The structural evolution of the Broken Hill Pb-Zn-Ag deposit, New South Wales, Australia
Creators: name: Webster, AE
Publisher: name: University Of Tasmania
Year: 2023

description: Broken Hill Type lead-zinc-silver deposits (BHT) are a sought after style of mineralisation because of their simple metallurgy and high metal grades. The purpose of this research project is to gain a deeper understanding of the complex BHT style of mineralisation through a comprehensive re-examination of the structure and stratigraphy of the 300 million tonne, multiply deformed and metamorphosed, type example, located at Broken Hill, in far western New South Wales, Australia. This study used the techniques of structural analysis of high-grade gneiss terrains and exploited 119 years of geological data collected during the mining exploitation and exploration of the Broken Hill (BH) mining field. The gneissic sulphide-silicatecarbonate rocks of the BH ore environment lend themselves well to such structural studies because of their mineralogical diversity, coarse grain-size and because of the presence of distinctive and persistent marker units. The result is a deposit-scale stratigraphic and structural model of this complex mineralised system, which allows an unparalleled view of the 'anatomy' of this giant deposit. The Palaeoproterozoic BH mineralised system is a stratified complex that contains at least nine separate economically significant mineralised horizons, known as 3 Lens (lowest); 2 Lens; 1 Lens Lower; 1 Lens Upper (southern 1 Lens); A Lode Lower (Southern A Lode); A Lode Upper and B Lode. It is hosted within a sequence of metasedimentary and metavolcanic rocks, and coeval intrusives, known as the Willyama Supergroup, a multiply deformed upper amphibolite to granulite fades gneiss terrane. The deposit lies at the southwestem end of a lens of quartzofeldspathic and mafic rocks on the boundary between the Thackaringa and Broken Hill Groups. The quartzofeldspathic and mafic rocks are interpreted to be a metamorphosed volcanic complex. A distinctive Mine Sequence hosts the orebodies and their distal equivalents and has been identified over a 25-kilometre strike length. It is a continuous sequence from the basal Footwall Quartzofeldspathic Gneiss to the Hangingwall Quartzofeldspathic Gneiss (interpreted to be an intrusive), but has considerable stratigraphic complexity. It can be subdivided into a Footwall Succession, Lode Sequence (host to the mineralisation) and a Hangingwall Succession. The Lode Sequence can be further subdivided into several lode rock units and associated metasediments in the southwestem and central parts of the field, including the '4.5' Horizon; the Upper Potosi Type quartzofeldspathic Gneiss; the B Lode Horizon and the Gamet Quartzite Horizon. In the near-ore position, the Footwall Succession is represented by the 'underwall zone', which is distinguished by a marked thinning of key horizons and interbedded elastic metasediments. Manganiferous garnet-rich rocks of various types and textures are associated with the main orebodies, as are calc-silicate-rich horizons, magnetite-bearing metasediments, thin \banded iron formation\" and mineralised psammite. The orebodies and their wall rocks have been affected by two major periods of regional metamorphism (Ml and M2). Ml coincided with the Olarian Orogeny and two protracted deformational events; D2 and D3 which commenced at or immediately prior to the peak of metamorphism. D2 took place at the culmination of Ml and D3 as it waned. Most deformation of the mineralised system took place during D2 and the first phase of D3 (D3A). Dl is only represented in the Mine Sequence by a pervasive Sl schistosity and by pegmatite dykes and melt segregations. M2 coincided with the Delamerian Orogeny and D4. Only a single phase of granulite to upper amphibolite grade regional metamorphism is recognised in the BH area. High grade south verging asymmetric F2 folds comprise all of the significant macroscopic folds in the Mine Sequence and cause much of the present orebody 'geometry. They are parasitic to and lie within the north limb of a major regional F2 antiform; the Airport Antiform. The intensity of F2 folding is greater in the northeastem end of the field and is associated with a pervasive galena-defined S2 axial plane foliation in 3 Lens and 2 Lens. Folding and transposition equally affected both ore and adjoining wall rocks. SO banding and syndepositional stratigraphic variations within the orebodies are folded around F2 axes and modified by syn-D2 mobilised sulphides. F2 folds also deform a well-defined layering in 2 Lens B Lode and A Lode Lower. All significant fluid phase sulphide mobilisation and most mechanical sulphide mobilisation took place within the orebodies during D2 and D3A. There is continuity from D2 folding to early retrograde (granulite to upper amphibolite grade) D3A ductile shearing and attenuation. D3A shearing extensively modified the folded geometry of the mineralised system dislocating and attenuating F2 folds and producing a series of deposit-wide anastomosing shear arrays in which narrow (&lt;15m) but intense D3A shears traverse the Lode Sequence at acute angles. D3A shears have a north block up sinistral sense of movement with horizontal displacements of up to 350 metres and the effects of shearing were particularly focussed in F2 fold limbs and synformal keels that contained large masses of metasediment-hosted ore. D3A shears contain locallised occurrences of mesoscopic F3 folds with a well-defined sillimanite biotite axial plane fabric (S3) and planes of intense transposition. Each successive stage of D3 is characterised by progressively lower grade metamorphic mineral assemblages in ore and wall rocks and the styles of deformation reflect decreasing ductility. The differing styles of deformation are interpreted to reflect stages in the waning of Ml. D3B shearing is characterised by a lower amphibolite to greenschist grade mineralogy and is recognised as belts of quartz-muscovite-biotite schist that are mainly focussed in areas strongly deformed during D3A. The effects of D3B are not evenly distributed in the mining field and the northeastem part of the deposit has been particularly influenced by these structures. M2 affected the orebodies during the Delamerian Orogeny and locally exceeded greenschist grade. M2 was associated with a fourth period of deformation (D4). Within the orebodies it caused the re-activation of Olarian D3A-D3B shears and produced a generation of brittle fault systems. Transgressive dolerite dykes intruded the ore system in three main belts. There were at least two distinct phases in D4 an earlier relatively high-grade phase D4A which reached lower amphibolite grade in places and with locally higher grades in faults associated with hydrothermal activity and a subsequent D4B phase which was possibly a distinct reactivation event. D4A was associated with locallised ductile deformation in the form of F4 folds and caused the major reversal in F2 plunges in the central mining field. F4 folds are closely associated with complex fault zones such as the British Fault System and refold the main orebodies. Mechanical sulphide mobilisation dismembered dolerite dykes within 2L and 3L. Hydrothermal activity along D4 faults produced alteration within mineralisation including sulphide mobilisation and impregnation on the margins of the dykes. D4 had particularly widespread effects in the northeastem part of the mines area being mainly manifested as brittle-ductile to brittle deformation that is most commonly represented by the development of the extensive fault joint and fracture systems. D4 faults dykes folds and quartz-siderite galena veins overprint Olarian structures within the orebodies. The elongate geometry and the stratigraphy of the BH mineralised system predate all of the deformation that has affected the region and it preserves syn-depositional textures internal stratification and layered gangue mineral distributions that predate Dl pegmatite intrusion. The stratigraphy of the mineralised complex is tectonically modified but the succession can still be readily discerned and the stratified orebodies and their associated 'lode rocks' are concordant with the surrounding stratigraphy. Structural fabrics have been formed in the ores and in their wall rocks during D2 to D4 and each of these events has affected Dl pegmatite intruded into the mineralisation and host sequence. F2 folds traverse the Lode Sequence stratigraphy including the Gamet Quartzite Horizon and five orebodies retain a trend that is 20¬¨‚àû clockwise of the F2 hinge orientation. D3 shear zones have subsequently accentuated the elongate form of the system. The effects of D3 shears have not been profound however and they are confined to relatively narrow and discrete planes. There has not been any large or medium scale tectonic mobilisation of syngenetic mineralisation into structural sites during D2 and the ore has not been moved on a mass scale by deformation. The high aspect ratio of the mineralised system is interpreted to be a primary feature and the orebodies are still largely in their site of deposition relative to the surrounding stratigraphy. The orebodies and most lode rocks were in place as a series of strongly elongated lenticular bodies of sulphide-silicate-calcite mineralisation and manganiferous rocks prior to deformation and metamorphism. The relationships between the stratigraphy of the mineralised system and the structures that have modified its geometry show that the BH orebodies are a part of the sedimentary succession in which they lie and have been deformed and metamorphosed along with the other rocks with which they are interlayered. Empirical exploration models for BHT style mineralisation have been developed based on the findings of this comprehensive reexamination."