DeGrussa, Conductor 1, 4 and 5, Red Bore, Monty/Springfield
Western Australia, WA, Australia
Cu Au Ag
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The DeGrussa, Conductor, Red Bore and Monty copper deposits are located within the Bryah Basin, part of the 2.0 to 1.8 Ga Capricon Orogen that separates the Yilgarn and Pilbara Cratons, approximately 900 km NNE of Perth and 150 km north of Meekatharra in Western Australia.
(#Location: DeGrussa - 25° 32' 18"S, 119° 19' 20"E).
Regional and Local Setting
The DeGrussa and related deposits were emplaced within the Bryah rift basin (Pirajno et al., 2000), which is situated along the northern margin of the Archaean Yilgarn Craton. It is one of the tectonic units of the Palaeoproterozoic Capricorn Orogen, located between the Yilgarn Craton in the south and the Pilbara Craton in the north (Cawood and Tyler, 2004), interpreted to be the result of collisional events between those cratons at ~1830 to 1780 Ma (Cawood and Tyler, 2004; Johnson et al., 2012; Johnson, 2013). Subsequent to these collisional events, later intracratonic reactivation occurred during the amalgamation of the West Australian and North Australian craton (Tyler, 2005), with renewed basin development and magmatism between ~1670 and 1620 Ma (Tyler et al., 1998). The Capricorn Orogen includes, from west to east, the Gascoyne Province, Edmund and Collier basins, Bryah Rift- Basin, Padbury Basin, Yerrida Basin and the Earaheedy Basin, collectively forming an ~1000 km long belt. The Yerrida, Bryah, Padbury basins and Earaheedy basins, form a series of depositional centres that extend for ~700 km east-west along the southeastern margin of the Capricorn Orogen and the northern margin of the Yilgarn Craton, covering a total area of ~70 000 km2 (Cawood and Tyler, 2004; Pirajno et al., 2004).
Mineralisation within the Bryah Basin is hosted by the 2 km thick Palaeoproterozoic (~2.0 Ga) Narracoota Volcanics, occurring as the DeGrussa, Conductor 1, 4 and 5, Red Bore and Monty/Springfield deposits/lenses. This sequence of volcano-sedimentary rocks is distributed over a strike length of 22 km.
The Narracoota Volcanics comprise basalts, basaltic hyaloclastites, sedimentary rocks, dolerite and gabbro and minor local mineralised quartz-carbonate breccias, jasper beds and banded iron formation. This sequence is overlain by the Karalundi Formation, which comprises metamorphosed and locally ferruginous shale and sandstone with metaconglomerate bands and lenses, and chert and siliciclastic metasedimentary rocks. Together, the Narracoota Volcanics and Karalundi Formation constitute the Bryah Group.
To the NE, the ENE trending belt of Narracoota Volcanics is separated from the Marymia Dome by the 1 to 4 km thick band of quartz-mica schist of the Karalundi Formation. The Marymia Dome is an inlier within the Capricorn Orogen, and comprises two Archaean greenstone belts intruded by granite. The Bryah Group and Marymia Dome are separated by the NE-trending Jenkin Fault which structurally influences the DeGrussa and Red Bore group of deposits.
DeGrussa, Conductor 1, 4 and 5
Pirajno et al. (2016) note that the sedimentary rocks hosting these deposits include numerous basaltic sills that display peperitic textures on both upper and lower contacts, indicating intrusion into un-lithified sediment. The massive sulphide mineralisation and associated minnesotaite-carbonate-chlorite-sulphide or chlorite-sulphide alteration has commonly replaced peperite textured portions of basalt-sediment contact zones (Adamczyk, 2013). Elsewhere in the deposit, breccias containing massive sulphide clasts, remnant chimney fragments and bedded sulphide rich sediments support the interpretation of the development of ore as sea floor sulphide mounds (Hawke et al., 2015).
The turbidite derived host sedimentary rocks have also been intruded by dolerite sills with a similar chemistry to those of the basalts, and are interpreted to have been derived from the same parent magma (Adamczyk, 2013). The footwall dolerite, a dolerite sill found immediately below the host unit, displays a peperite contact with the sediment, whereas the dolerite sills in the hanging wall of the deposit have sharp contacts and chilled margins (Pirajno et al., 2016, quoting Hilliard, 2015), indicating intrusion into dry, partly lithified sediment. This hanging wall dolerite has been dated at between 1991±7 and 2003±7 Ma (U-Pb zircon; Hawke et al., 2015). The DeGrussa orebody is considered to have originally been continuous with the Conductor 1 lens but is interpreted to have been separated from it by the intrusion of a post-mineralisation dolerite sill along a low-angle to lithological layering (Hawke et al., 2015).
The footwall dolerite is underlain by a sedimentary breccia, composed of poorly sorted, angular to sub-angular dolomite clasts, ranging from 4 cm to >5 m in diameter, set within a siltstone to sandstone matrix (Adamczyk, 2013).
The DeGrussa mineralisation is interpreted to have been emplaced prior to intrusion of the Hangingwall dolerites but after the basaltic sills that intruded wet, unlithified sediments at an earlier stage during the same magmatic event (Pirajno et al., 2016, quoting Hilliard, 2015). Hawke et al. (2015) dated molybdenite (Re-Os) in the DeGrussa orebody and received ages of mineralisation ranging from 2027±7 to 2011±7 Ma, which coincides with the age of primary pyrite of 2012±8 Ma (Re-Os; Pirajno et al., 2016).
NNE-SSW directed compression deformed the DeGrussa deposit following the cessation of mineralisation and mafic volcanism, leaving it located along the steeply south-dipping (60 to 70°) northern limb of a shallow WSW plunging, upright syncline that developed during this event. Two significant faults in the deposit area have estimated displacements of 500 and 80 m respectively are interpreted to have dismembers an original single, continuous zone of mineralisation into the now separate Conductor 1, DeGrussa, Conductor 4 and Conductor 5 orebodies (Pirajno et al., 2016, quoting Hilliard, 2015). Folding and faulting of the DeGrussa deposit is interpreted to have developed during the 1820 to 1770 Ma Capricorn Orogeny (Cawood and Tyler, 2004; Sheppard et al., 2005).
The ores at DeGrussa are classified as volcanic hosted massive sulphide (VHMS) style deposits, occurring as massive lenses of primary pyrite, chalcopyrite and pyrrhotite with minor magnetite, sphalerite, galena and arsenopyrite in a gangue of siderite, ankerite, stilpnomelane, minnesotaite, quartz and calcite.
The primary mineralisation has been subjected to oxidation and supergene enrichment near surface to produce a surface zone with native copper and siliceous cap with gold, overlying a layer of oxide/carbonate copper and a blanket of supergene chalcocite with tenorite, cuprite and complex tellurides.
The main DeGrussa deposit represents a 20 m-thick, steeply dipping to almost vertical body of high grade copper-gold mineralisation with lesser zinc and silver that lies structurally above Conductor 1. It is defined over a 180 m lateral strike extent and persists to a known vertical depth of more than 300 m. It is bounded by chloritised lithic sedimentary and volcaniclastic rocks on the structural hanging wall side and mostly basaltic lavas, doleritic and gabbroic rocks on the structural footwall. The footwall gabbro and dolerite are pervasively altered to an assemblage of epidote, Mg-rich chlorite, sericite, calcite, titanite (altered to leucoxene), albite and quartz, associated with a network of microfractures.
Deeper in the system, the medium-grained mafic rock (gabbroic) has been subjected to pervasive and complex alteration producing an assemblage that includes chlorite, titanite, sericite and epidote, associated with fractured feldspar with myrmeckitic-like (symplectic) intergrowths of quartz and sodic feldspar, replacing the primary feldspar. These are overprinted by epidote-Mg chlorite. Massive sulphides are found between depths of ~100 and 280 m.
Medium- to coarse grained clastic rocks for the hanging wall, with basaltic lava flows and sub-volcanic dolerite intrusions, many with peperite margins, as well as shale beds at the contact with a zone of disseminated sulphides, which grade into the massive sulphides.
At a depth of 290 m, the massive sulphides are composed of pyrite and chalcopyrite, accompanied by minor sphalerite in a talc-carbonate gangue. Pirajno et al. (2016) report three generations of pyrite, as follows: i). massive pyrite and intimately associated with chalcopyrite, with chalcopyrite infiltrating boundaries between pyrite blebs; ii). euhedral pyrite (py1); iii). spongy pyrite (py2).
Chalcopyrite and pyrite also also found as deformed, streaky patches, whilst spongy or irregular pyrite grains (py2) are usually associated with or overprinted by chalcopyrite. The sulphides are all enclosed within a matrix of granular quartz gangue cut by calcite and calcite-stilpnomelane veinlets. The massive pyrite-chalcopyrite, overprints grains and bands of magnetite. Small grains of sphalerite occurs as inclusions within the dominant pyrite-chalcopyrite assemblage in a gangue of mainly carbonate. In other sections, the ore contains bands of more massive and compact pyrite-chalcopyrite or more comminuted/brecciated pyrite-chalcopyrite-sphalerite-magnetite. A relatively narrow zone of stringer and brecciated ore, composed of fragmentary magnetite, pyrite and minor chalcopyrite in a gangue of phlogopite, occurs at one location within the massive sulphides (Pirajno et al., 2016).
The Conductor 1 deposit occurs as a number of stacked lenses and underlies the main DeGrussa ore zone. It averages 15 m in thickness and extends over a lateral strike length of 350 m, with a steep 75° south-west trending dip, persisting to a known vertical depth of 400 m.
The hanging wall side of the Conductor 1 ore zone comprises fine- to coarse grained immature siltstone, lithic sandstone and dark-green volcaniclastics, characterised by a chlorite-rich matrix, that is locally overprinted by sericitic patches and is cut by calcite veinlets. The footwall to the sulphide zones is occupied by a gabbroic rock, mostly composed of plagioclase, orthopyroxene, tremolite, hornblende and disseminated titanite (locally altered to leucoxene). This gabbro has been locally brecciated and veined and/or altered to chlorite, epidote and at greater depths, near the contact with massive sulphides, becomes texturally complex and pervasively altered. Quartz, tremolite-actinolite and epidote dominate this alteration assemblage. A zone of intense silicification and mylonitisation marks the contact with the massive sulphides. The latter is composed of pyrite, chalcopyrite, magnetite and minor sphalerite, with associated carbonate, white mica and stilpnomelane gangue minerals. Carbonate grains and patches or fragments, are overprinted by sulphide streaks. Acicular crystals of stilpnomelane occur in microfractures or form as patches and aggregates associated or included in carbonate. A dominant assemblage of pyrite-chalcopyrite occurs as a coarse-grained complex aggregate, with chalcopyrite generally formed interstitially within pyrite, possibly post-dating it, whilst in other instance the pyrite-chalcopyrite assemblage fills fractures in sphalerite. The massive sulphide zone is underlain by a stringer zone, comprising fine intergrowths of pyrite and chalcopyrite, minor sphalerite and chalco-pyrrhotite (a solid solution of CuFeS2 and FeS) associated with fragments of carbonate material, brecciated and overprinted by sulphides. These pyrite-chalcopyrite aggregates are locally cut by sinuous veins of chalcopyrite±sphalerite±stilpnomelane. Elsewhere a fragmented carbonate±quartz±stilpnomelane can be seen to be invaded by two generations of sulphides, namely, pyrite±chalcopyrite and chalcopyrite±sphalerite (Pirajno et al., 2016).
The smaller Conductor 4 deposit lies 120 m below and 200 m east of Conductor 1 and DeGrussa. A further small deposit, Conductor 5 has also been delineated, ~200 m further to the east.
In each of these zones, massive sulphide mineralisation occurs at one or more (up to six for Conductor 5) stratigraphic positions within the host sequence of interbedded siltstone, sandstone and conglomerate thought to have been deposited by turbidity currents and density flows (Adamczyk, 2013).
All are distributed along the steep, northeast-trending Shiraz Fault zone which offsets the ore bodies and is a significant local structure with varying widths of highly broken material. The Merlot and Pinot faults also have a local, but less pronounced interaction with the massive sulphide lenses.
The total indicated + inferred resources at DeGrussa and Conductor 1, 4 and 5 in September 2010 (Sandfire Resources website) were:
10.67 Mt @ 5.6% Cu, 1.9 g/t Au, 15 g/t Ag,
including a high grade zone of supergene enriched mineralisation of
0.25 Mt @ 17.6% Cu, 2.6 g/t Au, 21 g/t Ag.
Published ore reserves and mineral resources at 31 December 2015 (Sandfire Resources 2016) were:
DeGrussa underground, Proved - 0.9 Mt @ 5.8% Cu, 2.0 g/t Au
DeGrussa open pit, Proved - 2.8 Mt @ 1.2% Cu, 1.0 g/t Au
Conductor 1, Proved - 2.8 Mt @ 3.9% Cu, 1.5 g/t Au + Probable - 0.2 Mt @ 4.8% Cu, 1.0 g/t Au
Conductor 4, Proved - 0.9 Mt @ 3.7% Cu, 1.7 g/t Au + Probable - 1.3 Mt @ 4.0% Cu, 1.6 g/t Au
Conductor 5, Probable - 0.9 Mt @ 5.8% Cu, 2.0 g/t Au
Stockpiles, Proved - 0.1 Mt @ 4.4% Cu, 1.3 g/t Au
TOTAL reserves - 10.8 Mt @ 3.6% Cu, 1.5 g/t Au.
DeGrussa underground, Measured - 0.8 Mt @ 7.2% Cu, 2.3 g/t Au + Indicated - <0.1 Mt @ 1.8% Cu, 1.9 g/t Au
+ Inferred - <0.1 Mt @ 6.2% Cu, 3.0 g/t Au
DeGrussa open pit, Measured - 2.8 Mt @ 1.2% Cu, 1.0 g/t Au + Indicated - 0.2 Mt @ 0.2% Cu, 1.1 g/t Au
Conductor 1, Measured - 2.7 Mt @ 5.0% Cu, 1.9 g/t Au + Indicated - 0.2 Mt @ 5.7% Cu, 1.6 g/t Au
+ Inferred - 0.1 Mt @ 4.3% Cu, 0.7 g/t Au
Conductor 4, Measured - 1.7 Mt @ 5.8% Cu, 2.0 g/t Au + Indicated - 0.4 Mt @ 5.2% Cu, 2.0 g/t Au
+ Inferred - <0.1 Mt @ 4.5% Cu, 1.7 g/t Au
Conductor 5, Measured - 1.0 Mt @ 6.4% Cu, 2.9 g/t Au + Indicated - 0.6 Mt @ 6.1% Cu, 2.7 g/t Au
+ Inferred - 0.1 Mt @ 7.4% Cu, 2.9 g/t Au
Stockpiles, Measured - 0.1 Mt @ 4.4% Cu, 1.3 g/t Au
TOTAL resources - 10.7 Mt @ 4.5% Cu, 1.8 g/t Au.
Note: Mineral resources are inclusive of ore reserves.
The Red Bore prospect is located ~500 m southeast of the DeGrussa deposits, and appears to be similar to the mineralisation of DeGrussa, hosted by dolerite, gabbro and basaltic rocks of the Narracoota Formation. The ore minerals include chalcopyrite, pyrite, pyrrhotite, bornite, covellite and magnetite (Pirajno, unpublished data). Some sections are almost entirely composed of massive and disseminated granular magnetite, suggesting a transition to massive sulphides (Pirajno, unpublished data). Pirajno (2016) suggests relicts of pentlandite and violarite, which are also present, suggest the possibility of a Ni-Cu orthomagmatic mineral system, later overprinted by supergene sulphides (Pirajno, unpublished data). A preliminary JORC compliant mineral resource estimate by Thundelarra Exploration (2015) was 48 000 t @ 3.2% Cu, 3.6 g/t Ag, 0.4 g/t Au.
The Monty deposit at Springfield, 9 km east of DeGrussa was discovered in 2015 at a depth of 410 m within the same host sequence. It had a maiden resource (March, 2016) of 1.05 Mt @ 9.4% Cu, 1.6 g/t Au, comprising an elongate massive sulphide core containing 0.736 @ 12.1% Cu, 2.1 g/t Au, surrounded by a halo of 0.287 Mt @ 2.2% Cu, 0.3 g/t Au (Talisman Resources, 2016).
The most recent source geological information used to prepare this summary was dated: 2016.
Record last updated: 22/11/2016
This description is a summary from published sources, the chief of which are listed below.
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References to this deposit in the PGC Literature Collection:
Pirajno, F., Chen, Y-J., Li, N., Li, C. and Zhou, L-M., 2016 - Besshi-type mineral systems in the Palaeoproterozoic Bryah Rift-Basin, Capricorn Orogen, Western Australia: Implications for tectonic setting and geodynamic evolution: in Geoscience Frontiers v.7, pp. 345-357.|
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