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Discussions |
CSIRO Division of Exploration and Mining, PO Box 1130, Bentley, Western Australia 6102, Australia
E-mail: Raymond.E.Smith{at}csiro.au
Sir: Wilson et al. (2003), in their important paper on the Ridgeway Au-Cu deposit, describe hydrothermal alteration about the Ridgeway porphyry ore system. However, their findings are substantially more valuable if taken into account with the published works on regional burial metamorphism of the district (Smith, 1968, 1969). These works described burial metamorphism of the Ordovician volcanic sequences, including the hosts to the Ridgeway deposit, based on district and regional zonation of secondary mineral assemblages. This discussion briefly presents this context and focuses on the key issue of discriminating the porphyry-related hydrothermal alteration from the effects of burial metamorphism. Because porphyry-related hydrothermal minerals and later metamorphic mineral assemblages can be similar, their discrimination is crucial in mapping alteration styles as vectors in exploration for concealed ore deposits.
The burial metamorphism described by Smith (1968, 1969) ranges from prehnite-pumpellyite facies to middle greenschist facies. It is characterized by growth of hydrous alteration mineral assemblages developed particularly in mafic and intermediate volcanic rocks and associated sedimentary rocks. Increasing from lowest grade, the alteration zoning described by Smith (1969) is as follows: zone 1, quartz-albite-chlorite-epidote-carbonate; zone 2, quartz-albite-chlorite-epidote-prehnite-carbonate; zone 3, quartz-albite-chlorite-epidote-prehnite-pumpellyite-carbonate; zone 4, quartz-albite-chlorite-epidote- actinolite-carbonate; and zone 5, quartz-albite-chlorite-epidote-biotite-actinolite-carbonate. Wilson et al. (2003) describe similar mineral assemblages that are due to hydrothermal alteration arising from a subvolcanic, Ordovician porphyry Au-Cu system. The burial metamorphism described by Smith (1968, 1969) is interpreted to be due to accumulation of Ordovician and Silurian sequences, depth of burial, and a likely (ancient) gradient in regional heat flow.
Burial metamorphism would have overprinted earlier, porphyry-related hydrothermal alteration.
Wilson et al. (2003) describe relatively high temperature proximal alteration assemblages, their calc-potassic and potassic alteration assemblages, that can be discriminated from prehnite-pumpellyite facies assemblages. However, their distal alteration (inner propylitic, outer propylitic, and sodic alteration) would be difficult to distinguish from assemblages characteristic of zones 1 to 3, interpreted by Smith (1969) as products of regional burial metamorphism. In some of the areas surrounding Ridgeway, zones 4 and 5 occur and may present difficulties in distinguishing potassic or calc-potassic alteration related to concealed porphyry systems from burial metamorphic assemblages. Criteria for distinguishing porphyry-related hydrothermal mineral assemblages from burial metamorphism are, therefore, crucial in the use of alteration mapping in exploration.
The challenging issue of mapping porphyry-related hydrothermal alteration in burial metamorphic terrains would be fertile ground for a future important research topic. In addition to mineral assemblages discussed above, alteration style as seen in outcrop and drilling is important in distinguishing types of hydrothermal alteration and the effects of burial metamorphism. For example, Smith (1968) described conspicuously heterogeneous alteration in the Ordovician volcanic sequences of the Molong volcanic belt that hosts the Cadia porphyry deposits, including Ridgeway. Mesoscopic domains of epidote-quartz-sphene or pumpellyite-quartz-sphene that are set within an albite-chlorite "background" assemblage occur within 25 km of Ridgeway in burial metamorphic zone 3. The calc-silicate domains are clearly replacement features in which the original volcanic textures are commonly preserved.
In some cases, the boundaries between these burial metamorphic calc-silicate domains are joint- or fracture-controlled; in other cases, they are gradational or related to amygdular zones. Similar metamorphic development of heterogeneous assemblages occur widely in the district surrounding Ridgeway in basic and intermediate lavas and in pyroclastic and epiclastic rocks. This markedly heterogeneous burial metamorphism also has been described from other regions (Jolly and Smith, 1972; Smith, 1974; Smith et al. 1982). Similar intense alteration to epidote-quartz-sphene is described by Wilson et al. (2003) adjacent to their peripheral stage veins at Ridgeway. These authors recognize that burial metamorphism may have played a role in the development of these assemblages, stating that the veins may have had a protracted history of formation associated with repeated fault movement. This again emphasizes the need for studies that span porphyry-related hydrothermal alteration and burial metamorphism.
November 23, 2004
November 23, 2004; November 23, 2004
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References |
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 Top References |
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Jolly, W.T., and Smith, R.E., 1972, Degradation and metamorphic
differentiation of the Keweenawan tholeiitic lavas of northern Michigan, U.S.A.:Journal of Petrology
, v. 13, p. 273–209.
Smith, R.E., 1968, Redistribution of major elements in the alteration of some
basic lavas during burial metamorphism: Journal of Petrology, v. 9, p. 191–219.
——1969, Zones of progressive regional burial metamorphism in part of the
Tasman geosyncline, eastern Australia: Journal of Petrology, v.10, p.144–163.
——1974, The production of spilitic lithologies by burial metamorphism of flood basalts from the Canadian Keweenawan, Lake Superior, in Amstutz, G.C., ed., Spilites and spilitic rocks: Berlin, Springer-Verlag, p. 403–416.
Smith, R.E., Perdrix, J.L., and Parks, T.C., 1982, Burial metamorphism in the
Hamersley Basin, Western Australia: Journal of Petrology, v. 23, p. 75–102.
Wilson, A.J., Cooke, D.J. and Harper, B.J., 2003, The Ridgeway gold-copper
deposit: A high-grade alkalic porphyry deposit in the Lachlan fold belt, New
South Wales, Australia: Economic Geology, v. 98, p. 1637–1666.
This article has been cited by other articles:
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  A. J. Wilson, D. R. Cooke, and B. L. Harper THE RIDGEWAY GOLD-COPPER DEPOSIT: A HIGH-GRADE ALKALIC PORPHYRY DEPOSIT IN THE LACHLAN FOLD BELT, NEW SOUTH WALES, AUSTRALIA--A REPLY Economic Geology, January 1, 2005; 100(1): 177 - 178. [Full Text] [PDF]
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