Late Magmatic Stage Research Articles

Two types of noble metal mineralization in the Kaalamo massif (Karelia)

Noble metal mineralization of the syngenetic (Southern Kaalamo) and epigenetic (Surisuo) types are defined in the Kaalamo massif. The ƩPt, Pd, Au content is as high as 0.9–1.1 g/t. Syngenetic mineralization started at the late magmatic stage (at around 800°C) gradually evolving to cease during the hydrothermal–metasomatic stage (<271°C). Epigenetic mineralization was formed at temperatures ranging from 500 to <230°C in zones of intense shear deformations and low-temperature metasomatosis during the collisional stage of the Svecofennian tectono-magmatic cycle (approximately 1.85 Ga ago). Taking into consideration the geological position of the Kaalamo massif in the Raakhe–Ladoga metallogenic zone with widely developed intense shear dislocations, the epigenetic mineralization type seems to be more promising with respect to noble metals.

U–Pb zircon geochronology and geochemistry of Late Palaeozoic and Early Mesozoic igneous rocks of the Bujinhei area: implications for the tectonic evolution of south Great Xing'an Range

AbstractThe Bujinhei region, situated in the southern reach of the Great Xing'an Range, has experienced multistage tectonic‐magmatic activities, which have led to the formation of different stages and types of magmatic rocks. LA‐ICP‐MS zircon U–Pb dating was performed, and geochemical data were obtained for magmatic rocks samples from the Bujinhei area to analyse the times of magmatic rocks and their tectonic setting. Zircon U–Pb dating indicates that Middle Permian (269.8 Ma) magmatic event formed two‐mica granite, and Middle Jurassic (158–170 Ma) magmatic events formed monzogranite, quartz monzonite‐porphyry and granite porphyry. Chemical analyses indicate that the Middle Permian two‐mica granite is S‐type granite, rich in SiO2, Al2O3and K2O and low in Na2O, A/CNK &gt; 1. Middle Jurassic monzogranite is a metaluminous I‐type granite, rich in SiO2, Al2O3and K2O and low in Na2O and CaO. Quartz monzonite porphyry and granite porphyry are high in K2O and peraluminous and derived from a granitic liquid as a result of strong interaction with hydrothermal fluid during the late magmatic stage; they have high Zr saturation temperature and belong to A‐type granites. Geochemically, two‐mica granite formed in a compressional tectonic regime related to collision, whereas Middle Jurassic intrusive rocks have affinities with intrusive rocks from the extensional environment influenced by post‐collision. Therefore, the southern Great Xing'an Range has experienced the collisional orogenesis of the Central Asian Orogenic Belt in the Middle Permian, whereas in the Middle Jurassic, the southern Great Xing'an Range turned into a post‐collision extensional setting influenced by the Mongol–Okhotsk tectonic regime. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Geochemical constraints on the origin of Late Mesozoic andesites from the Ningwu basin in the Middle–Lower Yangtze Valley, South China

An integrated study of zircon U–Pb ages, Hf–O isotopes, whole-rock major-trace elements and Sr–Nd–Hf isotopes was carried out for Mesozoic andesites from the Ningwu basin in the Middle–Lower Yangtze Valley, South China. The results not only provide insights into the origin of anorogenic magmatism in the Middle–Lower Yangtze Valley but also shed light on the petrogenesis of continental arc andesites in general. The Ningwu andesites are mainly composed of trachyandesite and trachyte, with subordinate of basaltic trachy-andesites. They exhibit variable and low MgO contents of 0.1–3.7 wt.% and low Mg# values of 1.6–51.2, high K2O contents of 2.0–9.9 wt.%. They are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Furthermore, these rocks exhibit relatively enriched whole-rock Sr–Nd–Hf–Pb isotope compositions, with initial 87Sr/86Sr ratios of 0.7050–0.7082, negative εNd(t) values of −8.1 to −1.8, and negative εHf(t) values of −6.7 to 0.4. Zircon U–Pb dating yields consistent ages of 128±2 to 133±2Ma for magma emplacement through volcanic eruption. The zircon exhibits slightly high δ18O values of 5.7–7.8‰ and variable εHf(t) values of −5.8 to 0.7. All andesites can be categorized into two groups in view of their distinct geochemical compositions, an integrated interpretation of which allows deciphering of their petrogenesis. Group I andesite was produced by a relatively larger extent of partial melting of a more fertile and enriched mantle source that contains a greater amount of phlogopite, whereas Group II andesite was generated by a relatively smaller extent of partial melting of a less fertile and enriched mantle source that contains a less amount of phlogopite. In either case, these two mantle sources are less ultramafic in lithochemistry than normal mantle peridotite due to its reaction with metasediment-derived hydrous felsic melts. This metasomatism would take place during subduction of the Cathaysia oceanic crust beneath the Yangtze Craton in the Neoproterozoic, whereas the Mesozoic mafic magmatism in the Middle–Lower Yangtze Valley is essentially caused by reactivation of the fertile and enriched mantle sources due to lithospheric extension in response to the subduction of Pacific plate beneath the eastern China continent. Therefore, the geochemical composition of andesites is primarily governed by their mantle sources that were generated by reaction of the mantle wedge peridotite with hydrous felsic melts originated from the subducted terrigenous sediment in Andean-type subduction zones, though such sources may be partially melt either immediately following generation or at a later time. Late-stage magma evolution processes such as fractional crystallization and magma mixing would have further modified the lithochemical composition of andesites but they are not the governing factors for their trace element and radiogenic isotope compositions.

Mid-Neoproterozoic ridge subduction and magmatic evolution in the northeastern margin of the Indochina block: Evidence from geochronology and geochemistry of calc-alkaline plutons

The mid-Neoproterozoic medium- to high-K calc-alkaline magmatic rocks in the northeastern margin of the Indochina block, SW China, provide important insights into the relationship of the Indochina block with the Gondwana supercontinent. Here we report zircon LA-ICP-MS U–Pb data from the early and late stage plutons which yield weighted mean 206Pb/238U ages of 765Ma and 732–739Ma suggesting mid-Neoproterozoic emplacement. The zircon εHf(t) values show a range of −3.2 to +2.4 (average +0.1±0.9) with TDMC of 1510 to 1870Ma for the early plutons, and −5.4 to +5.1 (average +2.1 to −3.9) with TDMC of 1366 to 1985Ma for late plutons. Both groups show similar geochemical characteristics including high Mg#, enrichment of LILE and LREE, slight negative Eu anomalies, and strongly negative Nb, Ta and Ti anomalies, with all the samples falling within the continental/island arc field in tectonic discrimination diagrams. These features suggest that the early and late stage magmas were produced by the mixing of mantle-derived magma and crust-derived magma in different proportion within an active continental margin, in subduction-related continental-arc tectonic setting. The linear zoning and roughly parallel distribution of the two generations of intrusions with a hiatus of 20Ma might suggest an episode of ridge subduction with asthenosphere upwelling through the slab window that generated the second phase of plutons.

Chemical and O-isotope compositions of amphiboles and clinopyroxenes from A-type granites of the Papanduva Pluton, South Brazil: Insights into late- to post-magmatic evolution of peralkaline systems

Clinopyroxene and amphibole are the main mafic minerals in peralkaline granites of the Papanduva Pluton, Morro Redondo Complex, one of the most important granitic occurrences within the Graciosa A-type Province, S-SE Brazil. Clinopyroxene is present as three distinct late- to post-magmatic textural generations of aegirine, as well as post-magmatic aegirine–augite. Amphibole is predominantly magmatic ferrorichterite and arfvedsonite, and post-magmatic riebeckite. Major and trace element compositions reveal evolutionary trends characterized by increasing Fe3+ and Na and decreasing Fe2+, Ca, HFSE, and REE contents from core to crystal rims; trace element and REE partitioning is controlled by the composition of the coexisting melts or fluid phases, as well as by crystal chemistry. δ18O values for minerals investigated range from +2.3 to +6.4‰ and correlate both with increasing degrees of alkalinity and quasi-solidus (sub-magmatic) deformation affecting several granites of the Papanduva Pluton. These deformation events played a significant role in the concentration and circulation of residual melts and fluid phases from which amphiboles and clinopyroxenes precipitated. Combined textural, chemical, and O isotope findings suggest that these fluids evolved through more oxidizing (close to the FMQ buffer), alkali- (i.e. peralkaline), fluorine-, and 18O-rich conditions, associated with higher abundances of HFSEs and REEs within a temperature interval between 750 and 700°C (late-magmatic stages) and 400–450°C (post-magmatic stages).

Phanerozoic denudation across the Kola Peninsula, Northwest Russia: implications for long-term stability of Precambrian shield margins

Contrasting views exist on the stability of the Earth’s shield regions over the last 1 Ga that have major implications for reconstructing erosion patterns on shields and the supply of sediment to intra-cratonic and marginal basins. This paper explores Phanerozoic denudation rates and patterns on the northern part of the Fennoscandian Shield in the Kola Peninsula, Northwest Russia. This shield region was intruded by magmatic rocks of the Kola Alkaline Province (KAP) in the Devonian and Early Carboniferous. The KAP comprises a varied suite of alkali–ultramafic plutonic and hypabyssal intrusions, diatremes and dykes that was emplaced at various depths in the crust and allows assessment of depths and rates of erosion during and since the KAP magmatic episode. Further evidence of long-term denudation is provided by Mesoproterozoic and Neoproterozoic cover rocks found around the margins of the Kola Peninsula and in the White Sea. The burial and exhumation history is compared to available Apatite Fission Track (AFT) data for the Kola Peninsula and adjacent areas. Post-Devonian denudation rates on the shield rocks of the Kola Peninsula have varied in space and time. Around the periphery of the Kola Peninsula, low long-term denudation of shield rocks is indicated by the survival of Riphean cover rocks and Late Devonian lavas, kimberlite crater facies and near-surface emplacement of dykes. In contrast, in the main belts of KAP intrusions, 4–6 km of rock was removed in response to doming between 460 and 360 Ma. Deep denudation is indicated by the emplacement depths of alkaline intrusions and Phoscorite–Carbonatite pipes (PCPs). Erosion on the Kola Peninsula since 360 Ma has been far more limited. Extensive, shallow, late-stage magmatism associated with PCPs, dykes and the large alkaline intrusions in the KAP indicates that erosion depths nowhere exceeded 2 km. Post-Devonian denudation has removed <1 km of rock from the margins of the Kola Peninsula and from the backslope of the Saariselka–Karelia scarp in northern Finland. AFT data point to an important phase of erosion in the early Mesozoic but depths of unroofing of 3–5 km based on AFT cooling ages for this later phase are in conflict with the evidence of lesser erosion provided by the late-stage KAP intrusions and also require unrealistic depths of former Devonian to Triassic cover rocks. Mean denudation rates were greatest (up to 40 m/Myr) during the KAP magmatic phase. Post-Devonian rates across the Kola Peninsula and adjacent shield areas were much lower (<3–6 m/Myr) and are compatible with low long-term denudation rates for other cratons. Further resolution of longterm denudation patterns and rates on the Kola Peninsula requires the application of low-temperature thermochronometry, detailed examination of the regional geomorphology and firmer dating of ancient weathering episodes.

Last stage of Variscan granitoid magmatism in the Strzelin Massif (SW Poland): petrology and age of the biotite-muscovite granites

New petrographic and geochemical data show some differences between Variscan Bt-Ms granites occurring either as small plutons or dykes in the Strzelin Massif (SW Poland). The granites of the Gromnik and Gorka Sobocka plutons are rich in micas and crystallized from wet magmas; the granites in the dykes and in the Gebczyce pluton are mica-poorer and cordierite-bearing rocks, derived from “dryer” magmas. The lower initial eNd values in the Bt-Ms granites of the dykes, compared with those in the plutons, reflect a more signature of the former, possibly due to local crustal assimilation, via AFC, shortly before emplacement. Much more radiogenic initial 87 Sr/ 86 Sr ratios in the dykes, up to 0.726, further suggest the involvement of extraneous, hydrous crustal fluids enriched in 87 Sr during the evolution of late-stage magma derivatives. The new U-Pb SHRIMP zircon age of 296 ± 6 Ma for the Gebczyce Bt-Ms granite shows that this body belongs to the third stage of magmatism in the Strzelin Massif. The U-Pb SHRIMP zircon data for the Bt-Ms granite dykes provide ages similar to those of their host rocks: c. 295 Ma for the Gesiniec tonalite and the enclosed Bt-Ms granite, and c. 285 Ma for the Strzelin biotite granite and its Bt-Ms granite dykes. These new data from peraluminous rock-types complement our previous studies focused on the tonalites, granodiorites and biotite granites, and shed light on the late-stage igneous evolution of the Strzelin Massif.

Petrogenesis of primitive and evolved basalts in a cooling Moon: Experimental constraints from the youngest known lunar magmas

We have conducted high-temperature experiments over a range of pressures to constrain the petrogenesis of the youngest sampled lunar magmas, which have contrasting primitive and evolved compositions. Our results indicate that at ∼3 Ga, melting still occurred within the same mantle depth range that produced crystalline mare basalts for the previous ∼1 Ga, although our data cannot support or confirm that the shallowest extents of melting moved deeper into the mantle by ∼3 Ga, as is predicted by most thermal evolution models. Furthermore, melting still occurred in regions with low abundances of heat-producing elements. Basaltic lunar meteorite NEA 003A has some of the lowest abundances of incompatible trace elements among all mare basalts and no negative Eu anomaly. Our experiments show that NEA 003A is multiply saturated with olivine and low-Ca pyroxene on its liquidus at ∼1.1 GPa (∼215 km) and ∼1330 °C. If the primitive NEA 003A liquid composition is a minimally-modified melt, the relatively low Mg# of its source region (73–75), its lack of a Eu anomaly, and its chondritic initial Nd isotopic composition indicate its source region likely escaped mixing during mantle overturn with later-stage magma ocean cumulates that formed after plagioclase saturation. This condition would require the sources of the ultramafic glasses to have experienced cumulate mixing, or for assimilation of later-stage magma ocean cumulates by the ultramafic glass parental magmas before eruption in order to account for their higher Mg#'s and deeper negative Eu anomalies. Alternatively, NEA 003A may have undergone some fractional crystallization, in which case its more primitive source region would be deeper than 215 km and may approach the depth range of the ultramafic glass source regions. Iron- and incompatible trace element-rich basaltic lunar meteorites LAP 02205, NWA 032/479, and NWA 4734 have nearly identical bulk compositions and have a multiple saturation point on their high pressure liquidus at <0.6 GPa (<105 km) and 1150–1200 °C. The P–T conditions of this multiple saturation point are inconsistent with the expectation from thermal modeling of a thick, cold lunar elastic lithosphere at 3 Ga, and likely indicate these melts underwent fractional crystallization before eruption, thus increasing their abundances of incompatible trace elements. However, these LREE-enriched basalts possess a very deep negative Eu-anomaly but do not show other chemical fingerprints of KREEP, indicating that a component of Fe-rich, low-Ti late-stage magma ocean cumulates are required in their hybridized source region.

Protracted late magmatic stage of the Caleu pluton (central Chile) as a consequence of heat redistribution by diking: Insights from zircon data and thermal modeling

Zircon U–Pb geochronology and geochemistry are combined with whole-rock composition and thermal modeling to decipher the late magmatic stage of the composite Cretaceous Caleu pluton, which consists of four lithological zones: Gabbro–Diorite Zone (GDZ), Quartz Monzodiorite Zone (QMDZ), Granodiorite Zone (GZ) and Monzogranite Zone (MGZ). The four lithological zones include felsic dikes and veins of variable thickness and distribution. Zircons of four representative samples, each from the mentioned zones, were dated and chemically analyzed. The U–Pb ages exhibit sample-scale scatter derived from protracted zircon crystallization. At pluton scale the ages are substantially overlapped with a subtle decrease of ages from mafic to felsic sample; the latter has a normal age span distribution with a mean age of 94.68±0.71 (2σ confidence) and a MSWD of 0.95. Zircon grains from the uppermost zone of the pluton, where the QMDZ is emplaced, have the highest REE and HFSE contents. Zircon crystallization temperatures oscillate between 680 and 850°C, regardless of the zircon age and sample composition. Differences in temperature and age of zircon crystallization of up to 185°C and 2.6Myr were identified at sample scale, respectively. Numerical modeling indicates that the melts from which zircon crystallized are highly crystalline (mostly higher than 60% crystal) and resemble MGZ in compositions. Time-dependent thermal models were performed to account for preservation of the system above solidus temperature for long time intervals consistent with those of zircon crystallization. Two non-exclusive scenarios for the late-stage development of Caleu pluton were considered: (i) pluton construction by magma pulses assembled incrementally and (ii) upward transport of residual melts by diking through a mush system to yield heat redistribution to the levels where the samples collected. The first scenario does not preserve residual melts for intervals as long as 2.6Myr unless an extremely thick magma reservoir is considered. The second scenario is more favorable because it could provide enough heat that allows preserving residual melts above the solidus temperature depending on: (i) dike width, (ii) melt transport velocity and (iii) dike intensity (vol.% dike). For a dike width of 0.2m and dike intensity of 10%, consistent with field observations, a transport velocity of 300m/yr is required to maintain the upper mush zone above 700°C. The melt transport would have occurred as successive events to allow developing the protracted late magmatic stage of the Caleu pluton.

Origin of rhythmic anorthositic–pyroxenitic layering in the Damiao anorthosite complex, China: Implications for late-stage fractional crystallization and genesis of Fe–Ti oxide ores

The ∼1.7Ga Damiao anorthosite complex (DAC) in the North China Craton contains abundant Ti–magnetite-dominated ore deposits. Both the Fe–Ti–P-rich silicate rocks and massive Fe–Ti–(P) ores occur as discordant late-stage dikes cross-cutting early-stage anorthosites with irregular but sharp boundaries. Field and petrographic observations indicate that some late-stage dikes are composed of unique oxide–apatite gabbronorites (OAGNs), whereas others comprise well-developed alternating late-stage anorthosites and Fe–Ti–P-rich pyroxenites defining rhythmic layers. Massive Fe–Ti–(P) ores are closely related to the Fe–Ti–P-rich pyroxenites. Plagioclase and whole-rock compositions of different rock types were analyzed to constrain the late-stage magma evolution and genesis of the Fe–Ti oxide ores. The similar mineralogical assemblages, REE and HFSE patterns suggest that the different rock types formed by differentiation from a common parental magma. Early-stage anorthosites are characterized by positive Eu anomalies and low REE contents, whereas the late-stage dike-like rocks display no significant Eu anomalies and high REE contents. Plagioclase compositions in the late-stage rocks show a decrease of An contents when compared to that of the early-stage rocks. Based on field relations, petrography and well-defined linear compositional trends, the sequence of crystallization is inferred as: early-stage anorthosites+leuconorites+norites, OAGNs, late-stage anorthosites+Fe–Ti–P-rich pyroxenites+massive Fe–Ti–(P) ores, and massive Fe–Ti–(P) ores. The OAGNs which underwent relatively rapid crystallization represent an early phase during the residual magma evolution after anorthosite separation, whereas the rhythmic layers formed by slow but extensive fractional crystallization of interstitial melt. High solubility of phosphorous played an important role in the formation of rhythmic layering. Massive Fe–Ti–(P) ores crystallized and segregated directly from the magma of Fe–Ti–P-rich pyroxenites, thus representing the most evolved products of the residual magma. Progressive and extensive fractional crystallization of residual ferrobasaltic magma is probably responsible for the formation of large-scale massive Fe–Ti–(P) ores.

Platinum mineralization of the Svetly Bor and Nizhny Tagil intrusions, Ural Platinum Belt

This paper presents the results of a detailed mineralogical and micro analytical study of two Ural-Alaskan type intrusions in the Ural Platinum Belt: (1) the dunite-hosted mineralization of the Svetly Bor intrusion, and (2) the chromitite mineralization of the Nizhny Tagil intrusion. Two generations of platinum minerals are typical of both intrusions: magmatic Pt–Fe(Ni) alloys, and post-magmatic Pt(Fe,Ni,Cu) alloys. A trend from ferroan platinum to isoferroplatinum (Pt,Fe→Pt3Fe) is shown for magmatic alloys of the Svetly Bor intrusion. Magmatic alloys of the Nizhny Tagil intrusion are represented by ferroan platinum (with Ni) only, varying in Fe. The magmatic Pt–Fe alloys of both intrusions were depleted in Fe during the evolution of ore-forming systems and crystallized during the entire magmatic process, generally as fine cubic crystals and anhedral grains hosted by dunite at the magmatic stage during and after the crystallization of dunite (platinum–dunite type of the Svetly Bor intrusion). The evolution of mineral paragenesis was accompanied by a temperature drop, as well as increases in fO2 and fS2.Most of the platinum was concentrated in the residual melts together with chromium, and crystallized in the final stage of the magmatic process (platinum–chromite ore of the Nizhny Tagil intrusion). Post-magmatic Pt(Fe,Cu,Ni) alloys formed during the serpentinization of dunite evolved according to a general compositional trend from tetraferroplatinum and ferronickelplatinum to tulameenite and later to Pt–Cu alloy and Pt-oxide (PtFe)→Pt(Fe,Cu,Ni)→Cu3Pt→Pt–O. Platinum-rich mineralization of both intrusions was formed in the late magmatic stage from a melt rich in volatiles, regardless of their location in dunite or chromitites. Most probably, the rock structure (fracture systems) was the determining factor in the migration of PGE-rich residual melts and ore accumulation.

Minerals of the gadolinite-(Y)-hingganite-(Y) series in the alkali granite pegmatites of the Kola Peninsula

Minerals of the gadolinite-(Y)-hingganite-(Y) series pertaining to the gadolinite-datolite group have been found in the alkali granite pegmatites of the Kola Peninsula. Gadolinite-(Y) is distinguished by its unique natural crystalline state. The unit-cell parameters of this mineral have elevated values as compared with those of gadolinite-(Y) from other deposits and occurrences: (i) a = 10.11 A, b = 7.63 A, c = 4.79, V = 369.30 A3; (ii) a = 10.05 A, b = 7.69 A, c = 4.76, V = 367.99 A3. The increase in unit-cell parameters is not correlated with variation in chemical composition. The variable chemical compositions of particular individuals, especially as concerns REE and Y contents, assume two gadolinite-(Y) generations being contained in the intragranite pegmatites. Gadolinite-I is characterized by a high LREE content (LREE N /HREE N = 1.6) with a prevalence of total REE over Y (REE/Y = 1.36). Gadolinite-II is significantly depleted in LREE (LREE N /HREE N = 0.3) with a prevalence of Y over REE (REE/Y = 0.29). Hingganite-(Y), which has also been found in the alkali granite pegmatites of the Kola Peninsula for the first time, is characterized by elevated unit-cell parameters as well: a = 10.05 A, b = 7.72 A, c = 4.76 A, V = 369.12 A3. The mineral is enriched in Ca (up to 5 wt % CaO); and, by contents of REE and Y, the hingganite-(Y) from inter-granite pegmatites keeps the marginal position between its Y-dominant and REE-dominant varieties. The chondrite-normalized REE patterns assume that hingganite-(Y) crystallizes between the first and the second generations of gadolinite-(Y) and that alkali intragranite pegmatites are formed at the late magmatic stage, whereas amazonite-bearing pegmatites are formed under postmagmatic hydrothermal conditions.

Syntectonic emplacement of Late Cretaceous mafic dyke swarms in coastal southeastern China: Insights from magnetic fabrics, rock magnetism and field evidence

Magma flow directions for 6 Late Cretaceous mafic dyke swarms exposed in coastal southeastern China (SE China) were analyzed using anisotropy of magnetic susceptibility (AMS) and field evidence. Normal AMS fabrics are predominant. The AMS of the dyke swarms originates mainly from the distribution anisotropy of intersertal magnetite that crystallized during late stage magma flow or after the magma cooled. The AMS fabrics record tectonic stress combined with magma flow. Sub-vertical to vertical magma flow is inferred from symmetrical imbricated magnetic foliations of dyke walls and field evidence for 5 dyke swarms. The inferred (sub-) vertical flow directions also indicate that the magma chambers were probably just beneath the sampled locations. Low anisotropy degree, different orientations of principal AMS axes, and asymmetrical magnetic foliations of normal fabrics oblique to dyke walls indicate syntectonic emplacement of the Late Cretaceous dyke swarms under an extensional tectonic regime caused by Paleo-Pacific plate subduction.

Water content, δD and δ11B tracking in the Vanuatu arc magmas (Aoba Island): Insights from olivine-hosted melt inclusions

Abstract Ion microprobe measurements of H and B isotopic ratios and H2O, B and trace element contents are reported here for a series of melt inclusions typical of alkaline basalts of Aoba Island in the central part of Vanuatu arc (Southwestern Pacific). The melt inclusions, hosted in olivine Fo86–90, display large ranges in trace element concentrations and hydrogen (δD from − 48.2 to + 61.7‰) and boron (δ11B from − 11.9 to + 6.4‰) isotopic compositions. The high deuterium enrichment (δD ≥ 0‰) observed in a small subset of melt inclusions requires a proton diffusion loss through the olivine network, in addition to late-stage magma interactions with aqueous saline fluids. These melt inclusions are therefore not considered as representative of the magma from which the olivine grew. In most melt inclusions, positive correlations between H2O, K2O, Ba and Sr lead us to determine the K2O/H2O (1.5 ± 0.2), H2O/Ba (46 ± 3 × 10− 4) and H2O/Sr (29 ± 2 × 10− 4) ratios of Aoba basalts. Overall correlations between δ11B, B/Nb, and B/Nd testify to the mixing between slab-derived fluids, preferentially enriched in δ11B and fluid mobile elements and a relatively depleted MORB-type mantle wedge beneath Aoba Island. Heavy δ11B (on average 5.4 ± 0.7‰) indicate slab-derived fluids, possibly involving serpentine, which would have a mean δD value of − 28.4 ± 7‰. The chemical and isotopic variability recorded by Aoba magmas (melt inclusions) is consistent with the geodynamic context of ridge-arc collision in the central segment of Vanuatu arc.

Gold and silver in PGE–Cu–Ni and PGE ores of the Noril’sk deposits, Russia

Gold and silver contents in Noril’sk ore are controlled by the amount of sulphides and bulk Cu grade. Relative concentrations, re-calculated to 100 % sulphide, depend on type of ore: they are higher for disseminated ore than for massive ore and are the highest for low-sulphide platinum ore. Gold occurs mainly as high-fineness Au–Ag alloy in pyrrhotite-rich ore, whereas silver enters chalcopyrite mainly as solid solution. Increase in Cu grade correlates with an increase in the concentration of silver in chalcopyrite. Gold and silver form discrete minerals such as Au–Cu alloys, Au–Ag alloys, tellurides, sulphides, selenides, sulphobismuthides, Ag and Ag–Pd chlorides in Cu-rich ores; they also enter the structures of complex platinum-group minerals. The Au–Ag mineralisation is related to the post-magmatic hydrothermal stage under temperature conditions of 350–50 °C. Silver entered crystallizing chalcopyrite in solid solution in the late-magmatic stage, while all of the gold and the remainder of the silver and some platinum-group elements were transported predominantly as chloride and hydrosulphide complexes in hydrothermal fluids.

Bashkirian Sr-Nd age of the terminal stage of collisional magmatism in the Western Magnitogorsk zone of the South Urals

Magmatism in the Western Magnitogorsk zone ofthe Southern Urals took place on the accretionarybasement (island arc basement partly thrust over thecontinental margin) and is represented by mafic (lesscommonly, intermediate, ultramafic, and acid) rocksthat constitute intrusions of variable morphology: sillshaped sheets, laccoliths, lopoliths, chonoliths,stocks, dikes [1]. Based on their geological positionand petrological properties, these intrusions are unitedinto several complexes, which were formed during several stages. Each magmatic stage was characterized bylocal tectonic settings that existed from the Tournaisian to the Bashkirian ages [2, 3]. In our opinion, theoldest, Carboniferous rocks are related to the late stageof the collision between the Magnitogorsk island arcand passive margin of the East European continent,which is termed in [4] as a “soft collision,” laterreplaced in the Middle Carboniferous [5] by a “hardcollision,” i.e., a collision between the Kazakhstanand East European blocks. Until recently, the age estimates of magmatism in the Western Magnitogorskzone were based on geological and petrological features: stratified complexes with migmatites, intersection of intrusive bodies and dikes, and a compositionalanalogy with Tournaisian–Visean basalts of the neighboring Magnitogorsk–Bogdanovsk graben [6]. TheKhudolaz complex of differentiated and layered bodies with sulfide Cu–Ni mineralization recentlybecame the sole exception: baddelyite and zircongrains extracted from two autonomous gabbroid bodies yielded similar isotopic dates of 324.78 ± 0.46 and328.90 ± 0.78 Ma, respectively [7]. The terminal stageof magmatism in the Western Magnitogorsk zone isrepresented by a dike belt (dike complex), whichcrosses all the complexes of the Khudolaz syncline andstrikes at NE 10°–13° (Fig. 1).The available data on the Khudolaz complex implyits formation in the environment of early stabilizationof the island arc basement [6], i.e., in the subplatformal regime. This stage likely characterizes the tectonicpause between the two abovementioned collisiontypes. For a long time, it remained unclear whether ornot the dike complex is synplutonic, i.e., representinga terminal stage of magmatism in the Nibearing Khudolaz complex as was assumed by E.K. Buchkovskii etal. in their stored reports of 1971 and 1974, or if it ispostmagmatic, i.e., reflecting an autonomous latemagmatic stage as proposed in [1]. At the same time,inasmuch as no data on the upper geological limit ofthe dike complex were available at that time, we conceded both the Late Carboniferous and Early Permianage for the latter. The recently obtained date of 321 ±15 Ma supports our previous conclusion that the dikecomplex represents an autonomous stage of magmatism and precedes the commencement of hard collision in the Western Magnitogorsk zone. This was thelast pulse of magmatism in this structure.The thickness of dikes in the complex under consideration is sustained along the strike averaging 0.5–1.2 m. In areas where dikes cross the sedimentarycomplexes, the length of their outcrops is maximum(>1 km). The dikes are frequently bifurcated to joinagain or extend parallel to each other over a significantdistance.The mineralogical composition and texture ofrocks in dikes allow the following types to be defined:hornblende dolerites, dolerite porphyrites, gabbrodiorites, lamprophyres (spessartite, malchite), andgabbro dolerites. The rocks are variably altered:pyroxenes are partly or entirely replaced by secondaryhornblende (uralite, actinolite, tremolite); plagioclaseis frequently sossuritized; basaltic hornblende is commonly replaced by uralites and ore mineral. Fissures inrocks are filled with serpentine, chlorite, and epidote.Some relatively fresh varieties exhibit both unalteredplagioclase and clinopyroxene.

Tourmaline occurrences within the Penamacor-Monsanto granitic pluton and host-rocks (Central Portugal): genetic implications of crystal-chemical and isotopic features

Tourmalinization associated with peraluminous granitic intrusions in metapelitic host-rocks has been widely recorded in the Iberian Peninsula, given the importance of tourmaline as a tracer of granite magma evolution and potential indicator of Sn-W mineralizations. In the Penamacor-Monsanto granite pluton (Central Eastern Portugal, Central Iberian Zone), tourmaline occurs: (1) as accessory phase in two-mica granitic rocks, muscovite-granites and aplites, (2) in quartz (±mica)-tourmaline rocks (tourmalinites) in several exocontact locations, and (3) as a rare detrital phase in contact zone hornfels and metapelitic host-rocks. Electron microprobe and stable isotope (δ18O, δD, δ11B) data provide clear distinctions between tourmaline populations from these different settings: (a) schorl–oxyschorl tourmalines from granitic rocks have variable foititic component (X□ = 17–57 %) and Mg/(Mg + Fe) ratios (0.19–0.50 in two-mica granitic rocks, and 0.05–0.19 in the more differentiated muscovite-granite and aplites); granitic tourmalines have constant δ18O values (12.1 ± 0.1 ‰), with wider-ranging δD (−78.2 ± 4.7 ‰) and δ11B (−10.7 to −9.0 ‰) values; (b) vein/breccia oxyschorl [Mg/(Mg + Fe) = 0.31–0.44] results from late, B- and Fe-enriched magma-derived fluids and is characterized by δ18O = 12.4 ‰, δD = −29.5 ‰, and δ11B = −9.3 ‰, while replacement tourmalines have more dravitic compositions [Mg/(Mg + Fe) = 0.26–0.64], close to that of detrital tourmaline in the surrounding metapelitic rocks, and yield relatively constant δ18O values (13.1–13.3 ‰), though wider-ranging δD (−58.5 to −36.5 ‰) and δ11B (−10.2 to −8.8 ‰) values; and (c) detrital tourmaline in contact rocks and regional host metasediments is mainly dravite [Mg/(Mg + Fe) = 0.35–0.78] and oxydravite [Mg/(Mg + Fe) = 0.51–0.58], respectively. Boron contents of the granitic rocks are low (<650 ppm) compared to the minimum B contents normally required for tourmaline saturation in granitic melts, implying loss of B and other volatiles to the surrounding host-rocks during the late-magmatic stages. This process was responsible for tourmalinization at the exocontact of the Penamacor-Monsanto pluton, either as direct tourmaline precipitation in cavities and fractures crossing the pluton margin (vein/breccia tourmalinites), or as replacement of mafic minerals (chlorite or biotite) in the host-rocks (replacement tourmalinites) along the exocontact of the granite. Thermometry based on 18O equilibrium fractionation between tourmaline and fluid indicates that a late, B-enriched magmatic aqueous fluid (av. δ18O ~12.1 ‰, at ~600 °C) precipitated the vein/breccia tourmaline (δ18O ~12.4 ‰) at ~500–550 °C, and later interacted with the cooler surrounding host-rocks to produce tourmaline at lower temperatures (400–450 °C), and an average δ18O ~13.2 ‰, closer to the values for the host-rock. Although B-metasomatism associated with some granitic plutons in the Iberian Peninsula seems to be relatively confined in space, extending integrated studies such as this to a larger number of granitic plutons may afford us a better understanding of Variscan magmatism and related mineralizations.

Geochronology, geochemistry and metallogenic implications of the Boziguo'er rare metal-bearing peralkaline granitic intrusion in South Tianshan, NW China

We report a rare metal (Nb, Ta and rare earth elements)-bearing granitic intrusion from the Boziguo'er region in the South Tianshan Collisional Belt (STCB), which is located in the southwestern margin of the Central Asian Orogenic Belt (CAOB). LA-ICP-MS zircon dating reveals that the intrusion was emplaced during Early Permian at 290.1±1.4Ma. The granitoid is composed dominantly of porphyritic albite granite unit (unit I) and epigranular, medium-grained biotite–aegirine–arfvedsonite granite unit (unit II). Although the two units display different texture and mineral assemblages, there is no distinct boundary between them. The major Nb-, Ta- and rare earth element (REE)-hosting minerals in the intrusion are pyrochlore, xenotime, bastnasite and monazite. Geochemically, the intrusion is characterized by high contents of SiO2 (63.88–81.84wt.%), Na2O (2.51–7.66wt.%) and total alkalis (7.42–12.35wt.%). The trace element patterns are characterized by Rb, Nb, Ta, Zr and Hf enrichment and markedly negative Ba, Sr, P, Eu and Ti anomalies. The rocks show negative εNd(t) values (−4.4 to −3.1) and Mesoproterozoic two-stage Nd model ages (1.3 to 1.4Ga). Based on petrologic and geochemical features, we infer that, although the post-magmatic alteration (albitization and greisenization) has obliterated the original mineralogical constitution of the intrusion, this process was unlikely to have played a crucial role in the development of economic concentrations of Nb, Ta and REE. Alternately, the high concentration of these elements seems to be related to magmatic process. We propose that the crustal source region had been re-fertilized by supercritical fluids released from the asthenosphere-derived magmas and was initially enriched in fluorine, alkalis, (high-strength field elements) HFSEs and REEs. The original magma was formed by the moderate-degree partial melting in a crustal source, triggered by the underplating of the asthenosphere-derived magmas. During the magmatic differentiation, fluorine was largely preserved in the melt phase, even at the late magmatic stage because of the high emplacement depth. The presence of substantial fluorine in granitic melts would not only prolong the duration of the magmatic differentiation but can also act as a complexing agent with HFSEs and REEs. These complexes escape from being involved in the early stages of fractional crystallization, resulting in a HFSE- and REE-rich residual melt. Thus, the fluorine in the magmatic system was likely the key factor contributing to the rare metal enrichment. Tectonically, the Boziguo'er A-type granitic intrusion formed in an extensional setting, and therefore the onset of regional extension in the Chinese part of the SCTB is inferred to be earlier than 290Ma.

Composition and evolution of PGE mineralization in chromite ores from the Il’chir ophiolite complex (Ospa–Kitoi and Khara-Nur areas, East Sayan)

Data are presented on chromitites from the northern and southern sheets of the Il’chir ophiolite complex (Ospa–Kitoi and Khara-Nur (Kharanur) massifs). The new and published data are used to consider similarities and differences between ore chrome-spinel from the chromitites of the northern and southern ophiolite sheets as well as the species diversity of PGE minerals and the evolution of PGE mineralization. Previously unknown PGE minerals have been found in the studied chromitites.Ore chrome-spinel in the chromitites from the northern sheet occurs in medium- and low-alumina forms, whereas the chromitites from the southern sheet contain only medium-alumina chrome-spinel. The PGE minerals in the chromitites from the southern sheet are Os–Ir–Ru solid solutions as well as sulfides and sulfoarsenides of these metals. The chromitites from the northern sheet contain the same PGE minerals and diverse Rh–Pt–Pd mineralization: Pt–Ir–Ru–Os and isoferroplatinum with Ir and Os–Ir–Ru lamellae. Areas of altered chromitites contain a wide variety of low-temperature secondary PGE minerals: Pt–Cu, Pt–Pd–Cu, PdHg, Rh2SnCu, RhNiAs, PtAs2, and PtSb2. The speciation of the PGE minerals is described along with multiphase intergrowths. The relations of Os–Ir–Ru solid solutions with laurite and irarsite are considered along with the microstructure of irarsite–osarsite–ruarsite solid solutions. Zoned Os–Ir–Ru crystals have been found. Zone Os82–99 in these crystals contains Ni3S2 inclusions, which mark off crystal growth zones. Different sources of PGE mineralization are presumed for the chromitites from the northern and southern sheets.The stages of PGE mineralization have been defined for the chromitites from the Il’chir ophiolite belt. The Pt–Ir–Ru–Os and (Os, Ru)S2 inclusions in Os–Ir–Ru solid solutions might be relics of primitive-mantle PGE minerals. During the partial melting of the upper mantle, Os–Ir–Ru and Pt–Fe solid solutions formed syngenetically with the chromitites. During the late-magmatic stage, Os–Ir–Ru solid solutions were replaced by sulfides and sulfarsenides of these metals. Mantle metasomatism under the effect of reduced mantle fluids was accompanied by PGE remobilization and redeposition with the formation of the following assemblage: garutiite (Ni,Fe,Ir), zaccariniite (RhNiAs), (Ir,Ni,Cu)S3, Pt–Cu, Pt–Cu–Fe–Ni, Cu–Pt–Pd, and Rh–Cu–Sn–Sb. The zoned Os–Ir–Ru crystals in the chromitites from the northern sheet suggest dissolution and redeposition of Os–Ir–Ru primary-mantle solid solutions by bisulfide complexes. Most likely, the PGE remobilization took place during early serpentinization at 450–600 ºC and 13–16 kbar.During the crustal metamorphic stage, tectonic movements (obduction) and a change from reducing to oxidizing conditions were accompanied by the successive transformation of chrome-spinel into ferrichromite–chrome-magnetite with the active participation of a metamorphic fluid enriched in crustal components. The orcelite–maucherite–ferrichromite–sperrylite assemblage formed in epidote-amphibolitic facies settings during this stage.The PGE mineral assemblage reflects different stages in the formation of the chromitites and dunite-harzburgite host rocks and their transformation from primitive mantle to crustal metamorphic processes.

Isotope geochemistry and its implications in the origin of Yangla copper deposit, western Yunnan, China

The Yangla copper deposit is located in western Yunnan Province, China, with an estimated Cu reserve of approximately 1.2 million tons. It is a typical giant copper deposit, and its mining started only recently. The δ13CV–PDB values of the calcites studied vary from −5.1‰ to 1.0‰, implying that the hydrothermal fluids from which the calcites precipitated were derived from the granitic magma. The δ18OSMOW(H2O) and δDSMOW values of quartz fluid inclusions range from 0.11‰ to 2.50‰ and from −120‰ to −100‰, respectively. These data may suggest the following: (1) mixing between meteoric and magmatic fluids, or (2) the evolution of meteoric fluid by its interaction with igneous or metamorphic rocks. The δ34S values of sulfides range from −4.20‰ to 1.85‰ (average: −0.85‰), which is consistent with the magmatic origin. Based on the 3He/4He ratios of fluid inclusions trapped in sulfides of the deposit (0.14–0.17 Ra) and 40Ar/36Ar ratios of 301–1053, it can be inferred that the ore-forming fluids of the deposit were derived primarily from the crust with a minor mantle component during the metallogenic processes. Based on C, H, O, and S isotopic compositions, and the Yangla copper deposit is bordered primarily by gently dipping thrust faults near the Linong granodiorite. Moreover, the 187Re–187Os isochron age of molybdenite puts the time of metallogenesis at 233.3 ± 3 Ma, which is virtually coeval with the emplacement of the Linong granodiorite (235.6–234.1 Ma) and highlights the genetic link between the Yangla copper deposit and the Linong granodiorite. It is likely that the ore-forming fluids exsolved from the Linong granodiorite, which was formed by crustal melting induced by the intrusion of mantle-derived magma. During the late Early Permian, the Jinshajiang oceanic plate was subducted to the west, resulting in the formation of a series of gently dipping thrust faults in the Jinshajiang tectonic belt. Subsequently, the thrust faults was tensional during the early Late Triassic, which was a time of transition from collision-related compression to extension in the Jinshajiang tectonic belt; such conditions produced an environment favorable for the formation of ore fluids. This extension, in turn, induced the upwelling of hot asthenosphere, triggering intense melting in the lithospheric mantle and producing voluminous basaltic magma. Subsequently, the mantle-derived magma likely ascended along the fractures and faults to underplate the lower crust, which underwent partial melting to generate voluminous granitic magma. After the magma reached the base of the early-stage Yangla granodiorite, the platy granodiorite at the base of the Yangla body shielded the late-stage magma. Then, this magma cooled slowly, releasing some of its ore-forming fluids into the gently dipping thrust faults near the Yangla granodiorite and producing mineralization.