Intermediate Dikes Research Articles

Vestiges of the Kerguelen Mantle Plume in Southern Tibet: Evidence from 123–117 Ma Magmatism in the Dingri Area of the Central Tethys Himalaya

AbstractThe widely distributed Early Cretaceous magmatism in the Tethys Himalaya (TH) of southern Tibet is related to the Kerguelen mantle plume. Associated magmatic activity products are distributed in the eastern TH, where the active age is earlier than the peak ages of the Kerguelen mantle plume. This study investigated magmatic activity of the Dingri area in the central TH which was coeval with the Kerguelen mantle plume. The intrusion in the Dingri area contains diabases and monzonites. The zircon age of diabase is 123 ± 1 Ma, and that of monzonite is 117 ± 1 Ma. Geochemistry and Sr‐Nd isotopic analyses show that the mafic–intermediate dikes were formed in an intraplate extensional environment. The diabase is derived from the enriched lithospheric mantle and monzonite is derived from partial melting of the lower crust, with both magmatic evolutions being contaminated by crustal materials. These characteristics are similar to those of the Rajmahal–Sylhet basalt, a typical Kerguelen mantle plume product. The discovery of the Dingri mafic–intermediate dikes of the central TH suggests that the TH and Rajmahal–Sylhet Traps formed a continuous mantle plume overflow magmatic belt which was a product of the continuous eruption of the Kerguelen mantle plume.

Paragenesis of gold mineralization at the Kiyuk Lake Project, Kivalliq Region, Nunavut, Canada

ABSTRACT Exploration for gold in Nunavut has been primarily focused on Archean greenstone belts in the north and coastal regions of the territory, resulting in large areas of underexplored terrain in the south. The Kiyuk Lake property is located in the underexplored southwest corner of the Kivalliq Region of Nunavut within the Hearne domain of the ∼1.9 Ga western Churchill Province. The property is hosted by Proterozoic calc-silicate and clastic sedimentary units of the Hurwitz Group (<2.4–1.9 Ga) and the unconformably overlying Kiyuk Group (1.9–1.83 Ga). Gold mineralization in Proterozoic sedimentary rocks is rare in the Canadian Shield, so the Rusty Zone at Kiyuk Lake presents a unique opportunity to study the enigmatic gold mineralization hosted in such sedimentary rocks. Mineralization at the Rusty Zone is hosted by an immature lithic wacke cut by thin intermediate dikes that are associated with hydrothermal breccias composed of Fe-carbonate, calcite, calcic-amphibole, Fe-sulfide, Fe-oxide minerals, and gold. Textural and timing relationships suggest that the gold mineralization is post-sedimentary and syn- to post-intrusion of intermediate dikes. Stable isotope thermometry suggests that mineralization took place between 450 and 600 °C, and geochronological studies indicate that the intrusion and mineralization occurred before or about 1.83 Ga. Using basement breaching thrusts faults as conduits to the surface, over-pressurization along a later normal fault is thought to be the primary cause for the localized breccia pipe that controls gold mineralization. The hydrothermal fluids are postulated to be volatile-rich aqueous solutions exsolved from a source of cooling magmas at depth. Although sub-economic at present, the occurrence of high-grade gold in a Paleoproterozoic basin such as Kiyuk Lake could signal a new opportunity for exploration for gold in the Canadian Shield.

Late Paleozoic magmatic flare-up in the Nuoergong-Langshan Belt, Alxa Block: Insights into tectonic evolution of the southern Paleo-Asian Ocean

Magmatic flare-up events are widely recognized in continental arcs and have played a critical role in constructing arc crust. However, the geodynamic mechanism that triggers the Late-Paleozoic magmatic flare-ups in the Nuoergong-Langshan Belt (NLB) is still debated. We have identified a Late-Paleozoic magmatic flare-up in the Nuoergong-Langshan Belt (NLB), Alxa Block, based on new and published data. Zircon UPb dating shows that the diorites, intermediate dikes, and their host granites in the NLB crystallized at ca. 275–279 Ma, indicating contemporaneous emplacement of these rocks. Geochemical analyses, together with field observations, suggest that the intermediate dikes were probably formed through mixing of felsic and mafic magmas. The diorites have high MgO (3.2–4.0 wt%), Cr (70–80 ppm) contents, and enriched isotopic compositions and were probably derived from a lithospheric mantle source modified by subducted materials. The host granite of the intermediate dikes have similar isotopic compositions with the gabbro and diorite in the NLZ and were mainly generated by remelting of newly-formed mafic rocks and contaminated by ancient crustal components. Syntectonic folded veins and felsic veins intruding a fold core indicate a crustal shortening and thickening event in the early Carboniferous and early Permian in the NLZ. Our geochronological and geochemical data, coupled with published data, reveal that the magmatic flare-up events were simultaneous with crustal thickening in the southern CAOB, implying that flare-ups in continental arcs were probably related to crustal thickening. The temporal and spatial distribution of the late Paleozoic magmatic rocks in the northern Alxa Block reveals a retreating accretionary orogeny of the Paleo-Asian Ocean (PAO).

Eocene dike swarm and felsic stock in Central Iran: Roles of metasomatized mantle wedge and Neo-Tethyan slab

The Eocene felsic stock and dike swarm with intermediate composition from Central Iran (southwest of Jandaq) are studied to understand geodynamic evolution of the Central-East Iranian Microcontinent (CEIM). Field relationships show that the dikes, composed of trachyandesite and basaltic trachyandesite, cross-cut the felsic stock. Whole rock geochemical data reveal the calc-alkaline nature of stock and high-K calc-alkaline to shoshonitic nature of dikes. The felsic stock and intermediate dikes show LREE and LILE enrichment and HFSE (Ta, Nb and Ti) depletion, suggesting that they probably formed in a subduction zone and are similar to the volcanic arc rocks. The rock samples of felsic stock have geochemical characteristics of the high silica adakites (HSA) produced by melting of a subducted oceanic crust. On the other hand, the dikes have characteristics of low silica adakites (LSA) produced by partial melting of a lithospheric mantle peridotite previously metasomatized by slab-derived melts. The Eocene felsic stock and dikes probably formed by subduction of the CEIM-confining oceanic crusts beneath the CEIM. The field and geochemical studies indicate that the magma originated from melting of basic section (amphibolite) of the subducted slab (HSA) will arrive to the Earth surface before the magma formed by melting of mantle wedge peridotites (LSA).

К ВОПРОСУ О ЗОЛОТОНОСНОСТИ КОР ВЫВЕТРИВАНИЯ УЧАСТКА РЫБАЧИЙ (ЗАПАДНЫЙ СИХОТЭ-АЛИНЬ)

The author addresses an issue of gold presence in pre-Quaternary crusts of weathering that are widespread in sub-mountain areas of the doming-block geomorphological structures of the Amur Region, the Khabarovsk and Primorsky Territories along the periphery of large- and medium-sized Cenozoic basins and depressions. For a more detailed mining and geological study and assessment of the gold content of the weathering crusts, we recommended the Rybachy site as one of these objects (insufficiently studied), and the entire Mukhenskaya area as a promising one. These sites are located in the zone of articulation of the eastern side of the Middle Amur Depression with the folded structures of the West Sikhote-Alinsky volcanic belt. The author gives examples of large gold deposits, related to weathering crusts with their brief description (Olympiada, Kuranakh), as well as a brief description of alluvial deposits of weathering crust (sub-basalt cl.Paskhalny, an eluvial-alluvial placer of the Bolotisty Stream). The search work of the DVIMS (Far Eastern Institute of Mineral Raw Materials) in 2002 indicated the potential of weathering crusts within the Rybachy prospect for gold mineralization. This is supported by elevated gold in heavy mineral concentrates from the linear weathering crusts and residual blankets, as well as from non-commercial placers (from 0.01 to 4.0 g/t and higher); the occurrence of fracture zones, brecciation, secondary silicification close by fractures, and abundant Late Cretaceous silicic and intermediate dikes. Zones of hydrothermally altered rocks (silicification, chloritization, sericitization, argillization) are widespread here.

The Watershed Tungsten Deposit, Northeast Queensland, Australia: Permian Metamorphic Tungsten Mineralization Overprinting Carboniferous Magmatic Tungsten

Abstract The Watershed tungsten deposit (49.2 Mt avg 0.14% WO3) lies within the Mossman orogen, which comprises deformed Silurian-Ordovician metasedimentary rocks of the Hodgkinson Formation intruded by Carboniferous-Permian granites of the Kennedy Igneous Association. The Hodgkinson Formation in the Watershed area comprises skarn-altered conglomerate, psammite, and slate units that record four deformation events evolving from ductile, isoclinal, colinear folding with transposition (D1–D3) to brittle ductile shear zones (D4). Multiple felsic to intermediate dikes cut across the metasedimentary rocks at Watershed including the following: (1) Carboniferous, monzonite dikes (zircon U/Pb age of 350 ± 7 Ma) emplaced during D1–2; and (2) Permian granite plutons and dikes (zircon U/Pb ages of 291 ± 6, 277 ± 6, and 274 ± 6 Ma) and diorite (zircon U/Pb age of 281 ± 5 Ma) emplaced during D4. Tungsten mineralization is largely restricted to skarn-altered conglomerate, which preserves a peak metamorphic mineralogy formed during ductile deformation and comprises garnet (Grt40–87 Alm0–35Sps1–25Adr0–16), actinolite, quartz, clinopyroxene (Di36–59Hd39–61Jhn1–5), and titanite. A first mineralization event corresponds to the crystallization of disseminated scheelite in monzonite dikes (pre-D3) and adjacent units, with scheelite grains aligned in the S1–2 fabric and affected by D3 folding. This event enriched the Hodgkinson Formation in tungsten. The bulk of the scheelite mineralization formed during a second event and is concentrated in multistaged, shear-related, quartz-oligoclase-bearing veins and vein halos (muscovite 40Ar-39Ar weighted average age of 276 ± 6 Ma), which were emplaced during D4. The multistage veins developed preferentially in competent, skarn-altered conglomerate units and formed synchronous with four retrograde alteration stages. The retrograde skarn minerals include clinozoisite after garnet, quartz, plagioclase, scheelite, and phlogopite with minor sodium-rich amphibole, which formed during retrograde stages 1 and 2, accompanied by later muscovite, calcite, and chlorite formed during retrograde stage 3. Retrograde stage 4 was a late-tectonic, noneconomic sulfide stage. The principal controls on scheelite mineralization at Watershed were the following: (1) early monzonite dikes enriched in scheelite; (2) D4 shear zones that acted as fluid conduits transporting tungsten from source areas to traps; (3) skarn-altered conglomerate lenses that provide a competent host to facilitate vein formation and a source for calcium to form scheelite; and (4) an extensional depositional environment characterized by vein formation and normal faulting, which provide trapping structures for tungsten-bearing fluids, with decompression being a likely control on scheelite deposition. The coexistence of scheelite with oligoclase in monzonite dikes and veins suggests that tungsten was transported as NaHWO40. Exploration in the area should target Carboniferous monzonite, associated with later syn-D4 shear zones cutting skarn-altered conglomerate.

The large Bakyrchik orogenic gold deposit, eastern Kazakhstan: Geology, mineralization, fluid inclusion, and stable isotope characteristics

The large (>13 Moz Au, averaging 5–6 g/t Au) Bakyrchik Au deposit in eastern Kazakhstan is part of a district-scale mineralized zone, situated in an accretionary terrane. The deposit is localized within a shear zone in carbonaceous sedimentary rocks, which crosscuts the stratigraphy and comprises subconcordant dikes of high-K calc-alkaline (to shoshonitic?) rocks (lamprophyre to monzogabbro to monzodiorite). The tectonized/cataclased carbonaceous rocks contain numerous banded (bedded?) pyrite clasts and bands and abundant disseminated auriferous sulfides (pyrite and acicular arsenopyrite); the latter overprint pyritic clasts and occur proximal to (although mostly outside of) the concordant quartz veins and associated quartz-cemented hydrothermal breccias. Late granodiorite-plagiogranite dikes postdate auriferous sulfides and associated quartz veins/hydrothermal breccia but are overprinted by the late quartz veins containing free native gold and some Cu-Zn-Pb sulfides. This mineralization, however, is minor at the deposit.The most abundant stylolitic quartz veins accompanied by auriferous sulfide mineralization were formed from a homogenous and then boiling aqueous-carbonic fluid at temperatures of >315–300 to 265–240 °C and pressures of 0.8 to 1.2 kbar. Boiling was a dominant process during the formation of hydrothermal breccias accompanied by most intense auriferous sulfide (pyrite, arsenopyrite) mineralization at the temperatures of 250–230 °C and pressures of ~1.1 kbar. The late quartz veins were also formed from a homogenous and then boiling aqueous-carbonic fluid at temperatures of >350–315 to 235–225 °C and pressures of 1.1 to 1.3 kbar. The stable isotope data support a variable – from sedimentary to magma-derived - source of sulfur (δ34S = −20 to ~0‰), with a dominantly sedimentary to metamorphic source of carbon (δ13C = −27.4 to −22.5‰).The deposit is polygenetic and exhibits combined synsedimentary-exhalative, metamorphic to intrusion-related, magmatic-hydrothermal signatures, with strong structural control and close association with various dikes. Most of auriferous sulfide mineralization occurred after or nearly coeval to the intrusion of mafic to intermediate dikes, with a strong positive correlation of Au with As, Ssulf, Ag, Sb, and Hg. Homogenized δ34S values of auriferous pyrite and arsenopyrite (from −2.4 to −0.5‰), close to the meteorite standard, indicate dominantly magma-derived sulfur. Late quartz veins with free native gold and minor Cu-Zn-Pb sulfides are more distinctly related to granodiorite-plagiogranite intrusions.

Zircon U-Pb dating reveals Late Jurassic gold mineralization in the Jidong district of the northern North China Craton

The North China Craton (NCC) hosts numerous gold deposits clustering in several districts, which combined have proven reserves of ~7000 t Au. Geochronological studies have shown that the craton-wide gold mineralization occurred mostly in the 140–120 Ma interval, coincident with the peak of thinning and destruction of the lithospheric keel beneath the NCC. Here we integrate geological relations with LA-ICP-MS zircon U-Pb dating to reveal a new gold mineralization event in the Jidong district, northern NCC. The Xiajinbao, Yu’erya and Jinchangyu gold deposits in this district are characterized by crosscutting relationships between gold veins and various dikes. This relation suggests that zircon U-Pb dating of the pre-ore and post-ore dikes can provide tight constraints on the time of gold veining. LA-ICP-MS zircon U-Pb dating of pre- and post-ore dikes from the three gold deposits yields concordant U-Pb dates of 165–163 Ma, 165–164 Ma and 165–164 Ma, respectively. These indistinguishable U-Pb dates within analytical uncertainty are interpreted as the timing of gold deposition (165–163 Ma) and thus reveal a previously unrecognized Late Jurassic gold mineralization event in the Jidong district. A large variety of mafic to intermediate dikes in the Jidong district have zircon U-Pb dates of 166 ± 1 to 164 ± 2 Ma, consistent with ages of the three gold deposits investigated. The temporal consistency and abundant sulfide inclusions hosted in the mafic-intermediate intrusions, along with the close spatial relations between gold veins and various dikes within and surrounding the mines, indicate a possible magmatic source for ore fluids. Synthesis of regional geological, geochemical and isotopic data led us to conclude that the Late Jurassic gold mineralization in the northern NCC is genetically related to lithosphere destruction of the NCC. An additional implication from this study is that the Mesozoic decratonization of the NCC and associated gold mineralization are spatially and temporally heterogeneous and much more prolonged than commonly thought.

Middle Paleozoic intermediate-mafic rocks of the Tsoroidog Uul’ accretionary complex, Central Mongolia: Petrogenesis and tectonic implications

The Tsoroidog Uul’ accretionary complex is hosted by the Tsetserleg terrane in the southwestern Khangay-Khentey orogenic system (Central Mongolia), which represents the segment of the Central Asian Orogenic Belt and has significant regional implications for its tectonic evolution. This paper reports the results of field investigations and petrography, bulk-rock major and trace element geochemical, as well as SmNd isotopes of Middle Paleozoic intermediate-mafic rocks from the Tsoroidog Uul’ accretionary complex. We investigate a wide range of rock types which can be divided into 4 groups on the basis of their TiO2 and REE characteristics. Group 1 is characterized by moderate TiO2, relatively flat chondrite-normalized REE patterns (La/Smn = 1.0; Gd/Ybn = 1.2). These basalts are divided into two subgroups: (1) Nb/Thpm = 3.6, Nb/Lapm = 0.8, Zr/Nb = 24.6, and Ce/Ybpm = 0.8 (N-MORB type); (2) Nb/Thpm = 1.3, Nb/Lapm = 1.1, Zr/Nb = 11.9, and Ce/Ybpm = 1.6 (E-MORB type). Group 2 shows high TiO2 and LREE (La/Smn = 3.0), differentiated HREE (Gd/Ybn = 2.5), positive Nb anomalies shown in primitive mantle-normalized multi-element patterns (Nb/Thpm = 1.2; Nb/Lapm = 1.1), and low Zr/Nbav. ratios (~6). Group 3 displays low TiO2, high LREE (La/Smn = 3.6), Zr/Nbav. = 24.4, and low Nb (Nb/Lapm = 0.2). Group 4 exhibits moderate TiO2, flat REE patterns (La/Smn = 0.8; Gd/Ybn = 1.1), negative Nb anomalies (Nb/Thpm = 0.3; Nb/Lapm = 0.6) and Zr/Nbav. = 33. The εNd(t) values are positive for Group 1 and Group 2, but negative for Group 3. Based on their petrological and geochemical features, we suggest that the Group 1 and 2 mafic volcanic rocks were formed in an oceanic environment, and represent mid-oceanic ridge basalt (MORB) and oceanic-island basalt (OIB), respectively. In contrast, Group 3 intermediate dikes probably have supra-subduction origin with calc-alkaline features, whereas Group 4 represents arc tholeiite basalt including remnants of the continental volcanic arc. Overall, the Middle Paleozoic intermediate-mafic rocks of the Tsoroidog Uul’ accretionary complex were probably generated from heterogenous mantle sources. Thus, we propose that spatial and temporal changes of the Paleo-Pacific Oceanic lithosphere, which subducted under the continental margin of the Siberian Craton, resulted in the variable composition of the intermediate-mafic rocks of this complex. The accretionary complex of the Tsetserleg terrane, which extends into Ulaanbaatar terrane, was formed by subduction of the Paleo-Pacific Oceanic lithosphere or Mongol-Okhotsk Ocean.

Late Mesozoic magmatism in the Jiaodong Peninsula, East China: Implications for crust–mantle interactions and lithospheric thinning of the eastern North China Craton

A section from the Linglong gold deposit on the northwestern Jiaodong Peninsula, East China, containing Late Mesozoic magmatic rocks from mafic and intermediate dikes and felsic intrusions, was chosen to investigate the lithospheric evolution of the eastern North China Craton (NCC). Zircon U–Pb data showed that low-Mg adakitic monzogranites and granodiorite intrusions were emplaced during the Late Jurassic (~145 ​Ma) and late Early Cretaceous (112–107 ​Ma), respectively; high-Mg adakitic diorite and mafic dikes were also emplaced during the Early Cretaceous at ~139 ​Ma and ~118 ​Ma, and 125–145 ​Ma and 115–120 ​Ma, respectively. The geochemical data, including whole-rock major and trace element compositions and Sr–Nd–Pb isotopes, imply that the mafic dikes originated from the partial melting of a lithospheric mantle metasomatised through hydrous fluids from a subducted oceanic slab. Low-Mg adakitic monzogranites and granodiorite intrusions originated from the partial melting of the thickened lower crust of the NCC, while high-Mg adakitic diorite dikes originated from the mixing of mafic and felsic melts. Late Mesozoic magmatism showed that lithosphere-derived melts showed a similar source depth and that crust-derived felsic melts originated from the continuously thickened lower crust of the Jiaodong Peninsula from the Late Jurassic to Early Cretaceous. We infer that the lower crust of the eastern NCC was thickened through compression and subduction of the Palaeo-Pacific plate beneath the NCC during the Middle Jurassic. Slab rollback of the plate from ~160 ​Ma resulted in lithospheric thinning and accompanied Late Mesozoic magmatism.

Early Paleozoic arc magmatism in the Kalamaili orogenic belt, Northern Xinjiang, NW China: Implications for the tectonic evolution of the East Junggar terrane

The Kalamaili orogenic belt in northwestern China preserves a record of the magmatic and tectonic evolution of the Paleo-Asian Ocean and is thus an important region for reconstructing the Paleozoic evolution of the East Junggar terrane and the Central Asian Orogenic Belt. This study presents detailed field investigations, new SHRIMP U-Pb ages, whole-rock geochemistry, and in situ zircon Hf isotopic data for the early Paleozoic igneous rocks exposed in the Kalamaili orogenic belt. These igneous rocks are predominantly felsic plutons and mafic–intermediate volcanic rocks but include minor mafic–intermediate dikes. The igneous rocks have zircon U-Pb ages ranging from 463 Ma to 433 Ma and unconformably overlain by Late Silurian and Early Devonian sedimentary rocks. These early Paleozoic igneous rocks exhibit signatures similar to those of arc-related magmatic rocks such as enrichment in large-ion lithophile elements and depletion of high field strength elements. They have high positive εHf(t) values of +10.7 to +19.0, suggesting a juvenile source and considerable continental growth in the East Junggar terrane during the early Paleozoic. The geochemical signatures of the mafic–intermediate rocks suggest that they originated from the partial melting of mantle wedge material that had been metasomatized by slab-derived fluids; the felsic plutons were derived from the partial melting of juvenile crust. This early Paleozoic arc magmatic event, together with other evidence, indicates that the Kalamaili Ocean formed in the early Paleozoic, was subducted during Middle Ordovician–Early Silurian, and closed before Middle Silurian. Stratigraphic correlations reveal that the East Junggar terrane might not have experienced full extension to form a broad ocean during the late Paleozoic.

Intermediate-mafic dikes in the East Kunlun Orogen, Northern Tibetan Plateau: A window into paleo-arc magma feeding system

The contemporaneous mafic and intermediate dikes of a continental magmatic arc provide a window into the magma feeding system at depth. Here we integrate data on the elemental and Sr-Nd-Hf isotope geochemistry, petrology, mineralogy, and zircon geochronology of late Permian dikes in the East Kunlun Orogen, northern Tibetan Plateau, discuss the petrogenesis of the dikes, and reconstruct the nature and relationship of different magma reservoirs. The dikes are porphyritic diabases, lamprophyres and diorite porphyries that crystallized between 259 and 255 Ma, coeval with their host granodiorites. Geochemical and Sr-Nd-Hf isotopic data indicate the parental magma of the porphyritic diabases and lamprophyres was derived from enriched lithospheric mantle. This magma underplated the crust and underwent varying degrees of magma recharging, crustal assimilation, and fractional crystallization dominated by olivine, pyroxene, and hornblende. This evolved basaltic magmas ascended through the crust, undergoing further differentiation, and some magma entered mid-crustal reservoirs occupied by felsic crystal mushes, rejuvenating the mushes. The subsequent mixing of mafic and felsic materials formed the precursor magmas of the intermediate dikes. Our new data reveal that the magmas stagnated mainly in reservoirs at two levels, 26–32 and 9–18 km, based on hornblende barometry. The underplating, assimilation, and replenishment of basaltic magmas in the lower crust, and their eventual emplacement and differentiation in a mid-crustal reservoir, controlled the mineralogy and geochemistry of the mafic dikes, while the rejuvenation of crystal mushes, the mixing of mafic and felsic materials, and differentiation account for the compositional features of the diorite porphyries. This study shows that the trans-crustal magma feeding system, which controls the compositions of several dispersed but interconnected magma reservoirs, is the key to understanding the compositional diversity and igneous petrogenesis in continental arcs.

Petrogenesis of the Rambler Rhyolite Formation: Controls on the Ming VMS Deposit and geodynamic implications for The Taconic Seaway, Newfoundland Appalachians, Canada

The Ming Cu-Zn-Ag-Au volcanogenic massive sulfide (VMS) deposit is hosted by the Upper Cambrian-Lower Ordovician Rambler Rhyolite formation, which consists of a folded northeast plunging felsic dome complex, developed in the uppermost segment of the obducted supra-subduction Baie Verte oceanic tract, in the north central Newfoundland Appalachians. The deposit is overlain by interstratified mafic volcanic flows and volcaniclastic rocks of the Snooks Arm Group. The upper 1 km of the Rambler Rhyolite formation consists of a coherent felsic lithofacies at its base, overlain by a quartz-bearing volcaniclastic-dominated lithofacies (units 1.2 and 1.3). Geochemically, these rocks are light rare earth element (LREE)- and large ion lithophile element (LILE)-enriched ([La/Yb]pn = 4–20; Th = 2.58–4.05 ppm), and high field strength element (HFSE)-depleted (Y = 5.64–6.59 ppm) rhyodacite with calk-alkalic affinities (Th/Yb > 2; Zr/Y > 7). The last pulse of felsic volcanism (unit 1.3) is intimately associated with the massive sulfide lenses and has FI- and FII-type rhyolite signatures (high La/Yb ratios), commonly attributed to deep (≥ 30 km) crustal melting. The eNd(t) values of the rhyolites range from −2.5 to −1.3, indicating the rocks were influenced by crustal material. A 10 percent batch partial melt of a garnet-amphibole metamorphosed normal mid-oceanic ridge basalt (N-MORB) and island arc tholeiite (IAT) crustal material can generate a melt similar to the felsic rocks hosting the Ming deposit; however, the IAT reproduces much better the absolute abundances of LILE (Th, Sr), HFSE (Nb, Zr, Y), and middle rare earth elements (MREE) of the felsic rocks. It is interpreted that the felsic melt came from melting of a subducted slab with IAT affinity, thus explaining the depth of melt generation and FI-FII signatures of the felsic rocks. Moreover, the slab-derived siliceous melt may also have contributed magmatophile elements (for example, Ag, Au, Se, Te, Sn, Sb, Hg) into the hydrothermal system once it had reached near-surface depths, and may explain the epithermal suite element-enrichment in the Ming deposit. This study shows that strongly fractionated felsic volcanic rocks associated with boninites, such as those associated with the Ming VMS deposit, can be important hosts for precious metal-enriched VMS and may be as prospective as tholeiitic felsic volcanic sequences (for example, FIII-FIV-type rhyolite – low La/Yb ratios). The base of the Snooks Arm Group is comprised of spatially restricted sulfide-bearing mafic breccia, overlain by a regionally extensive sedimentary sequence (eNd(t) = +3.1 to +5.5), which is in turn overlain by interstratified high-Mg basalt (eNd(t) = +1.6), Th-enriched back-arc basin basalt (BABB), enriched mid-oceanic ridge basalt (E-MORB; eNd(t) = +6.6), and LREE-enriched/low-Ti tholeiitic tuffs (eNd(t) = −0.5). Four generations of mafic to intermediate dikes and sills cross-cut the Rambler Rhyolite formation and share similar geochemical characteristics to the extrusive rocks of the Snooks Arm Group, suggesting comagmatic relationships. The combination of variations in Nb/Yb, Th /Yb, and eNd(t) values within and between units suggest melts derived from depleted to enriched mantle material with melts synchronously generated by both slab-metasomatized mantle wedge and upwelling back-arc asthenosphere.

The age of young intrusions of Tsana Complex (Greater Caucasus) and isotope-geochemical evidence for their origin from hybrid magmas

This paper presents isotope-geochronological and petrological study of granitoids of the potentially ore-bearing (Au–As–Sb–Sn–Mo) Early Pliocene Tsana Complex, which are confined to the Main Caucasus fault zone (upthrow fault) in the central part of the Greater Caucasus Range. The Tsurungal and Karobi groups of magmatic bodies are distinguished based on spatial criterion. The Tsurungal group includes three small granite—granodiorite massifs (Tsurungal, Chorokhi, and Toteldash) and numerous acid and intermediate dikes in the upper reaches of the Tskhenistsqali River (Kvemo Svaneti, Georgia). The Karobi group comprises three subvolcanic rhyodacite bodies located in the upper reaches of the Chashuri River (Zemo Racha, Georgia) and numerous N–S-trending trachyandesite dikes near the axial zone of the Main Caucasus Range. The K-Ar and Rb-Sr isotope dating shows that the granitoid massifs and dike bodies of the Tsana Complex were formed in two different-age pulses of the Pliocene magmatism: phase I at 4.80 ± 0.15 and phase II at 4.15 ± 0.10 Ma. All hypabyssal rocks of the Karobi group, unlike those of the Tsurungal Group, were formed during the first pulse. Petrographic studies in combination with geochemical data indicate that most of the granitoids of the Tsana Complex are hybrid rocks (I-type post-collisional granites) and were derived through mixing of deep-seated mantle magmas with acid melts obtained by the upper crustal anatectic melting in the Main Caucasus fault zone. The granitoids of the Tsurungal Group define basic to acid evolution (diorite–granodiorite–granite–two-mica granite) possibly caused by both crystallization differentiation and increasing role of crustal contamination in the petrogenesis of the parental magmas of these rocks. This conclusion is also confirmed by the differences in the Sr isotope composition between granitoids of the early (87Sr/86Sr = 0.7053) and late (87Sr/86Sr = 0.7071) phases of the Tsana Complex. Main trends in spatiotemporal migration of magmatic activity in the central part of the Greater Caucasus in the Pliocene–Quaternary time were established using obtained and earlier published isotope-geochronological data.

Ordovician backarc‐basin metadolerite and metabasalt of the South Kitakami Terrane, Northeast Japan

AbstractBasement rocks that occur along the northern margin of the South Kitakami Terrane in Japan consist of Ordovician ultramafic rocks (Hayachine ultramafic complex), gneissose amphibolite (Kuromoriyama amphibolite), and mafic rocks (Kagura igneous rocks, KIR). The KIR are composed of metagabbro, metadolerite, metabasalt, and minor felsic–intermediate dikes. Although the KIR contain green hornblende due to metamorphism of greenschist to epidote–amphibolite facies, they rarely retain primary brown hornblende. Approximately 30% of the metabasalt shows porphyritic textures, with phenocrysts of saussuritized plagioclase and/or altered mafic minerals. The geochemistry of the common metadolerite and metabasalt of the KIR shows a tholeiite trend, a low TiO2 content, and high Th/Nb and Ti/V ratios. The KIR are therefore indicative of a supra‐subduction zone tectonic setting, which implies a backarc origin (as also indicated by discrimination diagrams). Trace element patterns of the KIR resemble those of the backarc‐basin basalt of the Japan and Yamato basins in the Japan Sea. We propose that the KIR formed during backarc spreading from the Ordovician to Early Silurian. This view is supported by the geochemical data, the tectonic setting of the Hayachine ultramafic rocks, and the provenance of clastics within Silurian sedimentary rocks.

Composition and genesis of the Konevinsky gold deposit, Eastern Sayan, Russia

The Konevinsky gold deposit in southeast Eastern Sayan is distinguished from most known deposits in this region (Zun-Kholba, etc.) by the geological setting and composition of mineralization. To elucidate the cause of the peculiar mineralization, we have studied the composition, formation conditions, and origin of this deposit, which is related to the Ordovician granitoid pluton 445–441 Ma in age cut by intermediate and basic dikes spatially associated with metavolcanic rocks of the Devonian–Carboniferous Ilei Sequence. Four mineral assemblages are recognized: (1) quartz–pyrite–molybdenite, (2) quartz–gold–pyrite, (3) gold–polysulfide, and (4) telluride. Certain indications show that the ore was formed as a result of the superposition of two distinct mineral assemblages differing in age. The first stage dated at ~440 Ma is related to intrusions generating Cu–Mo–Au porphyry mineralization and gold–polysulfide veins. The second stage is controlled by dikes pertaining to the Devonian–Carboniferous volcanic–plutonic association. The second stage is characterized by gain of Hg and Te and formation of gold–mercury–telluride paragenesis.

Genesis of garnet-bearing rocks at the Berezitovoe deposit, Upper Amur Region, Russia

The geological position, composition of mineral assemblages, and typomorphism of major minerals from garnet-bearing rocks at the Berezitovoe gold-base-metal deposit in the Upper Amur Region have been studied in detail. These are ore-bearing metasomatic rocks and metamorphosed porphyritic dikes. The garnet-bearing metasomatic rocks reveal zoning, which is caused by various degrees of metasomatic transformation of the Paleozoic porphyritic granodiorite that hosts the ore zone. The metasomatic replacement of granodiorite was accompanied by loss of Na, Ca, Ba, Sr and gain of K, Mn, and Rb. Garnet-bearing metamorphosed intermediate dikes occur within the metasomatic zone. The PT conditions of metamorphism and metasomatism are similar and estimated at 3.9 kbar and 500°C from various mineral equilibria. The results of physicochemical simulation of garnet-bearing mineral assemblages carried out by minimizing the Gibbs free energy and the geological data show that garnet-bearing mineral assemblages arose at the Berezitovoe deposit as a result of local high-temperature thermal metamorphism of previously formed low-temperature metasomatic rocks close in composition to classic beresite. In this connection, we propose considering garnet-bearing metasomatic rocks as high-temperature metamorphosed beresites.

A Late Carboniferous–Early Permian slab window in the West Junggar of NW China: Geochronological and geochemical evidence from mafic to intermediate dikes

The Late Carboniferous to Early Permian is a critical period for the formation of particular high-temperature magmatic associations in West Junggar, NW China and the geodynamic processes of ridge subduction and related slab windows in the Central Asian Orogenic Belt. Mafic to intermediate dikes, exposed in the central and southeastern West Junggar extensively, mainly consist of Nb-enriched and magnesian suites. The Nb-enriched dikes (305Ma) are characterized by high Nb contents (5.63–9.08ppm) and large variations of major element contents, exhibiting high-K to medium-K calc-alkaline characteristics with high Na2O/K2O (1.33–3.85) and low Sr/Y (9–32) ratios. The dikes have moderate Mg# (44.6–52.9), LREE-rich and sub-horizontal HREE patterns ((La/Yb)N=3.05–7.18; (Gd/Yb)N=1.43–1.78) with weak Eu anomalies (Eu⁎/Eu=0.81–1.10) and positive εNd (t) values (+5.2 to +8.0), and likely originate from partial melting of mantle peridotite that was metasomatized by subducted sediment-derived melts and slab fluids. The medium-K magnesian dikes (305Ma) are characterized by high MgO (3.05–7.28wt.%), Cr (22.3–311ppm) and Ni (32.6–199ppm) contents and all plot in the field of high-Mg andesite. Some have very high Mg numbers (Mg#>60), suggesting a primitive melt composition. These samples display positive Ba, Sr, K and negative Nb–Ta–Ti anomalies as well as strongly-depleted HREE patterns with weak positive Eu anomalies. They have high Sr/Y (27–126) and relatively high εNd (t) values (+6.0 to +8.3), which suggest a similar petrogenesis as the Nb-enriched dikes, but were metasomatized by adakitic melts. The high-K magnesian dikes (284Ma) are high-K calc-alkaline and have lower MgO (3.63–5.57wt.%) compared with the medium-K suite. They have high Sr (422–819ppm), Sr/Y (26–62) ratios and εNd (t) values (+7.0 to +7.1) with relatively flat REE patterns ((La/Yb)N=2.4–5.5), probably originating from partial melting of K-enriched mantle wedge peridotite metasomatized by adakitic slab melts. Based on these petrogenetic features, we suggest that a ridge subduction regime likely prevailed in West Junggar in the Late Carboniferous. The opening of related slab windows and upwelling asthenosphere have possibly triggered the formation of Late Carboniferous–Early Permian magmatic associations in West Junggar.

The Early Cretaceous Yangzhaiyu Lode Gold Deposit, North China Craton: A Link Between Craton Reactivation and Gold Veining

The Yangzhaiyu gold deposit is one of numerous lode gold deposits in the Xiaoqinling district, southern margin of the North China Craton. Gold mineralization is hosted in Neoarchean to early Paleoproterozoic amphibolite facies metamorphic rocks and consists of auriferous quartz veins and subordinate disseminated ores in the vein-proximal alteration zone. Ore-related hydrothermal alteration is dominated by sericite + quartz + sulfide assemblages close to gold veins, and biotite + quartz + pyrite ± chlorite ± epidote alteration generally distal from mineralization. Pyrite is the predominant sulfide mineral, locally coexisting with minor amounts of chalcopyrite, sphalerite, and galena. Gold occurs mostly as free gold enclosed in or filling microfractures of pyrite and quartz and is also present in equilibrium with Au-bearing tellurides, mainly petzite and calaverite coexisting with hessite, tellurobismuthite, and altaite. Fluid inclusion studies suggest that gold veins were deposited at intermediate temperatures (175°–313°C) from aqueous or aqueous-carbonic fluids with moderate salinity (5–14 wt % NaCl equiv). δ 34S values of sulfide minerals range mainly from 2.0 to 4.4‰, whereas auriferous quartz vein samples have δ 18O values of 12.4 to 9.6‰, with calculated δ 18OH2O values of 6.0 to 3.2‰. Gold-related pyrite grains yield elevated 3He/4He ratios (1.51-0.32 Ra) relative to crustal reservoirs and mantle-like 20Ne/22Ne and 21Ne/22Ne ratios (9.90-9.68 and 0.029, respectively). The stable and noble gas isotopes thus suggest deep-seated, most likely magmatic and mantle-derived, sources for the ore fluids, sulfur and, by inference, other components in the ore system. 40Ar/39Ar dating of ore-related sericite and biotite separates indicates two episodes of gold genesis at 134.5 to 132.3 and 124.3 to 123.7 Ma. The mineralization ages overlap zircon U-Pb ages of 141.0 ± 1.6 to 125.8 ± 1.4 Ma (2 σ ) for the Wenyu and Niangniangshan monzogranite Plutons and a number of mafic to intermediate dikes intruding these Plutons, all being proximal to the Yangzhaiyu gold deposit. The synchronism of gold genesis and magmatism provides additional weights of evidence for a magmatic derivation of ore fluids and sulfur. The geochronologic data also suggest that gold veining took place billions of years after the stabilization of the North China Craton and associated metamorphism in the Late Archean to early Paleoproterozoic. This contrasts sharply to lode gold deposits in other Precambrian cratons that formed predominantly in Late Archean to Paleoproterozoic, temporarily and genetically related to regional high-grade metamorphism and compressional or transpressional tectonism. Available data have demonstrated that the North China Craton was reactivated in the late Mesozoic, as marked by voluminous igneous rocks, faulted-basin formation, high crustal heat flow, and widespread metamorphic core complexes in the eastern part of the craton. It is thus suggested that the Yangzhaiyu gold deposit, together with other deposits of similar ages in the Xiaoqinling district, were products of this craton reactivation event. Lithospheric extension and extensive magmatism related to the craton reactivation may have provided sufficient heat energy, fluid, and sulfur required for the formation of the gold deposits.

The Kharapeh orogenic gold deposit: geological, structural, and geochemical controls on epizonal ore formation in West Azerbaijan Province, Northwestern Iran

The Kharapeh gold deposit is located along the northwestern margin of the Sanandaj–Sirjan Zone (SSZ) in the West Azerbaijan province, Iran. It is an epizonal orogenic gold deposit formed within the deformed zone between central Iran and the Arabian plate during the Cretaceous–Tertiary Zagros orogeny. The deposit area is underlain by Cretaceous schist and marble, as well as altered andesite and dacite dikes. Structural analysis indicates that the rocks underwent tight to isoclinal recumbent folding and were subsequently co-axially refolded to upright open folds during a second deformation. Late- to post-tectonic Cenozoic granites and granodiorites occur northeast of the deposit area. Mineralization mainly is recognized within NW-trending extensional structures as veins and breccia zones. Normal faults, intermediate dikes, and quartz veins, oriented subparallel to the axial surface of the Kharapeh antiform, indicate synchronous extension perpendicular to the fold axis during the second folding event. The gold-bearing quartz veins are >1 km in length and average about 6 m in width; breccia zones are 10–50 m in length and ≤1 m in width. Hydrothermal alteration mainly consists of silicification, sulfidation, chloritization, sericitization, and carbonatization. Paragenetic relationships indicate three distinct stages—replacement and silicification, brecciation and fracture filling, and cataclastic brecciation—with the latter two being gold-rich. Fluid inclusion data suggest mineral deposition at temperatures of at least 220–255°C and depths of at least 1.4–1.8 km, from a H2O–CO2±CH4 fluid of relatively high salinity (12–14 wt.% NaCl equiv.), which may reflect metamorphism of passive margin carbonate sequences. Ore fluid δ18O values between about 7‰ and 9‰ suggest no significant meteoric water input, despite gold deposition in a relatively shallow epizonal environment. Similarities to other deposits in the SSZ suggest that the deposit formed as part of a diachronous gold event during the middle to late Tertiary throughout the SSZ and during the final stages of the Zagros orogeny. The proximity of Kharapeh to the main tectonic suture of the orogen, well-developed regional fold systems with superimposed complex fracture geometries, and recognition of nearby volcanogenic massive sulfide systems that suggest a region characterized by sulfur- and metal-rich crustal rocks, collectively indicate an area of the SSZ with high favorability for undiscovered gold resources.