Geochemical characterisation of Pb‐Zn‐Cu and Ba mineralisation in the Lower Cretaceous sediments of the Coastal Basin, Gabon, Central Africa

Author(s): 
Mathieu Lacorde, Cherie Leeden and Regina Molloy
Date: 
Thursday, September 15, 2016
First presented: 
International Geological Congress, Johannesburg, September 2016
Type: 
Presentation
Category: 
Geology
Geochemistry

Over 150 sediment-hosted Pb-Zn-Cu and Ba occurrences are identified in Gabon along the contact between the Mesozoic to recent sedimentary Coastal Basin and the Archaean to Paleoproterozoic Kimezian basement of the West Congo Belt [1].  These occurrences were first described in the 1930s and systematically explored in the 1960s.  The Pb-Zn-Cu mineralisation is mostly comprised of disseminated coarse-grained galena and finer-grained sphalerite which is hosted in clastic sediments of the Upper Cocobeach Formation (Lower Cretaceous [2]).  The contact of the sediments with the basement follows kilometre-long embayments and is frequently marked by metre-large blocks of barite locally associated with pyrite and galena.

Exploration works in the 1960s and later in the 1970s focused on the high-grade Pb-Zn-Cu Kroussou prospect along the Dikaki embayment. The French Bureau de Recherches Géologiques et Minières conducted comprehensive exploration, targeting shallow and high-grade lead deposits. Detailed mapping, rock chip sampling, soil geochemistry, geophysics and Winkie drilling were conducted. The mineralisation style was not fully understood and both Mississippi Valley Type [1] (hosted in coarse-grained arkose i.e. Laisvall sub-type) and red-bed style [3] were proposed.

As part of a renewed interest in the area, two mapping projects were undertaken in 2014-2015 and 60 rock chip samples collected and assayed for major and trace elements, primarily in the Kroussou prospect.  A Principal Component Analysis conducted on 39 sediment samples highlighted two distinct metal associations.  The first Pb-Cu-Ag-Cr-W association is related to a sandstone-hosted, Pb-dominant mineralisation, more akin to MVT of the Laisvall type.  A Zn-Mo-As-Co association represents a carbonate-hosted, Zn-dominant mineralisation.  Both mineralisation types occur in close proximity in the field, suggesting a continuum between a Pb-Cu and a Zn end-members with the relative proportion of metals being controlled by the amount of carbonates in the host rocks. 

A geochemical characterisation of 12 samples of barite and barite-rich sandstone focused on the Rare Earth Element (REE) content.  The samples show a variable content of REE’s ranging from 7 – 118 ppm.  The chondrite-normalised REE distribution presents a similar pattern across the samples, with an enrichment in light REE’s and a slight enrichment to a depletion in heavy REE’s.  The chondrite-normalised REE patterns of the clastic sediments show a more pronounced enrichment than those of the baritic samples, particularly with respect to LREE’s.  The REE patterns of barite-bearing rocks were compared to published data on barite from hydrothermal and marine origins and are consistent with a low-temperature hydrothermal source [4].
 

References:

[1] Ministry of Mines of Gabon (1971) Mineral Plan, Partie Principale

[2] Arnould A. (1966) Synthèse Plomb Socle Crétacé Gabon, BRGM Lib. 66 A2

[3] Boujo A. and Heinty C. (1977) Recherche de plomb dans les couches détritiques de la base du Crétacé de Kroussou (Gabon), Campagne de sondages 1977, BRGM 77 Lib. 005

[4] Hein J.R. et al. (2007) Barite-forming environment along a rifted continental margin, Southern California Borderland, Deep-Sea Research II 54 1327-1349

 

Feature Author

Mathieu Lacorde

Mathieu Lacorde is a geologist with over six years of experience in mineral exploration. He conducted regional to prospect-scale geological and structural mapping for iron ore, base and precious metals in remote places of Africa (Gabon, Sierra Leone, Tanzania, Guinea and Mauritania), SE Asia (Indonesia, Malaysia) and South America. Mathieu was also involved in expert-driven and data-driven targeting studies in Australia and Ghana (uranium, gold and base metals). On several projects, Mathieu was responsible for drilling supervision and lithological/structural drill core logging (Guinea, Queensland, and Malaysia).

Senior Consultant (Geology)
MSc (Geosciences), MAIG
SRK Perth
SRK Turkey