Summary:
The Mandilla Gold Project is situated in the northern Widgiemooltha greenstone belt, approximately 70 kilometres south of the significant mining centre of Kalgoorlie, Western Australia.
Mineralisation is related to north-south trending major D2 thrust faults known as the “Spargoville Trend”. The Spargoville Trend contains four linear belts of mafic to ultramafic lithologies (the Coolgardie Group) with intervening felsic rocks (the Black Flag Group) forming a D1 anticline modified and repeated by intense D2 faulting and shearing. Flanking the Spargoville Trend to the east, a D2 Shear (possibly the Karramindie Shear) appears to host the Mandilla mineralisation along the western flank of the Emu Rocks Granite, which has intruded the felsic volcanoclastic sedimentary rocks of the Black Flag Group. This shear can be traced across the region, with a number of deflections present.
The Project comprises an east and west zone, both of which are dominated by supergene mineralisation between 20 and 50 m depth below surface. Only the east zone shows any significant evidence of primary mineralisation, generally within coarse granular felsic rocks likely to be part of the granite outcropping to the east. Minor primary mineralisation occurs in sediments.
The Mandilla prospect is located along the SE margin of M15/96 extending into the western edge of M15/633. It comprises an east and west zone, both of which are dominated by supergene mineralisation between 20 and 50 metres depth below surface. Only the east zone shows any significant evidence of primary mineralisation, generally within coarse granular felsic rocks likely to be part of the granite outcropping to the east. Minor primary mineralisation occurs in sediments.
Gold mineralisation appears as a series of narrow, high grade quartz veins with relatively common visible gold and grades over the width of the vein of up to several hundreds of grams per tonne. Surrounding these veins are lower grade alteration haloes. In places, these haloes can coalesce to form quite thick zones of lower grades (ten’s of metres). The mineralisation manifests itself as large zones of lower grade mineralisation from ~0.5 – 1.5 g/t Au with occasional high grades of +5 g/t Au over one or two metres.
Distal alteration comprises pale orange/red matrix porphyritic syenite. The alteration style is characterised by good textural preservation with the colouration likely to be hematite dusting. Observable minerals are mainly feldspar phenocrysts with 5% dark green secondary amphibole clusters, and possibly actinolite also present. Quartz veining is generally absent in this alteration style; however, quartz veining has been noted.
Another example of distal alteration comprises dark grey-green moderate to strongly texturally destructive alteration, comprising at least one amphibole, epidote-clinozoisite, chlorite and magnetite. The alteration resembles dark-coloured fracture-controlled alteration seen elsewhere at Mandilla. Diopside was also noted. This alteration appears zoned around the gold mineralised segment of the hole, but there is ample evidence that quartz veining and associated gold-related alteration overprints what is probably an earlier high-temperature calc-silicate alteration phase (possibly fault/shear zone). Drill orientation appears to be parallel to the crosscutting structures, hence a number of faults run at a high angle to the core axis.
The gold related alteration shows a degree of diversity which reflects variation in vein density and proximity to possible structures in the core of mineralised zones. More intense alteration is white to pale grey, locally with a pale brown or pink tinge in vein haloes, and probably is dominated by silicaalbite. Textural destruction is moderate to strong with replacement mineralisation of black biotite or hornblende that is also disseminated through the altered rock. Dark fractures containing biotite or hornblende sub-parallel to veining are also regularly distributed through the strongly altered zone. An increase in pyrite content is observed mainly close to veins or as blebby inclusions throughout the altered wall rock.
Vein density increases from one per metre to two to three per metre in the core of the mineralised zones, with individual veins up to 15cm thick, but typically one to then centimetres in thickness. Visible gold is commonly observed within and on the margin of quartz veins, and rarely observed in wall rock. Individual grains of gold, or small aggregates of grains are observed and can be coarse grained over one millimetre in size.
In some areas, such as in MDRCD151, the feldspar phenocrysts are albitised, standing out as white in a darker matrix.
Zones of intense, thin (1-10 mm scale) quartz fractures are locally developed within strongly altered zones. Oriented core indicates the fractures dip moderately to the SW, which appears to mimic the gross orientation of the gold mineralisation envelopes at Mandilla prospect. Such fracture zones may represent brittle structures which exert some control on the distribution of the gold mineralisation.
Most mineralised quartz veins are sub-horizontal extension veins (dip up to about 20° from horizontal) and form due to fluid overpressure. Extension vein distribution is probably controlled by multiple small-scale structures within the syenite but could extend ten’s of metres away from the structures, particularly into the hanging wall. It is likely small-scale structures (plus extensional veins) form an interlinked fault mesh pattern for allow for vertical fluid flow.
In addition to the granite-hosted mineralisation, a paleochannel situated above the granite/sediment contact contains significant gold mineralisation. The channel is about two kilometres in length, up to 50 metres wide, but only a few metres thick. Gold is contained within quartz sands and gravels, although is not consistently distributed throughout the paleochannel. An 800-metre stretch of the paleochannel was mined by Astral in 2006 and 2007, with gold production totalling 4,005 ounces, at a grade of almost 15 g/t Au (Fyfe, 2007).
The Project contains four discrete deposits that are separated spatially and with differing geological characteristics:
1. Theia is the main deposit and contains 81% of the gold ounces. The deposit is presently known to host gold mineralisation over a strike length of approximately 1,600 metres, is about 150 to 250 metres wide and extends to approximately 370 metres below the surface. The overall mineralisation at Theia strikes to the north-west at about 330°, with a sub-vertical dip. However, extensive structural logging from diamond core drilling of the quartz veins within the mineralised zones shows that majority dip gently (20° to 30°) towards SE to SSE (130° to 160°).
2. The Iris deposit contains about 9% of the gold ounces of the Project and has a similar trend and orientation as Theia. The mineralisation is currently known to extends over a strike length of approximately 600 metres, is about 200 metres wide and extends to approximately 200 metres below the surface.
3. Eos is at the southern boundary of the project and comprises paleochannel mineralisation that is currently interpreted to extend over a strike length of approximately 300 metres, is about 75 metres wide and up to 20 metres thick and occurs 40 to 50 metres below surface. Recent deeper drilling has also defined a zone of fresh rock mineralisation at Eos.
4. Hestia is on the western edge of the Project, with mineralisation known to extend over a strike length of approximately 800 metres and up to approximately 200 metres below surface. The stacked lodes are between two metres and ten metres thick, and dip steeply (75°) towards the WSW (250°). The mineralisation style is very different to the other deposits and is associated with a shear zone adjacent to a mafic/sediment contact.