The development of volcanic hosted massive sulfide and barite–gold orebodies on Wetar Island, Indonesia

Wetar Island is composed of Neogene volcanic rocks and minor oceanic sediments and forms part of the Inner Banda Arc. The island preserves precious metal-rich volcanogenic massive sulfide and barite deposits, which produced approximately 17 metric tonnes of gold. The polymetallic massive sulfides are dominantly pyrite (locally arsenian), with minor chalcopyrite which are cut by late fractures infilled with covellite, chalcocite, tennantite–tetrahedrite, enargite, bornite and Fe-poor sphalerite. Barite orebodies are developed on the flanks and locally overly the massive sulfides. These orebodies comprise friable barite and minor sulfides, cemented by a series of complex arsenates, oxides, hydroxides and sulfate, with gold present as <10 m free grains. Linear and pipe-like structures comprising barite and iron-oxides beneath the barite deposits are interpreted as feeder structures to the barite mineralization. Hydrothermal alteration around the orebodies is zoned and dominated by illite–kaolinite–smectite assemblages; however, local alunite and pyrophyllite are indicative of late acidic, oxidizing hydrothermal fluids proximal to mineralization. Altered footwall volcanic rocks give an illite K–Ar age of 4.7±0.16 Ma and a ⁴⁰Ar/³⁹Ar age of 4.93±0.21 Ma. Fluid inclusion data suggest that hydrothermal fluid temperatures were around 250–270°C, showed no evidence of boiling, with a mean salinity of 3.2 wt% equivalent NaCl. The ³⁴S composition of sulfides ranges between +3.3 and +11.7 and suggests a significant contribution of sulfur from the underlying volcanic edifice. The ³⁴S barite data vary between +22.4 and +31.0, close to Miocene seawater sulfate. Whole rock ⁸⁷Sr/⁸⁶Sr analyses of unaltered volcanic rocks (0.70748–0.71106) reflect contributions from subducted continental material in their source region. The ⁸⁷Sr/⁸⁶Sr barite data (0.7076–0.7088) indicate a dominant Miocene seawater component to the hydrothermal system. The mineral deposits formed on the flanks of a volcanic edifice at depths of ~2 km. Spectacular sulfide mounds showing talus textures are localized onto faults, which provided the main pathways for high-temperature hydrothermal fluids and the development of associated stockworks. The orebodies were covered and preserved by post-mineralization chert, gypsum, Globigerina-bearing limestone, lahars, subaqueous debris flows and pyroclastics rocks.     

U–Th–Pb monazite dating and the timing of arc–continent collision in East Timor

The Timor Orogen represents one of the youngest arc–continent collisions exposed on the Earth. It has the potential to provide some of the key parameters about how this style of orogeny evolves. However, the metamorphic age of the highest-grade rocks formed in the collision remains controversial. Using U–Th–Pb dating of monazite from amphibolite-grade sillimanite and garnet-bearing schists we show the peak metamorphism occurred at 5.5–4.7 Ma. The young age of the monazite and the presence of significant amounts of common Pb required the development of a new protocol to simultaneously account for the ²³⁰Th disequilibrium and the ²⁰⁷Pb common Pb correction. The new estimate of metamorphic age is consistent with the estimates based on plate reconstructions for the initiation of arc–continent collision in East Timor. The metamorphic event is a result of this collision.     

Temporal and spatial controls on the formation of magmatic PGE and Ni–Cu deposits

Magmatic PGE and Ni–Cu deposits form in contrasting geologic environments and periods. PGE deposits predominantly occur in large layered intrusions emplaced during the late Archean and early Proterozoic into stabilized, relatively S-poor cratonic lithosphere that provides enhanced preservation potential. The magmas ascend through intracratonic sutures where extension and rifting is limited. Crystallization under conditions of low regional stress, with limited magma-induced sagging due to underlying thick buoyant sub-continental mantle lithosphere, is consistent with their laterally continuous layering. Most of the global resources occur in three large intrusions: Bushveld, Great Dyke and Stillwater. Due to the large size (tens of kilometres) and limited complexity of the deposits, they are relatively easy to locate and delineate. As a result, the search space is relatively mature and few new discoveries have been made in the last few decades. The parental magmas to the intrusions are predominantly derived from the convecting mantle but, in addition, the involvement of the sub-continental lithospheric mantle is suggested by the relative Pt enrichment of most of the major deposits. In contrast to the PGE deposits, Ni–Cu deposits form throughout geologic time, but with the largest deposits being younger than ca. 2 Ga. The sulfide ores are concentrated under highly dynamic conditions within lava channels and magma conduits. The deposits are preferentially located near craton margins towards which mantle plumes have been channelled and where mantle magmas can readily ascend through abundant trans-lithospheric structures. Magma flow is focused and locally enhanced by shifting compressive–extensional tectonic regimes, and abundant S-rich crustal rocks provide an external S source that is required for the majority of deposits. The igneous bodies hosting the deposits tend to be irregular and small, tens to hundreds of metres in width and height, and are difficult to locate. As a result, the search space remains relatively immature. Understanding their tectonic setting helps reduce the prospective search space for world-class examples.     

The Cryogenian arc formation and successive high-K calc–alkaline plutons of Socotra Island (Yemen)

The Socotra Island belongs to the southern rifted margin of the Gulf of Aden and occupied in Neoproterozoic times a key position to constrain the age and the nature of the largely hidden Neoproterozoic rocks of the Arabian plate. Our integrated field, petrographic, geochemical and geochronological study in the Neoproterozoic rocks recognises three main successive events: (a) high-temperature ductile deformation and metamorphism forming probably in a compressive or transpressive regime; (b) mafic to intermediate intrusions as vertical sheets, kilometre-scale gabbro laccoliths, mafic dike swarm and lavas which present mainly a depleted arc signature with some evidences of evolution from an enriched-arc signature; (c) felsic intrusions mainly composed of highly potassic calc–alkaline and pinkish granites dated between 840 and 780 Ma. Relationships between the various petrographic types and U–Pb data suggest that these events occurred during a relatively short time span (80 Ma at max). Earlier high-temperature–low-pressure metamorphism stage as well as geochemical signature of mafic rocks show that development of Cryogenian formations of Socotra were controlled successively by an Andean-arc and a back-arc setting. These features cannot be easily reconciled with those of the Arabian–Nubian shield to the west of Socotra and of the Mozambique Belt to the south. We propose that the Socotra basement was developed at an active margin close to the India block in Cryogenian times.     

The hydrothermal system of Central Jebilet (Variscan Belt,Morocco): A genetic association between bimodal plutonism and massive sulphide deposits?

Central Jebilet (Moroccan Variscan Belt) hosts several Cu and Pb–Zn massive sulphide deposits that are associated with a suite of gabbroic and microgranitic, tholeiitic to alkaline, intrusions emplaced 330 Ma ago. The intrusions and ore bodies form structural lineaments within marine Visean shales that are affected by very low to low-grade post-Visean metamorphism and contemporaneous shortening accompanied by the development of conjugate ductile to brittle shear zones. The ductile shear zones are localised in thermally softened aureoles around magmatic intrusions, while brittle deformation is common far from the intrusions. The intrusions have induced a contact metamorphism that reaches the hornblende hornfels facies, and their emplacement was accompanied by hydrothermal activity that leached base metals from the felsic intrusions.The massive sulphide deposits consist of steeply dipping elongate lenses that are located in shear zones 1–1.5 km away from the intrusions. They are dominated by pyrrhotite (up to 90%), sphalerite, galena, chalcopyrite, pyrite and arsenopyrite forming a mylonitic texture. Their wall rocks are altered to syntectonic mineral assemblages similar to those found in the alteration zones associated with the magmatic intrusions. The massive sulphide deposits located near the felsic intrusions are rich in lead and zinc compared to those located near the mafic intrusions, which are copper deposits. These relationships indicate that the whole Central Jebilet hydrothermal system could be described in terms of a lateral secretion of base metals from source zones (i.e. bimodal intrusions) to discharge zones (i.e. the Jebilet sulphide deposits). The metapelites in the contact metamorphic zone around felsic intrusions contain zincian ilmenite that was probably related to interaction of the host rocks with chlorine-rich fluid carrying zinc and other metals leached from the microgranites.     

Arc–continent collision and orogenesis in western Tasmanides: Insights from reactivated basement structures and formation of an ocean–continent transform boundary off western Tasmania

Crustal architecture in formerly contiguous basement terranes in SE Australia, Tasmania and northern Victoria Land is a legacy of late Neoproterozoic–Cambrian subduction-related processes, culminating in formation of the Delamerian–Ross orogen. Structures of Delamerian–Ross age were subsequently reactivated during late Mesozoic–Cenozoic Gondwana breakup, strongly influencing the geometry of continental rifting and providing clues about the origins and configuration of the pre-existing basement structures. An ocean–continent transform boundary developed off western Tasmania follows the trace of an older Paleozoic strike-slip structure (Avoca–Sorell fault system) optimally oriented for reactivation during the final separation of Australia from Antarctica. This boundary cuts across rocks preserving an earlier record of arc–continent collision during the course of which continental crust was subducted to mantle depths and Cambrian mafic–ultramafic island arc rocks were thrust westwards over late Neoproterozoic–Cambrian passive margin sequences. Collision was accompanied by development of a foreland basin into which 520–600 Ma arc-derived detrital zircons were shed. Following a reversal in subduction polarity, and change to transcurrent motion along the Gondwana margin, Tasmania migrated northward along the proto-Avoca fault system before entering a subduction zone located along the Heathcote–Governor fault system, precipitating a second collision, south-vergent thrusting, and tectonic reworking of the already accreted Cambrian arc–forearc assemblages and underlying passive margin sequences.     

Juvenile granite in the Sanandaj–Sirjan Zone,NW Iran: Late Jurassic–Early Cretaceous arc–continent collision

The Sanandaj–Sirjan Zone (SSZ) of western Iran is characterized by numerous granitoids of mainly calc-alkaline affinities. Several leucogranite and monzonite bodies crop out in the eastern Sanandaj. Whole-rock Rb–Sr isochrons demonstrate that the Mobarak Abad monzonite (MAM) formed in two phases at 185 and 131 Ma. Low ⁸⁷Sr/⁸⁶Sr(i) (i represents initial) and high ¹⁴³Nd/¹⁴⁴Nd(i) ratios, resulting in positive ?^t _Nd, imply that the source magma originated from a depleted mantle; large ion lithophile element (LILE) and light rare earth element (LREE) enrichments imply that slab fluid was involved in the evolution of the parent magma. Geochemical characteristics of the MAM rocks show an affinity with I- and A-type granites, and the positive values of ?^t _Nd (+2 to +6), confirm that the MAM represents juvenile granite. Therefore, the MAM rocks are different from Himalayan, Hercynian, and Caledonian granites. Based on the geology of granitic host rocks that form the protoliths of metamorphic rocks, it is likely that the mafic part of the MAM formed in an island arc setting on Neo-Tethyan oceanic crust during Early to Middle Jurassic time. Subsequent collision of the island arc with the western part of the SSZ occurred in the Late Jurassic to Early Cretaceous. Metamorphism, accompanied by partial melting, occurred during collision. Finally, leucogranite magmas of the young Mobarak Abad dikes and the Suffi Abad body were generated in this collision zone. This new model suggests a Late Jurassic–Early Cretaceous arc–continental collision before final closing of the Neo-Tethys.     

Rapid development of negative Ce anomalies in surface waters and contrasting REE patterns in groundwaters associated with Zn–Pb massive sulphide deposits

Ground and surface waters collected from two undisturbed Zn–Pb massive sulphide deposits (the Halfmile Lake and Restigouche deposits) and active mines in the Bathurst Mining Camp (BMC), NB, Canada were analysed for the rare earth elements (REE). REE contents are highly variable in waters of the BMC, with higher contents typical of waters with higher Fe and lower pH. There are significant differences between ground- and surface waters and between groundwaters from different deposits. The REE contents of surface waters are broadly similar within and between deposit areas, although there are spatial variations reflecting differences in pH and redox conditions. Surface waters are characterised by strong negative Ce anomalies ([Ce/Ce*]_NASC as low as 0.08), produced by oxidation of Ce³⁺ to Ce⁴⁺ and preferential removal of Ce⁴⁺ from solution upon leaving the shallow groundwater environment. Groundwaters and seeps typically lack significant Ce anomalies reflecting generally more reducing conditions in the subsurface environment and indicating that Ce oxidation is a rapid process in the surface waters. Deeper groundwaters at the Halfmile Lake deposit are characterised by REE patterns that are similar to the host lithologies, whereas most groundwaters at the Restigouche deposit have LREE-depleted patterns compared to NASC. Halfmile Lake deposit groundwaters have generally lower pH values, whereas Restigouche deposit groundwaters show greater heavy REE-complexation by carbonate ions. Shallow waters at the Halfmile Lake and Stratmat Main Zone deposits have unusual patterns which reflect either the adsorption of light REE onto colloids and fracture-zone minerals and/or precipitation of REE–phosphate minerals. Middle REE-enrichment is typical for ground- and surface waters and is highest for neutral pH waters. The labile portion of stream sediments are generally more middle REE-enriched than total sediment and surface waters indicating that the REE are removed from solution by adsorption to Fe- and Mn-oxyhydroxides in the order middle REE≥light REE>heavy REE.     

Timing of juvenile arc crust formation and evolution in the Sapat Complex (Kohistan–Pakistan)

The combination of age determination and geochemical tracers allows understanding the source evolution during magmatism. We studied the Sapat Complex, in the exhumed Cretaceous Kohistan Paleo-Island Arc, to reconstruct the formation of the juvenile lower arc crust and the evolution of the mantle source during arc magmatism. High precision ID-TIMS U/Pb dating on zircon, shows that a protracted period of magmatic accretion formed the Sapat Complex between 105 and 99 Ma. Since continued melt percolation processes that formed the lower crust obscured the original bulk rock Nd–Pb–Sr isotopic composition, we rely on the Hf isotopic composition of zircons of different ages to unravel the source evolution. Nd and Pb bulk isotopic compositions coupled with Hf isotopic composition on zircons allow reconstructing a geodynamical scenario for the Sapat Complex, and the Cretaceous history of the Arc. We suggest that trenchward migration of the hot mantle source at 105 Ma explains the small heterogeneous εHf signal between + 14 and + 16. This heterogeneity vanished within ca. 2 million years, and the εHf of the source evolved from + 16 to + 14 at 99 Ma. Integrated to the Kohistan Cretaceous history, which has a baseline of εHf ≈ 14, these data pinpoint two geodynamical events, with slab retreat and the formation of the Sapat Complex followed by splitting of the Kohistan island arc at 85 Ma.     

Geochemical controls on the formation of unconformity-type Au–U deposits (Northern Transbaikalia)

This study provides geochemical, mineralogical, and isotope data for rocks and ores from Lower Proterozoic black shale formations of the Kodar–Udokan structural and formational zone, which host the Khadatkanda gold—uranium deposit. The results indicate that the uranium and gold mineralizations were formed at different times in relation to different geodynamic settings. The gold mineralization is associated with the inception of the Syulban fault and has a juvenile source. The later Th–U mineralization originated during tectonic rejuvenation of the Syulban fault zone, while the sources of radioactive elements were presumably the underlying sediments of the Kodar Group, which are widespread throughout the area of the Baikal mountain region (BMR). Based on the above results, the Au–U mineralization in the study area can be recognized as unconformity-type deposits, analogous to the well-known deposits of Australia and Canada. In this connection, the Baikal mountain region has a good potential for the discovery of Au—U deposits.     

Paleoproterozoic (1.90–1.86 Ga) arc volcanism in the Flin Flon Belt,Trans-Hudson Orogen,Canada

Geochemical and isotopic (Nd, Sr) data are reported on Paleoproterozoic (1904–1864 Ma), maficintermediate (<63% SiO₂), arc metavolcanic rocks from the Flin Flon greenstone belt, Manitoba and Saskatchewan. Major element criteria permit subdivision of the rocks into tholeiitic (TH), calc-alkaline (CA), alkaline, and boninitic (BO) magma series. Subaqueously erupted, TH and related CA basalt-basaltic andesite, and rare high-Ca boninites dominated between 1904 Ma and 1890 Ma. The TH rocks are similar to modern island are tholeiites, having low high-field-strength element (HFSE) and rare earth element (REE) abundances, and chondrite-normalized light REE depletion to slight enrichment. The boninites have even lower HFSE and REE abundances (1–2X chondrites). Along with their extreme ratios of refractory incompatible elements (e.g., high Al/Ti, Ti/Zr, low Ti/V, Zr/Y), these features indicate that the arc mantle source was strongly depleted, probably residual after MORB or back-arc basin basalt extraction. Elevated Th/Yb, Ba/La, La/Nb values, and the spread in Nd isotopic compositions (initial _Nd=–0.4 to +4.8) suggest recycling of small amounts (0–8%) of Archean and possibly older Proterozoic crust via sediment subduction and, locally, intracrustal contamination. Calcalkaline andesite-rhyolite and rare shoshonite and trachyandesite, erupted between 1890 Ma and 1864 Ma, are more strongly light REE enriched and have comparatively higher HFSE abundances, and higher Zr/Y and Nb/Y values. The rocks have strong arc trace element signatures (e.g., high Th/Nb, La/Nb), and initial _Nd values (+2.3 to +4.6) indicate that depleted mantle contributions to the magmas continued to be dominant. The geochemistry and geology of these younger volcanic rocks suggest a mature island arc setting in which the arc lithosphere was thicker than in the previous period, and a more fertile sub-arc mantle source was tapped. The pre-1890 Ma volcanism occurred in one or more separate arcs, probably characterized by rapid subduction of oceanic lithosphere, relatively thin, tholeiitic arc crust, and extensive backarc basin formation. In contrast, post-1890 Ma volcanism is dominantly calc-alkaline to (rarely) alkaline, and is interpreted to reflect crustal thickening due to longterm growth of arc edifice(s) and tectonic thickening associated with intraoceanic arc-arc (>1870 Ma) collision and subsequent intra-arc deformation.     

The nephelinitic–phonolitic volcanism of the Trindade Island (South Atlantic Ocean): Review of the stratigraphy,and inferences on the volcanic styles and sources of nephelinites

Trindade Island is located in the South Atlantic Ocean, 1170 km from the Brazilian coast, and represents the eastern end of the E–W Vitória–Trindade Chain. It shows the youngest plume-induced (ca. 3.7 to <0.17 Ma) subaerial volcanism on the South American plate, associated with the Trindade plume activity. Almeida (1961) recognized five volcanogenic successions at Trindade (in decreasing age): the Trindade Complex (TC, >2.4 Ma) and the Desejado (DF, ∼2.4 to 1.5 Ma), Morro Vermelho (MV, <0.17 Ma), Valado (VF, no age) and Paredão (PF, no age) formations, composed of effusive–pyroclastic deposits and subvolcanic intrusions associated with nephelinite–phonolite volcanic episodes. We revised the original Almeida's (1961) stratigraphy with additional field work and petrography to recognize eruptive styles and processes within the nephelinite–phonolite volcanism. Also, available geochemical databases were used to improve the stratigraphic correlation between nephelinites from different units and to characterize their mantle sources.The nephelinitic volcanism may represent Strombolian and Hawaiian–type activity of low viscosity and volatile–rich lavas interlayered with pyroclastic successions (fall–out deposits). Phonolitic deposits record explosive Vulcanian–style episodes of volatile–rich and higher–viscosity lavas interlayered with pyroclastic deposits (mostly pyroclastic flows). Geochemical data allowed the individualization of nephelinites as follows: (1) MV olivine–rich nephelinites and all olivine–free varieties are low K₂O/Na₂O, K₂O/TiO₂ and intermediate CaO/Al₂O₃ that may be derived from N–MORB and HIMU mantle components; (2) the VF olivine–rich nephelinites have high K₂O/Na₂O, K₂O/TiO₂ and CaO/Al₂O₃ that indicates both EM and HIMU mantle sources and; (3) the PF olivine–rich nephelinites show high K₂O/TiO₂ similar to those from VF, and intermediate CaO/Al₂O₃ as nephelinites from MV rocks, suggesting a mixed source with EM + HIMU > N–MORB components.We suggest that the HIMU and EM mantle types resulted from metasomatic episode(s) in the peridotitic mantle beneath the Trindade Island during the Brasiliano Orogeny and later, as previously pointed out by Marques et al. (1999). Thus, the major HIMU component would relate to recycled oceanic crust or lithospheric mantle (mostly CO₂–eclogites) whereas the less important EM component to recycled marine or continental sediments.     

Chemical and tectonic controls on the formation of sparry magnesite deposits — the deposits of the northern Greywacke Zone (Austria)

The sparry magnesite deposits of the northern Greywacke Zone are situated at the base of thrust sheets. Model calculations and natural examples suggest that an inverse temperature gradient can only be established at the base of a thrust sheet if thrusting is so rapid as to be geologically unrealistic. Independently of this thrusting induces a fluid flow from the lower units to the front of the moving thrust sheet. Stability data of magnesite and dolomite in contact with fluids with different Ca/(Ca+Mg) ratios suggest that this tectonically induced fluid flow produce magnesite from preexisting dolomite by metasomatism.

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Zusammenfassung Die Spatmagnesitvorkommen der Nördlichen Grauwackenzone liegen innerhalb von Deckengrenzen. Modellberechnungen zeigen, daß ein inverser Temperaturgradient sich nur dann an der Basis von Decken einstellen kann, wenn die Überschiebungsgeschwindigkeit so groß wird, daß sie geologisch unglaubwürdig ist. Deckenüberschiebungen erzeugen aber in jedem Fall in der überschobenen Einheit einen zur Deckenfront hin gerichteten Fluidstrom. Eine Betrachtung der Stabilitätsdaten für Magnesit und Dolomit im Gleichgewicht mit einer fluiden Phase mit unterschiedlichen Ca/(Ca+Mg)-Verhältnissen zeigt, daß durch einen solchen Fluidstrom metasomatisch Magnesit aus Dolomit gebildet werden kann.

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Résumé Les dépôts de magnésite spathique de la Grauwacken Zone septentrionale sont situés à la base d'unités charriées. Un calcul de modélisation et des exemples naturels montrent qu'un gradient inverse de température ne peut s'établir à la base d'une unité charriée qu'à la condition d'admettre une vitesse de charriage tellement élevée qu'elle est irréaliste. D'autre part, le phénomene de charriage induit un déplacement de fluide depuis les unités inférieures vers le front de la nappe. Les données relatives aux conditions de stabilité de la magnésite et de la dolomite en présence de fluides de divers rapports Ca/Ca+Mg permettent de déduire que le flux ainsi engendré par la tectonique peut engendrer la magnésite par métasomatose à partir de dolomite préexistante.

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Temporal variations in the concentration and isotopic signature of ammonium- and nitrate–nitrogen in soils under a breeding colony of Black-tailed Gulls (Larus crassirostris) on Kabushima Island,northeastern Japan

Temporal variations in the concentration and N isotopic ratios of inorganic N (NH₄– and NO₃–N) as affected by the soil temperature regime together with the input of bird excreta were analyzed in a sedentary soil under a dense colony (1.6 nests/m²) of breeding Black-tailed Gulls (Laruscrassirostris: a ground-nesting seabird). Surface soil samples were taken monthly from mid-March to late July 2005 from Kabushima Island, Hachinohe, northeastern Japan. The spatial concentration of inorganic N in the soils varied considerably on all sampling dates. There may be a statistically significant trend, showing increased NH₄–N content from settlement up to early June when the input of fecal N attains its maximum, and then decreases towards the end of breeding activity (early August). Abundant NO₃–N was observed in all soils, particularly in the later stage of breeding (up to 3800 mg-N/kg dry soil), refuting earlier claims that nitrification is unimportant in the soils. δ¹⁵N values of NH₄ in the soils showed unusually high values up to +51‰, reflecting N isotope fractionation due to volatilization of NH₃ during the mineralization. Mean δ¹⁵N values of the monthly collected totals of NH₄ and NO₃ were not significantly different at the 5% level based on ANOVA and significant differences were observed only among the three means of NO₃–N collected in mid-March (settlement of colony: δ¹⁵N = −0.2 ± 3.5‰) and late July (later stages of breeding: δ¹⁵N = +22.1 ± 7.0‰, +23.3 ± 7.8‰) at the 1% and 5% levels by t-test, respectively. Such an observation of significantly increased δ¹⁵N values for NO₃–N in soils from the fledgling stage indicates the integration of denitrification coupled with nitrification under a limited supply of fecal N.     

U–Pb geochronology and Pb isotope characteristics of the Chahgaz volcanogenic massive sulphide deposit,southern Iran

The Chahgaz Zn–Pb–Cu volcanogenic massive sulphide (VMS) deposit occurs within a metamorphosed bimodal volcano–sedimentary sequence in the south Sanandaj–Sirjan Zone (SSZ) of southern Iran. This deposit is hosted by rhyodacitic volcaniclastics and is underlain and overlain by rhyodacitic flows, volcaniclastics, and pelites. Peperitic textures between rhyodacite flows and contact pelites indicate that emplacement of the rhyodacite occurred prior to the lithification of the pelites. The rhyodacitic flows are calc-alkaline, and show rare earth and trace elements features characteristic of arc magmatism. Zircons extracted from stratigraphic footwall and hanging-wall rhyodacitic flows of the Chahgaz deposit yield concordant U–Pb ages of 175.7 ± 1.7 and 172.9 ± 1.4 Ma, respectively, and a mean age of 174 ± 1.2 Ma. This time period is interpreted to represent the age of mineralization of the Chahgaz deposit. This Middle Jurassic age is suggested as a major time of VMS mineralization within pull-apart basins formed during Neo-Tethyan oblique subduction-related arc volcano-plutonism in the SSZ. Galena mineral separates from the layered massive sulphide have uniform lead isotope ratios of ²⁰⁶Pb/²⁰⁴Pb?=?18.604–18.617, ²⁰⁷Pb/²⁰⁴Pb?=?15.654–15.667, and ²⁰⁸Pb/²⁰⁴Pb?=?38.736–38.769; they show a model age of 200 Ma, consistent with the derivation of Pb from a Late Triassic, homogeneous upper crustal source.     

The stages and duration of formation of gold mineralization at copper-skarn deposits (Altai–Sayan folded area)

Gold mineralization at copper-skarn deposits (Tardanskoe, Murzinskoe, Sinyukhinskoe, Choiskoe) in the Altai–Sayan folded area is related to different hydrothermal-metasomatic formations. It was produced at 400–150 ºC in several stages spanning 5–6 Myr, which determined the diversity of its mineral assemblages. Gold mineralization associated with magnetite bodies is spatially correlated with magnesian and calcareous skarns, whereas gold mineralization in crushing zones and along fault sutures in moderate- and low-temperature hydrothermal-metasomatic rocks (propylites, beresites, serpentinites, and argillizites) is of postskarn formation. Different stages were manifested with different intensities at gold deposits. For example, the Sinyukhinskoe deposit abounds in early high-temperature mineral assemblages; the Choiskoe deposit, in low-temperature ones; and the Tardanskoe and Murzinskoe deposits are rich in both early and late gold minerals. Formation of commercial gold mineralization at different copper-skarn deposits is due to the combination of gold mineralization produced at different stages as a result of formation of intricate igneous complexes (Tannu-Ola, Ust’-Belaya, and Yugala) composed of differentiated rocks from gabbros to granites.     

Geochronology,geochemistry, and petrology of adakitic Pliocene–Quaternary volcanism in the Şebinkarahisar (Giresun) area,NE Turkey

ABSTRACT

The Pliocene–Quaternary volcanics in NE Turkey are mainly hornblende–phyric trachyandesites having a narrow range of SiO₂ from 61.88 to 63.00 wt.% and exhibiting adakitic signatures with their Na₂O (3.67–4.27 wt.%), Al₂O₃ (16.19–16.80 wt.%), Y (14.1–16.5 ppm) contents and K₂O/Na₂O (0.87–1.12), Sr/Y (44.24–54.90), and La/Yb (36.80–43.88) ratios. Plagioclases as the main mineral phases show a wide range of compositions, and weak normal and reverse zoning. Hornblendes are generally edenite and pargasite (Mg#: 0.39–0.74). Clinopyroxenes are augite (Mg#: 0.58–0.76). Biotites have Mg# ranging from 0.45 to 0.66. The textural and compositional variations indicate disequilibrium crystallization possibly arising from magma mixing. The U–Pb zircon dating of the adakitic volcanics yielded 3.4–1.9 Ma. The studied rocks display moderate light rare earth element /heavy rare earth element ratios and enrichment in the lithophile element and depletion in high field strength element, implying that the parental magmas were derived from mantle sources previously enriched by slab-derived fluids and/or subducted sediments. The crystallization temperature and pressure estimations based on the clinopyroxene thermobarometry range from 1144 to 1186°C and from 3.92 to 7.97 kbar, respectively. Hornblende thermobarometry, oxygen fugacity, and hygrometer calculations yielded results as 908–993°C at a pressure of 2.87–5.22 kbar, water content of 4.4–8.4 wt.%, and relative oxygen fugacity (ΔNNO log units) of ?0.6 to 0.9, respectively. Biotite thermobarometry suggests relatively higher oxygen fugacity conditions (10^–13.33 to 10^–17.60) at temperatures of 676–819°C and at pressures from 1.15 to 1.76 kbar. In the light of the obtained data and modelling, it can be concluded that the magmas of the adakitic volcanics were derived from enriched mantle source through relatively higher partial melting and experienced magma mixing with melts at the crustal level. Additionally, the fractional crystallization and assimilation-fractional crystallization processes may have played an important role during the evolution of the studied volcanics.     

Pb–Pb isotopic systematics of orogenic gold deposits of the Baikal–Patom fold belt (Northern Transbaikalia,Russia) and estimation of the role of neoproterozoic crust in their formation

The paper reports the results of Pb isotope study of several gold deposits of the Russia’s largest metallogenic province of Northern Transbaikalia. Potential sources of the ore material are considered by the example of new and previously published Pb–Pb data on nine deposits and occurrences of different scales. The comparison of Pb–Pb isotope-geochemical characteristics of ores, Paleozoic granitoids, as well as metamorphic pyrite from barren metasedimentary sequences shows that the Neoproterozoic terrigenous–carbonate rocks of the Baikal–Patom fold belt (BPB) served as the main source of lead and other components in the mineral-forming systems of the deposits. Significant variations of Pb isotope ratios typical in general of the considered deposits of the BPB reflect the initial isotopic heterogeneity of Pb source. This heterogeneity is caused by mixing of two geochemical types of continental crust during sedimentation: old (Early Precambrian) crust of the Siberian craton with long-term geochemical evolution and newly formed Late Precambrian crust. Pb–Pb data serve in support of the hydrothermal–metamorphogenic hypothesis of the formation of gold deposits of the BPB.