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Abstract

The Cadjebut ore deposit is a Mississippi Valley-type (MVT) lead-zinc deposit within the Givetian “lower dolomite unit” of the Pillara Limestone platform on the Lennard Shelf in the Canning Basin. The ore-hosting sequence was deposited as a series of shoaling carbonate cycles. The upper portion of each cycle is characterized by an evaporitic cap of laminar dolomitic siltstone and mudstone with associated nodular anhydrite that formed a laterally extensive facies mosaic of sabkhas and salinas across an evaporitic mudflat. The originally shallow subaqueous or sabkha mudflat gypsum converted to nodular anhydrite during shallow burial. Regionally the depositional setting can be divided into two areas:(a) a higher energy shoal region in the vicinity of the mine and Cadjebut Fault and (b) a landward more restricted lower energy evaporitic lagoonal setting.

Two petrographically distinct forms of dolomite occur in the lower dolomite unit in the Cadjebut region: (a) a regional finer grained, nonferroan form (δ18O of -2 to -4%o) and, (b) a ferroan, coarser-grained, localized form (δ18O of - 3 to 7%o) that is associated with major growth faults in the mine area. The coarser grained dolomite overprints the finer grained stratabound dolomites as well as other facies. The finer grained non-ferroan dolomite formed by syndepositional brine reflux associated with the deposition of evaporites, while the coarser, more ferroan dolomite formed during burial from escaping basinal fluids. These same fluids carried metalliferous fluids from the Fitzroy Trough into sites of ore formation. Replacement dolomites from the Cadjebut mine are depleted in δ18O (10 to 12%o) and may reflect localized thermal conditions related to sulfide mineralization.

Interaction of a metalliferous basinal brine with sulfate that was derived from the dissolving anhydrite beds precipitated the various ore lenses at Cadjebut. Sulfide-mineralized horizons in the Cadjebut mine can be correlated on a one to one basis with fine rock-matrix breccia horizons in the immediate area around the mine and with remnant bedded sulfate units well away from the mine. The precursor host lithology to the various ore horizons at Cadjebut are massive to bedded evaporite units. The stratabound linear nature of the Cadjebut deposit reflects a time when metalliferous basinal fluids encountered bedded stratiform nodular anhydrites causing them to precipitate lead and zinc sulfides. This process locked in place a mineralization event at the stratigraphic position of each evaporite bed in the lower dolomite unit. Conduits for the metalliferous basinal fluids were the basin-margin-defining faults, such as the Cadjebut and Pinnacle Faults. Fine-rock-matrix breccias define subsurface evaporite dissolution both prior to and after the sulfide mineralization event. Today the remnant nodular anhydrite beds are rehydrating to satin-spar and poikilotopic gypsum as modem groundwaters seep into the units.

Lead-zinc mineralization in the Cadjebut mine occurs as three distinct ore types: (a) a stratiform rhythmically-banded zinc-rich ore, (b) a stratiform Type A ore-matrix breccia ore and (c) a crosscutting Type B ore-matrix breccia ore. These ore types represent a spectrum of textures that result from the simultaneous dissolution of evaporites and precipitation of sulfides. Such processes acted along a migrating dissolution front that ate its way into the evaporite units. The rhythmically banded ore formed under conditions where the potential for sulfide precipitation was equal to or greater than the rate of evaporite dissolution. This occurred in cm-sized laminar dissolution fronts that ate their way into the anhydrite beds. Type A ore-matrix breccias formed in irregular cm-dm stratiform to stratabound cavities. These formed at times when the rate of sulfide precipitation was less than the rate of sulfate dissolution. Such cavities where not sufficiently large enough to instigate roof collapse. TYpe B orematrix breccias formed in dm-meter sized crosscutting cavities that were sufficiently large enough to allow the collapse of roof blocks. The origin of Type B ore-matrix breccias can be related to similar evaporite dissolution and brecciation processes that formed the Type A ore-matrix breccias, however, subsequent hydrothermal karsting by overpressured fluids further enlarged the cavities.

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