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THE RIDGEWAY GOLD-COPPER DEPOSIT: A HIGH-GRADE ALKALIC PORPHYRY DEPOSIT IN THE LACHLAN FOLD BELT, NEW SOUTH WALES, AUSTRALIA—A REPLY

Alan J. Wilson

Newcrest Mining Limited, Level 2, 20 Terrace Road, East Perth, WA 6004, Australia

David R. Cooke

Centre for Ore Deposit Research, University of Tasmania, Sandy Bay, Hobart, Tasmania, 7001, Australia

Benjamin L. Harper

Newcrest Mining Limited, Cadia Road, Orange, NSW 2800, Australia

Corresponding author: e-mail address, wilsonal@newcrest.com.au

The first 20% of the full text of this article appears below.

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 . . . [Full Text of this Article]