In a granulite-facies spinel-bearing quartzite, corundum, orthopyroxene and sappbirine (and rarely cordierite and sillimanite) fonn partial rims separating spinel from quartz. Textures indicate the reactions: spinel + quartz = orthopyroxene + corundum, and spinel + quartz = orthopyroxene + sapphirine."hs, corundum and sapphirine are produced by reactions involving quartz. Tbe low Alantent of the orthopyroxene (0.5-2.8wt %) and low values for Mg-Fe distribution coefficient for spinel-sapphirine and spinel-orthopyroxene reflect low-temperature conditions during formation of the reaction products. Absence of zoning in spinel and a constant Mg-Fe distribution ooefficient for spinel-sapphirine and spinel-orthopyroxene, over a compositional range, indicate Mg-Fe equilibration. It is suggested that stable reactions such as spinel + quartz = cordierite or spinel + quartz = garnet + sillimanitc were overstepped and that metastable reactions give rise to the anomalous juxtaposition of corundum + quartz.
. The Cikidang gold deposit, discovered in 1991, is located within the Bayah dome, a Tertiary‐Quaternary volcanic zone at west end of Java, which is well known as a gold district (e.g., Pongkor and Cikotok mines). Typical low‐sulfidation quartz‐adularia‐sericite(‐calcite) vein deposits represent the gold deposit in the district. The Cikidang vein system comprises four sub‐parallel quartz‐adularia‐sericite(‐calcite) veins that are rich in manganese oxide and limonite with very poor amount of sulfides. These vary from 0.5 to 2.7 m thick and extend for up to 1,000 m long. The vein trends roughly N‐S and dip 60d̀ to 86° toward west. The ore grades vary from trace to 74.9 g/t Au and 1.2 to 225.0 g/t Ag. A K/Ar age determination on adularia yielded 2.4 Ma for the Cikidang vein. The ore minerals are represented by electrum, argentite, aguilarite and pyrite. Electrum shows the compositional ranges of Ag (50–65 atom %). The gangue minerals are dominated by quartz with variable amounts of calcite, sericite, adularia, clay minerals, manganese oxide and limonite. The vein textures are so variable as banded, colloform, comb, brecciated and massive. Host rocks, composed of Miocene lapilli tuff and breccia, suffered from pervasive hydrothermal alterations. Wall rocks adjacent to the vein are characterized by argillic and propylitic alteration. The fluid inclusion study of the Cikidang vein shows homogenization temperatures ranging from 170 to 260°C. Salinities are low, generally below 3 wt% NaCl equivalent. Oxygen isotope results suggest meteoric water in origin for ore fluids responsible for the Cikidang deposit.
Typological study, including paragenic, morphological, textural, and chemical characteristics of zircon from nine rare metal granitic stocks and associated greisens, was carried out in order to identify the metallogenic processes of their host granitoids. The investigated zircon-bearing granitoids and type occurrences can be categorized into magmatically and metasomatically specialized types. The magmatic type includes: (i) peralkaline, Zr + Nb-enriched, A 1 -granite (e.g. Um Hibal); (ii) metaluminous, Nb + Zr + Y-enriched, A 2 -type alkali granite (e.g. Hawashia and Ineigi); and (iii) peraluminous, Ta ³ Nb + Sn + Be ± W-enriched, Li-albite granite (e.g. Nuweibi, Igla and Abu Dabbab). The metasomatized granites are Nb>>Ta + Sn + Zr + Y + U ± Be ± Wenriched and hydrothermally altered alkali feldspar granite (i.e. apogranite; e.g. Um Ara, Abu Rusheid, and Um Naggat). Zircon of peralkaline granite is characteristically equant with well-developed pyramidal faces and short prisms (i.e. pseudo-octahedral form) with length/width ratios in the range of 2:1 -1:1. It is of Zr 0.990 Hf 0.007 SiO 4 composition and is associated with hypersolvus assemblage consisting of alkali feldspar, quartz, aegirine and minor reibeckite. Zircon of metaluminous alkali granites is of Zr 0.99 Hf 0.01 SiO 4 composition and is associated with sub-to transolvus assemblage of K-feldspar, quartz, plagioclase and annite-siderophyllite mica. It is prismatic with length/width ratios in the range of 5:1 -3:1, doubly terminated with small pyramidal faces. Compositionally, zircon of Li-albite granite ranges between Zr 0.925 Hf 0.075 SiO 4 and Zr 0.705 Hf 0.295 SiO 4 . It is idiomorphic with a simple combination of prism and bipyramidal terminations with a length/width ratio of 3:1 -2:1. This zircon commonly exhibits a normal zoning with rims consistently higher in Hf than cores. The higher Hf content, of this zircon coupled with its association with topaz, tantalite and lithian micas (e.g. zinnwaldite and Li-white mica), indicates a higher solubility of Hf-fl uoride complexes and their more stabilized state at lower temperature in Li-and F-rich sodic melts. Zircon of apogranite association ranges in composition between Zr 0.967 Hf 0.013 SiO 4 in the lower unaltered alkali feldspar granite zone and Zr 0.805 Hf 0.064 (Y, U, Th, heavy rare-earth elements) [HREE]) 0.125 SiO 4 in the apical metasomatized (i.e. microclinized, albitized, and greisenized) apogranite zones. This compositional change appears to refl ect a roofward increasing in KF , NaF , and HF of the exsolved fl uids. Columbite, xenotime, thorite, cassiterite, beryl and fl uorite are common associates of this zircon. This zircon is of bipyramidal to typical octahedral form with complete absence of prism concurrently with conspicuous development of pyramid, thus the zircon crystals have a length/width ratio of 1:1 -0.5:1. The neoformed metasomatic zircon commonly exhibits either normal or reverse zoning with rims consistently different in Hf, U, Y, and HREE than cores, refl ecting dise...
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