Authors / CoAuthors
Czarnota, K. | Gessner, K.
Abstract
The sediment-hosted Nifty Cu deposit is located 450 km east of Port Hedland in the Yeneena Basin of the Paterson Orogen in Western Australia. It is hosted within interbedded black carbonaceous shales and dolomitised micrites of the Broadhurst Formation. The host rocks have been folded and metamorphosed to lower greenschist facies in the Miles Orogeny (see also Czarnota et al. this volume). Textural relationships of the ore to host rock suggest syn-deformational (Miles Orogeny) timing of mineralisation (see also van der Wacht et al., this volume). Primary chalcopyrite preferentially replaces dolomitised micrite beds, occurs in black shales within the axial plane foliation, or as breccia infill. The ore and silica dolomite alteration envelopes trend from the keel of the Nifty Syncline and up the steeply dipping limb of the fold. There are two high grade ore trends (>1% Cu): one strikes NE-SW parallel to the fold axis and the other strikes N-S across the axis of the fold. Based on the inference that Nifty is a structurally controlled deposit that formed late, or after the establishment of the fold architecture, the question is why high grade ore is located in the keel and towards one limb of the asymmetric Nifty syncline. Assuming that post-folding dilation focussed flow of mineralising fluid(s) 2D and 3D coupled deformation/fluid flow simulations were carried out to examine why Nifty is in a syncline and what the controls on high grade ore trends may be. 2D models The 2D model geometry consists of a three layer stratigraphy folded in a series of asymmetric folds. The three-layer model represents the camp scale lithostratigraphy consisting of (i) a moderately competent and moderately permeable siltstone, (ii) a strong and permeable carbonate and (iii) a weak and impermeable shale. Contraction at hydrostatic pore pressure of this material layering resulted in focuses fluid flow down fluid pressure gradient occurred from the hinge of the syncline and up the steeply dipping limb of the fold driven by dilation higher up the limb. This dilation is a consequence of the location of a shear band that developed along the shallow dipping limb of the fold, above the competent carbonate unit, and intersected the steeply dipping limb of the syncline, adjacent to the syncline hinge. Models run using the same geometry but varying the stratigraphy to the mine sequence of shale-carbonate-shale showed focusing of fluid flow into anticlinal fold closures. This is a consequence of shear strain localisation below the competent carbonate unit and the intersection of the resultant shear band with the carbonate unit adjacent to the anticlinal fold closure. This scenario does not explain why Nifty is in a syncline. However this model may explain why the Telfer Ore deposit hosted in sediments with a similar competency contrast to this model (i.e. a sandstone unit between two weak carbonate units) is situated in a dome fold closure adjacent to the steeply dipping limb of the fold. Other models run on symmetrical folds showed similar results as the two models outlined above. However the shear bands in these models do not have preferential shallowly dipping fold limbs to localise on. 3D models A simple three layer 3D model of the Nifty syncline was constructed to examine the effects of (i) the nearby Vines Fault which was active during the Miles Orogeny as a major dextral strike-slip fault and (ii) the effects of the NW-SE directed Paterson Orogeny. The results of applying a dextral strike-slip velocity boundary velocity parallel to the NNW orientation of the Vines Fault produced high strain zones and associated dilation broadly coincident with the second direction of high grade ore trends. Deformation under the Paterson stress field i.e. perpendicular to the fold axis, resulted in shear strain localisation along inflections in the fold axis away from regions of mineralisation....
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nonGeographicDataset
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65188
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- External Publication
- ( Theme )
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- model
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
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2007-01-01T00:00:00
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