Turbidite-hosted Gold Deposits Research Articles

Source of the Chaoyangzhai Gold Deposit, Southeast Guizhou: Constraints from LA-ICP-MS Zircon U–Pb Dating, Whole-rock Geochemistry and In Situ Sulfur Isotopes

The Chaoyangzhai gold deposit is one of the newly discovered medium to large scale turbidite-hosted gold deposits in Southeast Guizhou, South China. In this study, laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) zircon U–Pb dating on the tuffaceous- and sandy-slates of Qingshuijiang Formation, Xiajiang Group, and gold-bearing quartz vein yielded similar age distributions, indicating that zircon grains in gold-bearing quartz vein originated from the surrounding tuffaceous- and sandy-slates. In addition, the youngest weighted mean ages of the zircon grains from the tuffaceous- and sandy-slates were 775 ± 13 Ma and 777 ± 16 Ma, respectively, displaying that the tuffaceous- and sandy-slates of the Qingshuijiang Formation were likely deposited in Neoproterozoic. Based on their major and trace element compositions, the tuffaceous- and sandy-slates were sourced from a felsic igneous provenance. The sandy slates have higher contents of Au (mostly ranging from 0.019 to 0.252 ppm), than those of the tuffaceous slates (mostly lower than 0.005 ppm). The δ34SV-CDT values of pyrite and arsenopyrite of the gold-bearing samples range from +8.12‰ to +9.99‰ and from +9.78 to +10.78‰, respectively, indicating that the sulfur source was from the metamorphic rocks. Together with the evidence of similar geochemical patterns between the tuffaceous- and sandy-slates and gold-bearing quartz, it is proposed that the gold might be mainly sourced from sandy slates. The metamorphic devolatilization, which was caused by the Caledonian orogeny (Xuefeng Orogenic Event), resulted in the formation of the ore-forming fluid. Gold was likely deposited in the fractures due to changes of the physico-chemical conditions, leading to the formation of the Chaoyangzhai gold deposit, and the large-scale gold mineralization in Southeast Guizhou.

Review of the structural architecture of turbidite-hosted gold deposits, Victoria, Australia

Turbidite-hosted gold deposits have variable structural architectures and it is important for exploration and mining that the basic controls on the structures and veins hosting mineralisation are well understood. Significant advances have been made in Victoria (primarily) during the last decade by implementing high-resolution stratigraphic logging. This technique has allowed very thorough correlation and interpretation at several gold fields. Detailed reconstruction of fold and fault geometries and an understanding of the significant lithological controls on major reef-forming structures and vein distribution have resulted. The insights arising from this work have enabled more detailed depictions of mineralised shoots for exploration targetting and resource modelling.

The Timing of Yellowknife Gold Mineralization: A Temporal Relationship with Crustal Anatexis?

In order to thoroughly evaluate the genesis of orogenic gold mineralization in the context of plate tectonics, absolute age data are required to link mineralization with lithological evolution. We show that pyrite, associated with orogenic gold mineralization from the Con deposit in the ca. 2700 Ma Yellowknife greenstone belt, has an Re-Os isochron age of 2591 ± 37 Ma. This is identical to the previously proposed timing of ca. 2592 Ma for S-type pluton-related lode-gold within ca. 2660 Ma graywacke-mudstone turbidites east of the greenstone belt. The Re-Os pyrite data indicate that orogenic gold mineralization was deposited ca. 60 to 100 m.y. after the formation of the host rocks in the Yellowknife district and, when combined with the regional geologic framework, indicate that the gold mineralization formed in a distinct geodynamic setting from that during host-rock formation. Initial Os values in the pyrite (Os i = 0.78 ± 0.17) indicate a significant crustal component of Os to the ore system, compatible with metals being derived from the older host rocks. Regionally preserved granulites, migmatites, and charnockites derived, in part, from the older supracrustal sequences, formed at 2595 to 2585 Ma, corresponding to the timing of gold mineralization. S-type plutons with ca. 2592 Ma crystallization ages in the upper crust reflect this high-temperature metamorphic event; these plutons have turbidite-hosted gold deposits associated with their metamorphic aureoles. Collectively, the isotopic and regional geologic data indicate that gold mineralization was related to regional-scale crustal anatexis, which likely resulted from crustal accretion or collision outside what is now preserved of the craton.

Paleoproterozoic, turbidite-hosted, gold deposits of the Ashanti gold belt (Ghana, West Africa): Comparative analysis of turbidite-hosted gold deposits and an updated genetic model

The Paleoproterozoic gold deposits of the Ashanti gold belt, Ghana, occur mainly as auriferous quartz veins in graphitic shears proximal to contacts between deformed and metamorphosed turbidites and tholeiitic volcanics (alternately block-faulted Tarkwaian sediments). They are suggested in this article to belong to that class of gold deposits recognized as ‘turbidite-hosted’. Hence, evidence from previous investigations of turbidite-hosted and turbidite-associated gold deposits occurring in circumstances roughly similar to the Ashanti deposits are used, where relevant, to produce an updated genetic model for the latter. Given the relatively significant amounts of graphite associated with the major Ashanti deposits, the role of non-carbonate carbon is assessed in the light of conclusions reached by work on other turbidite-hosted and turbidite-associated gold belts. Conclusions as to the origins of mineralizing fluids and temporary repositories of gold (auriferous interflow sediments) reached by work on these other belts are also analysed. A concept of the tectonic development of the Ashanti belt-Kumasi basin area, introduced in this article, is deemed to be critical to the origin of the Ashanti gold deposits. It encompasses the role of non-carbonate carbon, turbidites, and the primary sources and secondary repositories of Au and As in the genesis of the major Ashanti gold deposits. It examines the significance of the intimate relationship between calc alkaline volcanism and the basin sediments, the occurrences of felsic metasandstones in transition-zone sediments, and TTG granitic material and fragments of chemical sediments in turbidites: all of which are incompatible with one or the other of existing concepts of tectonic development in the area.

Gold deposits hosted in Ordovician sedimentary rocks of the Rinconada range (Jujuy- Argentina): implications for exploration.

La faja de sedimentitas ordovicicas de la sierra de Rinconada (Puna Jujena, Argentina), constituye un lugar clave para la exploracion de mineralizaciones del tipo 'fajas de pizarras auriferas en cinturones orogenicos'. Con el objetivo de establecer los controles geologicos sobre estas mineralizaciones, se llevaron a cabo estudios estratigraficos, estructurales y geoquimicos en distintos sectores de la sierra. De acuerdo a los resultados obtenidos los principales controles son: el arreglo estratigrafico de la secuencia, el estilo de plegamiento y la geoquimica de alteracion hidrotermal. Las secuencias constituidas por alternancias ritmicas de areniscas y pelitas, resultaron ser las mas favorables en funcion de su signatura geoquimica original y debido al comportamiento reologico durante el plegamiento. Las mineralizaciones se encuentran confinadas a lo largo de ejes anticlinales, siendo mas favorables aquellas estructuras de gran amplitud y longitud de onda. Si bien se reconocen multiples pulsos de silicificacion (hasta con 19 g/ton de Au en vetas) hay mineralizacion diseminada (0,6 g/ton de Au) en la caja sin indicios de silicificacion. Las caracteristicas geoquimicas demuestran la existencia de sistemas hidrotermales ricos en Au, As (Sb) con cantidades subordinadas de metales base (Pb, Zn, Cu (Mo)). A partir del estudio geoquimico de los metasedimentos se concluye que la razon As/Sb puede utilizarse como una buena guia para establecer intervalos optimos de mineralizacion, aunque esta relacion puede ser condicionada por los mecanismos de dispersion y fijacion, asi como la disponibilidad de elementos en el medio sedimentario. La conjuncion de factores como el ambiente de deposicion, el estilo de deformacion de la secuencia y el nivel estructural involucrado, es la clave para la localizacion de manifestaciones de interes economico. La similitud de estas mineralizaciones con yacimientos auriferos australianos, abre interesantes perspectivas para la exploracion minera del sector.

Controls on the formation of the Magdala gold deposit at Stawell, Western Victoria, Australia

The Stawell goldfield lies within a reworked portion of the Delamerian Fold Belt in western Victoria and has a structural complexity and a gold-lode style that is markedly different from similar ca 440 Ma aged deposits in the Lachlan Fold Belt such as Bendigo or Ballarat. Comparisons with these other turbidite-hosted gold deposits in Victoria suggest four major differences: (1) presence of a tholeiitic basaltic pile; (2) ductile deformation (D1-4) over at least 60 million years prior to gold mineralisation; (3) a preferential and localised reactivation of pre-existing structures within a progressively rotating stress field and (4) highly evolved hydrothermal alteration. Of these differences a key element has been the nature of the basaltic pile, which may have promoted alteration of onlapping sedimentary rocks, thereby creating a basis from which a high sulphur turbidite-hosted orogenic gold deposit was formed.

Hydrothermal alteration at the Magdala gold deposit, Stawell, western Victoria*

The Magdala deposit in the Stawell goldfield in western Victoria was formed during the 440 Ma gold event of the Lachlan Orogeny and is hosted by Cambrian quartz-rich turbiditic sedimentary rocks (Albion Formation) that onlap a thick pile of tholeiitic basaltic lavas (Magdala Basalt). Detailed petrographic and geochemical analyses suggest that the host-rock (Stawell Facies) was originally a turbiditic sedimentary rock that was hydrothermally altered in response to seawater interaction with the hot basaltic pile. Subsequent regional greenschist metamorphism and ductile deformation that lasted at least 10 million years culminated in the formation of the Magdala mineralised system and produced a complex pattern of hydrothermal alteration. Evolution of this alteration occurred over six stages: Stage 1, Fe-enrichment of sedimentary rock adjacent to the basalt pre-D1; Stage 2, chlorite (metamorphism), orbicular carbonate and pyrite, syn-D2; Stage 3, muscovite, siderite, ankerite and pyrrhotite, syn-D3 – D4a – b; Stage 4, stilpnomelane, siderite, pyrrhotite, arsenopyrite and pyrite, syn-D4c; Stage 5, silica, minnesotaite and magnetite, post-D4c – pre-D5; and Stage 6, Fe-rich chlorite, muscovite, calcite, arsenopyrite and pyrite, syn-D5. Comparisons with other turbidite-hosted gold deposits in Victoria (e.g. Bendigo and Ballarat) highlight four major differences: (i) presence of a tholeiitic basaltic pile; (ii) ductile deformation (D1 – 4) over at least 60 million years prior to gold mineralisation; (iii) highly evolved hydrothermal alteration; and (iv) source of sulfur. Of these differences the key element is the basaltic pile and its associated heat, which may have promoted the growth of micro-organisms in, and alteration of, the onlapping sedimentary rocks, thereby creating a basis from which an unusual turbidite-hosted orogenic-gold deposit was formed.

Chemical controls on mineral transport and precipitation in turbidite-hosted gold deposits

Chemical controls on mineral transport and precipitation in turbidite-hosted gold deposits

Controls to hydrothermal gold mineralization in the Witwatersberg Goldfield; situated in the floor to the south of the Bushveld Complex, South Africa

An exploration programme over the Blaauwbank group of gold deposits in the Witwatersberg Goldfield, which lies to the south and in the footwall of the Bushveld Complex, South Africa, has shown that these deposits possess many similarities and some differences to turbidite-hosted gold deposits. The dominant control to this hydrothermal mineralization is structural, with the associated quartz veins being localized along bedding planes, fold axes and a variety of semi-ductile to brittle faults. The deposits of the Witwatersberg Goldfield are also preferentially located in the Klapperkop Quartzite Member and overlying shales of the Timeball Hill Formation, Transvaal Supergroup. The rocks do not appear to form a particularly favourable protore but rather represent a favourable depositional site. The competency contrast of the shales, siltstones and sandstones in this stratigraphic interval is regarded as being important in facilitating vein formation, and the zone is also characterized by a high Fe 2+/Fe 3+ ratio and high C content, which may have reduced the mineralizing fluids and caused gold to precipitate. Fluid inclusion data show the quartz veins formed at 250–330 °C and pressures of 1.3–1.75 kb, from CaCl 2-rich brines. Comparison with structural studies to the north and with the Sabie-Pilgrim’s Rest Goldfield to the east suggest that the mineralization probably predates the peak of metamorphism related to the intrusion of the Bushveld Complex.

A fractal filtering technique for processing regional geochemical maps for mineral exploration

A regional geochemical map interpolated from point data, usually sampled in surficial media such as stream sediments or lake sediments, may contain a large amount of information critical for mineral exploration and environmental studies. The geochemical map is, however, not ‘ready-to-use’ for such tasks as the determination of a local ‘anomaly’ or the characterization of a regional trend of one or more chemical elements as may be required for the purpose of mineral resource prediction. This becomes possible only after the map has been clearly divided into different components. Fractal filtering, a recently developed technique for decomposing a map or image into different components, helps to separate the anomaly from background or to extract other meaningful patterns from the geochemical map using both frequency and spatial information. The fractal filters are formed by applying the fractal concentration-area model to the power spectrum of the processed geochemical field. They often constitute a group of irregularly shaped filters in the frequency domain that can separate the domain of wave numbers into distinct regions, each with a power spectrum following a similar power-law or fractal property. The corresponding patterns of the separate components are obtained after transformation back to the spatial domain. The fractal filter can be applied to decompose the original geochemical field into a set of map components with distinct scaling ranges and anisotropy. The analysis of relationships among these decomposed maps can provide useful information for the interpretation and evaluation of anomalies or trends. This paper briefly introduces the theory behind the fractal filtering technique. A case study of regional geochemical data of lake sediments from western Meguma Terrain, Southern Nova Scotia, Canada, is used to illustrate application of this technique to process the regional geochemical maps of the study area for the prediction of the turbidite-hosted gold deposits.

Pb isotope fingerprinting of mesothermal gold deposits from central Victoria, Australia: implications for ore genesis

New Pb isotope data from three major mesothermal lode gold deposits (Ballarat West, Tarnagulla, Maldon) in central Victoria support a model whereby the metals derived from a large reservoir with a long residence time in the crust below the Palaeozoic Lachlan Fold Belt. The Pb isotopic ratios of least radiogenic samples from these deposits are in close agreement with published Pb signatures for turbidite-hosted gold deposits, and for Devonian granites, elsewhere in the Lachlan Fold Belt. Despite their spatial distribution and variations in the geological setting, the Pb signatures point to the extraction and transport of metals from a crustal source area by long-lasting, large-scale hydrothermal systems, resulting in the prominent homogenisation of Pb isotopic ratios. The enduring interaction between large hydrothermal systems and an extensive crustal source reservoir were a vital pre-requisite in the formation of the Victorian gold province. In this regard, the prospectivity of Victoria is analogous to world-class ore provinces elsewhere, such as the Archaean Yilgarn Block in Western Australia.

A comparative study of isotopes and geochronology of the turbiditehosted gold deposits in Bendigo, Victoria, Australia and Rouergai, Sichuan, China

A comparative study of isotopes and geochronology of the turbiditehosted gold deposits in Bendigo, Victoria, Australia and Rouergai, Sichuan, China

Wallrock alteration associated with turbidite-hosted gold deposits. Examples from the Palaeozoic Lachlan Fold Belt in central Victoria, Australia

The study of wallrock alteration phenomena around slate belt-hosted gold deposits can provide valuable information regarding the physico-chemical conditions of the ore-bearing fluids and the timing relationships between deformation, peak metamorphism, granitoid emplacement and mineralisation. Thus, studying the processes involved in the interaction of the hydrothermal fluids with the host rock is critical in order to constrain genetic models and, ultimately, as a tool for exploration. Largely due to the characteristics of the host rocks, turbidite-hosted mesothermal gold deposits were historically believed to lack such diagnostic alteration haloes and their study has long been neglected. However, recent studies in the Palaeozoic Lachlan Fold Belt in southeastern Australia and in similar tectonic settings elsewhere have demonstrated that alteration around many of these deposits is more widespread and pervasive than previously thought. A review of existing information and new data strongly support the viability of studying hydrothermal alteration as a guide to turbidite-hosted mineralisation and as an indicator of the character of the solutions associated with ore deposition. Characteristically, the auriferous reef structures are surrounded by broad bleached zones, while disseminations of carbonate spots, pyrite and arsenopyrite porphyroblasts are the most obvious features of alteration. Depending on parameters such as fault geometry, fluid pressure, and reactivity and permeability of the host rocks, these disseminations can occur up to a kilometre away from the auriferous structures. Sericitisation and carbonatisation are commonly discernible within 3- to 10-m-wide haloes. Wallrock alteration chemistry of turbidite-hosted gold deposits in the Lachlan Fold Belt shows consistent enrichments of K 2O, CO 2, S and As, and the anomalous values of these elements also provide the most useful dispersion haloes for turbidite-hosted gold exploration due to both their regularity and extent.

Turbidite-hosted gold deposits of Central Victoria, Australia: their regional setting, mineralising styles, and some genetic constraints

Victoria is a gold province of international stature with primary gold mineralisation richly developed in the pre-Carboniferous rocks of the Lachlan Fold Belt. Total recorded production is in the order of 2,460,000 kg gold of which some 60% was sourced from extensive alluvial deposits, both of deep and shallow lead origin. Eleven mining centres each produced over 30 tonnes of gold with the major producer being Bendigo (684,300 kg). The Palaeozoic sequence in central Victoria is characterised by Cambrian metabasites or greenstones with associated volcanogenic and pelagic sequences followed by a Cambro–Ordovician to Early Devonian sequence of silici-clastic sediments. This succession is characterised by north–south trending open to tight folds and west-dipping thrust faults. Extensive S- and I-type granites, were emplaced from Late Silurian to Late Devonian, with the latter group, in part, associated with acid volcanic complexes. Mineralising styles include mesothermal deposits, either turbidite-hosted or dyke-affiliated, and these account for the bulk of primary gold mineralisation in the State. Less significant epithermal prospects and deposits are associated with either Cambrian calc-alkaline volcanics or sediments, typically of Siluro–Devonian age. Other styles of minor importance include Cambrian volcanogenic exhalative Cu–Zn mineralisation with gold as a minor credit (0.15–0.6 g/t Au), granite-hosted gold, and a potentially important group comprising disseminated gold in sediments. Examples of this latter group may exhibit spatially-related coticule development and by analogy with Nova Scotian disseminated intrametallic deposits, may have implications for a hitherto unrecognised style of gold mineralisation in Victoria. A gradation in lode-Au mineralising styles occurs across central Victoria. To the west in the Stawell metallogenic province, mineralisation was emplaced structurally and stratigraphically lowest in the crust. Mesothermal mineralisation is shear-hosted in Cambrian meta-sediments and greenstones exhibiting repeated deformation, multiple foliations, and extensive shearing. Host sediments were dominated by greenschist grade regional metamorphic conditions. Within the Ballarat metallogenic province, mesothermal mineralisation is hosted by dilational-site's style, in Ordovician sediments which were subjected to upper prehnite–pumpellyite facies conditions to the west and zeolite facies conditions to the east adjacent to the Melbourne metallogenic province. Ore fluids were of the low salinity C–O–H type with 1–10 wt% NaCl equivalent. CO 2 and CH 4 contents of fluid inclusions tend to be low with variable CO 2/CH 4 ratios. Depth of deposition (based on evidence from Wattle Gully mine) was in the order of 7 km and isotopic data suggest that ore fluids were derived either from an evolved igneous or from a devolatilised metamorphic source. Towards the eastern margin of the Ballarat metallogenic province and within the adjoining Siluro–Devonian Melbourne metallogenic province, epithermal style Au mineralisation is commonly emplaced in structurally and stratigraphically higher rocks. Trapping temperatures are lower than in deposits to the west and ore fluids are of low salinity C–O–H–N type with N 2>15 vol%. Gangue pyrite has elevated Sb contents and ore fluids were dominated by evolved meteoric water. δ 13 C values from vein carbonates suggest their derivation from oxidised organic carbon from within the Siluro–Devonian marine sediments. Alteration styles in gold deposits across central Victoria include sericitisation, carbonatisation, sulphidation, chloritisation, and (de-) silicification. Owing to the primary mineral assemblages and the physico-chemical characteristics of the ore-bearing fluids, visible evidence for wall rock alteration is commonly limited to 5–10 m wide zones of discolouration containing disseminated arsenopyrite and pyrite porphyroblasts and carbonate spotting. However, current petrographical and geochemical studies have demonstrated that the effects of pervasive wall rock alteration are more pronounced and extensive than previously recognised. Structural deformation within the Stawell and Ballarat metallogenic provinces, both from field evidence and 40 Ar / 39 Ar dating of cleavage phyllosilicates, indicates an Early Silurian event which migrated progressively eastwards with time. This Silurian event probably dates accretion of the proto Lachlan Fold Belt with the Kanmantoo Group of foreland Australia. Preliminary dating of hydrothermal sericites and felsic dykes associated with mineralisation, suggests episodic mineralising events 20–70 million years after accretion. These events demonstrate a spatial and temporal relationship with low-pressure regional metamorphism and/or granite plutonism; the latter being concentrated in two main pulses (400 Ma and 365 Ma) in central Victoria. The source of gold is equivocal with both igneous and devolatilised metamorphic sources being suggested. One potential reservoir source is the basement Cambrian greenstones of both MORB and boninitic affinity with associated sulphidic interflow sediments. Initial geochemical data indicate mean values of 65 ppb Au in these sulphidic interflow sediments which could have reacted with CO 2-rich fluids developed during Siluro–Devonian regional metamorphism and magmatism. Subsequent deposition of gold from such fluids could have occurred in structurally higher level dilational traps.

The geochemistry of wall rock alteration in turbidite-hosted gold vein deposits, central Victoria, Australia

Gold deposits hosted in Ordovician to Devonian turbidites in central Victoria, Australia, consist of steeply dipping quartz vein systems hosted mainly by reverse faults. Wall rock alteration of the host metasandstones, metasiltstones and shales (or slates) extends at least 20 m from the veins in the Bendigo-Ballarat zone (BBZ) and 10 m in the Melbourne zone (MZ) deposits. Alteration minerals include carbonates (ankerite, siderite and dolomite) chlorite, seriate, arsenopyrite, pyrite, chalcopyrite and sphalerite, with less common barite, albite and biotite in the BBZ and carbonates (siderite, ankerite, dolomite and ferromagnesite), sericite, chlorite, arsenopyrite, pyrite, and stibnite with less common chalcostibite in the MZ. SiO 2, Na 2O, MgO and Sr with P 2O 5 commonly decreasing during alteration while CO 2, S, As, Sb, Au, V, Al 2O 3, Ga, K 2O and Rb with Ni and Cr commonly increase. TiO 2, FeO, Fe 2O 3, MnO, Th, U, Nb, La, Ce, and Sc showed little change. Concentrations of Zn, Cu, Pb, and Ca are variable. The relatively large decrease of SiO 2 could account for most, if not all, quartz present in the ore veins. The Na 2O and MgO could have crystallized in the veins in the very minor albite and chlorite present. The addition of V in all and Ni and Cr in most deposits probably reflects a source enriched in these elements such as underlying greenstones. The source of both the volatile (S, As, Sb) and lithophile (K 2O and Rb) elements as well as Au is unknown, but they could have been derived from a magmatic source or from the metamorphism of Cambrian greenstones. CO 2, present as carbonate, was derived mainly by the reaction of graphite, originally present in the sediments, with the ore solutions. Al 2O 3, the only other major element after SiO 2, probably increased mainly due to the decrease of the latter.

Turbidite-Hosted Gold Deposits in the Bendigo-Ballarat and Melbourne Zones, Australia. II. Nature of Ore Fluids

Fluids in mesothermal, turbidite-hosted, gold vein deposits in central Victoria, Australia are characterized by temperatures ranging from 180 to 310° C and salinities of less than 10 wt% NaCl equivalent. From west to east there is a general decrease in temperatures, averaging near 300° C in the western Bendigo-Ballarat Zone (BBZ), 270° C in the eastern BBZ, and 230° C in the Melbourne Zone (MZ). Pre-mineralization veins generally are lower in temperature than those hosting mineralization, whereas post-mineralization veins have the lowest average temperatures. Salinities are low, <10 wt% NaCl equivalent; those in deposits in the MZ are slightly lower than those in the BBZ. Salinities are variable in a given deposit or even a single vein period, but there is a weak trend of dilution toward late mineralization and post-mineralization stages. Some pre-mineralization veins have higher salinities (to 9 wt%) than mineralized veins. Fluids in the BBZ have C-O-H compositions, whereas those in the MZ are C-O-H-N. I...

Turbidite-Hosted Gold Deposits in the Bendigo-Ballarat and Melbourne Zones, Australia. I. Geology, Mineralization, Stable Isotopes, and Implications for Exploration

Major gold provinces in central Victoria, Australia, are separated by the Heathcote Fault Zone into two zones: the Bendigo-Ballarat Zone (BBZ) and the Melbourne Zone (MZ). The gold-quartz vein deposits are hosted predominantly by reverse fault systems in the Ordovician to Early Devonian turbidites, which have undergone low-grade regional metamorphism as well as deformation. Mineralization in the two zones is distinct, predominantly Au-As in the BBZ and Au-Sb-As in the MZ. In the BBZ, quartz veins are large (up to 5 m wide) and are hosted by Ordovician metasediments. Large deposits are confined to the reverse faults or fold crests in the N-S-elongated narrow domal structures. The sequence of vein genesis is: early narrow, auriferous, laminated veins; massive barren veins; one or more periods of narrow, auriferous, brecciated veins; and late carbonate-quartz veins. Arsenopyrite, pyrite, chalcopyrite, and gold (fineness = 960) are found in the laminated veins, whereas a similar assemblage plus galena and sph...

Link between ridge subduction and gold mineralization in southern Alaska

40 Ar/ 39 Ar geochronology reveals that turbidite-hosted gold deposits in the southern Alaska accretionary prism are the same age as nearby near-trench plutons. These early Tertiary plutons and gold lodes formed above a slab window during subduction of an oceanic spreading center. Ridge subduction is a previously unrecognized tectonic process for the generation of lode gold.

Nature and evolution of metamorphic fluids associated with turbidite-hosted gold deposits: Hill End goldfield, NSW, Australia

AbstractThe Hill goldfield, NSW, Australia, is an example of a syntectonic, slate-belt gold deposit formed in a multiply deformed, Late Silurian slate-metagreywacke turbidite sequence. Gold is confined to bedding-parallel veins and discordant leader veins composed of as many as four generations of quartz, accompanied by phyllosilicates, carbonates and minor sulphides. Vein formation and gold deposition was apparently synchronous with Early Carboniferous metamorphism and deformation. Homogenisation temperatures (Th) for fluid inclusions in vein quartz demonstrate five groupings in the temperature intervals 350-280°C 280-250°C 250-190°C 190-150°C and 150-110°C corresponding to a variety of primary and secondary inclusions developed during four periods of vein quartz deposition under a generally declining temperature regime. Inclusion fluids are characterised by a low salinity of around 0.1 to 3.6 wt. % NaCl equivalent. The dominant gas phase present in the inclusion fluids varies from N2 in the early stages of the paragenesis, through CH 4 during the main episode of gold deposition, to CO2- rich fluids associated with late-stage mineralisation. δ18O values for vein quartz (range 15.1-17.1‰) and vein carbonate (range 11.3-13.4‰) are typical of metamorphic mineralisation. δD composition of hydrous minerals and inclusion fluids (range −53 to −138‰) suggest an influx of meteoric water in the later mineralising fluids. This conclusion is supported by δ13C data for vein calcite (range −2.5 to −9.7%0). δ34S composition of vein pyrrhotite and pyrite ranges from 6.9 to 7.8‰ early in the paragenesis, to lighter values (around 4.2 to 5.8%0) accompanying late gold deposition from more oxidising fluids. Sulphur isotope data imply a sulphur source from underlying turbidites and an increase in fluid oxidation state during mineralisation . Lead isotope measurements on vein pyrite, arseno py rite, galena and gold are characterised by two isotope populations with 207Pb/206Pb ratios of 0.862 and 0.860, which define two discrete mineralising events during vein formation. Consistency between data from vein minerals and lead isotope signatures for potential source rocks indicate that lead was derived from the sedimentary pile.

Gold mineralization in Lower Cambrian McNaughton Formation, Athabasca Pass, Canadian Rocky Mountains: structural, mineralogical, and temporal relationships

Gold-bearing quartz veins were recently discovered in archimetamorphic quartzite–rudite and quartzite–pelite sequences of the Lower Cambrian McNaughton Formation in the main ranges of the central Canadian Rocky Mountains. There are two distinct vein types: an early syntectonic, syn- to postmetamorphic, auriferous, bedding-parallel type, generated during repeated northeast-directed compressive tectonism; and a late, postpenetrative deformational discordant type, which contains only minor gold (&lt;500 ppb Au). Gold emplacement and discordant veining were confined to the onset of late compression leading to development of the Chatter Creek Fault.The spatial distribution and dimensions of the veins attest to mechanically founded lithologic anisotropy. Bedding-parallel veins are confined to less competent, volumetrically minor pelitic rock types. Discordant veins are confined to competent quartzitic units.Bedding-parallel vein filling took place in two paragenetic stages: a protracted, pre-gold stage (quartz ± minor white mica and pyrite) depositing over 90% of vein material; and a late gold-bearing–post-gold stage (quartz–pyrite–gold–galena ± white mica and Fe carbonate). Gold usually occurs in association with brecciated pelites and penecontemporaneous sulfides. Discordant veins (quartz ± minor pyrite) record a single stage of vein filling broadly coeval with gold deposition. Minor hydrothermal alteration (pyrite ± white mica ± carbonate) of adjacent wall rocks accompanied gold emplacement and is mineralogically congruent with the regional archimetamorphic and bedding-parallel vein assemblages.In terms of their setting and morphology, the Athabasca Pass gold lodes are viewed as a siliciclastic-hosted analogue of the turbidite-hosted class of gold deposits. Principle geochemical differences from published process models for turbidite-hosted gold deposits are a function of the mineralogical maturity of the siliciclastic host strata in the Athabasca Pass.