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Weathering or hydrothermal alteration?
CATENA
Vol. 10, 57-59
Braunschweig 1983
WEATHERING OR HYDROTHERMAL ALTERATION? C.D. Oilier, Armidale SUMMARY Altered granites are sometimes attributed to deep weathering, and sometimes to hydrothermal alteration. The nature of the evidence for the latter is discussed, and found inadequate in many instances. The alteration at Bega, Australia, and Dartmoor, England, is considered to result from deep weathering. In a recent contribution to this journal DIXON & YOUNG (1981) described the Character and Origin of Deep Arenaceous Weathering Mantles on the Bega Batholith, Southeastern Australia. The present note is largely a comment on that paper, but ranges rather wider. Granites are commonly altered so that the feldspars and micas are converted to clay, but the quartz grains remain unaffected, and the resultant clay with quartz grains may be described as an "arenaceous weathering mantle". There is some debate on whether "arenization" describes the changes better than "kaolinization". It seems to me that the preservation of the "arenite" depends merely on the presence of quartz grains in the original rock: whether the alteration of feldspars and micas reaches the kaolin stage depends on the nature and degree of weathering of these other minerals. In deep weathering the product depends on various parameters, but not the temperature at the ground surface. Except in extreme conditions of frozen ground, so long as water is present the silicate minerals will be altered to clays. Which particular type of clay depends on other facts such as the amount of leaching. I know of no techniques that allow either the conclusion that the mantles formed under tropical conditions (a very frequent assertion) or that "the arenaceous granitic materials.., seem to have formed under humid temperate climates similar to those now experienced in southeastern Australia" (DIXON & YOUNG 1981, 97). The weathering of a rock depends on its mineralogy, and the most weatherable mineral often provides the "weak link in the chain" that allows the whole rock to be more weatherable than an otherwise similar rock. Biotite is base-rich and highly weatherable, so it is no surprise that in the Bega Batholith the central biotite granodiorite is highly weathered with few corestones while the surrounding granite which is more acidic is less weathered and has more corestones. The biotite distribution is paralleled by an abundance of the more weatherable plagioclase feldspar in the granite. The original mineralogy is probably quite enough to account for the differences in weathering style. The evidence for hydrothermal alteration seems to amount to two observations, both explicable in other ways. 1. Alteration of biotite to chlorite. The formation of chlorite during weathering is well documented, and in no way amounts to proof ofhydrothermal alteration. 2. Shatter cracks are present in quartz grains. Cracking of quartz grains is evident in both fresh and weathered granites, and details of the cracks have been studied and described in some detail, as instanced by BISDOM (1967), MOSS (1966) and BAYNES & DEARMAN
58
OLLIER
(1978). There is nothing in the account by DIXON & YOUNG to indicate hydrothermal alteration. Merely as a matter of principle it should be noted that hydrothermal action, as a major geological phenomenon, seems to have been losing ground in the past two decades. Many economic deposits once attributed to hydrothermal activity are now related to weathering (AMSTUTZ & BERNARD 1973, OLLIER 1977). Late stage hydrothermal alteration should be characterised by geochemical indicators, for the volatiles are likely to be concentrated in the last hydrothermal stage. Chemical indicators of hydrothermal alteration include possible increases in metal such as Au, Sn, Ag, Cu, Zn, Pb, and anions such as S, CI, F, sulphates, arsenides and antimonides are typical. No such indicators are reported from the Bega Batholith. Hydrothermal alteration not only causes alteration of some silicate minerals to clay, but is accompanied by mineralisation. This is not reported from the Bega Batholith. Even the most optimistic exploration geochemist will expect such enriched areas to be rare, not on a regional extent like the described alteration of the Bega granodiorite. GILLULY, WATERS & WOODFORD (1975, 421) note that "Hydrothermal deposits are not associated with all parts ofa pluton. Where erosion has been deep enough to disclose the form of the pluton, the ore deposits appear to be clustered about the higher parts ..." and "These relations suggest that the ore-depositing solutions were hot volatiles ... concentrated in higher parts of the pluton ..." yet in the Bega Batholith it is the lower part that is thought to be hydrothermally altered. The kaolin deposits of SE England such as on Dartmoor, provide a useful comparison. The china clay deposits were once thought to be ofhydrothermal origin (e.g. EXLEY 1958). I was amazed on my first visit to the area to find that the china clay regions contained all the alteration features and corestone arrangements that are associated with deep weathering, and seemed to be indistinguishable from it. Indeed TOMKEIEFF (1958) had suggested a weathering origin. SHEPPARD (1977) eventually resolved the problem by the application of hydrogen and oxygen isotope studies, which showed that weathering, not hydrothermal alteration, was the process that created the saprolites of Dartmoor. In brief, since weathering is common and hydrothermal alteration rare, it seems reasonable to assume the former unless the latter can be really demonstrated. As pointed out by KONTA (1969, 1970) the proofs ofhydrothermal alteration should be as rigorous as those for weathering. The presence in weathered material of chlorite and some cracked quartz grains falls far short of proof, and even short of a reasonable suggestion. In the absence of further data I will continue to believe that the deep mantles of the Bega Batholith are indeed caused by deep weathering.
BIBLIOGRAPHY
AMSTUTZ, G.C. & BERNARD, A.J. (1973): Ores in sediments. Springer. BISDOM, E.B.A. (1967): The role of micro-cracksystems in the spheroidal weathering of an intrusive granite in Galicia (NW Spain). Geologie en Mijnbouw, 46, 333-340. BAYNES, J. & DEARMAN, W.R. (1978): The microfabric of a chemicallyweathered granite. Bulletin of the International Association of Engineering Geology, 18, 91-100. DIXON, J.C. & YOUNG, ILW. (1981): Character and origin of deep arenaceous weathering mantles on the Bega Batholith, southeastern Australia. CATENA, 8, 97-109. EXLEY,C.S. (1958):Magmaticdifferentiationand alteration in the St. Austell granite. QuarterlyJournal of the Geological Society,London, 114, 197-227.
WEATHERING,HYDROTHERMALALTERATION
59
GILLULY, J., WATERS, A.C. & WOODFORD, A.O. (1975): Principles of Geology. Freeman, San Francisco. KONTA, J. (1969): Comparison ofthe proofs ofhydrothermal and supergene kaolinization in two areas of Europe. Proceedings of the International Clay Conference, Tokyo, 1, 281-290. KONTA, J. (1970): Comparison of the proofs ofhydrothermal and supergene kaolinization in two areas of Europe (Discussion). Proceedings of the International Clay Conference, Tokyo, 2, 91-93. MOSS, A.J. (1966): Origin, shaping and significance of quartz sand grains. Journal of the Geological Society of Australia, 13, 97-136. OLLIER, C.D. (1977): Applications ofwcathering studies. Ch. 1, 9-50 in Applied Geomorphology, ed. J.R. Hails, Elsevier. TOMKEIEFF, S.I. (1958): Discussions of paper by EXLEY. Quarterly Journal of the Geological Society, London, 114, 197-227. SHEPPARD, S.M.F. (1977): The Comubian batholith, SW England: D/H and 180/160 studies of kaolinite and other alteration minerals. Quarterly Journal of the Geological Society, London, 133, 573-592.
Address of author: C.D. Oilier, Department of Geography, University of New England Arrnidale, 2351, Australia.
Vol. 10, 57-59
Braunschweig 1983
WEATHERING OR HYDROTHERMAL ALTERATION? C.D. Oilier, Armidale SUMMARY Altered granites are sometimes attributed to deep weathering, and sometimes to hydrothermal alteration. The nature of the evidence for the latter is discussed, and found inadequate in many instances. The alteration at Bega, Australia, and Dartmoor, England, is considered to result from deep weathering. In a recent contribution to this journal DIXON & YOUNG (1981) described the Character and Origin of Deep Arenaceous Weathering Mantles on the Bega Batholith, Southeastern Australia. The present note is largely a comment on that paper, but ranges rather wider. Granites are commonly altered so that the feldspars and micas are converted to clay, but the quartz grains remain unaffected, and the resultant clay with quartz grains may be described as an "arenaceous weathering mantle". There is some debate on whether "arenization" describes the changes better than "kaolinization". It seems to me that the preservation of the "arenite" depends merely on the presence of quartz grains in the original rock: whether the alteration of feldspars and micas reaches the kaolin stage depends on the nature and degree of weathering of these other minerals. In deep weathering the product depends on various parameters, but not the temperature at the ground surface. Except in extreme conditions of frozen ground, so long as water is present the silicate minerals will be altered to clays. Which particular type of clay depends on other facts such as the amount of leaching. I know of no techniques that allow either the conclusion that the mantles formed under tropical conditions (a very frequent assertion) or that "the arenaceous granitic materials.., seem to have formed under humid temperate climates similar to those now experienced in southeastern Australia" (DIXON & YOUNG 1981, 97). The weathering of a rock depends on its mineralogy, and the most weatherable mineral often provides the "weak link in the chain" that allows the whole rock to be more weatherable than an otherwise similar rock. Biotite is base-rich and highly weatherable, so it is no surprise that in the Bega Batholith the central biotite granodiorite is highly weathered with few corestones while the surrounding granite which is more acidic is less weathered and has more corestones. The biotite distribution is paralleled by an abundance of the more weatherable plagioclase feldspar in the granite. The original mineralogy is probably quite enough to account for the differences in weathering style. The evidence for hydrothermal alteration seems to amount to two observations, both explicable in other ways. 1. Alteration of biotite to chlorite. The formation of chlorite during weathering is well documented, and in no way amounts to proof ofhydrothermal alteration. 2. Shatter cracks are present in quartz grains. Cracking of quartz grains is evident in both fresh and weathered granites, and details of the cracks have been studied and described in some detail, as instanced by BISDOM (1967), MOSS (1966) and BAYNES & DEARMAN
58
OLLIER
(1978). There is nothing in the account by DIXON & YOUNG to indicate hydrothermal alteration. Merely as a matter of principle it should be noted that hydrothermal action, as a major geological phenomenon, seems to have been losing ground in the past two decades. Many economic deposits once attributed to hydrothermal activity are now related to weathering (AMSTUTZ & BERNARD 1973, OLLIER 1977). Late stage hydrothermal alteration should be characterised by geochemical indicators, for the volatiles are likely to be concentrated in the last hydrothermal stage. Chemical indicators of hydrothermal alteration include possible increases in metal such as Au, Sn, Ag, Cu, Zn, Pb, and anions such as S, CI, F, sulphates, arsenides and antimonides are typical. No such indicators are reported from the Bega Batholith. Hydrothermal alteration not only causes alteration of some silicate minerals to clay, but is accompanied by mineralisation. This is not reported from the Bega Batholith. Even the most optimistic exploration geochemist will expect such enriched areas to be rare, not on a regional extent like the described alteration of the Bega granodiorite. GILLULY, WATERS & WOODFORD (1975, 421) note that "Hydrothermal deposits are not associated with all parts ofa pluton. Where erosion has been deep enough to disclose the form of the pluton, the ore deposits appear to be clustered about the higher parts ..." and "These relations suggest that the ore-depositing solutions were hot volatiles ... concentrated in higher parts of the pluton ..." yet in the Bega Batholith it is the lower part that is thought to be hydrothermally altered. The kaolin deposits of SE England such as on Dartmoor, provide a useful comparison. The china clay deposits were once thought to be ofhydrothermal origin (e.g. EXLEY 1958). I was amazed on my first visit to the area to find that the china clay regions contained all the alteration features and corestone arrangements that are associated with deep weathering, and seemed to be indistinguishable from it. Indeed TOMKEIEFF (1958) had suggested a weathering origin. SHEPPARD (1977) eventually resolved the problem by the application of hydrogen and oxygen isotope studies, which showed that weathering, not hydrothermal alteration, was the process that created the saprolites of Dartmoor. In brief, since weathering is common and hydrothermal alteration rare, it seems reasonable to assume the former unless the latter can be really demonstrated. As pointed out by KONTA (1969, 1970) the proofs ofhydrothermal alteration should be as rigorous as those for weathering. The presence in weathered material of chlorite and some cracked quartz grains falls far short of proof, and even short of a reasonable suggestion. In the absence of further data I will continue to believe that the deep mantles of the Bega Batholith are indeed caused by deep weathering.
BIBLIOGRAPHY
AMSTUTZ, G.C. & BERNARD, A.J. (1973): Ores in sediments. Springer. BISDOM, E.B.A. (1967): The role of micro-cracksystems in the spheroidal weathering of an intrusive granite in Galicia (NW Spain). Geologie en Mijnbouw, 46, 333-340. BAYNES, J. & DEARMAN, W.R. (1978): The microfabric of a chemicallyweathered granite. Bulletin of the International Association of Engineering Geology, 18, 91-100. DIXON, J.C. & YOUNG, ILW. (1981): Character and origin of deep arenaceous weathering mantles on the Bega Batholith, southeastern Australia. CATENA, 8, 97-109. EXLEY,C.S. (1958):Magmaticdifferentiationand alteration in the St. Austell granite. QuarterlyJournal of the Geological Society,London, 114, 197-227.
WEATHERING,HYDROTHERMALALTERATION
59
GILLULY, J., WATERS, A.C. & WOODFORD, A.O. (1975): Principles of Geology. Freeman, San Francisco. KONTA, J. (1969): Comparison ofthe proofs ofhydrothermal and supergene kaolinization in two areas of Europe. Proceedings of the International Clay Conference, Tokyo, 1, 281-290. KONTA, J. (1970): Comparison of the proofs ofhydrothermal and supergene kaolinization in two areas of Europe (Discussion). Proceedings of the International Clay Conference, Tokyo, 2, 91-93. MOSS, A.J. (1966): Origin, shaping and significance of quartz sand grains. Journal of the Geological Society of Australia, 13, 97-136. OLLIER, C.D. (1977): Applications ofwcathering studies. Ch. 1, 9-50 in Applied Geomorphology, ed. J.R. Hails, Elsevier. TOMKEIEFF, S.I. (1958): Discussions of paper by EXLEY. Quarterly Journal of the Geological Society, London, 114, 197-227. SHEPPARD, S.M.F. (1977): The Comubian batholith, SW England: D/H and 180/160 studies of kaolinite and other alteration minerals. Quarterly Journal of the Geological Society, London, 133, 573-592.
Address of author: C.D. Oilier, Department of Geography, University of New England Arrnidale, 2351, Australia.