PLAYFORD, PHILLIP¹, MALCOLM WALLACE², NICOLE DE KEVER², and REBECCA MASON^2
1Geological Survey of Western Australia, Perth, WA
²School of Earth Sciences, The University of Melbourne, VIC

Abstract: Devonian Stromatolites and Exhalative Mineralization, Canning Basin, Western Australia

Columnar stromatolites, intergrown with barite, form conspicuous buildups at several localities in the Devonian reef complexes of the Canning Basin. They occur within the uppermost Gogo Formation and lowermost Virgin Hills Formation, in belts up to 3 km long and 200 m wide.

The buildups developed during the early to middle Frasnian as sedimentary-exhalative (SEDEX) deposits, beside or within permeable zones where water, methane, hydrogen sulfide, and mineralizing fluids were being expelled through early compaction of basinal shales in the Gogo Formation. A transitional relationship is evident between the stromatolite buildups and cannon-ball concretions in the shales ('Gogo nodules'). The buildups and concretions are thought to have been precipitated by chemosynthetic bacteria, living in anoxic mud and fuelled by exhalative fluids and gases, below water depths of 200-300 metres.

Most of the exposed stromatolite-barite buildups are cut by gossans, up to several metres wide and tens of metres long. These are mainly derived from iron sulfides, but anomalously high zinc values are present in some, and minor galena-sphalerite mineralization occurs in the subsurface.

The stromatolites consist predominantly of fan-shaped aggregates of inclusion-rich radial-calcite crystals, with some interspersed sediment layers. The well-preserved microstructure of the calcite indicates an original calcitic (rather than aragonitic) mineralogy. Isotopic studies are in progress.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas