I was undertaking a project on a private property near
Enmore, south of Armidale last weekend. This gave me the opportunity to visit
some abandoned mine sites and have a look at the country. The property I was on
consisted of two stratigraphic units, the Girrakool Beds and the Enmore
Monzogranite. The area I was most interested in was the boundary between the
two units. Where I was the boundary is defined by a fault known as the Borah
Fault. The fault zone is quite easily
observed through topographic and drainage features, but also there has
historically been some gold extraction from some locations along this fault
including two mines that I got to visit (Buffalo Ranche Mine and Sherwood Mine).
These mines make up part of the area
sometimes referred to as the Enmore-Melrose Goldfield.
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| Old mining equipment Sherwood Mine |
The regional geological mapping identifies that the north of
the Sherwood Fault are blocks of the Girrakool Beds. This geological unit is
dominated by mudstone (slate) and greywacke (lithic sandstones) with rare chert
and basalt (Gilligan et al 1986) and is sometimes considered of Permian age (e.g.
Binns 1966, Leitch 1974) but is more likely Carboniferous Gilligan et al 1986).
It appears to me that the Girrakool Beds in the Enmore area have not been
studied extensively but other areas well to the North east of Armidale have
been much more studied because in that area they have undergone extensive and
complex metamorphism.
South of the Borah Fault, as well as some fault bound blocks
to the north of it is the Enmore Monzogranite. The Enmore Monzogranite is a
name given to a biotite monzogranite of S-Type derivation (from melted
sedimentary rocks) commonly with a foliation (preferred direction of mineral
alignment). The quartz in the unit is usually of a blue colour and there is occasionally
amphibole. garnet and even some graphite present in some places too. It
commonly contains xenoliths. The Enmore Monzogranite has been classified as
part of the Hillgrove Supersuite. As far as I can find, the Enmore Monzogranite
has not been dated accurately and therefore only has an inferred age of
Carboniferous or Permian.
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| Remnants of the old Sherwood Mine |
The Borah fault can be traced for quite some distance
because the faulting has affected the rocks (which area now called mylonite,
breccia and fault gauge). The shearing stresses caused by movement along the
fault has recrystalised some of the rock and broken up other areas. Because of
this action the affected rocks have been weakened and are more susceptible to
erosion. This means that over time creeks have preferred to flow along the
fault strike. For example one creek, Postmans Gully flows along the fault
towards the north-east and another, Borah Creek flows along the same fault in
the opposite direction (towards the south-west).
Some old mining equipment still remains at Sherwood Mine,
with the remnants of a steam engine apparently manufactured about 1878 still
visible. Historical mining records (Henley 1985) show that approximately 7.9kg of
gold was extracted in 1893 then in the period up to 1937 a further 2.6kg was produced.
Follow up exploration was carried out from time to time, particularly in the
1970’s to 1990’s but no significant economic concentrations of gold were
identified. I note that the geology superficially appears similar to the nearby
Hillgrove mines area but on further inspection it appears that all of the
substantial mineral deposits lie in a thin zone around and along the fault
line. The mineral deposits are also of a quite different chemical make up with
low concentrations of Antimony, which distinguishes it from the major
mineralisation events that formed many of the Gold-Antimony deposits from the
nearby Hillgrove Gold Field.
As mentioned, the most significant gold occurrences in the
Enmore-Melrose Goldfield are located on, or adjacent to the Borah Fault (and
nearby Sherwood Fault). This indicates the faults are likely to be a structural
control on the gold mineralisation.
