South Australia, SA, Australia
Cu Au Mo
Super Porphyry Cu and Au|
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The Kalkaroo copper-gold-molybdenum deposit is located within the Curnamona province of northeastern South Australia, ~95 km WNW of Broken Hill and 440 km NNE of Adelaide.
(#Location: 31° 43' 57"S, 140° 31' 55"E).
The 200 x 400 km Curnamona Province, in eastern South Australia, encompasses a high-grade metamorphic and complexly deformed basement cropping out as the Willyama Inliers along its southern fringe (comprising, from west to east, the Olary, Mulyungarie and Broken Hill domains), and the Mt Painter and Mt Babbage Inliers to the northwest (the Moolawatana Domain). One of the dominant features of this basement is the now-buried Benagerie Ridge (Mudguard Domain), an 80 x 40 km north-south trending Proterozoic basement high, flanked by Cambrian basins. This basement ridge is situated to the east of Lake Frome, and forms the core of the province, which is rimmed by the exposed Willyama, Mt Painter and Mt Babbage inliers. The Benagerie Ridge represents a northward extension under cover of the NW-SE-trending exposed Mulyungarie domain in the central part of the Willyama Inlier.
The Neoproterozoic Adelaide rift basin (related to extension preceding the late Neoproterozoic Rodinia break-up) separates the Curnamona province from the Gawler craton to the west, and masks the mid- to late-Palaeoproterozoic suture between the Gawler craton and Curnamona province. The northeastern, eastern and southeastern margins of the preserved Curnamona province are determined by the Tasman Line, which marks the Rodinia break-up and separation of the eastern sections of the pre-breakup cratonic mass. Prior to the break-up of Rodinia, the Gawler craton, Curnamona Provinces, and cratonic elements in North America and Antarctica were all part of the larger Mawson craton.
The Curnamona province encompasses two major groups of rocks, the 4.5 to 13 km thick, 1720 to 1640 Ma Willyama Supergroup and the younger 1600 to 1580 Ma Ninnerie Supersuite. The Willyama Supergroup is divided into three lithostratigraphic packages, the lower Curnamona and overlying Saltbush and Strathearn groups (Conor, 2000; 2006). The Curnamona Group, which is predominantly composed of psammitic protoliths (now, quartzites, granofels, psammitic schists and schists), includes metamorphosed mafic and A-type felsic volcanic rocks, and comagmatic intrusions (Basso Suite), that were emplaced between ~1715 and ~1700 Ma (Ashley et al., 1996; Fanning et al., 1998). This sequence is interpreted to have been deposited in a rift setting (Willis et al., 1983; Plimer, 1986) or a back-arc setting (Giles and Nutman, 2003; Rutherford et al., 2006), although the presence of A-type magmatism and bimodal volcanism through the sequence favours extension and crustal thinning consistent with rifting (Conor 2004).
Typically, though not invariably, the Curnamona Group is overprinted by oxidised albite-magnetite alteration, resulting in Na-rich felsic igneous rocks with associated albitic psammopelites, massive to laminated albitite, quartzo-feldspathic gneisses and iron formation. The Curnamona Group also tends to show a higher degree of metamorphic alteration and partial melting (though not necessarily higher metamorphic grade) than the Saltbush and higher parts of the sequence. Copper-gold mineralisation associated with this alteration is preferentially developed in the lower sections of the Willyama Supergroup, but is concentrated near the top of the Curnamona and basal Saltbush groups (Conor, 2006).
The uppermost unit of the Curnamona Group, the Ethiudna Subgroup, is characterised by calc-silicate assemblages ±magnetite ±hematite bearing laminated albitite, minor Mn-rich rocks and local pyrite-dominated mineralisation. It contains variable basal quartzites and volcaniclastic rocks, and/or enrichments of quartz-magnetite±barite (e.g., the Cathedral Rock Formation), overlain by the Peryhumuck Formation, which is characterised by dark grey, well laminated, rather fine-grained, calc-albite-quartz metasediments, in which shallow water, sedimentary structures such as ripples, ripple crossbeds and flaser bedding are locally preserved. The calc-albitite is predominantly composed of finely granular albite (±K feldspar±biotite) and quartz, with minor to subordinate amounts of amphibole (actinolite, hornblende), clinopyroxene (diopside-hedenbergite), epidote and garnet (grossular-andradite), and trace amounts of titanite, magnetite and hematite. The Peryhumuck Formation is also characterised by its propensity for hydrothermal brecciation. Although not limited to this unit, brecciation is preferentially developed in the Ethiudna Subgroup which is interpreted to represent a dominantly evaporitic protolith (Conor, 2004; 2006).
The Ethiudna Subgroup and interpreted equivalents are disconformably(?) overlain by the Bimba and Portia formations, and the Ettlewood Calc-silicate Member/Cues Formation in the Olary, Mulyungarie and Broken Hill domains respectively.
The Bimba Formation, a thin regional marker, is the basal member of the Saltbush group. It is generally <50 m thick and commonly psammitic, but is characteristically also carbonate-bearing, variably represented by marble, calc-silicate-rich rocks (e.g. clinopyroxene, garnet, amphibole, epidote), pelitic schist, and gradations into bedded pyritic and pyrrhotitic zones with anomalous Cu, Pb, Zn, As and Co. The Bimba formation and overlying grey, partly graphitic, tuffaceous biotitic psammite of the 1693±3 Ma Plumbago Formation form a regional couplet known as the Larry Macs subgroup (Conor, 2006).
The Portia Formation is ~250 m thick, and can be generally divided into three units: a lower marble, middle albitic siltstone/calc-silicate and upper marble. These units are transitional and the thickness and lithology are variable. The formation is sulphide-rich, dominantly pyrite and pyrrhotite, which commonly form fine laminae, but also locally massive sulphide bodies. Dating of tuff units at the Portia Prospect (Teale 2000; Jagodzinski 2006) indicates a 1705 to 1700 Ma age. The Portia Formation is the principal host to epigenetic vein, breccia and replacement Cu-Au mineralisation in the western part of the Mulyungarie Domain, including the Kalkaroo and Portia deposits (Conor, 2006).
The Ettlewood Calc-silicate Member, which is up to ~10 m thick, consists of pale, layered, epidote, diopside- and/or clinozoisite-rich, calc-silicate rocks, both under- and overlain by grey graphitic tuffaceous metasiltstones, which has been dated at 1693±4 Ma.
In the bulk of the Olary and Mulyungarie domains the remainder of the overlying Saltbush and Strathearn groups, which total several hundred metres in thickness, are dominated by pelitic and psammopelitic lithologies, locally containing carbonaceous and aluminosilicate-bearing strata, banded iron formation and tourmalinite. The lead-zinc rich Saltbush Group in these two domains is equivalent to the Broken Hill and Sundown Groups of the Broken Hill Domain, while the overlying Strathearn Group is the direct equivalent of the Paragon Group in the respective domains (see the Broken Hill record).
The Ninnerie Supersuite is largely represented in the concealed Benagerie Ridge and followed a break related to the 1620 to 1600 Ma Olarian orogeny. It is largely composed of the 1594 to 1580 Ma S-type biotite and muscovite monzogranite of the Bimbowrie Suite granitoids on the Benagerie Ridge; the 1600 to 1581 Ma S-type granite and trondhjemite of the Coulthard suite in the Mount Painter Inlier; and the 1580 to 1568 Ma sodic or biotite granitoids, alaskite, leucocratic phlogopite trondhjemite and granodiorite of the Crocker Well suite in the Olary domain. Other members of this supersuite include a series of 1610 to 1550 (but dominantly 1590 to 1580 Ma) granites, diorites and syenites within the Olary Domain. On the Benagerie Ridge, the Ninnerie Supersuite also includes the comagmatic 1587 to 1581 Ma Benagerie Volcanics, variably composed of porphyritic rhyolite to rhyodacite, porphyritic amygdaloidal dacite, basaltic to andesitic breccia, orthoclase and plagioclase-quartz rhyolites. These volcanic rocks may contain apatite, zircon and minor fluorite as well as being subjected to extensive hematite-sericite-carbonate alteration.
Regional metamorphism reached granulite facies in the southern Broken Hill Domain and upper amphibolite facies conditions in the southern Olary Domain during the Olarian Orogeny at ~1600 Ma (Phillips, 1980; Clark et al., 1986; Page and Laing, 1992; Page et al., 2000). Metamorphic grade decreases northwards in the Olary and Broken Hill domains, only reaching upper greenschist facies on the Benagerie Ridge (Teale and Fanning, 2000). The Willyama Supergroup underwent three deformation events during the Olarian Orogeny, and several subsequent deformation events including those of the Delamerian Orogeny. The Olarian D1 and D2 high grade events occurred close to ~1600 Ma (Page and Laing, 1992), although some recent studies suggest D1 may have been as early as ~1690 Ma (Gibson, 1998, 2000). Upright folds and shear zones related to the retrograde D3 event were produced at ~1600 to 1585 Ma (Gibson, 2000; Page et al., 2000; Skirrow et al., 2000). The D2 and possibly D3 events were the most significant. The Delamerian Orogeny was from 515 to 500 Ma.
The Kalkaroo deposit lies on the northern margin of the Mulyungarie Domain, hosted by the Portia Formation, which also hosts most of the known significant Cu-Au deposits of the Curnamona province.
Prior to albitisation the host Portia Formation at Kalkaroo comprised finely laminated to planar bedded carbonaceous and non-carbonaceous shales, evaporitic and carbonate-rich beds and other saline silts and shales, punctuated by possible local disconformities. Flaser cross beds indicate intertidal conditions for some of the rock-types and much of the sequence is considered to have been deposited in shallow water (Teale, 2006).
The altered and metamorphosed sequence can be subdivided into three stratigraphic subdivisions, a lower marble, middle albitic siltstone/calc-silicate and upper marble, which have transitional contact, with both thicknesses and lithologies being variable. The magnetic susceptibility of the formation is generally low when compared with the footwall metasediments which are magnetite-bearing metapsammites and albitic metasiltstones. In addition to albite, the albitic metasiltstones contain K-feldspar, ±quartz, biotite, magnetite, scapolite, calcite, ±epidote, ±hematite. Scapolite and calcite become more abundant upwards, while hematite alteration is prominent in the lower part. The metasiltstones are generally layered, laminated and locally crossbedded, and become more coarser-grained downwards with thin (1 to 10 cm thick) beds of pelitic psammite. The base of the footwall metasiltstone has not been observed. Distribution of magnetite in these metasiltstones is variable, with many magnetite-rich layers (Zang and Conor, 2006).
There is a transitional contact between the footwall metasiltstone and Portia Formation, with increasing calcite and scapolite. The basal Portia Formation is generally defined
by the appearance of layered marble bands, although locally, laminated, dark-grey, pyritic pelites underlie the lower marble. The lower marble unit is generally layered to
laminated but also shows enhanced graded bedding and cross-bedding, suggesting a mid-shelf carbonate build-up, below or at the storm-wave base. It is mostly composed of calcite (up to 90%), with pelitic interlayers containing abundant scapolite and amphibole (tremolite/actinolite) and minor albite, although locally albitisation may be absent (Zang and Conor, 2006).
The middle unit of the Portia Formation is generally thinly layered to laminated and contains thin-bedded albitite or albitic pelite/psammopelite with minor calc-silicate. It is characterised by an assemblage of albite, feldspar, quartz, biotite, actinolite and chlorite ±magnetite. In drill core it occurs as albite-K feldspar-quartz layers with variably distributed round carbonate and biscuit-shaped albite nodules within bedding planes. Actinolite-carbonate-chlorite layers are present and commonly contain less albite. Pelitic layers are usually layered, but locally laminated and cross-layered, while dark-grey laminated, pyritic pelite lower in the unit has enhanced graded bedding and the upper parts of the unit are locally psammitic. These features and others may indicate a mid to outer shelf ramp setting, with deposition under shallowing upwards conditions (Zang and Conor, 2006).
The upper marble layer is very variable, from about 20 cm in one drill hole to >40 m thick in another. It is layered and transitional with the underlying psammopelite, and has a similar gradational contact with the overlying albitic pelite, although at one location a vuggy (karst ?) bed at the contact may suggesting a sedimentary break (Zang and Conor, 2006).
The pelitic hanging wall sequence in the Kalkaroo area comprises two units, a lower albitic, laminated pelite, and an upper deep-water graphitic pelite. The graphitic pelite is massive to laminated with local low-angle cross-bedding and enhanced graded bedding with scoured bases (Zang and Conor, 2006).
Teale (2006) notes that the juxtaposition of early, 1628±20 Ma pre-ore albitites and other brittle lithotypes with the more ductile carbonates is regarded to be one of the important influencing some of the Mo sulphide deposition at 1605±12Ma (Teale and Fanning, 2000), and Cu-Au and further Mo, mostly at ~1588 to 1583 Ma (Skirrow and Ashley, 2000).
The Kalkaroo deposit is masked by 40 to 70 m of transported cover, including Tertiary fluvial sands, lacustrine clays and Quaternary to recent alluvial clayey sand. Basement strata dip at ~40° around the NW-SE elongated Kalkaroo South Dome. This dome, as defined by the Portia Formation below the Cenozoic cover, is ~6 x 3.3 km. The same sequence is repeated in the 15 x 4.8 km Kalkaroo North Dome, which is ~ 2 km WNW of the South Dome. Both domes are cut by a series of steeply dipping, east-west to NE-SW faults which displace the limbs. The core of the Kalkaroo South Dome is reflected as strong magnetic anomaly, reflecting the magnetite-rich footwall rocks, although only part of the Kalkaroo North Dome has a similar magnetic signature. The main Kalkaroo deposit is located on the northwestern rim of the South Dome, dipping northwestward towards the North Dome. Copper-gold mineralisation, typically chalcopyrite-dominant, with associated molybdenite, occurs as:
i). as quartz breccias and stockworks developed within highly fractured and altered Portia Formation. Where transected by major fault zones, Cu, and especially Au grades, tend to be significantly higher, while Mo is generally absent. Breccia types are numerous and not always mineralised. Sulphidic and non-sulphidic milled breccias, fluidised injection breccias, crackle breccias and possible breccias developed by decompressive shock are observed. These breccias can be bedding parallel in stratigraphic and structurally lower domains, or cross-cutting and less sulphidic when structurally higher in the mineralised system. The bedding parallel shears and breccias are usually hosted by meta-carbonate and albitites and incorporate clasts of these rock-types, plus some vein quartz, vein carbonate and exotic rock fragments, and are interpreted to have developed during bedding parallel shear which may have taken place during the formation of the domal structures, and/or accompanied the introduction of mineralising fluids;
ii). stratabound replacement of carbonate, and of pyrite/pyrrhotite, which were in turn developed during diagenesis, replacing chemically reactive carbonate and sulphate bands and nodules in the host Portia Formation sequence. A SHRIMP U-Pb date on titanite from such mineralisation suggest at least a part of the mineralising event was at ~1588 to 1583 Ma (Skirrow and Ashley, 2000). Replacement fronts frequently emanate from veins that are either calcic or potassic. Molybdenite often replaces, fine-grained, delicate, early bedding parallel pyrite in albitised domains adjacent to quartz-carbonate±biotite±sulphide veins, but can also be developed in biotite selvedges adjacent to carbonate-biotite-quartz veins where it is often intergrown with biotite. These veins pass upwards into Mo-rich breccias. This style of mineralisation is characterised by moderate Cu and Au grades and often quite appreciable Mo; and
iii). as quartz breccias and stockworks in the crosscutting faults, both laterally outwards and below the mineralised Portia Formation. These faults are interpreted to have both fractured the favourable hosts and created open spaces for mineralisation, and acted as conduits for high temperature magnetite-K feldspar±biotite alteration. "Breccia swarm" mineralisation in these structures is sulphur-poor (quartz-K feldspar-calcite-magnetite-chalcopyrite-allanite±bornite) and extremely siliceous.
The Kalkaroo orebody has a gross geometry that defines an arcuate, NW-dipping sheet, that is disrupted by extensive faulting at its western and eastern ends. These 'ends' of the arc are defined by one of the NE-SW faults, the Kalkaroo Fault, which is also mineralised, to give the overall mineralised zone a 'D'-shape. An additional NW-SE-trending shear zone, the Western Shear, passes through the intersection of the Kalkaroo Fault and the host sequence on the western end of the mineralised sheet, adding to the structural complexity. The ore is sandwiched within an 80 to 120 m thick favourable host, between well defined footwall and hanging wall rocks, and is remarkably predictable and consistent over its entire 3.5 km of strike. Ore follows the host unit around the north-western margin of the dome, and follows the NE-SW Kalkaroo Fault down plunge to the south (Havilah Resources).
There is an overall zonation within the ore deposit, from the structural and stratigraphic base to top of: Mo, Mo-Cu, Cu-Mo-Au, Cu-Au, Au and Pb-Zn (Teale, 2006).
Molybdenite has been observed in many textural and/or mineralogical associations. Cross-cutting vein relationships suggest a long history of deposition, with some molybdenite considered to have developed as early as ~1630 Ma, making it older than the Cu-Au mineralisation. Mo is best developed structurally lower than the Cu-Au (Teale, 2006).
Gold at Kalkaroo is often observed as inclusions in pyrite and in chalcopyrite that is annealing shattered pyrite, and as gold grains that are in the sub-micron to 50µ range. The gold grains enclosed within pyrite are invariably associated with chalcopyrite-pyrrhotite inclusions that tend to occur as composite grains. Elsewhere, pyrite grains containing only chalcopyrite inclusions (no pyrrhotite) do not contain free gold. Gold is also present as inclusions in rare galena and within anatase. REE is dominantly carried by allanite and sphene (Teale, 2006).
Bedding parallel sphalerite and galena occur adjacent to the Kalkaroo deposit, replacing carbonate ellipsoids and as bedding parallel 'veins' which always tend to be coarser grained than the enclosing meta-sedimentary hosts. These veins are associated with albite-quartz-tourmaline-carbonate±garnet±epidote±pyrite. Pb-isotopic studies now indicate that galena shares a similar isotopic composition to altaite (PbTe) in the Cu-Au mineralisation (Teale, 2006).
During its post-depositional history, the orebody has undergone supergene leaching and enrichment, caused by oxidation of the primary sulphides in the weathering zone. This is manifest in a stratification of the ore minerals from top to bottom, forming four main ore types as follows (Havilah Resources):
i). Leached saprolite zone containing supergene free gold, with generally minor copper, largely recoverable by gravity methods.
ii). Native copper and gold proximal to the highest primary Cu grades, largely recoverable by gravity methods.
iii). Chalcocite dominant with gold, recoverable by conventional flotation, occurring as chalcocite coated chalcopyrite in the lower part of the profile, to malachite-dominant towards the top where gossanous material may exist (Dawson, 2000).
iv). Chalcopyrite dominant with gold, recoverable by conventional flotation. The shallowest intersections at around 110 m depth
The following lists the total resource sub-divided by ore type (Havillah Resources, 2012):
Saprolite gold - 18.69 Mt @ 0.74 g/t Au,
Native Copper - 13.12 Mt @ 0.55% Cu, 0.58 g/t Au,
Chalcocite - 29.52 Mt @ 0.56% Cu, 0.43 g/t Au,
Chalcopyrite - 81.86 Mt @ 0.47% Cu, 0.34 g/t Au,
Total ore - 144.00 Mt,
Resource estimates (JORC compliant), as of February 2012 (Havillah Resources) are:
Copper-gold ore, Measured resource - 85.89 Mt @ 0.52% Cu, 0.41 g/t Au,
Copper-gold ore, Indicated resource - 38.62 Mt @ 0.45% Cu, 0.33 g/t Au,
Copper-gold ore, Total Measured + Indicated resource - 124.51 Mt @ 0.50% Cu, 0.39 g/t Au,
Saprolite gold cap, Measured + Indicated resource - 18.7 Mt @ 0.74 g/t Au.
The most recent source geological information used to prepare this summary was dated: 2011.
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.
Conor. C.H.H. and Preiss, W.V., 2008 - Understanding the 1720-1640Ma Palaeoproterozoic Willyama Supergroup, Curnamona Province, Southeastern Australia: Implications for tectonics, basin evolution and ore genesis: in Precambrian Research v.166, pp. 297-317.|
Teale G S, 2006 - Structural and Stratigraphic Controls on the Zoned North Portia and Kalkaroo Cu-Au-Mo Deposits: in Korsch R J and Barnes R G, 2006 Broken Hill Exploration Initiative: Abstracts for the September 2006 Conference GeoScience Australia Record 2006/21 pp. 178-181|
Zang W L and Conor C H H, 2006 - Stratigraphic and depositional aspects of the Portia and Kalkaroo prospects, Mulyungarie Domain, SA: in Korsch R J and Barnes R G, 2006 Broken Hill Exploration Initiative: Abstracts for the September 2006 Conference GeoScience Australia Record 2006/21 pp. 202-206|
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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.
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