Abstract

The Cadjebut Zn-Pb deposit is one of several Mississippi Valley-type resources hosted in Devonian carbonates of the Lennard shelf, Western Australia. Several dolomitization events and at least three pulses of mineralization 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 mineralization event for the Lennard shelf.Two major mineralization events are recorded in the Cadjebut area. The earlier more weakly developed mineralization event is characterized by finely crystalline sphalerite and marcasite laths disseminated in dolomite cements that occlude primary porosity in evaporitic sequences. The later, mineralizing event forming the Cadjebut orebody is represented in terms of diagenesis either in cloudy calcites that replace evaporitic minerals or within blocky calcite cements in large secondary vugs and cavities. Timing relationships relative to diagenetic events in the area indicate that the first mineralization event occurred early in the diagenetic history prior to significant burial and is interpreted to be Middle Devonian (late Givetian-early Frasnian) in age. The later event occurred subsequent to significant chemical compaction and maximum burial and is dated as mid-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 sediments, support a shelfwide mid-Carboniferous event. Stratigraphic constraints suggest an absolute age for Cadjebut of 325 + or - 25 Ma. This second mineralization event coincides with a period of reefal platform emergence, second-order cycle regression, and minor uplift of the Lennard shelf, all temporally coinciding with the waning stages of the Alice Springs orogeny centered in central Australia 700 km to the southeast of Cadjebut.The two main pulses of Mississippi Valley-type mineralization on the Lennard shelf have distinctive patterns of Pb isotope compositions. The early burial small resource mineralization has heterogeneous Pb isotope compositions whereas later economic and subeconomic mineralization has more homogeneous signatures. The Pb isotope data, and the overall intensity of mineralization, suggest that large volumes of fluid were expelled during the later period of platform emergence, sea-level regression, distal orogenic events, and mineralization.

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