chalcopyrite) in host rocks and veins, and introduction of arsenic (up to 1 wt.%) to form arsenopyrite in veins and disseminated through host rock. Veins have little or no hydrothermal quartz, and up to 19 wt.% carbon as graphite. Late-stage oxidation of arsenopyrite in some graphitic veins has formed pharmacosiderite. Gold is closely associated with disseminated and vein sulphides in these two rock types, with grades of up to 3 ppm on the metre scale. Other rock types in the White River basement rocks, including biotite gneiss, hornblende gneiss, pyroxenite, and serpentinite, have not developed through-going fracture systems because of their individual mineralogical and rheological characteristics, and hence have been little hydrothermally altered themselves, have little hydrothermal gold, and have restricted flow of fluids through the rock mass. Some small post-metamorphic quartz veins (metre scale) have been intensely fractured and contain abundant gold on fractures (up to 40 ppm), but these are volumetrically minor. The style of gold mineralisation in the White River area is younger than, and distinctly different from, that of the Klondike area. Some of the mineralised zones in the White River area resemble, mineralogically and geochemically, nearby coeval igneous-hosted gold deposits, but this resemblance is superficial only. The White River mineralisation is an entirely new style of Yukon gold deposit, in which host rocks control the mineralogy and geochemistry of disseminated gold, without quartz veins.     

Regional fluid flow and the genesis of Irish Carboniferous base metal deposits

The Irish carbonate-hosted base metal deposits have long been an enigmatic subclass of deposit. Some of the Irish deposits (Harberton Bridge, Allenwood) are clearly epigenetic, hosted in breccia pipes and have close affinities to Mississippi Valley-type deposits. Others, are characterised by stratabound and sometimes stratiform mineralisation (Abbeytown, Navan), while a third group is associated closely with concordant dolomitic breccias and cavity fill mineralisation at the base of carbonate mud mounds (Lisheen, Ballinalack). When the stratigraphic and textural evidence is reviewed, it is apparent that all of the base metal mineralisation occurred in already compacted sediments. Hence, mineralisation probably occurred at depths of at least several 100 m depth below the sea bed. The mineralisation occurred either during transtensional Lower Carboniferous basin development, or subsequently, during the onset of Variscan shortening. Fluids may have been derived from dewatering of Variscan-driven deformation to the south of the carbonate platform, with fluid flow through major fracture zones and basal clastic aquifers. Alternatively, high heat flow produced by Lower Carboniferous extension may have driven the mineralising system. Syn-genetic models are extremely difficult to sustain.     

Gold mineralisation at the Lady Bountiful Mine,Western Australia: An example of a granitoid-hosted Archaean lode gold deposit

The Lady Bountiful granitoid-hosted lode gold deposit, located in the mid-greenschist facies metamorphosed Ora Banda greenstone sequence, is hosted predominantly by the late-tectonic Liberty Granodiorite. Gold mineralisation is localised along quartz-veined, sinistral, brittle fault-zone(s) that transect the boundary between the Liberty Granodiorite and Mt Pleasant sill. Quartz vein textures indicate two stages of a single gold-related vein-development event, with high-grade gold mineralisation restricted to the second stage. Ore minerals include pyrite, chalcopyrite, pyrrhotite, galena, sphalerite, Au−Ag−Bi−Pb-tellurides, and native gold. Fluid infitration has resulted in narrow (<1 m) bleached wallrock alteration envelopes to the fault zones comprising albite-K-mica ±chlorite±calcite±rutile assemblages. Temperature-pressure conditions varied from Stage I (300°±50°C, ≈2 kbar) to Stage II (250°±50°C, ≈0.5 to 2 kbar), with the hydrothermal fluid in both stages characterised by X(CO₂)≤0.15 and moderate salinity (≈1.28 m NaCl). Intermittent phase separation of Stage II mineralising fluids, initiated by pressure fluctuations in dilational sites, and/or fluid-dominated fluid: wallrock interaction, are invoked as the dominant depositional mechanisms. The granitoid-hosted Lady Bountiful lode gold deposit shares many features with other granitoid-hosted lode gold deposits in the Yilgarn Craton and the Superior Province. Granitoid-hosted lode gold deposits, such as the Lady Bountiful deposit, provide additional evidence that the dominant control on the localisation of gold mineralisation within a granitoid host is structure, with competency contrasts playing a significant role. Furthermore, the hydrothermal wallrock alteraction and orefluid chemistry characteristics of the granitoid-hosted lode gold deposits are comparable to those established for greenstone-hosted lode gold mineralisation.     

Origin and evolution of high-salinity, Zn–Pb mineralising fluids in the Variscides of Belgium

High-salinity, Na–Ca–Cl-rich fluids (˜20 wt% salts) in inclusions in gangue and ore minerals from Mesozoic Mississippi Valley-type (MVT) deposits in the Verviers Synclinorium (eastern Belgium) and in Cretaceous vein calcites at the Variscan front were investigated by microthermometric and crush-leach analysis. The MVT deposits formed at temperatures of ˜110 °C while the Cretaceous vein calcites were precipitated at temperatures <50 °C. Their Cl–Br content (Cl/Br ratio between 246 and 458) suggests that the fluids probably originated by the evaporation of seawater during basin development at the southern margin of the Caledonian Brabant Massif in the Late Palaeozoic. The Na–Ca–K content (Na: 29,700–49,600 ppm, Ca: 25,700–46,200 ppm, K: 1,000–5,620 ppm) is similar to that of the mineralising fluids in other Pb–Zn districts, interpreted to be of evaporative origin (e.g. Newfoundland, East Tennessee, Polaris). Furthermore, comparison of the Na–Ca–K content of the fluids with that of an evolved evaporitic brine enables the recognition of major water–rock interactions that modified the fluid composition. It indicates that the ambient fluids participated in the early diagenetic dolomitisation of Upper Palaeozoic carbonates and also in the albitisation of plagioclase in Lower Palaeozoic siliciclastics of the Caledonian basement. Illitisation of smectites or dissolution of K-feldspar probably controlled the K-content of the fluids. A model is proposed where the bittern brines migrated down into the deep subsurface because of their density during extension. After the Variscan orogeny, these fluids were finally expelled along extensional faults, resulting in the formation of Zn–Pb deposits. Received: 26 April 2000 / Accepted: 22 November 2000     

Characteristics of Cu–Mo Mineralization in the Chatree Mining Area,Central Thailand

Cu–Mo mineralization occurs in southern part of the Chatree Au–Ag deposit, central Thailand. Quartz veins of Cu–Mo mineralization are divided into five types: Types A, B, C, D and E. Quartz veins of Types A, B and C are hosted in altered granodiorite porphyry, and quartz veins of Types D and E occur in altered andesite lava. Mineral assemblages of Types A, B and C quartz veins are composed of qz–chl–ilt–mol–py–ccp, qz–chl–ilt–ccp–py and qz–chl–ilt–ccp–py–sp–po, respectively. Types D and E quartz veins consist of qz–chl–py–ccp–sp–po and qz–ep, respectively. Fluid inclusions of quartz veins are divided into liquid‐rich two‐phases fluid inclusion, vapor‐rich two‐phases fluid inclusion and multiphase solid‐bearing fluid inclusion. Coexistence of a halite‐bearing fluid inclusion having salinity of 37 equiv. wt.% NaCl and a vapor‐rich two‐phases fluid inclusion having salinity of 1 equiv. wt.% NaCl suggests that the Cu–Mo‐bearing quartz veins were formed at temperature of 450°C and pressure of 250 bars (depth of approximately 1.5 km from the paleosurface). Based on the formation temperature of 450°C of quartz veins and the δ¹⁸O values of quartz of the quartz veins, the δ¹⁸O value of fluid responsible for the Cu–Mo‐bearing quartz vein is estimated to be +9.9‰. The origin of fluid forming the Cu–Mo‐bearing quartz veins in the N prospect of the Chatree mining area would be magmatic water. Based on the characteristics of geology, age, mineral assemblage and the formation environment, Cu–Mo mineralization would be different from the epithermal Au–Ag mineralization of the Chatree mining area.     

Trace elements and cathodoluminescence of quartz in stockwork veins of Mongolian porphyry-style deposits

The combination of scanning electron microscope–cathodoluminescence (CL), fluid inclusion analysis and high-resolution electron probe microanalysis of Al, Ti, K and Fe in vein quartz has yielded results permitting a greater understanding of the complex mineralisation of the Central Oyu Tolgoi and Zesen Uul porphyry-style deposits, southern Mongolia. These data demonstrate the relationship between quartz precipitation, dissolution and ore deposition as the mineralising fluid chemistry changed through time. Four major quartz generations are identified in the A-type veins from the stockworks of both the Central Oyu Tolgoi (OTi to OTiv) and Zesen Uul deposits (ZUi to ZUiv). Despite differences in the associated alteration and mineralisation style, the observed CL textures and trace element signatures of the quartz generations are comparable between deposits. The OTi and ZUi stage formed both the primary network of A-type veins and pervasive silicification of the host rock. Using the Ti-in-quartz geothermometer, crystallisation temperatures for OTi and ZUi of between 598°C and 880°C are indicated. The main stage of sulphide mineralisation was accompanied by the dissolution of pre-existing quartz (OTi and ZUi) and precipitation of a weakly luminescent generation of quartz (OTii and ZUii) with a low Ti content, reflected in a calculated temperature drop from approximately 700°C to 340°C in Central Oyu Tolgoi and 445°C in Zesen Uul. OTii and ZUii stage quartzes show high and variable Al concentrations. The next stage of quartz in both deposits (OTiii and ZUiii) forms a fine network of veins in cracks formed in pre-existing quartz. OTiii and ZUiii quartz contain measurable fluid inclusions of moderate salinity (3–17.1 wt.% NaCl eq.), entrapped in the temperature range 256°C to 385°C. OTiii and ZUiii are not related to any sulphide mineralisation. The final OTiv and ZUiv stages are characterised by quartz–calcite micro-breccias that penetrate the A-type veins. Based on the calculated entrapment temperatures, the OTiv/ZUiv stage crystallised between 212°C and 335°C, and the quartz is characterised by elevated but variable Al and Fe contents. The CL and trace element signatures of the OTi to OTiii and ZUi to ZUiii stages of the two Mongolian porphyries show similar features to those observed in porphyry-style deposits from other regions. This suggests that a common sequence of quartz crystallisation occurs during the formation of early veins in many porphyry copper systems.     

Fluid Inclusions and Stable Isotopic Characteristics of the Yaoling Tungsten Deposit in South China: Metallogenetic Constraints

The Yaoling tungsten deposit is a typical wolframite quartz vein‐type tungsten deposit in the South China metallogenic province. The wolframite‐bearing quartz veins mainly occur in Cambrian to Ordovician host rocks or in Mesozoic granitic rocks and are controlled by the west‐north‐west trending extensional faults. The ore mineralization mainly comprises wolframite and variable amounts of molybdenite, chalcopyrite, pyrite, fluorite, and tourmaline. Hydrothermal alteration is well developed at the Yaoling tungsten deposit, including greisenization, silicification, fluoritization, and tourmalinization. Three types of primary/pseudosecondary fluid inclusions have been identified in vein quartz, which is intimately intergrown with wolframite. These include two‐phase liquid‐rich aqueous inclusions (type I), two‐ or three‐phase CO₂‐rich inclusions (type II), and type III daughter mineral‐bearing multiphase high‐salinity aqueous inclusions. Microthermometric measurements reveal consistent moderate homogenization temperatures (peak values from 200 to 280°C), and low to high salinities (1.3–39 wt % NaCl equiv.) for the type I, type II, and type III inclusions, where the CO₂‐rich type II inclusions display trace amounts of CH₄ and N₂. The ore‐forming fluids are far more saline than those of other tungsten deposits reported in South China. The estimated maximum trapping pressure of the ore‐forming fluids is about 1230–1760 bar, corresponding to a lithostatic depth of 4.0–5.8 km. The δD_H2O isotopic compositions of the inclusion fluid ranges from ?66.7 to ?47.8‰, with δ¹⁸O_H2O values between 1.63 and 4.17‰, δ¹³C values of ?6.5–0.8‰, and δ³⁴S values between ?1.98 and 1.92‰, with an average of ?0.07‰. The stable isotope data imply that the ore‐forming fluids of the Yaoling tungsten deposit were mainly derived from crustal magmatic fluids with some involvement of meteoric water. Fluid immiscibility and fluid–rock interaction are thought to have been the main mechanisms for tungsten precipitation at Yaoling.     

Evolution of a metamorphic fluid and its role in lode gold mineralisation in the Central lberian Zone

The Central Iberian Zone of Spain and Portugal contains numerous gold deposits of varying size, age and host lithology. Previous work concentrated on single deposits leading to a need to apply a range of techniques to the region as a whole, to form a model for metallogenesis throughout the region. With this aim, a number of occurrences of gold mineralisation in the south-western Central Iberian Zone were studied. The Central Iberian Zone consists essentially of greenschist facies metasedimentary units intruded by Hercynian granitoids. Hercynian deformation occurred in three main stages, the earliest (D1) being the most intense. Quartz veining is common throughout the region, and a vein chronology was established relative to these deformation events. V1 veins are pre-Hercynian, and may predate the Precambrian Sardic deformation. V2 veins are syndeformational (D1), and associated with peak greenschist metamorphism. V3 veins are late-kinematic, syn- to post-D2. V4 veins are syn- to post-D3, occurring in the final stages of the Hercynian, and often associated with or post-dating granites. Reactivation of vein sets by later fluids was common. Fluid chemistry studies of samples of known chronology indicate three main sources of fluid. The dominant fluid throughout the region from V2 to V4 is a dilute aqueo-carbonic fluid of varying composition. The combination of fluid inclusion analyses and geochemical studies of wallrock alteration indicates that the fluid was the result of devolatilisation and dehydration during prograde metamorphism at varying depths. The resulting aqueo-carbonic fluids flowed upwards, mixing with each other and with meteoric waters. Contact metamorphism associated with granite intrusions during the D3 deformation released further aqueo-carbonic fluids, which in some cases unmixed to form separate carbonic and aqueous inclusions. Circulation of meteoric waters was important during later stages (D3). Prolonged low temperature fluid-rock interaction produced highly saline aqueous fluids which post-date all major quartz veining. No single set of characteristics exists for gold mineralisation in the Central Iberian Zone. Deposits cover a wide spectrum, but three main types have been identified: (1) early (D1/V2) syn-peak metamorphic gold-quartz veins, associated with an aqueo-carbonic fluid with high N₂ and CH₄; (2) late-kinematic (D2/V3) veins, often associated with late brittle faults. Again, the fluid present at the time of formation was aqueo-carbonic, but with lower N₂ and CH₄. (3) Late (V4) low temperature veins, formed by a mainly meteoric fluid, which was also responsible for remobilisation and concentration of gold in earlier deposits. The gold mineralisation in the Central Iberian Zone supports the continuum model for lode gold deposits. Gold was leached from metasediments during prograde metamorphism, transported to shallower crustal levels, and deposited wherever suitable traps occurred. Channelling of fluid from deeper levels resulted in the localisation of deposits close to major shear zones. Depositional sites were usually dilational zones in faults, and more rarely folds, but locally lithological controls were important. H₂O-CO₂ unmixing was common, but was not necessarily related to gold deposition. Received: 19 February 1996 / Accepted: 11 February 1997