Pb Ag Deposit Research Articles

Lithostratigraphy of a portion of the Neoproterozoic Rosh Pinah formation: Host rocks of the Western Orefield, Rosh Pinah mine, southern Namibia

The Rosh Pinah Zn–Pb–Ag deposit in southern Namibia comprises a significant sulphidic Zn–Pb resource (19.94 Mt @ 7.3 wt% Zn, 1.83 wt% Pb and 27.7 g/t Ag) occurring mainly as sphalerite and galena. The ore minerals as well as pyrite are hosted by the Ore Equivalent Horizon (OEH), a part of the Neoproterozoic Rosh Pinah Formation. Details on original rock types and depositional environment of the OEH units remain unclear despite past work. Here, we combine detailed drillhole logging data, drillhole logging database and drillcore photographs, underground and near-mine outcrop observations coupled with detailed petrographic observations to elucidate original rock types and depositional environments. The OEH is comprised of three mappable lithofacies (carbonaceous argillite and siltstones, volcanic/volcaniclastic rocks, and carbonate rocks) enclosed within a sequence of arkose. The lateral transition from coarse-grained arkose to the fine-grained carbonaceous rocks of the OEH reflects subaqueous fan type deposition. The presence of diagenetic textures in the carbonate rocks in form of cloudy, isopachous fibrous dolomitic cements point to subaqueous depositional environments. The geometry of carbonate bodies in the OEH at the Rosh Pinah mine and in nearby surface outcrop exposures suggest local accumulations with a pronounced topographic relief above the surrounding sediments, reminiscent of carbonate buildups. The OEH appears to represent a limited interval of marine or lacustrine deposition that may be related to syn-sedimentary extensional conditions.

A New Alpine Metallogenic Model for the Pb-Ag Orogenic Deposits of Macôt-la Plagne and Peisey-Nancroix (Western Alps, France)

Understanding mass transfer associated with fluids circulation and deformation in the Alpine orogeny is often complex due to common multistage crystallization. For example, in two emblematic and historic Pb-Ag deposits of the French Alps, Macôt-la Plagne (MP) and Peisey-Nancroix (PN), a sedimentary or orogenic origin is still debated. To discriminate between the metallogenic models of the two deposits, an integrative methodology combining field, microstructural, mineralogical, thermobarometrical, and geochronological data was here applied for establishing detailed Pressure–Temperature–Time–Deformation (P-T-t-d) mineralization conditions. Both deposits are located in Permo-Triassic quartzite of the External Briançonnais domain along the Internal Briançonnais Front (Internal Western Alps). The ore mainly occurs as veins and disseminated textures containing galena, pyrite, and variable content of tetrahedrite–tennantite and chalcopyrite. Quartz porphyroclasts and sulfide microstructures indicate a dynamic recrystallization of the quartzite during the main fluid mineralization episode. Chlorites and K-white micas (phengite) chemical analysis and thermodynamic modeling from compositional maps indicate an onset of the mineralization at 280 °C, with a main precipitation stage at 315 ± 35 °C and 6.25 ± 0.75 kbar. In situ U-Pb dating on monazite, cogenetic with sulfides, gives ages around 35 Ma for both deposits. The integrative dataset converges for a cogenetic MP-PN Alpine Pb-Ag mineralization during deformation in relation to the thrusting of the “Nappe des Gypses” and the Internal Briançonnais at the metamorphic peak.

Geological and Mining Constraints on Historical Mine Production: The Case of Early Medieval Lead-Silver Mining at Melle, France

Early Medieval silver production for the Melle Pb-Ag deposit in western France has been estimated up to 15 tonnes per year over hundreds of years (Téreygeol, 2013) which would place it amongst the top silver mines of all times prior to the New World discoveries. However, this deposit has geological and mining characteristics economically unsuitable for substantial production, because it is thin, sub-horizontal and comprised of discontinuous pods. Furthermore, it is a Mississippi Valley type (MVT) base metal deposit, which are typically not major silver producers. There is no geological evidence for primary or secondary enrichment to generate silver-rich ore. Inaddition, the fact that Melle remained unmined in all later historical periods is enigmatic compared to the almost ubiquitous reworking of significant ore deposits elsewhere.In this paper, I discuss the geological characteristics, mining parameters, documented historical mining rates and workforce considerations, all of which can be used to constrain production estimates for Melle. The largest uncertainty for production estimates is the workforce size, which reflects scarce information about Carolingian Melle and its surroundings. A model employing realistic mining parameters and a workforce appropriate for a small mining village (100 miners/fire-setters, within a workforce of 250-300, from a settlement of at least 400-500) yields 52 tonnes of lead metal and 150 kg of silver per year. Doubling the workforce would double this estimate. Conversely, it could be half or less if mining was a seasonal activity between agricultural priorities such as harvesting and seeding.The previously claimed production rates require improbable mining assumptions together with at least 500 dedicated full-time miners and a population in the thousands. Furthermore, it would yield per capita silver productivity more than four times higher than in well-documented Early Modern operations that were leading silver deposits of their time. This seems unlikely. However, even at the much lower production levels, favoured here, Melle still might have been a major factor in the Carolingian economy, with its lead production perhaps as important as its silver.

Paleomagnetism of the Grum Zn–Pb–Ag deposit, Selwyn Basin, Yukon, Canada

The Grum deposit is one of five large clastic-dominated Zn–Pb–Ag deposits in the Anvil district of the Selwyn Basin, Yukon. Grum's mineralization occurs at the contact between the late Proterozoic to Cambrian Mount Mye Formation and the Cambrian to Early Ordovician Vangorda Formation. Paleomagnetic analyses of 113 specimens from 15 mineralized sites in the Grum open pit were undertaken to date Grum's mineralization. The analyses isolated a stable characteristic remanent magnetization (ChRM), mostly by thermal and then alternating field demagnetization. The main ChRM carrier is single- or pseudosingle-domain pyrrhotite with minor magnetite that give a paleopole at 67.8°N latitude and 317.4°E longitude (radius of the cone of 95% confidence, A95 = 7.2°). The paleopole yields an age of 176 ± 12 Ma for the Grum deposit after a clockwise rotation and northward translation of the basin to best fit the North American apparent polar wander path. Thus, the Zn–Pb–Ag Grum mineralization significantly predates intrusion of the nearby mid-Cretaceous Anvil Batholith, but is coeval with widespread Early Jurassic metamorphism in the Selwyn Basin that correlates with the Early Jurassic collision of the Intermontane Belt terranes with the North America craton.

Gold and aurostibite from the metaturbidite-hosted Au–Zn–Pb–Ag Hera deposit, southern Cobar Basin, central NSW, Australia: geochemical and textural evidence for gold remobilisation

AbstractThe Devonian Hera metaturbidite-hosted polymetallic Au–Zn–Pb–Ag deposit of central NSW, Australia, contained a total undepleted resource of 3.6 Mt @ 3.3 g/t Au, 25 g/t Ag, 2.6% Pb and 3.8% Zn. The deposit comprises a number of distinctive lodes with each containing a distinctive ore and alteration/gangue mineralogy, though generally the sulfide ore comprises various mixtures of sphalerite, galena, chalcopyrite, pyrrhotite and relatively common visible gold–electrum. The North Pod and Far West lodes are distinctly Sb rich and contain a more diverse ore mineralogy with arsenopyrite, native silver, native antimony, gudmundite, tetrahedrite-(Fe), argentotetrahedrite-(Fe), acanthite, dyscrasite, nisbite and breithauptite. From analysis of 52,760 assays from across the deposit it was found that there was a very poor correlation between gold and each of Fe, Zn, S, Pb, Cu, As and Ag, whereas Ag correlated reasonably well with both Pb and Zn. Results from EPMA shows that gold varies widely in composition from host-rock associated gold (96 wt.% Au) through more intermediate compositions (88–73 wt.% Au) to electrum (46–27 wt.% Au), commonly associated with Sb-phases and containing significant Sb within the gold itself (1.05–2.58 wt.% Sb). From the Far West lense, aurostibite occurs as distinctive rims around gold. Although aurostibite associated with gold contains no silver, the gold itself contains constant moderate amounts (10.87–12.27 wt.% Ag). We suggest that the aurostibite and other Sb phases formed from a late-stage Sb-rich hydrothermal during low-temperature retrograde skarn alteration. There is abundant evidence for both chemical and physical remobilisation at Hera and this remobilisation is largely responsible for the spectrum of gold compositions observed. The source for these fluids may be an underlying magmatic body, evidence for which occurs as granite pegmatite dykes in various locations throughout the deposit. Furthermore, gold with a moderate to high Sb content may be indicative of a low temperature of formation.

Trace element and sulfur isotope compositions of pyrite from the Tianqiao Zn–Pb–Ag deposit in Guizhou province, SW China: implication for the origin of ore-forming fluids

Trace element and sulfur isotope compositions of pyrite from the Tianqiao Zn–Pb–Ag deposit in Guizhou province, SW China: implication for the origin of ore-forming fluids

Multi-Media Geochemical Exploration in the Critical Zone: A Case Study over the Prairie and Wolf Zn–Pb Deposits, Capricorn Orogen, Western Australia

In this study, we compared traditional lithochemical sample media (soil) with hydrochemical (groundwater), biogeochemical (plant matter of mulga and spinifex), and other near-surface sample media (ferro-manganese crust), in a case study applied to mineral exploration in weathered terrain, through the critical zone at the fault-hosted Prairie and Wolf Zn–Pb (Ag) deposits in Western Australia. We used multi-element geochemistry analyses to spatially identify geochemical anomalies in samples over known mineralization, and investigated metal dispersion processes. In all near-surface sample media, high concentrations of the metals of interest (Zn, Pb, Ag) coincided with samples proximal to the mineralization at depth. However, the lateral dispersion of these elements differed from regional (several km; groundwater) to local (several 100′s of meters; solid sample media) scales. Zinc in spinifex leaves over the Prairie and Wolf deposits exceeded the total concentrations in all other sample media, while the metal concentrations in mulga phyllodes were not as pronounced, except for Ag, which exceeded the concentrations in all other sample media. These observations indicate potential preferential metal-specific uptake by different media. Pathfinder elements in vegetation and groundwater samples also indicated the Prairie Downs fault zone at the regional (groundwater) and local (vegetation) scale, and are, therefore, potentially useful tools to trace fault systems that host structurally controlled, hydrothermal Zn–Pb mineralization.

Mimetit z ložiska Ján Nepomuk pri Veľkom Poli (Slovenská republika)

A new occurrence of mimetite was recently discovered at the abandoned Ján Nepomuk Pb-Ag deposit near Veľké Pole, Žarnovica Co., Banská Bystrica Region, Slovakia. It forms light to bright yellow, prismatic crystals reaching up to 5 mm in size. Mimetite occurs in fractures and cavities of strongly altered and limonitised limestone with impregnations and relicts of primary galena and pyrite. It is associated with cerussite and calcite. The refined unit-cell parameters (for the hexagonal space group P63/m) are a 10.2437(4) Å, c 7.4437(6) Å and V 676.44(6) Å3. Except of dominant contents of Pb, As and Cl only minor amounts of Ca (up to 0.36 apfu), Al (up to 0.03 apfu), P (up to 0.03 apfu) and V (up to 0.02 apfu) were detected in four mimetite samples studied by EMPA-WDS.

Geological, geophysical, and geochemical characteristics of the Ban Kiouchep Cu–Pb–Ag deposit and its exploration significance in Northern Laos

The Simao–Indochina Basin is located between the Loei–Luang Prabang–Dien Bien Phu fold belt and the Jinghong–Nan-Uttaradit suture zone, composed of Mesozoic and Cenozoic red beds. It overlies the marine clastic rocks and volcanic sedimentary rocks of a Paleozoic back-arc foreland sequence. The Late Triassic Indosinian orogeny and subsequent superimposition and intracontinental shearing and structural superimposition may have facilitated the formation of various deposits in this basin, including hydrothermal, skarn, and porphyry types of mineralization. The Ban Kiouchep Cu–Pb–Ag deposit is a medium–low-temperature hydrothermal deposit that is largely controlled by faults and is hosted in carbonate rocks and volcanic (-sedimentary) rocks. The formation of this deposit includes a series of geological and metallogenic processes (i) neritic–littoral facies (volcanic-) sedimentation in a Late Paleozoic to Late Triassic back-arc foreland setting, (ii) Indosinian orogenic transformation, (iii) Mesozoic–Cenozoic tectonic-hydrothermal overprinting, and (iv) epigenetic oxidized leaching. The ore minerals are primarily malachite, azurite, and galena, followed by sphalerite, chalcocite, and chalcopyrite, with associated silver minerals. Using induced polarization (IP) geophysical surveys and soil geochemical surveys, large-scale and highly overlapping coincident anomalies and a mineralized zone at a scale of 100–110-m size (ore body I) was discovered in the area. Two anomalies, P1-C1 and P5-C4, with the best metallogenic conditions and the deepest extensions of the known ore bodies, were further selected as engineering verification targets. The two following verifying drill holes revealed Pb–Zn–Ag mineralization in the fault zone; however, no mineralized ore bodies consistent with the anomalies were found. The high background value of the Fe–Cu–Pb–Zn (Ag) elements, strong fracturing, and hydrothermal alteration, as well as later oxidized leaching with lateritization, resulted in in-situ dispersion and local secondary enrichment of the metallogenic elements. These may be the primary causes of the overlapping anomaly distribution along the strata and faults therein. Lateritic Ag mineralization has been also found in the area. The geological, geophysical, and geochemical characteristics and the prospecting are of great importance for further research and prospecting for other related deposits, for example, the VHMS, SEDEX, hydrothermal Cu polymetallic deposits and porphyry, skarn, epithermal, sediment-hosted/orogenic Au deposits, with lateritic Ag (Au) deposits in the Simao–Indochina Basin of Northern Laos.

Geology, fluid inclusions, C–O–S–Pb isotopes and genesis of the Ahangaran Pb-Ag (Zn) deposit, Malayer-Esfahan Metallogenic Province, western Iran

The Malayer-Esfahan Metallogenic Province (MEMP) of western Iran hosts many Pb-Zn deposits and occurrences, largely hosted within a sequence of clastic detrital and carbonate host rocks. A well-known example is Ahangaran which is composed of several hypogene orebodies with estimated resources of ~1.5 Mt sulfide ore grading 4 wt% Pb, 0.5 wt% Zn and 200 g/t Ag, plus a supergene iron-bearing oxyhydroxide ore with 1 Mt reserve and grading 30–35 wt% FeO. The sulfide orebodies are primarily hosted in Lower Cretaceous dolostone and sandstone along the marginal-southern part of the NW–SE trending Kolehbid Syncline. Galena and pyrite, together with chalcopyrite and sphalerite in lesser amounts, comprise the primary sulfide ores. Minor sulfosalt minerals including bournonite, tetrahedrite, freibergite, famatinite and meneghinite have also been identified by electron probe microanalysis. Three paragenetic stages are recognized: (1) pre-sulfide stage (oxide stage); (2) main sulfide stage (including three sulfide sub-stages); and (3) supergene stage. Fluid inclusion studies conducted on pre- and main-sulfide stages identified two end-member fluids: a hot (~290 °C) and saline fluid (~20 wt% NaCl eq.) and a cooler (<150 °C), lower salinity fluid (~7 wt% NaCl eq.). C–O–S isotope studies suggest that these fluids originated as an 18O-enriched, heated basinal brine that carried Pb, Ag, and Ba, and a low salinity, SO42−- and H2S-bearing fluid derived from Mesozoic seawater. Sulfide required for ore formation was generated by thermochemical sulfate reduction and the sulfur of sulfate minerals was derived directly from dissolved SO42−. The Pb isotope compositions are homogeneous with 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios ranging from 18.458 to 18.501, 15.623 to 15.708 and 38.512 to 38.715, respectively. These ratios are indicative of a continental crustal reservoir as the source of lead and presumably other metals. Taken together, our observations suggest that the Ahangaran sediment-hosted Pb-Ag (Zn) deposit is an epigenetic, structurally-controlled and strata-bound deposit with fluids and metals derived from the Paleozoic basement rocks. This model for formation of the Ahangaran deposit is different to magmatic-hydrothermal, sedimentary-exhalative or Mississippi Valley-types, and to some extent is similar to Irish-type deposits.

Atmosphere oxygen cycling through the Proterozoic and Phanerozoic

Variations in atmosphere oxygen and ocean sulfate concentrations through time are regarded as important controls on the cycles of sediment-hosted and volcanic-hosted ore deposits. However, estimates of atmosphere oxygen in the Proterozoic have been frustrated by the lack of a direct measurement method and conflicting evidence from various proposed geochemical proxies. Studies in the 1970s to 1990s suggested a relatively oxygenated atmosphere (> 3 wt% O2) in the Proterozoic. However, since the late 1990s, new proxies and modelling have suggested very much lower levels of oxygen (< 0.02 wt% O2). Focusing on redox-sensitive trace elements, here we combine a dataset of over 3000 LA-ICP-MS trace-element analyses on sedimentary pyrite, standardised against Berner’s Phanerozoic O2 modelling and direct measurement of oxygen concentrations in fluid inclusions in sedimentary halite, to develop the first detailed estimate for atmosphere O2 concentration and secular variation from 2200 Ma to the present. The estimates suggest dynamic cycles of atmosphere oxygen that increased in frequency through time. There were possibly three first-order cycles in the Proterozoic varying from 400 to 600 million years in length and a further five first-order cycles in the Phanerozoic from 60 to 120 million years in length. Our estimates of oxygen concentration are at odds with most previous estimates. We suggest, rather than very low atmosphere oxygen in the Proterozoic, the mean concentration was about 7 wt%, rising to a mean of about 10 wt% in the Phanerozoic, but with significant cyclic variation of up to a maximum concentration of possibly over 30 wt%. We observe that the proposed oxygen cycles correlate with biodiversity cycles and to the timing of major stratiform base-metal deposits in sedimentary basins. For example, minima in atmosphere oxygenation correlate with mass extinction events and stratiform Zn–Pb–Ag deposits, whereas maxima in oxygenation correlate with major evolutionary events, global periods of evaporite formation and the timing of stratiform copper deposits.

The origin of the carbonate-hosted Huize Zn–Pb–Ag deposit, Yunnan province, SW China: constraints from the trace element and sulfur isotopic compositions of pyrite

The Huize Zn–Pb–Ag deposit in SW China has an ore reserve in excess of 5 Mt. with an ore grade of ≥25 wt% Zn + Pb and 46–100 g/t Ag. This deposit is hosted in Carboniferous dolostone and limestone. Sulfide mineralization is dominated by sphalerite, galena, and pyrite. Four types of pyrite (Py1 to Py4) are temporally and spatially related to mineralization distinguished on the basis of textural features and mineral associations. Pyrite 1 to 3 corresponds to the pyrite-sphalerite sub-stage, whereas Py4 corresponds to sphalerite-galena-pyrite sub-stage. Pyrite 1 shows zoned texture composed of an inclusion-rich core and an inclusion-free rim, whereas Py2, Py3, and Py4 show replacement relic or overgrowth textures. The zoned texture in Py1 was formed by multiple stages of ore fluids, whereas replacement relic texture in Py2 to Py4 was formed by replacement of pyrite by late Pb-Zn-rich fluids. Trace element variation in pyrite results from a combination of mineral inclusions, co-precipitating minerals, and various fluid compositions. Sphalerite, pyrite, and galena have δ34S values of 10.4–23.5‰, suggesting that sulfur was probably derived from the thermochemical reduction of marine sulfates. The Huize pyrite has Co and Ni concentrations (0.02–9.5 ppm and 0.08–143 ppm, respectively) and Co/Ni ratios (~0.01–2.63) similar to pyrite from sedimentary exhalative deposits, submarine hydrothermal vents, and sedimentary pyrite, which may be due to pyrite precipitation from low-temperature (<~250 °C), basin brines or seawater. The Co/Ni ratios of the Huize pyrite are lower than those (~0.2–7.2) of pyrite from Mississippi Valley-type Zn-Pb deposits. The Huize pyrite has Co and Ni contents lower than those associated with volcanogenic massive sulfide, iron oxide copper gold, and porphyry Cu deposits, arguing against the involvement of a magmatic/volcanic component and a direct genetic link between Permian Emeishan basalts and Pb-Zn mineralization. Combining with fluid inclusion temperatures and silver grade, we define the Huize deposit as a transitional type between Mississippi Valley-type and high-temperature carbonate-replacement Zn-Pb deposits.

Signature and geological significance of the specularite from the Laojiezi Pb-Ag deposit in the Chuxiong basin, central Yunnan, SW China

Signature and geological significance of the specularite from the Laojiezi Pb-Ag deposit in the Chuxiong basin, central Yunnan, SW China

Geochemistry and Zircon U–Pb age of the Yao’an pseudoleucite porphyry, Yunnan Province, China

The Yao’an Pb–Ag deposit, located in the Chuxiong Basin, western Yangtze Block, is an important component of the Jinshajiang–Ailaoshan alkaline porphyry–related polymetallic intrusive belt. This complex suite of rock bodies includes a vein of pseudoleucite porphyry within deposits of syenite porphyry and trachyte. The pseudoleucite is characterized by a variable greyish, greyish-white, and greyish-green porphyritic texture. Phenocrysts are mainly pseudoleucite with small amounts of alkali feldspar and biotite. In an intense event, leucite phenocrysts altered to orthoclase, kaolinite, and quartz. Both the pseudoleucite porphyry and the syenite porphyry samples were typical alkali-rich, K-rich, al-rich rocks with high LaN/YbN ratios; enriched in light rare earth elements and large-ion lithophile elements, and depleted in high field strength elements; and with strongly negative Ta, Nb, and Ti (TNT) anomalies and slightly negative Eu anomalies—all characteristics of subduction-zone mantle-derived rock. We obtained a LA-ICP-MS zircon U–Pb age of 34.1 ± 0.3 Ma (MSWD = 2.4), which is younger than the established age of the Indian and Eurasian Plate collision. The magma derived from a Type-II enriched mantle formed in a post-collisional plate tectonic setting. The geochemical characteristics of the Yao’an pseudoleucite porphyry are powerful evidence that the porphyry’s development was closely linked to the Jinshajiang–Ailaoshan fault and to the Indian-Eurasian collision.

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

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

Structural evolution of the Palaeoproterozoic Sala stratabound Zn-Pb-Ag carbonate-replacement deposit, Bergslagen, Sweden

A structural investigation of the Sala Zn–Pb–Ag deposit in the Bergslagen mining district of southern Sweden shows that it is associated with two tectonic structures: the N–NW-trending Storgruveskölen shear zone (SSZ), which is parallel to the strike of the mined ore bodies, and the F1 Sala syncline with a fold hinge plunging c. 35° towards NNW, which is parallel to the plunge of the entire mineralised system. The Sala syncline was refolded by F2 folds, leading to flattening and local reversals in the plunge of F1 folds and the ore bodies. Field evidence suggests that the SSZ represents both a phase of D3 reverse dip-slip shearing and a later (D4) phase of dextral strike-slip reactivation. However, a high concentration of pre- to syn-D1 skarn- and sulphide-bearing vein networks and breccias adjacent to the SSZ, which are gradational into the mined massive sulphide ore bodies, suggest that stages in the formation of the SSZ predated D3 and D4. It may consequently constitute a reactivated pre- to syn-D1 structure. The distribution of breccia and hydrothermal alteration together with the highly discordant nature of the deposit are consistent with a pre- to syn-D1 timing of ore formation, involving of cross-stratal fluid flow along the proto-SSZ and subordinate fluid flow parallel to volcanic interbeds in the host carbonate rock. Three δ34S determinations on sphalerite (2.1–2.4‰) and galena (1.2‰), respectively, are consistent with a magmatic source for ore sulphur, as has been suggested for many other sulphide deposits in Bergslagen.

Evidence for SEDEX-style mineralization in the 1.7 Ga Tawallah Group, McArthur Basin, Australia

The Paleoproterozoic McArthur Basin (McArthur Group) of northern Australia hosts world-class sedimentary ‘exhalative’ (SEDEX) McArthur type Zn–Pb deposits, which are largely hosted within a sequence of 1.64Ga pyritic carbonaceous shales deposited in an extensional rift setting. A well-known example of these is McArthur River (or Here's Your Chance [HYC] Zn–Pb–Ag deposit). The ~1.78Ga McDermott and ~1.73Ga Wollogorang formations (Tawallah Group) both contain carbonaceous shales deposited in similar environments. Our observations suggest the carbonaceous facies of the Wollogorang Formation were deposited under mostly euxinic conditions, with periodically-high concentrations of sedimentary pyrite deposition. The carbonaceous shales in the older McDermott Formation contain considerably less early pyrite, reflecting a mostly sulfide-poor, anoxic depositional environment. Localized fault-bound sub-basins likely facilitated lateral facies variations, which is evident from synsedimentary breccias.The presence of evaporitic oxidized facies within the McDermott and Wollogorang formations, alongside evidence for synsedimentary brecciation in reduced shales are favourable criteria for SEDEX-style base metal deposition. Both formations overlie volcanic units, which could have been sources of base metals. Detailed X-ray petrography, new geochemical data and sulfur isotope data from historical drill cores indicate multiple horizons of stratiform and sediment breccia-hosted base metal sulfide within carbonaceous shale units, with high-grade Zn concentrations. A close association between sphalerite and ferromanganean dolomite alteration draws comparisons with younger SEDEX mineralization at HYC. Additionally, SEDEX alteration indices, used demonstrably as a vector to the younger orebodies, indicate the sedimentary rocks analyzed in this study are marginally below the ore window when compared to the overlying mineralized stratigraphy.Our data imply that localized active circulation of metalliferous brines occurred in the Tawallah Group basin. High-grade sulfide deposition in reduced facies alteration may represent distal expressions of larger SEDEX-style deposits. Furthermore, abundant pyrite and high molybdenum in the Wollogorang Formation suggest the global oceanic sulfate concentration was sufficient by ~1.73Ga to engender intermittent but strong bottom-water euxinia during shale deposition, thus providing a robust chemical trap for base metal sulfide mineralization.

The Jabali nonsulfide Zn–Pb–Ag deposit, western Yemen

The Jabali Zn–Pb–Ag deposit is located about 110km east of Sana'a, the capital of Yemen, along the western border of the Marib-Al-Jawf/Sab'atayn basin. The economic mineralization at Jabali is a nonsulfide deposit, consisting of 8.7milliontons at an average grade of 9.2% zinc, derived from the oxidation of primary sulfides. The rock hosting both primary and secondary ores is a strongly dolomitized carbonate platform limestone of the Jurassic Shuqra Formation (Amran Group). The primary sulfides consist of sphalerite, galena and pyrite/marcasite. Smithsonite is the most abundant economic mineral in the secondary deposit, and is associated with minor hydrozincite, hemimorphite, acanthite and greenockite. Smithsonite occurs as two main generations: smithsonite 1, which replaces both host dolomite and sphalerite, and smithsonite 2, occurring as concretions and vein fillings in the host rock. At the boundary between smithsonite 1 and host dolomite, the latter is widely replaced by broad, irregular bands of Zn-bearing dolomite, where Zn has substituted for Mg. The secondary mineralization evolved through different stages: 1) alteration of original sulfides (sphalerite, pyrite and galena), and release of metals in acid solutions; 2) alteration of dolomite host rock and formation of Zn-bearing dolomite; 3) partial dissolution of dolomite by metal-carrying acid fluids and replacement of dolomite and Zn-bearing dolomite by a first smithsonite phase (smithsonite 1). To this stage also belong the direct replacement of sphalerite and galena by secondary minerals (smithsonite and cerussite); 4) precipitation of a later smithsonite phase (smithsonite 2) in veins and cavities, together with Ag- and Cd-sulfides.The δ18O composition of Jabali smithsonite is generally lower than in other known supergene smithsonites, whereas the carbon isotope composition is in the same range of the negative δ13C values recorded in most supergene nonsulfide ores. Considering that the groundwaters and paleo-groundwaters in this area of Yemen have negative δ18O values, it can be assumed that the Jabali smithsonite precipitated in different stages from a combination of fluids, possibly consisting of local groundwaters variably mixed with low-temperature hydrothermal waters. The carbon isotope composition is interpreted as a result of mixing between carbon from host rock carbonates and soil/atmospheric CO2.The most favorable setting for the development of the Jabali secondary deposit could be placed in the early Miocene (~17Ma), when supergene weathering was favored by major uplift and exhumation resulting from the main phase of Red Sea extension. Low-temperature hydrothermal fluids may have also circulated at the same time, through the magmatically-induced geothermal activity in the area.

Regional and local ice-flow history in the vicinity of the Izok Lake Zn–Cu–Pb–Ag deposit, Nunavut

Ice-directional indicators were compiled from detailed field mapping at the Izok Lake volcanogenic massive sulphide deposit, in the Point Lake region of western Nunavut and eastern Northwest Territories. Cross-cutting erosional relationships and depositional landforms indicate that the Izok Lake area was affected by four ice-flow phases. It is this new glacial history interpretation that is used to explain the gahnite indicator mineral dispersal train down-ice of the Izok Lake deposit as being the net effect of all ice-flow phases. Its fan-shaped morphology is a function of the duration and intensity of two dominant glacial trajectories. Field-based mapping of all glacial-flow indicators are essential, and must be properly interpreted, to detect palimpsest dispersal trains. The resultant dispersal fan serves as a model for future exploration in the glaciated terrain of the north-central part of the Slave Province.

Depositional systems in the Mt Isa Inlier from 1800 Ma to 1640 Ma: Implications for Zn–Pb–Ag mineralisation

Two facies models are proposed to explain siliciclastic and carbonate depositional systems of 1800 Ma to 1640 Ma age in the Western Fold Belt of the Mt Isa Inlier. Both models record the response of depositional systems to storm-driven processes of sediment transport, dispersal and deposition on a shallow water shelf. The same suite of facies belts can also be identified in sedimentary successions of the Eastern Fold Belt. Slope driven processes of sediment transport and dispersal characterise turbidite and debrite deposits of the Soldiers Cap Group and Kuridala Formation and provide evidence for significantly greater water depths in this part of the basin from ca 1685 Ma. Through the recognition of unconformity surfaces, their correlative conformities, maximum flooding and ravinement surfaces, the facies belts are packaged into seven supersequences for the interval ca 1800–1640 Ma. The new correlations are shown in an event chart that correlates linked depositional systems across the entire Mt Isa Inlier. Thick successions of turbidite and debrite deposits are restricted to the eastern parts of the Mt Isa Inlier and do not occur in the Western Fold Belt. A major phase of extension and rifting commenced at ca 1740 Ma and by ca 1690 Ma led to significant crustal thinning and increased rates of accommodation over an area east of the Mount Dore Fault and Burke River Structural Belt. In the Mitakoodi Anticline and Kuridala-Selwyn Block, the rapid transition from shallow-water shelf depositional systems of the Prize Supersequence to significantly deeper-water slope environments of the Gun Supersequence coincided with the development of a platform margin, the deposition of turbidite and debrite deposits in deep water on the continental slope and the intrusion of mafic sills and dykes. Turbidite and debrite depositional systems of the Soldiers Cap Group and Kuridala Formations are restricted to a lowstand wedge of siliciclastic facies deposited basinward of a platform margin. Basin geometries and sediment architectures associated with this extensional event and recorded in the Gun Supersequence (ca 1685 Ma to 1650 Ma) provide an explanation for the geographic separation and fluid evolution pathways responsible for the Mt Isa Type and Broken Hill Type Zn–Pb–Ag deposits.