THE STRATIGRAPHIC AND STRUCTURAL SETTING OF THE BALCOOMA VOLCANOGENIC MASSIVE SULFIDE LENSES, NORTHERN QUEENSLAND

The Balcooma prospect is one of three volcanogenic massive sulphide deposits in the (?)Cambro-Ordovician Balcooma metamorphic belt in northern Queensland. The Balcooma metamorphic belt is an assemblage of multiply deformed metasedimentary and felsic metavolcanic rocks that has undergone amphibolite grade metamorphism. The deposit occurs in a metapelitic lens within a sequence of metagreywacke containing minor interbedded volcaniclastic rocks. Massive sulphide lenses at Balcooma are interpreted to occur at three distinct stratigraphic horizons within the metapelite. The central horizon contains reserves of 3.5 Mt of 3.0% Cu and occurs in the core of an antiformal syncline. The upper and lower horizons are dominated by Zn-Pb mineralization and occur in the limbs of folds. Chloritic alteration occurs in a pipe below the main Cu-rich lenses, whereas pyritic quartz-muscovite alteration is associated with Zn-Pb-rich lenses. Four generations of cleavage have been recognized at the surface, and two of these (S2 and S3) are pervasive. The first three deformation events (D1-3) are apparently coaxial with fold axes plunging to the southwest at 20-degrees. The prospect occurs on the eastern limb of an open D3 anticline and is bisected by a northwest trending D2 tear fault. To the northeast of this fault the structure is dominated by a completely overturned D2 anticline/syncline pair, but to the southwest of the fault the structure is dominated by moderately southeast dipping monocline that steepens down dip.