Geology of the Mt. Leyshon gold deposit, Australia: a study of breccia pipe formation, facies and brecciation mechanisms

Wormald, Peter John (1993) Geology of the Mt. Leyshon gold deposit, Australia: a study of breccia pipe formation, facies and brecciation mechanisms. PhD thesis, James Cook University of North Queensland.

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The facies architecture of the Mt. Leyshon breccia complex is reinterpreted as a subvolcanic intrusive breccia and igneous complex. The main pipe breccia defines the margins of the complex, which also contains a number of smaller breccia pipes and dykes, associated with porphyritic intrusives. The grossly rhombohedral shaped main pipe breccia has antler-like geometries with inter-fingering, intrusive host rock contacts. The faulted granite-metasediment contact, controlled the location of the complex within a regional wrench system.

The main pipe breccia is composed of distinctive fine and coarse variants with steeply dipping, gradational contacts. Localised zones of accretionary pellets are interpreted to result from particle accretion during brecciation and gaseous fluid transfer, and are therefore not necessarily diagnostic of pyroclastic lithologies. A juvenile igneous component has not been identified, although the rare presence of cognate igneous fragments suggest the prior existence of a magma system.

An interactive period followed, where smaller breccia bodies are characterised by the presence of cognate and juvenile igneous fragments and the digestion of a fragmental component by the magma. The Mt. Leyshon breccia pipe (the main gold-bearing unit) formed first, followed closely by emplacement of the fragmental component of porphyry unit II, the Mt. Hope breccia, and felsic igneous intrusions (porphyry unit II-IV). The latter partly engulfs and reworks the last two breccia units. Finally, late breccia bodies formed as steeply and gently dipping units containing abundant cognate and juvenile igneous particles (tuffisite breccia dykes and late breccia dykes).

The mobile/transported components of breccia units have characteristics suggesting forceful injection into host rocks. No unambiguous evidence for pyroclastic lithologies exists, nor is there evidence that the complex vented. Facies characteristics suggest the main pipe breccia resulted from prolonged mobilisation and upwards transport of fragments in the presence of a gaseous fluid phase, combined with pervasive host rock extension of basement. Subsequent pipe forming events have characteristics dominantly suggesting processes of localised host rock extension, in conjunction with minor to moderate gaseous fluid release. Strike slip reactivation of ductile basement structures is implicated in brittle extension in the complex. Facies characteristics of the final breccia dykes suggest they resulted from processes of prolonged fragment mobility/transport, as a consequence of gaseous fluid release.

Four main episodes of igneous emplacement, breccia generation and hydrothermal evolution are defined by the geological and paragenetic studies on the Mt Leyshon breccia complex. In the first episode, extensional quartz veins formed in host rock and emplacement of a magmatic system at depth probably contributed to main pipe breccia formation. Episode II began with argillic I alteration of the main pipe breccia, suggesting a significant time interval between breccia formation and high temperature fluid flow resulting in later potassic and propylitic alteration. In a the latter part of episode II, the emplacement of intermediate then felsic dykes preceded sericitic I alteration and quartz-molybdenite vein formation. Repeated breccia formation and (intermediate and felsic) emplacement, beginning with the Mt Leyshon breccia, characterises episode III (interactive magma/breccia sequence). Argillic II alteration affecting the episode III sequence, suggests a time break between rock unit formation and high temperature (episode IV) fluid flow through vein and fault style rebrecciation of rock, in the absence of igneous emplacement. The bulk of gold mineralisation in the Mt. Leyshon ore body, is concentrated within re-break zones (secondary porosity) in breccia cavities and veins.

The breccia pipe is divided into six fades, based on fragmentation characteristics, the proportion of mixed fragment zones and fragment types. The six facies are interpreted to reflect variations in three fundamental mechanisms of pipe formation: (1) Host rock fragmentation. (2) Host rock expansion. (3) Fragment transport/mobility. Fades characteristics suggest the presence of a fluid phase during brecciation.

A pre-pipe host rock stratigraphy is defined by variably in situ fragmented host rock and change of dominant fragment type. Portions of the pipe dominated by fragmentation progressions are merely in situ fragmented host rock, and probably formed by fault induced fragmentation. Regions of the pipe dominated by jigsaw fit textures and highly expanded fragments in matrix indicate horizontal and vertical extension of the host rock during pipe formation. As the degree of host rock fragmentation, expansion and transport/mobility increases in a given portion of the pipe, jigsaw fit textures and mixed fragment zones become dominant over fragmentation progressions reflecting stages in the progressive evolution of the breccia pipe. Zones of fragment transport occur in · narrow regions of maximum extension throughout the pipe and in a steeply dipping irregular region at the northern end of the pipe; the probable focus of fluid release.

An early association of magma with the main pipe is implied by cognate igneous fragments within the main pipe breccia. Subsequent, early igneous intrusion was associated with relatively closed system behaviour (culminating in quartz-molybdenite mineralisation). In a later interactive period of tectonically induced extension, intrusion of igneous bodies were genetically related to and partially consumed breccia units. Current models for magmatic evolution associated with porphyry-style (Cu, Au) mineralised systems require modification to reflect this more realistic geologic situation.

The magnetite bearing Mt Leyshon igneous units are divided into a felsic and intermediate group based on petrography and geochemistry. A late stage petrogenetic model for the Mt. Leyshon igneous units, proposes fractional crystallisation of the observed phenocryst phases controlled their evolution. Periodic input of more basic magma into the system occurred, inducing minor magma mingling (implying volatile transfer), and during extension, resulted in periodic igneous emplacement and volatile loss into the Mt Leyshon complex.

Breciation in the Mt Leyshon complex is related to a repeated magmatic cycle: (1) Recharge of more basic magma and volatile transfer to the felsic magma system. (2) Fractionation to felsic magma compositions. (3) Intrusive breccia generation during vapour release. The Mt. Leyshon igneous rocks have characteristics (e.g. K-feldspar megacrysts in felsic units genetically related to brecciation) implying they were not saturated with an aqueous fluid phase at the time of emplacement. Instead, a vapour phase with significant non-aqueous species (C0₂, HCl, S0₂, etc.) is favoured for brecciation.

Wide variation in breccia types can be produced by changing the rate and duration of structural, fluid phase and magmatic processes. A dynamic model for the Mt. Leyshon breccia complex proposes five breccia forming events linked to four geological cycles. Brecciation results from variable coupling of: (1) Brittle host rock extension, interpreted to result from seismic rupture perturbation at dilational strike slip fault irregularities, as a consequence of the earthquake (crustal/fluid pressure) cycle. (2) Vapour evolution as a consequence of periodic recharge of basic magma and vapour, combined with fractional crystallisation. (3) Explosively driven hydrofracture as a consequence of rapid magmatic vapour decompression (first boiling), evolving to time dependent vapour discharge, and as a result, fragment transport/mobilisation in breccia pipes and dykes. (4) Fluid redistribution in conjunction with after shock activity.

The Mt. Leyshon breccia complex records the passage of a magmatic system through the upper crust during transcurrent faulting. Three general mechanisms of brecciation are envisaged within this time-space framework: (1) Tectonic extension dominant (eg. main pipe, and Mt. Leyshon breccias). (2) Magma recharge and vapour evolution dominant (e.g. tuffisite breccia dykes). (3) A complex regime where both mechanisms interact (e.g. "porphyry" unit II).

Gold mineralisation postdates all coeval igneous emplacement and occurs late in the hydrothermal evolution. Gold occurs in three principal sites within the host rocks: (1) Primary porosity reflecting the breccia characteristics. (2) Secondary porosity resulting from re-break zones cutting across rock units. (3) Tertiary porosity resulting from dissolution of rock and alteration minerals. The characteristics of the ore body (geometry, timing and gold distribution) suggest gold mineralisation was controlled by fault reactivation under high fluid pressure conditions.

The steeply dipping and bulbous geometry of the Mt. Leyshon ore body is consistent with control by the irregular nature of porosity and permeability on gold deposition. At lower levels the ore body consists of linear NE and SE trending en-echelon segments, interpreted to result from movement on strike slip faults. Upper levels of the ore body arc expanded, probably resulting from an upward branching fault geometry and/or rheological contrast between host rocks to the ore body.

These characteristics suggest that at the regional scale the Mt Leyshon ore body was controlled by secondary porosity, resulting from the intersection of strike slip fault segments linked to the Mt. Leyshon and Mt. Dean corridors. The geological model suggests hydrothermal fluid evolution resulted from complex mixing of diffusing fluid into dilated regions of the complex, with trapped magmatic vapour released during after shocks.

Item ID: 47307
Item Type: Thesis (PhD)
Keywords: breccia formation; breccia pipes; breccia production; brecciation; Charters Towers; chimneys; facies; geology; gold deposits; gold mineralization; igneous rocks; Mt. Leyshon; petrology
Date Deposited: 17 May 2017 04:57
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 20%
04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 60%
04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 20%
SEO Codes: 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840105 Precious (Noble) Metal Ore Exploration @ 100%
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