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.

Rocks in the Rocky River

Rocky River Monzogranite (Bungulla Suite).
The Monzogranite here contains large crystals of twinned pink K-feldspar.

The Rocky River Road is a very quiet, scenic and out of the way route to travel. It is slow and windy, but a pretty alternative to the Bruxner Highway route between Drake and Tenterfield. I had the pleasure of a trip along Long Gully Road and Rocky River Road just last week. I enjoyed it very much for the scenery and the clear water of the Rocky River (also known as the Timbarra River). The area is also very interesting in a geological sense. The rock that is found along Rocky River Road (the Rocky River Monzogranite) is actually remnants of outer part of a very large batholith that makes up Timbarra Tableland.

Previously, understanding of the inner rocks of the Timbarra Tableland were incorrectly thought to be Moonbi Supersuite, while the outer rocks were correctly part of the Stanthorpe Supersuite. Having two parts of an intrusion being apparently related to different Suites was all quite confused. Mustard (2004) suggested an informal renaming of the Bungulla Monzogranite in the area of Rocky River to the Rocky River Monzogranite. The Rocky River Monzogranite would in turn be part of the Bungulla Suite. The Bungulla Suite being rocks that are I-type (derived from melted igneous rocks) of the Stanthorpe Supersuite.  Although the nomenclature by Mustard (2004) was suggested as informal it is quite reasonable to adopt the name of Rocky Creek Monzogranite as formal. The previous identification of some rocks in the Timbarra Tableland as Moonbi Supersuite has since been shown to be incorrect - they are all Stanthorpe Supersuite.

The Rocky River Monzogranite is in the extensive eastern edge of the Timbarra Tablelands. It is comprised mainly of the rock monzogranite. This rock is comprised of abundant quartz and roughly equal proportions of plagioclase feldspar (sodium and calcium feldspar) and potassium feldspar. There are also smaller amounts of dark biotite mica and amphibole in the rock. The Rocky River Monzogranite is quite a course grained and the crystals are very, very large. The monzonite is notable as it has many 'inclusions' called xenoliths. These are blobs of rock are of a less granitic composition. They are very, very common in some areas as the rock comprises of about 10% or more xenoliths. The xenoliths indicate that mixing of different composition magmas was occurring when the intrusion formed.

A monzogranite tor in the sandy bed of the Rocky River.
Note different sized irregular shaped xenoliths.

Along the very margin of the intrusion (I didn't get to see this) the crystals are smaller in size and the feldspars are even more potassium rich forming the rock syenite. The central area of the Timbarra tablelands is comprised of granitic rocks that were high in fluids when the rock was crystallizing. These fluids (formed by residual enrichment of the original magma chamber), has resulted in the concentration of metals, most notably gold (Mustard 2004). The Timbarra gold mine targeted this inner zone of the tablelands as the outer granite (Rocky Creek Monzogranite) do not contain nearly as much gold. The erosion of the gold has led to alluvial gold deposits in the Rocky River and Clarence Rivers but the gold is very fine grained so fossickers panning can be tricky.

The many components of the Timbarra tablelands intrusion were emplaced in the Triassic period. They intruded the Drake Volcanics. The size of the granite plutons has caused significant contact metamorphism, creating a large metamorphic aureole around the intrusion.

There is much more to say about the zones in the Timbarra tablelands intrusion described by Mustard (2004). This includes the neatness of the tablelands cross section, the way that the slightly different granites tapped different parts of a deeper magma chamber and the way that differentiation of granite types occurred are all worthy of a discussion. Though, this needs more than just a few paragraphs and so I will have to cover these matters in future posts. In the mean time I hope this post gives a taste for some of the 'granite'.

References/bibliography:
*Mustard, R. 2004. Textural, mineralogical and geochemical variation in the zoned Timbarra Tablelands pluton, New South Wales. Australian Journal of Earth Sciences, 51.

Tharz gold in them hills!

Jim Belshaw, blogger and New England self government advocate has several interesting blogs. I thought I'd take the opportunity to share his latest New England History blog post on gold in the Timbarra/Rocky River area.

Jim has a very interesting writing style and I enjoy his blogs. He also seems to capture many parts of the New England landscape and history that go poorly documented. I understand this fascinating article was also published in the Armidale Express Extra which is not available online.

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.

Drake mining: managing a muddy mess

Sorry it has taken some time for me to post. I have had very little time of late because of some health problems my daughter has been having. But she is better than ever so time to get some time back into geology matters again.

Drake has a history of gold mining spanning back to 1886 when gold was dredged from Plumbago Creek. Since then the source of much of the alluvial gold was found just to the north of Drake. Many pits were created in the search for gold since the 1920s. These pits were relatively large mines in themselves and were given names such as White Rock, Carrington, Strauss, Lady Hampden and others. The mines were a source of wealth (during the good times) and a source of debt (during the bad times) with the mining operations completely ceasing in the 1990’s.


One of the old pits at Drake shortly after treatment with red mud

The formation of gold in the gold fields just north of Drake are a little difficult to put together as there seems to be more than one period of mineral formation in the rock. The parent rock is lavas and pyroclastic deposits including tuff which is of andesite to rhyolite in chemical composition. These rocks are called the Drake Volcanics which are part of the spatially significant Wandsworth Volcanic Suite. It appears that a caldera once developed in the area and fluids heated by magma transported gold and other metals and concentrated them. This is called an epithermal mineral deposit. However, Houston (1999) demonstrated that overprinting much of this epithermal stage is another chemically different period of mineralisation possibly related to different intrusive introducing mineralised fluids. And finally much of the area has been affected by supergene enrichment, which is enrichment caused by natural transport of minerals in groundwater and the percolation of rainwater.

Because financial stresses encourage people to take shortcuts to save money several things have happened at Drake that has caused elevated metal contamination to the environment of Plumbago Creek, a tributary of the Clarence River. Though sometimes people are just lazy or even ignorant of the possible impacts of incorrectly disposing of waste materials (Just like at home). Mineral deposits of the type at Drake contains minerals called sulphides, these include pyrite (iron sulphide), chalcopyrite (copper-iron sulphide) and sphalerite (zinc-iron sulphide). When exposed to air and water these minerals break down creating acids (called acid mine drainage) that cause the metals to be dissolved in any waters and therefore easily discharged into the environment. This is what has happened at the old pits near Drake and also at the waste dumps and even the access roads which were surfaced with waste rock.

But the story of the Drake mines also involve another waste material deliberately brought in from central Queensland. This material is referred to as Red Mud and is caustic (highly alkaline) waste material from aluminium refineries. But this is actually a good news story! Basic chemistry demonstrates that when you add acid and alkaline material together the material becomes neutral and metal contaminants precipitate out meaning any discharged water is decontaminated. Essentially an environmentally serious problem (disposal of aluminium refinery waste) has actually proven to be a resource. The trials and remediation of the pits was so successful that the technique was patented and a commercial product developed out of the Red Mud and given the name TerraB.

Application of the Red Mud was both as slurry pumped by ‘sprinkler’ directly into contaminated water left at the site or incorporated into waste rock or used as treatment liners. The picture shows one of the pits that I visited more than a decade ago when this technique was being trialled. It may look bad but really it is just suspended sediment that will settle out, while the acid and heavy metals have been neutralised. Some trials in waste rock have even found that Red Mud can actually reduce the uptake of heavy metals by plants, better than traditional rehabilitation techniques such as lime (Maddocks et al 2009).

The area around drake is interesting for many a geological reason, from its formation, the minerals found, the historical mining, contamination and rehabilitation. Who would have thought that adding two waste products together would fix both problems?! Two wrongs do make a right!

References/bibliography:

*Clark, M.W., Walsh, S.R. & Smith, J.V. 2001. The distribution of heavy metals in an abandoned mining area; a case study of Strauss Pit, the Drake mining area, Australia: implications for the environmental management of mine sites. Environmental Geology v40.

*Houston, M.J. 1999. The Geology and Mineralisation of the Drake Mine Area, Northern New South Wales. Papers, New England Orogen Conference, Armidale 1999.

*Maddocks, G., Lin, C. & McConchie, D. 2009. Field scale remediate of mine wastes at an abandoned gold mine, Australia II: Effects on plant growth and groundwater. Environmental Geology

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.

The New England tablelands seem to be upside down

The geomorphology of the Northern Rivers and New England region can be quite complex. There are many features around the region that have developed as a direct result of the underlying geology. Whether it be the great escarpment, the Ebor Volcano, the backward Clarence River or various other situations, there is always a geological reason for the landscape we see today. In a previous post on the Maybole Volcano near Guyra I quickly mentioned that there is an “inverted topography” which has been created following the deposition of the lava from this volcanic area. Maybole is not isolated in this situation, indeed according to Coenraads & Ollier (1992) much of the basalts in the New England region from Armidale, Walcha, Llangothlin and even places on the other side of the watershed and great dividing range of the Northern Rivers such as Nundle or Inverell show what is technically referred to as relief inversion.

The area around Armidale is actually a good example of the relief inversion, as most hills actually demonstrate the situation nicely. Take, for example, the hill that the University of New England is situated on. The Hill is capped with Cenozoic (Miocene) aged calc-alkaline olivine basalt (part of the Central Volcanic Province) just to the east of the hill (in the paddock below the university carparks) below the level of the lowest basalt flow is a fossil soil horizon, known as a palaeosol. This palaeosol has been affected by lava being deposited on it and has been turned into a material known as silcrete (soil which has been cemented with silica). The old soil was developed on rocks of the Carboniferous aged Sandon Beds. The Sandon Beds outcrop on the lower slopes and in the valleys in and around Armidale but once they were the hills themselves.

The basalts were erupted to the surface the chemical composition of the lava meant that they were quite low in viscosity, that is it was very liquid and consequently the lavas flowed down the valleys that existed at the time. The valleys tended to fill up to varying degrees, leaving only a thin layer of volcanic rock on the existing hill crests of the Sandon Beds or none at all. In the following millions of years the process of erosion would be more effective on the non-volcanic rock and the hills would eventually become incised, turning into gullies and eventually larger valleys. The basalt in the old valleys would remain relatively un-eroded and be become the modern hills.

Evidence of this process can be seen from historic mining of some of the gold around Armidale. The ‘old timers’ would dig under the basalt along ‘deep leads’ which were originally gravel and sand deposits associated with old creeks and rivers. These deep leads had been alluvial gold deposits preserved by the basalt flows. Many of these were mined in the 1800’s and early 1900’s in many areas of the New England district including one quite recently in the Tilbuster area (Ashley & Cook 1988). The silcrete deposits mentioned previously are also examples of the process.

References/bibliography:

*Ashley, P.M. & Cook, N.D.J. 1988. Geology of the Whybatong gold prospect and associated Tertiary deep lead, Puddledock, Armidale District. New England Orogen - Tectonics and Metallogenesis. Conference Papers presented at the University of New England.
*Coenraads, R.R. & Ollier, C.D. 1992. Tectonics and Landforms of the New England Region. 1992 Field Conference - New England District. Geological Society of Australia Queensland Division.