scholarly journals Element mobility and spatial zonation associated with the Archean Hamlet orogenic Au deposit, Western Australia: Implications for fluid pathways in shear zones

2019 ◽  
Vol 514 ◽  
pp. 10-26 ◽  
Author(s):  
Shawn B. Hood ◽  
Matthew J. Cracknell ◽  
Michael F. Gazley ◽  
Anya M. Reading
Keyword(s):  
Western Australia ◽  
Shear Zones ◽  
Element Mobility ◽  
Spatial Zonation

Petrogenesis of amphibole – biotite – calcite – plagioclase alteration and laminated gold – silver quartz veins in four Archean shear zones of the Norseman district, Western Australia

1992 ◽  
Vol 29 (3) ◽  
pp. 388-417 ◽  
Author(s):  
Andreas G. Mueller
Keyword(s):  
Western Australia ◽  
Shear Zones ◽  
Wall Rock ◽  
Alteration Zone ◽  
Mining District ◽  
Low Grade ◽  
Fluid Temperature ◽  
Quartz Veins ◽  
The North ◽  
Gold Silver

The Norseman mining district in the Archean Yilgarn Block, Western Australia, has produced 140 t of gold and about 90 t of silver from 11.24 × 106 t of ore. The district is located within a metamorphic terrane of mafic and minor ultramafic greenstones, intruded by granite cupolas and swarms of porphyry dykes. The orebodies consist of laminated quartz veins, controlled by narrow (0.5–5 m) reverse shear zones that, in general, follow the contacts of metapyroxenite or porphyry dykes. Petrological studies of four shear zones, exposed on the Regent shaft 14 level, Ajax shaft 10 level, and in the stope above the North Royal shaft 5 level, show that the host rocks were metamorphosed to hornblende–plagioclase amphibolites and actinolite–chlorite rocks at temperatures of 500–550 °C prior to mineralization.At the localities studied, intense wall-rock replacement and low-grade (0.5 g/t) gold mineralization are confined to ductile or brittle–ductile shear structures. Alteration is similar in both ultramafic and mafic greenstones, and consists of an inner zone of biotite–quartz–calcite–plagioclase rock with minor actinolitic hornblende and quartz–calcite–actinolite veinlets, and an outer zone, locally developed, of chlorite–calcite–quartz rock. At an estimated pressure of 3 kbar (300 MPa), fluid temperatures during wall-rock alteration are constrained by the hydrothermal mineral assemblages to 480 ± 30 °C in two shear zones on the Regent shaft 14 level, and to 450 ± 20 °C in one shear zone in the North Royal shaft 5 level stope. The mole fraction of CO2 of the fluids is estimated at [Formula: see text], and the sulphur fugacity at 10−6 bar (10−1 kPa) (at 450 °C), based on the assemblage pyrrhotite + pyrite ± arsenopyrite. The development of an outer chloritic alteration zone at North Royal is related to the lower fluid temperature at this locality.High-grade (up to 75 g/t Au, 283 g/t Ag) veins formed within three of the shear zones studied at fluid temperatures of 400 °C and less, by the successive accretion of quartz laminae, separated by films of retrograde chlorite and sericite. The assemblage of ore minerals in the veins differs from that in the altered wall rocks, and includes disseminated galena, Pb–Bi–Ag tellurides, and native gold, which coprecipitated with the quartz. The orebodies at Norseman show affinities to Phanerozoic and Archean gold skarn deposits.

The geometry and evolution of magma pathways through migmatites of the Halls Creek Orogen, Western Australia

1997 ◽  
Vol 61 (404) ◽  
pp. 3-14 ◽  
Author(s):  
N. H. S. Oliver ◽  
T. D. Barr
Keyword(s):  
Western Australia ◽  
Mafic Magma ◽  
Shear Zones ◽  
Magma Source ◽  
Felsic Magma ◽  
Melt Migration ◽  
Metasedimentary Rocks ◽  
Fracture Systems ◽  
Magma Migration

AbstractIn the Halls Creek Orogen of north-western Australia, the distance of melt migration through migmatitic metasedimentary rocks and adjacent metabasites is partly constrained by relationships of leucosomes and small mafic magma veins to rock boundaries and structural elements. Stromatic leucosomes in metasediments are cut by a network of small extensional fractures and shear zones, oriented steeply during melt migration. These shear zones allowed cm- to 10 m-scale migration of felsic magma derived by in situ anatexis. In the adjacent metabasite layers, a similar shear array allowed injection of H2O-undersaturated mafic to ultramafic magma, locally dehydrating and chemically modifying these rocks. However, these mafic to ultramafic veinlets are too mafic to be explained by in situ anatexis, necessitating an external magma source. Also, the lack of felsic veinlets cutting metabasites, and mafic veinlets cutting metasediments, requires that vertical inter-connectivity of these fracture systems was restricted. We propose along-layer migration of mafic to ultramafic magma through the metabasite, assisted by horizontal connection of the shear zones. This migration occurred independantly of metre-scale felsic magma migration in the adjacent metasediments, even though these two deformation-assisted magma migration systems may have been operating at the same time.

Porphyry intrusions and albitites in the Bardoc–Kalgoorlie area, Western Australia, and their role in Archean epigenetic gold mineralization

1992 ◽  
Vol 29 (8) ◽  
pp. 1609-1622 ◽  
Author(s):  
W. K. Witt
Keyword(s):  
Western Australia ◽  
Gold Mineralization ◽  
Shear Zones ◽  
Rock Composition ◽  
Gold Mines ◽  
Mid Ocean Ridge ◽  
History Of ◽  
Compatible Elements ◽  
Ocean Ridge ◽  
Textural Evidence

Minor intrusions in the Menzies – Kambalda greenstone belt of the Archean Eastern Goldfields Province, Western Australia, range from quartz–feldspar porphyry to plagioclase–hornblende porphyry. The porphyries display enrichment of mobile and incompatible elements (K to Zr) and depletion of relatively compatible elements, with negative Nb, P, and Ti anomalies, on mid-ocean-ridge basalt-normalized spidergrams. The composition and timing of emplacement of the porphyries are consistent with a genetic relationship with spatially related granitoids. Porphyries occur in 30% of gold mines in the Menzies–Kambalda belt. The association appears to be largely structural, since both the intrusions and the mineralizing fluids exploit zones of weaknesses, such as lithological contacts and shear zones. Porphyries have been modified to varying degrees by hydrothermal alteration, especially pervasive albitization. Textural evidence indicates that secondary albite and associated sodic amphibole formed late in the deformation history of the greenstones and were broadly contemporaneous with secondary phyllosilicate, carbonate and sulphide minerals related to gold mineralization. Recent studies in the Alleghany district of California suggest the initial rock composition may critically influence the nature of alteration associated with gold mineralization. Therefore, albitization of porphyries may be caused by the same hydrothermal fluids that deposit gold and produce potassic alteration in mafic rocks.

Chapter 33: Neoarchean Eastern Goldfields of Western Australia

2020 ◽  
pp. 709-734
Author(s):  
Gerard I. Tripp ◽  
Richard M. Tosdal ◽  
Thomas Blenkinsop ◽  
Jamie R. Rogers ◽  
Scott Halley
Keyword(s):  
Western Australia ◽  
Spatial Association ◽  
Shear Zones ◽  
Gold Deposits ◽  
Gold Deposition ◽  
Structural Environment ◽  
World Class ◽  
Greenstone Belts ◽  
Crustal Shear Zones ◽  
Host Rocks

Abstract Neoarchean greenstone-hosted gold deposits in the Eastern Goldfields Superterrane of the Yilgarn craton of Western Australia are diverse in style, timing with respect to magmatic activity, structural environment, host rocks, and geochemical character. Geologic constraints for the range of gold deposits indicate deposit formation synchronous with volcanism, synchronous with syn- and postvolcanic intrusion, synchronous with postvolcanic deformation in faults and shear zones, or some combination of superposed events over time. The gold deposits are distributed as clusters along linear belt-parallel fault zones internal to greenstone belts but show no association with major terrane boundary faults. World-class gold districts are associated with the thickest, internal parts of the greenstone belts identified by stratigraphic preservation and low metamorphic grades. Ore-proximal faults in those regions are more commonly associated with syn- and postvolcanic structures related to greenstone construction and deformation rather than major terrane amalgamation. Using the Kalgoorlie district as a template, the gold deposits show a predictable regional association with thicker greenstone rocks overlain unconformably by coarse clastic rock sequences in the uppermost units of the greenstone stratigraphy. At a camp scale, major gold deposits show a spatial association with unconformable epiclastic and volcaniclastic rocks located above an unconformity internal to the Black Flag Group. Distinct episodes of gold deposition in coincident locations suggest fundamental crustal structural controls provided by the fault architecture. Late penetrative deformation and metamorphism overprinted the greenstone rocks and the older components of many gold deposits and were accompanied by major gold deposition in late quartz-carbonate veins localized in crustal shear zones or their higher order fault splays.

scholarly journals Chloritization of granites in shear zones: an open window on fluid pathways, equilibrium length-scales and porosity formation down to nanoscale

2020 ◽  
Author(s):  
Laura Airaghi ◽  
Benoit Dubacq ◽  
Gloter Alexandre ◽  
Verlaguet Anne ◽  
Bellahsen Nicolas
Keyword(s):  
Shear Zones ◽  
White Mica ◽  
Ductile Deformation ◽  
Partial Replacement ◽  
Low Grade ◽  
Element Mobility ◽  
Rock Interaction ◽  
The Matrix ◽  
Mineral Replacement ◽  
Compositional Variations

<p>Strain localisation in the upper crust is strongly influenced by the presence of phyllosilicates (e.g. white mica, biotite, chlorite), systematically observed in shear zones in granites. Identifying reactions involving phyllosilicates at low-grade metamorphic conditions is crucial to understand crust mechanics and fluid-granite interactions during deformation. In the 305 Ma old basement of the Bielsa massif (Axial Zone, Pyrenees), extensive pre-orogenic (i.e. pre-Alpine) alteration related to feldspar sericitization and chloritization of biotite and amphibole occurred at temperatures of 270–350°C at 230–300 Ma. This event was followed by mylonitization and fracturing at 40–70 Ma, and fluid–rock interaction at 200–280°C marked by replacement and new crystallization of chlorite and white mica. In undeformed parts of the granite, compositional maps reveal in situ reaction, high local heterogeneities and low element mobility (migration over few µm) for most elements. Transmission electron microscopy (TEM) shows disconnected reaction-induced nanoporosity in chloritized amphiboles and ripplocations in chloritized biotite. Chloritization reaction varies over tens of nanometres, indicating high variability of element availability. Equilibrium is reached locally due to isolation of fluid in pockets. In samples with fractures, both elemental maps and TEM images show two chlorite groups: alpine chlorites in fractures have homogeneous composition while pre-alpine chlorites in the matrix show patchy compositions. Channelization of fluids in fractures and sealing by chlorite prevented replacement of the matrix chlorite. High element mobility was therefore limited to fractures. In mylonites, compositional maps show secondary chlorites up to 1 mm around cracks and only partial replacement of chlorite within the matrix. This suggests fluids could percolate from cracks to the matrix along chlorite grain boundaries. TEM images show nanocracks at the boundary of chlorite crystallites where replacement is localised. Crystallites were individually replaced by dissolution-reprecipitation reactions and not by intra-crystallite mineral replacement, explaining the patchy compositional variations. While fracturing did not allow chlorite sheets to be progressively re-oriented, a continuous, brittle-ductile deformation in mylonites did, making preferential fluid pathways progressively change.  Despite high strain, chlorite replacement was not complete even in mylonites. Replacement appears to be controlled by matrix-fracture porosity contrasts and the location and connection of nanoporosity between crystallites, criteria that may be only transiently met in space during deformation. These mechanisms need to be taken into account when attempting to reconstruct the metamorphic history of shear zones as well as the evolution of their mechanical behaviour since they affect the scale of the thermodynamic equilibrium and the preservation of hydrothermal metamorphism in granites.</p>

Kinematic nature and origin of regional-scale ductile shear zones in the central Yilgarn Craton, Western Australia

2004 ◽  
Vol 394 (3-4) ◽  
pp. 139-153 ◽  
Author(s):  
She Fa Chen ◽  
John W. Libby ◽  
Stephen Wyche ◽  
Angela Riganti
Keyword(s):  
Western Australia ◽  
Regional Scale ◽  
Shear Zones ◽  
Yilgarn Craton ◽  
Ductile Shear Zones ◽  
Ductile Shear

Chapter 13: Boddington: An Enigmatic Giant Archean Gold-Copper (Molybdenum-Silver) Deposit in the Southwest Yilgarn Craton, Western Australia

2020 ◽  
pp. 275-288
Author(s):  
Stephen J. Turner ◽  
Graeme Reynolds ◽  
Steffen G. Hagemann
Keyword(s):  
Western Australia ◽  
Melt Inclusion ◽  
Gold Mineralization ◽  
Extended Period ◽  
Granite Gneiss ◽  
Shear Zones ◽  
Gold Deposits ◽  
Yilgarn Craton ◽  
Local Evidence ◽  
Host Rocks

Abstract Boddington is a giant, enigmatic, and atypical Archean Au-Cu deposit hosted in a small, remnant greenstone belt within granite-gneiss and migmatite of the Southwest terrane of the Yilgarn craton, Western Australia. Primary Au and Cu (and Mo) mineralization consists of a network of thin fractures and veins, controlled by shear zones, and dominantly hosted by early dioritic intrusions and their immediate wall rocks, which comprise felsic to intermediate-composition volcanic and volcaniclastic rocks. The pre-~2714 Ma host rocks are typically steeply dipping and strongly deformed, with early ductile and overprinting brittle-ductile fabrics, and have been metamorphosed at mid- to upper greenschist facies. Features consistent with porphyry-style mineralization, classic orogenic shear zones, and intrusion-related Au-Cu-Bi mineralization are all recognized, giving rise to a variety of genetic interpretations. It is clear that Boddington does not fit any classic Archean orogenic gold deposit model, having a general lack of quartz veins and iron carbonate alteration, a Cu (Mo and Bi) association, zoned geochemical anomalism, and evidence of high-temperature, saline ore-forming fluids. Detailed petrographic, geochemical, and melt inclusion studies suggest a late-stage ~2612 Ma, monzogranite intrusion as one of the principal sources of the mineralizing fluids. However, there is also local evidence for older, perhaps protore, porphyry-style Cu (±Au) in the dioritic intrusions and patchy, locally high-grade, orogenic-style gold mineralization associated with enclosing shear zones and brittle-style deformation, which was focused on the relatively competent dioritic intrusions. The relative contributions of metals from these components to the system may not be resolvable. It appears that the Boddington deposit has been a locus for multiple episodes of intrusion, alteration, and mineralization over an extended period of time, as has been demonstrated in a number of other large Canadian and Australian gold deposits, including the Golden Mile near Kalgoorlie.

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