Abstract

At the Cadjebut orebody, Lennard shelf, Western Australia, there are at least six fluid pulses related to two main ore events which are both texturally and spatially distinct. Early rhythmically banded ore formed under essentially hydrostatic conditions, with the stratiform mineralization and its associated zoned dispersion halo formed by replacement of evaporites. Replacement occurred under a very weak extensional stress field, with ore-stage fluids infiltrating upward along a fault beneath the orebody. Postore-stage fluids were focusedalong more regional normal fault structures, and replacive calcite in the rhythmically banded ore of this stage yields delta 13 C values between -15 to -25 per mil (PDB) and delta 18 O values between -7 and -10 per mil (SMOW). In contrast, the later crosscutting breccia ore precipitated along low-angle reverse faults, and as breccia cements and veins. Each of four successive breccia ore-forming events is attributed to stages of overpressured fluid. Calcite which formed during the fourth, postore stage has delta 13 C values between -2 and -15 per mil (PDB); these values are very different from those of calcite of the rhythmically banded ore. The delta 18 O values of this calcite, -5 to -16 per mil (SMOW), have a wider, but overlapping range.Textural and structural evidence shows that rhythmically banded ore precipitated subsequent to regional normal faulting and close to maximum burial. Based on overgrowths of breccia ore mineralization on a late dolomite cement, the breccia ore event is considered post maximum burial. Geologic and stable isotopic evidence shows that Cadjebut is the product of two separate ore-forming events in a single, progressively developing, hydrothermal system whose duration spanned a transition from weakly hydrostatic to overpressured conditions. Metal deposition at Cadjebut lasted from the Late Devonian to mid-Carboniferous time, a period of up to 35 m.y. The higher hydrocarbon content of the later breccia ore suggests that oil and gas generation in the basin, triggered by a fall in sea level, may have been a contributing factor to the generation of overpressured fluid which deposited the breccia ore.

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