Evaporites; in situ sulfur source for rhythmically banded ore in the Cadjebut mississippi valley-type Zn-Pb deposit, Western Australia

Abstract

The Cadjebut orebody, northern Western Australia, is a Mississippi Valley-type deposit with reserves of 3.5 million metric tons of 17 percent combined Zn + Pb. Mineralization is hosted by carbonate-evaporite units in the Givetian lower dolomite sequence of the Pillara Limestone platform and occurs as two types: (1) an early Zn-rich, stratiform, rhythmically banded ore, and (2) a later crosscutting, breccia fill, Pb-rich ore.The rhythmically banded ore comprises 0.1- to 1.0-m-thick layers of repetitive 5- to 15-mm-thick bands of sphalerite, galena, Fe sulfide, and calcite. Three generations of sphalerite are developed symmetrically about a dark brown, hydrocarbon-bearing lamination of sucrose dolomite, detrital quartz, and feldspar. Galena is associated with the second-generation colloform sphalerite, and marcasite occurs as a post-main ore mineral phase. Late-stage blocky calcite fills open cavities.Evidence for precursor evaporitic laminites as the host rock to the rhythmically banded ore includes: (1) preservation of rhythmically banded primary evaporites in stratigraphically equivalent units to the lower dolomite near Cadjebut; (2) the ubiquitous occurrence of ore calcite pseudomorphous after primary lenticular laths, rosettes, and mosaic mesh textures in ore-equivalent horizons, both inside and outside the orebody; (3) antiformal tepee structures within the banded ore; (4) enterolithic structures in unmineralized and mineralized ore-equivalent horizons; (5) solution breccias in the rhythmically banded ore horizons; and (6) homogeneous, heavy sulfur isotope signatures of the banded ore sulfides, with values just below those of lower dolomite evaporites and mid-Devonian seawater.Mineralization at Cadjebut occurred during burial of the reefal platform in the mid-late Carboniferous. During platform burial, the conversion of gypsum to anhydrite is interpreted to have resulted in volume loss and creation of an anastomosing network of microgeode-like cavities along stylolitized cyanobacteria mat laminations. These open cavities provided the passageway for the introduction of early Zn-saturated, hydrocarbon-bearing solutions, focused along regional and local faults, to the evaporites. The hydrocarbons introduced with the ore fluids acted as a catalyst for sulfate reduction of the host evaporites, resulting in the precipitation of base metal sulfides and concomitant enlargement of the geode cavities. The formation of the banded ore is fabric-mimicking, such that primary sedimentary structures remained intact during the process, despite the fact that it did not involve replacement sensu stricto.The presence of evaporites constrains the chemical model for Cadjebut mineralization, in that only a single fluid mechanism is required for base metal transport and precipitation. In situ reduction of oxidized sulfur species acquired at the site of ore deposition is implied. Sulfur isotope values yield disequilibrium, yet rather homogeneous, values that range from 4.5 to 0.5 per mil less than the interpreted source lower dolomite evaporites. The data are consistent with reduction of the host sulfates by thermochemical sulfate reduction. The direct correlation between evaporite host rocks and base metal mineralization at Cadjebut is considered an important relationship, not only for genetic models for this style of mineralization in northern Australia and elsewhere but also for Mississippi Valley-type exploration models in general.