An analysis of the sedimentary hosts to mineralisation at the Cattlegrid Orebody, Mt. Gunson.

Abstract

The Cattlegrid Orebody is located at the disconformable contact between the Willouran Pandurra Formation and the overlying Marinoan Whyalla Sandstone. Mineralisation is located on a palaeo high of Pandurra quartzite known as the Pernatty Culmination.

The Pandurra Formation at the Cattlegrid Mine is a moderately to moderately well-sorted, cross-bedded quartz arenite. Grainsize analysis and cross-bedding data indicates it to be of fluviatile origin, while packing and compaction of the grains suggests a considerable thickness of quartzite has been removed by erosion. The bimodal Whyalla Sandstone is a texturally­inverted subfelsarenite, with well-rounded and spherical quartz grains in a poorly- to moderately-sorted fabric. Ripple marks, truncation of sandy and clay laminations, and a grain-size analysis suggests the Whyalla Sandstone to be a beach deposit. Syndepositional faulting, thickness of the beach deposit and the textural inversion suggest the sandstone to have been reworked by successlve transgressions and regressions.

It is in the fractured and brecciated upper Pandurra Formation that the bulk of the mineralisation (chalcocite, bornite, chalcopyrite) is found in fractures and vughs. Basal Whyalla Sandstone is also richly mineralised.

Throughout the Cattlegrid Mine, one, occasionally two, predominant lenticular clay bands show evidence of acting as a glide plane. Squeezing of the clay into fractures of the overlying quartzite, striations, and sharp transitions from the Upper Cattlegrid Breccia to the underlying mosaic breccia and displacement of sand wedges, all suggest sliding.

The topmost Cattlegrid Breccia is thought to be a bajada breccia underlain by the mosaic breccia, considered to be due to a normal weathering cycle and mild tectonic activity.

Gelifluction or periglacial mass movement processes are considered to be the mechanism responsible for the formation of tepee-like anticlines and sand wedges. Cryostatic pressures resulting from the melting of the active and talik layers and lateral movement over the permanently-frozen ground and/or clay band is considered to result in the water-escape anticlinal structures. Tensional fractures resulting from this movement initiated ice wedges.

Fluid inclusion data suggest a calcium-rich, highly saline, dense brine precipitated copper-sulphide mineralisation. Waters of a surface origin e.g., connate, meteoric, are therefore ruled out due to the high Ca content and paucity of Na and K salts.

YPMA has suggested that these dense brines created instability in the brecciated quartzite due to excessive pore pressures. This instability has caused slumping along a decollement surface and formation of the anticlinal structures and sand wedges. Similar structures are observed in Upper Mississippi Valley Pb-Zn districts.

The mixing of this dense copper­-bearing brine with less saline connate waters of the Whyalla Sandstone has caused precipitation of copper sulphides at the disconformable contact.