Abstract
Abra is an upper Proterozoic ( approximately 1,100 m.y.) sediment-hosted strata-bound Cu-Pb-Ba mineralization located some 900 km north-northeast of Perth in Western Australia. It was discovered in 1981, is spatially related to coincident magnetic and gravity anomalies, and is covered by 230 to 450 m of unmineralized clastic sediments. Drilling of eight exploration holes (5,000 m total length) during 1981 to 1983 established an estimated 130 million metric tons of mineralized rocks with an average grade of 1.15 percent lead, 0.25 percent copper, and 2.5 percent barium. The silver content is in the range of 2 to 50 ppm; the gold content is usually low and is irregularly distributed with a maximum value of 6.9 ppm over 6 m of drill core.Abra occurs near the eastern margin of the Jillawarra basin, a west-east-trending graben structure, in the central Bangemall basin which covers an area of 143,000 km 2 and consists of shallow-water sediments dated at 1,100 m.y. The host-rock sequence, in the lower Bangemall Group, comprises over 500 m of intertidal clastics, overlain by some 40 m of subaqueous evaporitic iron-formation deposited in a fault-bounded restricted basin. These ferruginous laminites (black zone) grade upward into hematitic sabkha sediments (red zone) which form the top of the mineralization. The hanging wall consists of more than 300 m of unmineralized coarse clastics and shallow-water turbidites.The major ore minerals are galena, chalcopyrite, barite, subordinate tetrahedrite, and minor pyrite. Most remarkable is the general paucity of zinc and the dominance of stratiform barite (40 m of drill core with more than 20% Ba) at the top of the mineralization.Relative metal abundances show a vertical zonation pattern of Cu-(Zn)-Pb-Ba, where Cu is restricted to the discordant feeder zone, Pb has an intermediate position, and some Ba is of concordant nature within the sabkha sediments. Mineralization is accompanied by chloritization and albitization. Sulfur isotope analysis yielded delta 34 S values of 21 per mil for sulfides and 40 per mil for barite.The mineralizing processes are linked to the geotectonic evolution of the Bangemall and the Jillawarra basins; a genetic model involves mobilization of metal-bearing fluids from an arkose of the basal Bangemall Group in response to anomalous heat flow. Emplacement of mineralization was crosscutting and replacive, within unconsolidated diagenetic sediments. Ascent of the hydrothermal fluids was induced by the generation of deep-seated faults in the coarse of the very first intracratonic rifting of the Bangemall basin.
