Perkins, W.G. (1997) Mount Isa lead-zinc orebodies: replacement lodes in a zoned syndeformational copper-lead-zinc system? Ore Geology Reviews, 12 (2). pp. 61-110.

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Abstract

Examination of the microstructural and textural development of the lead-zinc mineralisation at Mount Isa reveals that it formed cogenetically with the large-scale Cu orebodies by late syndeformational replacement. The constraints on mineralisation provided by the sedimentological framework (Neudert, 1984) and timing criteria place the lead-zinc and its associated pyrrhotite and layer-parallel pyrite as forming late in the last major phase of deformation affecting the mine area. This is the same event as that which formed the copper ore (Perkins, 1984; Swager, 1985a) and indicates copper-lead-zinc to be a zoned late-stage epigenetic system. A succession of stratigraphically-constrained samples traced into the orebody reveals the progressive development of an alteration system that formed from well-laminated carbonaceous siltstones and mudstones. At the periphery, alteration consists of bleaching by destruction of carbonaceous seams and enhanced growth of dolomite. Progressively further inwards, it changes to mica and albite-bearing 'buff alteration', commonly into siderite, magnetite and stilpnomelane, through a chloritic zone, to ultimately massive 'silica-dolomite' alteration. Paralleling this alteration was the formation of abrupt pyrite-pyrrhotite transitions, and a zonation from sphalerite to galena to chalcopyrite. The characteristic folded and breccia ores were not the result of ductile or fluid-assisted deformation of preexisting sulphide-siltstone interlayers, but instead formed by shear-controlled carbonate alteration that was ultimately replaced by sulphides. Layer-parallel mineralisation commonly terminates abruptly on or near extensional dolomite veins that formed progressively through the last major deformation event in the mine. Layer-parallel mineralisation formed at the same time as one in folded and brecciated zones. Evidence cited in previous studies for large-scale remobilisation and recrystallisation during deformation, which is a logical consequence if mineralisation had been an integral part of the sedimentary succession, can be more consistently interpreted as resulting from a single episode of sulphide deposition. Both bedding-parallel and cross-cutting structures that control the localisation of mineralisation existed prior to the precipitation of sulphides. Dissolution of sulphides only occurs where there is a paragenetic sequence of sulphides, with fine-grained pyrite locally overprinted by all other economic sulphides (Grondijis and Schouten, 1937). Sphalerite is overprinted by galena, and both are overprinted by chalcopyrite, which everywhere is paragenetically the youngest sulphide. Copper-lead-zinc concentrations formed from hydrothermal solutions entering the region of the ramp of the Paroo Fault and then migrating outwards to form an evolving alteration system during the formation of both the layer-parallel and cross-cutting steep cleavages. This was overprinted by a relatively short-lived mineralisation event to form the zoned copper-lead-zinc system. Implications of this interpretation for other similar deposits around the world are profound. It indicates a necessity to reexamine many other deposits using techniques that relate ore textures to associated structures at all scales. A completely different ore genesis paradigm may result and exploration concepts may need to be substantially modified.

Item ID:	38413
Item Type:	Article (Research - C1)
ISSN:	0169-1368
Keywords:	lead-zinc orebodies; mineralisation; alteration; Mount Isa; Cu-Pb-Zn system
Date Deposited:	22 Apr 2015 01:39
FoR Codes:	04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 34% 04 EARTH SCIENCES > 0403 Geology > 040311 Stratigraphy (incl Biostratigraphy and Sequence Stratigraphy) @ 33% 04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 33%
SEO Codes:	84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840102 Copper Ore Exploration @ 50% 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840108 Zinc Ore Exploration @ 50%
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