THE SCOTT SIMPSON LECTURE

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1 Economic geology of granite THE SCOTT SIMPSON LECTURE THE ECONOMIC GEOLOGY OF GRANITE - AN INTEGRATED VIEW D.A.C. MANNING Manning, D.A.C The economic geology of granite - an integrated view. Geoscience in South-West England, 13, Long associated with metallic ore minerals and industrial minerals such as china clay, granitic rocks are, in some parts of the world, also sources of petroleum and potentially geothermal energy. They may also have value as fertilizers, given that two of their constituent minerals (K-feldspar and mica) have been identified as sources of K for plant nutrition. This paper reviews briefly different aspects of the economic geology of granites, culminating in a conceptual generic model intended to provoke discussion and lateral thought. School of Civil Engineering & Geosciences, Newcastle University, Newcastle upon Tyne, NE1 7RU, U.K. ( david.manning@ncl.ac.uk) Keywords: granite, mineral deposits, industrial minerals, petroleum, geothermal energy, fertilizer. INTRODUCTION Granite, in the broadest sense, includes a suite of rocks with a mineralogical composition that characteristically includes quartz, alkali feldspars and biotite. Granitic rocks commonly occur in continental crust, as part of the crystalline basement beneath younger sedimentary cover. They vary in age, ranging from Precambrian to as little as 0.8 Ma, in the case of very young granites exposed in the Hida Mountain Range, central Japan (Ito et al., 2013). Following decades of controversy (Tuttle and Bowen, 1958), the origin of granite is now widely accepted to be magmatic, although many features of granites can best be explained by invoking the role of volatiles, which accumulate during crystallization. In addition to water, incompatible fluxes such as fluorine (Manning, 1979, 1981) and boron (Pichavant, 1981) become concentrated in late stage rocks, reducing melt viscosity (especially B) and allowing residual melts to persist to low temperatures (especially F). Once magmatic crystallisation is complete, residual aqueous fluids escape, carrying specific ore-forming elements in solution and leading to the eventual formation of mineral deposits. The economic value of granite (and related or similar rocks) has long been recognized, with a history of mining of graniteassociated metal ores dating back to prehistoric times (Dines, 1956). At the present time, granite is immediately associated with the production of tin and tungsten, and is associated with a wide range of other ores and industrial minerals such as fluorite and china clay. The potential of granites as sources of geothermal energy has a relatively long history, which continues. What is less well known is the occurrence in some parts of the world of economic deposits of petroleum within granitic rocks. Additionally, their constituent minerals possibly have use as alternatives to chemical fertilizers, enabling granites to respond to the need to feed the rising world population. In this paper, a brief overview is given of the economic geology of granite, in its broadest sense, with the intention of stimulating novel thinking behind the development of the conceptual models that synthesise the value of granite to humanity. GRANITES AND METAL ORE DEPOSITS The association between granites and metal ore deposits was investigated rigorously in South-West England long ago. The zoned distribution of minerals around granite cusps was described by Hosking (1951; Figure 1), and remains a valuable field-based observation. Further work by Jackson et al. (1989) describes the spatial distribution of minerals in more detail. Although Figure 1 emphasises the importance of mineral veins, the accumulation of volatiles as crystallization of the melt proceeds gives rise to low viscosity magmas that precipitate pegmatites (London, 2008). Pegmatites vary greatly in their mineralogical composition, but can be associated with enrichment in incompatible elements that are increasingly important as raw materials for modern technology. An example of a deposit with a history of producing tantalum is Tanco in Manitoba (Stilling et al., 2006), where a wide range of minerals Figure 1. Mineral zoning around granitic cusps in Cornwall and Devon (from Hosking, 1951). Reproduced with permission from the Royal Geological Society of Cornwall. 248

2 D.A.C. Manning occur in a complex pegmatite (Figure 2). In South-West England, although there are similarities between the topaz granites and the rare metal pegmatites (Manning et al., 1996), metallic mineral deposits typically occur as veins, filling fissures or faults (Figure 3) or as stockworks of Figure 2. Underground face in the Tanco pegmatite, showing coarse mineral textures and association of beryl with columbite-tantalite. Figure 4. Stockwork of thin quartz-wolframite veins at Cligga Head. Figure 3. Tin lode in Geevor Mine (Coronation Lode, 14 Level), photographed in thin closely spaced veins (Figure 4). The ore minerals include oxides (cassiterite, wolframite) in the highest temperature zones, with sulphides (chalcopyrite and other copper sulphides, galena, sphalerite, arsenopyrite, etc.) more important moving further from the granite into ores formed at lower temperatures. The concept of mineral zoning associated with granites underpinned genetic models of the North Pennine Orefield (Dunham, 1990), but was challenged in the early 1960s when a deep borehole discovered that the Weardale Granite was older than and unconformably overlain by the mineralisation s host rocks (Dunham et al., 1965). Figure 5. General distribution of kaolinisation within granite (based on Bristow and Exley, 1994). 249

3 Economic geology of granite INDUSTRIAL MINERALS Following the heyday of metallic mineral production in South-West England, the industrial minerals took precedence, especially with the production of china clay, a kaolin-rich commodity. Kaolinisation of granite in South-West England, especially the St Austell Granite (Psyrillos et al., 1998; Manning and Exley, 1984), massively alters the pre-existing rock to a matrix of kaolinised feldspars that can be mined by washing. The zones of kaolinisation relate to the occurrence of veins, faults and joints within the granite (Figure 5), which are amenable to working in large open pits (Figure 6). In these, as well as occurring as a kaolinised mass of granite, kaolin also occurs as discrete veins (Figure 7). Figure 6. Large scale working of kaolinised granite (Lee Moor, 2000). Figure 7. Kaolin vein approximately 1 cm wide, in kaolinised granite (St Austell). Table 1. Amount and value (2012 prices) of minerals produced in historical times from granites in England (based on Burt, 1998; Beer and Scrivenor, 1982; Dunham, 1990). The value of mineral products from granite is substantial, as summarized in Table 1, which estimates a total product value of almost 35 billion from the orefield throughout its total life, perhaps as much as 2,000 years. The Cornish deposits account for most of this, having produced 90% of the value mined. However, despite the size of this sum, it is only 1% of the value of petroleum produced from the North Sea over the last 50 years. 250

4 D.A.C. Manning ENERGY FROM GRANITE Exploration for geothermal energy from granite started in earnest in South-West England with the hot-dry rock project in the late 1970s, when three deep boreholes were drilled at Rosemanowes quarry in the Carnmenellis Granite (Downing and Gray, 1986; Richards et al., 1994). After about 15 years the work came to an end, contributing much to knowledge about granites as a source of geothermal energy, but with no recovery of energy from the boreholes drilled at the site. In 2004, a 1-km deep borehole (Figure 8) was drilled to investigate the geothermal potential of the Weardale Granite at Eastgate, County Durham (Manning et al., 2007). This penetrated the overlying Carboniferous sequence (including the Whin Sill), and was intended to intersect a major mineral vein (the Slitt Vein) and associated splays, which was known to be a pathway for fluid flow (Manning and Strutt, 1990). Considerable amounts of warm water were encountered in fissures 150 m below the unconformity, and pumping trials produced water with a yield of 1,600 m3day-1 at 46 C largely from fissures at m depth. This source of geothermal energy has yet to be developed into a commercial reality. In addition to geothermal energy, granite is also a source of petroleum, from fractured basement reservoirs. These are significant in a number of petroleum provinces, including offshore Vietnam, where production rates of up to 15,000 barrels per day from granodiorite are reported (Cuong and Warren, 2009). Descriptions of the reservoir rock character published in the scientific literature are rather few, but images of cores from these fields (Figure 9), and of associated conceptual models, show much that is similar to observations that could be made at field exposures of the South-West England granites, especially in the St Austell china clay pits. The link between the petroleum system and kaolinisation in the granitic rocks of South-West England has been explored by Psyrillos et al. (2003), who investigated the formation of kaolin within granite with techniques used to understand diagenetic changes in petroleum reservoir rocks. They showed that kaolinisation could have occurred as a consequence of reaction between the granite and basinal brines, thus having no genetic connection with the hydrothermal systems that resulted in metal ore formation. Both basinal brines and meteoric water could be drawn into the granite (a source of heat due to its natural radioactivity), rising to its crest and in the process reacting with the rock to produce the secondary mineral assemblage seen today. This concept could equally well be applied to the North Pennine Orefield, given that the brines responsible for mineralization and the geothermal waters encountered at Eastgate clearly postdate the exposure and weathering of the granite. GRANITES Figure 9. Cores from the Bach Ho Field, Vietnam, showing oil (brown; indicated by red arrows) in fractured granodiorite. Scale bar 5 cm. Reproduced with permission from Cuong and Warren (2009). AND FOOD SECURITY Looking to the future, there is no doubt that the world s soils will struggle to feed the anticipated population of 9 billion people in The key nutrients that plants remove from soils are N, P and K, and of these both P and K are derived ultimately solely from geological sources. At the present time, P fertilizers are produced from phosphate rock, predominantly sedimentary calcium phosphates but with some coming from igneous phosphate rock. K fertilizers are produced from evaporites, targeting minerals such as sylvite (KCl: 63% equivalent K2O) and carnallite (MgCl2.KCl.6H2O: 17% equivalent K2O). Granites contain potassium feldspar as an essential mineral, and in principle this mineral is a source of K, like carnallite containing up to 17% K2O. A number of studies have attempted to use potassium feldspar as a source of K, dating back 90 years (Goldschmidt, 1922). In general, although biotite can be shown to be equally effective as KCl (Mohammed et al., 2014), these studies have failed to show a convincing link that demonstrated feldspar to be an effective source of K, and this is partly due to 251 Figure 8. Conceptual design of the Eastgate borehole (from Manning et al., 2007). Figure 10. Weathered surface of potassium feldspar from soil developed on granite, St Austell.

5 Economic geology of granite Figure 11. Distribution of P2O5 in stream sediments in the Carnmenellis Granite; the P2O5 content increases with degree of shading of the symbols, to a maximum of >2,300 ppm (Hosking, 1964). Reproduced with permission from the Royal Geological Society of Cornwall. the design of the experiments used to test the feldspar. Recent work (Mohammed et al., 2014) has shown that plants are able to access K in feldspars, although more research still needs to be done in this area. Nevertheless, there is no doubt that feldspars do weather in soils, and that soil microbes are involved in that process (Figure 10). In addition to K, feldspars are the most abundant mineral host for P. The berlinite substitution Al+P = 2Si allows potassium feldspar to approach a hypothetical composition corresponding to KAl2PSiO8 (Manning, 2008). The P content of feldspars is typically less than 1% P2O5 by weight, but exceeds 0.1%. Given the very common occurrence of feldspar, this provides a widely available low level source of P. An indication of the P content of granitic feldspar comes from Hosking s work on determining P in stream sediments from the Carnmenellis Granite (Hosking, 1964). These show zoning (Figure 11), with up to 1,491-2,300 ppm P2O5 in the outer zone of the granite, and 701-1,030 ppm in the central zone. Although aimed at metallic mineral exploration, this work by Hosking demonstrates the occurrence of a dispersed plant nutrient in weathered material. Hosking was cited at the start of this paper, and his observations on P bring it to an end, in the context of the use of granite as a possible source of mineral nutrients to meet the needs of food security. 252

6 D.A.C. Manning Figure 12. Conceptual model of the economic geology of granites. A CONCEPTUAL MODEL: THE ECONOMIC GEOLOGY OF GRANITE This brief overview of links between mineral deposits (in their broadest sense) and granitic rocks leads to a conceptual model that accommodates different geological processes (Figure 12). First, the primary magmatic processes that lead to the formation of granite from a magma create the host rock, with late-stage pegmatites that in some cases (such as Tanco) have value as sources of rare metals. This is followed by the earliest hydrothermal stage, in which high temperature mineral veins are formed, with tin and tungsten mineralization progressing with cooling through copper to zinc and lead. A close spatial association with the cooling granite may be associated with a magmatic source for these metals, especially tin and tungsten. Then, the role of the granite changes to being dominated by its character as a permeable quartz-feldspar rock that is capable of generating or focusing heat flow, and so fluid flow. Interaction with neighbouring sedimentary basins allows petroleum (and associated waters) to enter the granite, and if trapped by virtue of cover with impermeable sediments, the granite can become a petroleum reservoir. The movement of hot water gives potential for geothermal energy, but such waters may be associated with the early stages of kaolinisation, a process that is dominated by surface weathering which demonstrates the attractive nature of granite minerals to soil biota. There is therefore a continuum of geological activity associated with granites that in favorable circumstances creates resources of economic interest. CONCLUSIONS This review has briefly explored a range of aspects of the economic geology of granite. History has shown that granite has value as a source of ore minerals, and it continues to do so. Valuable products such as dimension stone have been ignored, but the use of granite as a source of china clay, a term covering a range of kaolin products, has to some extent picked up where metals left off, at least in South-West England. The concept of granite as a petroleum reservoir rock is much less familiar, but very important in some parts of the world, and offers a new paradigm for understanding the origin of the South-West England kaolin deposits. Recent work on granite-associated geothermal energy in northern England has stimulated interest in such resources more widely, and we might expect more activity in South-West England in the coming years. Granite contains minerals that are known as sources of K for plant nutrition, especially biotite, and the surfaces of weathered feldspars indicate that biologically-mediated soil processes encourage feldspar corrosion. Importantly, feldspars are a globally significant host for P, and it may well be that this is what attracts the microbes responsible for accelerating weathering. Granites play a role in economic geology in very many fields. They have done so ever since mining began, and they will no doubt continue to do so as long as society requires geological resources. REFERENCES BEER, K.E. and SCRIVENER, R.C Metalliferous mineralization. In: DURRANCE, E.M. and LAMING, D.J.C. (Eds), The Geology of Devon. University of Exeter Press, Exeter, BURT, R History of metalliferous mining. In: SELWOOD, E.B., DURRANCE, E.M. and BRISTOW, C.M. (Eds), The Geology of Cornwall. University of Exeter Press, Exeter, CUONG T.X. and WARREN J.K Bach Ho field, a fractured granitic basement reservoir, Cuu Long Basin, offshore SE Vietnam: a buried hill play. Journal of Petroleum Geology, 32, DINES, H.G The Metalliferous Mining Region of South-West England, Volume 1. Memoir of the Geological Survey of Great Britain, London. DOWNING, R.A. and GRAY, D.A. (Eds) Geothermal energy - the potential in the United Kingdom. British Geological Survey. H.M.S.O., London. DUNHAM, K.C Geology of the Northern Pennine Orefield, Volume 1 - Tyne to Stainmore. Economic Memoir of the British Geological Survey. H.M.S.O., London. DUNHAM, K.C., DUNHAM, A.C., HODGE, B.L. and JOHNSON, G.A.L Granite beneath Visean sediments with mineralization at Rookhope, northern Pennines. Quarterly Journal of the Geological Society of London, 121, GOLDSCHMIDT, V.M Oversikgtskart over utbredelsen av de forskjellige kalimineraler I norsk fjeldgrund. Norsk Landmansblad, 41, HOSKING, K.F.G Primary ore deposition in Cornwall. Transactions of the Royal Geological Society of Cornwall, 18, HOSKING, K.F.G Permo-Carboniferous and later primary mineralization of Cornwall and south-west Devon. In: HOSKING, K.F.G. and SHRIMPTON, G.J. (Eds), Present views of some aspects of the Geology of Cornwall and Devon. Royal Geological Society of Cornwall, Truro,

7 Economic geology of granite ITO, H., YAMADA, R., TAMURA, A., ARAI, S., HORIE, K. and HOKADA, T Earth s youngest exposed granite and its tectonic implications: the Ma Kurobegawa Granite. Scientific Reports 02/2013, 3, DOI: /srep JACKSON, N.J., WILLIS-RICHARDS, J., MANNING, D.A.C. and SAMS, M.S Evolution of the Cornubian Orefield of SW England. Part II: Mineral deposits and ore-forming processes. Economic Geology, 84, LONDON, D Pegmatites. Canadian Mineralogist Special Publication 10, Mineralogical Association of Canada, Quebec. MANNING, D.A.C An experimental study of the effect of fluorine, in addition to water, on crystallisation in the system Qz-Ab-Or, and its application to Cornish granitic rocks rich in fluorine. Proceedings of the Ussher Society, 4, MANNING, D.A.C The effect of fluorine on liquidus phase relationships in the system Qz-Ab-Or with excess water at 1 Kb. Contributions to Mineralogy and Petrology, 76, MANNING, D.A.C Introduction to Industrial Minerals. Chapman and Hall, London. MANNING, D.A.C Phosphate minerals, environmental pollution and sustainable agriculture. Elements, 4, MANNING, D.A.C. and EXLEY, C.S The origins of late-stage rocks in the St Austell granite - a reinterpretation. Journal of the Geological Society of London, 141, MANNING, D.A.C. and STRUTT, D.W Metallogenetic significance of a North Pennine springwater. Mineralogical Magazine, 54, MANNING, D.A.C., HILL, P.I. and HOWE, J.H Primary lithological variation in the kaolinised St. Austell Granite, Cornwall, England. Journal of the Geological Society of London, 153, MANNING, D.A.C., YOUNGER, P.L., SMITH, F.W., JONES, J.M., DUFTON, D.J. and DISKIN, S A deep geothermal well at Eastgate, Weardale, UK: a novel exploration concept for low-enthalpy resources. Journal of the Geological Society of London, 164, MOHAMMED, S.O., BRANDT, K., GRAY, N.D., WHITE, M.L. and MANNING, D.A.C Comparison of silicate minerals as sources of K for plant nutrition in sandy soil. European Journal of Soil Science, 65, DOI: /ejss PICHAVANT, M An experimental-study of the effect of boron on a water saturated haplogranite at 1-kbar vapor-pressure - geological applications. Contributions to Mineralogy and Petrology, 76, PSYRILLOS, A., MANNING, D.A.C. and BURLEY, S.D Geochemical constraints on kaolinisation in the St. Austell Granite, Cornwall, England. Journal of the Geological Society of London, 155, PSYRILLOS, A., BURLEY, S.D, MANNING, D.A.C. and FALLICK, A.E Coupled mineral-fluid evolution of a basin and high: kaolinization in the SW England granites in relation to the development of the Plymouth Basin. In: PETFORD, N. and McCAFFREY, K.J.W. (Eds), Hydrocarbons in Crystalline Rocks. Geological Society Special Publication, 214, RICHARDS, H.G., PARKER, R.H., GREEN, A.S.P., JONES, R.H., NICHOLLS, J.D.M., NICOL, D.A.C., RANDALL, M.M., RICHARDS, S., STEWART, R.C. and WILLIS- RICHARDS, J The performance and characterisitics of the experimental hot dry rock geothermal reservoir at Rosemanowes, Cornwall ( ). Geothermics, 23, STILLING, A., CERNY, P. and VANSTONE, P.J The Tanco pegmatite at Bernic Lake, Manitoba. XVI. Zonal and bulk compositions and their petrogenetic significance. Canadian Mineralogist, 44, TUTTLE, O.F. and BOWEN, N.L Origin of granite in the light of experimental studied in the system NaAlSi3O8-KAlSi3O8-SiO2-H2O. Geological Society of America Memoir, 74, New York. 254