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.

Rockvale Arsenic mine

Rockvale Monzogranite and my toes for scale

It has been a long time since I wrote a blog post. Life has got in the way with a unexpected sorrow in the death of my little lady. It has been a year of change and that includes work and new ventures. Now, I begin the journey of working for myself. In saying that, if you have any work that might interest me I’d love to chat to you about it. I don’t have a web page at this stage, but I have my contact details on this page. Already, I have had the opportunity to work in the North Coast again, renewing old and making new contacts. I am still based in the New England area and as always it is an adventure to explore the land and environment.

The first post I want to do is one about an arsenic mine I visited near Armidale. When I visited the Rockvale Arsenic Mine it was quite evident that the site has contaminated the soil. The most widespread contaminant in the region is antimony metal which when mined (along with gold) in the Hillgrove district and was discharged in large volumes into the Macleay River catchment. This antimony continues to be dissolved in water flowing from the old Hillgrove and other abandoned areas. However, further upstream from Hillgrove, in the Wollomombi River catchment from is Rockvale which has a similar geological history.

Rockvale Arsenic mine

Possible evidence of contaminated soil transport (gully erosion)

Rockvale is named for its rocks! Granite types that cover a very large area. The rock unit is called the Rockvale Monzogranite of the Hillgrove Supersuite. It was formerly known as the Rockvale Adamellite, in the old nomenclature. According to Kent (1994) the Rockvale Monzogranite consists of 20 individual plutons all intruded during the Carboniferous (at approximately 303Ma). The surrounding rock into which it intruded and metamorphosed the Girrakool Beds. The metamorphic effects are quite significant and extend for quite some distance from the Rockvale Monzogranite. Interestingly, there are many hydrothermal mineral deposits in the Rockvale Monzogranite and adjacent metamorphic rocks, yet the mineral deposits were formed 50Ma later (approximately 250Ma). This situation is similar to the nearby Hillgrove mines which although spatially appear to be directly related to the Hillgrove Monzogranite but are actually later concentrations of hydrothermal minerals.

The number of old mines is the Rockvale area is quite significant. Some have been rehabilitated, some badly rehabilitated and others still discharge metal contaminants into the receiving environment. The Rockvale Arsenic mine is a good example, it appears to be the northern most of several mineral deposits that occur along a line a couple of kilometres long, possibly terminating at the Ruby Silver Mine. The main mineral mined from the hillgrove arsenic mine was arsenopyrite, though other less common arsenic minerals were also part of the ore. The arsenic was 'roasted' to drive of volatile elements and concentrate the ore.

The legacy of historical operations can be seen from the photograph which shows extensive bare areas and gully erosion due to the metal toxicity and acid mine waste from oxidised pyrite, arsenopyrite and other sulfide minerals. So, although the metal contamination from downstream Hillgrove is well known, a baseline study will obviously provide some indication that contamination is coming from other upstream sources too. It will be interesting to see how much.

References/bibliography

*Ashley, P. & Graham, B. 2001. Heavy metal loadings of streams in the Macley River catchment. Report to the Mid North Coast Catchment Management Board, NSW Department of Mineral Resouces & Armidale Dumaresq Council.
*Craw, D., Wilson, N. & Ashley, P.M. 2004. Geochemical controls on the environmental mobility of Sb and As at mesothermal antimony and gold deposits. Applied Earth Sciences (Transactions of the Institute of Mining and Metallurgy) vol 133 B3.
*Kent, A.J.R. (1994) Geochronology and geochemistry of Palaeozoic intrusive rocks in the Rockvale region, southern New England Orogen, New South Wales. Australian Journal of Earth Sciences 14:4
*McClatchie & Sylvester. 1970. The Tulloch Silver mine. Records of the Geological Survey of New South Wales. Vol 12, part 1.

History Snippet: Drake

I noticed that the Northern Star Newspaper has an interesting snippet about historical copper mineral exploration near Drake. I thought It would be worth directing readers to the article.


http://www.northernstar.com.au/news/history-copper-hidden-in-the-hills-at-drake/2998850/


I've been meaning to do a detailed post on the geology outlined by Grace Cumming who in 2011 did a very detailed survey of the region and put together an interesting model which illustrates that the mines and prospects just north of Drake are actually the remnant of a very large 400 square kilometre volcanic caldera. I will get to that soon! There is always a million interesting things in our region that I seem to never get the chance to cover them all. In the mean time I can only point you to one post I've done about the 'Drake mines'. http://nrgeology.blogspot.com.au/2012/12/drake-mining-managing-muddy-mess.html


If you are interested in the current explorer and operator of the mines just to the north of Drake here is the link to White Rock Minerals Mt Carrington project overview.
http://www.whiterockminerals.com.au/projects/mt-carrington/overview/

How wonderfully marbleous!

There are some rock types that are very common around the country and around the world that just don’t seem to rate much of a mention in the Northern Rivers. One very common rock is limestone formed from corals in a shallow sea, just like the Great Barrier Reef. Limestone is made almost entirely of the mineral calcite. Some parts of the world have vast terrains dominated by limestone called karst landscapes and it is quite distinctive. Limestone terrains sometimes form amazing subterranean cave systems as the stone is dissolved by rainwater infiltration into the formation. These karst terrains include north-west Mexico and other parts of North America, a giant band through northern England and a wide area of South Australia along the Great Australian Bight. However, it is a landscape absent from the Northern Rivers.

Outcrop of limestone north west of Tabulam

Having said that vast areas of limestone don’t exist in the region it is worth noting that they do exist in small areas here and there within the older rocks of the New England Orogen. The reason for this is interesting. The New England Orogeny was a period of mountain building during periods of plate collision which included a period of subduction of an oceanic plate under the Australian continental landmass during the Silurian period. The material on the surface of the oceanic plate was often accreted, that is scraped off and squashed onto the Australian continent. Seamounts are old islands in the middle of the sea. Such as, those around modern day Hawaii or Fiji. The seamounts were accreted onto the continental mass where they created little pockets of limestone in midst of the jumbled, squashed mass of deep seafloor sediments.

This means that if you find limestone in the New England area you are actually finding the preserved remnants of a little tropical island reef or lagoon. An especially nice thought, when you find some limestone on a cold frosty New England winter morning. One relatively accessible place to see some limestone is an old quarry on the Pretty Gully Road just north-west of the town of Tabulam which sits on the Bruxner Highway crossing of the Clarence River. The stratigraphic unit that the limestone of the area is part is the Emu Creek Formation which also includes areas of interesting fossils (more about that in yet another post). However, the quarry is interesting for more reasons than just as an occurrence of limestone.

Following the period of subduction and accretion a period occurred where intrusions of molten magma pushed their way into the accretionary sedimentary rocks. It occurred a couple of times including during the Late Permian to Early Triassic and created one part of what is referred to as the New England Batholith. The batholith is an array of granitic rocks that stretches through the whole New England Tablelands. The intrusions of the Late

fresh face of limestone - note the sparkles from the calcite crystals

Permian to Early Triassic included the emplacement of the Bruxner Monzogranite, a type of granite pluton (more about this specific rock in a future post). This pluton heated up and metamorphosed the rocks around it and one of which was that body of limestone near Tabulam. Contact metamorphism of limestone creates the rock called marble and this has happened at Tabulam. Although, the quality of marble is questionable because of the amount of impurities.

Other things happened to the limestone during metamorphism too. The transfer of fluids into and out of the cooling magma created chemical reactions which concentrated elements such as iron. This process develops what is called a skarn, a body of altered limestone with sometimes economic amounts of minerals. The minerals in a skarn can be diverse and very, very valuable but the minerals are based on the chemistry of the granite pluton. In the case of the chemistry of the Bruxner Monzogranite, there was not much of value except lots of iron which formed abundant amounts of the minerals magnetite and haematite. This has been considered for mining in the past but the small size and low grade means it is not a viable iron mine.

There are other small limestone deposits all around the New England and all of them are interesting for one reason or another. Some north of Inverell have lovely caves, others near Tamworth are mined for lime on a large scale. While others, just have interesting little features that illustrate what happened during the formation of our region.

References/bibliography:

*Bryant, C.J., Arculus, R.J. & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cordilleran Tonalitic I-type Intrusions in Eastern Australia. Journal of Petrology. v38.

*Lishmund, S.R., Dawood, A.D. & Langley, W.V. 1986. The Limestone Deposits of New South Wales. 2nd Ed. Geological Survey of New South Wales

Antimony and the Macleay River

Antimony is a metal that is very well represented in our region. Many people have not heard of antimony as it is one of those elements that is ‘hidden away’ in many metal alloys and plastics and therefore often outshone by the more well known ones such as Iron, Nickel, Cobalt etc. It is a very important element for use in electronics and to modify the properties of rubber and plastics. It is even used in the cosmetics industry and HIV treatment medication (Wilson et al 2010). The main antimony mineral is called stibnite, an antimony sulphide mineral with the chemical formula Sb2S3, though there are many other less common antimony minerals.

The geographical distribution of antimony mineralisation in the Northern Rivers and New England closely follows certain geological units intruded by granite type plutons during the Permian (Ashley & Craw 2004). Essentially these deposits fall into the category of mesothermal mineral deposits meaning that they were formed through the action of hot fluids under pressure within the earth. The heat source is from regional heat increase due to the intrusion of many granites and sometimes from the actual contact zone of individual intrusions. The source of the fluids can be existing water in sedimentary rock pore space and/or derived from the breakdown of hydrous minerals such as clays. This hot water (often accompanied by elevated salts) can dissolve elements such as antimony as well as others such as gold and silver and then as they cool these elements are redeposited. In practice this tends to mean that the elements are located within veins of quartz or carbonate.

Probably the best known deposit of antimony is the Hillgrove Mine east of Armidale. The mine is in the headwaters of the Macleay River and was first mined for gold at the end of the nineteenth century. Indeed Hillgrove had a gold rush of such size that it was much bigger than Armidale (now its population is less than a hundred, I think). But many other areas have extensive mineralisation of antimony such as the area to the west of Bowraville in the headwaters of the Nambucca River catchment, areas north of Dorrigo in the headwaters of the Nymboida River catchment and even areas as far north as Tooloom which is to the north of Drake in the upper portions of the Clarence River catchment. Some of these deposits have been mined historically, though in the main gold has been the target and antimony just a by-product.

Antimony is an interesting element because it is chemically closely related to arsenic and therefore behaves in a similar way. This means it can also be dangerous in high concentrations and its environmental impact can be significant at even moderate to low levels, however, the nature of antimony has not been as extensively researched as arsenic and therefore the drinking water and environmental limits in Australia have been set lower than arsenic to increase the safety margin in assessing whether there is likely to be an adverse impact (Ashley et at 2004).

Interestingly, unlike many other elements that can be mobilised by the creation of sulphuric acid during the oxidation of the parent sulphide mineral, antimony tends not to remain in solution for long because the nature of the mineralisation model is such that carbonates are often present which neutralises the acids and leads to settling out of the antimony from the water column with iron and other metals. However, if the sediment is transported then this can be deposited a huge distance from its source and in some situations can be re-mobilized because of local stagnant water during dry periods combined with the presence of natural humic acids. This behaviour has been observed in the Macleay River catchment as suspended sediment from the areas around Hillgrove has been deposited on the flood plains as far away as Kempsey, very low concentrations of antimony are usually found in clear, clean water in the region. However, Wilson et al (2010) has shown that sometimes high antimony contents of alluvial soils can lead to uptake by flora and therefore this contaminant can then be accumulated in animals that graze on these plants.

References/bibliography:

*Ashley, P.M. & Craw, D. 2004. Structural controls on hydrothermal alteration and gold-antimony mineralisation in the Hillgrove area, NSW, Australia. Mineralium Deposita v39.
*Ashley, P.M., Craw, D., Graham, B.P. & Chappell, D.A. 2003. Environmental mobility of antimony around mesothermal stibnite deposits, New South Wales, Australia and southern New Zealand. Journal of Geochemical Exploration v77
*Craw, D, Wilson, N. & Ashley, P.M. 2004. Geochemical controls on the environmental mobility of Sb and As at mesothermal antimony and gold deposits. Applied Earth Science (Transactions of the International Mineralogy and Metallurgy Bulletin). v 113.
*Wilson, S.C., Lockwood, P.V., Ashley, P.M., & Tighe, M. 2010. The chemistry and behaviour of antimony in the soil environment with comparisons to arsenic: a critical review. Environmetnal Pollution v158.

In the hills of Valla and Nambucca Heads

The Valla Adamellite now termed the Valla Monzogranite to reflect modern naming conventions is an interesting small to medium sized intrusion about 10km north east of Nambucca Heads. It is one of the suites of coastal granites which are mostly I-Types (melted igneous material), this means that the coastal granites show abundances of ore minerals within the granite or in the surrounding metamorphosed country rocks. A monzogranite is a granite with roughly equal proportions of (alkali-feldspar (potassium and sodium rich) and plagioclase feldspar (calcium rich)). The monzogranite is thought to have formed during the Triassic period.

The metamorphic aureole for the Valla Monzogranite is actually quite interesting as it shows a classic zonation of metamorphism (high grade at the contact grading to low grade further away) and also excellent examples of mineral zonation associated with metasomatism (hot-water or fluid alteration of rock). The Valla Monzogranite has been shown to be associated with gold, silver, arsenic and molybdenum mineralisation (as well as others). The rock that the monzogranite has been intruded into is called the Nambucca Beds which are part of the Nambucca Block. The Nambucca Beds are Permian to Carboniferous in age and are mainly comprised of the regionally metamorphic rock type called phyllite which was originally deposited on the sea floor. The Nambucca Block was accreted onto the Australian continent in the New England Orogen and this caused the regional metamorphism of the beds.

The Nambucca Beds are intruded by the Monzogranite. The Beds are extensive and
extend far into the rugged Nambucca Hinterland. This photo is west of Bowraville.

The Valla Monzogranite seems to be a Climax Molybdenum Deposit named after the Climax Mine in North America. This means that when the Monzogranite was cooling the upper portion of the pluton became residually enriched with fluids, metals and silica. These fluids cause alteration of the upper portion of the pluton forming what is called greisen and also are injected into the surrounding rock through veins and sometimes aggressively through breccia pipes. One of the first minerals to form in these veins is silica, quartz with metal sulphide such as molybdenite (molybdenum ore) and wolframite (tungsten ore). Further away from the intrusion the degree of alteration becomes less grading through potassic through to argillic which are defined alteration zones based on the changes in the rock forming minerals. As the degree of alteration becomes less so the types of metal ores change with increasing amounts or arsenic, gold and silver. Further out in the alteration zone minerals such as galena form (lead ore) and finally stibnite (antimony ore). These ore deposits seem to be fairly common in the New England area with Glen Eden being the most studied (Somarin 2001, Somarin & Ashley 2004) and have in some areas been extensively explored such as Kingsgate east of Glen Innes.

Some attempts of mining have occurred in the Valla Monzogranite in the past, the most significant being the Valla Gold mine which was located just to the north of Valla Beach. The mine was abandoned with very little rehabilitation and therefore has become an environmental problem for the local creek. However, rehabilitation efforts have recently been undertaken, though these will need another post to discuss in more detail.

References/bibliography:

*Somarin, A.K. 2011. Petrography, Geochemistry, and Petrogenesis of Late-Stage Granites: An Example from the Glen Eden Area, New South Wales, Australia. Earth and Environmental Sciences.
*Somarin, A.K. & Ashley, P.M. 2004. Hydrothermal Alteration and Mineralisation of the Glen Eden Mo-W-Sn deposit: A Leucogranite related hydrothermal system, southern New England Orogen, NSW, Australia. Mineralium Deposita.