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     

Geochemistry and genesis of the Lengenbach Pb-Zn-As-Tl-Ba-mineralisation,Binn Valley,Switzerland

 The Lengenbach Pb-Zn-As-Tl-Ba mineralisation is located in Triassic dolostones of the Penninic zone in the Swiss Alps where Alpine metamorphism reached upper greenschist to lower amphibolite grade. Geochemical data are used to constrain the origin of this unique occurrence. Two metamorphic redox environments are present: the As(III)-rich zone is controlled by barite-pyrite while the reduced zone contains graphite or pyrrhotite-pyrite and formally zerovalent As. The As(III)-rich zone is characterised by a mineral assemblage consistent with fO₂ in the stability field of barite+pyrite. An As-(Pb, Tl)-rich sulphide melt coexisted with a hydrothermal fluid at >kk300 °C in this zone. Mineralised dolostones are anomalous in As, Pb, Ag, Tl, Hg, Zn, Ba, Cd, Fe, Cu, Mo, U, V, B, Ga, Cr and possibly Sn and Au (in order of decreasing enrichment). As, Pb and Zn are present in the 0.1 to 1% range, Tl and Ag reach several hundred ppm. Uraninite is concentrated in silicate-rich bands and yields a late Alpine U-Pb age of 18.5±0.5 Ma. Pb- and S isotopic variations are interpreted by metamorphic overprinting and re-equilibration within an isochemically metamorphosed mineralisation. Hydrothermal sulphides are more strongly affected by uranogenic Pb than massive Pb-As-sulphides representing a former sulphide melt. The least overprinted mineralisation is characterised by ²⁰⁶Pb/²⁰⁴Pb U003U=18.44−18.56, ²⁰⁷Pb/²⁰⁴Pb=15.60−15.75, ²⁰⁸Pb/²⁰⁴Pb =38.44−38.84 and δ³⁴S (sulphide)=−25±2‰. S isotopic variations are largely a result of sulphide-sulphate re-equilibration yielding temperatures of 450± 30 °C. ⁸⁷Sr/⁸⁶Sr ratios of mineralised samples are lower than or equal to host dolostones, precluding major infiltration of basement-derived fluids during Alpine metamorphism. The Sr source (⁸⁷Sr/⁸⁶Sr close to 0.708) probably was seawater with a radiogenic, detrital mineral component. The genesis of the unique Lengenbach mineralisation is interpreted as the result of isochemical metamorphic overprinting of a carbonate hosted stratiform sulphide mineralisation. Well-crystallised sulphide minerals in fissures and druses formed during retrograde cooling of a sulphide melt in equilibrium with a hydrothermal fluid. The primary mineralisation was probably formed at or close below the sea floor and fed by sulphide-poor hydrothermal fluids. Sulphide was largely derived from seawater by open system bacterial sulphate reduction. U, V and Mo may be seawater-derived. Received: 1 February 1995/Accepted: 10 January 1996     

Evaluation of metallogenic potential of the Nuggihalli greenstone belt,South India

Chromite from the north–south-trending schist belt of Nuggihalli (southern Karnataka), belonging to the greenstone belt of South India (3.0–3.5 Ga) and crystallising at 1178 °C under oxygen fugacity of ?6.67 shows komatiitic affinity. The chemical data of the Cr-spinels when plotted in the form of the discriminant function of Johan (1979) show a remarkable follow-up of the curve A, which delineates the boundary between the ‘barren’ and ‘fertile’ (with respect to the plausible presence of sulphide phases) Cr-spinel bodies. Only a few samples plot marginally within the barren region (Fig. 1), suggesting that the chromite bands should hardly hold the possibility of becoming host of sulphide mineralisation of commercial relevance.     

Metamorphic Rock-hosted Disseminated Gold Deposits—A Case Study of the Xiaotongjiapuzi Gold Deposit of Eastern Liaoning

The Xiaotongjiapuzi gold deposit occurs in a Palaeoproterozoic rift accretionary terrane in eastern Liaoning and is hosted by a carbonate formation of the Dashiqiao Formation, Liaohe Group. The metamorphic grade of the host rock is low amphibolite-high greenschist facies. Gold, which is mainly invisible, is distributed in pyrite and arsenopyrite. The grains of Au-bearing sulphide minerals are fine, ranging from 0.0 n to 1 mm. The wall-rock alterations are characterized by low-temperature silification, hydro-sericitization, sericitization and carbonatization. The homogenization temperatures of the fluid inclusions in quartz closely associated with gold mineralization range from 140 to 240° C. The metallogenic age represented by the Ar-Ar isotopic age of sericite is 167 Ma. Comparisons and studies show that the Xiaotongjiapuzi gold deposit can be classified as the submicron-sized disseminated deposit (analogous to the Carlin-type) hosted in metamorphic rocks.     

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.     

Archaean lode gold mineralisation in banded iron formation at the Kalahari Goldridge deposit, Kraaipan Greenstone Belt, South Africa

The Kalahari Goldridge Mine is located within the Archaean Kraaipan Greenstone Belt, about 60 km southwest of Mafikeng in the North West Province, South Africa. The ore body thickness varies from 15 to 45 m along a strike length of about 1.5 km within approximately N–S striking banded iron formation (BIF). The stratabound ore body is hosted primarily by BIF, which consists of alternating chert and magnetite–chlorite–stilpnomelane–sulphide–carbonate bands of millimetre- to centimetre scale. A footwall of sericite–carbonate–chlorite schist underlain by mafic amphibolite occurs to the west and carbonaceous metapelites in the hanging wall to the east. Overlying the hanging wall, carbonaceous metapelites, units of coarse-grained metagreywackes fining upwards, become increasingly conglomeratic up the stratigraphy. Small-scale isoclinal folds, brecciation, extension fractures and boudinage of cherty BIF units reflect brittle-ductile deformation. Fold axial planes have foliation, with subvertical plunges parallel to prominent rodding and mineral lineation in the footwall rocks. Gold mineralisation is associated with two generations of quartz–carbonate veins, dipping approximately 20° to 40° W. The first generation consists of ladder-vein sets (group IIA) preferentially developed in centimetre-scale Fe-rich mesobands, whereas the second generation consists of large quartz–carbonate veins (group IIB), which locally crosscut the entire ore body and extend into the footwall and hanging wall. The ore body is controlled by mesoscale isoclinal folds approximately 67° E, orthogonal to the plane of mineralised, gently dipping veins, defining the principal stretching direction and development of fluid-focussing conduits. The intersections of the mineralised veins and foliation planes of the host rock plunges approximately 08° to the north. Pervasive hydrothermal alteration is characterised by chloritisation, carbonatisation, sulphidation and K-metasomatism. Gold is closely associated with sulphides, mainly pyrite and pyrrhotite, and to a lesser extent, with bismuth tellurides and carbonate minerals. Mass balance transfer calculations indicate that hydrothermal alteration of BIF involved enrichment of Au, Ag, Bi, Te, S and CO₂ (LOI), MgO, Ba, K and Rb, but significant depletion of SiO₂ and, to a lesser extent, Fe₂O₃. Extensive replacement of magnetite and chlorite in BIF and other pelitic sedimentary rocks by sulphide and carbonate minerals, both on mesoscopic and microscopic scales, is evidence of interaction of CO₂- and H₂S-bearing fluids with the Fe-rich host rocks. The fineness of gold grains ranges from 823 to 921, similar to that of other epigenetic Archaean BIF-hosted gold deposits, worldwide.     

Geochemical alteration halos around the Mount Morgan gold-copper deposit, Queensland, Australia

The Mount Morgan Au-Cu pyritic massive sulphide deposit occurs in a north-trending belt of Middle Paleozoic volcanic rocks located in south-central Queensland. The host rocks for the deposit are a normal sequence of rhyolitic tuff that have a north-northwest regional strike and easterly dips of 20° to 30°. The tuff contains thin units of chert, jasperoid and carbonate.The Mount Morgan deposit was represented by a zone of sulphide mineralization 600 m long, 100–200 m wide and 300 m deep that transects stratigraphy and can be divided into: (1) an oxidized zone, characterized by a hematitic, Au-enriched gossan with minor stratiform sphalerite-argillite; and (2) a primary zone which can be subdivided into an upper zone of greater than 50% sulphide minerals (Main Pipe orebody), and a lower siliceous stockwork zone with approximately 20% sulphide minerals (Sugarloaf orebody). Pyrite is the most abundant sulphide mineral in both the upper and lower primary zones with lesser pyrrhotite and accessory chalcopyrite, sphalerite and gold. A zone of silicification forms an envelope around the orebody and extends stratigraphically downwards in a pipe-like zone for greater than 750 m. The orebody contained 67 Mt of 4.87 g/t Au and 0.70% Cu.The distribution and variation of between 7 and 29 elements and specific conductance were examined in 1252 samples of the host rocks taken from diamond drill core and surface outcrop. The host rocks in the immediate vicinity of the deposit are marked by the development of three distinct but overlapping chemical and mineralogical zones representing an outward progression from the most intensive to a less intensive alteration. A 50-m-thick siliceous inner zone of intensely altered rocks, depleted in all investigated elements except Si, surrounds the orebody. This zone passes outward into a 100-m-thick middle zone of dominantly sericite-pyrite characterized by high concentrations of K, Fe, Cu and Co. The sericite-pyrite zone, in turn, passes into an outer 100-m-thick chlorite zone with high Fe, Mg, Mn and Zn concentrations. High concentrations of H₂O⁺ are associated with the sericite-pyrite zone and the chlorite zone. The alteration pipe underlying the Mount Morgan orebody is characterized by depletions in Na, Ca and K and enrichments in Fe and Mg. A non-economic pyrite body contained within the alteration pipe has spatially restricted enrichment halos of Fe, Mg, Zn, Cu and Co.     

Experimental and numerical simulation study of the mineral sequestration mechanism of the Shiqianfeng saline aquifers in the Ordos Basin,Northwest China

This study focused on typical injection layers of deep saline aquifers in the Shiqianfeng Formation used in the Carbon Capture and Sequestration Demonstration Projects in the Ordos Basin, Northwest China. The study employed experiments and numerical simulations to investigate the mechanism of CO₂ mineral sequestration in these deep saline aquifers. The experimental results showed that the dissolved minerals are plagioclase, hematite, illite–smectite mixed layer clay and illite, whereas the precipitated minerals are quartz (at 55, and 70 °C) and kaolinite (at 70 °C). There are rare carbonate mineral precipitations at the experimental time scale, while the precipitation of quartz as a product of the dissolution of silicate minerals and some intermediate minerals rich in K and Mg that transform to clay minerals, reveals the possibility of carbonate precipitation at the longer time scale. These results are consistent with some results previously reported in the literature. We calibrated the kinetic parameters of mineral dissolution and precipitation by these experimental results and then simulated the CO₂ mineral sequestration under deep saline aquifer conditions. The simulation results showed that the dissolved minerals are albite, anorthite and minor hematite, whereas the precipitated minerals are calcite, kaolinite and smectite at 55 and 70 °C. The geochemical reaction of illite is more complex. At 55 °C, illite is dissolved at the relatively lag time and transformed to dawsonite; at 70 °C, illite is precipitated in the early reaction period and then transformed to kaolinite. Based on this research, sequestrated CO₂ minerals, which are mainly related to the temperature of deep saline aquifers in Shiqianfeng Fm., are calcite and dawsonite at lower temperature, and calcite at higher temperature. The simulation results also establish that calcite could precipitate over a time scale of thousands of years, and the higher the temperature the sooner such a process would occur due to increased reaction rates. These characteristics are conducive, not only to the earlier occurrence of mineral sequestration, but also increase the sequestration capacity of the same mineral components. For a sequestration period of 10,000 years, we determined that the mineral sequestration capacity is 0.786 kg/m³ at 55 °C, and 2.180 kg/m³ at 70 °C. Furthermore, the occurrence of mineral sequestration indirectly increases the solubility of CO₂ in the early reaction period, but this decreases with the increase in temperature.     

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.     

Paleofluid analysis from fracture-fill cements in the Asmari limestones of the Kuh-I-Mond field, SW Zagros, Iran

Kuh-I-Mond field in the Zagros foreland basin is a conventional heavy oil resource and is composed of fractured carbonates whose fractures were filled by calcite, dolomite, and anhydrite cements. Oil inclusions occurred within the fracture-fill cements indicate that fractures were open and played an active role during oil migration and charge. The highest measured values for secondary porosities belong to fractures in Asmari Formation, which is characterized by significant amounts of vug- and fracture-filling cements. Fractures facilitated fluid circulation and subsequently dissolution of allochems and high Mg carbonates. In contrast, fine-grained carbonate facies were less cemented, and thus, porosity enhancement by cement dissolution was insignificant. Temperature profiles of oil inclusions in the dolomite, calcite, and anhydrite minerals characterized by distinct variations in the homogenization temperatures (Th) that are divided into two ranges below 50°C in anhydrites and from 45°C to 125°C in dolomites and calcites. The lower Th ranges for anhydrite suggests that it may have formed at shallower burial depths during early to middle diagenesis. The oil inclusions display trend for increasing temperature downward which conform to Formation geothermal gradient. In other word, the decreasing trend of Th temperatures upward within Asmari Formation that can be observed in Th versus depth plot is consistent with the uplift events at Late Miocene time and later that caused removal of about 1,300 m of the crest of the Kuh-I-Mond anticline. Vitrinite reflectance data from source rock intervals in the field area do not support vertical migration of locally generated hydrocarbons into the Kuh-I-Mond accumulation, and long-distance lateral oil migration and charge from a source kitchen to the southwest is proposed. Vitrinite reflectance data from this dolomite and limestone reservoir suggest low maturation levels corresponding to paleotemperatures less than 50°C. The observed maturation level (<0.5% Ro) does not exceed values for simple burial maturation based on the estimated burial history. Also, homogenization temperatures from fluid inclusion populations in calcite and dolomites show expected good correlation with reflectance-derived temperatures. The Th data represent pore fluids became warmer and more saline during burial. As aqueous fluid inclusions in calcite veins were homogenized between 22°C and 90°C with a decrease in salinity from 22 to 18 eq.?wt.% NaCl. The Th values suggest a change in water composition and that dolomite and calcite cements might have precipitated from petroleum-derived fluids. The hydrocarbon fluid inclusions microthermometry data suggest that the reservoir was being filled by heavy black oils in reservoir during Cenozoic. Aqueous fluid inclusions hosted by calcite equant sparry/fossil cavity fills suggest low cementation temperatures (<45°C) and high salinities (19 eq.?wt.% NaCl), while those in dolostones are characterized by highly variable homogenization temperature (52°C to 125°C) and salinities (6.5 to 20 eq.?wt.% NaCl).     

The role of intrusions in the formation of Irish-type mineralisation

The Stonepark Prospect is located in County Limerick, south-central Ireland. Multiple zones of Zn–Pb mineralisation have been identified at Stonepark and these are approximately 5 km west of the Pallas Green Prospect. At Stonepark, the sulphide bodies are hosted within the Waulsortian Limestone and closely resemble other Irish-type deposits. The mineralisation is composed of pyrite-marcasite, sphalerite and galena with gangue Fe-dolomite and calcite cements. A key difference at Stonepark is the presence of Chadian-aged volcanic rocks (Knockroe Volcanics) that intrude into and overlie the Waulsortian Limestone. Subsequent hydrothermal brecciation of the Waulsortian Limestone and Knockroe intrusions resulted in the formation of tabular polymict breccia bodies containing mixed carbonate and clasts of intrusive rocks. These have then been overprinted by massive sulphide mineralisation. Further syn-mineralisation brecciation has overprinted the earlier breccias. Drilling has demonstrated a spatial relationship between the volume of intrusive rocks (dykes and polymict breccias) and Zn–Pb mineralisation. This association suggests that the intrusive rocks provided a mechanism for the introduction of the mineralising fluids into the breccia bodies. This is significant as to date no large controlling fault has been identified, as is seen at other Irish-type deposits. Further work is required to understand the alteration process of the intrusive rocks and how this may relate to the mineralising process.     

Age constraints on Tarkwaian palaeoplacer and lode-gold formation in the Tarkwa-Damang district, SW Ghana

Two major epigenetic gold-forming events are recorded in the world-class gold province of southwest Ghana. A pre-Tarkwaian event was the source of the world-class Tarkwa palaeoplacers whereas post-Birimian and Tarkwaian deformation, which was related to the Eburnean orogeny, gave rise to the world-class (e.g. Prestea) to giant (e.g. Obuasi) orogenic gold deposits which have made the region famous for more than 2,500 years. A maximum age of 2133±4 Ma for Tarkwaian sedimentation is provided by 71 of 111 concordant SHRIMP II U–Pb dates from detrital zircons in Tarkwaian clastic rocks from Damang and Bippo Bin, northeast of Tarkwa. The overall data distribution broadly overlaps the relatively poorly constrained ages of Birimian volcanism and associated Dixcove-type granitoid emplacement, indicating syntectonic development of the Tarkwaian sedimentary basin. These zircon ages argue against derivation of the palaeoplacer gold from an orogenic gold source related to the compressional phase of an orogeny significantly older than the Eburnean orogeny. Instead, they suggest that the gold source was either orogenic gold lodes related to an earlier compressional phase of a diachronous Eburnean orogeny or ca. 2200–2100 Ma intrusion-related gold lode. The CO₂-rich fluid inclusions in associated vein-quartz pebbles are permissive of either source. At the Damang deposit, an epigenetic, orogenic lode-gold system clearly overprinted, and sulphidised low-grade palaeoplacer hematite–magnetite gold occurrences in the Banket Series conglomerate within the Tarkwaian sedimentary sequence. Gold mineralisation is demonstrably post-peak metamorphism, as gold-related alteration assemblages overprint metamorphic assemblages in host rocks. In alteration zones surrounding the dominant, subhorizontal auriferous quartz veins, there are rare occurrences of hydrothermal xenotime which give a SHRIMP U–Pb age of 2063±9 Ma for gold mineralisation. The similar structural timing of epigenetic gold mineralisation in Tarkwaian host rocks at Damang to that in mainly Birimian host rocks elsewhere in southwest Ghana, particularly at Obuasi, suggests that 2063±9 Ma is the best available age estimate for widespread orogenic gold mineralisation in the region. Argon–argon ages of 2029±4 and 2034±4 Ma for hydrothermal biotite from auriferous quartz veins appear to represent uplift and cooling of the region below about 300 °C, as estimates of the temperature of gold mineralisation are higher, at around 400 °C. If peak metamorphism, with temperatures of about 550 °C, is assumed to have occurred at about 2100 Ma, the biotite ages, in combination with the xenotime age, suggest a broadly constant uplift rate for the region of about 1 km per 10 million years from about 2100 to 2025 Ma.     

Syn-amphibolite facies archaean lode gold mineralisation in the Mt. York District,Pilbara Block,Western Australia

The Archaean lode gold deposits in the Mt. York District, Pilbara Block, Western Australia are hosted in banded iron formation (Main Hill/Breccia Hill prospect) of the ca. 3.33 Ga Gorge Creek Group and in amphibolites (Zakanaka prospect) of the ca. 3.46 Ga Warrawoona Group. Gold mineralisation at the Main Hill/Breccia Hill prospect is associated with breccias comprising quartz clasts in a quartz-pyrrhotite matrix, and quartz-amphibole veins, with löllingite being the major host for gold. Minimum temperatures for gold mineralisation at the prospect are constrained as 455°C to 550°C by arsenopyrite thermometry. Gold mineralisation at the Zakanaka prospect is spatially associated with quartzclinopyroxene-calcite-microcline-calcic-amphibole veins and biotite altered wallrock adjacent to the veins. Temperatures for vein emplacement are estimated as 480°C to 570°C using both plagioclase-amphibole thermometry and mineral equilibria with respect to T and X_CO2. The timing of gold mineralisation relative to the peak of metamorphism is constrained by mineral textures and the relative temperatures of hydrothermal alteration and metamorphism. Gold mineralisation at both deposits was broadly synchronous with the peak of regional amphibolite facies metamorphism, which reached temperatures of 520°C to 640°C based on amphibole-plagioclase and garnet-biotite thermometry. In this respect, the deposits are similar to other well documented syn-amphibolite facies lode gold deposits from the Archaean Southern Cross greenstone belt in the Yilgarn Block, and represent the deeper section of a crustal continuum of lode gold deposits that includes mesothermal deposits such as those at Kalgoorlie at higher crustal levels.     

The genesis of Pb‐Zn sulphide occurrences in the Lower Devonian Buchan Group,Victoria

Six minor sulphide occurrences hosted by the Lower Devonian Buchan Group have been investigated. Sulphide minerals and associated phases are hosted by both dolostone and limestone lithologies along stylolitized bedding planes, cross‐cutting fractures, low angle minor faults and in cavities. Mineralization was closely associated with minor structures of inferred Tabberabberan age (Middle Devonian), which it appears to have post‐dated, but was largely strata‐bound in nature. The mineralogy of the occurrences is simple and characterized by the following generalized paragenesis which reflects the increasing oxidation state and pH of the mineralizing fluids: pyrite (pseudomorphous after marcasite)‐galena ± sphalerite ± pyrite‐dolomite ± barite‐calcite ± fluorite ± dolomite.

The sulphur isotope composition of sulphide minerals varies from ‐32.1 to +4.1‰, with iron and base metal sulphide minerals forming two distinct populations around ‐25‰ and 0‰, respectively. A single barite sample gives a sulphur isotope composition of +22.4‰, which is similar to that estimated for Early Devonian seawater. Fluid inclusions in fluorite and calcite homogenize at temperatures in the range 160 to 212° C and have average salinities of approximately 10 wt% NaCI eq. Sphalerite contains up to 1.81 wt% iron which correlates with colour, and up to 1.43 wt% cadmium. The Pb isotopic pattern of galena suggests a source region with U/Pb(= μ) lower than the crustal average, and a high Th/U.

A genetic hypothesis is proposed which involves the circulation of saline fluids through the Snowy River Volcanics, which directly underlie the Buchan Group, during or at some time after the Tabberabberan Orogeny. Although the Buchan occurrences show features characteristic of both Mississippi Valley‐type and stratiform ore deposits, they are most directly comparable to the epigenetic zones of Irish carbonate‐hosted base‐metal deposits. However, Pb‐Zn sulphide mineralization at Buchan appears to have been associated with minor compressional structures, suggesting that a simple correlation with the Irish deposits is not directly applicable.     

Timing and genesis of base-metal mineralisation in black shales of the Upper Permian Ravnefjeld Formation,Wegener Halvø, East Greenland

Bituminous mud shales of the Upper Permian Ravnefjeld Formation (Zechstein 1 equivalent) are mineralised with zinc, lead and copper within a ca. 50 km² area on Wegener Halvø in central East Greenland. The occurrence of base-metal sulphides in shale nodules cemented prior to compaction indicates an early commencement of base-metal mineralisation. In other cases, post-compactional sulphide textures are observed. Homogeneous lead isotope signatures of galena and sphalerite from the shales (²⁰⁶Pb/²⁰⁴Pb: 18.440–18.466; ²⁰⁷Pb/²⁰⁴Pb: 16.554–16.586; ²⁰⁸Pb/²⁰⁴Pb: 38.240–38.326) suggest that all base metals were introduced during a single hydrothermal event. Therefore, post-compactional textures are believed to result from recrystallisation of early diagenetic sulphides during deep burial in the Upper Cretaceous to Tertiary. Lead isotope signatures of galena hosted in Upper Permian carbonate build-ups are relatively heterogeneous compared to those of the shale-hosted sulphides. The observed relations indicate a shared lead source for the two types of mineralisation, but different degrees of homogenisation during mineralisation. This suggests that lead was introduced to the carbonate rocks and black shales during two separate events. δ³⁴S of base-metal sulphides in the Ravnefjeld Formation lie between –12 and –4‰, whereas synsedimentary and early diagenetic pyrite in unmineralised shales in general have δ³⁴S between –47 and –16.5‰. Early diagenetic pyrite in the Wegener Halvø area in general has δ³⁴S 15 to 20‰ higher than the same pyrite morphotype in Triaselv in the western part of the basin. This relatively high δ³⁴S can be explained by extensive microbial sulphate reduction within persistent euxinic (super-anoxic) bottom waters under which supply of isotopically light seawater sulphate (and disproportionation of intermediate sulphur compounds) was restricted. The sulphur in the base-metal sulphides is believed to represent sulphide-dominated pore water, enriched in ³⁴S due to preferential removal of ³²S by sulphate-reducing bacteria and precipitation of diagenetic pyrite in the near-seafloor environment. We suggest that the sulphide-dominated pore water was trapped in the shale formation prior to introduction of base-metal-bearing fluids through fractures in the underlying carbonates, and that sulphide precipitation took place when the two fluids met. δ³⁴S values of carbonate-hosted base-metal sulphides fall within the same range as the shale-hosted ones. The relationship between barite and sulphides and evidence for pre-mineralisation entrapment of liquid hydrocarbons in the carbonates suggest that the sulphide in this case is derived by in-situ thermochemical sulphate reduction (TSR). Measured fractionation between sulphide and sulphate ranges from 18.5 to 24.4‰, suggesting temperatures of TSR around 70 to 100 °C. Vitrinite reflectance measurements in mineralised shale samples are all between 1.7 and 2.0%, except for samples taken close to a Tertiary dyke giving ca. 3.0%. Vitrinite reflectance data are comparable to previously published data from unmineralised shale samples in the area and could not be proven to correlate with the degree of mineralisation. This indicates that any early hydrothermal effect has been overprinted later, probably during deep burial in the Late Cretaceous to Early Tertiary as previously proposed.     

Mineralisation of amethyst-bearing geodes in Ametista do Sul (Brazil) from low-temperature sedimentary brines: evidence from monophase liquid inclusions and stable isotopes

Fluid inclusion studies in combination with hydrogen, oxygen and sulphur isotope data provide novel insights into the genesis of giant amethyst-bearing geodes in Early Cretaceous Paraná continental flood basalts at Amestita do Sul, Brazil. Monophase liquid inclusions in colourless quartz, amethyst, calcite, barite and gypsum were analysed by microthermometry after stimulating bubble nucleation using single femtosecond laser pulses. The salinity of the fluid inclusions was determined from ice-melting temperatures and a combination of prograde and retrograde homogenisation temperatures via the density maximum of the aqueous solutions. Four mineralisation stages are distinguished. In stage I, celadonite, chalcedony and pyrite formed under reducing conditions in a thermally stable environment. Low δ³⁴S_V-CDT values of pyrite (?25 to ?32?‰) suggest biogenic sulphate reduction by organotrophic bacteria. During the subsequent stages II (amethyst, goethite and anhydrite), III (early subhedral calcite) and IV (barite, late subhedral calcite and gypsum), the oxidation state of the fluid changed towards more oxidising conditions and microbial sulphate reduction ceased. Three distinct modes of fluid salinities around 5.3, 3.4 and 0.3 wt% NaCl-equivalent characterise the mineralisation stages II, III and IV, respectively. The salinity of the stage I fluid is unknown due to lack of fluid inclusions. Variation in homogenisation temperatures and in δ¹⁸O values of amethyst show evidence of repeated pulses of ascending hydrothermal fluids of up to 80–90 °C infiltrating a basaltic host rock of less than 45 °C. Colourless quartz and amethyst formed at temperatures between 40 and 80 °C, while the different calcite generations and late gypsum precipitated at temperatures below 45 °C. Calculated oxygen isotope composition of the amethyst-precipitating fluid in combination with δD values of amethyst-hosted fluid inclusions (?59 to ?51?‰) show a significant ¹⁸O-shift from the meteoric water line. This ¹⁸O-shift, high salinities of the fluid inclusions with chloride-sulphate composition, and high δ³⁴S values of anhydrite and barite (7.5 to 9.9?‰) suggest that sedimentary brines from deeper parts of the Guaraní aquifer system must have been responsible for the amethyst mineralisation.     

The Syama and Tabakoroni goldfields,Mali

Gold deposits in the Syama and Tabakoroni goldfields in southern Mali occur along a north-northeast trending mineralised litho-structural corridor that trends for approximately 40 km. The deposits are interpreted to have formed during a craton-wide metallogenic event during the Eburnean orogeny. In the Syama goldfield, gold mineralisation in 9 deposits is hosted in the hanging-wall of the Syama-Bananso Shear Zone in basalt, greywacke, argillite, lamprophyre, and black shale. Gold is currently mined primarily from the oxidised-weathered zone of the ore bodies. In the Syama deposit, mineralisation hosted in altered basalt is associated with an intense ankerite–quartz–pyrite stockwork vein systems, whereas disseminated style mineralisation is also present in greywackes. In contrast, the Tellem deposit is hosted in quartz–porphyry rocks.In the Tabakoroni goldfield, gold mineralisation is hosted in quartz veins in tertiary splay shears of the Syama-Bananso Shear Zone. The Tabakoroni orebody is associated with quartz, carbonate and graphite (stylolite) veins, with pyrite and lesser amounts of arsenopyrite. There are four main styles of gold mineralisation including silica-sulphide lodes in carbonaceous fault zones, stylolitic quartz reefs in fault zones, quartz–Fe–carbonate–sulphide lodes in mafic volcanics, and quartz–sulphide stockwork veins in silicified sediments and porphyry dykes. The several deposit styles in the goldfield thus present a number of potential exploration targets spatially associated with the regional Syama-Bananso Shear Zone and generally classified as orogenic shear-hosted gold deposits.     

Thrust-controlled gold mineralisation at the Elandshoogte Mine,Sabie-Pilgrim's Rest goldfield,South Africa

Stratiform quartz-sulphide-gold veins, locally termed reefs are hosted within the Proterozoic Transvaal Sequence sedimentary succession, in the Sabie-Pilgrim's Rest goldfield, eastern Transvaal. These deposits have produced about 180 tonnes of gold and share many characteristics with those of Telfer, Western Australia. Detailed examination of the Elandshoogte Mine shows that gold deposition occurred in two stages, both linked to bedding-parallel thrust faulting within the sedimentary pile. Deformation being concentrated within incompetent shale beds, interlayered within more competent units. The majority of gold was introduced in the second stage of mineralisation and occurs within fractures in early-formed sulphide minerals. Deposition of competent quartz veins accompanying early sulphide and gold mineralisation resulted in a change in deformation style within the reef zone, from early shearing in shales to later duplex faulting of the quartz-reef. Fluids accompanying faulting are implied to have transported gold, and a magmatic source of mineralisation is suggested.     

Oxygen isotope and fluid inclusion study of the Sorkhe-Dizaj iron oxide-apatite deposit,NW Iran

The Sorkhe-Dizaj orebody is located 32 km southeast of Zanjan within the Tarom subzone of the Alborz-Azarbaijan structural zone. It is hosted mainly in quartz monzonite-monzodiorite and, to a lesser extent, in volcanoclastic rocks. Mineralization occurs in the form of stockwork and veins, comprising predominantly magnetite and actinolite, with minor pyrite and chalcopyrite. Two generations of magnetite and apatite are inferred: the first as disseminations in the host rock and the second mainly as an alteration product of actinolite, secondary K-feldspar, silica, sericite, chlorite and epidote. Fluid inclusion studies were carried out on second-generation apatite, and late-stage quartz to understand the geochemical evolution of the ore-bearing fluids. Fluid inclusions are of three types, i.e. primary, secondary, and pseudo-secondary. These inclusions are liquid or vapour single-phase, two-phase rich in liquid or vapour, and three-phase. Homogenization temperatures of second-generation apatite are inferred to be between 209°C and 520°C (mostly between 290°C and 320°C), indicating salinities of 9.08–21.61 wt.% NaCl equiv. At 342°C, the δ¹⁸O values range from 9‰ to 11.32‰ for the second-generation magnetite associated with coeval apatite. Fluid inclusions in the late-stage quartz veins are inferred to have homogenized between 186°C and 263°C, with δ¹⁸O values ranging between 2.5‰ and 7.4‰ at 220°C. Oxygen isotopes in the late-stage carbonate veins have values of 3.28–6.14‰ at 100°C. These data in the late-stage veins imply introduction of a cooler, less saline, isotopically depleted fluid, probably meteoric water. Field observations, mineral parageneses, and fluid inclusion?+?oxygen isotope data suggest that the magnetite-apatite veins formed from a predominantly magmatic-derived fluid. Introduction of cooler meteoric water in the final stage of mineralization reduced δ¹⁸O values, facilitating precipitation of sulphides, quartz, and carbonate veins.     

The Kuh-e-Surmeh mineralization, a carbonate-hosted Zn-Pb deposit in the Simply Folded Belt of the Zagros Mountains, SW Iran

The Kuh-e-Surmeh carbonate-hosted zinc-lead deposit, located within the Simply Folded Belt of the Zagros Mountains in southwestern Iran, is an orogen-related Mississippi Valley type deposit originally formed in the foreland Thrust Belt of the Zagros Mountains. Structural and textural observations indicate that ore deposition took place as open-space fillings in brecciated carbonate rock and as internal sediments consisting of fine-grained ore minerals interlayered with carbonates. The preferred genetic model for the concentration of the ore metals is that of dewatering of the Zard-Kuh basin due to regional tectonic compaction tectonism and expulsion of basin-derived fluids into the highly porous and brecciated dolomitized rocks of the Dalan Formation. The metals precipitated from dense basinal brine (15 wt% equiv. NaCl) at low temperatures (less than 200 °C), typically within strata of a Late Paleozoic carbonate platform. Received: 21 July 1998 / Accepted: 20 August 1999