Regional Geology

Regional location of tenements
Regional location of tenements

Bedrock within the area covered by EL 6874 is dominated by Middle and Late Ordovician meta-sediments and the Carboniferous Burraga Granite. The stratigraphy of EL6463 is poorly understood with different workers providing substantially different interpretations, especially with respect to the relationship of the Burraga sequence to the rest of the stratigraphy. The following descriptions of the rocks with EL6463 is a summary of the ‘consensus’ stratigraphy.

The oldest rocks in the tenement are the middle Ordovician Adaminaby Group which is comprised mainly of variably deformed quartz sandstone and carbonaceous shale. The Adaminaby Group underlies the south eastern parts of the licence area, east of the ‘Lloyds syncline’.

Conformably overlying the Adaminaby Group is the Triangle Formation of the middle Ordovician Kenilworth Group. The Triangle Formation consists of mafic volcaniclastic sandstone, meta-basalt, slate, phyllite, schist, siliceous carbonaceous slate, chert, quartzite and sandstone. The Triangle Group, is host to the Lucky Draw and Hackney’s Creek gold deposits in the north eastern part of EL6463.

The Triangle Formation rocks are unconformably overlain by the Middle to Late Silurian Campbells Formation of the Mumbil Group. Typically, the formation comprises siltstones overlain by interbedded slate and fine to coarse grained feldspathic meta-sandstone. The Campbells Formation is broadly correlatable with the Anson Formation; host to the McPhillamys Hill gold deposit in the Blayney-Orange district to the northwest.

Barraga district projects
Barraga district projects

In faulted contact above the Triangle Formation are sediments of the Early Devonian Crudine Group. The Crudine Group comprises the Dunchurch Formation (feldspathic quartz sandstone with minor slate, ashstone and dacite) and the Buckburraga Slate (laminated silty slate).

Within the ‘Lloyds syncline’ is a sequence of strongly deformed rocks with complex structural and stratigraphic relationships. This sequence comprises a basal slate mapped as the Buckburraga slate overlain by the Excelsior Porphyry. Petrographic analysis of the Excelsior Porphyry shows that is in fact a highly altered volcanic tuff. Above the Excelsior Porphyry is the Hanrahan’s Agglomerate which is actually a polymict breccia of tectonic origin. The Hanrahan’s Agglomerate includes clasts of limestone, amorphous silica and Excelsior Porphyry. Most of the Burraga copper mineralisation occurs in the Hanrahan’s Agglomerate. Fine grained schist, phyllite and minor limestone of the Lovett’s Formation occurs above the Hanrahan’s Agglomerate.

The age and stratigraphic relationship of the ‘Lloyds syncline’ sequence to rocks outside the ‘syncline’ have not been resolved. The sequence is variably interpreted as part of the Late Ordovician Rockley Volcanics or as part of the Silurian Mumbil Group.

In the northeast of EL 6463 is the western margin of the Carboniferous Burraga Granite that has intruded rocks of the Adaminaby Group and Triangle Formation. The Burraga Granite is described as a medium to coarse-grained leucocratic biotite granodiorite that comprises two phases; a massive medium-grained two mica I-type granodiorite and a medium-grained garnet-muscovite granodiorite that has S-type affinities. The garnet-muscovite phase occurs in the northwest part of the pluton (within EL 6463) adjacent to the Lucky Draw gold deposit. Intruded Ordovician Adaminaby Group sediments have been contact metamorphosed to micaceous quartzite and pelitic quartz-mica schists containing quartz-albite-biotite + cordierite and quartz-biotite-muscovite-albite-andalusite-cordierite assemblages. Intruded Ordovician Triangle Formation sediments have been contact metamorphosed to quartz-feldspar-biotite schist and tremolite-chlorite schist (Rockley Volcanics?). The contact metamorphic aureole associated with the intrusion of the Burraga Granite is reported to be 75-100 metres wide.

Minor Quaternary alluvium and gravels are located adjacent to streams in the central part of the tenement where these streams drain part of the Burraga Granite.

The tenement area has undergone a complex structural and metamorphic history.

Government mapping on the Oberon 1:100,000 geological map sheet shows that the Godolphin Fault, (a significant control on the recently discovered 2.96 Moz McPhillamys gold deposit) extends through EL6463 where it juxtaposes Silurian Campbells Formation (to the west) and Ordovician Triangle Formation.

A recent structural interpretation of the Burraga area suggests that the ‘Lloyds syncline’ is not a syncline, but a NNE trending structural corridor (Lloyds corridor) and that the synformal shape suggested by the outcrop pattern and magnetic signature of the Hanrahan’s agglomerate (breccia) is in fact two separate shear / alteration zones that intersect in the Burraga South area giving rise to a synform like outcrop pattern. There is no evidence for a synformal structure in the Lloyds corridor. There is good evidence for polyphase folding and faulting within the Lloyds corridor.