62 research outputs found
STRUCTURAL GEOLOGICAL CONTROL ON THE MINERALIZATION ON TABUAN ISLAND,SEMANGKO BAY, SOUTH SUMATERA, INDONESIA
Get PDFMineralization have been discovered on Tabuan Island, Semangko Bay, South Sumatera, Indonesia. Tabuan Island belongs to the Neogene Sunda-Banda magmatic arc system. Tabuan Island is a tectonic horst structure which belongs to the subduction-related, magmatically active Barisan zone along the active continental margin of western Sumatera. Basaltic-andesitic volcanics of the late Oligocene to earliest Miocene Hulusimpang Formation are distributed in a broad zone along and subparallel to the regional Semangko Fault and are hosts for several epithermal-style auriferous deposits. The occurrence of hydrothermal mineralization was first suggested from seismic identification of small intrusive bodies which form elongated northwest-southeast ridges passing through the island. Surface sampling campaigns on the island revealed significant hydrothermal alteration and mineralization with pervasive occurrences of sulphide minerals. Detailed mineralogical and geochemical studies at the Federal Institute for Geoscience and Natural Resources show pronounced disseminations and vein-type mineralization. Mineralization shows moderate enrichments in Au, Ag, Zn, Pb, Cu, As, Sb, Ba, and Mn. The association of subaerial island arc volcanism and subvolcanic intrusive bodies, the regional extensional and strike-slip structural regime, and the occurrence of epithermal-style alteration and mineralization in the same volcanic sequence along the coastal zone of Semangko Bay and on Tabuan Island reveal the great potential of this region for epithermal type Au-Ag and base metal deposits. On Tabuan Island, delineation of structural blocks and fault systems suggests that normal faults and margins of grabens may have acted as fluid channelling structures.
Key words: structural geology, mineralization, Tabuan Island, Semangko Ba
Major, trace element and Sr-Nd-Hf-Pb isotopic compositions of basalts from the southern Central Indian Ridge and the Rodrigues Triple Junction.
Get PDFExperimental study of liquid immiscibility in the Kiruna-type Vergenoeg iron–fluorine deposit, South Africa
Get PDFIn this study we experimentally assess whether the bulk composition of the Kiruna-type iron–fluorine Vergenoeg deposit, South Africa (17 wt.% SiO2 and 55 wt.% FeOtot) could correspond to an immiscible Fe-rich melt paired with its host rhyolite. Synthetic powder of the host rhyolite was mixed with mafic end-members (ore rocks) in variable proportions. Experimental conditions were 1–2 kbar and 1010 C, with a range of H2O and F contents in the starting compositions. Pairs of distinct immiscible liquids occur in experiments saturated with fluorite, under relatively dry conditions, and at oxygen fugacity conditions corresponding to FMQ 1.4 to FMQ+1.8 (FMQ = fayalite-magnetite-quartz solid buffer). The Si-rich immiscible liquids contain 60.9–73.0 wt.% SiO2, 9.1–12.5 wt.% FeOtot, 2.4–4.2 wt.% F, and are enriched in Na2O, K2O and Al2O3. The paired Fe-rich immiscible melts have 41.0–49.5 wt.% SiO2, 20.6–36.1 wt.% FeOtot and 4.5–6.0 wt.% F, and are enriched in MgO, CaO and TiO2. Immiscibility does not develop in experiments performed under water-rich (aH2O > 0.2; a = activity) and/or oxidized (>FMQ+1.8) conditions. In all experiments, solid phases are magnetite, ±fayalite, fluorite and tridymite. Our results indicate that the rocks from the Vergenoeg pipe crystallized in a magma chamber hosting two immiscible silicate melts. Crystallization of the pipe from the Fe-rich melt explains its extreme enrichment in Ca, F and Fe compared to the host rhyolitic rocks. However, its low bulk silica content compared to experimental Fe-rich melts indicates that the pipe formed by remobilization of a mafic crystal mush dominated by magnetite and fayalite. Segregation of evolved residual liquids as well as the conjugate immiscible Si-rich melt produced the host rhyolite. The huge amount of fluorine in Vergenoeg ores ( 12 wt.% F) can hardly be explained by simple crystallization of fluorite from the Fe-rich silicate melt (up to 6 wt.% F at fluorite saturation). Instead, we confirm a previous hypothesis that the fluorite enrichment is, in part, due to the migration of hydrothermal fluids
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Porosity and distribution of water in perlite from the island of Milos, Greece
Get PDFA perlite sample representative of an operating mine in Milos was investigated with respect to the type and spatial distribution of water. A set of different methods was used which finally provided a consistent view on the water at least in this perlite. Infrared spectroscopy showed the presence of different water species (molecular water and hydroxyl groups / strongly bound water). The presence of more than 0.5 mass% smectite, however, could be excluded considering the cation exchange capacity results. The dehydration measured by thermal analysis occurred over a wide range of temperatures hence confirming the infrared spectroscopical results. Both methods point to the existence of a continuous spectrum of water binding energies. The spatial distribution of water and/or pores was investigated using different methods (CT: computer tomography, FIB: scanning electron microscopy including focused ion beam technology, IRM: infrared microscopy). Computer tomography (CT) showed large macropores (20 – 100 μm) and additionally revealed a mottled microstructure of the silicate matrix with low density areas up to a few μm in diameter. Scanning electron microscopy (FIB) confirmed the presence of μm sized pores and IRM showed the filling of these pores with water. In summary, two types of pores were found. Airfilled 20 – 100 μm pores and μm-sized pores disseminated in the glass matrix containing at least some water. Porosity measurements indicate a total porosity of 26 Vol%, 11 Vol% corresponding to the μm-sized pores. It remains unsolved wether the water in the μm-sized pores entered after or throughout perlite formation. However, the pores are sealed and no indications of cracks were found which indicated a primary source of the water, i.e. water was probably entrapped by quenching of the lava. The water in these pores may be the main reason for the thermal expandability which results in the extraordinarily porous expanded perlite building materials
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