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Winu, Ngapakarra |
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Western Australia, WA, Australia |
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Cu Au Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Winu - Ngapakarra copper-gold-silver deposit pair are located within the Paterson Province, on the western margin of the Great Sandy Desert in Western Australia, ~125 NNW of Telfer and 330 km ESE of Port Hedland (#Location: Winu - 20° 43' 26"S, 121° 44' 28"E; Ngapakarra - 20° 43' 55"S, 121° 46' 5"E).
The Winu deposit was discovered by Rio Tinto Exploration in late 2017, the result of follow-up of structural and geophysical targets defined in 2016. In July 2020 a maiden resource was published, with mining provisionally planned to commence in 2024.
The deposit is hosted in a sequence of gently folded metamorphosed sandstone and siltstone, with lesser mafic rocks, which are preliminarily correlated with the upper Malu Formation of the Yeneena basin, that hosts the Telfer gold-copper deposit. These sequences are part of the extensive Centralian Superbasin which occupied much of central Australia during the Neoproterozoic. The structure in the deposit area is dominated by a gently plunging, pre-800 Ma Miles Orogeny monocline which was refolded during the ~550 Ma Paterson Orogeny to form a half dome. Copper-gold mineralisation is localised in a series of 350 to 750 m long en echelon lodes with northerly trends and moderate easterly dips, surrounded by a more extensive halo of low-grade mineralisation occurring as numerous sulphide-bearing veins and breccias with progressively lower temperature mineral assemblages. Two gold rich lodes in the south-eastern part of the Winu deposit strike roughly E-W towards a fault offset gold rich satellite deposit called Ngapakarra, ~2 km to the east. The hypogene mineralisation is overlain by a well-developed supergene chalcocite blanket. Element associations and vein and alteration textures and mineralogy have been interpreted to classify Winu as a 658 to 655 Ma Neoproterozoic intrusion related copper-gold deposit genetically related to an, as yet unidentified, parental granitoid batholith. Mineralisation is regarded as was most likely having been triggered by rapidly changing stress fields and cooling of multiple fluid pulses around an intruding granitoid pluton during the broadly north-south compressive Paterson orogeny.
Following the discovery of Telfer in the early 1970s and later the Nifty sediment hosted copper deposit, the Paterson Province was intensely explored, resulting in the location of a number of smaller gold, copper and tungsten deposits/occurrences. However, the concealed northern extension of the province, the Anketell Shelf, received significantly less attention, although petroleum exploration seismic profiles revealed relatively shallow Proterozoic sedimentary rocks below younger cover on the western margin of the Canning Basin. Systematic mineral exploration began in the early 1990s, and included a regional aircore drilling programme combined with GEOTEM surveys by BHP and more focussed exploration programmes by Croesus/Gindalby Gold, Teck, NRG, Glengarry, Centaurus and Antipa Minerals, culminating in the discovery of the fully concealed Magnum and Calibre gold-copper deposits by Antipa Minerals.
Rio Tinto Exploration pursued an opportunity for a large tonnage copper-gold deposit under shallow cover in the Anketell Shelf. This firstly took the form of a joint venture with Antipa Minerals from October 2015 to learn from the known modest resources at Calibre and Magnum; then as an independent appraisal of available public data based on a model for a deep seated 'porphyry style' deposit related to a concealed granitoid. This granite/porphyry association is suggested by the mineralisation style at Telfer, by the metasomatic W-Pb-Zn-Cu O'Callaghans deposit and small 17 Mile Hill, possibly porphyry style copper deposit, both near Telfer. This work involved identifying deep granite signatures in geophysical data, in proximity of shallow favourable stratigraphy and structures. Three areas of interest were identified and tenements applications lodged in July-September 2016 along the western margin of the Anketell Shelf. Two small airborne EM geophysical surveys were flown in July 2017. One of thesse revealed the presence of a shallow late-time EM anomaly coincident with the Winu target. An Exploration Licence covering the target, was granted in October 2017 and two two reverse circulation holes were drilled in December 2017, both intersecting visible copper in a sequence of Neoproterozoic sandstones, siltstones and minor dolerite dykes. Assays revealed an intersection of just over 100 m @ 0.8% Cu, 0.28g/t Au from ~70 to 174 m (ASX release 2019), beneath <50 m of Permian cover. Weather interrupted work which was resumed in April 2018 drilling a target 3 km to the east of Winu, which intersected narrow, but significant zones of gold mineralisation. This was named Ngapakarra. These results heralded an ongoing resource definition program.
Regional Setting
The Paterson orogen comprises a NNW-trending belt of folded and metamorphosed Proterozoic sedimentary and igneous rocks (e.g. Williams and Meyers, 1990). These include rocks of the Palaeoproterozoic Rudall Complex and the Neoproterozoic Yeneena Basin. The latter comprises the >9 km thick Yeneena Supergroup sedimentary rock sequence, a succession developed in a NNW trending arm of the Centralian Superbasin that is thicker than most of the remainder of the superbasin. The Yeneena Supergroup hosts most of the significant mineral deposits of the region (Williams and Meyers, 1990, Sagas, 2004, Maidment et aI., 2010). It is subdivided into the:
• Throssell Range Group - which commences with basal conglomerates, followed by a thick package of fluviatile sandstones and fine-grained sedimentary rocks, largely carbonaceous, shale to pelitic schist units;
• Lamil Group, which is made up of the:
- Isdell Formation - mainly carbonates;
- Malu Formation - which consists of turbiditic sandstones interbedded with calcareous siltstones and is host to the Telfer copper-gold deposit;
- Puntapunta Formation - that is composed of finely bedded silty to sandy carbonates, which in higher metamorphic domains to the east are often converted to calc-silicate rocks.
- Wilki Formation - which comprises mature quartzites and sandstones that often form prominent outcrop ridges.
Deposition of the passive margin to rift hosted Yeneena Supergroup took place during an ~850 to 800 Ma, NE-SW directed extensional event. Related mafic to intermediate intrusions have been dated at 837 to 815 Ma. Yeneena Supergroup deposition was terminated by basin inversion during the Miles Orogeny between ~820 and 810 Ma (Huston et al., 2010). This orogeny produced NE to NNW-trending folds and thrusts in the Yeneena basin and predominantly greenschist facies metamorphism (Czarnota et aI., 2009, Maidment et aI., 2010). This event comprises two pulses of regional deformation overprinting the Palaeoproterozoic D1 and D2 fabrics of the Rudall Complex. The first, D3, produced dextral strike-slip and reverse fault reactivation of the prominent NW to NNW trending extensional phase growth faults (e.g., Vines and Camel-Tabletop faults), and recumbent folding in the Throssell Range Group. D4 was responsible for broad folds, conjugate faulting and greenschist facies metamorphism. A progressive 15° anti-clockwise rotation from NNW D3 to NW D4 axes resulted in a series of domal structures, including the Telfer Dome (Bagas, 2004). Widespread mafic dykes and sills were intruded between 750 and 700 Ma, followed by extensive 645 to 605 Ma granitic intrusion. The subsequent Paterson Orogeny involved early D5 open folds and a late D6 episode of NW trending dextral and ENE striking sinistral faults (e.g., Parallel Range and McKay faults) at ~550-540 Ma (Maidment, et al., 2010). This relates to NNE-SSW compression and comprises conjugate dextral and sinistral faults, and E-W trending open folds. The Paterson Orogeny is most likely equivalent to the ~550 Ma Petermann Orogeny in central Australia and was preceded by localised intrusion of granitoids (Sagas, 2004). See the geological map in the Telfer deposit record for locations.
The largely concealed Anketell Shelf is a gently NE-dipping Proterozoic basement high, separated from the Barnicarndy Graben of the Palaeozoic Canning Basin to the west, and the exposed Yeneena Supergroup sediments to the SW by the major NW-trending Anketell Fault. The Barnicarndy Graben represents a marked thickening of the Canning Basin cover sequence, bounded by the Anketell and Camel-Tabletop faults to the east and west respectively. The stratigraphy of the Anketell Shelf differs from that of the wider Yeneena Basin. Carbonate rocks are comparatively rare, while there is a thick banded magnetic package composed of magnetite bearing quartzites, calc-silicates and amphibolites which may be correlated to the Puntapunta Formation further south. The remainder of the sequence is mainly mica-schists, meta-sandstones/siltstones and quartzites with lesser graphitic shales, carbonate/calc-silicate bearing rocks and amphibolites. The structure of the shelf is dominated by kilometre-scale, SE-plunging anticlines, separated by tight, faulted synclines. The anticlines are often intruded by both magnetic and non-magnetic granitoids many of which have now been intersected by exploration drillholes. This drilling demonstrates an increase in metamorphic grade to the north and east with upper-greenschist to lower amphibolite facies conditions encountered around Winu and granulite facies conditions further to the north and east. Metamorphism reached as much as sillimanite stable conditions in locations across the Anketell Shelf, often in the vicinity of granitoid intrusions.
Deposit Geology
The Winu mineralisation is hosted in metamorphosed sub-arkosic sandstone, lesser siltstone, minor greywacke, calc-silicate and mafic rocks. The sandstones are mostly thick bedded and massive, with subordinate thin bands that are cross-bedded and parallel or wavy laminated sandstone and siltstone. Coarser sandstones with a higher content of matrix and characteristic subangular blue quartz grains with serrated grain boundaries, i.e., 'gritstones' or arkosic greywackes, are mainly restricted to the M3 unit (see below) at Winu and also at Ngapakarra. Dark grey to black siltstone layers occur as individual units that vary from a few centimetres to just over 10 m (averaging 2 m) in thickness. The siltstone-rich sequences can be geochemically distinguished from sandstones by their higher Al, Ti and Sc contents. Geochemistry, utilising ratios of immobile trace elements, together with lithological logging has been employed to define the main lithological packages of the host sequence which have been subdivided into a series of 'marker units' that can be correlated between drill holes. These are denoted, from the base:
M1 - >150 m of sandstone and siltstone;
M2A and B - 200 to 250 m of sandstone;
M3A, B and C - 165 to 200 m of sandstone and siltstone;
M4A - 50 to 60 m of sandstone;
M4B - 40 to 50 m of siltstone and sandstone; and
M5A and B - >300 m of sandstone and siltsone.
These are unconformably overlain by Permian cover that transgresses the whole section.
At Ngapakarra, mineralisation is hosted in lower stratigraphic units, M3 toM4, which generally correspond to an increase in mica schists and calc-silicate rocks.
Two generations of mafic intrusive rocks have been identified in the deposit area, i). an early strongly altered and mineralised, bedding-parallel sill of basalt to alkali basalt affinity; and ii). younger, N to NNW trending, thin, cross-cutting, post-mineral dykes of andesitic to basaltic composition, often with a porphyritic texture and brecciated margins. These dykes have subvertical dips.
Pelitic protoliths contain regional metamorphic assemblages that include muscovite, biotite and lesser chlorite, corresponding to upper greenschist facies, with a contact metamorphic overprint of poikiloblastic garnet, biotite and rare zoned andalusite-cordierite porphyroblasts. At Winu, the latter are nearly always altered to quartz-K feldspar intergrowths with chlorite and/or biotite rims. In calc-silicate rocks, contact metamorphic assemblage include poikiloblastic garnet, clinopyroxene, actinolite and epidote.
To the south and NE of Winu, granites have intruded the metasedimentary sequence, where they are massive, undeformed, and rich in K feldspar. Although they mostly do not have a magnetic signature, they include phases that contain fine grained disseminated magnetite.
Structure
The dominant structure at Winu is a mine scale, NNW-trending, moderately inclined, W-vergent monocline that has a moderate to steep WSW-dipping western flank, a subhorizontal eastern flank and a subhorizontal to gently SSE plunge. It has a weakly developed axial planar foliation that trends NNW and dips ~80° ENE. The structure is interpreted to have formed during the Miles Orogeny, with less intense WNW-ESE oriented interference folding during the Paterson Orogeny forming a half-domal structure.
At Ngapakarra, sharp N-vergent, gently WNW-plunging kink folds up to tens of metres across are mapped, with small thrust faults cutting the steep limbs of the kinks. These kink folds post-date the more ductile WSW-verging Winu anticline, and are therefore interpreted to have formed during the Paterson Orogeny. The Winu and Ngapakarra deposits are disconnected by the Thorny Devil Fault, a significant NW-trending normal fault with dextral displacement. This fault is occurs as an intensely deformed zone of cataclasites, surrounding zones of brecciated and altered metasedimentary rocks containing angular to rounded clasts, some of which are mineralised with sulphides in a carbonate rich matrix. The total displacement on this structure is only of the order of 70 m. Other faults off-setting mineralised veins include reverse faults, gently WSW to E dipping thrusts, and lesser normal faults that are steeply WSW to ENE dipping.
Hypogene Mineralisation
High-grade copper and gold mineralisation at Winu occurs in a series of northerly trending, en echelon, left stepping, lodes with strike lengths of between 350 and 750 m. These lodes are surrounded by an extensive halo of low-grade mineralisation. Whilst high copper grade lodes remain open at depth (2021), gold grades are higher in the shallower levels of the deposit.
In the SE of the Winu deposit, two gold-rich lodes trend towards Ngapakarra. In this area, there appears to be some stratigraphic control, particularly as regards gold distribution, which follows bedding trends. To the end of 2021, the mineralisation at Winu had been outlined over a strike length of >2 km, whilst the Iow grade mineralised vein halo had a width of up to 600 m. The deepest intersections were to ~750 m below surface.
The highest copper grades at Winu coincide with a relatively narrow, strike limited core of K feldspar dominant alteration, enveloped by an extensive phengitic white mica dominant assemblage and an extensive zone of unmineralised quartz veins. The Ngapakarra mineralisation is generally gold-rich and copper-poor, and has been delineated over a 600 m strike length, with a width of up to 150 m. Alteration at Ngapakarra is predominantly phengitic white mica, with chlorite and quartz, although a K feldspar zone is absent. Wallrock alteration assemblages are variable, ranging from from K feldspar → biotite → sericite dominant often along a single vein, and depend upon wallrock composition.
In more detail, the Winu copper-gold mineralisation is hosted within and in the immediate selvages of numerous thin, brittle, discontinuous veins and breccias that are each up to a few metres in thickness. In the main section of the deposit, namely on the west dipping limb of the anticline, these veins have a dominant north-south trend and a moderately steep easterly dip. However, in the south-eastern part of the deposit they have a more variable orientation, with the addition of an important ENE-WSW trending, steeply dipping set of gold-rich veins. Most of the 'native gold' at Winu described below is in electrum with 20 to 50% Ag.
At Ngapakarra, gold-copper mineralisation is predominantly within numerous steeply dipping veins that range from centimetres to several metres in width, and trends NW with steep dips ranging from NE to SW. Native gold contains less silver than at Winu.
In a broad sense, the main section of the Winu deposit, as defined by the 0.2% Cu envelope, forms a thick tabular mass that strikes NNW-SSE and dips at ~60°ENE, with a thickness of up to 350 m and strike length of ~1500 m. It parallels the main lodes and both the 0.5% and 1% Cu envelopes which it envelopes, and at least in part is centred on the fold axis of the of the upper (extensional) monoclinal axis. To the SE it swings towards the east. Copper mineralisation extends from the M5B marker unit to the sandstones and siltstones of the M1 marker unit. In contrasts, golf mineralisation is best developed in the M3A to M5B marker units, tapering downward in both width and grade.
Four distinct veining and alteration events associated with copper-gold mineralisation have been recognised in the Winu deposit:
• V1 - which occur as early narrow, stockwork style veinlets of K feldspar and sericite, with lesser albite, chlorite and quartz. These are particularly prevalent in siltstones. These early stockwork veinlets mostly have flat dips or are bedding parallel following siltstone-sandstone contacts. Sandstone affected by V1 type veining commonly contains minor disseminated sulphides, mainly pyrrhotite-pyrite-chalcopyrite.
• V2, which crosscut the early stockwork and are 'pegmatitic' quartz-K feldspar-chalcopyrite-pyrite veins that have dogtooth or comb textures, with syntaxial, euhedral to subhedral prismatic K feldspar and quartz crystals growing perpendicular to the wallrock contacts. Anhedral carbonate, sulphides and quartz are concentrated in the vein centres, and are transitional between magmatic and hydrothermal. They also contain lesser albite, biotite, carbonate, wolframite, scheelite, molybdenite and bismuth/gold. Most trend NE to N and are subvertical, and rarely contain bismuth minerals. They are absent from Ngapakarra.
• V3 - which represent the main stage mineralising event, cut both V1 and 2 and commence with Stage V3A mineralised quartz-chalcopyrite-pyrite veins, containing accessory biotite, K feldspar, carbonate, chlorite, muscovite, wolframite, scheelite, anhydrite and bismuth/gold. Virtually coeval Stage V3B veins are characterised by aold-rich quartz-bismuth-sulphide-carbonate-sericite assemblage. Stage V3C is slightly younger with pyrite-chalcopyrite-carbonate dominated veins and breccias that have a similar orientation to the quartz dominated veins. They also contain lesser quartz, chlorite, biotite, Pb/Zn sulphides and bismuth/gold. All three V3 veins and breccias mostly dip moderately at ~55°E, although there is a subordinate ENE-WSW trending V3B set with subvertical dips, often with anomalous high gold and bismuth, and lower sulphide content. At Ngapakarra, gold-bismuth rich veins mostly trend NW-SE and have steep dips. Sulphides in the V3 Main Stage veins are commonly hosted in fractures and minor faults which cut the vein fill as well as the immediate adjacent host rocks. A final set, Stage V3D, occurs as chalcopyrite-pyrite filled reverse faults that mostly dip gently to moderately steeply to the SW or N to NE.
Stage V3A and C can exhibit the full transitoin from undeformed to intensely brecciated, with the most strongly developed breccias being several metres thick, coinciding with the highest grades of copper and gold in the deposit. These breccia are never cut by stage V3A or B veins, implying they are paragenetically relatively late. However, they often contain rounded fragments of vein quartz enclosed in a sulphide matrix that is suggestive of hydrothermal milling. The matrix of these breccias is composed of chalcopyrite, wolframite, chlorite, amphibole and carbonate with trace pyrrhotite, sphalerite and galena. Wolframite is relatively early in these breccias, and is mostly highly fractured, with late chalcopyrite dominantly filling those fractures. Some massive chalcopyrite-pyrite accumulations within these veins and breccias have 'durchbewegung' textures, indicating that localised deformation took place after breccia/vein formation. These breccia zones have also affected a reverse or reverse-dextral offset of the mafic sill of the order of metres to a few tens of metres.
Within stage V3B veins, which carry the highest gold content, very fine bismuth and gold occur as a cloud in quartz cut by lenses and fractures containing chalcopyrite and pyrite. In detail, native bismuth, as well as maldonite [Au2Bi], are found as minute (<4 to 50 µm) droplet shaped inclusions in quartz grains, and are aligned along several cross-cutting lines. Many of these inclusions contain small grains of gold intergrown with bismuth. These droplets are cut by larger grains of native bismuth, the telluro-bismuth minerals JoseiteB [Bi4(S,Te)3], hedleyite [Bi7Te3] and pilsenite [Bi4Te3]) and native gold. These minerals are distributed along quartz grain boundaries, narrow fractures within the quartz, or in narrow terminations of wider chalcopyritepyrite filled fractures. The native bismuth, gold, hedleyite and pilsenite tend to occur as inclusions in larger joseiteB crystals. Early native bismuth, bismuthinite, sulphides and native gold occurring as droplet shaped grains with symplectitic intergrowth textures, are preserved, and inferred to suggest disequilibrium. Bismuthinite and native gold also occur within larger lenses and veins of chalcopyrite, and as narrow rims on earlier formed bismuth minerals in fractures. Native gold may be found as rare small grains included in chalcopyrite or quartz.
• V4 veining represent the final mineralising stage, cutting all of the veins and breccias described above. The veins are up to several metres thick and only carry weak mineralisation. They are composed of milky white quartz with chlorite, muscovite, carbonate and biotite, and minor sulphides. Contacts with wall rocks are often irregular and diffuse, and their orientation is highly variable, although they dominantly dip moderately to steeply to the W to SW. Biotite from a V4 vein returned an Ar-Ar age spectrum of 619.0 ±8.1 Ma. Unmineralised quartz veins, presumably late V4, are common in the host rocks immediate surrounding the deposit.
• V5 represents several late generations of carbonate-chlorite-sericite bearing veinlets, fractures and faults that cut the mineralised vein system at Winu. These structures contain variable pyrite/marcasite and minor galena.
Supergene Mineralisation
Much of the Winu deposit lies beneath at least 50 m of generally deeply weathered Permian sandstones, diamictites and mudstones. Post-Permian weathering and erosion has removed section of this cover sequence and exposed sections of the upper parts of the Winu deposit to produce supergene upgrading of the mineralisation to depths of as much as 340 m. Three distinct supergene zones have been differentiated; i). an upper leached/depleted zone with abundant iron oxides and kaolinite alteration; ii). a middle, strongly enriched interval, dominated by steely and sooty chalcocite and sporadic native copper; and iii). a lower weakly enriched zone with sooty chalcocite and hypogene chalcopyrite. However, the supergene enriched zone is split into two, specifically, where more of the total copper within an assay is contained within the acid soluble component of the analysis this interval is classified as a zone of copper oxide and silicate minerals (malachite-azurite-chrysocolla), whereas, iIf more of the total copper within an interval is contained within the cyanide soluble component of the assay this interval likely belongs to a zone of secondary sulphide (chalcocite-bornite) enrichment.
Evolution of the deposit
Mineralisation at Winu is interpreted to have developed during a progressive deformation, intrusive and mineralisation event that occurred during the Paterson Orogeny. This event was superimposed upon a pre-existing NNW-SSE trending Miles Orogeny monocline which folded the Yeneena Basin sedimentary sequence. The host sequence at Winu is interpreted to be a correlative of the Upper Malu Formation of the Yeneena Supergroup. The mineral assemblages of the host sequence at Winu are consistent with regional upper greenschist facies metamorphism during the Miles Orogeny deformation which formed the SSE plunging Winu Monocline. This deformation generated weak to moderate axial planar fabrics in metasedimentary rocks.
The regional metamorphism was overprinted by hornblende hornfels facies contact metamorphism. The first appearance of garnet in calcareous rocks, together with biotite, chlorite and epidote/clinozoisite indicates contact metamorphic temperatures of the order of 450 to 550°C (Spear, 1993). This would correspond to upper greenschist to lower amphibolite facies contact metamorphism. Similarly diopside accompanied by epidote in calc-silicate rocks suggest lower amphibolite facies peak metamorphic conditions (Spear, 1993).
The early stages of the main mineralising event is characterised by hot and relatively reduced fluids producing vein stages V1 and 2 which are accompanied by abundant K feldspar, molybdenite and wolframite. These evolved towards the relatively lower temperature and more oxidised fluids of vein stages V3 and 4 with biotite, chlorite, carbonate and anhydrite. This regression is also often evident within individual veins, taken to suggest that whilst the entire fluid system cooled through its lifetime, within this broad period of cooling there were individual pulses of high temperature fluid introduction and veining that cooled more rapidly, leading to precipitation of ore minerals.
The paragenesis of the mineralisation of the deposit relative to the veining has been interpreted as follows:
• V0 - thin, 0.2 to 5 cm, unmineralised grey quartz veins, oriented subparallel to bedding. Many are folded with gently ESE or WNW plunging fold hinges.;
• V1 - major K feldspar and phengitic sericite/muscovite, with minor quartz, albite, chlorite and pyrrhotite;
• V2 - moderate K feldspar and quartz with minor albite, biotite, phengitic sericite/muscovite, chlorite and carbonate with moderate molybdenite, wolframite and minor pyrite and chalcopyrite;
• V3A - moderate to strong quartz, weak K feldspar and biotite, weak to moderate pyrite and chalcopyrite;
• V3B - strong quartz, weak biotite, with weak pyrite and chalcopyrite, strong bismuth/Te-bismuth and gold;
• V3C - moderate to weak quartz, weak biotite, moderate carbonate, strong pyrite, moderate chalcopyrite, weak bismuth/Te-bismuth and gold;
• V3D - weak chlorite and carbonate, weak pyrite, strong chalcopyrite, moderate bismuthinite and gold;
• V4 - strong quartz weak biotite, weak chlorite and carbonate;
Initial age dating presented (Dalstra et al., 2021) indicates the mineralising event took place at ~658 to 655 Ma during the Paterson Orogeny.
The en echelon distribution of the lodes at Winu are consistent with dextral-reverse transpression in a NNE-SSW compressive regime most, assumed to have been active during the Paterson Orogeny. Other indications of a similar compressive syn-mineralisation event are WNW-ESE trending reverse fractures and faults often with sulphide fill at Winu and Ngapakarra and brittle folds at Ngapakarra. Superimposing NNE-SSW compression on competent, thick bedded meta-sandstones that were folded into a SSE plunging monocline is regarded as likely to lead to the fracturing and veining pattern which hosts the Winu mineralisation. However, the high variability in V1 to V4 vein orientations indicates the stress regime was rapidly changing during the mineralisation event, possibly influenced by the inferred granitoid intrusion.
Given the degree of brittle veining and brecciation, it is reasonable to assume the system was under overpressure during the mineralising event, related to the escape of fluids ponded in the apex of a deep segregating batholith. The inhomogeneity of the superimposed Paterson stress field on the Miles Orogeny monocline would result in the overpressured fluids fracturing and inflating the host sequence (natural fracking) by opening dilational zones consistent with the prevailing or previous stress field. Stress field controlled dilational zones would likely be related to axial planar foliation, and at least in part, in the extensional curvature of the upper part of the monoclinal structure.
Dalstra et al. (2021 and 2023) interpret the vein and alteration textures and the mineral assemblages, as well as the element associations at Winu to have many characteristics of an intrusion related gold-copper deposit of Neoproterozoic age. They suggest it also shares features with the reduced porphyry copper-gold group (e.g., Rowins, 2000). However, they go on to say, other features, such as the dominance of pyrite over pyrrhotite and the presence of trace amounts of sulphate minerals in the mineralisation, are more in accordance with an oxidised intrusion-related system. They therefore consider it appropriate to classify Winu as a wallrock hosted intrusion-related copper-gold deposit that is transitional between a reduced and oxidised porphyry system, genetically related to a deep granitoid pluton. They suggest it preserves evidence of an early, reduced intrusion-related gold system overprinted by a more oxidised intrusion-related copper system, and reflects the episodic cooling of multiple fluid pulses above an intruding granite pluton that progressively decreased in temperature over time.
Mineral Resources
The published JORC compliant Mineral Resource at Winu as of 31 December, 2021 at a 0.2% CuEquiv. were (Rio Tinto Notice to ASX, 23 February, 2022):
Supergene
Indicated Resource - 26 Mt @ 0.64% Cu, 0.52 g/t Au, 3.54 g/t Ag = 0.68% CuEquiv.
Inferred Resource - 29 Mt @ 0.45% Cu, 0.46 g/t Au, 2.12 g/t Ag = 0.51% CuEquiv.
TOTAL Supergene Resource - 55 Mt @ 0.54% Cu, 0.49 g/t Au, 2.8 g/t Ag = 0.59% CuEquiv.
Hypogene
Indicated Resource - 223 Mt @ 0.43% Cu, 0.31 g/t Au, 2.62 g/t Ag = 0.53% CuEquiv.
Inferred Resource - 330 Mt @ 0.37% Cu, 0.27 g/t Au, 1.94 g/t Ag = 0.45% CuEquiv.
TOTAL Hypogene Resource - 552 Mt @ 0.54% Cu, 0.28 g/t Au, 2.21 g/t Ag = 0.49% CuEquiv.
Hypogene + Supergene
Indicated Resource - 249 Mt @ 0.45% Cu, 0.33 g/t Au, 2.72 g/t Ag = 0.55% CuEquiv.
Inferred Resource - 358 Mt @ 0.37% Cu, 0.28 g/t Au, 1.95 g/t Ag = 0.46% CuEquiv.
TOTAL Resource - 608 Mt @ 0.40% Cu, 0.30 g/t Au, 2.26 g/t Ag = 0.49% CuEquiv.
This summary is largely drawn from Dalstra, H., Black, A., Chembeya, E., Maguire, J., Ewington, D., Rayner, M. and Mudrovska, I., 2021 - The Winu-Ngapakarra deposit in the Great Sandy Desert of Western Australia, discovery of a new intrusion related copper-gold deposit; NewGenGold 2021, Conference Proceedings, Perth Western Australia, November 9-10, Paydirt Media; pp. 129-143. with input also from Dalstra, H., Black, A. and Mudrovska, I., 2023
The most recent source geological information used to prepare this decription was dated: 2021.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Winu Ngapakarra
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Dalstra, H., Black, A. and Mudrovska, I., 2023 - Geology of Winu-Ngapakarra, Great Sandy Desert of Western Australia, a Recently Discovered Intrusion-Related Cu-Au Deposit: in Econ. Geol. v.118, pp. 967-998. doi.org/10.5382/econgeo.5005.
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Dalstra, H., Black, A., Chembeya, E., Maguire, J., Ewington, D., Rayner, M. and Mudrovska, I., 2021 - The Winu-Ngapakarra deposit in the Great Sandy Desert of Western Australia, discovery of a new intrusion related copper-gold deposit: in NewGenGold 2021 Conference, Perth Western Australia, November 9-10, 2021, Paydirt Media, Proceedings pp. 129-143.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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