Petrology, mineral chemistry, and exploration significance of Fe-sulfides from the metal dispersion halo surrounding the Cadjebut Zn-Pb MVT deposit, Western Australia

The Cadjebut MVT mineralization is hosted by carbonate-evaporite units in the Givetian lower dolomite sequence of the Pillara platform, northern Western Australia, and occurs as 2 ore types: (1) An early Zn-rich, stratiform and rhythmically banded ore and its laterally equivalent halo of banded marcasite, barite +/- calcite; and (2) a later cross-cutting, breccia-fill, Pb-rich ore. The 2 ore types are exploited through 2 mining lenses; an upper lens 1 and lower lens 2. The orebody is sited N of the normal, basin-side down, Cadjebut Fault. Rhythmically banded marcasite completely envelopes the orebody in both lenses, whereas banded barite occurs N of the lower lens 2 only where the barite is partially to completely replaced by late cloudy calcite + marcasite. Barite replacement by marcasite and cloudy calcite increases northwards. Petrographic and trace element data indicate 3 types of Fe-sulfides: (1) low-Pb Fe-sulfides with Pb contents 30 000 ppm Pb. Only the 'colloform' pyrites exhibit Pb contents above 30 000 ppm, and SEM images show these elevated Pb contents are due to micron-scale galena inclusions. Mineral chemical data from sulfides in the stratiform Fe-sulfide and barite halos indicate that main ore-stage fluids of the rhythmically banded ore event were enriched in Zn, Pb, Cd, As and, to a minor extent Ni and Cu. Late-stage fluids of the rhythmically banded ore event were not saturated with respect to galena, precipitating low-Pb marcasites along the very outer fringes of the orebody, and replacing rhythmically banded barite in lens 2. Only the low-Pb marcasites show various stages of pyrite inversion. Petrographic and trace element mineral chemistry of the 2 stratiform halos are best explained by a model in which main ore stage fluids of the early rhythmically banded ore event emanated from below the Cadjebut orebody along an inferred antithetic reverse fault. The presence of a laterally zoned alteration halo with a pattern of proximal higher Pb and trace element-bearing Fe-sulfides provides a vector towards the ore zone, and has important exploration significance for this style of base-metal mineralization. (C) 1997 Elsevier Science Ltd.