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.

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

Some musings on coal seam methane

I thought I’d get myself in trouble again on the coal seam gas subject by having some musings on the public presentation by two academics at Southern Cross University last night. Of course the topic relates to the potential impact to the environment by the Coal Seam Gas Industry in the Northern Rivers. Research has been conducted by Dr Isaac Santos and Dr Damien Maher from the School of Environment, Science and Engineering. The subject of the presentation was a preliminary report on the status of water and air chemistry research at Tara in the southern Queensland Darling Downs which are being conducted Dr Santos and Dr Maher. This is up my alley and so I couldn’t help have some of my own thoughts of the matter. It is important to note that the research has not yet been published in a scientific journal as yet, but when or if it is, I will return to this topic. It is even more important to note that this post has no references as it is just some personal musings on the matter.

To summarise very, very broadly what the research appears to have found is evidence of methane directly attributable to geological derived methane gas stores (called thermogenic methane) as opposed to biogenic methane sources such as animals or organic matter decay in stagnant water, etc. This methane was measured in both surface water and in the air in an active coal seam gas extraction area in southern Queensland. The isotopes (isotopes are elements with slightly different numbers of neutrons in the atom, e.g. Carbon usually has either 13 or 14 neutrons) of the carbon that makes up methane (methane is a combination of carbon and hydrogen (CH4)) were measured during this process. The slightly different masses in these isotopes means that they have different stability in different environments (e.g. heating of coal would preferentially release one isotope over another). The isotopic composition can therefore be used to give an indication of whether the methane is biogenic or thermogenic. 

Where the interesting bit comes in is where Dr Santos and Dr Maher then draw on some comparisons. They had a look at the Richmond River valley and took observations. The observations were different, the Richmond River valley showed lower concentrations of thermogenic methane relative to Tara. Their suggestion then arises that it is the active gas field in Queensland has higher levels of fugitive emissions as a direct result of the coal seam gas industry operating in there. This suggestion can then be extended to presume that should the industry develop in the Richmond River Valley the levels of methane in the air and water would also increase. Now, this is a reasonable chain of assumptions but there are some things missed if the data is not interpreted properly. From my cursory understanding of the data sets gathered by Dr Maher and Dr Santos there are two (potentially contradictory) variables that could significantly affect their findings:

1. The actual amount of thermogenic methane may be underestimated because CSIRO researcher John Smith has shown that during or following catagenesis (generally geological formation of gas), some methane may be converted into carbon dioxide depending on the abundance of dissolved oxygen in any formation waters, this carbon dioxide can then be naturally reconverted into methane. The process during reconversion from carbon dioxide into methane preferentially uses the biogenic indicator isotope. This can then give the false impression that the methane is biogenic and therefore actually underestimating the effect of the coal seam gas industry on gas emissions.
2. The assumption that the natural levels of gas dispersal though normal processes of gas dispersal is the same in south Queensland (Surat Basin and Gunnedah Basins) is the same as that in the Northern Rivers (southern Clarence-Moreton Basin) may be erroneous. The geological units are different both in the depositional, structural, deformational, and erosional history for these basins. This is important because sources of thermogenic gas can be close to the surface in the Surat and Gunnedah Basins but in the Richmond River Valley the gas bearing layers are mostly trapped by a thick succession impermeable rocks such as the Kangaroo Creek Sandstone and Grafton Formation. This may mean a comparison is not a reasonable thing to do and therefore that the gas in Queensland is naturally occurring and not as a result of anthropogenic impacts. The low level of thermogenic methane in the Richmond River valley may simply be a reflection of the geology and any development of the industry in the northern rivers may actually not increase the amount of methane in the local water and air.

During the survey carbon dioxide levels were also measured. The weird thing is that the levels of carbon dioxide in the Tara area was higher than that in the Richmond Valley. When coal seam gas is targeted the presence of carbon dioxide sometimes means that the gas has been affected by some process that has degraded the quality of the gas and therefore carbon dioxide rich resevoirs are not as good economically as low carbon dioxide ones. As such these high CO2 reservoirs tend not to be the tapped to the same degree. The abundance of CO2 is therefore confusing as it does not seem to fit cleanly into the picture of an athropogenically affected local atmosphere.

Dr Maher and Dr Santos have both demonstrated many useful and interesting environmental studies over the years. I have no question of their ability to collect good quality data. It is important to note though that Dr Maher did caution about jumping to the immediate conclusion that the results they observed were totally due to the gas industry. Both of the researches did say that it was important that further data was required to give a more definitive answer by determining whether the geological assumptions were correct.

My view at this stage is that we should not jump to conclusions that it is actually the gas industry that is causing the measurable difference in methane from one location to another… though it may well be. When people say the science demonstrates this or that it is important to note that there are assumptions made in interpreting data and those assumptions could actually understate impacts or even the opposite, overstate them. One thing I think is important to note is that we really don’t understand much of the world around us and therefore we don’t really know if what we see is due to us or not. Slowly we learn more and this helps, but there is so still far to go.

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.

Walloon Coal Measures of the Southern Clarence-Morton Basin

In previous posts I’ve briefly discussed the upper most layers of the Clarence-Moreton Basin. The Grafton Formation which overlies the Kangaroo Creek Sandstone which in turn overlies the Woodenbong Beds/MacLean Sandstone Member. The MacLean Sandstone Member is a member of a larger unit called the Walloon Coal Measures and it is this unit that I will briefly comment on now.

I’ve often heard people mistakenly say that the Walloon Coal Measures is a coal seam. This is not correct because the balance of the unit is actually made up of mixed rocks. According to Wells & O’Brien (1994) the coal measures include sandstones (made from volcanic rock fragments), carbonaceous siltstone, shale, mudstone, coal and clayey siltstones. Also clayey ironstone and infrequently oil shale and limestone can be found. Apparently tree stumps remaining in their growth position have also been found, though these have become carbonised (coal). The coal layers themselves are thin (millimetre scale) to occasionally thick (30-40cm) in the Southern Basin but the whole unit of all the different rock types that make up the Walloon Coal Measures totals at least 200 metres of thickness and is variable from location to location.

The coal in the measures is formed from peat that grew in a moist but temperate environment during the early to middle Jurassic in this area (smack in the middle of the age of the dinosaurs). The depositional environment appears to have been mainly flood-plain and meandering stream environments. Boggy mires forming the peat were common, but layers of volcanic ash from occasional volcanic eruptions from close by are preserved. This makes some of the coal seams high in ash content which reduces the quality of the coal. The environment was thought to be reflective of a period of high sea level.

The Walloon Coal Measures in Bexhill Brick Pit at Bexhill

Interestingly, the Walloon Coal Measures are some of the most extensive and continuous sedimentary rock formations in eastern Australia. They are correlated with almost identical units in the Surat Basin and the Maryborough Basin making the potential spatial extent of the unit huge. The outcrop of the Walloon Coal Measures is fairly limited with much obscured by the Grafton Formation, Kangaroo Creek Sandstone and Woodenbong Beds as well as Cenozoic aged volcanic rock especially associated with the Focal Peak and Tweed Volcanic areas. In our region the best exposures are in the Nimbin area and further north but also at Coaldale where the Clarence-Moreton Basin has been deformed creating a bulge which has been eroded exposing the Walloon Coal Measures. Areas to the south of MacLean show some outcrop and on the other side of the Basin, the Kangaroo Creek and areas near Tabulam show good exposures. Other places have exposures of the Walloon Coal Measures because of local faulting and folding that has occurred in places like the Richmond Range.

I understand that coal mining was historically carried out near Tabulam, Kangaroo Creek and Nimbin but the size of the deposits was such that these were only small and fairly short lived enterprises, though Murwillimbah did have a power station earlier last century which was fueled on local coal transported from the area around Tyalgum. Of course now the Walloon Coal Measures has been frequently under discussion regarding its gas potential especially in the form of coal seam gas (CSG) also known as coal bed methane.

The presence of gas in the coal measures is a natural function of coal and the formation of coal when it was formed. As the rock is gently ‘cooked’ following its deposition as peat gases are given off. Peat is made from decayed plant and animal matter which when broken down into its elemental constituents is mainly hydrogen (H) and carbon (C) atoms. The hydrogen is bonded to the carbon in oxygen poor environments and forms methane (CH4) and sometimes more slightly moe complex organic molecules such as C2H6, C3H10 etc, or if conditions are right the molecules are big enough and complex enough to form oils. In the case of the Southern Clarence-Moreton Basin Walloon Coal Measures the conditions were too hot for oil to be stable so the smaller gas molecules are formed. Gas may be trapped in the layers of coal within voids and cracks (called cleats) or they may sometimes migrate to other layers where they can be trapped. This is actually the difference between ‘conventional’ gas and coal seam gas, i.e. all conventional gas was once coal seam gas. Oil shale and shale gas are also present in some areas of the Walloon Coal Measures but these are very rare and are small deposits (I might do a post on these in the future but given their insignificance I might not get there). Russell 1994 noted that the best quality gas, mature or 'dry gas' was likely to be found abundantly in the eastern portion of the basin, whereas wetter gas and oils were likely to be more prevalent in the west. Interestnigly it is thought that the maturity is a response to the thermal changes in the Earths crust during the formation of the Tasman Sea.

The Walloon Coal Measures contains both conventional and coal seam gas and very little oil. Indeed, I understand that substantial amounts of conventional gas was first discovered in the Hogarth Ranges about 40 years ago and that more recently Metgasco have discovered significant amounts at Kingfisher which I think is to the south of Casino. As far as coal seam gas goes, if Walloon Coal Measures are present there is coal and so there is also a chance that gas may also be present.

References/bibliography:

*O’Brien, P.E., Korsch, R.J., Wells, A.T., Sexton, M.J. Wake-Dyster, K. (1994) Structure and Tectonics of the Clarence-Morton Basin. In Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*O'Brien, P.E., Powell, T.G. & Wells, A.T. (1994). Petroleum Potential of the Clarence-Moreton Basin in Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*Russell, N.J. 1994. A Palaeogeothermal study of the Southern Clarence Moreton Basin in Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*Wells, A.T. and O'Brien, P.E. 1994. Lithostratigraphic framework of the Clarence-Moreton Basin. In Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.

A big pluton cut by the Clarence River

Yulgibar Bridge on the Clarence River

It has been some time since I spent a lot of time in the Clarence River area but occasionally I’ve got back there. Not too long ago I travelled along the Clarence Way. I took a quick detour down Lionsville Road when I came to the village of Baryulgil. Only a few kilometres down the road there is a quaint long low thin bridge over the Clarence. Just on the opposite side is a spot reguarly used as a swimming spot, there is also a tourist attraction for geologists (A road cutting).

The road cutting and the stream bank expose boulders of ‘granite’ rock which make up part of the New England Batholith. Right there at the bridge is a great spot to see one of rocks that make up what is called the Clarence Supersuite, a suite of ‘granites’ that have been derived from the melting of older igneous rocks. According to Bryant et al (1997) the Clarence River Supersuite for which this rock is a member is a type of ‘granite’ called an I-type. The ‘I’ stands for melted igneous in origin (as apposed to S-type for melted sedimentary). There is a lot to say about the New England batholith, its different granite types and its models of formation, such that I will do several blog posts in the future to cover this topic better.

The actual pluton in this area is called the Dumbudgery Granodiorite and it extends a few more kilometres to the north and for many kilometres to the south. Good outcrops can be seen in the hills on the southern side of Lionsville Road if you continue to the west a bit further. Indeed quartz veins in this area also contain small amounts of cinnabar (mercury ore), others contain some gold. If you want to take a sample or have a look at a fresh piece, a hammer (preferably a big one) with appropriate safety goggles is required. It is hard rock! But a fresh piece of granodiorite reveals a lovely white, pink and black speckled appearance. It can be so pretty that is is worth going on display.

Dumbudgery Granodiorite, fresh samples are very bright coloured

The colours of the rock reflect the mineral composition. Normal granite has a large proportion of alkali feldspar (sodium and potassium rich) relative to the amount of plagioclase (calcium and sodium rich) feldspar. A granodiorite like the Dumbudgery Granodiorite contains more plagioclase than alkali feldspar but still enough to be common in the rock. In the specimens at the Clarence River the plagioclase is a cloudy grey colour, sometimes difficult to distinguish from the quartz (tends to clearer) but the other alkali feldspar is a lovely bright pink colour. The lighter colours are contrasted by the two black minerals which are hornblende and biotite. The hornblende is identified by its hardness relative to the biotite which is a form of mica and therefore very easy to scratch. The Dumbudgery Granodiorite has been previously dated at 249 million years (very early Triassic period which is part of the mesozoic era).

Oddly the mass of granodiorite is actually bisected by the Clarence River. This is surprising given the prominent hills (and very hard rock) that the Dumbudgery Granodiorite is made from compared with the relative softness of the Clarence-Moreton Basin sedimentary rocks a short distance to the east. I’ve discussed why it is likely and surprising that the river has created this route in a previous post. Additonally, I’ve quickly discussed in another post the nearby unusual rock called the Gordonbrook Serpentinite which was mined at Baryulgil for asbestos. The Gordonbrook Serpentinite forms the eastern contact with the Dumbugery Granodiorite in Baryulgil area.

The Clarence river here is quite wide with large sand and gravel deposits moving every time it floods and altering its course. Historically some gold was found in this sand and gravel and is thought to be mainly sourced from gold in mineralised granitic rocks further up the river and in its tributaries. Some of the little deposits in the hills and much of the river itself was mined by the old timers, around the end of the 19th Century.

The area around Baryulgil is off the beaten track and Baryulgil itself is a bit of a delapidated little community but the area is worth a visit for its wonderful scenery and geological significance given its location at the edge of the mountainous New England region and the edge of the Clarence-Moreton Basin. Apparently the swimming and fishing are lovely too.

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.

Mining and the Bible?!

This is a bit of a different type of post than I usually do but I find this quite interesting! If you don't mind I'll put my 'philosophical hat' on.

I've reproduced a part of the Bible below - it is sort of an account of 'mining and gemstones', but more than that it is also a comment on how people are often motivated in life and see things like struggling for the riches of the earth as the most important thing. I agree with the author that the wisdom and understanding described are the most important things in life, not the search for personal riches.

It is also worth noting that in 'gemology' what some of the precious stones are considered to represent (these are the only ones I know, maybe someone else knows the ones I am missing):

• Lapis Lazuli - wisdom;
• Gold - wealth;
• Oryx - overcoming adversity;
• Silver - love;
• Topaz - loyalty or being righteous; and
• Rubies - desire

It is interesting to read the passage, but in particular the section 12-28 by replacing the gemstones with their popular meanings, it actually emphasizes the message even more.

Job 28
1 There is a mine for silver
and a place where gold is refined.
2 Iron is taken from the earth,
and copper is smelted from ore.
3 Mortals put an end to the darkness;
they search out the farthest recesses
for ore in the blackest darkness.
4 Far from human dwellings they cut a shaft,
in places untouched by human feet;
far from other people they dangle and sway.
5 The earth, from which food comes,
is transformed below as by fire;
6 lapis lazuli comes from its rocks,
and its dust contains nuggets of gold.
7 No bird of prey knows that hidden path,
no falcon’s eye has seen it.
8 Proud beasts do not set foot on it,
and no lion prowls there.
9 People assault the flinty rock with their hands
and lay bare the roots of the mountains.
10 They tunnel through the rock;
their eyes see all its treasures.
11 They search the sources of the rivers
and bring hidden things to light.

12 But where can wisdom be found?
Where does understanding dwell?
13 No mortal comprehends its worth;
it cannot be found in the land of the living.
14 The deep says, “It is not in me”;
the sea says, “It is not with me.”
15 It cannot be bought with the finest gold,
nor can its price be weighed out in silver.
16 It cannot be bought with the gold of Ophir,
with precious onyx or lapis lazuli.
17 Neither gold nor crystal can compare with it,
nor can it be had for jewels of gold.
18 Coral and jasper are not worthy of mention;
the price of wisdom is beyond rubies.
19 The topaz of Cush cannot compare with it;
it cannot be bought with pure gold.

20 Where then does wisdom come from?
Where does understanding dwell?
21 It is hidden from the eyes of every living thing,
concealed even from the birds in the sky.
22 Destruction and Death say,
“Only a rumor of it has reached our ears.”
23 God understands the way to it
and he alone knows where it dwells,
24 for he views the ends of the earth
and sees everything under the heavens.
25 When he established the force of the wind
and measured out the waters,
26 when he made a decree for the rain
and a path for the thunderstorm,
27 then he looked at wisdom and appraised it;
he confirmed it and tested it.
28 And he said to the human race,
“The fear of the Lord—that is wisdom,
and to shun evil is understanding.”

From deep within the earth lies Baryulgil

Deep within the earth below the seas (so deep in fact we begin to enter the Earths upper mantle) we find material that is solid but so hot that it is viscous. This material is very low in quartz and when we see this rock on the surface it is unusual. The only way for such rock to come to the surface is through great wedges being thrust on to the edges of continents as the great oceanic plates move on the mantle. The upper units of rock from oceanic plates is greywacke from turbidites from collapsing continental shelves or the pelagic sediment accumulated over vast periods of time. But also you will find volcanic rocks erupted under the water at mid-ocean ridges and below these great thicknesses of basalt cooled into columns and even further below these great plutons of the mafic rock called gabbro which is the source of the basalt on the surface. Yet even deeper we start transitioning into the mantle and here we find rock that contains very little silica (ultramafic rocks) but is rich instead in iron and magnesium. These are called peridotites and dunites when found in rock form. From top to bottom the section is called an ophiolite sequence and these occur infrequently on the earths surface.

Given that the highlands of the New England region are derived from accretionary material scrapped off the sea floor during collision with the Australian Plate we have a good chance to find some. And we are in luck. I know of three significant areas in this region where ophiolite is preserved the two biggest are located north of Tamworth along the peel fault and at Port Macquarie. A smaller area can be found north-west of Grafton at the little village of Baryulgil, located midway between Tabulam and Copmanhurst.

Sepentinite from a location south of Baryulgil, the host rock for the asbestos

The ophiolite at Baryulgil is unusual because only a portion of the ophiolite is preserved, this being the peridotite and dunite altered to a rock called serpentinite and a small area of gabbro. It is also worthy of note because of the damage such a rock has caused the local people. The serpentinite at Baryulgil is known as the Gordonbrook Serpentinite and includes such serpentine minerals as chrysotile – better known as a mineral of the asbestos group. Mining of this industrial mineral by Australian Asbestos and later by James Hardie occurred at Baryulgil for quite some time and it is this that has caused many problems.

Stepping slightly into the area of politics and aboriginal relations (and then quickly away again) the Baryulgil asbestos mine was often held as a wonderful example of how an indigenous population could be assimilated into the good things of western culture. Alas, as we know too well today that model of assimilation was flawed, in part in the case of Baryulgil because of the harm to its workers from such a carcinogenic material. Reportedly the mine and its processing plant had an appalling reputation for dust which is the main mechanism that causes the entry into the body and the subsequent long term damage including a massive increase in the risk of cancer. As an aside, it is worth noting that even the Nazi party in Germany before the Second World War (and greater than 40 years before the closure of the Baryulgil mine) introduced regulations to ensure that dust was minimised when working with asbestos because of the probable heath effects.

The Gordonbrook Serpentinite is a body approximately 25km long elongated unit right on the edge of the New England Fold Belt accretionary terrain. Geophysical surveys including gravity and magnetics indicate that the unit probably much larger than the area exposed as it appears to underlie the Clarence Morton basin just to the east of Baryulgil. The unit shows a gravity anomaly given its composition from heavy minerals and the magnetic signature shows up because of the richness of iron when compared to the more recent Jurassic aged sediments (Laytons Range Conglomerate and Gatton Sandstone) of the Clarence Moreton Basin and the accretionary complex meta-sediments to the west.

The gabbro unit of the ophiolite sequence is present as a small remnant unit on the north western most part of the serpentinite body on the northern side of the Clarence River. Interestingly the Clarence River pretty much runs straight though the middle of the serpentinite as it meanders from the mesozoic clarence moreton basin sediments into and out of the older accretionary terrain. This meandering has implications for indicating the history of the river development of the Clarence. But more about the Clarence River in another future post.

The minerals present in the serpentinite are mainly comprised of serpentine (a type called antigorite) but there is asbestos (chrysotile) occurring naturally in vein systems. Altered serpentinite also locally forms magnesite which is a white chalk like mineral formed through the affects of carbon dioxide rich ground water. The nature of the serpentinite and ground water alteration and reposition of secondary minerals is such that metals such as arsenic, and particularly nickel and cobalt are also quite rich in small patches. But these minerals are hard to come by unless intersected by cuttings or mine workings.

If you pass through that way to explore the more remote corners of our region take note of the roads. The councils that managed the area have previously maintained and unpgraded the roads with locally sourced rock. This means that the road base is often made from serpentinite. This has caused made road management problematic because the current Clarence Valley Council to minimise the risk of exposure to asbestos when staff or contractors are maintaining the roads!

Another feature of the Baryulgil Serpentinite is that it helps to demonstrate a theory about a major period of deformation in Eastern Australia. This formed tectonic features called the Coffs Harbour Orocline and the Texas Orocline, but there is too much to discuss about this now so I will have to dedicate a post about this in the future.

References/bibliography:

*Cornwell, J 2004 Hitlers Scientists: Science, War and the Devil's Pact. Penguin Books
*Henley, H.F. , Brown, R.E. , Brownlow, J.W. , Barnes, R.G. , Stroud, W.J. 2001 Grafton-Maclean 1:250 000 Metallogenic Map SH/56-6 and SH/56-7: Metallogenic Study and Mineral Deposit Data Sheets Geological Survey of New South Wales.
*Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.

Radioactive paradise (slightly)

The areas of the Tweed Valley, Nightcap National Park and Byron Bay are often seen as fresh clean and natural. Well, I can argue that especially Byron Bay may be a little unnatural but certainly there is a feeling of 'freshness' with the rainforests and the beaches. Given this, few people would think that you'd get a bigger dose of radiation from living in these areas than you would in Brisbane or Sydney (even living near the Lucas Heights Reactor).

Few people realise that radiation occurs naturally in the environments in which we live. Yes, most of you would know that the Sun is a thermonuclear power station bombarding Earth with gamma radiation on a daily basis. But it is also a natural part of the earth and actions either natural or man made can result in these areas being elevated in radiation. In the cases below the sources are formed through different ways but all provide an increase in radiation sometimes thousands of times higher or more than what would be considered background.

Let us look at the little village of Uki first. This little place is located in the Tweed River valley and is known for its rainforest surroundings and rugged, scenic landscapes. Geologically some of the area around Uki is situated on mesozoic aged rhyolite of the Chillingham Volcanics and this rock type provides an added level of radiation due to the minerals that exist naturally in it. But even more interesting is that a mineral exploration company discovered a very tiny sized but significant anomaly in the radiation levels just south of the village. The source was not clear but sampling showed that a five square metre anomaly existed in the already slightly elevated rhyolite terrain background radiation. Analysis showed a nearly 0.05% concentration in uranium which is quite high. This is many thousands of times higher than the normal level expected. The reason for this anomaly remains unknown.

Byron Bay is located on the southern side of expansive active and historic beach systems. Much of the Byron Bay area (and much of the north coast itself) was subjected to heavy mineral mining up until the 1980's but this has ceased now. The heavy minerals sought after were mainly titanium rich ilmanite and rutile and there are other heavy minerals too such as zircon and monazite. These minerals were naturally enriched through the processes of wave and tidal action which created zones amongst the dunes that were targeted for mining. But many of the left over heavy mineral sands were not needed once the rutile and ilmanite were removed. So the left over mineral sand was discarded in some cases used as fill for future building sites. Little did people realise monazite rich left over sand would cause issues which may be unsafe for building homes on. This is because monazite is a radioactive mineral and when the residually enriched sands were dumped this increased the concentration of thorium and uranium and the associated radiation. In fact this situation didn't just occur at Byron Bay but all along the north coast.

More broadly, but less significantly many areas where rhyolite or granite is the underlying rock also have higher than normal background radiation. This too is because of radioactive minerals being enriched naturally when these sorts of magmas are being formed. So this would apply to areas in or close to the national parks of the nightcap ranges and many areas inland in the headwaters of the northern rivers such as the Clarence or Bellinger Rivers and large expanses of the New England tablelands.

References/bibliography:

 *Pechiney Resources (1970). Report on air and ground prospection, Clarence-Moreton Basin, EL 278, Nimbin - Murwillumbah area. Unpubl. Exploration Progress Report.