Copper and gold in skarn at Brown's Creek, Blayney, N.S.W

• Published in: Journal of the Geological Society of Australia Vol. 30; no. 3-4; pp. 431 - 442
• Main Author: Taylor, Geoffrey R.
• Format: Journal Article
• Language: English
• Published: Taylor & Francis Group 01.12.1983
• Subjects: calcsilicate rock; copper; Economic mineralisation; gold; metasomatism; Ordovician; skarn
• ISSN: 0016-7614
• DOI: 10.1080/00167618308729268

Mineralisation at Brown's Creek, Blayney, N.S.W., occurs in calcsilicate skarns at the contact between the Carcoar Granite and Ordovician limestones within the Angullong Tuff. Stratabound skarns have formed at the contact between limestone and several interbedded tuffaceous mudstone units. Vein-skarns in the limestone emanate from several sub-vertical fractures, and are possibly derived from underlying stratabound skarn. Within the calcsilicate skarns, several discrete mineral assemblages represent increasing degrees of carbonate replacement. Stratabound skarns, consisting of fine-grained bands of quartz, calcite and idocrase alternating with bands of epi-dote and sulphides, indicate a period during which diffusion processes were effective. Coarse-grained stratabound skarn is comprised of wollastonite, garnet, diopside, sulphides (pyrrhotite, chalcopyrite, arsenopyrite, tennantite) and gold. Vein-skarns are also composed of wollastonite, garnet and diopside, but are accompanied by bornite, chalcopyrite, tetrahedrite and gold. Field and textural relations suggest that the vein-skarns are probably of infiltration origin. A period of supergene alteration, associated with post-skarn faulting, is indicated by the development of chalcocite, covellite, copper oxides, native copper and silver sulphosalts in the vein-skarns. Epidote and actinolite in the stratabound skarn are indicative of a high fO 2 and high total fluid pressure, respectively, and the high fO 2 has facilitated the transport of gold in solution. The possible sources of the base and precious metals in the skarns are either magmatic or volcanic (derived from the tuffaceous beds in the Angullong Tuff, which shows evidence of possible volcanic exhalative activity). Metasomatism has resulted in an influx to the carbonate units of silica, iron, and base and precious metals.