Summary:
The Fortnum Gold Operations can be subdivided into three major geological domains:
• Fortnum;
• Horseshoe - Cassidy;
• Peak Hill.
Fortnum
The project sits within the Fortnum Wedge, which hosts the Fortnum mining centre, and surrounding areas in the northwestern corner of the Bryah Basin. The Fortnum Wedge is a fault-bounded package of volcaniclastic rocks of the Narracoota Formation, bounded to the north by the Fortnum Fault and to the east and west by the Ravelstone Formation immediately around the Fortnum mining centre. To the north and west, mineral prospects are hosted by Labouchere Formation units.
There are two main styles of gold mineralisation identified within the Fortnum Project, and particularly the Fortnum Wedge; mafic-jasperoid associated deposits (Yarlarweelor and Tom’s), and structurally controlled vein stockworks associated with tuffaceous sediments and siltstone units bound by competent crystal tuff units (Starlight Group, Callie’s and Eldorado).
Other major deposits that occur in the project area outside of the Wedge share similar structural controls though host lithologies are different. Chert hosted mineralisation at Labouchere shares many features with the jasperoid associated style, and the vein stockworks hosted in siltstones, and bounded by sandstone-conglomerate units at Nathan’s are broadly similar to the Starlight Groups style.
Horseshoe – Cassidy
The Horseshoe group of deposits share several geological features with the Fortnum Wedge: mineralisation is hosted within a fault bounded package of mafic volcanic and volcaniclastic rocks of the Narracoota Formation, bounded by the Ravelstone Formation.
The main zone of mineralisation at Horseshoe – Cassidy is developed within a horizon of extremely silica altered magnesian basalt. The silicification appears to predate mineralisation, and represents a broad zone of brecciation that has undergone intense silica flooding. Core from the margins of this zone show relict, partly replaced breccia fragments, cross-cut by mineralisation associated veining. Later potassic alteration related to gold mineralisation is spatially associated with strong vein stock-works that are confined to the altered mafic. Alteration consists of two types; stockwork proximal silica-carbonate-fuchsite-haematite-pyrite and distal silica-haematite-carbonate+/- chlorite.
Peak Hill
The Peak Hill covers a marginal part of a Protozoic orogenic belt (Capricorn Oregon) that developed around the northern edge of the Yilgarn craton. Rocks of the Capricorn Orogen separate the Archean rocks of the Yilgarn Craton to the south from the Pilbara Craton to the north.
The Peak Hill district represents remnants of a Proterozoic fold belt comprising completely deformed trough and shelf sediments and mafic / ultramafic volcanics, which in part are moderately metamorphosed.
The Peak Hill area is dominated by the Early Proterozoic Peak Hill Schist, a highly deformed and metamorphosed sequence of uncertain origin. The Peak Hill Schist is locally broken down into three stratigraphic units comprising:
• The Intermediate/Footwall Sequence;
• The Mine Sequence;
• The Hangingwall Sequence.
Deposit Types
The gold deposits at FGO are consistent with the Proterozoic (copper) gold deposit model.
There are three main styles of gold mineralisation identified within the Fortnum Project, and particularly the Fortnum Wedge; mafic-jasperoid associated deposits (Yarlarweelor and Tom’s), structurally controlled vein stockworks associated with tuffaceous sediments and siltstone units bound by competent crystal tuff units (Starlight Group, Callie’s, Eldorado, Horseshoe – Cassidy – Pod and Fiveways), and secondary gold mineralisation found in lateritic profiles developed over bedrock mineralisation.
Dimensions
• The Yarlarweelor mineral resource extends over 1,400m in strike length, 570m in lateral extent and 190m in depth;
• The Tom's and Sam's mineral resource extends over 650m in strike length, 400m in lateral extent and 130m in depth;
• The Eldorado mineral resource extends over 240m in strike length, 100m in lateral extent and 100m in depth.