JaLCDOI 10.18926/ESR/13851
Title Alternative ガーナ, ビルム・ダイヤモンド産地のアクァティア地域に分布する沖積層の重鉱物分析
FullText URL ESR_12_7.pdf
Author Samuel Dampare| Daniel Asiedu| Bruce Banoeng-yakubo| Shibata, Tsugio|
Abstract Heavy mineral analysis was carried out on Pliocene to Recent alluvial sediments from the Birim diamondiferous field od Ghana. The main objective of the study was to examine the mineral composition of heavy fractions in order to identify : (1) the heavy mineral assenblage that occur in the sediments, (2) particular diamond indicator minerals associated with the diamonds, and (3) the provenance of the alluvial sediments. The heavy minerals are essentially composed of staurolite, ilmenite and magnetite in varying proportions, with trace amounts of leucoxene, rutile, garnet and zircon. The heavy mineral assenblage and chemical composition of ilmenite and gamet suggest their derivation from phyllites and schists which reflect directly the composition of the basement rocks developed in the study area. The absence of diamond indicatior minerals such as apatite, pyrope garmet, chromian spinel, and picroilmenite in the heavy fraction is unlikely to be due to their destruction during intense weathering and/or diagenesis but rarher their non-occurrence in the area.
Keywords Heavy minerals provenance sediments diamonds Ghana
Publication Title Okayama University Earth Science Report
Published Date 2005-12-31
Volume volume12
Issue issue1
Start Page 7
End Page 14
ISSN 1340-7414
language 英語
Copyright Holders 岡山大学地球科学研究報告編集委員会
File Version publisher
NAID 120002310657
JaLCDOI 10.18926/ESR/13853
Title Alternative ガーナ,南部アシャンティ帯の原生代プリンスタウン・グラニトイド岩体に関する主成分地球化学
FullText URL ESR_12_15.pdf
Author Samuel Dampare| Shibata, Tsugio| Daniel Asiedu| Shiloh Osae|
Abstract The Pateoproterozoic metavolcanic rocks of the southern Ashanti greenstone belt of Ghana are intruded by three major suites of granitoids, locally called Prince's Town, Dixcove and Ketan plutons. The Prince's Town pluton is the largest intrusive body in the Axim area, and tends to separate the Axim volcanic branch from the Cape Three Points branch. The Pluton consists of granitic to dioritic rocks, which are generally massive but occasionally display alignment of ferromagnesian minerals. The rocks contain mainly plagioclase, K-feldspar, quartz, amphibole, biotite and opaques. The feldspars are mostly sericitized and saussuritized, and alteration of amphibole and biotite to epidote and chlorite is common. Accessory minerals include apatite, sphene and zircon. The geochemical data indicate that the rocks are tonalitic to granodioritic in composoition, metaluminous (ASI<1) and have I-type characteristics. The granitoids have the SiO2 content of 63-70% ; total iron, as Fe2 O3 of 3.10-5.80% ; (Na2O+K2O) content of 5.01-6.96% and Na2O/K2O ratios from 1.34 to 2.70 ; and are characterized by Mg# ranging from 53 to 48. The Fe*(=FeOtot/FeOtot+MgO) and modified alkali-lime index (MALI) of the rocks indicate that the Prince's Town pluton is dominantly magnesian and calcic in nature. Higher values in molar CaO/(MgO+FeOtot) coupled with low molar AI2O3/(MgO+FeOtot) may suggest their derivation from partial melting of metabasaltic to metatonalitic sourcc, with a possible contribution from metagreywacke, but preclude any contribution from metapelitic sources. The Birimaian metavolcanic rocks are the likely source material candidate for the rocks. CIPW norm calculations yielded a crystallization temperature of~650-685℃ and a pressure of 4-7kb for the rocks, suggesting a lower crustal souree. The Prince's Town plutonic rocks also show characteristice of plutons emplaced in a volcanic are tectonic setting environment. This observation is largely consistent with previous studies conducted on granitoids from other parts of the southern Ashanti greenstone belt c and the belt-type granitoids of Ghana as a whole.
Keywords Geochemistry tectonic setting granitoids Birimian Ghana
Publication Title Okayama University Earth Science Report
Published Date 2005-12-31
Volume volume12
Issue issue1
Start Page 15
End Page 30
ISSN 1340-7414
language 日本語
Copyright Holders 岡山大学地球科学研究報告編集委員会
File Version publisher
NAID 120002310649