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Mineralogy, geochemistry and emplacement of the Conakry Igneous Complex, Guinea: implications for the Ni–Cu–PGE mineralization

Published online by Cambridge University Press:  15 April 2018

Thierry Augé ,
Éric Gloaguen ,
Matthieu Chevillard  and
Laurent Bailly
Thierry Augé
Affiliation:
Bureau de Recherches Géologiques et Minières (BRGM), BP 36009, 45060 Orléans cedex 2, France
Éric Gloaguen*
Affiliation:
Bureau de Recherches Géologiques et Minières (BRGM), BP 36009, 45060 Orléans cedex 2, France
Matthieu Chevillard
Affiliation:
Bureau de Recherches Géologiques et Minières (BRGM), BP 36009, 45060 Orléans cedex 2, France
Laurent Bailly
Affiliation:
Bureau de Recherches Géologiques et Minières (BRGM), BP 36009, 45060 Orléans cedex 2, France
*
*E-mail: e.gloaguen@brgm.fr
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Abstract

The Conakry Igneous Complex is a mafic-ultramafic intrusion emplaced contemporaneously with the opening of the Atlantic, forming a complex, 55 km x 5 km dyke-like body within which three main episodes of injection have been recognized, characterized by a lack of mineral layering. Unit 1 consists of dunite and related facies, Unit 2 of wehrlite and pyroxene peridotite and Unit 3 corresponds to various gabbro facies. Units 1 and 2 constitute the Kaloum Peninsula; Unit 3 is its NW extension, forming the 1010 m high Mount Kakoulima. Unit 3 intrudes the two previous units and corresponds to a tholeiitic liquid that crystallized in an almost closed system, and thus exhibits a strong differentiation trend, in contrast to Units 1 and 2. Mineral compositions suggest the existence of a deeper magma chamber where a first stage of differentiation occurred.

Disseminated base-metal sulfides (BMS) are present in all units of the complex and earlier descriptions have mentioned a “massive sulfide layer” with 2 to 4 g/t PGE. Platinum-group minerals (PGM) are almost everywhere included in or attached to composite Ni–Fe–Cu sulfides. Most PGM grains form complex associations resulting either from exsolution or alteration. It is characteristic of the Conakry Igneous Complex PGM, described here for the first time, to be dominated by (Pd,Pt)(Te,Bi) minerals with rare Pd,Sn and Pd,Pb compositions and an absence of Pt,Pd sulfides and Pt,Pd antimonides.

The constant association of the PGM with the BMS shows that the magmatic sulfide liquid acts as an efficient collector of PGE. In such a dynamic environment, the process leading to the formation of massive sulfides must be sought in the accumulation of sulfides in the conduit following host-rock assimilation. Accordingly, considering the multiple injection processes that characterize the whole intrusion, the potential for discovering additional Ni–Cu–PGE mineralization in the Conakry Igneous Complex remains high.

Keywords

Conakry Igneous ComplexGuineaplatinum-group elements
Type
Article
Information
Mineralogical Magazine , Volume 82 , Special Issue 3: Special Issue dedicated to the work and memory of Professor Hazel M. Prichard (1954–2017) , June 2018 , pp. 593 - 624
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

This paper is published as part of a thematic set in memory of Professor Hazel M. Prichard

Associate Editor: John Bowles

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