The principal exploration target within the Mt Piper exploration license area (ELA) is disseminated, sulphidic, structurally- controlled quartz-poor stockwork and veined Au-As-Sb mineralisation, similar to that successfully mined at the nearby Fosterville, Nagambie and Costerfield gold mining centres.
Reported recent developments at Fosterville, in particular, are very encouraging for exploration for this style of gold mineralisation. At Fosterville, plunge-persistent high-grade gold mineralization has been reported at depths greater than 800m), adding very substantially to expected mine life.
General Geological Background
The Mt Piper ELA area is located within the Melbourne 1:250,000 and the Woodend and Yea 1:100,000 geological sheet areas (which have not been mapped at that scale).
The oldest known rocks in the area are Cambrian metabasites and metasedimentary rocks, poorly exposed in the hanging wall of the regional-scale Mt William thrust fault. The Cambrian rocks have only a restricted areal extent and are conformably overlain in the west by Ordovician greywacke-turbidite and slate of lower greenschist facies grade
To the east of the Mt William Fault, the ELA area is dominated by Silurian to Early Devonian sedimentary rocks, mostly pelitic with subordinate sandstone, which were affected by two main folding events, and which are of lower greenschist, or even lower, metamorphic grade.
The Cambrian to Early Devonian sequences, and the Mt William Fault, have been intruded and cut by Late Devonian granite. A regionally-important phase of Au-As-Sb mineralisation of central Victoria (Fosterville-style) appears to be associated with later and minor post-granite deformation and the re-activation of the various faults in the area.
An earlier phase of simple, “nuggety” Au-As-quartz vein-hosted mineralisation was probably emplaced around the time of Silurian deformation of these rocks or during a later Early Devonian mineralising event.
Torrens’ exploration target is disseminated, sulphidic, quartz-poor stockwork bodies that contain gold-arsenic-antimony mineralisation, like those of the Fosterville and Nagambie mines further to the north-west and the north-east. This style of mineralisation is considered to be represented by the Mt Piper Au-Sb mineralisation and other prospects within or near the area under application.
The fine-grained nature of primary gold and paucity of quartz veins and associated alluvial gold occurrences is a feature of some deposits of this type, combined with the antiquity and uncertain quality of past regional exploration and different mining economics today, suggests that potential deposits might have been completely missed by previous explorers in the 19th and 20th centuries (e.g. as was discovered and mined at Nagambie in the 1980’s).
Historical gold mining or prospecting activity has been identified or recorded at several sites within the area. Outside the area and excluded from exploration within the Mt Piper Conservation Reserve is the previously-mined Mt Piper Au-Sb mineralisation, which is hosted by an intensely brecciated porphyry intrusive. These occurrences, and accumulated geochemical data, are regarded as prima facie evidence that Au-Sb mineralising processes having been active in the target region.
Previous explorer Oroya conducted systematic geochemical exploration, which identified several geochemically anomalous zones, some associated with historical gold workings. Due to corporate restructuring, Oroya did not complete the follow-up work recommended by Dr. Martin Hughes, thus creating a significant commercial opportunity for Torrens to acquire and explore the priority targets designated by Hughes and to generate new targets.
Figure 2. (in yellow) Principal anomalous geochemical drainage anomaly source areas defined by Hughes.
Torrens Mining’s exploration license EL5455 covers the northern, strongly geochemically anomalous, extension of the Buldah Shear Zone, a structural splay off the regional-scale Combienbar Fault in East Gippsland, Victoria. Along the shear zone, there are number of reported gold and base metal mineralisation occurrences.
The Company’s objective is to discover one or more standalone gold and/or base metal resources. The Project, which covers 23km² in the headwaters of Buldah Creek, was granted to Terrace Mining on 21 October 2013, for 5 years. Itis located within state forest, much of which has previously been harvested for timber. The area is centred 40km north of Cann River township and adjacent to the small Buldah farming district. The Eastern Gas Pipeline and an all-weather access road passes through the eastern sector of the Project area. The area is also readily accessed by the sealed Cann River Highway thence by numerous roads and tracks.
Between 2006 and 2012, the Buldah Project area was explored by Oroya Mining Limited, which focused on the mineral potential along the regional-scale Combienbar Fault zone. Initial drainage geochemical sampling along the Buldah Shear Zone defined large copper, zinc and gold anomalous zones, along with other anomalous metals.
Exploration was undertaken in 2009 by Independence Group NL, as part of a joint venture with Oroya, and included geochemical grid soil sampling over an area of approximately 7km2in two phases, the first at a sampling density of 200m by 200m, and the second being an infill phase at a sampling density of 100m by 100m, in the headwaters of Buldah Creek.
Zones of gold, lead, zinc, silver, copper, arsenic, molybdenum, bismuth and antimony anomalism were defined, including a gossanous zone of quartz veined and brecciated metasediments, from which rock chip assays of 0.3 g/t Au and up to 1.77% Pb were reported. These anomalies are spatially associated with the regional-scale, sulphide-mineralized and quartz-veined Buldah Shear Zone.
Independence Group, however, withdrew from the Project without conducting any drilling to test the geochemical anomalism delineated by the soil, rock chip and stream sediment sampling. Earlier exploration by other companies extended only into the southern-most part of EL5455. This exploration was largely driven by 1890s reports of gold mineralisation in the Buldah Shear Zone. The Granite Creek prospect, just to the south of EL5455, was drill tested in the 1980s. A silicified, strongly pyritic (5-10% pyrite over 25 metres width) shear zone was intersected, however with only trace gold reported.
Figure 1 - Location Plan EL5455
Pink • Devonian granite, purple = Devonian ‘red-bed’ sediments,
Blue • Ordovician sedimentary rocks, solid and dashed black lines represent faults.
The Project area is located within the Mallacoota 1:250,000 and the Craigie 1:100,000 geological sheet areas.
The oldest rocks in the area are Ordovician Pinnak Sandstones; a thick sequence of turbiditic sandstones, mudstones and black shales which were tightly-folded and deformed during Silurian-Devonian orogenic deformation. During the Lower Devonian, the Pinnak Sandstones were intruded by granitic rocks of the regionally extensive Bega Batholith, which are marginally exposed in the EL area. Later, during the Upper Devonian, the older rocks were unconformably overlain by terrestrial coarse fluvial red-bed͛ sediments of the Combyingbar Formation.
Regional deformation in the Buldah area culminated during the Carboniferous with the regional development of the distinctively-shaped en-echelon, fault and shear-bounded Bemm River, Combienbar and Buldah Synclines. Gold and base metal mineralisation is thought to be related to reactivation of faults along the Buldah Shear Zone and other structures during the late Devonian and Carboniferous.
The field evidence points to a high certainty that the Buldah drainage and soil geochemical anomalies are directly related to the Buldah Shear, or splays off the main shear/fault. The exploration target is economically significant tonnages of sulphidic, structurally-controlled, polymetallic mineralisation. The strike length of known geochemical anomalism within EL5455 is at least 3.5km, which is considered sufficient to accommodate a body of mineralisation of economic proportions. Torrens is continuing its exploration initiatives at Buldah.