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Gruyere Project - Gruyere, Yam 14, Attila, Alaric, Argos, Montagne, Orleans, Central Bore

Western Australia, WA, Australia

Main commodities: Au
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The Gruyere gold deposit is located 200 km east of Laverton, ~1000 km NE of Perth, ~400 km NE of Kalgoorlie and ~145 km NNW of the Tropicana mine, in the easternmost known greenstone belts of the Yilgarn Craton, Western Australia. Other deposits included within the Gruyere Project resource inventory occur within 30 km of the main Gruyere deposit and include Yam 14, Attila, Alaric, Argos, Montagne, Orleans and Central Bore. (#Location: Gruyere - 27° 58' 58"S, 123° 51' 58"E).

The blind Gruyere deposit was discovered by Gold Road Resources Limited in 2013. Prior to the discovery, a detailed airborne magnetic survey was conducted ahead of an 18-month geological interpretation and targeting study. Followup of identified targets commenced in 2013 and very early delineated a gold anomaly in rotary air blast drilling over the area where the deposit is located. Subsequent reverse circulation drilling confirmed rnineralisation, and discovery of the deposit was announced in October 2013 (Osborne et al., 2017). A 50:50 joint venture was formed with Gold Fields Australia in late 2016 and mining commenced in late 2018, with all construction completed during 2019 and the first gold produced in June 2019. It is planned in 202 that the mine comprise three open pits to extract the current Ore Reserve and seven open pits and one underground mine for the known Mineral Resource. All ore mined is processed in the central Gruyere plant (Gold Fields Annual Report, 2019).

Regional Setting

The deposit is located on the western margin of the Yamarna Terrane, the easternmost segment of the Yilgarn Craton. The western margin of the terrane, which encompasses the Yamarna and Dorothy Hills greenstone belts, is defined by the 350-km long, east dipping Yamarna shear zone that separates it from the older Burtville Tenane to the west (Pawley et al., 2012). The Yamarna Terrane is extensively covered by Permian and younger sedimentary sequences with only minor outcropping Archaean basement.

For detail of the regional setting of the Yamarna Terrane, see the Yilgarn Craton Overview record.

The NNW-SSE trending Yamarna Greenstone Belt is >250 km in length and ranges from 3 to 30 km in width. The Dorothy Hills Greenstone Belt, which also trends NNW-SSE and is ~25 km to the east, is sigmoidal in shape and 90 km long. Both greenstone belts are metamorphosed from greenschist to amphibolite facies, and are intruded by 2667 ±4 to 2634 ±3 Ma monzogranite plutons (Pawley et aI., 2012). The belts are separated by 2711 ±6 Ma or older orthogneiss (Pawley et aI., 2012).

The geology of the Yamarna Greenstone Belt is dominated by siliceous, high-thorium basaltic lithologies in the north, whilst similar rocks but with a low-thorium composition occur to the south. The 2737 ±26 Ma Argus anorthosite complex intrudes the western part of the belt (Wingate, Kirkland and Romano, 2011). The central section of the belt is dominated by a thick volcano-sedimentary sequence of felsic volcanic rocks and vo1canoclastic sandstone of the Toppin Hill Formation (Pawley et aI, 2012; Romano, Doyle and Jones, 2010). Dacites of this latter formation yielded ages of 2677 ±7 Ma and 2699 ±5 Ma (U-Pb zircon), whilst a maximum depositional age of 2682 ±5 Ma is indicated for sandstone within the unit (Pawley et aI., 2012; Sircombe et aI., 2007). The Toppin Hill Formation is separated from a thick sandstone sequence to the east by a district-wide chert band. The sandstone includes lenses of pyroxenite and siliceous high-thorium mafic units in the east. A range of sills and dykes, varying from differentiated dolerite to feIsic, occur throughout the belt.

The Dorothy Hills Greenstone Belt does not contain an equivalent to the Toppin Hill Formation. Low-thorium pillow basalts dominate the eastern part of this belt (Pawley et aI., 2012; Romano, Doyle and Jones, 2010), whilst volcaniclastic sedimentary and ultramafic rocks, and high-thorium siliceous basalt make up the central part. Felsic intrusions are found throughout the belt, including the Gruyere Porphyry which hosts the ore deposit. The Gruyere Porphyry is a medium‐grained porphyritic granitoid composed of plagioclase, quartz and ferromagnesian minerals. Tunjic et al. (2109) report that geochronological results across the Dorothy Hills Belt indicate all rocks formed or were deposited in the Mesoarchaean, between 2840 to 2810 Ma. This includes the Gruyere Porphyry, crystal rich tuffs found in the footwall of the Gruyere Porphyry and siltstones located in the western part of the belt, and that the crystallisation age of the Gruyere Porphyry is only slightly younger than the volcanoclastic sedimentary rocks into which it is intruded. The geochemically equivalent Ziggy Monzogranite, SE of Gruyere, has a crystallisation age of 2832 ±4 Ma (U-Pb zircon; Wingatel Kirkland and PawIey, 2010).

Geology

The lithological succession at Gruyere commences with an upper greenschist-facies tholeiitic basalt which is overlain by mafic to intermediate volcaniclastic rocks and thinly bedded epiclastic sedimentary rocks. The Gruyere Porphyry has been intruded into this sequence along the NNW-SSE trending Dorothy Hills Shear Zone. To the south of the NW-SE trending crosscutting early thrust, the Alpenhorn Fault, the immediate hanging wall of the porphyry is occupied by basalt, whilst to the north, and in the footwall, volcaniclastic rocks of intermediate composition are dominant. Another, but only weakly mineralised felsic porphyry, texturally and mineralogically similar to the Gruyere Porphyry, occurs in the footwall, to the south of the Alpenhorn fault (Osborne et al., 2017).

Mineralisation is almost entirely restricted to the Gruyere Porphyry, which is well mineralised over a strike length of ~1500 m. It ranges from 5 to 10 m wide on its northern and southern extremities, and reaches as much as 190 m in width in its centre. Where undeformed, it is composed of 1 to 2 mm albite phenocrysts, pseudomorphed after plagioclase, set in a matrix of albite (after andesine/oligoclase and orthoclase) with ~20% quartz and minor biotite. It contains xenoliths of country rock, most abundant proximal to the hanging wall contact, and is also intruded by variably deformed mafic, intermediate and felsic dykes (Osborne et al., 2017). A persistent, steeply dipping 1 to 5 m wide mafic dyke occurs proximal to the hangingwall whilst multiple thin sub‐parallel, intensely sheared, mafic to intermediate rocks occur internal to the porphyry, interpreted to be dykes and/or rafts of the initial shear zone that have been caught up in the porphyry during intrusion (Gold Road Resources Limited, 2015).

Structure

The Gruyere deposit is situated at the northern end of a flexure in the Dorothy Hills Shear Zone, within the Dorothy Hills Greenstone Belt. This steeply north to NE-dipping, sigmoidal flexure comprises an ~8 km long north-south interval of the Dorothy Hills Shear Zone, which to the north and south trends NNW-SSE. The shear zone is a regional feature which runs the entire length of the greenstone belt. Aeromagnetic data indicate that the NNW trending segments of the shear zone are subparallel to regional stratigraphy, whilst the north-south segment is transgressive. The stratigraphy immediately to the east of the north-south segment is tightly folded and butts against the shear zone, whilst to the west, NNW striking stratigraphy is truncated by the shear and wraps into the shear zone with an apparent gross dextral sense of movement (Osborne et al., 2017). Although shearing is variably developed in the hanging wall and footwall rocks and within porphyry, the intensity is very strong at the porphyry margins, although the contact itself on both the hanging wall (east) and footwall (west) is sharp. Within the porphyry there is a strong foliation fabric, interpreted to be invoked by the Dorothy Hills Shear Zone. It is steeply dipping at 70 to 80°E and striking to the north, parallel to the orientation of the porphyry. The foliation intensity within the porphyry varies from very weak to very strong, with kinematic indicators showing both sinistral, dextral, reverse and normal movement senses, suggesting a complex structural history. In areas of increased deformation, a crenulation of the foliation has formed, producing a steep down‐dip lineation (Gold Road Resources Limited, 2015).

In addition to the NNW trending Dorothy Hills Shear Zone, a set of NW striking thrust faults, initially interpreted from magnetic data and changes in stratigraphy, are interpreted to be an important second order control on mineralisation. These faults appear to represent early activity, offsetting the regional stratigraphy, but not the Dorothy Hills Shear Zone or Gruyere Porphyry. They also appear to be coincident with zones of thickening of the porphyry e.g., the Alpenhorn Fault, and areas of higher‐grade development in the north, e.g., the Northern Fault. These second order mineralisation controls constrain the strike extent of the Gruyere mineralised system. The intersection of the Alpenhorn Fault with the Gruyere Porphyry also defines a steep plunge, which is considered the gross plunge of the mineralised system (Gold Road Resources Limited, 2015).

The alignment of the peak metamorphic mineral assemblages define both the axial planar foliation of upright folds, and the shear fabric of the Dorothy Hills Shear Zone, suggesting the folding and shearing were broadly coeval. The geometry and these relationships imply the north-south segment of the shear zone formed a restraining bend during sinistral shearing, and likely reactivated earlier-formed structures during east-west compression across the Yamarna Terrane (Osborne et al., 2017). This is typical of structures commonly attributed to D2 in the Eastern Goldfields, as synthesised by Wyche et al. (20l2).

The Gruyere porphyry is a steeply NE-dipping, laterally attenuated body that has been delineated in detail over a strike length of >2 km and down dip extent of >1 km. Within and proximal to the porphyry, there is a strong down-dip lineation within high-strain zones, defined by the elongation of rock fabrics, such as sedimentary clasts, amygdales, crenulations (as detailed above) and elongate albite aggregates within the peak metamorphic mineral assemblage. This lineation plunges parallel to the porphyry margins. However, the dominant fabric within the porphyry is the alignment of the peak metamorphic mineral assemblage, which forms a variably developed, predominantly NNW trending biotite-foliation. This foliation has been variably folded about moderate north-plunging and subhorizontal axes that are locally truncated against the porphyry margins. These foliated zones commonly have a mylonitic fabric which separate blocks of massive unfoliated porphyry (Osborne et al., 2017).

Gold mineralisation is most commonly associated with the following sets of structures (after Osborne et al., 2017):
i). Tabular quartz-albite ±sulphide veins (pyrite and pyrrhotite ±arsenopyrite, sphalerite and galena), with distinctive, strong to intense albite + sulphide ±white mica alteration haloes which overprint and obliterate the pre-existing biotite-albite foliation. However, these veins are commonly sub-parallel to the early biotite-foliation, but are folded about tight, moderately north-plunging and shallow NNW- and SSE-plunging folds. These folds and the porphyry margins are dislocated by steep ESE-dipping shears;
ii). Fractures filled with chlorite (chamosite) ±biotite ±pyrite ±pyrrhotite, often with centimetre-scale, strong albite selvages overprinting earlier biotite-albite foliation, commonly with a strong preferred orientation similar to the previous vein set;
iii). Lenticular quartz veins with chamosite ±biotite selvages that have apparently exploited the chlorite fractures of the previous stage;
iv). Quartz stockwork veins which are fine, generally <0.5 cm, and occur within zones of intense, overprinting albite-sulphide alteration that commonly occur in conjunction with fine, random chamosite ±sulphide-filled fractures.

The veins and chamosite-fractures are found in a variety of orientations, although steeply ENE-dipping and shallower SE-dipping sets predominate. The chlorite fractures often transect the tabular veins, although cross-cutting relationships, opening of the fractures by mineralised quartz veins and similar alteration haloes imply they formed during progressive deformation and mineralisation of the porphyry (Osborne et al., 2017).

Alteration and Mineralisation

Gold mineralisation is widespread but unevenly distributed throughout the Gruyere Porphyry, emplaced in response to a complex reverse shearing structural event, with both sinistral and dextral movement indicated. The porphyry is more competent and brittle compared to the more ductile country rocks, and as a consequence suffered relatively greater cracking and fracturing. This resulted in increased permeability allowing gold bearing mineralising fluids to permeate through the rock mass. Mineralisation is closely associated with multiple quartz ± carbonate vein sets and chlorite-filled fractures, with gold grades correlating with the abundance of veining and fracturing and the width of guartz veins. Conversely, poorly mineralised and unmineralised areas coincide with the absence of veining and fractures, and where biotite-foliation or early stage red hematite-dusted albite zones are well preserved (Gold Road Resources Limited, 2015; Osborne et al., 2017).

The entire Gruyere Porphyry is altered to varying degrees of intensity. Early alteration generated a brick‐red hematite‐magnetite assemblage which has only background, <0.3 g/t gold mineralisation. Weak to strong gold mineralisation is increasingly associated with the progression from sericite → sericite‐chlorite → chlorite‐muscovite → chlorite‐muscovite-albite → strong albite alteration. Sulphides are commonly associated with the gold mineralisation throughout, with pyrite dominant in the upper areas and pyrrhotite increasing with depth. Arsenopyrite is commonly associated with quartz veining in areas of highest grade gold mineralisation. Sphalerite and galena occur within veins and their presence, as well as abundant arsenopyrite in altered wall rock, are common indicators of higher gold grades (Gold Road Resources Limited, 2015; Osborne et al., 2017).

In areas of weak deformation, altered Gruyere Porphyry contains abundant preserved 1 to 2 mm, stubby, euhedral tabular feldspar phenocrysts. Their morphology is consistent with plagioclase being the precursor mineral in the protolith. The porphyry has moderate (~20 vol.%) quartz, indicating it had the composition of a quartz monzonitic prior to alteration. To the east of the Gruyere Porphyry, tholeiitic basalt is composed of a metamorphic assemblage of actinolite, hornblende, andesine and quartz. They have been subjected to biotite-calcite metasomatism in the immediate hanging wall of the porphyry. This metasomatism is interpreted to have occurred during the development of the strong NNW-trending fabric. A four stage paragenesis of alteration and mineralisation has been established at Gruyere, based on lithological relationships, petrography, geochemistry and mineral/elemental mapping (after Osborne et al., 2017):
Stage 1, which includes emplacement of folded quartz ±albite veins that are offset by brittle fractures, boudinaged and/or folded by the deformation that produced the dominant NNW-striking steeply east-dipping foliation.
Stage 2 has been divided into three substage alteration assemblages, namely Stage 2a albite-oligoclase-muscovite-K feldspar; Stage 2b red albite-quartz-oligoclase-biotite-muscovite-magnetite; and late Stage 2c K-feldspar-biotite-calcite veining. Stage 2 extends beyond the Gruyere Porphyry, being detected as far afield as a granite 1 km to the SSE of the deposit.
Stage 3, alteration which accompanied, and hence is spatially associated with the gold mineralisation. It has been subdivided into three substages as follows: Stage 3a quartz ±arsenopyrite ±gold veins; Stage 3b alteration is pervasive and overprints the Stage 3a veining with assemblages of albite + biotite + chamosite + iron sulphides + calcite ±arsenopyrite ±gold, and appears to involve some quartz dissolution in the rock matrix; Stage 3c quartz-arsenopyrite veins which cross-cut the previous alteration, indicating a later pulse of reduced arsenic-rich fluids under quartz-saturated conditions.
  This stage involved a complex progression of interactions between structures, fluids and metals, as the three substages indicate. Mineralising fluids appear to have been introduced via a fracture network, the chamosite-fractures, to permeate through the Gruyere Porphyry with gold precipitated in veins in those fractures. The wall rock bordering these structures was intensely altered to an albite-dominant rock, grading to massive albite in the most intense zones, where iron in the form of pyrite, pyrrhotite ±arsenopyrite and base metal sulphides, mainly sphalerite and galena, were precipitated. The mineralising fluid penetrated deep into the wall rock, likely along grain boundaries and microfractures, as suggested by the extensive sulphide halos up to tens of metres wide surrounding the fracturing. Albite replaced biotite, and in doing so is interpreted to have consumed quartz and released iron and other mobile elements from the biotite which were mobilised into the fractures. The mobilised iron reacted with the mineralising fluid to precipitate chamosite (±biotite), sulphides and precipitated gold. Titanium, likely released by the destruction of biotite, remained in the alteration halo and precipitated as rutile. Ongoing deformation, including folding, and over-pressured fluids resulted in the opening of favourably oriented chamosite-filled fractures, into which further mineralised quartz veins were emplaced (Osborne et al., 2017).
Stage 4 late stage calcite veins which cross-cut all other alteration assemblages.

Regolith

The Gruyere deposit is not exposed at surface, and is entirely masked by 1 to 3 m of Quaternary aeolian sands, with localised dunes that are 5 to 10 m high. The aeolian sand overlies a semi-consolidated sequence of Cenozoic channel sediments (previously though to be Permian, e.g., Gold Road Resources Limited, 2015) that increases in thickness from being absent over the southern Gruyere deposit, to gradually thicken to 25 to 30 m at the northern end. The underlying preserved weathered Archaean rocks gradually increases from south to north, with the depth to top of fresh rock averaging 45 m in the south to 85 m in the north. Much of the weathering profile in the Archaean rocks has been stripped by Cenozoic erosion, with depth to the base of oxide varying from 1 to 2 m to absent in the south, to 15 m in the north, with only saprock remaining. The thickness of saprock varies from 30 to 45 m along strike. The transition zone from saprock to fresh rock varies from 5 to 15 m in thickness. Mineralisation occurs within the cover and underlying profile is distributed as follows (Gold Road Resources Limited, 2015):
Dispersion Blanket - a small, flat lying, thin zone of mineralisation hosted at the saprock‐oxide boundary within both hanging wall and footwall lithologies, accounting for <1% of mineralisation.
Saprock and saprolite within the Gruyere Porphyry, accounting for ~7% of mineralisation.
Fresh and transitional mineralisation hosted within the Gruyere Porphyry makes up the remaining ~93% of the reserves.

Other Deposits

In addition to the main Gruyere deposit there a numerous other smaller deposits, prospects and occurrences distributed over a strike length of 140 km of the Yamarna Shear Zone, including the Attila, Alaric, Argos, Montagne, Orleans and Central Bore resources. Most of these are concentrated over 16 km interval within and near the shear zone, and are directly opposite the Gruyere deposit and Yam 14 resource on the Dorothy Hills Shear Zone. While Gruyere is on the northern end of the north-south section of the flexure in the Dorothy Hills Shear Zone, where its trend changes to NNW-SSE, Yam 14 is at the southern end in a mirror image structural setting.

Reserves and Resources

Pre-mining Ore Reserve and Mineral Resource estimates as at 31 December 2016 (Osborne et al., 2017) were:
    Measured resource - 13.86 Mt @ 1.18 g/t Au;
    Indicated resource - 91.12 Mt @ 1.29 g/t Au;
    Inferred resource - 42.73 Mt @ 1.35 g/t Au;
  TOTAL Mineral Resources - 147.71 Mt @ 1.30 g/t Au for 192 tonnes of contained gold (inclusive of Ore Reserves)
    Proved reserve - 14.9 Mt @ 1.09 g/t Au;
    Probable reserve - 76.7 Mt @ 1.22 g/t Au;
  TOTAL Ore Reserves - 91.6 Mt @ 1.20 g/t Au.

Mineral resources at the Gruyere Project, as of 31 December, 2019 (Gold Fields Mineral Resources and Mineral Reserves Supplement, 2019) were:
  Gruyere Project
    Measured resource - 11.454 Mt @ 1.23 g/t Au;
    Indicated resource - 129.428 Mt @ 1.34 g/t Au;
    Inferred resource - 9.394 Mt @ 1.66 g/t Au;
  TOTAL Mineral Resources - 150.276 Mt @ 1.35 g/t Au (inclusive of Ore Reserves)
  The Measured + Indicated + Inferred Mineral Resources of this combined total were distributed as follows:
    Gruyere Deposit - 134.57 Mt @ 1.23 g/t Au for 165 tonnes of contained gold;
    Yam 14 - 0.854 Mt @ 1.21 g/t Au;
    Alaric - 2.414 Mt @ 1.52 g/t Au;
    Montagne - 3.148 Mt @ 1.27 g/t Au;
    Argos - 2.150 Mt @ 1.21 g/t Au;
    Orleans - 1.006 Mt @ 1.64 g/t Au;
    Attila - 5.894 Mt @ 1.64 g/t Au.

The most recent source geological information used to prepare this summary was dated: 2019.    
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.


  References & Additional Information
   Selected References:
Gold Road Resources Limited  2015 - Gruyere resource increases to 5.62 million ounces; Yamarna mineral resource fully JORC 2012 compliant: in    ASX Announcement by Gold Road Resources Limited, 16 September, 2015,    79p.
Osborne, J.P., Levett, J., Donaldson, J., Berg, R., Davys, C., Prentice, K., Tullberg, D., Lubieniecki, L.Z., Tunjic, J.A., Bath, A.B. and Libby, J.W.,  2017 - Gruyere gold deposit, Yamarna: in Phillips, G.N., (Ed.), 2017 Australian Ore Deposits, The Australasian Institute of Mining and Metallurgy,    pp. 267-272.
Tunjic, J.A., Davys, C., Donaldson, J., Berg, R. and Osborne, J.P.,   2019 - Yamarna Geology: Foundations for Further Discovery: in    Australasian Exploration Geoscience Conference (AEGC 2019): From Data to Discovery - 2 to 5 September, 2019, Perth, Australia   Proceedings, 5p.


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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