The Inata deposit,Belahouro District,northern Burkina Faso

The Inata gold deposit is hosted in the Bouroum greenstone belt of northern Burkina Faso and contains ca. 5 Moz of gold resource. The greenstone belt is divided into 4 distinct domains: The Pali West, Pali-Minfo and Fété Kolé domains comprised of variable proportions of mafic to intermediated volcanic, volcaniclastic and sedimentary rocks, and the Sona Basin comprised of feldspathic sandstones and turbidites. Potential Tarkwaian-like conglomerates are rarely observed on the eastern margin of the basin. The stratigraphy is crosscut by a series of intrusions between 2172 ± 15 Ma and 2122 ± 4 Ma. A complex deformation sequence is recorded in the rocks and has been interpreted in a five stage scheme: early syn-depositional basin margin faults reactivated through time and partitioning all subsequent regional deformation (De_B); N–S compression (D1_B > 2172 Ma); E-W compression (D2_B, < ca 2122 Ma); NW–SE compression (D3_B), and a late N–S compression (D4_B). D2_B-D4_B overprint all rocks, including those of the Sona Basin and Tarkwaian-like conglomerates. Peak metamorphism is mid- to upper-greenschist facies.Mineralisation at Inata is hosted in black shales and volcaniclastic rocks of the Pali-Minfo domain and comprises shear-zone hosted quartz-tourmaline-ankerite veins with associated sulphides dominated by pyrite and arsenopyrite. Three generations of pyrite (py1, py2, py3) and one generation of arsenopyrite (apy2) have been identified. Py1 is parallel to bedding and early D1_B foliation and not associated with gold. Py2 and apy2 are coeval, contain up to 1 ppm gold and are spatially associated with auriferous quartz veins. Py3 locally overprints previous assemblages and is also associated with Au. Fluid inclusions in quartz indicate H₂O to H₂O–CO₂–NaCl fluids in auriferous quartz veins.Microscopic to macroscopic observation of fabric-mineral-vein crosscutting relationships indicate that mineralisation is syn-D2_B, disrupted and remobilised during D3_B. All observations and data are consistent with Inata representing an orogenic style of gold mineralisation formed relatively late in the evolution of the host terrane.     

Modification of a Palaeoproterozoic porphyry-like system: Integration of structural,geochemical, petrographic,and fluid inclusion data from the Aitik Cu–Au–Ag deposit,northern Sweden

The Aitik Cu–Au–Ag deposit in the Gällivare area in northern Sweden is Sweden's largest sulphide mine with an annual production of 35 Mt of ore, and the biggest open pit operation in northern Europe. It is proposed in the present study that the Aitik deposit represents a Palaeoproterozoic, strongly metamorphosed porphyry copper deposit that was affected ca. 100 Ma later by a regional IOCG-type hydrothermal event. Consequently, the Aitik deposit might represent a mixed ore system where an early copper mineralisation of porphyry type has been overprinted by later regional IOCG mineralisation.Several attempts have previously been made to genetically classify the Aitik Cu–Au–Ag deposit as a distinct ore type. New geochemical, petrographic, structural, and fluid inclusion results combined with published data have provided the opportunity to present new ideas on the genesis and evolution of the Aitik Cu–Au–Ag deposit. The emplacement of a ca. 1.9 Ga quartz monzodiorite that host the ore at Aitik was related to subduction processes and volcanic arc formation, and synchronous with quartz vein stockwork formation and porphyry copper mineralisation. Highly saline aqueous (38 wt.% NaCl) fluid inclusions in the stockwork veins suggest entrapment at 300 °C and a pressure of nearly 3 kbar, a high pressure for a typical porphyry copper ore, but consistent with conditions at associated deep root zones of intrusion-related magmatic–hydrothermal systems. The highly saline fluid formed disseminated and vein-type ore of mainly chalcopyrite and pyrite within comagmatic volcaniclastic rocks, and caused potassic alteration (biotite, microcline) of the host rocks. The early porphyry copper mineralising event was followed, and largely overprinted, by CO₂ and aqueous medium- to high-salinity (16–57 wt.% salts) fluids related to a ca. 1.8 Ga tectonic and metamorphic event (peak conditions 500–600 °C and 4–5 kbar). Extensive deformation of rocks and redistribution of metals occurred. Magnetite enrichment locally found within late veins, and late amphibole–scapolite and K feldspar alterations within the deposit, are some of the features at Aitik implying that aqueous fluids responsible for IOCG-mineralisation (200–500 °C and ~ 1 kbar) and extensive Na–Ca alteration in the region during the 1.8 Ga tectonic event also affected the Aitik rocks, possibly leading to addition of copper ± gold.     

Early Mesozoic metamorphism and tectonic significance of the eastern segment of the Lhasa terrane,south Tibet

The metamorphic belt in the Basongco area, the eastern segment of Lhasa terrane, south Tibet, occurs as the tectonic blocks in Paleozoic sedimentary rocks. The Basongco metamorphic rocks are mainly composed of paragneiss and schist, with minor marble and orthogneiss, and considered previously to be the Precambrian basement of the Lhasa terrane. This study shows that the Basongco metamorphic belt experienced medium-pressure amphibolite-facies metamorphism under the conditions of T = 640–705 °C and P = 6.0–8.0 kbar. The inherited detrital zircon of the metasedimentary rocks yielded widely variable ²⁰⁶Pb/²³⁸U ages ranging from 3105 Ma to 500 Ma, with two main age populations at 1150 Ma and 580 Ma. The magmatic cores of zircons from the orthogneiss constrain the protolith age as ca. 203 Ma. The metamorphic zircons from all rocks yielded the consistent metamorphic ages of 192–204 Ma. The magmatic cores of zircons in the orthogneiss yielded old Hf model ages (T_DM2 = 1.5–2.1 Ga). The magmatic zircons from the mylonitized granite yielded a crystallization age of ca. 198 Ma. These results indicate that the high-grade metamorphic rocks from the Basongco area were formed at early Jurassic and associated with coeval magmatism derived from the thickening crust. The Basongco metamorphic belt, together with the western and coeval Sumdo and Nyainqentanglha metamorphic belts, formed a 400-km-long tectonic unit, indicating that the central segment of the Lhasa terrane experienced the late Paleozoic to early Mesozoic collisional orogeny.     

Timing and rate of granulite facies metamorphism and cooling from multi-mineral chronology on migmatitic gneisses,Sierras de La Huerta and Valle Fértil,NW Argentina

Migmatitic paragneisses of the Valle Fértil–La Huerta Ranges at the Western margin of the Sierras Pampeanas are composed of garnet–cordierite–plagioclase–biotite–quartz-bearing units that experienced peak metamorphic conditions of ca. 800 °C at 6–7 kbar. Based on petrological studies, pseudosection modeling and petrographic observations, an anticlockwise P–T path with a small pressure increment is proposed. Rare earth element LA-ICP-MS patterns acquired from rutile bearing garnets suggest a single stage of garnet growth at high-T at pressures above the ilmenite–rutile transition. U–Pb dating of zircon rims from the migmatites indicates two distinct metamorphic U–Pb ages of 525 ± 9 Ma and 478 ± 9 Ma. The older age is suggested to record an amphibolite facies event of the Pampean orogeny. The younger metamorphic age is contemporary with igneous zircons from metatonalites and pegmatites that yield 478 ± 4 Ma. We suggest that the prograde high-T metamorphic Famatinian event is associated with the emplacement of large magmatic bodies in which large-scale magmatic activity gave rise to an increased geothermal gradient of about 35 °C/km. Sm–Nd garnet ages of 447 ± 3 Ma indicate a time span of around 30 Ma for which temperatures above the garnet closure temperature prevailed. Using U–Pb, Sm–Nd and Rb–Sr isotope systems, a cooling rate of 3 to 6 °C/Myr is inferred.     

Structural evolution of the Mokrsko-West,Mokrsko-East and Čelina gold deposits,Bohemian Massif,Czech Republic: Role of fluid overpressure

The studied Mokrsko-West (90–100 t Au), Mokrsko-East (30 t Au) and Čelina (11 t Au) deposits represent three spatially and genetically interrelated deposits of supposed affiliation to the intrusion-related gold deposit type. The deposits differ in their dominant host rocks, which are represented by ca 354 Ma old biotite tonalite (Mokrsko-West) and Neoproterozoic volcanic and volcanosedimentary rocks (Mokrsko-East, Čelina). Another difference lies in the style of veining — densely spaced networks of 0.1–5 mm thin veins (Q₂) within the tonalite, compared to thick (usually 5–20 cm; Q_1–2) and widely spaced veins within the Neoproterozoic rocks.Five generations of quartz veins, referred to as Q₀ through Q₄ were distinguished: Q₀ veins are the oldest and ore-barren, Q₁ veins mark the onset of the Au-ore formation, Q₂ veins its culmination and Q₃ veins its fading. Late quartz gangue (Q₄) is associated with uneconomic Ag–Pb–Zn vein-type ores hosted by calcite–barite–(quartz) veins.Quartz vein thickness (~ 0.3 to ~ 300 mm), spacing (~ 3 mm to ~ 500 mm), distribution, and related extensional strain (ca. 3–25%) evolve systematically across the studied ore district, reflecting both the major host rock and other tectonic factors. Detailed study of vein dimension parameters (thickness, length, width, aspect ratios) allowed estimation of the probable depth of the fluid source reservoir (~ 2 km or ~ 4 km) below the present surface. The depth to the fluid source seems to increase through time, being the shallowest for the Q₀ veins and the deepest for the Q₂ veins. Two independent methods of estimating fluid overpressure are discussed in the paper. Fluid overpressure during vein formation decreases from the Q₀ through the Q₂ veins, from 10 to 4 MPa or from 26 to 10 MPa, depending on the assumed tensile strength of the tonalite (5.5 and 15 MPa, respectively).The origin of joints and veins is discussed in terms of the stress orientation and crack-seal and crack-jump mechanisms. Field relationships unambiguously indicate that the veins hosted by Neoproterozoic rocks originated by reopening of the pre-existing extension joints (J₁) due to fluid overpressure. The origin of the densely-spaced thin veins (Q₂) hosted by the tonalite at the Mokrsko-West deposit is, however, less certain. It is probable that the tonalite was already affected by microfracturing analogous to the J₁ joints prior to the formation of quartz veins.The formation of the Q_1–2 veins at the Mokrsko-East deposit was constrained by the Re–Os dating of molybdenite to 342.9 ± 1.4 Ma. The ore-bearing hydrothermal system is thus ca 12 Ma younger than the tonalite that hosts the Mokrsko-West deposit. A similar ca 15–2 Ma difference between the age of the host-intrusion and the age of the hydrothermal event was encountered in several other gold deposits in the vicinity of the Central Bohemian Plutonic Complex. Two hypotheses to explain this are discussed in the paper.     

Isotopic analysis of the super-large Shuangjianzishan Pb–Zn–Ag deposit in Inner Mongolia,China: Constraints on magmatism,metallogenesis, and tectonic setting

The super-large Shuangjianzishan Pb–Zn–Ag deposit is a newly discovered deposit located in the Huanggang–Ganzhuermiao polymetallic metallogenic belt of Inner Mongolia, NE China. The deposit's resource includes 0.026 Mt Ag, 1.1 Mt Pb, and 3.3 Mt Zn. The deposit is controlled by a NW-trending ductile shear zone and NE- and NW-trending faults in black pelite assigned to the lower Permian Dashizhai Formation. LREE enrichment, HREE depletion, Nb, Ta, P, and Ti depletion, and Zr and Hf enrichment characterize felsic magmatic rocks in the Shuangjianzishan Pb–Zn–Ag district. The ages of porphyritic monzogranite, rhyolitic crystal–vitric ignimbrite, and porphyritic granodiorite are 254–252, 169, and 130 Ma, respectively. Pyrite sampled from the mineralization has Re–Os isochron ages of 165 ± 7 Ma, which suggest the mineralization is associated with the ca. 169 Ma magmatism in the Shuangjianzishan district.Zircons extracted from the porphyritic granodiorite yield ε_Hf(t) values of − 11.34 to − 1.41, with t_DM2 dates of 1275–1901 Ma. The ε_Hf(t) values of zircons in the rhyolitic crystal–vitric ignimbrite and the ore-bearing monzogranite porphyry are 7.57–16.23 and 10.18–15.96, respectively, and their t_DM2 ages are 177–733 and 257–632 Ma, respectively. Partial melting of depleted mantle resulted in the formation of the ca. 254–252 Ma ore-bearing porphyritic monzogranite and the ca. 169 Ma rhyolitic crystal–vitric ignimbrite; dehydration partial melting of subducted oceanic crust resulted in the formation of the ca. 130 Ma porphyritic granodiorite. The porphyritic monzogranite was emplaced during the late stages of closure of the Paleo-Asian Ocean during the transformation from a collisional to extensional tectonic setting. The ca. 170 and ca. 130 Ma magmatism and mineralization in the Shuangjianzishan district are related to subduction of the Mongolia–Okhotsk Ocean and subduction of the Paleo-Pacific Ocean Plate, respectively.     

Fluid inclusion geochemistry and 40Ar/39Ar geochronology constraints on the genesis of the Jianchaling Au deposit,China

The giant Jianchaling gold deposit is located in the Shaanxi Province, China. The mineralization is hosted by WNW-trending faults in the Mianxian-Lueyang-Yangpingguan (MLY) area. The mineralization can be divided into three stages based on mineralogical assemblages and crosscutting relationships of mineralized quartz veins. These stages, from early to late, are characterized by the mineral assemblage of: (1) quartz – coarse-grained pyrite – pyrrhotite – pentlandite – dolomite; (2) quartz – pyrite – gold – sphalerite – galena – carbonate – arsenopyrite – fuchsite; and (3) dolomite – calcite – quartz – fine-grained pyrite – realgar – orpiment.Three types of fluid inclusions have been recognized in this study based on petrographic and microthermometric measurements, including pure CO₂ and/or CH₄ (PC-type), NaCl-H₂O (W-type), and NaCl-CO₂-H₂O (C-type) fluid inclusions. These fluid inclusion types are present in quartz from the Stage 1 and 2 assemblages, whereas the Stage 3 quartz only contains W-type fluid inclusions. The Stage 2 assemblage is associated with the mineralization at the Jianchaling deposit. Fluid inclusions of Stage 1 quartz homogenize mainly between 250° and 360 °C, with salinities up to 15.6 wt.% NaCl equiv., whereas the Stage 3 dolomite with homogenization temperatures of 160° – 220 °C and salinities of 1.1–7.4 wt.% NaCl equiv. This indicates that the ore fluid system evolved from CO₂-rich, probably metamorphic hydrothermal to CO₂-poor, meteoric fluid. All three types of fluid inclusions can be observed in the Stage 2 quartz, suggesting that this heterogeneous association was trapped from a boiling fluid system. These inclusions homogenized at temperatures of 200°–250 °C and salinities of 1.2–12.4 wt.% NaCl equiv. The estimated trapping pressures of the fluid inclusions are between 117 and 354 MPa in Stage 1, suggesting an alternating lithostatic–hydrostatic fluid system, which was controlled by a fault-valve at the depth of ~ 12 km.Two fuchsite samples collected from the Stage 2 polymetallic-quartz veins yielded well-defined ⁴⁰Ar/³⁹Ar isotopic plateau ages of 197 ± 2 and 194 ± 2 Ma, and ³⁹Ar/³⁶Ar-⁴⁰Ar/³⁶Ar normal isochrones of 198 ± 2 and 199 ± 2 Ma. This indicates that the mineralization at Jianchaling is Early Jurassic (ca. 198 Ma) in age. We propose that Jianchaling is an orogenic gold deposit, and formed during continental collision related to the northward subduction of the Mian-Lue oceanic plate during the Early Jurassic. We also conclude that the beginning of the continental collision between the Yangtze and the North China Cratons took place around 200 Ma.     

Paleogene magmatism and gold metallogeny of the Jinping terrane in the Ailaoshan ore belt,Sanjiang Tethyan Orogen (SW China): Geology,deposit type and tectonic setting

The Jinping terrane is situated in the southern segment of the Ailaoshan ore belt, Sanjiang Tethyan Orogen (SW China). The Paleogene intrusions in Jinping consist of syenite porphyry, fine-grained syenite and biotite granite stocks/dikes, and contain relatively low TiO₂ (0.21–0.38 wt%), P₂O₅ (0.01–0.35 wt%), and high Na₂O (2.00–4.62 wt%) and K₂O (4.48–7.06 wt%), belonging to high-K alkaline series. Paleogene gold mineralization in Jinping comprises four genetic types, i.e., orogenic, alkali-rich intrusion-related, porphyry and supergene laterite. The NW–NNW-trending faults and their subsidiaries are the major ore-controlling structures. The orogenic Au mineralization, dominated by polymetallic sulfide-quartz veins, occurs in the diorite and minor in Silurian-Devonian sedimentary rocks. It contains a CO₂-rich mesothermal fluid system generated from the mixing of mantle-derived fluids with crustal-derived metamorphic fluids, and the ore-forming materials were upper crustal- or orogenic-derived. The alkali-rich intrusion-related Au mineralization is hosted in the Ordovician-Silurian sedimentary rocks and minor in the Paleogene alkaline intrusions, and the Au orebodies occur predominantly in the alteration halos. It contains a CO₂-bearing, largely metamorphic-sourced mesothermal fluid system, and the ore-forming materials were derived from the ore-hosting rocks and minor from the alkali-rich intrusions. The porphyry Cu-Mo-Au mineralization occurs in the granite/syenite porphyries and/or along their contact skarn, with the mineralizing fluids being magmatic-hydrothermal in origin. The former two hypogene Au mineralization types in Jinping were mainly formed in the late Eocene (ca. 34–33 Ma) and slightly after the porphyry Cu-Mo-Au mineralization (ca. 35–34 Ma), which is coeval with the regional Himalayan orogenic event. Subsequent weathering produced the laterite Au mineralization above or near the hypogene Au orebodies.     

Systematic changes in metamorphic styles along the Dabie–Hongseong and Himalayan collision belts,and their tectonic implications

Different continental collision belts show contrasting metamorphic trend along their length, including the distribution of extreme metamorphism; i.e., ultrahigh-pressure (>100 km depth) and ultrahigh-temperature (900–1150 °C) metamorphisms. However, no previous study has succeeded in explaining these trends. The present study investigates the main factors that control the metamorphic trends along collision belts, with reference to the Dabie–Hongseong collision belt between the North and South China blocks and the Himalayan collision belt between the Indian and Asian blocks. In the Dabie–Hongseong collision belt, collision began in the east before 245 Ma and propagated westward until ca. 220 Ma. In the eastern part of the belt, the amount of oceanic slab that subducted before collision was insufficient to pull down the continental crust to the depths of ultrahigh-pressure metamorphism; however, ultrahigh-pressure metamorphism occurred in the western part of the belt. Slab break-off also migrated from east to west, with a westward increase in the depth of break-off (from ca. 10 kbar in the west to ca. 35 kbar in the east). These lateral trends along the belt resulted in a westward change from ultrahigh-temperature (915–1160 °C, 9.0–10.6 kbar) to high-pressure (835–860 °C, 17.0–20.9 kbar) and finally ultrahigh-pressure metamorphism (680–880 °C, 30–40 kbar). In the Himalayan collision belt, collision started from the west at 50 Ma and propagated eastward. The amount of oceanic slab subducted prior to collision was sufficient to pull down the continental crust to the depths of ultrahigh-pressure metamorphism in the west, but not in the east. Slab break-off started in the west at ca. 46 Ma and propagated eastward, with an eastward decrease in the depth of slab break-off from 27–29 to 17–18 kbar. Consequently, the metamorphic trend along the belt changes eastward from ultrahigh-pressure (690–750 °C, 27–29 kbar) to high-pressure and finally high-pressure granulite facies metamorphism (890 °C, 17–18 kbar). The differences in metamorphic trend between the Dabie–Hongseong and Himalayan collision belts reflect the amount of oceanic crust subducted prior to collision and the depth and timing of slab break-off along each belt.     

U-Pb and Ar/Ar geochronological constraints on the exhumation history of the North Qinling terrane, China

The amphibolite facies grade North Qinling metamorphic unit forms the centre of the Qinling orogenic belt. Results of LA-ICP-MS U-Pb zircon, ⁴⁰Ar/³⁹Ar amphibole and biotite dating reveal its Palaeozoic tectonic history. U-Pb zircon dating of migmatitic orthogneiss and granite dykes constrains the age of two possible stages of migmatization at 517 ± 14 Ma and 445 ± 4.6 Ma. A subsequent granite intrusion occurred at 417 ± 1.6 Ma. The ⁴⁰Ar/³⁹Ar plateau ages of amphibole ranging from 397 ± 33 Ma to 432 ± 3.4 Ma constrain the cooling of the Qinling complex below ca. 540 °C and biotite ⁴⁰Ar/³⁹Ar ages at about 330–368 Ma below ca. 300 °C. The ages are used to construct a cooling history with slow/non-exhumation during 517– 445 Ma, a time-integrated cooling at a rate < 2.5 °C/Ma during the period of 445–410 Ma, an acceleration of cooling at a rate of 8 °C/Ma from 397 Ma to 368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma. The data show a significant delay in exhumation after peak metamorphic conditions and a long period of tectonic quiescence after the suturing of the North China and South China blocks along the Shangdan suture. These relationships exclude classical exhumation models of formation and exhumation of metamorphic cores in orogens, which all imply rapid cooling after peak conditions of metamorphism.     

Zircon U–Pb geochronology of the Mesozoic metamorphic rocks and granitoids in the coastal tectonic zone of SE China: Constraints on the timing of Late Mesozoic orogeny

The coastal Changle-Nan’ao tectonic zone of SE China contains important geological records of the Late Mesozoic orogeny and post-orogenic extension in this part of the Asian continent. The folded and metamorphosed T₃–J₁ sedimentary rocks are unconformably overlain by Early Cretaceous volcanic rocks or occur as amphibolite facies enclaves in late Jurassic to early Cretaceous gneissic granites. Moreover, all the metamorphic and/or deformed rocks are intruded by Cretaceous fine-grained granitic plutons or dykes. In order to understand the orogenic development, we undertook a comprehensive zircon U–Pb geochronology on a variety of rock types, including paragneiss, migmatitic gneiss, gneissic granite, leucogranite, and fine-grained granitoids. Zircon U–Pb dating on gneissic granites, migmatitic gneisses, and leucogranite dyke yielded a similar age range of 147–135 Ma. Meanwhile, protoliths of some gneissic granites and migmatitic gneisses are found to be late Jurassic magmatic rocks (ca. 165–150 Ma). The little deformed and unmetamorphosed Cretaceous plutons or dykes were dated at 132–117 Ma. These new age data indicate that the orogeny lasted from late Jurassic (ca. 165 Ma) to early Cretaceous (ca. 135 Ma). The tectonic transition from the syn-kinematic magmatism and migmatization (147–136 Ma) to the post-kinematic plutonism (132–117 Ma) occurred at 136–132 Ma.     

Migration of brines in the basement rocks of the Athabasca Basin through microfracture networks (P-Patch U deposit,Canada)

The unconformity-type uranium deposits of the Athabasca Basin (Saskatchewan, Canada) are hosted near the unconformity between a middle Proterozoic intracratonic sedimentary basin and an Archean to Paleo-Proterozoic metamorphic and plutonic basement. These deposits, which are considered to be the richest U deposits in the world, are the result of massive basinal fluid migrations in the basement rocks.This study shows that basinal brines have strongly penetrated into the basement not only through faults and major pathways but also by way of dense networks of microfractures which favoured the percolation of fluids down to considerable depths (hundred metres below the unconformity) and their chemical modification (salinity increase) by interaction with basement lithologies. These processes are one of the major causes of uranium mobility within the basement rocks and the formation of unconformity-type mineralization.Microfracture networks, which opened during the basinal brine stage (ca. 1600–1400 Ma) are interpreted as sets of mode I cracks corresponding to a specific stage of deformation and occur as fluid inclusion planes after healing. The stress field at that stage (σ₁ = N130–150 °E, subvertical) partly reopened the earlier microcrack networks (σ₁ = N80–110 °E and N130–150 °E, subvertical) issued from the Trans-Hudson Orogeny late retrograde metamorphic stage (ca. 1795–1720 Ma). The circulation of the two types of fluids (carbonic and brines) occurs thus at two distinct events (Trans-Hudson Orogeny late retrograde metamorphism for carbonic fluids and maximal burial diagenesis for brines) but the same main microfissure geometry was used by the fluids. This demonstrates the existence of a similar stress field direction acting before and after the basin formation. Moreover, the brine circulations in the basement acted in a wider volume than the clay-rich alteration halo surrounding the U-ores, generally considered as the main envelope of fluid percolation outside the fault systems. The data on the chemistry of the fluids and on the geometry of their migration at various scales emphasise the fundamental role of the basement in the chemical evolution of highly saline brines linked to unconformity-related uranium mineralization in the Athabasca Basin.     

Zircon geochemistry of two contrasting types of eclogite: Implications for the tectonic evolution of the North Qaidam UHPM belt,northern Tibet

Compared to the extensively documented ultrahigh-pressure metamorphism at North Qaidam, the pre-metamorphic history for both continental crust and oceanic crust is poorly constrained. Trace element compositions, U–Pb ages, O and Lu–Hf isotopes obtained for distinct zircon domains from eclogites metamorphosed from both continental and oceanic mafic rocks are linked to unravel the origin and multi-stage magmatic/metamorphic evolution of eclogites from the North Qaidam ultrahigh-pressure metamorphic (UHPM) belt, northern Tibet.For continental crust-derived eclogite, magmatic zircon cores from two samples with U–Pb ages of 875–856 Ma have both very high δ¹⁸O (10.6 ± 0.5‰) and mantle-like δ¹⁸O (averaging at 5.2 ± 0.7‰), high Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, and steep MREE-HREE distribution patterns (chondrite-normalized) with negative Eu anomalies. Combined with positive ε_Hf (t) of 3.9–14.3 and T_DM (1.2–0.8 Ga and 1.3–1.0 Ga, respectively), they are interpreted as being crystallized from either subduction-related mantle wedge or recycled material in the mantle. While the metamorphic rims from the eclogites have U–Pb ages of 436–431 Ma, varying (inherited, lower, and elevated) oxygen isotopes compared with cores, low Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, and flat HREE distribution patterns with no Eu anomalies. These reflect both solid-state recrystallization from the inherited zircon and precipitation from external fluids at metamorphic temperatures of 595–622 °C (T_Ti-in-zircon).For oceanic crust-derived eclogite, the magmatic cores (510 ± 19 Ma) and metamorphic rims (442.0 ± 3.7 Ma) also show distinction for Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, and the REE patterns and Eu anomalies. Combined with the mantle-like δ¹⁸O signature of 5.1 ± 0.3 ‰ and two groups of model age (younger T_DM close to the apparent ages and older > 700 Ma), two possible pools, juvenile and inherited, were involved in mixing of mantle-derived magma with crustal components. The relatively high δ¹⁸O of 6.6 ± 0.3‰ for metamorphic zircon rims suggests either the protolith underwent hydrothermal alteration prior to the ~ 440 Ma oceanic crust subduction, or external higher δ¹⁸O fluid activities during UHP metamorphism at ~ 440 Ma.Therefore, the North Qaidam UHPM belt witnesses multiple tectonic evolution from Late Mesoproterozoic–Neoproterozoic assembly/breakup of the Rodinia supercontinent with related magmatic emplacement, then Paleozoic oceanic subduction, and finally transition of continental subduction/collision related to UHP metamorphism.     

Basin architecture and evolution in the Mount Isa mineral province,northern Australia: Constraints from deep seismic reflection profiling and implications for ore genesis

Deep seismic reflection profiling confirms that the Paleo- to Mesoproterozoic Mount Isa mineral province comprises three vertically stacked and partially inverted sedimentary basins preserving a record of intracontinental rifting followed by passive margin formation. Passive margin conditions were established no later than 1655 Ma before being interrupted by plate convergence, crustal shortening and basin-wide inversion at 1640 Ma in both the 1730–1640 Ma Calvert and 1790–1740 Ma Leichhardt superbasins. Crustal extension and thinning resumed after 1640 Ma with formation of the 1635–1575 Ma Isa Superbasin and continued up to ca. 1615 Ma when extensional faulting ceased and a further episode of basin inversion commenced. The 1575 Ma Century Pb–Zn ore-body is hosted by syn-inversion sediments deposited during the initial stages of the Isan Orogeny with basin inversion accommodated on east- or northeast-dipping reactivated intrabasinal extensional faults and footwall shortcut thrusts. These structures extend to considerable depths and served as fluid conduits during basin inversion, tapping thick syn-rift sequences of immature siliciclastic sediments floored by bimodal volcanic sequences from which the bulk of metals and mineralising fluids are thought to have been sourced. Basin inversion and fluid expulsion at this stage were entirely submarine consistent with a syn-sedimentary to early diagenetic origin for Pb–Zn mineralisation at, or close to, the seafloor. Farther east, a change from platform carbonates to deeper water continental slope deposits (Kuridala and Soldiers Cap groups) marks the position of the original shelf break along which the north–south-striking Selwyn-Mount Dore structural corridor developed. This corridor served as a locus for strain partitioning, fluid flow and iron oxide–copper–gold mineralisation during and subsequent to the onset of basin inversion and peak metamorphism in the Isan Orogeny at 1585 Ma. An episode of post-orogenic strike-slip faulting and hydrothermal alteration associated with the subvertical Cloncurry Fault Zone overprints west- to southwest-dipping shear zones that extend beneath the Cannington Pb–Zn deposit and are antithetic to inverted extensional faults farther west in the same sub-basin. Successive episodes of basin inversion and mineralisation were driven by changes in the external stress field and related plate tectonic environment as evidenced by a corresponding match to bends in the polar wander path for northern Australia. An analogous passive margin setting has been described for Pb–Zn mineralisation in the Paleozoic Selwyn Basin of western Canada.     

Geology,fluid inclusion geochemistry,and 40Ar/39Ar geochronology of the Wulasigou Cu deposit,and their implications for ore genesis,Altay, Xinjiang,China

The Wulasigou Cu deposit occurs as veins controlled by a NW-trending structure in a Devonian volcano-sedimentary basin of the Altay orogenic belt, Xinjiang, China. Igneous and sedimentary rocks exposed in the area have undergone greenschist-facies metamorphism. The ore-forming process can be divided into early, middle, and late stages, represented by, respectively, pyrite-quartz, polymetallic sulfide-quartz, and carbonate–quartz veins, veinlets, and/or replacement bodies. The early veins were deformed and brecciated during a compressional or transpressional event. The middle-stage veinlets filled fractures in the early-stage vein and alteration assemblages, and are undeformed, suggesting a tensional shear setting. The late-stage veinlets are mainly open-space fissure fillings that cut veins and replacement bodies formed in the earlier stages.Four types of fluid inclusions (FIs), including aqueous (W-type), mixed carbonic-aqueous (M-type), purely carbonic (C-type) and daughter mineral-bearing (S-type), have been identified in copper-related quartz and calcite from the Wulasigou deposit. The early-stage quartz contains M- and W-type primary FIs that completely homogenized at temperatures of 322–412 °C with low salinities of 0.9–6.5 wt.% NaCl equiv. In contrast, the late-stage quartz or calcite contains only the W-type FIs with homogenization temperatures of 101–234 °C, and salinities of 0.9–2.9 wt.% NaCl equiv. This indicates that the metallogenic system evolved from CO₂-rich, metamorphic to CO₂-poor, through input of meteoric fluids. All four types of FIs can only be observed in the middle-stage minerals, where they show evidence of vein formation during an episode of fluid immiscibility. These FIs homogenized at temperatures ranging mainly from 230 to 347 °C, with salinities clustering 2.7–10.2 wt.% NaCl equiv for the W-, M- and C-types, and 34.7–38.2 wt.% NaCl equiv for the S-type, respectively. The metal precipitation resulted from a decrease in copper solubility during the fluid immiscibility episode. The estimated trapping pressures for the middle-stage fluids are 1.55–3.55 kbar, suggesting an alternating lithostatic-hydrostatic fluid-system, controlled by fault-valve activity at a depth of 13–15.5 km.Muscovite separates from the middle-stage polymetallic-quartz veinlets yield a well-defined ⁴⁰Ar/³⁹Ar isotopic plateau age of 219.41 ± 2.10 Ma, and an ³⁹Ar/³⁶Ar - ⁴⁰Ar/³⁶Ar isochron age of 219.73 ± 2.17 Ma. This age postdates the final Paleo-Asia Ocean closure (at ca. 250 Ma) by about 30 Ma, and indicates that the Cu mineralization at Wulasigou has occurred in the Triassic continental collision setting. Hence, the Wulasigou Cu deposit may be the first example of orogenic lode Cu deposits formed in accretionary orogeny or continental collision.     

Multi-stage metamorphic evolution of retrograde eclogite with a granulite-facies overprint in the Zhaigen area of the North Qinling Belt,China

Retrograde eclogite from the central part of the Qinling Complex, Zhaigen area of the North Qinling Belt, was studied using integrated petrology, mineral chemistry, pseudosection modeling, and geochronology. Microstructures and mineral relationships reveal five metamorphic stages and associated mineral assemblages as follows: (1) pre-peak stage M₁, which is recorded by the inner cores of garnets together with mineral inclusions of clinopyroxene (Cpx₁) + amphibole (Am₁) + plagioclase (Pl₁) ± quartz ± rutile, occurred under conditions of 760–770 °C and 11.4–14.0 kbar; (2) eclogite-facies stage M₂, recorded by garnet cores + relic omphacite (with a high jadeite content up to 31%) + rutile + quartz under conditions of > 16.7 kbar and 679–765 °C; (3) high-pressure granulite-facies stage M₃, characterized by clinopyroxene (Cpx₂) + plagioclase (Pl₂) symplectites under conditions of 14.5–15.6 kbar and 800–850 °C; (4) medium-pressure granulite-facies stage M₄, characterized by the growth of plagioclase + orthopyroxene coronas around garnet under conditions of 8.3–10 kbar and 795–855 °C; and (5) retrogressive amphibolite-facies stage M₅, which is represented by amphibole (Am₃) + plagioclase (Pl₃) kelyphitic rims around garnet at conditions of < 4 kbar and < 620 °C. Based on Laser Raman analysis of mineral inclusions, cathodoluminescence images, in situ trace element concentrations from different domains within zircon grains, and LA-ICP-MS and SHRIMP U–Pb dating, the protolith age of the Zhaigen retrograde eclogite is suggested at 786 ± 10 Ma and the eclogite-facies metamorphic age recorded by metamorphic zircon cores is limited within 501–497 Ma. The retrograde zircon rims display ages of 476–447 Ma and 425 Ma that probably reflect the timing of two stages of retrograde metamorphism, respectively. The mineral assemblages, P–T conditions, and zircon U–Pb data define a clockwise P–T–t path for the retrograde eclogite, suggesting that the Neoproterozoic protolith of the retrograde eclogite might evolved into continental subduction and eclogite-facies metamorphism during 501–497 Ma before undergoing retrograde metamorphism during an initial stage of exhumation to middle–upper crust level at 474–447 Ma and subsequent exhumation to shallow upper crust at ~ 420 Ma.     

Rapid cooling of the Yanshan Belt,northern China: Constraints from 40Ar/39Ar thermochronology and implications for cratonic lithospheric thinning

The Yanshan Orogenic Belt is located in the northern part of the North China Craton (NCC), which lost ∼120 km of lithospheric mantle during Phanerozoic tectonic reactivation. Mesozoic magmatism in the Yanshan fold-and-thrust belt began at 195–185 Ma (Early Jurassic), with most of the granitic plutons being Cretaceous in age (138–113 Ma). Along with this magmatism, multi-phase deformational structures, including multiple generations of folds, thrust and reverse faults, extensional faults, and strike-slip faults are present in this belt. Previous investigations have mostly focused on geochemical and isotopic studies of these magmatic rocks, but not on the thermal history of the Mesozoic plutons. We have applied ⁴⁰Ar/³⁹Ar thermochronology to biotites and K-feldspars from several Lower Cretaceous granitic plutons to decipher the cooling and uplift history of the Yanshan region. The biotite ⁴⁰Ar/³⁹Ar ages of these plutons range from 107 to 123 Ma, indicating that they cooled through about 350 °C at that time. All the K-feldspar step-heating results modeled using multiple diffusion domain theory yield similarly rapid cooling trends, although beginning at different times. Two rapid cooling phases have been identified at ca. 120–105 and 100–90 Ma. The first phase of rapid cooling occurred synchronously with widespread extensional deformation characterized by the formation of metamorphic core complexes, A-type magmatism, large-scale normal faults, and the development of half-graben basins. This suggests rapid exhumation took place in an extensional regime and was a shallow-crustal-level response to lithospheric thinning of the NCC. The second phase of rapid cooling was probably related to the regional uplift and unroofing of the Yanshan Belt, which is consistent with the lack of Upper Cretaceous sediments in most of the Yanshan region.     

Metamorphic evolution of the Maud Belt: P–T–t path for high-grade gneisses in Gjelsvikfjella,Dronning Maud Land,East Antarctica

A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P–T–t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M₁) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P–T–t path for the Pan-African orogeny. Peak metamorphic (M_2b) conditions recorded by most rocks in the area (T = 709–785 °C and P = 7.0–9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M_2a).The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent–continent collision at the end of the Mesoproterozoic (M₁; 1090–1030 Ma) and again during the Pan-African orogeny (M_2a, M_2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M_2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma.     

Young ores in old rocks: Proterozoic iron mineralisation in Mesoarchean banded iron formation,northern Pilbara Craton,Australia

The origin of bedded iron-ore deposits developed in greenstone belt-hosted (Algoma-type) banded iron formations of the Archean Pilbara Craton has largely been overlooked during the last three decades. Two of the key problems in studying these deposits are a lack of information about the structural and stratigraphic setting of the ore bodies and an absence of geochronological data from the ores. In this paper, we present geological maps for nearly a dozen former mines in the Shay Gap and Goldsworthy belts on the northeastern margin of the craton, and the first U-Pb geochronology for xenotime intergrown with hematite ore. Iron-ore mineralisation in the studied deposits is controlled by a combination of steeply dipping NE- and SE-trending faults and associated dolerite dykes. Simultaneous dextral oblique-slip movement on SE-trending faults and sinistral normal oblique-slip movement on NE-trending faults during initial ore formation are probably related to E–W extension. Uranium–lead dating of xenotime from the ores using the sensitive high-resolution ion microprobe (SHRIMP) suggests that iron mineralisation was the cumulative result of several Proterozoic hydrothermal events: the first at c. 2250 Ma, followed by others at c. 2180 Ma, c. 1670 Ma and c. 1000 Ma. The cause of the first growth event is not clear but the other age peaks coincide with well-documented episodes of orogenic activity at 2210–2145 Ma, 1680–1620 Ma and 1030–950 Ma along the southern margin of the Pilbara Craton and the Capricorn Orogen farther south. These results suggest that high-grade hematite deposits are a product of protracted episodic reactivation of a structural architecture that developed during the Mesoarchean. The development of hematite mineralisation along major structures in Mesoarchean BIFs after 2250 Ma implies that fluid infiltration and oxidative alteration commenced within 100 myr of the start of the Great Oxidation Event at c. 2350 Ma.     

Zircon ages and Nd–Hf isotopes in UHT granulites of the Ider Complex: A cratonic terrane within the Central Asian Orogenic Belt in NW Mongolia

The Ider Complex of the Tarbagatai Block in northwestern Mongolia is part of a Precambrian microcontinental terrane in the Central Asian Orogen Belt and has experienced a polymetamorphic tectono-metamorphic evolution. We have investigated an enderbitic gneiss, derived from a quartz diorite and a charnockite, derived from a leucogranite, and zircon SHRIMP data reveal late Archaean protolith ages of 2520–2546 Ma for these rocks. Metamorphic overgrowth on these zircons as well as newly-formed metamorphic zircons document a high-temperature metamorphic event (T = 930–950 °C) at about 1855–1860 Ma. Nd whole-rock isotopic systematics show these and other gneisses of the Ider Complex straddling the CHUR-line in a Nd isotope evolution diagram, suggesting both crustal reworking and input of some juvenile material, with Nd model ages ranging between ca. 2.5 and 3.1 Ga. Hf-in-zircon isotopic data provide a similar pattern and also yielded Archaean Hf crustal model ages. The metamorphic zircons seem to have inherited their Hf isotopic composition from the igneous grains, suggesting a complex process of dissolution, transportation, and re-precipitation involving a fluid phase during high-grade metamorphism. The zircon age patterns do not make it possible to unambiguously assign the Tarbagatai Block to any of the cratons bordering the Central Asian Orogenic Belt, since age peaks at ca. 2520–2550 and ca. 1860 Ma are common in the Siberian, North China and Tarim cratons.