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.

Monday 11 June 2018

The NSW/QLD border geological mess and other matters - Talk at Binna Burra

This year marks the 85th birthday of the Binna Burra Wilderness Lodge in Southern Queensland. As part of the 85th celebrations the lodge has invited many people to give talks at the lodge between the 20th and 24th of June. There are many interesting science and nature talks open to the public on these days and two of the talks will have a geological theme. The Lodge is situated on the northern side of the Tweed volcano and the landscape and ecology of the area is intimately connected with the geological history of the area.

I will be giving a talk on the evolution of our understanding of the Tweed Volcano over the years and how politics can affect how we scientifically look at our part of the world. My talk will be on Friday 22nd June at 9.30am. On Sunday 24th, Warwick Willmott will be giving a more geology overview walk and talk including discussions on how the Tweed Volcano and Hawaiian Volcanoes have many similar characteristics.

UPDATE: Due to personal matters I have had to cancel my talk. The Talk by Warwick Willmott on Sunday is unaffected.

The details of my talk are as follows:

Talk title:
Our understanding of the Tweed Volcano: A Learning, Unlearning, Forgetful and Confused Experience.

The landscape of the NSW/QLD Border (Lamington and Tweed areas) being the result of a single volcanic centre, has been recognised formally for less time than the establishment of Binna Burra Lodge (only 70 years). Since this first realisation, many researchers have added to understanding of how the landscape has evolved. However, sometimes even in our modern and scientific world new knowledge can get lost, be ignored, or repeat old myths. This talk will cover some of the evolution of our understanding of the Tweed Shield Volcano and examine some of the persistent ‘popular science’ myths of this landscape.

For details of events being held at Binna Burra Wilderness Lodge you can visit their facebook page:

Saturday 26 May 2018

Blog Update #12

Firstly, I need to apologize to all the commenters who have not had their comments published. The old regular visitor crowd will have noticed the blog has become fairly inactive over the last few months. Given the number of comments placed and the continuing interest (including an invitation to speak just this morning) I should really do a lot more... and I will.

I have recently had a few interesting journeys which included work near Nymboida on a new quarry and also visited an old quarry with some really interesting fossils. I've also noticed some interesting rock alteration in a road cutting on the way to Valla and I've been spending some time at a friends place near Glencoe at a place called Mount Mitchell. All of these are worth blog posts and I shall try and get to them at a rate of at least one a fortnight.

Thanks to everyone continuing to support this page. Please feel free to continue to comment but if you have something specific please feel free to email or phone me. My details are on this page:

Thursday 25 January 2018

Nambucca Heads earthquakes follow up

Yesterday, following the rare experience of a noticeable earthquake in our region, I provided a very rough map of the location of earthquakes in preceding 24hours to the one many people felt. I have since incorporated earthquakes since and for the week before. I've also increased the area we are looking at.

Mapping is all about enough data, and enough decent data. Collecting the data is essential and then looking at it so see whether it adds to the story helps us understand the world in which we live. Geoscience Australia provide an excellent online service to the public showing all the earthquake activity in our Country. Yesterday's post had enough data to demonstrate that there appears to be little relationship between inferred or known faults in the region and the location of earthquakes. More data since means more interesting features!

A comment from a reader asked about the trace of the Demon fault system (a system that is very evident even today despite it being apparently inactive for many millions of years). I have included a thick pale blue line that roughly approximates the Demon Fault system strike.

The map I have just updated implies that there is a fault system previously unknown. I have included a line that approximates the pattern of many quakes in area. What is interesting is that the apparent line of most quakes is perpendicular to the old Demon Fault. It is also interesting that the 'major' 4.2 quake is separate from the apparent line of quakes experienced in the last week or so. It is likely that the quakes in the Nambucca area are a parallel line of stress to the most frequent quake trend.

Either way, the map says more that I can so have a look and tell me what you think.

 *Update: At least three more quakes have been identified (one occurred since the above map) and another couple which was subsequently identified after cleaning out the background 'noise.

I also note that the quakes were almost entirely situated in a structural block known as the Nambucca Block. I guess another post will have to be done since the Nambucca Block provides insights into the way the New England Orogen has formed (Shaanan, Rosenbaum & Ranauld 2016).

Wednesday 24 January 2018

Yes, I felt that Earthquake too

Earthquakes are a bit infrequent in our part of the world. So, when one is big enough to feel happens there is usually much excitement.

While in my home office a few hours ago (1.30pm this afternoon) I felt a strange sensation and the roof of my studio shook. About 1 minute later, my wife came up from the house and asked if I had felt something, because the house shook for a moment. So, onto trusty Geoscience Australia Earthquakes page I went. Sure enough a substantial (by our standards) quake hit somewhere under the ground near Urunga.

I've put together a quick map which shows all the mapped major faults in the area (noting that most of them are actually inferred). I have overlain the last 24hrs worth of quakes to see if there is a correlation. There appears to be very little if any. But that is unsurprising given most faults have probably been inactive for millions of years. But I suppose interesting anyway. Note that the data had not been double checked by Geoscience Australia when I put this together.*
*Since posting the above, the earthquake epicentre has been updated to near Nambucca Heads the hypocentre being approximately 10km below Nambucca Heads.