Minerály pegmatitových hnízd z okolí Jablonce nad Nisou (krkonošsko-jizerský pluton) - část I. silikáty

We have studied silicate minerals in pegmatite nests from the Tanvald Granite (4 sites) and the Liberec Granite (1 site) in the vicinity of Jablonec n. Nisou, situated within the Variscan Krkonoše-Jizera Pluton. They contain major quartz, K-feldspar and plagioclase (An0-11), subordinate biotite, muscovite and locally schorl. Accessory phases include garnet (spessartine-almandine), andalusite, Hf-rich zircon and thorite. In addition, zinnwaldite was found in a single sample. The studied pegmatites show simple internal structure including aplitic, granitic and coarse-grained “blocky” units; the central zone commonly contains miarolitic cavity which is sometimes filled by tourmaline. The mineral composition and fractionation degree largely reflect those of the host granite; the more fractionated are pegmatites hosted by the Tanvald Granite. The pegmatite nest from Nová Ves nad Nisou II exhibits distinct mineral assemblage with zinnwaldite, pure albite and lack of biotite and garnet, therefore we suggest here a substantial modification of mineral assemblage by superimposed processes. Moreover, tourmaline (schorl) composition with local increasing of Mg toward rim indicates a possible contamination derived from adjacent rocks during tourmaline crystallization.

Recognition of Late Triassic Cu-Mo Mineralization in the Northern Yidun Arc (S.E. Tibetan Plateau): Implications for Regional Exploration

The Yidun arc, located in the southeastern Tibetan Plateau, was formed by the westward subduction of the Ganze-Litang Paleo-Tethys ocean in Late Triassic. It is well-known for the formation of numerous Mesozoic porphyry-skarn Cu-Mo-(Au) deposits in the arc. To date, more than 20 Cu-Mo-(Au) deposits (>10 million tonnes Cu resources) have been discovered in the southern Eastern Yidun arc. However, few Cu-Mo deposits have been discovered in the northern Eastern Yidun arc. In recent years, some Cu-Mo deposits or occurrence are successively discovered in the northern Eastern Yidun arc, but their ore-forming ages are not well constrained. It remains unclear whether such Cu-Mo mineralization formed by similar metallogenic event and geodynamic setting as the Cu-Mo-(Au) mineralization in the south. In order to determine the metallogenic age and shed light on potential links between Cu-Mo mineralization and regional magmatic events, we present molybdenite Re-Os and zircon U-Pb ages to constrain the timing of two types of Cu-Mo mineralization in the northern Eastern Yidun arc (type I and type II). Molybdenite ICP-MS Re-Os dating results show that type I mineralization was formed at 217.7 ± 3.6 Ma, which is highly consistent with the formation ages of the host granite (218.1 ± 1.5 Ma, 2σ, n = 15, MSWD = 0.92) and aplite dyke (217.3 ± 1.3 Ma, 2σ, n = 16, MSWD = 0.50) within error. While the type II mineralization has a relatively younger formation age of 211.8 ± 4.7 Ma than the host granite (217.1 ± 1.5 Ma, 2σ, n = 14, MSWD = 0.96) and type I Cu-Mo mineralization. These data indicate that the Cu-Mo mineralization in the northern Eastern Yidun arc was temporally and spatially related to the Late Triassic magmatism in the region. Rhenium (Re) concentrations in the molybdenite from type I mineralization, ranging from 12.77 to 111.1 ppm (typically > 100 ppm), indicate that the ore-forming metals were derived mainly from a mantle source. However, Re contents in molybdenite from the type II mineralization, ranging from 7.983 to 10.40 ppm, indicate that the ore-forming metals were derived from a mixed mantle and crustal source with a predominantly crustal component. This study confirms that the northern Eastern Yidun arc exists Late Triassic Cu-Mo metallogenesis, and thus much attention should be paid on this region to find more Late Triassic Cu-Mo resources.

Y-REE mineralization in biotite-arfvedsonite granites of the Katugin rare-metal deposit, Transbaikalia, Russia

We provide the results of study of the extremely enriched in Y-REE carbonate-fluorine isolations from biotite-arfvedsonite granite of the Katugin rare metal deposit. New chemical data of isolations mineral phases - gagarinite-(Y), tveitite-(Y), fluocerite-(Ce), basnaesite, fluornatropyroclore, are delivered. Carbonate-fluoride globule in quartz of hosting granite gives possibility to estimate crystallization order. This finding might be the evidence of silicate-fluorine immiscibility suggested before for Y-REE segregations in the Katugin granites. Fluorine melt segregation took likely place at the magmatic stage of biotite-arfvedsonite granite formation. It causes host granite depletion with fluorine and redistribution of REE and Y in fluorine salt melt.

Mineral compositions of magmatic dikes cutting across the Khe Phen granites (Huong Tra, Thua Thien Hue, Central Vietnam)

The Khe Phen granite quarry located in Huong Tra district (Thua Thien Hue province) has been confirmed as a part of the Ba Na granitoid complex (G/K₂<em>bn</em>), mostly composed of two-mica granite and porphyritic granite. Field survey data show that the granites here are cut across by five distinct narrow dikes (about 50-70 cm wide) including granite pegmatite, granite aplite, aplite, granodiorite and lamprophyre diorite. Mineral compositions of the granite pegmatite and aplite dikes are similar with those of the host granite, which are mainly comprised of quartz (27-35 %), orthoclase (45-58 %), plagioclase (4-15 %), biotite (1-3 %) and a few opaque minerals. Meanwhile, the granodiorite and lamprophyre diorite dikes are melanocratic and compositionally much more mafic, particularly lamprophyre diorite, evidenced by a presence of hornblende (50-55 %), plagioclase (33-40 %), quartz (3-15 %), calcite (5-17 %)... Origin and emplacement age of the latter dikes have not been reported so far, and thus are needed for further studies based on geochemical and isotopic data.

Precise Sm–Nd and U–Pb isotopic dating of the supergiant Shizhuyuan polymetallic deposit and its host granite, SE China

The supergiant Shizhuyuan W–Sn–Bi–Mo deposit is hosted by the Qianlishan granite, a small, highly fractionated granitic pluton (∼10 km2) with multiple phases of intrusions within the Early Yanshanian granitoid province of SE China. Strong alteration of skarn and greisen that formed in the contact zone between the first and second phases of granite intrusions and Devonian limestone is responsible for the polymetallic mineralizations. SHRIMP U–Pb zircon analysis indicates that the two early phases of the Qianlishan granite formed contemporaneously at 152±2 Ma. Metasomatic minerals (garnet, fluorite and wolframite) separated from the skarn and greisen yield a Sm–Nd isochron age of 149±2 Ma that is interpreted as the formation age of the Shizhuyuan deposit. Therefore, the mineralization of the supergiant Shizhuyuan polymetallic deposit formed contemporaneously with, or very shortly after, the intrusion of the small, highly fractionated Qianlishan granite.

Plagioclase-rich microgranular inclusions from the late-Caledonian Galway Granite, Connemara, Ireland

We report on the occurrence, petrology and geochemistry of recently recognized leucocratic plagioclase-rich microgranular inclusions hosted by two granite facies in the late-Caledonian Galway Granite, Connemara, Ireland. They have been recorded at 66 localities along an ESE trending, 4 km wide corridor which incorporates the contact zone between their host granites (i.e. The Megacrystic Granite and the Mingling and Mixing Zone Granodiorite). The inclusions are discoidal in shape and oriented parallel to the general ESE trending foliation in the granites with the most elongate (6.0 × 0.6 cm) occurring in zones of strongest fabric intensity. Contacts between the inclusions and the host granite are sharp with no chilled margin visible. They display a fine-grained (<1 mm) interlocking texture with occasional crystals of plagioclase ranging up to 2 mm in length. Microprobe analysis shows that the plagioclase is essentially oligoclase (An22–32) in composition and is similar to that (i.e. An21–30) occurring in the host granites. Furthermore, the oligoclase accounts for between 61 and 78% of the mode which is reflected in the major element chemistry of the inclusions. Other minerals (in decending order of abundance) include K-feldspar, quartz, biotite and magnetite. The origin of the inclusions is unclear. However, the results of the microprobe analysis provide evidence of a link between them and their host granites.

The chemical composition of uraninite in Variscan granites of the Erzgebirge, Germany

Uraninite is widespread as an accessory mineral in the Erzgebirge granites. It occurs throughout the entire comagmatic series of strongly peraluminous S-type Li-mica granites and has been discovered in more evolved transitional I-S type biotite and two-mica granites, but is rare in those of A-type affinity. Textural relationships and chemical ages imply that uraninite is of magmatic origin. Its composition is variable with a proportion of U plus radiogenic Pb between 71 and 99 mol.%. Uraninite has incorporated Th, Y, and the REE in total amounts between 1 and 29 mol.%. Elements such as P, Si, Al, Ca, and Fe are subordinate. Uraninite from two-mica and Li-mica granites is low in ThO2 (0.8–6.5 wt.%), Y2O3 (0–0.8 wt.%) and REE2O3 (0.1–0.6 wt.%). In contrast, biotite granites from the Kirchberg pluton contain uraninite which is enriched in these components (in wt.%) (ThO2 = 5.6–11.0, Y2O3 = 0.6–5.5, Ce2O3 = 0.1–0.6, Dy2O3 = 0.2–1.1). Commonly, the lanthanide and actinide contents in uraninite correlate poorly with those in the host granite. In S-type Li-mica granites as well as fractionated two-mica and biotite granites, uraninite is the dominant contributor to the bulk-rock U content. Here the proportion of U approaches 80–90%.