資源地質
Online ISSN : 2185-4033
Print ISSN : 0918-2454
ISSN-L : 0918-2454
フィジー諸島共和国ナモシ地域の銅探鉱プロジェクト
森田 誠也田中 隆之長根 将格幕内 歩高畑 裕之古野 正憲高橋 武春安永 健太郎村上 浩康
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ジャーナル フリー

2002 年 52 巻 1 号 p. 19-35

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In the Namosi district located 30 km of Suva, the capital of Fiji, there are more than fifteen porphyry Cu-Au prospects (Waisoi; Wainabama; Waivaka Corridor). Of these, however, the Waivaka Corridor area in the center part of the Namosi distirict have not yet been investigated systematically.
We conducted geological, geochemical, and geophysical survey in the Waivaka Corridor area and evaluated the utility of principle features as a survey tool for exploration for porphyry Cu-Au deposits.
Summaries are as follows:
(1) Late Oligocene to Middle Miocene Wainimala formation and Middle Miocene to Pliocene Madurausucu formation distributed in the area, are mainly composed of andesite lava, tuff, and pyroclastic rocks. Late Miocene to Early Pliocene mineralized stocks and dikes, composed mainly of quartzdioritic and dioritic porphyries, are emplaced in those rocks.
(2) The Waivaka corridor, one of the biggest mineralized zones, is situated in an area which is enclosed by two major faults striking ENE-WSW. On the other hands, most quartz and sulfide veins crossing ENE-WSW systems occur along NE-SW to E-W fractures.
(3) The close spatial and temporal relationships between Cu-Au mineralization and hydrothermal alteration accompanied with hypabyssal dikes of Late Miocene age (5.3Ma) suggested a genetic relationship between calc-alkaline magmatism and Cu-Au mineralization.
(4) The Cu contents of soil distributed around dioritic porphyries have commonly more than 250 ppm. In particular, potentially mineralized zones with soil having Cu value of more than 500 ppm are characterized by the presence of the silicified rock and quartz veins.
(5) There are two geochemical features of soil which can be distinguished as a series of erosion. Soil anomalies with Cu-Au-Mo in and around outcropping dioritic porphyries suggest that the porphyry system has suffered erosion. In contrast, soil anomalies with Au-Mo indicate that top of the porphyry system may be still preserved. In both geochemical features, intermediate chargeability zones (14 to 20 mV/V) of IP geophysical survey that may reflect bornite and chalcopyrite, are superimposed beneath these anomalies and it is expected that the presence of promising porphyry Cu-Au system in the underground.
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