Geochemical investigations associated with the Wilga and Currawong massive sulphide deposits, Benambra, Victoria

The Wilga and Currawong copper-zinc massive sulphide deposits in northeastern Victoria occur within a sequence of Silurian volcanics and sediments. The Wilga deposit which was discovered in mid 1978 consists of a single lens while the Currawong deposit, discovered in early 1979, consists of at least two lenses.The first indication of the presence of base metal mineralization in the area was provided by an assessment of stream sediment geochemical data contained in open-file Exploration Licence reports at the Victorian Department of Minerals and Energy.The massive sulphide mineralization does not outcrop, but the ore horizons are weakly mineralized and give rise to stringer gossans as far as 150 m up dip from ore grade mineralization. These can be identified by their trace element chemistry (anomalous values of Bi, Fe, As, Au, Pb, Hg, Se, Co, Ag and Mn) corresponding to the trace element signature of both stringer and massive sulphides.Soils in the area are essentially skeletal and residual with some colluvial movement on the steeper slopes. The soils are highly anomalous in Cu, Pb, and Zn over the projected horizon of the Wilga mineralization and the No. 2 lens at Currawong.The stream sediment responses at both Wilga and Currawong result from a combination of chemical and elastic dispersion. Downslope from the surface expression of the Wilga mineralization a spring discharges directly into the Tambo River. The spring has a very low pH and is rich in base metals resulting in enhanced metal values in both stream water and stream sediments.Analyses of selected samples of the more prominent vegetation species have failed to show a clear relationship to the mineralization.     

The importance of different types of magmatism in VHMS mineralisation: evidence from the geochemistry of the host volcanic rocks to the Benambra massive sulphide deposits, Victoria, Australia

Summary The Wilga and Currawong Cu-Zn massive sulphide deposits in southeastern Australia are hosted by a deformed sequence of Upper Silurian basaltic to rhyolitic volcanic and sedimentary rocks. The syn-volcanic mineralisation occurs immediately above a thick package of rhyolitic volcanic rocks and volcaniclastic rocks (Thorkidaan Volcanics), and is overlain by relatively thin intercalated sills, intrusive domes and flows of basalt, andesite and dacite (Gibson's Folly Formation). The Thorkidaan Volcanics haveNd(420Ma) = -2.2 to - 9.8 and are considered to have been derived by partial melting of older crustal rocks, whereas the basalt-andesite-dacite hangingwall sequence has(Nd(415Ma) = -0.5 to + 2.0 suggesting derivation from a relatively undepleted mantle source. Relatively high-Ti andesitic to dacitic rocks from the Bumble Creek area haveNd(415Ma) = +5.2 to +5.9 suggesting affinities with Ordovician volcanic rocks elsewhere in the Lachlan Fold Belt. The Thorkidaan Volcanics display a limited silica range (73 to 79 wt.%), but have distinctive minor and trace element variations indicating a substantial fractionation history involving feldspar and several accessory phases. Major and trace element compositions of the basalt-andesite-dacite suite display regular variations consistent with a cogenetic relationship by fractional crystallisation. The basaltic rocks mostly have low TiO₂ (< 0.8 wt.%) and other chemical characteristics such as high Zr/Nb and La/Nb which suggest formation in a subduction-related setting; probably an embryonic back-arc basin developed on stretched continental lithosphere, or in small pull-apart basins developed adjacent to a transtensional margin. The magmatic history and paleogeography reflect an extensional tectonic and magmatic cycle comprising uplift, rhyolitic magmatism from crustal melting, extension, subsidence, and penetration of a mantle-derived basalt-andesite-dacite suite up extensional faults to the sea floor. Massive sulphide ores are located exactly at the stratigraphic change from rhyolitic to more mafic mantle-derived magma types. Consideration of the types of mineralisation associated with crustal, S-type granitoids, coupled with thermal constraints limiting the capacity of small bodies of silicic magma to initiate and sustain hydrothermal convection cells of reasonable size, suggests that in the absence of coeval mafic magmatism, S-type crustal-derived silicic volcanic packages are likely to be barren of VHMS deposits. Mineralisation occurs in association with mantle-derived basalt-andesite-dacite suites that either provide the necessary heat to facilitate leaching of the footwall volcanic rocks, or contribute metal-rich hydrothermal solutions during fractional crystallisation, or both.

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Die Bedeutung verschiedener Typen von Magmatismus für VHMS Mineralisationen: Die Geochemie der vulkanischen Nebengesteine der massiven Sulfid-Lagerstätte von Benambra, Victoria, Australien

Zusammenfassung Die massiven Cu-Zn-Sulfid-Lagerstätten von Wilga und Currawong in Südostaustralien treten in einer tektonisch beanspruchten Abfolge von obersilurischen, basaltischen bis rhyolitischen Vulkaniten und Sedimenten auf. Die syn-vulkanische Vererzung ist unmittelbar oberhalb einer mächtigen Abfolge von rhyolitischen Vulkaniten und Vulkanoklastiten (Thorkidaan Vulkanite) zu finden, darüber folgen relativ dünne wechsellagernde Ge, Intrusiv-Dome und Ergüsse von Basalt, Andesit und Dacit (Gibson's Folly Formation). Die Thorkidaan Vulkanite habenNd(420Ma) =-2,2 bis - 9,8 und dürften durch partielle Aufschmelzung älterer krustaler Gesteine entstanden sein. Die Basalt-Andesit-Dacit-Abfolge im Hangenden hat jedochNd(415Ma) = -0,5 bis + 2,0, was auf Herkunft aus einer relativ wenig verarmten Mantelquelle hinweist. Relativ Ti-reiche andesitische bis dacitische Gesteine aus dem Gebiet vom Bumble Creek zeigenNd(415) = +5,2 bis + 5,9. Dies weist auf Beziehungen mit Ordovicischen Vulkaniten in anderen Teilen des Lachlan-Gürtels hin. Die Thorkidaan Vulkanite zeigen SiO₂-Gehalte von 73 bis 79 Gew. %, und charakteristische Variationen der Haupt- und Spurenelementgehalte. Diese lassen eine signifikante Fraktionierung erkennen, an der Feldspäte und verschiedene andere Nebengemengteile beteiligt waren. Die Haupt- und Spurenelementzusammensetzungen der Basalt-Andesit-Decit-Abfolge zeigt normale Variationen, die auf co-genetische Beziehungen mit fraktionierter Kristallisation hinweisen. Die basaltischen Gesteine haben niedrige Gehalte an TiO₂ (< 0, 8 gew. %) sowie hohe Zr/Nb und La/Nb Verhältnisse, die auf Bildung in einem Subduktionsbereich, wahrscheinlich in einem embryonischen Back-Arc Becken, das sich auf ausgedünnter kontinentaler Lithosphäre oder in einem kleinen Pull-Apart Becken in der Nähe eines transtensionalen Randes entwickelt hat, hinweisen. Die magmatische Entwicklungsgeschichte und die Palägeographie weisen auf Dehnungs-Tektonik und auf einen magmatischen Zyklus hin, der Hebung,rhyolitischen Vulkanismus infolge von Krustenaufschmelzung, Extension, Absenkung und Durchdringung mit einer vom Mantel bezogenen Basalt-Andesit-Dacit-Suite entlang von Verwerfungen auf den Meeresboden erkennen läßt. Massive Sulfiderze kommen genau an der stratigraphischen Grenze von rhyolitschen zu mafischen Magmatypen mit Mantel-Ursprung vor. Hier ist es erforderlich, die Vererzungstypen, die zusammen mit krustalen S-Typ Granitoiden vorkommen, ebenso zu erwägen, wie die thermalen Aspekte, die die Fähigkeit kleiner saurer Magmenkörper limitieren, hydrothermale Konvektionszellen ausreichender Größe in Gang zu setzen und zu erhalten. Das weist darauf hin, daß bei Fehlen von gleichaltrigem mafischem Magmatismus saure vulkanische Abfolgen mit Krustenherkunft sehr wahrscheinlich keine VHMS-Lagerstätten führen können. Diese Vererzungen sind an Basalt-Andesit-Dacit-Abfolgen mit Mantelherkunft gebunden, die entweder die notwendige Wärmequelle für die Auslaugung vulkanischer Gesteine im Liegenden lieferten und/oder metallreiche hydrothermale Lösungen während fraktionierter Kristallisation verfügbar machten.

     

The Mount Saddleback bauxite deposit, southwestern Australia

Bauxite within the laterite horizon was first identified as being of economic significance in the Mt. Saddleback area of Western Australia in the 1960's. The Mt. Saddleback bauxite is derived from a mafic volcanic (greenstone) parent material and has high Fe and low silica contents compared with other bauxite in the region. It occurs as two distinct lithological units, a hard, massive, Fe-rich layer (hardcap) overlying a more friable, nodular earthy horizon (B zone). The bauxite averages 6 m in thickness and is underlain by a deep clay horizon. The major minerals of the bauxite are gibbsite, goethite and hematite with kaolin increasing with depth towards the base of the B zone. Minor components are quartz and anatase with traces of corundum, boehmite and maghemite in hardcap.     

Contact metamorphism around the Stawell granite, Victoria, Australia

The contact metamorphosed metapelitic and metapsammitic rocks surrounding the Stawell granite, western Victoria, Australia, are divided into three zones: the low-grade zone, the medium-grade zone and the high-grade zone. Detailed petrological study shows consistency of element distributions, implying that equilibrium was widely attained in the rocks, although equilibrium volumes are generally small (millimetre scale) and considerable mineral chemical variations exist between adjacent domains. The metamorphic mineral assemblages are generally of high variance (KFMASH variance ≤ 2). Consequently, the chemical evolution of assemblages is controlled largely by bulk composition and metamorphic temperature, the former factor being more important in most rocks. The chemographic relations of mineral assemblages in low- and medium- to high-grade zones are presented in compatibility diagrams projected from biotite, quartz and H₂O, and biotite, K-feldspar and H₂O, respectively. These compatibility diagrams have the advantage of showing both quartz-bearing and quartz-absent assemblages. The metamorphic reactions are modelled successfully by a calculated petrogenetic grid that combines both KFASH and KMASH equilibria. Based on petrographic observations and with constraints from the calculated petrogenetic grid, the following KFMASH reactions, in the order of increasing metamorphic grade, are responsible for producing the various mineral assemblages in the Stawell rocks: chl + mu + q = bi + cd + V, chl + q + cd = g + V, mu + bi + q = ksp + cd + V, mu + q = ksp + and + cd + V (or KASH mu + q = ksp + and + V), mu + cd = ksp + and + bi + V, mu + bi + and = ksp + sp + V, and + bi = ksp + sp + cd + V, mu + bi = ksp + cor + sp + V, mu = ksp + cor + and + sp + V (or KASH mu = ksp + cor + V), bi + cd + q = g + ksp + V. The combined KFASH and KMASH grid provides constraints on reaction coefficients in the above sequence of reactions and on temperature and pressure of metamorphism.     

Flexural-slip generated bedding-parallel veins from central Victoria, Australia

Thin continuous laminated bedding-parallel quartz veins (BPVs) with slip-striated and fibred vein walls occur within slates, or at their contact with sandstones, on the limbs of chevron folds in the Bendigo-Castlemaine goldfields, southeastern Australia. Two microstructural Types of BPV (I and II) have been previously recognized, and are confirmed in this study. Both types are concluded to have formed during and/or after crenulation cleavage (the first tectonic axial planar structure) in the wallrock slates, and during flexural-slip folding. Type I BPVs consist of syntaxial phyllosilicate inclusion trails, parallel to bedding, enclosing inclined inclusion bands, the latter formed by detachment of wallrock phyllosilicate particles from the walls of pressure solution-segmented discordant tension veins. Type I BPVs are formed by bedding-parallel shear, and grow in width by propagation of the discordant veins into the BPV walls. Type II veins are composed of quartz bands separated by wallrock slate seams which have split away from the vein wall during dilatant shear opening. They incorporate numerous torn-apart fragments of crenulated wallrock slate. Type I BPV inclusion band average spacing of 0.5 mm probably represents the magnitude of slip increments during stick-slip flexural-slip folding activity.     

The geology and palaeohydrology of the Kow Swamp fossil hominid site,Victoria, Australia

During a lake‐full phase at the end of the Pleistocene Period, lacustrine silts and aeolian sands were deposited around the eastern margins of Kow Swamp. These sediments have yielded the remains of a large population of early Australian man, retaining certain archaic Homo erectus characteristics.

The lake‐lunette system in which the burials were made provides a geographically isolated situation for the examination of late Pleistocene and Holocene hydrological changes which have left their imprint on the sediments, soils and vegetation.     

The hydrogeology and hydrogeochemistry of the Barwon Downs Graben aquifer, southwestern Victoria, Australia

The Barwon Downs Graben lies on the northern flanks of the Otway Ranges and is situated approximately 70 km southwest of Geelong, Victoria, Australia. The major lower Tertiary Barwon Downs Graben aquifer comprises highly permeable sands and gravels interbedded with clays and silts of the hydraulically interconnected Pebble Point, Dilwyn and Mepunga Formations. Groundwater flows east into the Barwon Downs Graben from the Barongarook High, and yields ¹⁴C ages up to ～20 ka implying that recharge rates are low and, consequently, that the resource could be impacted by overabstraction. The presence of three different lithological units has led to the development of localized flow systems that has resulted in a lack of regular spatial variations in groundwater chemistry. Stable isotopic data suggests that groundwater was recharged under similar climatic conditions as of today. The major ion chemistry of the freshest groundwater is dominated by Na and HCO₃ while higher TDS groundwater, from the confining Narrawaturk Marl, is dominated by Na and Cl. Cl/Br ratios are close to rainfall suggesting that halite dissolution is not the principle source of salts. An excess of Na relative to Cl in fresher groundwater suggests that feldspar dissolution has occurred, however, water–rock interaction is limited. The concentrations of Ca, Mg, and SO₄ are controlled by silicate dissolution and ion-exchange reactions with clays.     

The evolution of groundwater in the Tyrrell catchment, south-central Murray Basin, Victoria, Australia

The Tyrell catchment lies on the western margin of the Riverine Province in the south-central Murray Basin, one of Australia’s most important groundwater resources. Groundwater from the shallow, unconfined Pliocene Sands aquifer and the underlying Renmark Group aquifer is saline (total dissolved solids up to 150,000 mg/L) and is Na-Cl-Mg type. There is no systematic change in salinity along hydraulic gradients implying that the aquifers are hydraulically connected and mixing during vertical flow is important. Stable isotopes (¹⁸O+²H) and Cl/Br ratios indicate that groundwater is entirely of meteoric origin and salts in this system have largely been derived by evapotranspiration of rainfall with only minor halite dissolution, rock weathering (mainly feldspar dissolution), and ion exchange between Na and Mg on clays. Similarity in chemistry of all groundwater in the catchment implies relative consistency in processes over time, independent of any climatic variation. Groundwater in both the Pliocene Sands and Renmark Group aquifers yield ages of up to 25 ka. The Tyrrell Catchment is arid to semi-arid and has low topography. This has resulted in relatively low recharge rates and hydraulic gradients that have resulted in long groundwater residence times.     

Low-pressure regional metamorphism in the Omeo Metamorphic Complex, Victoria, Australia

The Omeo Metamorphic Complex forms the southern end of the Wagga Metamorphic Belt, which is the main locus of Palaeozoic low-pressure metamorphism in the Lachlan Fold Belt, south-eastern Australia. It comprises metamorphosed Ordovician quartz-rich turbidites originally derived from Precambrian cratonic rocks. Prograde regional metamorphism occurred in the early Silurian, very soon after sedimentation had ceased. The sequence of metamorphic zones, with increasing grade, is: chlorite, biotite, cordierite, andalusite–K-feldspar and sillimanite–K-feldspar. Migmatites occur in the sillimanite–K-feldspar zone, but large bodies of S-type granite were derived from rocks underlying the exposed Ordovician sequence. P and T estimates for the highest grade rocks are T = 700°C and P = 3.5 kbar, indicating a very high P–T gradient of 65°C/km.
The high heat flow during prograde metamorphism probably resulted from a combination of a thermal anomaly persisting from a pre-metamorphic back-arc basin environment, and intrusion of hot, mantle-derived magmas into the lower and middle crust.
Regional retrograde metamorphism coincided with a general reheating of the crust in the Siluro-Devonian, accompanied by intrusion of many I-type plutons and resetting of the K–Ar dates of some earlier plutons. The Omeo Metamorphic Complex was exposed to erosion at this time.     

Palaeozoic Victoria,Australia: Igneous rocks,ages and their interpretation

A reconnaissance traverse across Victoria yields 160 K‐Ar dates on igneous rocks from 94 localities. These are supplemented by Rb‐Sr dating in critical cases, and major‐element analyses (some new) on a proportion of the samples. All dates quoted in text and tables, new and previously‐published, have been revised in terms of the latest decay‐constant conventions.

The dates range from Early Ordovician (480 Ma) in the west to Late Devonian (360 Ma) among the high‐level intrusives of Central Victoria. The relatively complex age pattern, and the petrochemical character of the rocks, are compared with the published chronology of neighbouring States, and are interpreted in terms of a long‐duration regime of westwards compression, which began in the deformations of the Adelaide System of South Australia, and continued until the Mid‐Devonian Tabberabberan Movement.     

Partially Fused Granite Blocks from Mt. Elephant, Victoria, Australia

Petrological and chemical evidence is presented to show thatsmall patches of brown glass found in blocks of granite eruptedin basaltic scoria at Mt. Elephant are the result of the partialfusion of biotite and quartz. The glass has moderate SiO₂ (63–66per cent), high AI₂O₃ (17.5–19 per cent), high FeO (3.8–5.2per cent) and very high K₂O (7.0–7.8 per cent) relativeto Na₂O (3.3–3.7 per cent) and is similar in compositionto many high K alkali syenites and trachytes.     

The late orogenic timing of mineralisation in some slate belt gold deposits,Victoria, Australia

Gold mineralisation in classic Australian slate belt gold deposits at Ballarat, Bendigo, St. Arnaud and Inglewood occurred very late in the orogenic history of these rocks rather than during formation of the main slaty cleavage. This has been revealed through the examination of microstructural relationships in gold-bearing quartz veins and their host rocks from these deposits, which has established a D₁ to D₄ deformation-stage history and consistent timing for gold mineralisation over a wide area. The gold was deposited synorogenically but during the fourth deformation stage (D₄) of the orogeny, a relatively weak event occurring two deformations after the main slaty cleavage producing event, D₂. Previously, D₂ had been regarded as both the source and control of gold mineralisation as most of the quartz veins that occur in these deposits formed before or during this deformation event. However, most gold is hosted in breccia veins that formed during D₄. The wallrock clasts within these breccia veins contain a young rotated foliation and the breccia veins are spatially associated with a paragenetically consistent alteration of the host rocks in the deposits. This alteration both crosscuts and preferentially mineralises wallrock S₄ allowing the timing of the breccia veins, alteration and gold deposition to be defined as syn-D₄ in age.     

The geochemistry of wall rock alteration in turbidite-hosted gold vein deposits, central Victoria, Australia

Gold deposits hosted in Ordovician to Devonian turbidites in central Victoria, Australia, consist of steeply dipping quartz vein systems hosted mainly by reverse faults. Wall rock alteration of the host metasandstones, metasiltstones and shales (or slates) extends at least 20 m from the veins in the Bendigo-Ballarat zone (BBZ) and 10 m in the Melbourne zone (MZ) deposits. Alteration minerals include carbonates (ankerite, siderite and dolomite) chlorite, seriate, arsenopyrite, pyrite, chalcopyrite and sphalerite, with less common barite, albite and biotite in the BBZ and carbonates (siderite, ankerite, dolomite and ferromagnesite), sericite, chlorite, arsenopyrite, pyrite, and stibnite with less common chalcostibite in the MZ. SiO₂, Na₂O, MgO and Sr with P₂O₅ commonly decreasing during alteration while CO₂, S, As, Sb, Au, V, Al₂O₃, Ga, K₂O and Rb with Ni and Cr commonly increase. TiO₂, FeO, Fe₂O₃, MnO, Th, U, Nb, La, Ce, and Sc showed little change. Concentrations of Zn, Cu, Pb, and Ca are variable.The relatively large decrease of SiO₂ could account for most, if not all, quartz present in the ore veins. The Na₂O and MgO could have crystallized in the veins in the very minor albite and chlorite present. The addition of V in all and Ni and Cr in most deposits probably reflects a source enriched in these elements such as underlying greenstones. The source of both the volatile (S, As, Sb) and lithophile (K₂O and Rb) elements as well as Au is unknown, but they could have been derived from a magmatic source or from the metamorphism of Cambrian greenstones. CO₂, present as carbonate, was derived mainly by the reaction of graphite, originally present in the sediments, with the ore solutions. Al₂O₃, the only other major element after SiO₂, probably increased mainly due to the decrease of the latter.     

Fire frequency analysis for different climatic stations in Victoria,Australia

Frequent occurrence of fire events will have severe impact on Victoria’s water supply catchments. Hence, it is important to perform fire frequency analysis to obtain fire frequency curves (FFC) on fire intensity using Forest Fire Danger Index (FFDI) at different parts of Victoria. FFDI is a measure of fire initiation, spreading speed and containment difficulty. FFC will guide water harvesting by providing information with regard to future fire events and the subsequent impact on catchment yield. Five probability distributions, namely normal, Log Pearson Type III (LPIII), gamma, log-normal and Weibull distributions were used for the development of FFCs at ten selected meteorological stations spread all over Victoria. LPIII distribution was identified as the best fit distribution for Victoria and subsequently applied for an additional 30 more stations to show spatial variability for the entire Victoria.     

Volcanic-hosted rare-metals deposit at Brockman,Western Australia

Rare-metals mineralization at Brockman, Western Australia, is the product of early pyroclastic eruption of trachytic magma enriched in volatiles and incompatible elements such as Zr, Hf, Nb, Ta, Be, Y and REE and Ga. The mineralization is fine-grained (<20 m) and is the result of alteration and re-mobilization of comparatively simple magmatic precursor minerals such as columbite and zircon by F-rich deuteric solutions that were retained in an ash-flow tuff (the Niobium Tuff) following eruption. Chondrite normalized REE distributions show strong enrichment in HREE. Gel-zircon is the principal residence of the HREE, disseminated bastnaesite (±parisite and synchisite) carries the LREE and bertrandite, in late-stage calcite veins, is the host for Be. Ga occurs in K-mica in the groundmass. Trachytic flows overlying the Niobium Tuff contain many of the same ore minerals, but in trace amounts.     

New constraints on fluid sources in orogenic gold deposits,Victoria, Australia

Fluid inclusion microthermometry, Raman spectroscopy and noble gas plus halogen geochemistry, complemented by published stable isotope data, have been used to assess the origin of gold-rich fluids in the Lachlan Fold Belt of central Victoria, south-eastern Australia. Victorian gold deposits vary from large turbidite-hosted ‘orogenic’ lode and disseminated-stockwork gold-only deposits, formed close to the metamorphic peak, to smaller polymetallic gold deposits, temporally associated with later post-orogenic granite intrusions. Despite the differences in relative timing, metal association and the size of these deposits, fluid inclusion microthermometry indicates that all deposits are genetically associated with similar low-salinity aqueous, CO₂-bearing fluids. The majority of these fluid inclusions also have similar ⁴⁰Ar/³⁶Ar values of less than 1500 and ³⁶Ar concentrations of 2.6–58 ppb (by mass) that are equal to or much greater than air-saturation levels (1.3–2.7 ppb). Limited amounts of nitrogen-rich fluids are present at a local scale and have the highest measured ⁴⁰Ar/³⁶Ar values of up to 5,700, suggesting an external or distinct source compared to the aqueous fluids. The predominance of low-salinity aqueous–carbonic fluids with low ⁴⁰Ar/³⁶Ar values, in both ‘orogenic’ and ‘intrusion-related’ gold deposits, is attributed to fluid production from common basement volcano-sedimentary sequences and fluid interaction with sedimentary cover rocks (turbidites). Aqueous fluid inclusions in the Stawell–Magdala deposit of western Victoria (including those associated with N₂) preserve mantle-like Br/Cl and I/Cl values. In contrast, fluid inclusions in deposits in the eastern structural zones, which contain more abundant shales, have elevated molar I/Cl ratios with maximum values of 5,170 × 10⁻⁶ in the Melbourne Zone. Br/I ratios in this zone range from 0.5 to 3.0 that are characteristic of fluid interaction with organic-rich sediments. The maximum I/Cl and characteristic Br/I ratios provide evidence for organic Br and I released during metamorphism of the shales. Therefore, the regional data provide strong evidence for the involvement of sedimentary components in gold mineralisation, but are consistent with deeper metamorphic fluid sources from basement volcano-sedimentary rocks. The overlying sediments are probably involved in gold mineralisation via fluid–rock interaction.     

The Mesoproterozoic Abra polymetallic sedimentary rock-hosted mineral deposit,Edmund Basin,Western Australia

Abra is a blind, sedimentary rock-hosted polymetallic Fe–Pb–Zn–Ba–Cu ± Au ± Ag ± Bi ± W deposit, discovered in 1981, located within the easterly trending Jillawarra rift sub-basin of the Mesoproterozoic Edmund Basin, Capricorn Orogen, Western Australia. The Edmund Basin contains a 4–10 km thick succession of siltstone, sandstone, dolomitic siltstone, and stromatolitic dolomite. The age of the Edmund Group is between 1.66 and 1.46 Ga. The Abra polymetallic deposit is hosted in siltstone, dolostone, sandstone and conglomerate of the Irregully and Kiangi Creek Formations, but the mineralised zones do not extend above an erosion surface marking the change from fluvial to marine facies in the lower part of the Kiangi Creek Formation. The Abra deposit is characterised by a funnel-shaped brecciated zone, interpreted as a feeder pipe, overlain by stratiform–stratabound mineralisation. The stratiform–stratabound mineralisation includes a Red Zone and an underlying Black Zone. The Red Zone is characterised by banded jaspilite, hematite, galena, pyrite, quartz, barite, and siderite. The jaspilite and hematite cause the predominant red colouration. The Black Zone consists of veins and rhythmically banded sulphides, laminated and/or brecciated hematite, magnetite, Fe-rich carbonate and scheelite. In both zones, laminations and bands of sulphide minerals, Fe oxides, barite and quartz commonly exhibit colloform textures. The feeder pipe (Stringer Zone) merges with Black Zone and consists of a stockwork of Fe-carbonate-quartz, barite, pyrite, magnetite and chalcopyrite, exhibiting fluidised and/or jigsaw textures.The Abra mineral system is characterised by several overprinting phases of hydrothermal activity, from several stages of brecciation and fluidisation, barite and sulphide veining to barren low-temperature chalcedonic (epithermal regime) veining. Hydrothermal alteration minerals include multi-stage quartz, chlorite, prehnite, Fe-rich carbonate and albite. Albite (Na metasomatism) is an early alteration phase, whereas Fe-rich carbonate is a late phase. Fluid inclusion studies indicate that the ore fluids had temperatures ranging from 162 to 250 °C, with salinities ranging from 5.8 to about 20 wt.% NaCl. In the course of our studies, microthermometric and Raman microprobe analyses were performed on fluid inclusions in carbonate, quartz and barite grains. Fluid inclusions in quartz show homogenisation temperatures ranging from 150 to 170 °C with calculated salinities of between 3.7 and 13.8 wt.% NaCl.The sulphur isotopic system shows δ³⁴S values ranging from 19.4 to 26.6‰ for sulphides and from 37.4 to 41.9‰ for barite (Vogt and Stumpfl, 1987, Austen, 2007). Sulphur isotope thermometry between sulphides and sulphide–barite pairs yields values ranging from 219 to 336 °C (Austen, 2007).Galena samples were analysed for Pb isotope ratios, which have been compared with previous Pb isotopic data. The new Pb isotope systematics show model ages of 1650–1628 Ma, consistent with the formation of the host Edmund Basin.Re–Os dating of euhedral pyrite from the Black Zone yielded an age of ~ 1255 Ma. This age corresponds to the 1320–1170 Ma Mutherbukin tectonic event in the Gascoyne Complex. This event is manifested primarily along a WNW-trending structural corridor of amphibolite facies rocks, about 250 km to the northwest of the Abra area. It is possible that the Re–Os age represents a younger re-activation event of an earlier SEDEX style system with a possible age range of 1640–1590 Ma.A genetic model for Abra is proposed based on the above data. The model involves two end-members ore-forming stages: the first is the formation of the SEDEX style mineral systems, followed by a second multi-phase stage during which there was repeated re-working of the mineral system, guided by seismic activity along major regional faults.     

Silicate glass and sulfides in ultramafic xenoliths,Newer Basalts,Victoria, Australia

The Newer Basalts of Victoria contain an abundance of inclusions, the majority being Iherzolites of an accidental origin. During ascent of the host magma a CO₂-rich vapor phase ‘boiled’ off and penetrated the inclusions, resulting in ubiquitous fluid inclusions and partial melting along some fractures and intergranular boundaries. Fe+Ni+Cu sulfide globules distributed within the glass and along planes in the primary silicates apparently formed during ascent and originated with a sulfur component in the invading vapor and not with remobilization of free sulfides in the xenoliths themselves.     

Geochemistry of a peraluminous granitoid suite from North-eastern Victoria,South-eastern Australia

The Koetong Suite of Silurian, 2-mica granitoids was derived from a metasedimentary source and emplaced into Ordovician sediments and metasediments along the eastern margin of the Western Metamorphic Belt of South-eastern Australia. Whole-rock geochemical considerations preclude derivation of the magmas represented by the granitoids from exposed Ordovician metasediments. The magmas were generated by partial melting of material similar in composition to garnet-cordierite gneisses exposed in the adjacent metamorphic belt. Melting at pressures in excess of 5 Kb and temperatures about 750°C produced peraluminous magmas and, when the degree of partial melting approached 25–30%, these magmas became mobile and moved vertically into the overlying Ordovician sediments. During movement from the source region to the zone of emplacement, separation of the melt and refractory residue components of the magma resulted in a range of compositions so that whole-rock analyses of the granitoids are linearly related on major and trace element variation diagrams. Processes such as crystal fractionation and crystal accumulation may have operated locally. The magmas were largely composed of solid material throughout their emplacement histories and the amount of melt may not have exceeded 30–45% at any stage. Metasedimentary inclusions are a reflection of source heterogeneity.After emplacement of the magmas, in situ crystallization of a relatively anhydrous assemblage of minerals led to water contents in residual, intercrystalline, melts sufficiently high for muscovite to begin crystallization at pressures around 4 Kb. Subsequent saturation of intercrystalline residual melt and loss of the resultant volatile phase caused the development of eutectoid intergrowths involving muscovitebiotite-quartz and alkali feldspar.     

A geochronological framework for orogenic gold mineralisation in central Victoria, Australia

New ⁴⁰Ar/³⁹Ar geochronological data support, and significantly expand upon, preliminary age data that were interpreted to suggest an episodic and diachronous emplacement of gold across the western Lachlan fold belt, Australia. These geochronological data indicate that mineralisation in the central Victorian gold province occurred in response to episodic, eastward progressing deformation, metamorphism and exhumation associated with the formation of the western Lachlan fold belt. Initial gold formation throughout the Stawell and the Bendigo structural zones can be constrained to a broad interval of time between 455 and 435 Ma, with remobilisation of metals into new structures and/or new pulses of mineralisation occurring between 420 and 400 Ma, and again between 380 and 370 Ma, linked to episodic variations in the regional stress-field and during intrusion of felsic dykes and plutons. This separation of ages is incompatible with the view that gold emplacement in the western Lachlan fold belt was the result of a single, orogen-wide event during the Devonian. A distinct phase of gold mineralisation, characterised by elevated Cu, Mo, Sb or W, is associated with both Late Silurian to Early Devonian (~420 to 400 Ma) and Middle to Late Devonian (~380 to 370 Ma) magmatism, when crustal thickening and shortening during the ongoing consolidation of the western Lachlan Fold Belt led to extensive melt development in the lower crust and resulted in widespread magmatism throughout central Victoria. These ~420 to 400 Ma and ~380 to 370 Ma occurrences, best exemplified by the Wonga deposit in the Stawell structural zone and many of the Woods Point deposits in the Melbourne structural zone, but also evidenced by occurrences at Fosterville and Maldon in the Bendigo structural zone, clearly formed synchronous with, or post-date, the emplacement of plutons and dykes, and thus are spatially (if not genetically) related to melt generation at depth. This later, magmatic-associated and polymetallic type of gold mineralisation is economically subordinate to the earlier, metamorphic-associated type of gold deposition in the Stawell and Bendigo structural zones, but tends to be the dominant style in the Melbourne Zone. These new geochronological constraints, together with zircon U-Pb data from felsic intrusive rocks of known relationship to gold mineralisation, demonstrate that initial hydrothermal alteration associated with gold emplacement in the western Lachlan fold belt was metamorphic-related, predating the emplacement of granite plutons by as much as 80 million years. This timing differs from other important orogenic gold districts where gold deposition is closely associated spatially with felsic magmatism. The early introduction of metamorphically derived fluids well before magmatism may reflect variations in the timing of peak metamorphic conditions at different crustal levels in an accretionary prism undergoing simultaneous deformation and erosion. Consequently, no genetic link exists between the main phase(s) of gold mineralisation and magmatism in the central Victorian gold province. With the exception of formation of a minor magmatism-related and geochemically-distinct mineralisation style at about 420 to 400 Ma, and again at about 380 to 370 Ma, the apparent spatial relationship between gold mineralisation and felsic intrusions is merely the result of melts and fluids being channelised along the same structures.