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Island Dam
South Australia, SA, Australia
Main commodities: Cu


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The Island Dam mineralised system is located in the northern half of Andamooka Island which is on the western margin of Lake Torrens, ~15 km NW of the Murdie Murdie prospect, ~70 km SE of Olympic Dam, ~45 km north of Carrapateena, and ~35 km NE of Oak Dam West in northern South Australia.
(#Location: 30° 48' 17"S, 137° 29' 51"E).

  It lies within the Olympic IOCG Province, which incorporates all of the significant known IOCG mineralised systems of the Gawler Craton, distributed within Palaeo- to Mesoproterozoic rocks along the eastern edge of the currently preserved craton. See the Gawler Craton and Olympic IOCG Province record for details of the geological setting and the geological units mentioned below.

  Mineralisation in the Olympic Cu-Au Province has been spatially and temporally linked to volcanism and plutonism during generation of a ~1.6 Ga siliceous, large igneous province, represented by the Hiltaba Intrusive and Gawler Range Volcanic suites and equivalents. However, that mineralisation is emplaced in a range of different host lithofacies and structures, such that the hydrothermal mineralising fluids generate differing alteration assemblages when reacting with the different hosts. For example, the Olympic Dam deposit is hosted within the Hiltaba Suite granitic and mafic intrusive hosts taking the form of a hematite-breccia deposit. In contrast, the Island Dam mineralised sytem is hosted within a sequence of metasedimentary and volcanic rocks of the Wallaroo Group that include carbonate protoliths which have reacted with mineralising fluids that consequently formed an alteration assemblage that included iron oxides and skarn minerals, as detailed below. Similar contrasts and comparisons may be made with the Carrapateena and Oak Dam granite hosted, and the Prominent Hill and Hillside volcano-sedimentary hosted deposits.

NOTE: The Island Dam mineralised system does not appear to have yielded any Mineral Resource, Ore Reserve or potential economic intersections as of July 2022. However, it is part of the greater Olympic IOCG Province and an example of a Wallaroo Group hosted system that has associated skarn alteration assemblages, copper sulphide mineralisation and iron oxide alteration. It is also the subject of detailed research on its geology and mineralogy recorded in Keyser et al. (2017), (2022) as cited below, and used as the source of information on which this description is based.

  Mineralisation at Island Dam is hosted within meta-sedimentary and volcanic rocks of the Wallaroo Group that were deposited at ~1765 to 1740 Ma over a basement of the ~1850 Ma gneissic granite of the Donington Magmatic Suite (Cowley et al., 2003). The Wallaroo Group includes impure limestone and dolostone horizons, as well as thin banded iron formation-like layering, which is also observed/inferred elsewhere such as in the northern part of the Olympic Dam district (Keyser et al., 2017). These iron oxide layers pre-date the ~1600 to 1585 Ma Hiltaba Igneous Suite equivalents that are related to IOCG mineralisation in the province. The host sequence and basement are intruded by the ~ 820 Ma Gairdner Dolerite (Wingate et al., 1998) and are masked by ~250 to 350 m of unconformably overlying Neoproterozoic and younger cover. The latter include the flat lying to shallowly dipping Nuccaleena Dolomite and the Tregolana Shale. Extensive fault reactivation took place during the ~500 Ma Delamerian Orogeny associated with regional-scale fold-thrust complexes (Foden et al., 2006) and inversion of rifting between the Gawler Craton and Curnamona Block.

  The Island Dam mineralisation and associated alteration is reflected by a pronounced east-west magnetic anomaly that straddles the similarly trending Andamooka Fault Zone, a major fault complex interpreted to provide an important conduit for IOCG fluid flow during the ~1.6 Ga regional Hiltaba Suite magmatic event (Skirrow et al., 2007). Hiltaba Suite intrusives are mapped as being cut by this structure below the Neoproterozoic unconformity within 10 km to the west and at a similar distance to the east. SHRIMP U-Pb dating of igneous zircon within a megacrystic granite intersected in a drill hole yielded an age of 1860 ±4 Ma (Jagodzinski 2005), confirming the presence of a Donington Suite basement. Igneous biotite from the same granite yielded a 40Ar/39Ar plateau age of 1593 ±12 Ma, interpreted to reflect thermal resetting during the Hiltaba Suite silicic-dominated large igneous province magmatism (Skirrow et al., 2007).

  On the basis of limited drilling, the geological model of the mineralised area is divided by the east-west Andamooka Fault Zone into a northern and southern block, with offset sequences that can be correlated across the structure. To the west, the main fault zone splits into three west diverging splays, one on either side of the main through-going structure. These splays seperate two west thickening wedges, one to the west and the second to the NW, separating the main blocks of Wallaroo Group volcanic and sedimentary rocks to the north and south. The NW wedge uplifts and juxtaposes Donington Suite basement granitoids across normal faults on its southern and northeastern margins. The western wedge comprises skarn altered rocks that have been uplifted by normal faulting relative to the Wallaroo Group arkose on its southern flank, although the Donington granitoid wedge to its north has been have been uplifted to a relatively greater degree. All of these faults are steep to moderately dipping with a normal sense of movement, implying an extensional regime. All of these rocks are unconformably overlain by Neoproterozoic and younger cover.

Representative lithologies within the Island Dam mineralised zone include:
Donington Suite granites, which are recognised by the presence of K feldspar megacrysts and their sheared and foliated character. The granites contain preserved phenocrysts of K feldspar, both orthoclase and microcline, magnesio-hornblende and partially replaced biotite, whereas plagioclase is largely replaced by albite and sericite. Syn-orogenic deformation of K feldspar phenocrysts is associated with the development of myrmekitic intergrowths between quartz and plagioclase. Replacement by later generations of feldspar is prominently developed along foliation defined by muscovite-chlorite, becoming progressively obvious in strongly sheared granite, whilst shears are marked by patches of anhedral, elongated K feldspar and phlogopite. The foliation of more melanocratic granites is defined by magnesio-hornblende, phlogopite and K feldspar. Variably replaced magmatic accessories include Fe-Ti oxides, zircon and apatite, whilst secondary apatite is a common phase formed along domains of foliation and shearing. The apatite within foliations and shears in the ~1860 Ma Donington Suite granite has been dated at 1594 ±35 and 1575 ±35 Ma, postdating emplacement of the host by ~250 Ma, suggesting a Hiltaba age overprinting mineralisation-alteration event.
Felsic and mafic volcanic to sub-volcanic rocks. Felsic volcanic rocks occur over distinct intervals within the arkose, whilst fine-grained, dark green, mafic subvolcanic rocks occur as thin, metre-sized intervals in drill core and are interpreted as either sills or dykes. Felsic volcanic bands within the Wallaroo Group volcanic-sedimentary sequence are interbedded with impure dolostone, which may be the Mg-, Ca-, K-bearing protoliths to skarn alteration. The felsic meta-volcanic rocks are composed of K feldspar, microcline and quartz, and contain accessory phases that include zircon and apatite. Mafic dykes have aphanitic textures with carbonate pseudomorphs, after clinopyroxene or olivine phenocrysts, and plagioclase within a groundmass of finer-grained plagioclase and titanomagnetite containing up to 27 wt.% TiO
2.
Siliceous, K feldspar-rich lithologies with up to 80 wt.% SiO
2 and ~7.5 wt.% K2O, interpreted to represent arkosic protoliths by Keyser et al. (2022), although they may, at least in part, have a volcano-sedimentary origin. They are composed of K feldspar, quartz, and to a lesser degree, carbonates and sericite.
Various rhythmically banded iron oxide containing lithologies. Like the arkoses, the banded lithologies represent distinctive packages within the Wallaroo Group, and both have a wide range of colours, from pink to brown and green, and vary from fine- to coarser-grained. These colours are due to K feldspar, disseminations of iron-oxides, and the presence of green minerals such as actinolite and phlogopite, as well as siderite and calcite. The ubiquitous iron oxides variably produce rhythmic bands with silicates, carbonates, chlorite and K feldspar, as well as with skarn silicates. Those lithologies enriched in iron-oxides that are rhythmic banding with quartzitic intervals usually also contain K feldspar as either porphyroblasts or as fine-grained bands. These banded iron-rich rocks are regarded as likely representing Fe-rich metasedimentary protoliths, analogous to banded iron formations (BIFs). Whilst skarn minerals are scarce or absent in such Si-rich banded Fe-rich metasedimentary rocks, skarn silicates, with or without K feldspar, are major components of Si-poor, Mg-rich sediments, referred to by Keyser et al. (2022) as skarn altered.
Rocks with pervasive alteration attributed to metasomatism, principally skarn alteration. The skarn assemblage comprises actinolite/phlogopite, K feldspar and magnetite, suggesting calcareous protoliths and high-temperature alkali-calcic alteration in the early stages of IOCG mineralisation. The actinolite and phlogopite are considered by Keyser et al. (2022) as index minerals for those rocks defined as calcic exoskarn and magnesian endoskarn, respectively. In more detail, the skarn alteration zones are characterised by amphibole and/or biotite ±K feldspar, magnetite and quartz. Amphibole varies from euhedral to acicular (needle-like), with core-to-rim or patchy zoning. The cores of these amphiboles have compositions corresponding to either magnesio-hornblende or a more Fe-rich actinolite, whilst rims are a Mg-richer actinolite. Dark micas have both more or poorer Fe-rich compositions, but are still Mg-dominant and thus have been classified as phlogopite. A characteristic of the Island Dam skarn alteration is the presence of both magnetite and hematite. Euhedral magnetite occurs either as a component of the matrix, or as disseminations along bands, or intergrown with silicate minerals. In all, this magnetite contains up to 2.7 wt.% SiO2 at the micron-scale, wich is defined as 'silician magnetite'. This magnetite is compositionally zoned, from Fe-rich cores to Si-rich rims, with oscillatory zoning with respect to silicon. Whilst the magnetite displays variable pseudomorphic replacement by hematite/martite, it may also have complex equilibrium relationships with coarse grained lamellar hematite. There is a compositional variability in lamellar hematite between cores and margins, or an oscillatory or patchy zoning. Hematite within arkose and the banded Fe-rich horizons is characterised by acicular morphologies within polygranular hematite accumulations. Relict magnetite occurs within both the acicular and polygranular hematite. The polygranular hematite is generally Si-rich, whereas acicular hematite (and relict magnetite inclusions) contains dusty inclusions of scheelite. The iron oxides described above represent three main types: i). silician magnetite, ii). coarse-grained lamellar hematite, and iii). hematite within arkose and Fe-rich ("BIF") horizons that are characterised by acicular morphologies within polygranular hematite.
  The skarn alteration zone contains abundant accessory minerals. Zoned tourmaline, which is more common within the phlogopite skarns, has 'bluish' cores and green rims in transmitted light, corresponding to brighter (Fe-rich) and darker (Mg-rich) intensities, respectively, in back-scattered electron images. Fluorapatite, without any obvious compositional zoning, is found throughout all lithologies and has an intimate relationship with silician magnetite, or with K feldspar within more siliceous lithologies. Lozenge-shaped pseudomorphs, after pre-existing titanite, are more common within actinolite skarns. Assemblages after replacement of titanite, include fine-grained intergrowths of Fe-Ti oxides, which may carry magmatic accessories such as zircon, or rutile + chlorite + calcite, that could have formed contemporaneous with martitisation of silician magnetite. Rutile associated with lamellar hematite is zoned with respect to tungsten and contains fine-grained inclusions of scheelite. Carbonates and fluorite are common within veins and within accumulations of Ca- REE-phosphates and -fluorocarbonates such as apatite and bastnäsite; whilst the latter is partially replaced by synchysite [Ca(Ce,La)(CO
3)2F]. Molybdenite is less common, but is found within phlogopite-K feldspar assemblages.

  Sulphide mineralisation occurs as chalcopyrite, pyrite, bornite and lesser sphalerite, galena, chalcocite and molybdenite, set in a gangue of hematite, chlorite, carbonates, fluorite, barite and quartz. The sulphides occur as disseminations, within veins and as replacements of preexisting skarn alteration minerals. Chalcopyrite is commonly associated with lamellar hematite, generally within pockets and as veins, but may also occur as selective replacement of skarn assemblages along banding. Copper-Fe-sulphide species can co-exist e.g., chalcopyrite and bornite, with exsolution of bornite evident in chalcopyrite. Chalcopyrite, Pb- and Ag-Bi-selenides, clausthalite (PbSe) and bohdanowiczite (AgBiSe
2) occur as symplectic intergrowths illustrating the complexity of the ore mineral assemblages. The selenides are found as fields of dusty inclusions, rod-like lamellae within chalcopyrite, as well as two-component inclusions. Molybdenite is found as discrete bundles tied to phlogopite-bearing assemblages. Assemblages of pyrite, chalcocite, galena and sphalerite are associated with the breakdown of Fe-Ti oxides within mafic dykes.

  Dating of skarn related lamellar hematite, which is intergrown with Cu-Fe-sulphides, places the age of an alteration-mineralisation phase at 1594 ±28 Ma, contemporaneous with the ~ 1.59 Ga IOCG mineralising event recorded across the eastern Gawler Craton. On the basis of textures, an epigenetic crystallisation of this lamellar hematite is consistent with its coarse grain size, fan-like morphology and occurrence infilling pockets and vugs. Oscillatory zoned silician magnetite assemblages have a trace element signature that is comparable to the early alkali-calcic alteration stage associated with the pre-main stage onset of IOCG mineralisation observed in the outer shell of the Olympic Dam deposit and the nearby Wirrda Well prospect. It has the strongest enrichment in transitional metals and lithophile elements compared to lamellar and acicular-polygranular hematite, containing thousands of ppm Mg, Al, Si and Ca, hundreds of ppm Mn, Zn and V, and tens of ppm Cr, Co and Ni. The geochemical signature of hematite from these two skarn associations and that from the banded Fe-rich metasedimentary rocks share a common enrichment in W, Sn, Mo, Th and U, as is also seen in hematite from IOCG-style mineralisation across the Gawler Craton. However, the acicular and polygranular hematite from the banded Fe-rich metasedimentary ("BIF") rocks and arkoses contain magnetite inclusions/relicts indicative of a complex history of iron-oxide interactions. In addition, to the geochemical signature they share with the skarn hematites, they are relatively enriched in chalcophile elements such as As and Sb, transitional metals that include Co and Ni, as well as Mo and U, and as such are quite distinct from the HFSE, W-Sn signature of the lamellar hematite. These geochemical characteristic are interpreted (Keyser et al., 2022) to indicate that the different overall signature of the acicular-polygranular hematite reflects pre-existing, genetically unrelated, iron-rich lithologies that have been overprinted and modified by the 1.59 Ga mineralising event. The polygranular hematite, which also contains the highest mean Ni concentrations of ~33 ppm, may be related to fluids from an earlier source, such as an earlier Hiltaba phase or ~1760 Ma granites, or detrital oxides. This means that not all iron oxide within the mineralised system was introduced by that system.

The information in this description is dominantly drawn from Keyser et al. (2017), (2022) as cited below.

The most recent source geological information used to prepare this decription was dated: 2022.    
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.


Island Dam

  References & Additional Information
   Selected References:
Keyser, W., Ciobanu, C.L., Ehrig, K., Dmitrijeva, M., Wade, B.P., Courtney‑Davies, L., Verdugo‑Ihl, M. and Cook, N.,  2022 - Skarn‑style alteration in Proterozoic metasedimentary protoliths hosting IOCG mineralization: the Island Dam Prospect, South Australia: in    Mineralium Deposita   v.57, pp. 1227-1250.
Keyser, W.M., Ciobanu, C.L., Cook, N.J., Courtney-Davies, L., Ehrig, K., Gilbert, S. and McPhie, J.,  2017 - Links between sedimentary protoliths and IOCG-skarn alteration, Island Dam, South Australia: in   Symposium 3, IOCG-IOA ore systems and their magmatic-hydrothermal continuum: A family reunion? Proceeding of the 14th SGA Biennial Meeting, 20-23 August 2017, Quebec City, Canada,   Proceedings, pp. 935-938.


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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