THE CADJEBUT DEPOSIT AS AN EXAMPLE OF MISSISSIPPI VALLEY-TYPE MINERALIZATION ON THE LENNARD SHELF, WESTERN-AUSTRALIA - SINGLE EPISODE OR MULTIPLE EVENTS

The Cadjebut Zn-Pb deposit is one of several Mississippi Valley-type r esources hosted in Devonian carbonates of the Lennard shelf, Western A ustralia. Several dolomitization events and at least three pulses of m ineralization are documented in the Cadjebut area, indicating multiple episodes of fluid expulsion during basin evolution. The data contrast with previous models which propose a single dolomitization and minera lization event for the Lennard shelf. Two major mineralization events are recorded in the Cadjebut area. The earlier more weakly developed m ineralization event is characterized by finely crystalline sphalerite and marcasite laths disseminated in dolomite cements that occlude prim ary porosity in evaporitic sequences. The later, mineralizing event fo rming the Cadjebut orebody is represented in terms of diagenesis eithe r in cloudy calcites that replace evaporitic minerals or within blocky calcite cements in large secondary vugs and cavities. Timing relation ships relative to diagenetic events in the area indicate that the firs t mineralization event occurred early in the diagenetic history prior to significant burial and is interpreted to be Middle Devonian (late G ivetian-early Frasnian) in age. The later event occurred subsequent to significant chemical compaction and maximum burial and is dated as mi d-Carboniferous (late Tournaisian-Westphalian). Anomalous values of Zn and Pb in early Carboniferous sediments of the Lennard shelf, and the apparent absence of mineralization in late Carboniferous to Permian s ediments, support a shelfwide mid-Carboniferous event. Stratigraphic c onstraints suggest an absolute age for Cadjebut of 325 +/- 25 Ma. This second mineralization event coincides with a period of reefal platfor m emergence, second-order cycle regression, and minor uplift of the Le nnard shelf, all temporally coinciding with the waning stages of the A lice Springs orogeny centered in central Australia 700 km to the south east of Cadjebut. The two main pulses of Mississippi Valley-type miner alization on the Lennard shelf have distinctive patterns of Pb isotope compositions. The early burial small resource mineralization has hete rogeneous Pb isotope compositions whereas later economic and subeconom ic mineralization has more homogeneous signatures. The Pb isotope data , and the overall intensity of mineralization, suggest that large volu mes of fluid were expelled during the later period of platform emergen ce, sea-level regression, distal orogenic events, and mineralization.