1. Introduction

4. 3-Dimensional Geological Mapping

Structural and Lithological Controls on Gold Mineralisation at the Wassa Mine, Ghana Rebecca Strachana, David Holwella, Christopher Bonsonb, Mitchel Waselc

aDepartment of Geology, University of Leicester, University Road, Leicester, LE1 7RH, rcs26@student.le.ac.uk bSRK Consulting, 5th Floor Churchill House, 17 Churchill Way, Cardiff, CF10 2HH , cGolden Star Resources, Accra, Ghana

6. References

2. Regional Structure

3. Aims

Figure 1[1] - Ghana’s Ashanti Greenstone belt is host to a number of Paleoproterozoic hydrothermal gold deposits, including Golden Star Resources’ structurally complex Wassa Mine[2].

Figure 3 - Block models of gold assay grade >2g/t looking NW of section A (Figure 3a) and section C (Figure 3b). Gold appears to be distributed mostly within the hinges of the F3 folds.

Figure 2 – Map view of the assay data taken from Leapfrog. The red represents >2g/t and blue <0.1g/t. Cross-section lines are shown.

Gold assay data analysed in Leapfrog 3D illustrates the large F4 fold present at Wassa. The assay data also highlights F3 folds refolded around F4, with higher grades concentrating along the extenuated limbs and hinges of F3.

Using this data 3D models were produced to show the areas of localisation of the gold mineralisation (Figure 3). In addition to assay data, lithology data was used to create a geological map and cross-sections (Figure 4).

Figure 4 – a) A geological map of Wassa with lines of cross-section for Figure 3 a & b and Figure 4b, b) a geological cross-section of section B.

The main aims of this study are to: • Use Leapfrog Mining to analyse distribution of

gold grade in the assay data. • Produce an updated lithological map and cross-

sections. • Analyse samples to clarify geochemistry and

petrology of the lithologies, in particular the Banded Magnetic Unit.

• Create a paragenesis of temporal relationships between mineralisation and folding.

5. Mineralisation Styles

Structurally, the Ashanti Belt is extremely complex having undergone five deformation events during the Eoeburnean and Eburnean Orogenies, 2187-1980 Ma, and a later D6 event ~600Ma[2][3]: • D1 – N-S shortening and initial mineralisation • D2 – Extensional phase • D3 – NW-SE shortening and remobilisation of

mineralisation • D4 – NNW-SSE shortening • D5 – Vertical shortening • D6 – NE-SW shortening[2]

a)

b)

a) b)

[1] Castle Peak Mining, 2012, http://www.castlepeakmining.com/s/Akorade_Geology.asp. [2] Perrouty, S., Ailleres, L., Jessell, M.W., Baratoux, L., Bourassa, Y, 2012, , Revised Eburnean geodynamic evolution of the gold-rich southern Ashanti Belt, Ghana, with new field and geophysical evidence of pre-Tarkwaian deformations; Precambrian Research, v.204-205, p.12-39 [3] Bourassa, Y., 2014, The Wassa gold mine: An Eoeburnean deposit within the Ashanti belt; Prospectors and Developers Association of Canada (PDAC) Convention. (Powerpoint)

Style 4 Style 5 Style 6 Style 3 Style 2 Style 1 a) b) Pyrite following highly magnetic stripe in Banded Magnetic Unit

Late mineralisation phase – pyrite over-printing previous mineralisation

Elongated pyrite deformed by D1 following folded S1 cleavage

Wallrock hosted pyrite contained within a fold hinge

Undeformed pyrite associated with carbonate alteration and veining

Pyrite within the mafic bands of phyllite, closely following folding

Figure 5 – six different examples of mineralisation styles that occur at Wassa. Current work aims to link the different styles of mineralisation seen.

Figure 6 – SEM images taken from a mineralised sampled of graphitic phyllite. a) Gold mineralisation seen to be contained within a gold-sliver telluride. Also present is the lead telluride, altaite, and chalcopyrite infilling cavities within the pyrite. b) Gold mineralisation as electrum in pyrite, with chalcopyrite infilling the fractures within the pyrite. This suggests chalcopyrite formed syn or post pyrite mineralisation.

Chalcopyrite

Gold Gold-silver telluride

Altaite

Chalcopyrite

Pyrite

Pyrite