Summary:
Mineralization
At Hermes the Hawkeye, Trapper, Klinger, Winchester & Blake deposits are sub-parallel, northeast trending, mineralised zones. Mineralisation at all deposits is typically associated with quartz veins at or near the sheared contact of mafic amphibolite footwall and hangingwall quartz-biotite-sericite schists. Graphitic schist occurs to a minor extent on the hangingwall and footwall sides of the mineralisation at Trapper, Klinger and Blake. The base of strong oxidation varies from 20-30 m vertically below surface at Hawkeye & Winchester, and from 25-45 m at Trapper, Klinger & Blake. Transition to fresh rock occurs at approximately 30-50 m and 40-65 m, respectively.
In general, the mineralised quartz veins, foliation and relict bedding are steeply NW-dipping to sub-vertical in both the Hawkeye and Trapper deposits and high-grade shoots are interpreted to plunge shallowly to the north within the mineralised plane.
Hermes South
Gold mineralisation occurs within a predominantly metasedimentary sequence of the Proterozoic Peak Hill Schist and mafic units. The Peak Hill Schist comprises quartz-sericite schist and quartz-muscovite schist and is located on the south-western extreme of the Marymia Inlier.
Deposit Type
In the Hermes Gold Mine and BBJV Projects there are two broad mineralisation styles (Outhwaite, 2013) referred to here as the Peak Hill Type and the Bryah Type.
The Peak Hill Type gold deposits are hosted in rocks that are generally highly deformed (four or more fold events) and metamorphosed (up to amphibolite facies), generally represented by the Peak Hill Schist Formation. Mineralisation is early in the paragenesis, (syn- to post-D1 isoclinal folding) with folded mineralisation commonly observed. Previously mined examples include Peak Hill Main/Five-Ways, Mt Pleasant, Jubilee, Wilgeena and St Crispin. The Hermes deposits may be examples of mineralisation that belongs in this category.
The local controls on this deposit class are difficult to identify, because of the effects of strong, postmineralisation deformation, and probable remobilisation of Au. Despite this problem, some similarities between the deposits have been recognised:
• Located on a major structural and metamorphic gradient. The Peak Hill camp, Hermes, Wilgeena and Central Bore all lie around a significant gradient from high-strain, highly metamorphosed rocks at the core of uplifted basement blocks, outwards to lower-strain, less metamorphosed rocks. This gradient is a transitional structural zone up to a few kilometres wide. This zone was probably a major fluid conduit during early deformation, and hence a first-order corridor for Au exploration;
• Located at or around mafic-sedimentary contacts. With the exception of Central Bore, which is on a granite-sedimentary contact, all of the mentioned Peak Hill-style occurrences are closely associated with mafic rocks within dominantly sedimentary sequences. The mafic rocks are typically amphibolite, after dolerite or basalt, but high-Mg to ultramafic rocks are noted at Hermes, Hermes South, and possibly the Peak Hill camp (high Cr and Mg);
• Early structures. A likely early age on the controlling structural corridor has been recognised, based on the repetition of stratigraphy, which was then folded together during the identifiable fold events. The identification of early, controlling structures has proven difficult, due to the overprint of subsequent deformation events;
• Strong plunge controls. Due to the multiple folding events these deposits have strong plunge controls, meaning they can have small surface footprints – especially when considering their general lack of pathfinder anomalism (see below). Exploration must take this fact into account;
• Limited geochemical signature. Wilgeena, Central Bore, St Crispin, Pelorus and possibly Peak Hill have a limited range of pathfinder elements associated with the Au mineralisation – scattered W, Bi, Pb and Zn, but very little else. Hermes is a clear exception however, as limited multi-element analysis has shown that this system has a strongly developed pathfinder association, similar to that found in typical Archean Au systems: As, Sb, Mo, W, Bi, etc.