Mallee Bull prospect: Widening the horizons within the Cobar Superbasin

Dr Nancy Vickery – University of New England                                                                                Rugby Club, 5:50pm, Thursday, May 31, 2012

 

In 2010 Peel Mining took up an exploration licence covering the old May Day gold mine and historic 4 Mile and Gilgunnia workings, 100 km south of Cobar.  Previous exploration work in the 4 Mile area included detailed geological mapping, surface geochemistry surveys and geophysical surveys.  Peel’s exploration strategy was to target both shallow and deeper mineralisation, based on features of Cobar-style deposits such as CSA.

Initial geophysical surveys conducted by Peel (heliborne VTEM) identified a coincident EM and magnetic anomaly under the historic 4 Mile workings.  A shallow <60m RAB program followed and failed to identify signs of a geochemical anomaly but proved useful in refining geological contacts.  Follow up fixed loop EM confirmed a strong conductor and provided the impetus to undertake a short RC drilling programme.  From March to July 2011 three RC drill programmes were undertaken to intersect the conductor, with all holes intersecting anomalous base metals. Down hole EM was conducted to further refine the location of the conductor and used to site a program of diamond drilling. In August a diamond tail was drilled on hole 6, reaching the conductor and returning high grade base metal mineralisation.  Subsequent drilling has also intersected strong base metal mineralisation and further work is planned.

The Mallee Bull prospect is hosted by Late Devonian-Early Silurian sediments of the Cobar Superbasin, tentatively within the Shume Formation and proximal to the overlying Upper Amphitheatre Group.  The deposit is located on a moderately to steeply dipping western limb of a major antiform.  Mineralisation is stratabound but is discordant to stratigraphy.  The hosting sequence comprises turbidite, including poorly to well bedded mudstones, siltstones and sandstones, exotic volcanic and limestone bodies and minor conglomerates.  The sequence has suffered weak to locally intense fabric development and generally good preservation of sedimentary features such as bedding, grading, cross beds and erosional features are noted.

Mineralisation and associated hydrothermal alteration of the hosting package is typical of many well documented Cobar-style deposits (eg CSA, Endeavor, Chesney, Shuttleton).  Alteration assemblages are dominated by green chlorite and locally intense silicification.  Quartz veining is locally abundant and is spatially associated with copper mineralisation.

Mineralisation is complex and includes massive sulphide lenses dominated by pyrrhotite, pyrite, sphalerite and lesser galena up to several metres wide.  Chalcopyrite is most abundant in stringer zones under massive sulphide intervals.  Silver is significantly enriched and gold is highly anomalous.  Metal zonation with depth is apparent in all drill holes and possible remobilisation, particularly of zinc and copper, is apparent.  Complex overprinting relationships of sulphide minerals indicates multiphase metal introduction.  The metal inventory includes Cu, Zn, Pb, Ag, Co, Bi, As, Au, Co and Sb.

Dr Nancy Vickery

Department of Earth Sciences

University of New England

Armidale NSW 2351