The development of Early Permian sequences and hydrothermal alteration in the Myall Syncline, central eastern New South Wales

The Myall Syncline, within the southern Tamworth Belt of the New England Fold Belt, contains a sequence of concentric meridionally folded Carboniferous to Early Permian shallow marine to fluvial‐lacustrine sedimentary and bimodal terrestrial volcanic rocks. The sequence spans a cycle of marine regression and transgression. The Late Carboniferous Koolanock Sandstone is overlain by the Muirs Creek Conglomerate, a possibly diachronous unit, which is succeeded apparently conformably by the Early Permian Markwell Coal Measures. Deposition of the coal measures took place during the intervals between extrusion of the members of the Alum Mountain Volcanics: the basal rhyolitic to dacitic predominantly ash flow Sams Road Rhyolite Member (new name); the Burdekins Gap Basalt Member; and the flow dominated Lakes Road Rhyolite Member. The Markwell Coal Measures had previously been interpreted as overlying the Alum Mountain Volcanics. The Late Permian marine Bulahdelah Formation disconformably overlies the Alum Mountain Volcanics but transgressed them without a significant hiatus.

Three epithermal‐type hydrothermal alteration systems have been recognized within the Alum Mountain Volcanics, the Markwell Coal Measures and the lower part of the Bulahdelah Formation. The largest and most intense system has affected a 6 km long part of the eastern limb of the Myall Syncline in the vicinity of Bulahdelah Mountain. An early quartz‐illite phase is overprinted by a sheet‐like zone of intense advanced argillic (alunite‐kaolinite) alteration imposed largely on the Sams Road Rhyolite Member but also extending down dip for almost 1 km into the Muirs Creek Conglomerate in the form of a funnel shaped zone. High concentrations of As, Sb and Hg and minor Au are present in the northernmost 1 km of the alteration zone and to a lesser extent along the southernmost 1 km of the zone.

The alteration was caused by hydrothermal fluids ascending steep fractures which intersect reactive sub‐horizontal zones of high porosity, resulting in extensive lateral hydrothermal alteration. There are indications that regional deformation post‐dates hydrothermal alteration, and hence occurred after the deposition of the Bulahdelah Formation.

Key words:

• Alum Mountain Volcanics
• alunite
• hydrothermal alteration
• kaolinite
• marcasite
• New England Fold Belt
• Sams Road Rhyolite Member