Friday 21 December 2018

Faulty Gold - The Enmore Goldfields

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.

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.

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.