During the year low level exploration advanced two gold prospects within the tenement portfolio, at Wildhorse Plain and Napoleons Hat.
Historic core was sourced from the PIRSA core library and composite samples taken and assayed for gold and multi-elements.
Hole A405_2 reported an average of 16.5m @ 0.32g/t Au, 7g/t Ag and 146 ppm Mo from 32m down hole. The mineralised intervals appear as quartz filled voids, with flourite and hydraulic breccias, indicating a possible epithermal-style deposit. The type of gold system may yield higher grades at depth.
Additionally hole A405_3A located some 200m NE of hole A405_2 (figure 1), reported an intercept of 5.8m @ 0.48 g/t Au and 2.93 g/t Ag from 31.4m down-hole.
EL 4668 was granted to Archer on 21s February 2011 for a period of 2 years. The EL covers the historic Wonna gold workings.
Napoleons Hat is located immediately north of Archer’s North Burra (EL 4266) tenement which reported gold in rock chip samples in 2010 (figure 2).
Hydrothermal fluids carrying gold in solution require changes in either pressure or temperature or both to enable gold to precipitate out of solution. Structural flexures like the one that exists on Napoleons Hat offer very large ‘openings’ and they may provide the important depositional environment needed to precipitate gold. The importance of the large flexure at Napoleons Hat is increased due to the fact that known favourable lithological units such as the Appila Tillite, the Watervale Sandstone and the Cox Sandstone occur in the flexure.
The extensive historic Wonna workings have been described as being alluvial gold deposits. Archer inspected several of the shafts and workings and it is clear that the gold mineralisation is not alluvial, but rather related to sheeted quartz veining within competent sediments.
Gold in the district is reported to have a very high nugget variance making sampling for gold a “hit-and-miss” proposition. It has been well documented that the surface gold is associated with iron which is a product of possible weathering of sulphides.
Archer’s intention is to explore for a primary gold at depth. Underground lodes in the district have been documented by government mine inspectors in the early 1900’s as appearing to be associated with strong sulphide alteration and +1oz gold grades. At the time the miners could not extract the gold associated with sulphides and this form of mineralisation was not mined. It is these occurrences that Archer will be exploring for as they may represent potential high grade and even bonanza grade opportunities.
Future exploration is likely to include geophysics to identify sulphide roots followed by drilling.
Sampling of quartz veins within the Watervale Sandstone indentified gold with associated arsenopyrite, figure 3. Additional test work led to the conclusion that the occurrence is similar to other gold occurrences in the area in that it is nuggety. It is thought that electrical methods may be able to define highly charged deep responses thus assisting to better target potential gold lodes.
Drilling in the region has been noted as being very costly due to the broken nature of the weathered surficial rocks, which has often led to the abandonment of drill holes before targets are reached.
Limited work has been performed on the tenement (figure4) where the exploration target is the Watervale Sandstone for gold mineralisation. Historic occurrences of gold were reported from a prospectors drill hole (that has not been located by previous explorers), which recorded as having gold mineralisation at;
- @168m 3.3g/t
- @195m 2.5g/t
- @335m 2.8g/t
Similar to the approach for the other gold targets in the Burra area, future exploration is likely to include an electrical based survey to identify prospective targets within and along the Watervale Sandstone. If the geophysics identifies conductive bodies they will be tested by drilling.