The continuity challenge: old gold in West African deformed terranes

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SRK News | Issue 57Exploration Geology: Keys to success

Wayne Barnett, Principal Consultant


Exploration in highly deformed orogenic environments is challenging because of the potential for extensive deformation of the mineralised target zone. One of the key factors impacting the continuity of economic mineralisation is timing of mineralisation relative to deformation. If gold mineralisation is relatively old with respect to the deformation, then the structural deformation patterns overprinting the gold distribution need to be well understood for greenfield or brownfields exploration, and resource characterisation and exploitation on the mines.

Project experience by SRK in Liberia and the Paleoproterozoic Birimian terranes of Burkina Faso between 2010 and 2018 included at least 5 projects with gold mineralisation that clearly predates at least two phases of penetrative, ductile or brittle-ductile deformation. The gold mineralisation on all these projects is closely associated with quartz vein systems and sulphides (such as pyrite, pyrrhotite, chalcopyrite, arsenopyrite), and typically associated with magnetite, sericite alteration and sometimes more general silicification. These systems were most likely originally formed as orogenic gold deposits.

The distribution of gold mineralisation on these projects has been structurally investigated on all scales, from (i) regional airborne and ground magnetics, and soil sampling, to (ii) mine and outcrop scale mapping and grade control, and through (iii) drill core assay, mineralogical and textural observations.

The early rock structural fabrics, the gold mineralisation and the associated minerals including quartz veins, are isoclinally deformed (transposed) by an early deformation phase associated with a penetrative foliation. The patterns observed at all scales include zones of isoclinal folding, pinching and swelling, and stretching along high strain zones and fold axes. These deposits have then subsequently been folded by a second and often third phase of deformation, creating a complex interference pattern.

Only when the control on mineralisation is identified can the deformed and often simplified structural geometry be used for further targeting. Lineament-type interpretation is useless. Rather, structural investigations based on geophysics, mapping and core logging should be applied without exception to define the deformation patterns and gold trends. Understanding the structural patterns in combination with soil sampling allows exploration targets to be accurately defined.

In contrast to the “old gold”, in at least one of these West African projects, there is also a later phase of gold mineralisation associated with more typical structural targets, such as dilation zones and complex fault intersection areas favourable for fluid permeability. Such “late gold” is well documented at other locations, notably along the Birimian fault systems in Ghana at Obuasi Mine. 

Wayne Barnett: wbarnett@srk.com

 

SRK Latin America