Timing and metal sources for carbonate-hosted Zn-Pb mineralization in the Franklinian Basin (North Greenland): Constraints from Rb-Sr and Pb isotopes

Carbonate-hosted Zn-Pb (± barite, fluorite) occurrences in the Franklinian Basin of North Greenland were studied using the Rb-Sr method, applied to sphalerite, and combined Sr and Pb isotope analysis of ore and gangue minerals, to place constraints on their age and genesis. The occurrences are located in the easternmost part of the basin in Peary Land and Kronprins Christian Land, and are hosted by Upper Ordovician to lowermost Silurian dolostones of the Turesø Formation.Sphalerite samples from the Zn-Pb occurrence at Børglum, in Peary Land, reflect undisturbed Rb-Sr systems and return an isochron age of 388 ± 4 Ma (MSWD = 1.6, ⁸⁷Sr/⁸⁶Sr_i = 0.70930 ± 1). Sphalerites and their fluid inclusion fractions from an adjacent Zn-Pb occurrence, at Tvilum, exhibit various degrees of isotopic disturbance caused by secondary fluid overprint. They yield no age, however, reconstructive modelling of their Rb-Sr and Pb-Pb signatures indicates original isotope systematics consistent with the Børglum data. Likewise, due to a lack of suitable samples, no Rb-Sr age could be determined for a third occurrence in southeastern Peary Land at Løgum, however, the time-integrated Pb isotopic evolution of fluorite and calcite from this mineralization over c. 390 Ma is compatible with modelled initial signatures for Tvilum and Børglum ores. Consequently, a contemporaneous formation at c. 390 Ma can be assumed for all studied locations in Peary Land. For the Zn-Pb occurrence in Kronprins Christian Land farther east, a Rb-Sr sphalerite age of c. 360–365 Ma has been obtained through a combined isochron and paleomixing line approach.The absolute ages obtained at the studied occurrences, corresponding to Middle to Upper Devonian ages, are in no conflict with the age of the host dolostones of the Turesø Formation, which placed a maximum age limit for the mineralization. Actually, the fact that mineralization was emplaced at least 50 Ma after deposition of the host rocks, which would have been completely lithified by then, warrants the classification of the studied occurrences as epigenetic Mississippi Valley-Type, rather than diagenetic Irish-type. Furthermore, the obtained ages are contemporaneous to the Middle or Upper Devonian to the Lower Carboniferous Ellesmerian Orogeny, indicating that the mineralization likely formed from basinal brines expelled by tectonism and/or hydraulic head caused by Ellesmerian orogenic uplift, as previously suggested for the Polaris carbonate-hosted Zn-Pb deposit (Cornwallis Island, Canada).Pb isotope systematics of base metal mineralization in the Franklinian Basin point to principal metal sources located in the crystalline basement and in basement-derived clastic sediments. These two reservoirs define radiogenic and unradiogenic end components from which lead was mobilized and mixed in different proportions during discrete periods of hydrothermal activity. Distinct thorogenic Pb isotope signatures indicate that specific local sources (lower crustal basement, carbonate rocks and possibly organic-rich shales) were also involved in mineralization.     

Constraints of Sr isotopic compositions of apatite and carbonates on the origin of Fe and Cu mineralizing fluids in the Lala Fe-Cu-(Mo,LREE) deposit,SW China

Numerous Fe-Cu deposits with mineralization styles similar to iron oxide-copper gold (IOCG) deposits form the Kangdian Fe-Cu metallogenic province, southwestern (SW) China. As one of the largest deposits in the region, the ~ 1.0 Ga Lala Fe-Cu deposit is hosted in a Paleoproterozoic volcanic-sedimentary succession named the Hekou Group which is alternately intruded by ~ 1.0 Ga doleritic plutons. This deposit has a paragenetic sequence evolving from Stage I of Na-alteration to Stage II of Fe mineralization, and finally to Stage III of Cu-(Mo, REE) mineralization, coeval with mafic-felsic intra-plate magmatism in the region. This study conducted in-situ Sr isotopic analyses on apatite and carbonate, aiming to resolve the long controversial issue regarding the origin of the Fe and Cu mineralizing fluids in the deposit. Apatite of Stage II has ⁸⁷Sr/⁸⁶Sr ratios varying from 0.71380 to 0.72733, much higher than those of synchronous igneous rocks in the region (0.7074 to 0.7091), but similar to the Paleoproterozoic host rocks (0.71368 to 0.71837 at ~ 1.0 Ga). This similarity indicates that radiogenic Sr of the Fe mineralizing fluid was dominantly sourced from the host rocks. Apatite and calcites of Stage III have ⁸⁷Sr/⁸⁶Sr ratios (0.75758–0.79293) much higher than apatite of Stage II and the host rocks but similar to the Archean basement rocks (as high as 0.80 at ~ 1.0 Ga) beneath the cover of the Yangtze Block, suggesting that the highly radiogenic Sr isotopic composition of the Cu mineralizing fluid was mainly inherited from the old basement rocks. In combination with previous C-O-S isotopic data indicating a magma-hydrothermal origin, it was suggested that the Fe mineralizing fluid was exsolved from a mafic magma that generated the ~ 1.0 Ga doleritic plutons, and inherited radiogenic Sr from the host rocks during fluid-rock interaction. By contrast, the Cu mineralizing fluid might have been sourced from another pulse of magmatic, Cu-Mo-REE- and CO₂-rich fluid which have once interacted with Archean basement rocks prior to mineralization. The source of such a Cu-Mo-REE-rich fluid was not well constrained in current study but was inferred to be exsolved from a hidden felsic magma. We propose that intrusions of the bimodal magmas in Kangdian are responsible for regional hydrothermal circulation which led to Fe-Cu-(Mo, REE) mineralization in the Kangdian province.     

Ages (U–Pb SHRIMP and LA ICPMS) and stratigraphic evolution of the Neoproterozoic volcano-sedimentary successions from the extensional Camaquã Basin,Southern Brazil

During the Ediacaran, southern Brazil was the site of multiple episodes of volcanism and sedimentation, which are best preserved in the 3000 km² Camaquã Basin. The interlayered sedimentary and volcanic rocks record tectonic events and paleoenvironmental changes in a more than 10 km-thick succession. In this contribution, we report new U–Pb and Sm–Nd geochronological constraints for the 605 to 580 Ma Bom Jardim Group, the 570 Ma Acampamento Velho Formation, and a newly-recognized 544 Ma volcanism. Depositional patterns of these units reveal the transition from a restricted, fault-bounded basin into a wide, shallow basin. The expansion of the basin and diminished subsidence rates are demonstrated by increasing areal distribution and compressed isopachs and increasing onlap of sediments onto the basement to the west. The Sm–Nd isotopic composition of the volcanic rocks indicates mixed sources, including crustal rocks from the adjacent basement. Both Neoproterozoic and Paleoproterozoic sources are indicated for the western part of the basin, whereas only the older Paleoproterozoic signature can be discerned in the eastern part of the basin.     

Geological setting of the Wassa gold deposit,SW Ghana

Including past production, current indicated and inferred resources, Wassa is a 5 Moz poly-deformed early-orogenic gold deposit located on the eastern flank of the Ashanti Belt, in southwest Ghana. It is hosted by metamorphosed volcanic, intrusive and sedimentary rocks of the Sefwi Group (ca. 2260–2160 Ma). Early mineralization has an Eoeburnean age (2164 ± 22 Ma, Re–Os on pyrite) and is characterized by quartz veins, by a carbonate alteration of the host rocks, and by deformed gold-bearing pyrite. Remobilization of this gold occurred during the late stages of the Eburnean Orogeny (~ 2.1 Ga) and is associated with quartz-carbonate veins with visible gold and euhedral pyrites.     

Relative contribution of magmatic and post-magmatic processes in the genesis of the Thompson Mine Ni-Co sulfide ores,Manitoba, Canada

The Ni-Co-(PGE) sulfide deposits of the Thompson Nickel Belt (TNB) in Northern Manitoba, Canada are part of the fifth largest nickel camp in the world based on contained nickel; past production from the TNB deposits is 2500 kt Ni. The Thompson Deposit is located on the eastern and southern flanks of the Thompson Dome structure, which is a re-folded nappe structure formed during collision of the Trans-Hudson Orogen with the Canadian Shield at 1.9–1.7 Ga. The Thompson Deposit is almost entirely hosted by P2 member sulfidic metasedimentary rocks of the Paleoproterozoic Ospwagan Group. Variably serpentinised and altered dunites, peridotites and pyroxenites contain disseminated sulfides and have a spatial association with sediment-hosted Ni sulfides which comprise the bulk of the ore types. These rocks formed from rift-related komatiitic magmas that were emplaced at 1.88 Ga, and subsequently deformed by boudinage, thinning, folding, and stacking.Disseminated sulfide mineralization in the large serpentinised peridotite and dunite intrusions that host the Birchtree and Pipe Ni-Co sulfide deposits typically has 4–6 wt% Ni in 100% sulfide. The disseminated sulfides in the less abundant and much smaller boudinaged serpentinised peridotite and dunite bodies associated with the Thompson Deposit have 7–10 wt% Ni in 100% sulfide. The majority of Thompson Mine sulfides are hosted in the P2 member of the Pipe Formation which is a sulfidic schist developed from a shale prololith; the mineralization in the schist includes both low Ni tenor (<1 wt% Ni in sulfide) and barren sulfide (<200 ppm Ni) and a Ni-enriched sulfide with 1–18 wt% Ni in 100% sulfide. The semi-massive and massive sulfide ores show a similar range in Ni tenor to the metasediment-hosted mineralization, but there are discrete populations with maximum Ni tenors of ∼8, 11 and 13 wt% Ni in 100% sulfide. The variations in Ni tenor are related to the Ni/Co ratio (high Ni/Co correlates with high Ni tenor sulfide) and this relationship is produced by the different Ni/Co ratios in sulfides with a range in proportions of pyrrhotite and pentlandite. Geological models of the ore deposit, host rocks, and sulfide geochemical data in three dimensions reveal that the Thompson Deposit forms an anastomosing domain on the south and east flanks of a first order D3 structure which is the Thompson Dome. In detail, a series of second order doubly-plunging folds on the eastern and southern flank control the geometry of the mineral zones. The position of these folds on the flank of the Thompson Dome is a response to the anisotropy of the host rocks during deformation; ultramafic boudins and layers of massive quartzite in ductile metasedimentary rocks control the geometry of the doubly-plunging F3 structures. The envelope of mineralization is almost entirely contained within the P2 member of the Pipe formation, so the deposit is clearly folded by the first order and second order D3 structures. The sulfides with highest Ni tenor (typically >13 wt% Ni in sulfide) define a systematic trend that mirrors the configuration of the second order doubly-plunging F3 structures on the flanks of the Dome. Although moderate to high Ni tenor mineralization is sometimes localized in fold hinges, more typically the highest Ni tenor mineralization is located on the flanks of the fold structures.There is no indication of the mineralogical and geochemical signatures of sedimentary exhalative or hydrothermal processes in the genesis of the Thompson ores. The primary origin of the mineralization is undoubtedly magmatic and this was a critical stage in the development of economic mineralization. Variations in metal tenor in disseminated sulfides contained in ultramafic rock indicate a higher magma/sulfide ratio in the Thompson parental magma relative to Birchtree and Pipe. The variation in Ni tenor of the semi-massive and massive sulfide broadly supports this conclusion, but the variations in metal tenor in the Thompson ores was likely created partly during deformation. The sequence of rocks was modified by burial and loading of the crust (D2 events) to a peak temperature of 750 °C and pressure of 7.5 kbar. The third major phase of deformation (D3) was a sinistral transpression (D3 event) which generated the dome and basin configuration of the TNB. These conditions allowed for progressive deformation and reformation of pyrrhotite and pentlandite into monosulfide solid solution as pressure and temperature increased; this process is termed sulfide kinesis. Separation of the ductile monosulfide solid solution from granular pentlandite would result in an effective separation of Ni during metamorphism, and the monosulfide solid solution would likely be spread out in the stratigraphy to form a broad halo around the main deposit to produce the low Ni tenor sulfide. Reformation of pentlandite and pyrrhotite after the peak D2 event would explain the broad footprint of the mineral system. The effect of the D3 event at lower pressure and temperature would have been to locally redistribute, deform, and repeat the lenses of sulfide.The understanding of the relationships between petrology, stratigraphy, structure, and geochemistry has assisted in formulating a predictive exploration model that has triggered new discoveries to the north and south of the mine, and provides a framework for understanding ore genesis in deformed terrains and the future exploration of the Thompson Nickel Belt.     

Age and origin of uranium mineralization in the Camie River deposit (Otish Basin,Québec,Canada)

The Camie River uranium deposit is located in the southeastern part of the Paleoproterozoic Otish Basin (Québec). The uranium mineralization consists of disseminated and vein uraninite and brannerite precipitated close to the unconformity between Paleoproterozoic fluviatile, pervasively altered, sandstones and conglomerates of the Matoush Formation and the underlying sulfide-bearing graphitic schists of the Archean Hippocampe greenstone belt. Diagenetic orange/pink feldspathic alteration of the Matoush Formation consists of authigenic albite cement partly replaced by later orthoclase cement, with the Na₂O content of clastic rocks increasing with depth. Basin-wide green muscovite alteration affected both the Matoush Formation and the top of the basement Tichegami Group. Uraninite with minor brannerite is mainly hosted by subvertical reverse faults in basement graphitic metapelites ± sulfides and overlying sandstones and conglomerates. Uranium mineralization is associated with chlorite veins and alteration with temperatures near 320 °C, that are paragenetically late relative to the diagenetic feldspathic and muscovite alterations. Re-Os geochronology of molybdenite intergrown with uraninite yields an age of 1724.0 ± 4.9 Ma, whereas uraninite yields an identical, although slightly discordant, 1724 ± 29 Ma SIMS U-Pb age. Uraninite has high concentrations in REE with flat REE spectra resembling those of uraninite formed from metamorphic fluids, rather than the bell-shaped patterns typical of unconformity-related uraninite. Paragenesis and geochronology therefore show that the uranium mineralization formed approximately 440 million years after intrusion of the Otish Gabbro dykes and sills at ∼2176 Ma, which constrains the minimum age for the sedimentary host rocks. The post-diagenetic stage of uraninite after feldspathic and muscovite alterations, the paragenetic sequence and the brannerite-uraninite assemblage, the relatively high temperature for the mineralizing event (∼320 °C) following the diagenetic Na- and K-dominated alteration, lack of evidence for brines typical of unconformity-related U deposits, the older age of the Otish Basin compared to worldwide basins hosting unconformity-related uranium deposits, the large age difference between basin fill and mineralization, the older age of the uranium oxide compared to ages for worldwide unconformity-related U deposits, and the flat REE spectra of uraninite do not support the previous interpretation that the Camie River deposit is an unconformity-associated uranium deposit. Rather, the evidence is more consistent with a PaleoProterozoic, higher-temperature hydrothermal event at 1724 Ma, whose origin remains speculative.     

Reprint of “Depositional environment and tectonic implications of the Paleoproterozoic BIF in Changyi area,eastern North China Craton: Evidence from geochronology and geochemistry of the metamorphic wallrocks”

The Changyi banded iron formation (BIF) in the eastern North China Craton (NCC) occurs within the Paleoproterozoic Fenzishan Group. Three types of metamorphic wallrocks interbedded with the BIF bands are identified, including plagioclase gneisses and leptynites, garnet-bearing gneisses and amphibolites. Protolith reconstruction suggests that the protoliths of the plagioclase gneisses and leptynites are mainly graywackes with minor contribution of pelitic materials, the garnet-bearing gneisses are Fe-rich pelites contaminated by clastics, and the amphibolites are tholeiitic rocks. Trace elements of La, Th, Sc and Zr of the plagioclase gneisses and leptynites and the garnet-bearing gneisses support that these meta-sedimentary rocks were probably derived from recycling of Archean rocks with felsic and mafic materials differentiated into different rock types. ²⁰⁷Pb/²⁰⁶Pb ages of detrital zircons from the meta-sedimentary rocks concentrate at 2.7–3.0 Ga, confirming their derivation from the Archean rocks. The presence of several Paleoproterozoic detrital zircons (2240 to 2246 Ma), however, also suggests minor involvement of Paleoproterozoic materials. The Archean detrital zircons have ε_Hf(t) values varying from − 0.7 to 7.6, which mainly fall between the 3.0 Ga and 3.3 Ga average crustal evolution lines on the age vs. ε_Hf(t) diagram, further illustrating that the rocks providing materials for the meta-sedimentary rocks mainly originated from partial melting of a Mesoarchean crust. This is strongly supported by their crust-like trace element distribution patterns (such as Nb, Ta, P and Ti depletion) and ancient Nd depleted mantle model ages (T_DM = 2.9–3.4 Ga). In addition, the remarkably high ε_Hf(t) values (7.5 to 9.3) of the Paleoproterozoic detrital zircons constrain the Paleoproterozoic materials to originate from a depleted mantle. The amphibolites show low SiO₂ (46.5 to 52.8 wt.%) and high MgO (5.68 to 10.9 wt.%) contents, crust-like trace element features and low ε_Nd(t) values (− 4.5 to − 0.3), suggesting that these ortho-metamorphic rocks were mainly derived from subcontinental lithospheric mantle with some contamination by Archean crustal materials. Since an intra-continental environment was required for the formation of the above metamorphic rocks, these rocks not only confine the depositional environment of the Changyi BIF to be an intra-continental rift, but also support the rifting processes of the eastern NCC during Paleoproterozoic.     

The Basil Cu–Co deposit,Eastern Arunta Region,Northern Territory,Australia: A metamorphosed volcanic-hosted massive sulphide deposit

The Basil Cu–Co deposit, Harts Range, central Australia, is hosted by the Riddock Amphibolite, a sequence that has been metamorphosed at upper-amphibolite- to granulite-facies conditions at 480–460 Ma (Larapinta Event), and subsequently reworked at amphibolite-facies conditions (450–300 Ma). As a result, many of the primary mineralization textures and other features that could characterise ore genesis have been obliterated. However, preserved textures and mineral relationships in the mineralized zone, allow some constraints to be placed on the genetic history of the deposit using mineralogical, petrographic and geochemical studies of host rocks and sulphides.Results of this study permit at least two genetic models to be ruled out. Firstly, whole rock geochemistry and garnet compositions suggest that the deposit is not a skarn system. Secondly, the lack of any significant Ni-signature, and the presence of abundant zircons in the host amphibolite (indicating that not all host rocks are mafic in composition and/or magmatic in character), make an orthomagmatic Ni–Cu–(PGE) system unlikely. Alternatively, Basil is assigned to a volcanic-hosted massive sulphide (VHMS)-style of mineralization, formed on the seafloor, within basaltic and sedimentary host rocks, typical of deposits occurring in such settings. The lack of a recognisable hydrothermal alteration zone is consistent with either destruction of the alteration zone during metamorphism or detachment of the ore from alteration during later deformation.The occurrence of sulphide inclusions within garnet and amphibole indicates that the sulphides must be syn-metamorphic or earlier. Partitioning of trace elements between pyrite and co-existing pyrrhotite suggests that (re)crystallization occurred under equilibrium conditions. The composition of sphalerite coexisting with pyrite and pyrrhotite indicates crystallization at pressures of at least 10 kbar, consistent with peak metamorphism during the Early Ordovician Larapinta Event. Zr-in-titanite geothermometry indicates peak temperatures of 730–745 °C.     

Chemical and boron isotopic compositions of tourmaline from the Paleoproterozoic Houxianyu borate deposit,NE China: Implications for the origin of borate deposit

The Houxianyu borate deposit in northeastern China is one of the largest boron sources of China, hosted mainly in the Paleoproterozoic meta-volcanic and sedimentary rocks (known as the Liaohe Group) that are characterized by high boron concentrations. The borate ore-body has intimate spatial relationship with the Mg-rich carbonates/silicates of the Group, with fine-grained gneisses (meta-felsic volcanic rocks) as main country rocks. The presence of abundant tourmalinites and tourmaline-rich quartz veins in the borate orebody provides an opportunity to study the origin of boron, the nature of ore-forming fluids, and possible mineralization mechanism. We report the chemical and boron isotopic compositions of tourmalines from the tourmaline-rich rocks in the borate deposit and from the tourmaline-bearing fine-grained gneisses.Tourmalines from the fine-grained gneisses are chemically homogeneous, showing relatively high Fe and Na and low Mg, with δ¹¹B values in a narrow range from +1.22‰ to +2.63‰. Tourmalines from the tourmaline-rich rocks, however, commonly show compositional zoning, with an irregular detrital core and a euhedral overgrowth, and have significantly higher Mg, REE (and more pronounced positive Eu anomalies), V (229–1852 ppm) and Sr (208–1191 ppm) than those from the fine-grained gneisses. They show varied B isotope values ranging from +4.51‰ to +12.43‰, which plot intermediate between those of the terrigenous sediments and arc rocks with low boron isotope values (as represented by the δ¹¹B = +1.22‰ to +2.63‰ of the fine-grained gneisses of this study) and those of marine carbonates and evaporates with high boron isotope values. In addition, the rim of the zoned tourmaline shows notably higher Mg, Ti, V, Sn, and Pb, and REE (particularly LREEs), but lower Fe, Co, Cr, Ni, Zn, Mn, and lower δ¹¹B values than the core. These data suggest that (1) the sources of boron of the borate ore-body are mainly the Paleoproterozoic meta-volcanic and sedimentary rocks, and (2) the ore-forming fluids should be the high temperature metamorphic fluids related to the amphibolite-facies metamorphism of the Paleoproterozoic foldbelt, which leach boron from the boron-rich meta-volcanic and sedimentary rocks of the Liaohe Group, and the boron-rich metamorphic fluids subsequently interacted with the marine Mg-rich carbonates and evaporates, forming borate deposit, the tourmaline overgrowth in the rim and the tourmaline-rich rocks.     

Chemical sedimentary protoliths in the > 3.75 Ga Nuvvuagittuq Supracrustal Belt (Québec,Canada)

Analyses of chemical sedimentary precipitates such as banded iron formation (BIF) provide a direct means to explore the nature and composition of the early hydrosphere. The recently discovered > 3750 Myr old Nuvvuagittuq Supracrustal Belt (NSB) in the Northeast Superior Province (Québec, Canada) hosts a suite of iron oxide-rich (± pyroxene and amphibole) units that are interpreted to be the metamorphosed equivalents of Fe oxide-facies BIF, and a collection of BIF-like Ca–Fe–Mg silicate rocks. The NSB rocks provide a rare glimpse of trace metal availability in Eoarchean (ca. 3800 Ma) seawater. As they may be contemporaneous with the relatively well-studied Isua Supracrustal Belt of southern West Greenland, their comparison provides an opportunity to enhance our basic understanding of the Eoarchean oceans at a global scale. Work since the initial discovery of the NSB in 2001 has established the basic lithological, geochemical and petrographic characteristics of these BIF and BIF-like rocks. Here we review the current state of knowledge of NSB rocks of probable chemical sedimentary origin, including aspects of their geology, likely origin and age. We conclude by examining the implications of results thus far for our understanding of early seawater compositions, and for the emergence of life in the context of early metallo-enzyme evolution.     

Microplaty hematite ore in the Yilgarn Province of Western Australia: The geology and genesis of the Wiluna West iron ore deposits

The Wiluna West small (~ 130 Mt) high-grade bedded hematite ore deposits, consisting of anhedral hematite mesobands interbedded with porous layers of acicular hematite, show similar textural and mineralogical properties to the premium high-grade low-phosphorous direct-shipping ore from Pilbara sites such as Mt Tom Price, Mt Whaleback, etc., in the Hamersley Province and Goldsworthy, Shay Gap and Yarrie on the northern margin of the Pilbara craton. Both margins of the Pilbara Craton and the northern margin of the Yilgarn craton were subjected to sub-aerial erosion in the Paleoproterozoic era followed by marine transgressions but unlike the Hamersley Basin, the JFGB was covered by comparatively thin epeirogenic sediments and not subjected to Proterozoic deformation or burial metamorphism. The Joyner's Find greenstone belt (JFGB) in the Yilgarn region of Western Australia was exhumed by middle to late Cenozoic erosion of a cover of unmetamorphosed and relatively undeformed Paleoproterozoic epeirogenic sedimentary rocks that preserved the JFGB unaltered for nearly 2 Ga; thus providing a unique snapshot of the early Proterozoic environment.Acicular hematite, pseudomorphous after acicular iron silicate, is only found in iron ore and BIF that was exposed to subaerial deep-weathering in early Paleoproterozoic times (pre 2.2 Ga) and in the overlying unconformable Paleoproterozoic conglomerate derived from these rocks and is absent from unweathered rocks (Lascelles, 2002). High-grade ore and BIF weathered during later subaerial erosion cycles contain anhedral hematite and acicular pseudomorphous goethite. The acicular hematite was formed from goethite pseudomorphs of silicate minerals by dehydration in the vadose zone under extreme aridity during early Paleoproterozoic subaerial weathering.The principal high-grade hematite deposits at Wiluna West are interpreted as bedded ore bodies that formed from BIF by loss of chert bands during diagenesis and have been locally enriched to massive hematite by the introduction of hydrothermal specular hematite. No trace of chert bands are present in the deep saprolitic hematite and hematite–goethite ore in direct contrast to shallow supergene ore in which the trace of chert bands is clearly defined by goethite replacement, voids and detrital fill. Abundant hydrothermal microplaty hematite at Wiluna West is readily distinguished by its crystallinity.The genesis of the premium ore from the Pilbara Region has been much discussed in the literature and the discovery at Wiluna West provides a unique opportunity to compare the features that are common to both districts and to test genetic models.     

The Vorontsovskoe Au-Hg-As ore deposit (Northern Urals,Russia): Geological setting,ore mineralogy,geochemistry, geochronology and genetic model

The large Vorontsovskoe Au-Hg-As deposit in the Urals is located in the exocontact of the Early Devonian Auerbah gabbro-diorite-granodiorite massif, which intrudes volcano-sedimentary rocks. The orebodies are confined to a tectonic contact of calcareous and tuffaceous rocks. They are composed of 6 types of disseminated ores, but the main reserves of gold are associated with the following ore types: gold-pyrite-arsenopyrite in altered tuffaceous rocks, pyrite-realgar ores in limestone breccia with a carbonate-volcanogenic cement, and gold-oxide-clay from regolith with residual gold. Early ore associations have been formed at 450–300 °C, whereas the late ores have been formed at lower temperature of 260–110 °C. We propose a model for the genesis of the Vorontsovskoe deposit based on synchronicity of mineralization with the formation of the Auerbah volcano-plutonic complex. The Ar-Ar age of hydromica from the gold-arsenopyrite association is 391.1 ± 4.9 Ma, which coincides with the age of igneous rocks of the Auerbah complex. The main sources of water and carbon dioxide were composed of the fluid derived from the magma chamber and the metamorphic water equilibrated with carbonate sedimentary rocks. Magmatic fluid dominated during the development of skarns, jasperoids and quartz veins, whereas metamorphic water was dominant during quartz-sericite alteration of volcano-sedimentary rocks and calcareous breccias. The bulk of the sulfur was supplied by a deep magma reservoir, however this source prevailed only during skarn ore formation. The mixing between deep-sourced sulfur and sedimentary or biogenic sulfur was established for other ore types. Gold and other ore components were possibly introduced during the volcanic and intrusive activity and also extracted from host sedimentary rocks.     

Origin of the Alxa Block,western China: New evidence from zircon U–Pb geochronology and Hf isotopes of the Longshoushan Complex

The NW–SE trending Longshoushan is in the southwestern margin of the Alxa Block, which was traditionally considered the westernmost part of the North China Craton (NCC). Precambrian crystalline basement exposed in the Longshoushan area was termed the “Longshoushan Complex”. This complex's formation and metamorphism are significant to understand the geotectonics and early Precambrian crustal evolution of the western NCC. In this study, field geology, petrology, and zircon U–Pb and Lu–Hf isotopes of representative orthogneisses and paragneisses in the Longshoushan Complex were investigated. U–Pb datings reveal three Paleoproterozoic magmatic episodes (ca. 2.33, ca. 2.17 and ca. 2.04 Ga) and two subsequent regional metamorphic events (ca. 1.95–1.90 Ga and ca. 1.85 Ga) for metamorphic granitic rocks in the Longshoushan Complex. U–Pb dating of the detrital magmatic zircons from two paragneisses yields concordant ²⁰⁷Pb/²⁰⁶Pb ages between 2.2 Ga and 2.0 Ga, and a small number of metamorphic zircon rims provide a ca. 1.95 Ga metamorphic age, suggesting that the depositional time of the protolith was between 2.0 and 1.95 Ga and that the sedimentary detritus was most likely derived from the granitic rocks in the Longshoushan Complex itself. Zircon Lu–Hf isotopic analyses indicate that nearly all magmatic zircons from ca. 2.0 Ga to ca. 2.17 Ga orthogneisses have positive εHf(t) values with two-stage Hf model ages (T_DMC) ranging from 2.45 to 2.65 Ga (peak at ca. 2.5 Ga), indicating that these Paleoproterozoic granitic rocks were derived from the reworking of the latest Neoarchean–early Paleoproterozoic juvenile crust. Detrital magmatic zircons from two paragneisses yield scattered ¹⁷⁶Hf/¹⁷⁷Hf ratios, εHf(t) and T_DMC values, further indicating that the sedimentary detritus was not only derived from these plutonic rocks but also from other unreported or denuded Paleoproterozoic igneous rocks. The ca. 2.15 Ga detrital magmatic zircons from one paragneiss have negative εHf(t) values with T_DMC ranging from 2.76 to 3.04 Ga, indicating another important crustal growth period in the Longshoushan region. These data indicate that the Longshoushan Complex experienced Neoarchean–Early Paleoproterozoic crustal growth, approximately ca. 2.3–2.0 Ga experienced multiphase magmatic events, and approximately ca. 1.95–1.90 Ga and ca. 1.85 Ga experienced high-grade metamorphic events. The sequence of tectonothermal events is notably similar to that of the main NCC. Together with the datasets from an adjacent area, we suggest that the western Alxa Block was most likely an integrated component of the NCC from the Neoarchean to the Paleoproterozoic.     

U–Pb and Lu–Hf isotopes in detrital zircon from Neoproterozoic sedimentary rocks in the northern Yangtze Block: Implications for Precambrian crustal evolution

A combined study of Lu–Hf isotopes and U–Pb ages for detrital zircons from sedimentary rocks can provide information on the crustal evolution of sedimentary provenances, and comparisons with potential source regions can constrain interpretations of paleogeographic settings. Detailed isotopic data on detrital zircons from Neoproterozoic sedimentary rocks in the northern part of the Yangtze Block suggest that these rocks have the maximum depositional ages of ~ 750 Ma, and share a similar provenance. In their source area, units of late Archean (2.45 to 2.55 Ga) to Paleoproterozoic (1.9 to 2.0 Ga) U–Pb ages made up the basement, and were overlain or intruded by magmatic rocks of Neoproterozoic U–Pb ages (740 to 900 Ma). Hf isotopic signatures of the detrital zircons indicate that a little juvenile crust formed in the Neoarchean; reworking of old crust dominates the magmatic activity during the Archean to Paleoproterozoic, while the most significant juvenile addition to the crust occurred in the Neoproterozoic. Only the Neoproterozoic zircon U–Pb ages can be matched with known magmatism in the northern Yangtze Block, while other age peaks cannot be correlated with known provenance areas. Similar zircon U–Pb ages have been obtained previously from sediments along the southeastern and western margins of the Yangtze Block. Thus, it is suggested that an unexposed old basement is widespread beneath the Yangtze Block and was the major contributor to the Neoproterozoic sediments. This basement had a magmatic activity at ~ 2.5 Ga, similar to that in North China; but zircon Hf isotopes suggest significant differences in the overall evolutionary histories between the Yangtze and North China.     

Metallogeny of South Greenland: A review of geological evolution,mineral occurrences and geochemical exploration data

South Greenland has been the site of historic mining of cryolite, copper, graphite and gold, hosts mineral deposits with gold, uranium, zinc, niobium, tantalum, zirconium, hafnium, REE, iron, titanium, vanadium, fluorite and graphite, and has additional potential for lithium, beryllium, phosphorus, gallium and thorium. Data from stream sediment geochemical surveys document that South Greenland is enriched in a range of these elements relative to the rest of Greenland and to estimates of the upper crust composition. Distribution patterns for individual elements within south Greenland exhibit enriched regions that are spatially related to lithological units, crustal structure and known mineralisation.The Northern Domain of South Greenland includes the southernmost part of the orthogneiss-dominated North Atlantic craton. Orogenic gold mineralisation is hosted by quartz veins and hydrothermally altered rocks associated with shear zones intersecting the Mesoarchaean Tartoq Group of mafic metavolcanic rocks. Geochemical exploration indicates that additional potential for gold mineralisation exists within Palaeoproterozoic supracrustal rocks overlying the Archaean basement.Rocks formed during the Palaeoproterozoic Ketilidian orogeny occupy a major part of South Greenland and has been divided into two domains. The Central Domain is underlain by the Julianehåb igneous complex forming a 100 km wide ENE–WSW zone centrally across South Greenland. Intrusive and extrusive, mostly felsic magmatic rocks were emplaced in two main stages (1850–1830 and 1800–1780 Ma) in a continental arc setting. Positive anomalies in aeromagnetic data indicate that mafic plutons are common in the late igneous complex. Intra-arc mafic metavolcanic rocks contain syngenetic stratabound copper sulphide and epigenetic shear zone-hosted copper–silver–gold mineralisation at Kobberminebugt and Kangerluluk, whereas metasedimentary and metapyroclastic rocks contain stratabound uraninite mineralisation. Orthomagmatic iron–titanium–vanadium mineralisation is hosted by a gabbro. A potential for porphyry-type mineralisation related to the late intrusive stages of the Julianehåb igneous complex is suggested by showings with copper, molybdenum and gold together with stream sediment anomalies for these elements. Vein-type uranium mineralisation occurs in fault zones in the Julianehåb igneous complex related to Mesoproterozoic rifting.The Southern Domain contains an assemblage of Palaeoproterozoic metasedimentary and metavolcanic rocks that underwent moderate to strong deformation, peak HT–LP metamorphism and partial melting with subsequent retrograde exhumation at 1790–1765 Ma. The supracrustal rocks contain syngenetic Au, As, Sb, U, and Zn mineralisation in volcanic or graphite- and sulphide-rich sedimentary environments; graphite was mined historically at two sites. Many stream sediment gold anomalies are located in a NE-trending belt along the boundary between the early Julianehåb complex and the supracrustal rocks to the south. They reflect a number of auriferous quartz vein occurrences, including the Nalunaq gold deposit, hosted in a system of shear zones and probably generated as orogenic gold during Ketilidian accretion. The 1755–1730 Ma, A-type Ilua plutonic suite is the latest magmatic event in the Ketilidian orogen.The 1300–1140 Ma Gardar period involved continental rifting, sedimentation and alkaline magmatism. Numerous dykes and 10 ring-shaped intrusion complexes were formed across South Greenland. An orthomagmatic iron–titanium–vanadium deposit is hosted by troctolitic gabbro. Residual magmas and fluids resulting from extreme magmatic differentiation, possibly combined with assimilation of older crust, created mineral deposits including cryolite that was mined at Ivigtut, large low-grade deposits of uranium–rare earth elements–zinc at Kvanefjeld and tantalum–niobium–rare earth element–zirconium at Kringlerne, in the Ilímaussaq complex, as well as tantalum–niobium–rare earth elements at Motzfeldt Sø in the Igaliko complex.The South Greenland crustal evolution records effects of mantle processes, such as lithospheric extension, subduction and underplating, which resulted in recurrent magma emplacement in tectonically active environments. As such, the geology of South Greenland reflects events and circumstances that are favourable to the generation and preservation of hydrothermal ore-forming fluid systems during the Ketilidian orogeny as well as to the development of extreme rock compositions within the Gardar alkaline igneous province.     

Zircon and titanite U-Pb dating of the Zhangjiawa iron skarn deposit,Luxi district,North China Craton: Implications for a craton-wide iron skarn mineralization

The North China craton hosts numerous iron skarn deposits containing more than 2600 Mt of iron ores, mostly with an average grade of >45 wt% Fe, which have been among the most important source of high-grade iron ores for the last three decades in China. These deposits typically form clusters and can be roughly divided into the western and eastern belts, which are located in the middle of Trans-North China orogen and to the west of the Tan-Lu fault zone in the eastern part of North China craton, respectively. The western belt mainly consists of the southern Taihang district, as well as the Linfen and Taiyuan ore fields, whereas the eastern belt comprises the Luxi and Xu-Huai districts. The Zhangjiawa deposit in the Luxi district has proven reserves of 290 Mt at an average of 46% Fe (up to >65%). The iron mineralization occurs mainly along contact zones between the Kuangshan dioritic intrusion and middle Ordovician marine carbonate rocks that host numerous evaporite intercalations. Titanite grains from the mineralized skarn are closely intergrown with magnetite and retrograde skarn minerals including chlorite, phlogopite and minor epidote, indicating a hydrothermal origin. The titanite grains have extremely low REE contents and low Th/U ratios, consistent with their precipitation directly from hydrothermal fluids responsible for the iron mineralization. Ten hydrothermal titanite grains yield a weighted mean ²⁰⁶Pb/²³⁸U age of 131.0 ± 3.9 Ma (MSWD = 0.1, 1σ), which is in excellent agreement with a zircon U-Pb age (130 ± 1 Ma) of the ore-related diorite. This age consistency confirms that the iron skarn mineralization is temporally and likely genetically related to the Kuangshan intrusion. Results from this study, when combined with existing isotopic age data, suggest that iron skarn mineralization and associated magmatism throughout both the eastern and western belts took place coevally between 135 and 125 Ma, with a peak at ca. 130 Ma. As such, those deposits may represent the world's only major Phanerozoic iron skarn concentration hosted in Precambrian cratons. The magmatism and associated iron skarn mineralization coincide temporally with the culmination of lithospheric thinning and destruction of the North China craton, implying a causal link between the two.     

Age and petrogenesis of the Neoproterozoic Chon-Ashu alkaline complex,and a new discovery of chalcopyrite mineralization in the eastern Kyrgyz Tien Shan

Neoproterozoic volcanics and granitoids formed at Rodinia margins within a time span of 880 Ma–700 Ma, are well-documented in many terranes of the southern Central Asian Orogenic Belt (CAOB). Ages younger than 550 Ma corresponding to the opening of the Terskey Ocean are also common. However, so far, there were very few published ages in the range 700 Ma–550 Ma from the Kyrgyz Tien Shan. In this paper we present new data for the alkaline Chon-Ashu complex emplaced at the end of the Cryogenian Period of the Neoproterozoic (850–635 Ma, Gradstein et al., 2012). The alkaline complex intrudes the Precambrian metamorphic rocks north of the Nikolaev Line which separates the Northern and Middle Tien Shan terranes in the eastern Kyrgyzstan. The undeformed shallow level alkaline rocks range from olivine gabbro to nepheline and cancrinite syenites and leucosyenites. The differentiated rock assemblage can be explained by fractional crystallization of high-silica mineral phases which drives nepheline-normative melts away from the silica saturation boundary. The alkaline rocks of Chon-Ashu are enriched in LILE and HFSE indicative of their origin from lithospheric mantle. An age of 678 ± 9 Ma (U–Pb, SHRIMP) was obtained for a protolith of country gneiss, and an age of 656 ± 4 Ma was obtained for the crosscutting alkaline rocks of the Chon-Ashu complex. Seven zircon grains recovered from gneiss and alkaline rocks had bright overgrown rims which yielded a cumulative age of 400 ± 8 Ma. A metamorphic event, followed by uplift and emplacement of shallow level alkaline complex, constrains the geodynamic setting. Alkaline rocks usually form in an extensional setting and originate from lithospheric mantle. The 690 Ma xenoliths of mafic granulite from the NW Tarim have been interpreted to originate by mafic underplating. This mafic underplating may have been responsible for metamorphism in the middle crust prior to emplacement of the Chon-Ashu complex. The 670 Ma–630 Ma period of extension and emplacement of enriched alkaline rocks can be also traced on a regional scale through southern Kazakhstan and the northern Tarim. We tentatively interpret these events as a result of mafic underplating and subsequent rifting related to the break-up of Rodinia. During field work at Chon-Ashu, rich chalcopyrite mineralization has been discovered in carbonate veinlets in leucosyenite alkaline dikes and has also been found in the adjacent Cambrian gabbro and granites shown on the map as undivided Devonian–Silurian. Stockwork mineralization predominates though disseminated mineralization is also present. The Cu content reaches 16,184 ppm and is associated with elevated concentrations of Pb, Zn and Ag. The polyphase structural evolution of the area suggests that mineralization could have formed in several genetically unrelated stages. Based on structural and mineralogical evidence we tentatively relate the earliest stage of chalcopyrite mineralization to the late magmatic CO₂-rich fluids emanating from the Cryogenian alkaline complex. The Early Devonian thermal event registered by growth of new zircon at 400 Ma has important metallogenic implications on a regional scale. However the origin of two zones of alteration in the undivided Silurian–Devonian granites is ambiguous because their age was not determined geochronologically. The 522 ± 4 Ma Cambrian gabbro of the Tashtambektor Formation is strongly foliated along the splays of the Nikolaev Line, indicating a Hercynian origin of the fabric. Superimposed mineralized stockwork postdates the foliation and suggests a late-Hercynian age of mineralization in gabbro. The new data enable a reassessment of the metallogenic potential of the Eastern Kyrgyz Tien Shan. Presence of not eroded high-level mineralized Neoproterozoic alkaline intrusions points to a previously underestimated metallogenic potential of pre-Hercynian granitoids which may host preserved porphyry systems, skarns and shear-related mineralization. Finally, the Devonian magmato-metamorphic event which caused formation of a number of ore deposits in central Kyrgyzstan and Kazakhstan could also create potential exploration targets in eastern Kyrgyzstan.     

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 1.64 Ga 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 ~ 1.78 Ga McDermott and ~ 1.73 Ga 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 ~ 1.73 Ga to engender intermittent but strong bottom-water euxinia during shale deposition, thus providing a robust chemical trap for base metal sulfide mineralization.     

Formation and evolution of Precambrian continental lithosphere in South China

An overview is presented for the formation and evolution of Precambrian continental lithosphere in South China. This is primarily based on an integrated study of zircon U–Pb ages and Lu–Hf isotopes in crustal rocks, with additional constraints from Re–Os isotopes in mantle-derived rocks. Available Re–Os isotope data on xenolith peridotites suggest that the oldest subcontinental lithospheric mantle beneath South China is primarily of Paleoproterozoic age. The zircon U–Pb ages and Lu–Hf isotope studies reveal growth and reworking of the juvenile crust at different ages. Both the Yangtze and Cathaysia terranes contain crustal materials of Archean U–Pb ages. Nevertheless, zircon U–Pb ages exhibit two peaks at 2.9–3.0 Ga and ~ 2.5 Ga in Yangtze but only one peak at ~ 2.5 Ga in Cathaysia. Both massive rocks and crustal remnants (i.e., zircon) of Archean U–Pb ages occur in Yangtze, but only crustal remnants of Archean U–Pb ages occur in Cathaysia. Zircon U–Pb and Lu–Hf isotopes in the Kongling complex of Yangtze suggest the earliest episode of crustal growth in the Paleoarchean and two episodes of crustal reworking at 3.1–3.3 Ga and 2.8–3.0 Ga. Both negative and positive ε_Hf(t) values are associated with Archean U–Pb ages of zircon in South China, indicating both the growth of juvenile crust and the reworking of ancient crust in the Archean. Paleoproterozoic rocks in Yangtze exhibit four groups of U–Pb ages at 2.1 Ga, 1.9–2.0 Ga, ~ 1.85 Ga and ~ 1.7 Ga, respectively. They are associated not only with reworking of the ancient Archean crust in the interior of Yangtze, but also with the growth of the contemporaneous juvenile crust in the periphery of Yangtze. In contrast, Paleoproterozoic rocks in Cathaysia were primarily derived from reworking of Archean crust at 1.8–1.9 Ga. The exposure of Mesoproterozoic rocks are very limited in South China, but zircon Hf model ages suggest the growth of juvenile crust in this period due to island arc magmatism of the Grenvillian oceanic subduction. Magmatic rocks of middle Neoproterozoic U–Pb ages are widespread in South China, exhibiting two peaks at about 830–800 Ma and 780–740 Ma, respectively. Both negative and positive ε_Hf(t) values are associated with the middle Neoproterozoic U–Pb ages of zircon, suggesting not only growth and reworking of the juvenile Mesoproterozoic crust but also reworking of the ancient Archean and Paleoproterozoic crust in the middle Neoproterozoic. The tectonic setting for this period of magmatism would be transformed from arc–continent collision to continental rifting with reference to the plate tectonic regime in South China.     

加拿大拉布拉多铀矿化类型和铀矿勘探潜力

拉布拉多(Labrador)地处加拿大地盾东部边缘,其U矿化类型多样,但最重要的是赋存于变质火山岩和沉积岩内的矿化,主要分布于Makkovik省中心矿带区(CMB)的古元古代表壳岩和侵入岩中。该区矿化有岩浆型(伟晶岩和酸性火山岩)、后生--热液型(有些类似于Fe氧化物--Cu--Au(IOCG)型矿床)、剪切带型(可能是变质--交代成因)和砂质沉积岩中的层状矿化,但未发现典型的不整合型。根据其矿化类型,结合已发现矿床的成矿地质特征和矿化地质资料分析,得出该区的一些成矿远景区有较大的勘探潜力,圈定White Bear湖U矿勘查战略选区和Nash湖U矿勘查战略选区。