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

Abstract

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 <5300 ppm; high-Pb Fe-sulfides with Pb contents <30 000 ppm and, extremely high-Pb Fe-sulfides with > 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.