EVAPORITES - IN-SITU SULFUR SOURCE FOR RHYTHMICALLY BANDED ORE IN THECADJEBUT MISSISSIPPI VALLEY-TYPE ZN-PB DEPOSIT, WESTERN-AUSTRALIA

The Cadjebut orebody, northern Western Australia, is a Mississippi Val ley-type deposit with reserves of 3.5 million metric tons of 17 percen t combined Zn + Pb. Mineralization is hosted by carbonate-evaporite un its in the Givetian lower dolomite sequence of the Pillara Limestone p latform and occurs as two types: (1) an early Zn-rich, stratiform, rhy thmically banded ore, and (2) a later crosscutting, breccia fill, Pb-r ich ore. The rhythmically banded ore comprises 0.1- to 1.0-m-thick lay ers of repetitive 5- to 15-mm-thick bands of sphalerite, galena, Fe su lfide, and calcite. Three generations of sphalerite are developed symm etrically about a dark brown, hydrocarbon-bearing lamination of sucros e dolomite, detrital quartz, and feldspar. Galena is associated with t he second-generation colloform sphalerite, and marcasite occurs as a p ost-main ore mineral phase. Late-stage blocky calcite fills open cavit ies. Evidence for precursor evaporitic laminites as the host rock to t he 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 ca lcite pseudomorphous after primary lenticular laths, rosettes, and mos aic 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-equival ent horizons; (5) solution breccias in the rhythmically banded ore hor izons; and (6) homogeneous, heavy sulfur isotope signatures of the ban ded ore sulfides, with values just below those of lower dolomite evapo rites and mid-Devonian seawater. Mineralization at Cadjebut occurred d uring burial of the reefal platform in the mid-late Carboniferous. Dur ing platform burial, the conversion of gypsum to anhydrite is interpre ted to have resulted in volume loss and creation of an anastomosing ne twork of microgeode-like cavities along stylolitized cyanobacteria mat laminations. These open cavities provided the passageway for the intr oduction 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 reducti on of the host evaporites, resulting in the precipitation of base meta l sulfides and concomitant enlargement of the geode cavities. The form ation of the banded ore is fabric-mimicking, such that primary sedimen tary structures remained intact during the process, despite the fact t hat it did not involve replacement sensu stricto. The presence of evap orites constrains the chemical model for Cadjebut mineralization, in t hat only a single fluid mechanism is required for base metal transport and precipitation. In situ reduction of oxidized sulfur species acqui red at the site of ore deposition is implied. Sulfur isotope values yi eld disequilibrium, yet rather homogeneous, values that range from 4.5 to 0.5 per mil less than the interpreted source lower dolomite evapor ites. The data are consistent with reduction of the host sulfates by t hermochemical sulfate reduction. The direct correlation between evapor ite 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 Mi ssissippi Valley-type exploration models in general.