|
|
 |
|||||||||||||||||
|
|||||||||||||||||
 |
|||||||||||||||||
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Discussions |
Newcrest Mining Limited, Level 2, 20 Terrace Road, East Perth, WA 6004, Australia
Centre for Ore Deposit Research, University of Tasmania, Sandy Bay, Hobart, Tasmania, 7001, Australia
Newcrest Mining Limited, Cadia Road, Orange, NSW 2800, Australia
Corresponding author: e-mail address, wilsonal{at}newcrest.com.au
Sir: In his commentary on the Wilson et al. (2003) paper on the Ridgeway alkalic porphyry Au-Cu deposit, Smith (2005) highlights some of the difficulties in distinguishing between zones of propylitic alteration that characterize the distal portions of porphyry (and other) hydrothermal systems and mineralogically similar assemblages related to regional low-grade burial metamorphism. This problem has long been recognized, and explorers and researchers of porphyry deposits have to contend with it on a daily basis. The problem has recently been reviewed by Gifkins et al. (2005), who provide criteria to help discriminate between hydrothermal and metamorphic mineral assemblages in altered volcanic rocks.
Smith (2005) highlights that some of the subgreenschist facies metamorphic assemblages documented throughout the Ordovician volcanic sequence of the wider Ridgeway-Cadia region (zones 1, 2, and 3 of Smith, 1969) are mineralogically similar to distal (inner propylitic, outer propylitic, and sodic) hydrothermal alteration zones documented from Ridgeway and other porphyry deposits of the Cadia district (Tedder et al., 2001; Holliday et al., 2002). However, these distal alteration zones are precisely that—zones of alteration that have a near-concentric distribution around the higher temperature (potassic and calc-potassic) alteration assemblages in the core of the Ridgeway deposit (Wilson et al., 2003). This spatial arrangement differs dramatically from regional-scale metamorphic zonation.
The inner propylitic assemblage (chlorite-albite-hematite-magnetite-epidote) at Ridgeway extends between 200 and 500 m from the mineralized potassic core of the deposit, which, in turn, is centered within and around a pipelike quartz monzonite porphyry complex. The outer limit of the inner propylitic assemblage is defined by the disappearance of hematite, a product of magmatic-hydrothermal oxidation (Wilson et al., in prep.). Alteration intensity within the inner propylitic zone at Ridgeway diminishes upward and outward from the central zone of calc-potassic alteration. The concentric spatial distribution of this alteration assemblage around the quartz monzonite porphyry complex supports the interpretation that it is genetically related to the Ridgeway magmatic-hydrothermal system.
The sodic (albite-quartz ± pyrite) alteration zone at Ridgeway occurs as sporadically developed "screens" close to the contact between the inner and outer propylitic alteration zones (Wilson et al., 2003). These screens have variable intensities of hydrothermal alteration and include some of the most intensely developed, texturally destructive secondary mineral assemblages that have been developed in the Ridgeway host rocks. The alteration assemblage is not restricted to any particular facies of the Forest Reef Volcanics, and its spatial distribution and variable intensity over short distances vertically and laterally (dimensions of up to 100 x 100 x 400 m) are difficult to reconcile with regional metamorphic processes. Instead, we consider the spatial distribution of this alteration assemblage to be consistent with metasomatism that occurred in an outflow zone to the Ridgeway magmatic-hydrothermal system.
The outer limits of the outer propylitic zone at Ridgeway are poorly defined mineralogically and texturally, and it is this assemblage that cannot be discriminated with any confidence from background regional metamorphism (burial metamorphic zone 1: quartz-albite-chlorite-epidote-carbonate of Smith, 1969). The scope of the Wilson et al. (2003) study precluded an examination of the secondary mineral assemblages farther than 1 km away from the center of the Ridgeway deposit. Such a study would prove challenging, owing to the presence of Quaternary alluvial sediments, Tertiary basalt, and Silurian sedimentary rocks that obscure the Ordovician volcano-sedimentary sequences. Nevertheless, such work may help to answer some of the remaining questions highlighted by Smith (2005).
In consideration of higher grade metamorphic assemblages, Smith (2005) notes the occurrence of his zone 4 (quartz-albite-chlorite-epidote-actinolite-carbonate) and zone 5 (quartz-albite-chlorite-epidote-biotite-actinolite-carbonate) mineral assemblages in the general vicinity of Ridgeway. A review of the metamorphic mineral assemblages used by Smith (1969) to define these higher grade metamorphic zones suggests that the assemblages have been superimposed spatially on each other and onto lower grade "metamorphic" assemblages in many areas covered by the Dubbo 1:250,000 sheet (Barron, 1997). Their juxtaposition cannot be explained readily by a single episode of burial metamorphism but can be explained easily if they are the product of the steep thermal gradients and metasomatic processes that occur above and to the sides of magmatic-hydrothermal systems.
Smith’s (1969) zone 4 and zone 5 mineral assemblages may be difficult to distinguish mineralogically from hydrothermal potassic and calc-potassic alteration assemblages that are related to porphyry systems such as Ridgeway. However, at both Ridgeway and the nearby Cadia East deposit (Tedder et al., 2001), in addition to the Goonumbla porphyry deposits, which are also related to similar-aged Ordovician volcano intrusive complexes west of the Cadia district (Heithersay and Walshe, 1995; Lickfold et al., 2003), mineral assemblages that contain actinolite and/or biotite have a strong spatial association with the core of the porphyry gold-copper mineralized systems and the associated quartz monzonite porphyry intrusions. They are also associated with locally abundant magnetite veining and alteration, which has resulted in rocks that contain up to 5 to 10 wt percent magnetite. Such chemical transformations most likely require intensive metasomatism, most readily achieved by hot and saline hydrothermal fluids. It is difficult to conceive how isochemical metamorphism could produce this degree of hypogene iron enrichment.
From an exploration perspective, particularly in a region now known to contain globally significant economic mineral deposits, the presence of secondary biotite should always warrant close examination. Perhaps the occurrence of Smith’s (1969) zones 4 and 5 mineral assemblages in some of the areas surrounding Ridgeway and elsewhere in the Ordovician volcanic belts of New South Wales should be reexamined for their porphyry mineral potential in light of the relatively recent recognition of porphyry-style alteration and mineralization in the Cadia district.
 |
References |
|---|
 Top References |
|---|
Barron, L., 1997, Overview of metamorphic zonation on the Dubbo 1:250 000 geological sheet, in Morgan, E.J., ed., Geology and metallogenesis of the Fifield-Peak Hill-Wellington-Gulgong-Mudgee region, New South Wales, Field Conference Guide, New South Wales Geological Survey Report GS1997/080, p. 53–55.
Gifkins, C., Herrmann, W. and Large R., 2005, Altered volcanic rocks; Hobart, Tasmania, Centre for Ore Deposit Research, University of Tasmania, 275 p.
Heithersay, P.S., and Walshe, J.L., 1995, Endeavour 26 North; a porphyry
copper-gold deposit in the Late Ordovician, shoshonitic Goonumbla volcanic
complex, New South Wales, Australia: Economic Geology, v. 90, p. 1506–1532.
Holliday, J.R., Wilson, A.J., Blevin, P.L., Tedder, I.J., Dunham, P.D., and Pfitzner, M., 2002, porphyry gold-copper mineralisation in the cadia district, eastern lachlan fold belt, New South Wales, and its relationship to shoshonitic magmatism: Mineralium Deposita, v. 37, p. 100–116.[CrossRef][Web of Science][GeoRef]
Lickfold, V, Cooke, D.R., Smith, S.G. and Ullrich, T.D., 2003, Endeavour
copper-gold porphyry deposits, Northparkes, New South Wales: Intrusive history
and fluid evolution: Economic Geology, v. 98, p. 1607–1636.
Smith, R.E., 1969, Zones of progressive regional burial metamorphism in part
of the Tasman geosyncline, eastern Australia: Journal of Petrology, v. 10, p.144
–163.
——2005, The Ridgeway gold-copper deposit: A high-grade alkalic porphyry
deposit in the Lachlan fold belt, New South Wales, Australia—a discussion:Economic Geology
, v. 100, p. 175–176.
Tedder, I.J., Holliday, J., and Hayward, S., 2001, Discovery and evaluation drilling of the Cadia Far East gold-copper deposit: NewGen Gold 2001—Case Histories of Discovery: Perth, Australian Mineral Foundation, p. 171–184.
Wilson, A.J., Cooke, D.R. and Harper, B.L., 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.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
