Reimar Seltmann

MELT INCLUSIONS IN QUARTZ FROM AN EVOLVED PERALUMINOUS PEGMATITE : GEOCHEMICAL EVIDENCE FOR STRONG TIN ENRICHMENT IN FLUORINE-RICH AND PHOSPHORUS-RICH RESIDUAL LIQUIDS

Abstract We have investigated the magmatic evolution of a late-stage, F- and P-rich, pegmatite-forming aluminosilicate liquid and the geochemical controls on magmatic mineralizing processes by remelting totally-crystallized melt inclusions in quartz and analyzing the quenched glass by EPMA and SIMS. The quartz phenocrysts were sampled from a pegmatite that occurs in a Variscan granite genetically associated with cassiterite- and wolframite-mineralized greisen veins at the Ehrenfriedersdorf SnW deposit, central Erzgebirge, SE Germany. The melt inclusion compositions imply that the pegmatite-forming liquid achieved extreme levels of chemical differentiation. It contained high abundances of Sn, F, P, Li, Rb, Cs, Nb, Ta, and Be and abnormally low concentrations of Ca, Y, Sr, and REE for a granite, and it was strongly peraluminous (the molar [Al 2 O 3 /CaO + Na 2 O + K 2 O] ranged from 1.3 to 2.0). Fractions of the pegmatite-forming liquid were extremely enriched in P 2 O 5 + F + Al 2 O 3 , and the molar abundances of (F + P) in the glasses correlate strongly with moles of network-modifying Al ions implying that the bulk liquid included F-, P-, and Al-bearing complexes. Formation of these complexes reduced the activities of F, P, and Al in bulk liquid, suppressed the crystallization of magmatic topaz and P-rich minerals, and allowed the liquid to become enriched in these constituents. Some fractions of the Ehrenfriedersdorf aluminosilicate liquid contained 1000–2000 ppm Sn. These levels of Sn enrichment were up to 2 orders of magnitude greater than that ever reported for nonmineralized, metaluminous and peraluminous igneous materials and are consistent with some experimentally-derived Sn solubilities in cassiterite-saturated granitic liquids at geologically relevant pressures and temperatures. This concordance implies that cassiterite could have crystallized directly from this highly evolved, P- and F-rich peraluminous granitic liquid without the involvement of hydrothermal fluids.

Age and source constraints for the giant Muruntau gold deposit, Uzbekistan, from coupled Re-Os-He isotopes in arsenopyrite

The Muruntau gold deposit, Uzbekistan, is one of the largest gold deposits known worldwide, but its origin remains controversial. We used Re-Os arsenopyrite geochronology to precisely determine the age of main-stage gold mineralization at Muruntau to be 287.5 ± 1.7 Ma, which overlaps the emplacement of proximal post-tectonic granitoid magmatism. Additionally, we suggest that arsenopyrite growth may have occurred over an interval of at least 2 m.y. Os initial ratios derived from arsenopyrite were coupled with He isotopic data from fluid inclusions within arsenopyrite to constrain the source of ore metals and fluids. Muruntau arsenopyrite yields relatively unradiogenic initial Os (0.37 ± 0.27) and elevated 3He/4He ratios (0.23–0.33 R a) relative to purely crustal Os-He reservoirs. These data suggest the presence of a mantle-derived component in the ore system that was probably introduced during the generation of the granitoid magmas. These new timing and source constraints provide important new insight into the generation of this giant gold deposit, and they necessitate reexamination of genetic models for Muruntau and potentially other giant “orogenic gold” deposits worldwide.

Metallogeny of Siberia: tectonic, geologic and metallogenic settings of selected significant deposits*

Siberia has a prominent position in Russia, in terms of mineral resources and mineral production including copper, nickel, PGMs, uranium, molybdenum, tungsten, tin, manganese, gold, silver, lead, tantalum–niobium, rare earths, diamonds and many other mineral commodities. These resources are represented by a vast array of mineral systems and deposit styles in their respective terranes spanning the Precambrian and Phanerozoic geological history. These mineral systems include VHMS and SEDEX lead–zinc, orogenic gold, sediment- and shear-zone hosted to intrusive-related silver to silver-tin, alkaline gold to gold–uranium and uranium, porphyry copper and copper–molybdenum, epithermal gold, gold–silver, silver, gold–antimony, mercury, uranium–fluorite, various granite-related deposits (W, Mo, Sn, Be, Ta, Co–Ni, etc.) including those associated with peralkaline granites (Nb–Ta–Zr–REE), skarn iron, lead–zinc, gold, tungsten, carbonatite tantalum–niobium, niobium–REE and REE, magmatic copper–nickel–PGM sulfide, PGM and mafic intrusion-hosted iron–titanium–vanadium deposits, and diamondiferous kimberlites. Some deposits are large and superlarge including the well-known Norilsk nickel–copper–PGM and Udokan copper deposits, the Sukhoi Log, Olympiada, Nezhdaninskoe, Kubaka, Kupol gold deposits, the Dukat and Prognoz silver deposits, and the Yakutian diamondiferous kimberlites. Apart from the above-mentioned giant deposits, several others are poorly known and/or unknown to western geoscience. The study of these mineral systems can significantly contribute to our further understanding of the metallogeny of cratons and orogenic belts, orogenic collages, and anorogenic settings. This provides additions to, and further development of, existing classifications and genetic models of mineral systems, allowing researchers to elucidate unknown or poorly studied mineral systems and styles found in Siberia, and to search for some other important styles that appear to be missing, although they are present in other regions with similar geological and tectonic settings.

Geodynamics of late Paleozoic magmatism in the Tien Shan and its framework

The Devonian-Permian history of magmatic activity in the Tien Shan and its framework has been considered using new isotopic datings. It has been shown that the intensity of magmatism and composition of igneous rocks are controlled by interaction of the local thermal upper mantle state (plumes) and dynamics of the lithosphere on a broader regional scale (plate motion). The Kazakhstan paleocontinent, which partly included the present-day Tien Shan and Kyzylkum, was formed in the Late Ordovician-Early Silurian as a result of amalgamation of ancient continental masses and island arcs. In the Early Devonian, heating of the mantle resulted in the within-plate basaltic volcanism in the southern framework of the Kazakhstan paleocontinent (Turkestan paleoocean) and development of suprasubduction magmatism over an extensive area at its margin. In the Middle-Late Devonian, the margins of the Turkestan paleoocean were passive; the area of within-plate oceanic magmatism shifted eastward, and the active margin was retained at the junction with the Balkhash-Junggar paleoocean. A new period of active magmatism was induced by an overall shortening of the region under the settings of plate convergence. The process started in the Early Carboniferous at the Junggar-Balkhash margin of the Kazakhstan paleocontinent and the southern (Paleotethian) margin of the Karakum-Tajik paleocontinent. In the Late Carboniferous, magmatism developed along the northern boundary of the Turkestan paleoocean, which was closing between them. The disappearance of deepwater oceanic basins by the end of the Carboniferous was accompanied by collisional granitic magmatism, which inherited the paleolocations of subduction zones.Postcollision magmatism fell in the Early Permian with a peak at 280 Ma ago. In contrast to Late Carboniferous granitic rocks, the localization of Early Permian granitoids is more independent of collision sutures. The magmatism of this time comprises: (1) continuation of the suprasubduction process (I-granites, etc.) with transition to the bimodal type in the Tien Shan segment of the Kazakhstan paleocontinent that formed; (2) superposition of A-granites on the outer Hercynides and foredeep at the margin of the Tarim paleocontinent (Kokshaal-Halyktau) and emplacement of various granitoids (I, S, and A types, up to alkali syenite) in the linear Kyzylkum-Alay Orogen; and (3) within-plate basalts and alkaline intrusions in the Tarim paleocontinent. Synchronism of the maximum manifestation and atypical combination of igneous rock associations with spreading of magmatism over the foreland can be readily explained by the effect of the Tarim plume on the lithosphere. Having reached maximum intensity by the Early Permian, this plume could have imparted a more distinct thermal expression to collision. The localization of granitoids in the upper crust was controlled by postcollision regional strike-slip faults and antiforms at the last stage of Paleozoic convergence.

Early Carboniferous volcanic rocks of West Junggar in the western Central Asian Orogenic Belt: implications for a supra-subduction system

The paper presents new U–Pb zircon ages and geochemical data from early Carboniferous volcanic rocks of the Wuerkashier Mountains in the northern West Junggar region, NW China, and of the Char suture–shear zone in East Kazakhstan. The study included analysis of geological setting, major and trace elements, and rock petrogenesis. Both localities host early Carboniferous volcanic units dominated by plagioclase-porphyry andesites and dacites. A West Junggar dacite yielded a 206Pb/238U age of 331 ± 3 Ma. The Junggar volcanic rocks are tholeiitic, and the Char samples are intermediate between tholeiitic and calc-alkaline. Both the Junggar and Char volcanic units are characterized by LREE enriched rare-earth spectra (La/Smn = 1.1–2.4) with Eu negative anomalies (Eu/Eu* = 0.12–1.0) and Nb-Ta minimums (Nb/Thpm = 0.15–0.35; Nb/Lapm = 0.3–0.7) on multi-element spectra. The Junggar andesites and dacites have higher REE and HFSE (Ti, Nb, Zr, Y, and Th) compared with the Char rocks, suggesting their derivation from a different mantle source. The melting modelling in the Nb-Yb system showed that the Junggar volcanic rocks formed by low- to medium- (2–5%) degree melting of depleted mantle harzburgite and spinel lherzolite. The Char volcanic rocks formed by high-degree melting (15–20%) of spinel lherzolite and garnet-bearing peridotite. The regional geology of West Junggar and East Kazakhstan and the geochemical features of the rocks under study (i.e. depletion in Nb, Ta, and Ti and enrichment in Th, and combination of LREE enrichment and HFSE depletion) all suggest a subduction-related origin of both Junggar and Char volcanic rocks. The early Carboniferous volcanic rocks of West Junggar possibly formed by subduction of the Junggar-Balkhash ocean beneath an active margin of the Kazakhstan continent, whereas those of East Kazakhstan formed by subduction of the Irtysh-Zaisan Ocean beneath an intra-oceanic arc at the active margin of the Siberian continent.

Tin deposits of the Sikhote–Alin and adjacent areas (Russian Far East) and their magmatic association

The Sikhote–Alin accretionary belt along the northwestern Pacific Plate hosts the most important tin province of Russia. Here, more than 500 ore deposits were formed between 105 and 55 Ma at transform and active subduction margins. Petrological models suggest an active role of the mantle in the mineralisation processes. The deposits can be divided into three groups according to their mineral content and associated magmatism. The first group, a cassiterite–quartz group is defined by tin-bearing greisens as well as quartz–cassiterite and quartz–cassiterite–feldspar veins and stockworks. The mineralisation shows distinct genetic relationships with S- and A-type granites. The deposits are located mainly in Jurassic accretionary prisms adjacent to the Bureya–Khanka Paleozoic continental terrane margin. The second group is represented by the economically important cassiterite–silicate–sulfide deposits, which produce about 80% of Russian tin. Mineralisation in this group is represented by metasomatic zones or veins related to I-type granitoids. The orebodies consist of cassiterite–tourmaline–quartz or cassiterite–chlorite–quartz associations and contain variable amounts of sulfides. The third group comprises tin deposits containing cassiterite and sulfides with the most complicated ore composition with abundant sulfides and sulfostannates accounting for 60–80% of the total ore mass. In some deposits, zinc, lead and silver dominate, whereas tin is sub-economic. The deposits of this group are generally associated with magmatic rocks of the Sikhote–Alin volcano-plutonic belt. The different associations are found together in the same districts and, locally, also in individual deposits. These are characterised by polychronous and polygenetic mineral systems, formed during long periods of time and in different tectonic settings. This testifies to changes in the many physico-chemical parameters of ore formation and, probably, of ore sources. We suggest that the complex mineral and element compositions of some of the ores were caused by the long-lasting composite tectono-magmatic processes.

Closed-vessel microwave digestion technique for lichens and leaves prior to determination of trace elements (Pb, Zn, Cu) and stable Pb isotope ratios

A reliable and robust procedure using closed-vessel microwave digestion of lichens and leaves for precise and accurate determination of trace elements (Pb, Zn and Cu) and stable Pb isotope ratios is presented. The method was developed using certified reference material CRM 482 Pseudovernia furfurea (Lichens), NIST 1515 (Apple Leaves) and NIST 1547 (Peach Leaves) and tested on lichens from a mining site in Russia. A mixture of 3 mL of HNO3, 3 mL of H2O2, 2 mL of H2O and 0.8 mL of HF ensured complete sample dissolution with 100 ± 5% recovery for Pb, Zn and Cu at a maximum temperature of 210°C and pressure of 350 psi. The amount of HF and microwave pressure significantly influenced Pb, Zn and Cu recovery. Comparison between EMMA-XRF and ICP-AES showed a good correlation between Pb, Zn and Cu concentrations. Using the newly developed digestion method, Pb isotopes in lichens from the mining site were determined with an internal precision better than 0.02%.

Mineralogy and formation conditions of ores in the Bereznyakovskoe ore field, the Southern Urals, Russia

AbstractThe Bereznyakovskoe ore field is situated in the Birgil’da-Tomino ore district of the East Ural volcanic zone. The ore field comprises several centers of hydrothermal mineralization, including the Central Bereznyakovskoe and Southeastern Bereznyakovskoe deposits, which are characterized in this paper. The disseminated and stringer-disseminated orebodies at these deposits are hosted in Upper Devonian-Lower Carboniferous dacitic-andesitic tuff and are accompanied by quartz-sericite hydrothermal alteration. Three ore stages are recognized: early ore (pyrite); main ore (telluride-base-metal, with enargite, fahlore-telluride, and gold telluride substages); and late ore (galena-sphalerite). The early and the main ore stages covered temperature intervals of 320–380 to 180°C and 280–300 to 170°C, respectively; the ore precipitated from fluids with a predominance of NaCl. The mineral zoning of the ore field is expressed in the following change of prevalent mineral assemblages from the Central Bereznyakovskoe deposit toward the Southeastern Bereznyakovskoe deposit: enargite, tennantite, native tellurium, tellurides, and selenides → tennantite-tetrahedrite, tellurides, and sulfoselenides (galenoclausthalite) → tetrahedrite, tellurides, native gold, galena, and sphalerite. The established trend of mineral assemblages was controlled by a decrease in

Geophysical and geochemical nature of relaminated arc-derived lower crust underneath oceanic domain in southern Mongolia

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