Sediment-hosted Zn-Pb Deposits Research Articles

Pyrite trace element halos to northern Australian sediment-hosted Zn–Pb–Ag deposits

Subtle primary halos form around many sedimenthosted Zn-Pb deposits. Northern Australian deposits may also have pyrite halos, representing the first paragenetic stage of the mineralising fluids. The occurrence of trace elements within this pyrite has potential toprovide vectors to the orebody or delineate mineralised from batten systems. Pyrite trace element compositions from rocks surrounding the McArthur River sediment-hosted Zn-Pb deposit. Northern Territory, Australia have been analysed by LA-ICPMS. Analysis was conducted for 25 elements. Over 95% of all pyrite analysed contained_> 0.10 wt% of trace elements, of which, Co, Ni, Cu, Zn, As, Sb, Ba, Tl, Pb, Mn, Mo, Ag, Bi and Sn are found in abundances significantly greater than detection limits. The host sediments contain no significant levels of Cu, Co, Ni, Pb, Zn. Concentrations of Co, Cu, Ni, Tl and Zn are significantly elevated at the orebody and hangingwall equivalent stratigraphy. As, Ba, Mo, Sb, Sn are slightly elevated at the orebody equivalent stratigraphy. Spatial distribution patterns of the trace elements are erratic, possibly reflecting variations in pH, oxidation state, or temperature across the basin at the time of formation.

Geologic, fluid inclusion and isotopic characteristics of the Jinding Zn–Pb deposit, western Yunnan, South China: A review

With a reserve of ∼ 200 Mt ore grading 6.08% Zn and 1.29% Pb (i.e., a metal reserve of ∼ 15 Mt) hosted in Cretaceous and Tertiary terrestrial rocks, the Jinding deposit is the largest Zn–Pb deposit in China, and also the youngest sediment-hosted super giant Zn–Pb deposit in the world. The deposit mainly occurs in the Jinding dome structure as tabular orebodies within breccia-bearing sandstones of the Palaeocene Yunlong Formation (autochthonous) and in the overlying sandstones of the Early Cretaceous Jingxing Formation (allochthonous). The deposit is not stratiform and no exhalative sedimentary rocks have been observed. The occurrence of the orebodies, presence of hangingwall alteration, and replacement and open-space filling textures all indicate an epigenetic origin. Formation of the Jinding Zn–Pb deposit is related to a period of major continental crust movement during the collision of the Indian and Eurasian Plates. The westward thrusts and dome structure were successively developed in the Palaeocene sedimentary rocks in the ore district, and Zn–Pb mineralisation appears to have taken place in the early stage of the doming processes. The study of fluid inclusions in sphalerite and associated gangue minerals (quartz, celestine, calcite and gypsum) shows that homogenisation temperatures ranged from 54 to 309 °C and cluster around 110 to 150 °C, with salinities of 1.6 to 18.0 wt.% NaCl equiv. Inert gas isotope studies from inclusions in ore- and gangue-minerals reveal 2.0 to 15.6% mantle He, 53% mantle Ne and a considerable amount of mantle Xe in the ore-forming fluids. The Pb-isotope composition of ores shows that the metal is mainly of mantle origin, mixed with a lesser amount of crustal lead. The widely variable and negative δ 34S values of Jinding sulphides suggest that thermo-chemical or bacterial sulphate reduction produced reduced sulphur for deposition of the Zn–Pb sulphides. The mixing of a mantle-sourced fluid enriched in metals and CO 2 with reduced sulphide-bearing saline formation water in a structural–lithologic trap may have been the key mechanism for the formation of the Jinding deposit. The Jinding deposit differs from known major types of sediment-hosted Zn–Pb deposits in the world, including sandstone-type (SST), Mississippi Valley type (MVT) and sedimentary-exhalative (SEDEX). Although the fine-grained ore texture and high Zn/Pb ratios are similar to those in SEDEX deposits, the Jinding deposit lacks any exhalative sedimentary rocks. Like MVT deposits, Jinding is characterised by simple mineralogy, epigenetic features and involvement of basinal brines in mineralisation, but its host rocks are mainly sandstones and breccia-bearing sandstones. The Jinding deposit is also different from SST deposits with its high Zn/Pb ratios, among other characteristics. Most importantly, the Jinding deposit was formed in an intracontinental terrestrial basin with an active tectonic history in relation to plate collision, and mantle-sourced fluids and metals played a major role in ore formation, which is not the case for SEDEX, MVT, and SST. We propose that Jinding represents a new type of sediment-hosted Zn–Pb deposit, named the ‘Jinding type’.

Molecular Geomicrobiology.: J. F. BANFIELD, J. CERVINISILVA, AND K. M. NEALSON, EDITORS. 294 Pp. Mineralogical Society of America, Reviews in Mineralogy and Geochemistry. Volume 59. 2005. ISBN: 093995071-5. US$40.00.

The field of geomicrobiology is rapidly expanding as both geoscientists and biologists have come to recognize the importance of both disciplines for understanding nature’s complexities. Bacteria and other microorganisms are extremely important in their direct and indirect effects on geochemical cycling of metals and organic matter throughout Earth history. Indeed, classically trained microbiologists are currently being hired by geoscience departments, which will ensure future beneficial collaborations in the field of geomicrobiology. Southam and Saunders (2005) reviewed much of the recent geomicrobiology literature in Economic Geology , trying to highlight important advances that might impact ore-forming processes and, perhaps, even geochemical prospecting techniques. No doubt, bacteria are directly responsible for forming a number of types of low-temperature ore deposits such as roll-front uranium deposits and Superior-type iron formations. In addition, bacteria have produced reduced sulfur for some types of higher-temperature deposits such as sediment-hosted Zn-Pb deposits. Bacteria have even been found as fossils in sulfide minerals. Thus, there are many aspects of bacterial metabolism that warrant understanding by economic geologists. Furthermore, as we explore for more deeply buried mineral deposits below the water table, exploration geochemists are going to have to factor in the effects of microbial processes in controlling dispersion of metals away from ores …

COMPOSIÇÃO E GEOTERMOMETRIA DOS FLUIDOS MINERALIZANTES EM DEPÓSITOS DE Zn-Pb DAS COBERTURAS NEOPROTEROZÓICAS DO CRÁTON DO SÃO FRANCISCO: IMPLICAÇÕES METALOGENÉTICAS

Fluid inclusion studies in primary and pseudo-secondary inclusions in sphalerite from sediment-hosted Pb-Zn deposits of the Sao Francisco Craton Neoproterozoic basins were performed to obtain minimum formation temperatures, and fluid composition. These data could be used to interpret the physico-chemical environment of the ore deposition and for exploration programs. In the Morro Agudo deposit, fluid inclusions in sphalerite indicate moderate saline solutions, around 14%wt eq. NaCl. The wide range of salinity within the orebodies is apparently related to a zonal distribution in relation to the main fault zone that controls the mineralization. Total homogenization temperatures (T H ) indicate also a zoned pattern closely related to the main fault zone. They are relatively higher in the ‘A’ Block, closer to the fault zone, varying from 122° to 283°C with a mean of 170°C in the JKL orebody. Away from the main fault , to the west, paleotemperatures are gradually lower, ranging from 80° to 168°C with a mean at 155°C, in the ‘C’ block. The stratiform ‘N’ orebody, in the upper part of the section exhibits the lowest temperature interval, varying between 120° and 144°C with a mean of 138°C. In the Nova Redencao deposit, fluid inclusions show moderate to high salinity aqueous solutions, with 24.3 wt%. NaCl eq., probably containing Ca, K and Mg salts. The homogenization temperatures range from 140° to 190°C with a mean of 185°C. In the Irece deposits, limited determinations in primary inclusions of sphalerite show homogenization temperatures (T H ) between 140 and 200°C and salinity between 3 and 10% NaCl equivalent. When compared with T H and salinities from typical MVT, SEDEX and Irish deposits, the obtained data fall in the field of Irish type.

Distribution and geochemical characteristics of barite and barium-rich rocks associated with the Broken Hill-type Gamsberg Zn-Pb deposit, Namaqua Province, South Africa

Research Article| March 01, 2005 Distribution and geochemical characteristics of barite and barium-rich rocks associated with the Broken Hill-type Gamsberg Zn-Pb deposit, Namaqua Province, South Africa M. Stalder; M. Stalder Department of Geology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa, email: mstalder@freesurf.ch Corresponding author: ar@sun.ac.za Search for other works by this author on: GSW Google Scholar A. Rozendaal A. Rozendaal Department of Geology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2005) 108 (1): 35–50. https://doi.org/10.2113/108.1.35 Article history first online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation M. Stalder, A. Rozendaal; Distribution and geochemical characteristics of barite and barium-rich rocks associated with the Broken Hill-type Gamsberg Zn-Pb deposit, Namaqua Province, South Africa. South African Journal of Geology 2005;; 108 (1): 35–50. doi: https://doi.org/10.2113/108.1.35 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Bedded barite forms a distinct component of four Broken Hill-type deposits in the amphibolite-facies metamorphosed Bushmanland sequence of South Africa. In the Gamsberg Zn-Pb deposit, barite has been effectively fractionated from the base metal sulphide facies and concentrated as a separate deposit towards the eastern part of the Gamsberg inselberg. The barite occurs as massive to laminated layers, associated with other oxidised lithologies, such as hematite-quartz rocks and manganiferous iron formations. Geochemical and isotopic evidence favours a non-marine, hydrothermal origin of the Ba and supports a close genetic relationship with the base metal sulphide ores. Fractionation of these hydrothermal components occurred due to redox conditions within the depositional basin, which restricted base metal precipitation to an anoxic basin facies and barite to an oxidised shelf facies. The spatial distribution of barite and other oxidised lithologies indicate that redox transitions occurred immediately before and after deposition of the base metal sulphide ores. Such rapid facies changes are typical for sediment-hosted Zn-Pb deposits, reflecting tectonically induced sub-basin formation and reactivation of feeder conduits, which represent important prerequisites for the formation of these deposits. In addition, the presence of barite in the Bushmanland deposits indicates that the ore-forming fluids were reduced, since Ba cannot be transported in the presence of sulphate. Reduced fluids are sensitive to temperature changes and a decrease in brine temperature most probably resulted in the termination of base metal sulphide deposition. Postdating deposition of the sulphide ores, Fe, Mn and Ba were still supplied to the system and precipitated as manganiferous iron formations and bedded barite lenses that overlie the sulphide horizon. Anomalous concentrations of Ba within the wall rocks of the Aggeneys-Gamsberg deposits indicate that hydrothermal discharge commenced before and continued after peak base metal sulphide deposition. The presence of this Ba halo has implications for exploration of similar deposits in the Namaqua Province. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Sulphur isotope patterns from the Bleiberg deposit (Eastern Alps) and their implications for genetically affiliated lead?zinc deposits

A set of 354 sulphur isotope data from the Bleiberg deposit, the type deposit of Alpine low temperature carbonate hosted Pb–Zn deposits (APT deposits), is critically evaluated applying statistical methods. The sulphur isotope patterns vary significantly among the ore horizons. This suggests a long lasting and polyphase mineralisation system. The sulphur isotope composition of barite corresponds to that of Carnian seawater (i.e. 16‰ δ34S). The δ34S values of the iron sulphides correspond to data from sedimentary iron sulphides. Pb and Zn sulphides are characterized by three normally distributed δ34S populations with mean values of −6 to −8‰, −13 to −18‰, and −25 to −29‰. Heavy sulphur (>−10‰ δ34S) indicates contribution of sulphide sulphur from epigenetic-hydrothermal fluids, whereas light sulphide sulphur (<−21‰ δ34S) was produced by sulphur – reducing bacteria. The intermediate population is explained by mixing of sulphur derived from these two sources. Other sources of local importance, however, can not be excluded. The isotope populations correspond only partly to the paragenetic ore stages. The sulphur isotope patterns in the APT deposits are regionally different. Data from other low-temperature sediment-hosted Pb–Zn deposits support the proposed interpretation. A comparison demonstrates that the sulphur isotope patterns of APT deposits correspond to patterns of the Irish type deposits, but are different to those of Mississippi Valley type deposits.

Origin of Ore-Forming Brines in Sediment-Hosted Zn-Pb Deposits of the Basque-Cantabrian Basin, Northern Spain

Origin of Ore-Forming Brines in Sediment-Hosted Zn-Pb Deposits of the Basque-Cantabrian Basin, Northern Spain

Origin of Ore-Forming Brines in Sediment-Hosted Zn-Pb Deposits of the Basque-Cantabrian Basin, Northern Spain

Fluid inclusion data (microthermometry and Na-K-Li-Cl-Br chemistry) from Mississippi Valley-type Zn-Pb deposits in the Basque-Cantabrian basin, north Iberian Peninsula, indicate that fluid mixing occurred during mineralization. Cl/Br ratios of the ore-forming brines suggest that the high salinity was primarily acquired by evaporation of seawater. Only in deposits near salt domes (Orduna and Murgia diapirs) do the ore-forming brines have halogen signatures indicative of halite dissolution. Mixing of fluids frequently cannot be detected using only microthermometry or halogen data, although microthermometric data can indicate mixing where one of the end members is fresh water or a highly diluted fluid. Combining the two allows both the recognition of mixing and an estimation of the relative proportion of the different fluids involved. The effects of mixing on the salinity and halogen ratios of the resulting mixture have been calculated using four types of fluids: (1) seawater, (2) evaporated seawater before the onset of halite precipitation, (3) seawater evaporated past the point of halite precipitation, and (4) a halite-saturated brine at 25oC derived from halite dissolution. The calculated Cl/Br-Na/Br mixing curves have been compared to data from the Zn-Pb deposits of the Basque-Cantabrian basin and show that mixing between a residual brine and a halite-dissolution brine can account for the deposits in the Western Biscay district (Txomin, Matienzo, Barambio). Most of the ore-forming brines from the studied deposits around the Orduna and Murgia salt domes (Altube, Monteleon, Jugo) originated by halite dissolution, although there is also significant contribution (20‐50 wt %) from a highly evaporated brine.

Processes of ore formation in the stratiform sediment-hosted Zn [sbnd]Pbdeposits of Northern Australia: testing the Century model

Numerical simulations of chemical processes of ore formation are used to test hypotheses concerning the genesis of northern Australian Proterozoic stratiform sediment-hosted Zn Pb deposits. Exhalation of metal-bearing brines into anoxic marine waters is shown to be effective for massive sulfide ore formation. The ‘Century’ model, late diagenetic massive sulfide deposition involving metal-laden brines interacting with hydrocarbon reservoirs, is also demonstrated to have potential as an ore-forming process. It requires that H 2S (g) is available in the gas reservoir and that acidification is prevented by dissolution of wall rock carbonate. Seawater mixing is capable of producing laminated sulfide ore. Brine-sour gas interaction will most likely produce discordant Zn Pb ore due to carbonate dissolution.

CEMENTATION, HYDROTHERMAL ALTERATION, AND Zn-Pb MINERALIZATION OF CARBONATE BRECCIAS IN THE IRISH MIDLANDS: TEXTURAL EVIDENCE FROM THE COOLEEN ZONE, NEAR SILVERMINES, COUNTY TIPPERARY

Sir: We are delighted to respond to the comments of Reed and Wallace on our paper on the origin of breccia-hosted base-metal mineralization in the Irish ore field (Lee and Wilkinson, 2002). Reed and Wallace’s thesis is that the mineralized breccias we have described within the Mississippian Waulsortian Limestone Formation at Cooleen are not of sedimentary origin and therefore, by extension, that synsedimen-tary breccias do not exist elsewhere in the ore field. If correct, one of a number of lines of evidence that have been used to infer a near–sea-floor origin for at least a component of sulfide mineralization in Ireland—that of reworked clasts of sulfide within sedimentary breccias—would be invalidated. Contrary to the suggestion of Reed and Wallace, the existence of sedimentary breccias spatially associated with mineralization in the Waulsortian is universally accepted in the literature. Mine-based investigations that include underground observations as well as drill core data and sedimentological studies have concluded that the stratiform, wedge-shaped breccia bodies commonly developed within the Waulsortian Limestone facies are sedimentary debris flows of Courceyan age (Boyce et al., 1983; Taylor, 1984; Andrew, 1986; Clifford et al., 1986; Somerville et al., 1992). Submarine fault scarp sedimentary breccias are common in sediment-hosted Zn-Pb deposits (e.g., Goodfellow et al., 1993) and display the same characteristics as the breccias at Cooleen. Hitzman and Beaty (1996, p. 125) also recognized that synsedimentary breccias are present within the Waulsortian at both Silvermines and Lisheen but went on to state that textural and field relationships clearly show that the principal episode of brecciation postdated regional burial dolomiti-zation, inferring a hydrothermal origin for the ore-hosting breccias. While we would not agree with this conclusion, particularly in relation to the Silvermines area, we accept that the ore-hosting breccias at Lisheen, Galmoy, and Silvermines have been …

The Importance of Oxidized Brines for the Formation of Australian Proterozoic Stratiform Sediment-Hosted Pb-Zn (Sedex) Deposits

A two-fold subdivision for stratiform sediment-hosted Pb-Zn (sedimentary exhalative, sedex) deposits is proposed, based on fundamental differences in the chemistry of the mineralizing brines. The type of sedimentary basin from which the ore fluids are derived, and the lithologies contained within the basin, control these differences in fluid chemistry. The two discrete brine types capable of transporting Zn and Pb are oxidized brines and reduced, acidic brines. McArthur-type deposits (e.g., McArthur River, Mount Isa, Hilton) precipitate from oxidized (SO42—predominant), acidic to near-neutral brines that evolve from sedimentary basins dominated by carbonates, evaporites, and hematitic sandstones and shales. Selwyn-type deposits (e.g., Sullivan, Rammelsberg, sedex deposits of the Selwyn basin) precipitate from acidic, reduced (H2S-predominant) connate brines that evolved in reduced siliciclastic and shale basins. Temperature decrease and dilution (fluid mixing), addition of H2S, and pH increase can all be effective depositional processes for Zn and Pb from reduced (Selwyn-type) brines. In contrast, sulfate reduction and/or addition of H2S (via fluid mixing or interaction with earlier formed pyrite) may be the important processes for sphalerite and galena deposition from oxidized (McArthur-type) brines. McArthur-type sedex deposits are intimately associated with siderite or ferroan carbonate alteration halos and most likely precipitate from lower temperature brines than Selwyn-type deposits. The redox state of the mineralized brines (sulfate or sulfide predominant) is important for controlling minor element associations in the two classes of sedex deposits. Weakly acidic to weakly alkaline oxidized brines can precipitate siderite but are incapable of carrying significant gold, tin, and barium in solution, and as such, McArthur-type deposits do not contain anomalous concentrations of these elements. Reduced, acid brines can carry high concentrations of barium, explaining the common association with barite in these deposits. If reduced sulfur concentrations were sufficient in the mineralizing brines, individual Selwyn-type deposits may contain anomalous or ore-grade gold. If the brines were highly reduced (pyrrhotite-stable), they may have carried high concentrations of tin (e.g., Sullivan). The lack of sulfide-bearing feeder systems in McArthur-type deposits and their common occurrence in Selwyn-type deposits probably also relate to the redox state of the brines. From a mineral exploration perspective, oxidized sedimentary brines are more likely to produce large tonnage Zn-Pb-Ag deposits that have siderite or ankerite alteration halos and commonly lack barite lenses and vent complexes. By contrast, deposits that form in reduced siliciclastic and shale-dominated basins are more likely to be lower tonnage and to contain barite, vent complexes and may have minor gold or tin credits.

Geological and Isotopic Constraints on the Metallogenic Evolution of the Proterozoic Sediment-Hosted Pb-Zn (Ag) Deposits of Brazil

Integrated studies of seven Proterozoic sediment-hosted, Pb-Zn-Ag sulfide deposits of Brazil, permit the estimation of the age of the hosting sequence and the mineralization, the nature of the sulfur and metal sources, the temperature range of sulfide formation and the environment of deposition. These deposits can be classified into three groups, according to their ages. (a) Archean to Paleoproterozoic: the Boquira deposit, in Bahia state, consists of stratiform massive and disseminated sulfides hosted by parametamorphic sequences of grunnerite-cummingtonite+magnetite that represent a silicate facies of the Boquira Formation (BF). Lead isotope data of galena samples indicate a time span between 2.7 and 2.5 Ga for ore formation, in agreement with the stratigraphic position of the BF. The relatively heavy sulfur isotope compositions for the disseminated and stratiform sulfides (+8.3 to +12.8 ‰ CDT)suggest a sedimentary source for the sulfur. (b) Paleo to Mesoproterozoic: stratiform and stratabound sulfides in association with growth faults are present in the Canoas mine (Ribeira, in Paraná state) and in the Caboclo mineralization (Bahia state). They are hosted by calcsilicates and amphibolites in the Canoas deposit, whereas in the Caboclo area the mineralization is associated with hydrothermally altered dolarenites at the base of the 1.2 Ga Caboclo Formation. The interpreted Pb-Pb age of the Canoas mineralization is coeval with the 1.7 Ga host rocks. Sulfur isotopic data for Canoas sulfides (+1.2 to +16 ‰ CDT) suggest a sea water source for the sulfur. The range between −21.1 and +8.8 ‰ CDT for the Caboclo sulfides could suggest the action of bacterial reduction of seawater sulfates, but this interpretation is not conclusive. (c) Neoproterozoic: stratiform and stratabound sulfide deposits formed during the complex diagenetic history of the host carbonate rocks from the Morro Agudo (Bambui Group), Irecê and Nova Redenção (Una Group), yield heavy sulfur isotope values (+18.9 to +39.4 ‰ CDT). The uniform heavy isotope composition of the barites from these deposits (+25.1 to +40.9 ‰) reflect their origin from Neoproterozoic seawater sulfates. The late-stage, and most important, metallic concentrations represent sulfur scavenged from pre-existing sulfides or from direct reduction of evaporitic sulfate minerals. Lead isotope data from the Bambui Group suggest focused fluid circulation from diverse Proterozoic sediment sources, that probably was responsible for metal transport to the site of sulfide precipitation. (d) Late Proterozoic to Early Paleozoic: lead-zinc sulfides (+pyrite and chalcopyrite) of Santa Maria deposits, in Rio Grande do Sul, form the matrix of arkosic sandstones and conglomerates, and are closely associated with regional faults forming graben structures. Intermediate volcanic rocks are intercalated with the basal siliciclastic members. Lead isotope age of the mineralization (0.59 Ga) is coeval with the host rocks. Sulfur isotopic values between −3.6 and +4.1 are compatible with a deep source for the sulfur. Geological, petrographic and isotopic data of the deposits studied suggest that they were formed during periods of extensional tectonics. Growth faults or reactivated basement structures probably were responsible for localized circulation of metal-bearing fluids within the sedimentary sequences. Sulfides were formed by the reduction of sedimentary sulfates in most cases. Linear structures are important controls for sulfide concentration in these Proterozoic basins.

Peloids in stratabound Zn-Pb deposits and their genetic importance

Peloids from sediment-hosted Zn-Pb deposits in Belgium, Ireland and Poland are composed of Zn-calcite, siderite, smithsonite, silica, pyrite, melnikovite, sphalerite, galena, thiosulphates and Zn-As-bearing limonite. The size of peloids is between 7 and 60 μm. The core and the shell of peloids are composed of different minerals and the shell is usually coarsely crystalline. Peloids are present in collapse breccias of karst cavities, stromatactis cavities, debris-flow breccias, and fractures cross-cutting carbonates hosting the mineralization. In places peloids form a major microtexture of the sulfide mineralization. Peloids are cemented and replaced by sulfides. Organic acids extracted from sulfide peloids are composed of amido-acids considered to be characteristic of bacterial origin. The size and specific texture of peloids are independent of mineralogy, location and age of the mineralization. Therefore the peloids disussed are considered as forms induced by bacterial activity rather than forms precipitated inorganically.

Basement controls on sediment-hosted Zn-Pb deposits; a Pb isotope study of Carboniferous mineralization in central Ireland

Lead isotope ratios of galenas from carbonate-hosted Zn-Pb (+ or - Ba + or - Cd + or - Ag) deposits in central Ireland are homogeneous within individual deposits but vary between deposits. The variation correlates directly with geographic position, defining isoplumbs, and not with style of mineralization, age of host rock, or structural or physico-chemical factors. The least radiogenic Pb occurs in the northwest and the most radiogenic in the southeast. This pattern elegantly reflects Irish pre-Caledonian basement geology: Lewisian gneisses in the northwest and late Proterozoic, upper crustal rocks in the southeast. That the northwestern end member of the Irish Pb-mixing trend is ultimately Lewisian is demonstrated by the concordance of the Irish Pb isotope trend with the trends defined in Proterozoic to Tertiary rocks and ores of Scotland which contain Pb of demonstrably Lewisian derivation. Although geologic relations and Pb isotope ratios unambiguously implicate basement in the ore-forming process, we are unable to discriminate between Lewisian gneisses themselves and detritus derived from Lewisian rocks in the Caledonian metamorphic complex as the direct source of Pb.Comparison of the Irish deposits with McArthur River and Mississippi Valley deposits leads us to conclude that style of mineralization (syngenetic, syndiagenetic, epigenetic) is not a fundamental aspect of sediment-hosted Zn-Pb deposits but is a reflection of local stress regimes and physico-chemical conditions. The fundamental link between the types of sediment-hosted Zn-Pb deposits may be the onset of a condition, such as extension or compression, that provides a mechanism for driving fluids. In extensional regimes, seismic pumping processes may channel fluids toward the surface or into fractures or favorable pore spaces, resulting in a wide variety of styles of mineralization. The lack of dilatency in sediments in compressional regimes, however, may inhibit surface exhalation and favors epigenetic styles of mineralization.

Deformation and recrystallization of pyrite

AbstractA detailed study of pyrite in a number of metamorphosed, stratiform, sediment-hosted Pb-Zn deposits has shown the importance of cataclastic deformation, pressure-solution, and grain growth in the deformation and textural development of pyrite. Primary depositional or early diagenetic microstructures are preserved in pyritic ores deformed or metamorphosed at grades up to mid-upper greenschist facies, whereas at higher temperatures only metablastic or annealed pyrite textures are found. Brittle deformation is found at all metamorphic grades and is favoured by coarse grain-sizes. Pressure-solution is a major deformation mechanism in fine-grained pyritic ores in low-grade metamorphic environments. Grain growth and annealing dominate at higher metamorphic temperatures and are likely to have obliterated any evidence of deformation by dislocation processes. Significant macroscopic ductility of fine-grained pyritic ores in low-grade environments may be accounted for by a combination of pressure-solution, grain boundary sliding, and cataclastic flow.

Phase relationships in the system zirconia-thoria

In China, sediment-hosted Pb―Zn deposits represent a significant type of base-metal deposits are major sources of Pb and Zn and significant sources of Ag and Ge. In this paper, we focus discussion on MVT (Mississippi Valley type), SEDEX (sedimentary exhalative) and Jinding-type deposits because they constitute the major subtypes of sediment-hosted Pb―Zn deposits in China. For these deposits, which occur in a wide variety of carbonate and siliciclastic rocks having no obvious genetic association with igneous activity, we present an overview and re-evaluation with regard to: (1) nature, diversity, and geological settings, (2) spatio-temporal distributions, (3) structural controls on mineralization, (4) metallogenic process, and (5) C, O, S and Pb isotope geochemistry. Several of these deposits with known reserves are concentrated in eight main metallogenic belts. Available data from these deposits in China show the highest combined Zn and Pb reserves exist in the Qinling Mountains metallogenic belt, followed by the northern margin of the North China Craton (NCC), the Sanjiang fold belt, the Upper Yangtze River belt, the South China fold system, the Tianshan–Beishan Mountains belt, the Lower Yangtze River belt, and the Qilian Mountains belt. In these belts, the sediment-hosted Pb―Zn deposits have formed in a variety of tectonic settings representing a range of processes that have occurred within two billion years of earth history, but large-scale mineralizations have occurred notably in the Proterozoic, Late Paleozoic and Cenozoic. The Proterozoic era was important for the formation of SEDEX Pb―Zn deposits such as those in the interior and along the edges of the NCC and Yangtze Craton. The Guanmenshan is the only known MVT deposit formed within an intra-continental basin in the Proterozoic era. The Late Paleozoic was a productive period for MVT Pb―Zn deposits in China; in particular, there were significant MVT mineralizations during Devonian to Permian as a result of the amalgamation of the NCC and the South China Craton. The Cenozoic was a significant metallogenic era in China. For example, in the Sanjiang area during the Cenozoic, the Lanping Basin has evolved from a passive margin to open ocean basin and then has experienced ocean basin closure followed by the India–Asia collision and related tectonic activities including thrusting, and formation of strike–slip extensional basins, in which the Jinding-type deposits developed.