Deformation history of the Naraku Batholith,Mt Isa Inlier,Australia: Implications for pluton ages and geometries from structural study of the Dipvale Granodiorite and Levian Granite

Plutons of the Naraku Batholith were emplaced into Proterozoic metasediments of the northern portion of the Eastern Fold Belt of the Mt Isa Inlier during two intrusive episodes approximately 200 million years apart. Structural relationships and geochronological data suggest that the older plutons (ca 1750 Ma) are contemporaneous with granites of the Wonga Batholith to the west. The Dipvale Granodiorite and the Levian Granite represent these older intrusive phases of the Naraku Batholith, and both contain an intense tectonic foliation, S₁, which is interpreted to have formed during the north‐south shortening associated with D₁ of the Isan Orogeny. The geometry of S₁ form surfaces at the southern end of the Dipvale Granodiorite, and of the previously unrecognised sheeted contact, defines a macroscopic, steeply south‐southwest‐plunging antiform, which was produced by the regional D₂ of the Isan Orogeny. S₁ form surfaces in the Levian Granite define open F₂ folds with wavelengths of several hundred metres. The structural age of emplacement of the Dipvale Granodiorite and the Levian Granite is interpreted to be pre‐ or syn‐ the regional D₁. An intense foliation present in some of the younger (ca 1505 Ma) granites that comprise the bulk of the Naraku Batholith is interpreted to represent S₃ of the Isan Orogeny. Foliations commonly have similar styles and orientations in both the pre‐D₁ and younger plutons. This emphasises the simplicity with which regional fabrics can be, and probably have been, miscorrelated in the Eastern Fold Belt, and that the classification of granites in general on the basis of structural and geometric criteria alone is fraught with danger.     

Long-lived crustal-scale fluid flow: the hydrothermal mega-breccia of Hidden Valley, Mt. Painter Inlier, South Australia

The Palaeozoic Hidden Valley breccia in the Northern Flinders Ranges of South Australia is exceptional for (1) its size of about 10 km², (2) the large continuous range in clast sizes from tens of microns to hundreds of metres and (3) thorough mixing of lithologies of different provenance, some originally kilometres apart stratigraphically. The size distribution follows a single fractal dimension of about unity over at least 6 orders of magnitude, implying that a single process was responsible for diminution from the 100 m scale, down to < mm. The breccia formed during >12 km exhumation which lasted about 200 Myrs, starting during the ~500 Ma Delamerian Orogeny and continuing during the Alice Springs Orogeny. Fluids released during exhumation were structurally focussed towards Hidden Valley, where an estimated 20 (5–30) km³ total fluid volume caused the extensive brecciation. Brecciation initiated in Neoproterozoic cover metasedimentary rocks, at a level that is now fully exhumed. As hydrothermal fluid ascent continued with ongoing exhumation, the level of brecciation shifted down into the underlying Mesoproterozoic basement rocks, taking with it clasts of cover rocks. In this model, rocks pass through the zone of brecciation, which can thus incorporate a variety of lithologies that were originally kilometres apart.     

Early Mesoproterozoic metamorphism in the Barossa Complex,South Australia: links with the eastern margin of Proterozoic Australia

LA-ICP-MS U–Pb geochronological data from metamorphic monazite in granulite-facies metapelites in the Barossa Complex, southern Australia, yield ages in the range 1580–1550 Ma. Metapelitic rocks from the Myponga and Houghton Inliers contain early biotite–sillimanite-bearing assemblages that underwent partial melting to produce peak metamorphic garnet–sillimanite-bearing anatectic assemblages. Phase equilibrium modelling suggests a clockwise P–T evolution with peak temperatures between 800 and 870°C and peak pressures of 8–9 kbar, followed by decompression to pressures of ～6 kbar. In combination with existing age data, the monazite U–Pb ages indicate that the early Mesoproterozoic evolution of the Barossa Complex is contemporaneous with other high geothermal gradient metamorphic terranes in eastern Proterozoic Australia. The areal extent of early Mesoproterozoic metamorphism in eastern Australia suggests that any proposed continental reconstructions involving eastern Proterozoic Australia should share a similar tectonothermal history.     

扬子陆块西缘中元古代晚期元谋花岗岩的时代、成因及构造意义

扬子陆块西缘晚中元古代地质演化史一直存在较大争议,本文选择以扬子西缘元谋杂岩中一套二长花岗岩为研究对象,开展岩相学、锆石U-Pb年代学、全岩地球化学等综合研究,为认识和理解扬子西缘晚中元古代地质演化提供支撑。两件元谋二长花岗岩样品的LA-ICP-MS锆石U-Pb年龄分别为1086±10 Ma（MSWD=1.4,n=50）和1099±10 Ma（MSWD=1.8,n=58）。所有样品具有高硅（SiO₂为69.44%~73.98%）、富碱（Na₂O+K₂O为6.11%~7.72%）、贫钙（CaO为0.39%~1.46%）、贫镁（MgO为0.52%~0.76%）、低钛（TiO₂为0.30%~0.59%）的特点,同时表现出强过铝质（A/CNK=1.19~1.35）及中钾钙碱性–钾玄岩系列特征。它们具有高的稀土元素总量（∑REE=211.60×10⁻⁶~349.01×10⁻⁶）,呈现轻稀土元素富集和重稀土元素亏损（（La/Yb）_N=4.32~7.36）;富集Rb、U、Th等大离子亲石元素和Zr、Th、Hf等,亏损Nb、Ta、Ba等元素,并具有明显的负Eu异常（δEu=0.46~0.59）,锆石饱和温度介于827~912℃之间,展示了A型花岗岩的属性。这些二长花岗岩可能是通过中上地壳的中酸性火成岩的部分熔融形成,结合前人的研究成果,它们最可能形成于弧后的伸展环境,综合扬子陆块周缘晚中元古代的岩浆记录,元谋杂岩中1.09 Ga二长花岗岩的形成应与扬子陆块开始参与Rodinia超大陆聚合有关。

     

Age of the Mt Boggola volcanic succession and further geochronological constraint on the Ashburton Basin,Western Australia

The age of strata in the Palaeoproterozoic Ashburton Basin is not well constrained, particularly the generally homogeneous, turbiditic and thick Ashburton Formation containing only a small fraction of volcanics suitable for geochronological examination. The Mt Boggola volcanic succession is one of these rare occurrences, consisting of mafic pillow lavas and breccia overlain by BIF, chert, ferruginous pelite, mafic volcaniclastics and possible felsic tuffs identified in the course of mineral exploration. A locality proximal to the volcanic succession is interpreted as a fragmental volcaniclastic unit derived with minimal reworking from a tuff. Zircon extracted from this unit has yielded a SHRIMP ²⁰⁷Pb/²⁰⁶Pb weighted‐mean age of 1829 ± 5 Ma (95% conf.: χ² 1.0). This age is significantly older than that of the June Hill Volcanics in the northwest of the Ashburton Basin that had previously been surmised to be potentially coeval, and provides a further constraint on the evolution and diachroneity of the Ashburton Formation.     

Thermoluminescence dating of the eruption at Mt Schank,South Australia

Mount Schank, a young volcano in southeastern South Australia, has been dated by thermoluminescence. The dated material was quartz from a former beach dune overlain by the lava flow. Disequilibrium in the uranium decay series required a detailed analysis of the isotopic concentrations in the sand. The samples dated yielded an average age of 4930 ± 540 years BP which is consistent with palaeomagnetic measurements. Combined thermoluminescence, palaeomagnetic and radiocarbon evidence leave unresolved the relative chronologies of Mt Schank and nearby Mt Gambier.     

Timing and nature of fluid flow and alteration during Mesoproterozoic shear zone formation, Olary Domain, South Australia

The development of shear zones at mid‐crustal levels in the Proterozoic Willyama Supergroup was synchronous with widespread fluid flow resulting in albitization and calcsilicate alteration. Monazite dating of shear zone fabrics reveal that they formed at 1582 ± 22 Ma, at the end of the Olarian D3 deformational event and immediately prior to the emplacement of regional S‐type granites. Two stages of fluid flow are identified in the area: first an albitizing event which involved the addition of Na and loss of Si, K and Fe; and a second phase of calcsilicate alteration with additions of Ca, Fe, Mg and Si and removal of Na. Fluid fluxes calculated for albitization and calcsilicate alteration were 5.56 × 10⁹ to 1.02 × 10¹⁰ mol m^?2 and 2.57 × 10⁸–5.20 × 10⁹ mol m^?2 respectively. These fluxes are consistent with estimates for fluid flow through mid‐crustal shear zones in other terranes. The fluids associated with shearing and alteration are calculated to have δ¹⁸O and δD values ranging between +8 and +11‰, and ?33 and ?42‰, respectively, and ?Nd values between ?2.24 and ?8.11. Our results indicate that fluids were derived from metamorphic dehydration of the Willyama Supergroup metasediments. Fluid generation occurred during prograde metamorphism of deeper crustal rocks at or near peak pressure conditions. Shear zones acted as conduits for major crustal fluid flow to shallow levels where peak metamorphic conditions had been attained earlier leading to the apparent ‘retrograde’ fluid‐flow event. Thus, the peak metamorphism conditions at upper and lower crustal levels were achieved at differing times, prior to regional granite formation, during the same orogenic cycle leading to the formation of retrograde mineral assemblages during shearing.     

New age constraints on metamorphism,metasomatism and gold mineralisation at Plutonic Gold Mine,Marymia Inlier,Western Australia

The Plutonic Well Greenstone Belt (PWGB) is located in the Marymia Inlier between the Yilgarn and Pilbara cratons in Western Australia, and hosts a series of major Au deposits. The main episode of Au mineralisation in the PWGB was previously interpreted to have either accompanied, or shortly followed, peak metamorphism in the late Archean at ca 2650 Ma with a later, minor, event associated with the Capricorn Orogeny. Here we present new Pb isotope model ages for sulfides and Rb–Sr ages for mica, as well as a new ²⁰⁷Pb–²⁰⁶Pb age for titanite for samples from the Plutonic Gold Mine (Plutonic) at the southern end of the PWGB. The majority of the sulfides record Proterozoic Pb isotope model ages (2300–2100 Ma), constraining a significant Au mineralising event at Plutonic that occurred >300 Myr later than previously thought. A Rb–Sr age of 2296 ± 99 Ma from muscovite in an Au-bearing sample records resetting or closure of the Rb–Sr system in muscovite at about the same time. A younger Rb–Sr age of 1779 ± 46 Ma from biotite from the same sample may record further cooling, or resetting during a late-stage episode of metasomatism in the PWGB. This could have been associated with the 1820–1770 Ma Capricorn Orogeny, or a late-stage hydrothermal event potentially constrained by a new ²⁰⁷Pb–²⁰⁶Pb age of 1725 ± 26 Ma for titanite in a chlorite–carbonate vein. This titanite age correlates with a pre-existing age for a metasomatic event dated at 1719 ± 14 Ma by U–Pb ages of zircon overgrowths in a sample from the Marymia Deposit. Based on the Pb-isotope data presented here, Au mineralising events in the PWGB are inferred to have occurred at ca 2630, 2300–2100 Ma, during the Glenburgh and Capricorn orogenies, and 1730–1660 Ma. The 2300–2100 Ma event, which appears to have been significant based on the amount of sulfide of this age, correlates with the inferred age for rifting of the Marymia Inlier from the northern margin of the Yilgarn Craton. The texturally-later visible Au may have been deposited during the Glenburgh and Capricorn orogenies.     

Geology of the Acropolis prospect,South Australia,constrained by high-precision CA-TIMS ages

Abstract

Acropolis is an Fe-oxide–copper–gold prospect ～20?km from Olympic Dam, South Australia, and marked by near-coincident gravity and magnetic anomalies. Prospective Fe-oxide–apatite?±?sulfide veins occur in Mesoproterozoic and Paleoproterozoic volcanic and granitoid host units beneath unmineralised sedimentary formations. We have produced a geological map and history of the prospect using data from 16 diamond drill holes, including LA-ICPMS and high-precision CA-TIMS ages. The oldest unit is megacrystic granite of the Donington Suite (ca 1850?Ma). A non-conformity spanning ca 250 My separates the Donington Suite and felsic lavas and ignimbrites of the Gawler Range Volcanics (GRV; 1594.03?±?0.68?Ma). The GRV were intruded by granite of the Hiltaba Suite (1594.88?±?0.50?Ma) and felsic dykes (1593.88?±?0.56?Ma; same age as the Roxby Downs Granite at Olympic Dam). The felsic dykes are weakly altered and lack Fe-oxide–apatite–sulfide veins, suggesting that they post-date the main hydrothermal event. If correct, this relationship implies that the main hydrothermal event at Acropolis was ca 1594?Ma and pre-dated the main hydrothermal event at Olympic Dam. The GRV at Acropolis are the same age as the GRV at Olympic Dam and ca 3–7 My older than the GRV exposed in the Gawler Ranges. The gravity and magnetic anomalies coincide with sections through the GRV, Hiltaba Suite and Donington Suite that contain abundant, wide, Fe-oxide veins. The GRV, Hiltaba Suite and Donington Suite are unconformably overlain by the Mesoproterozoic Pandurra Formation or Neoproterozoic Stuart Shelf sedimentary formations. The Pandurra Formation shows marked lateral variations in thickness related to paleotopography on the underlying units and post-Pandurra Formation pre-Neoproterozoic faults. The Stuart Shelf sedimentary formations have uniform thicknesses.

1. KEY POINTS

2. Fe-oxide–apatite?±?sulfide veins are hosted by the Gawler Range Volcanics (1594.03?±?0.68?Ma), the Hiltaba Suite granite (1594.88?±?0.50?Ma) and Donington Suite granite (ca 1850?Ma).

3. The age of felsic dykes (1593.88?±?0.56?Ma) interpreted to be post-mineralisation implies that the main hydrothermal event at Acropolis was ca 1594?Ma.

4. The Gawler Range Volcanics at Acropolis are the same age as the Gawler Range Volcanics at Olympic Dam and ca 3 to 7 My older than the Gawler Range Volcanics exposed in the Gawler Ranges.

     

Structural geology of the Dugald River Zn---Pb---Ag deposit, Mount Isa Inlier, Australia

The sediment-hosted Zn---Pb---Ag deposit at Dugald River is situated 87 km northeast of Mount Isa, NW Queensland. It is a mid-scale base metal accumulation restricted to a black slate sequence of low metamorphic grade. The orebody is tabular and consists of fine- to medium-grained sulphides with a dominant mineralogy of sphalerite, pyrrhotite, pyrite, galena, quartz and muscovite. Three different ore types have been recognized based on mineralization textures; laminated, banded and brecciated. The present reserve stands at 38 million tons of ore averaging 13.0% Zn, 2.1% Pb and 42 g/t Ag. A structural investigation has revealed that six stages of deformation have affected the metasediments in the Dugald River area. The first four (D₁, D₂, D₃ and D₄) are characterized by the extensive development of folds and associated axial plane cleavage. They were all generated in a ductile regime and are of considerable significance for the structural evolution of this region as well as for the emplacement and localization of the sulphide mineralization. D₅ provides a transition towards brittle deformation developing strong kink folds with subhorizontal axial planes. D₆ was a brittle event, producing E-W-trending open folds and major NE and NW strike-slip faults crosscutting all the pre-existing structural elements plus segmenting the orebody. Correlation between the development of deformation and the formation of mineralization can be observed from macro- to microscales. Relationships of mineralization with folds and cleavage indicate a post-D₂ (dominant deformation event) and probably syn-D₄ deformation timing for the Zn---Pb---Ag mineralization at Dugald River, as suggested by the ubiquitous truncations of D₂ fabrics by ore mineral assemblages throughout the deposit.     

Metamorphic evolution of the Georgetown Inlier,northeast Queensland,Australia; evidence for an accreted Palaeoproterozoic terrane?

Mineral textures, coupled with thermodynamic modelling in the MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (MnNCKFMASH) model system, of mid‐amphibolite facies metapelites from the Georgetown Inlier, northeast Australia, point to the recording of two separate and unrelated metamorphic events. The first occurred contemporaneously with Palaeo‐ to Mesoproterozoic orogenesis and involved heating and burial to temperatures and pressures of approximately 600–650 °C and 6.0–7.0 kbar. Textural evidence for the up‐temperature (and pressure) prograde part of this path is inferred from the inclusion of garnet in biotite and staurolite. The second metamorphic event resulted in a low‐pressure thermal overprint that is equated with the advective addition of heat to the terrane via the emplacement of the Forsayth Batholith (c. 1550 Ma). This event is inferred from subsequent growth of andalusite and randomly orientated fibrolitic sillimanite after garnet, biotite and staurolite. This two stage metamorphic evolution, when coupled with a number of other distinct geological characteristics, suggests that the Georgetown Inlier is dissimilar to the other Australian Palaeoproterozoic terranes with which it is commonly correlated.     

南岭大东山花岗岩的形成时代与成因——SHRIMP锆石U-Pb年龄、元素和Sr-Nd-Hf同位素地球化学

中国东南部南岭地区广泛出露以弱过铝质黑云母二长花岗岩和黑云母钾长花岗岩为主的燕山早期花岗质岩石,其成因有待进一步研究。大东山岩体岩性主要为黑云母二长花岗岩和黑云母钾长花岗岩,两个样品的SHRIMP锆石U-Pb 年龄为165±2 Ma 和159±2 Ma,与区域南岭系列的黑云母花岗岩的主要形成时代一致。花岗岩样品以高硅（SiO2 > 72%）、高钾（K2O/Na2O > 1.6）、富碱（K2O + Na2O = 7.36% ~ 9.31%）和弱过铝质（集中于ASI = 1.00 ~ 1.11）为特征。微量和稀土元素组成上,岩体富Rb, Th 和LREE,贫Ba, Nb, Sr, P 和Ti, Eu 负异常显著（δEu = 0.06 ~ 0.34）。多数样品的Zr,Ce, Nb 和Y 含量总和小于350×10-6,10 000 × Ga/Al 值低于典型的A 型花岗岩。同位素组成上,样品具有高I sr（ 0.7123 ~ 0.7193）和低εN（d t）（-9.3~ -11.5）的特点,两阶段Nd 模式年龄为1.70~1.89 Ga ;与全岩εNd（t）不同,岩浆锆石的εHf（t）具有较大的变化范围（-3.5~ -11.8）。矿物学及地球化学结果表明大东山是一个高分异的I 型花岗岩岩体。岩体岩浆很可能是由元古代火成岩石部分熔融形成,并伴随有少量年轻或新生幔源物质的加入,岩浆上升侵位的过程中发生混合、结晶分异作用。     

Supergene alteration of the Mt Windarra nickel sulphide ore deposit,Western Australia

Three main zones of progressive oxidation, termed the transition, violaritepyrite and oxide zones, can be delineated in the supergene profile of the Mt Windarra massive/matrix ore deposit. In the broad transition zone from pure primary ore, pentlandite is progressively oxidised to an iron rich violarite of composition Co_0.02Fe_1.38Ni_1.60 S₄, releasing Fe²⁺ and Ni²⁺ ions into solution. Up to 43% of this Ni²⁺ moves to nearby pyrrhotite margins which are replaced firstly by nickeliferous smythite and then by a second lamellar-textured violarite with an even higher iron content but lacking in cobalt (approximately Fe_1.6Ni_1.4S₄). On completion of violaritisation of the pentlandite, violaritisation of the pyrrhotite also ceases and the remainder of the pyrrhotite is rapidly replaced by secondary pyrite/marcasite, siderite and void space, this reaction defining the top of transition zone. Both sulphur and nickel are extracted from solution and further Fe²⁺ ions are released into solution. The violarite-pyrite zone is characterised by the absence of pentlandite and pyrrhotite and continued stability of violarite and secondary iron disulphides. Most, if not all, of the iron generated by these oxidation reactions precipitates as magnesian siderite at the expense of magnesite, giving rise to solutions containing mainly Mg²⁺ and Ni²⁺ ions. At and just above the water table atmospheric oxygen is reduced while the sulphides are oxidised to sulphate and hydroxides. Much of the iron remains in situ as characteristic goethite relicts while nickel and copper are leached, producing the enrichment below the water table. The overall genetic model proposed is electrochemical and is analogous to the corrosion of a piece of metallic iron partially immersed in differentially aerated water.     

Linking deformation, migmatite formation and zircon U–Pb geochronology in polymetamorphic orthogneisses, Sveconorwegian Province, Sweden

Absolute ages of migmatization in the polymetamorphic, parautochthonous basement of the Sveconorwegian Province, Sweden, have been determined using U–Pb ion probe analysis of zircon domains that formed in leucosome of migmatitic orthogneisses. Migmatite zircon was formed by recrystallization whereas dissolution–reprecipitation and neocrystallization were subordinate. The recrystallized migmatite zircon was identified by comparison of zircon in mesosomes and leucosomes. It is backscatter electron‐bright, U‐rich (800–4400 ppm) with low Th/U‐ratios (generally 0.01–0.1), unzoned or ‘oscillatory ghost zoned’, and occurs as up to 100 μm‐thick rims with transitional contacts to cores of protolith zircon. Protolith ages of 1686 ± 12 and 1668 ± 11 Ma were obtained from moderately resorbed, igneous zircon crystals (generally Th/U = 0.5–1.5, U < 300 ppm) in mesosomes; protolith zircon is also present as resorbed cores in the leucosomes. Linkage of folding, synchronous migmatization and formation of recrystallized zircon rims allowed direct dating of south‐vergent folding at 976 ± 7 Ma. At a second locality, similar recrystallized zircon rims in leucosome date pre‐Sveconorwegian migmatization at 1425 ± 7 Ma; an upper age bracket of 1394 ± 12 Ma for two overprinting phases of deformation (upright folding along gently SSW‐plunging axes and stretching in ESE) was set by zircon in a folded metagranitic dyke. Lower age brackets for these events were set at 952 ± 7 and 946 ± 8 Ma by zircon in two crosscutting and undeformed granite–pegmatite dykes. Together with previously published data the present results demonstrate: (i) Tectonometamorphic reworking during the Hallandian orogenesis at 1.44–1.42 Ga, resulting in migmatization and formation of a coarse gneissic layering. (ii) Sveconorwegian continent–continent collision at 0.98–0.96 Ga, involving (a) emplacement of an eclogite unit, (b) regional high‐pressure granulite facies metamorphism, (c) southvergent folding, subhorizontal, east–west stretching and migmatization, all of which caused overprint or transposition of older Mesoproterozoic and Sveconorwegian structures. The Sveconorwegian migmatization and folding took place during or shortly after the emplacement of Sveconorwegian eclogite and is interpreted as a result of north–south shortening, synchronous with east–west extension and unroofing during late stages of the continent–continent collision.     

Relative timing of deformational,metamorphic and mineralisation events at the Mt. Todd (Yimuyn Manjerr) Mine,Pine Creek Inlier,Northern Territory,Australia

The Mt. Todd Mine (also known as the Yimuyn Manjerr Mine), located approximately 40 km northwest of the township of Katherine, in the Northern Territory, Australia, is host to several discrete ore bodies that strike NNE within a broad NE-trending corridor of gold mineralisation. The mine lies in the southern region of the Central Domain of the Pine Creek Inlier (PCI) and is hosted by a Palaeoproterozoic sequence of rocks termed the Burrell Creek Formation, which is dominated by greywacke, siltstone, sandstone and shale that exhibit sedimentary features akin to those of a river-dominant delta front to prodelta environment. The formation is conformably overlain by volcanoclastic and volcanolithic sedimentary rocks of the Tollis Formation (∼1890 Ma).Three deformation events are recognised in the Mt. Todd region, D1, D2 and D3. The earliest deformation, D1, is characterised by close to tight, NE to N to NW-trending asymmetric folds (F1), and a continuous axial–planar cleavage (S1). The deformation is associated with the development of conjugate buck–quartz veins and was preceded by the emplacement of the Yenberrie Leucogranite, which produced contact metamorphism of the sedimentary rocks of the Burrell Creek Formation to hornblende–hornfels facies (H1), with the development of cordierite porphyroblasts (type C1). D1 was coincident with peak regional metamorphism to greenschist facies.D2 is associated with westerly trending open folds (F2), and a spaced disjunctive to fracture cleavage (S2) in transection to the folds. It was preceded by the emplacement of the Tennysons Leucogranite of the Cullen Batholith (1835–1820 Ma), which produced contact metamorphism of the Yenberrie Leucogranite and the sedimentary rocks of the Burrell Creek and Tollis formations to hornblende–hornfels facies (H2), with the development of cordierite porphyroblasts (type C2).D3 is characterised by the reactivation of strike-slip faults (mostly sinistral), a steeply dipping Type S3-C type foliation, and mesoscopic en échelon folds (F3) that trend oblique to the faults in a left stepping (sinistral) array.The age of emplacement of the Tennysons Leucogranite, and the timing of D1 and D2 are broadly constrained by the age of emplacement of the Cullen Batholith at 1835–1800 Ma. D1 and D2 are correlated with deformation during the Maud Creek Event (∼1850 Ma), while D3 is correlated with deformation during the Shoobridge Event (∼1780 Ma). The age of the Yenberrie Leucogranite is constrained to the age of emplacement of granite batholiths at 1870–1860 Ma.A temporal and broad structural relationship exists between D2 structures, the Tennysons Leucogranite, and the several gold-bearing quartz–sulphide veins and lode systems of the Mt. Todd Mine. The systems appear to have formed after peak thermal metamorphism associated with the emplacement of the pluton at about 1825 Ma, and early in D2, prior to the development of the regional S2 fabric. W–Mo–Sn–Bi–Cu greisen-type mineralisation in the carapace of the Yenberrie Leucogranite of the Yenberrie Wolfram Field constitutes a discrete mineralising event that preceded the emplacement of the Tennysons Leucogranite.     

Wallrock alteration at the western system of the CSA mine,cobar, New South Wales,Australia

Petrological and geochemical characteristics of the sequence from chloritic siltstone through “siliceous siltstone” to “elvan”, which occurs approaching ore on the hanging wall of the Western System in the CSA Mine, Cobar, indicate its formation by extensive dilational silicification together with minor addition of S and K very close to the ore. The manner whereby more chloritic members of this sequence developed from ‘background’ muscovite-rich siltstones remote from mineralization is less clear. “Elvan” is not an exhalative chert. The distribution of wallrock alteration products and characteristics of the ores do not support suggestions that Cobar orebodies are remobilized stratiform deposits. Alteration preceded epigenetic ore emplacement in the early stages of a regional metamorphic episode which took place soon after Early Devonian turbidite sedimentation without associated igneous activity. Mineralization is attributed to diagenetic dewatering or metamorphic dehydration of the sediment pile.     

Structural geology of the Dugald River ZnPbAg deposit, Mount Isa Inlier, Australia

The sediment-hosted ZnPbAg deposit at Dugald River is situated 87 km northeast of Mount Isa, NW Queensland. It is a mid-scale base metal accumulation restricted to a black slate sequence of low metamorphic grade. The orebody is tabular and consists of fine- to medium-grained sulphides with a dominant mineralogy of sphalerite, pyrrhotite, pyrite, galena, quartz and muscovite. Three different ore types have been recognized based on mineralization textures; laminated, banded and brecciated. The present reserve stands at 38 million tons of ore averaging 13.0% Zn, 2.1% Pb and 42 g/t Ag. A structural investigation has revealed that six stages of deformation have affected the metasediments in the Dugald River area. The first four (D₁, D₂, D₃ and D₄) are characterized by the extensive development of folds and associated axial plane cleavage. They were all generated in a ductile regime and are of considerable significance for the structural evolution of this region as well as for the emplacement and localization of the sulphide mineralization. D₅ provides a transition towards brittle deformation developing strong kink folds with subhorizontal axial planes. D₆ was a brittle event, producing E-W-trending open folds and major NE and NW strike-slip faults crosscutting all the pre-existing structural elements plus segmenting the orebody. Correlation between the development of deformation and the formation of mineralization can be observed from macro- to microscales. Relationships of mineralization with folds and cleavage indicate a post-D₂ (dominant deformation event) and probably syn-D₄ deformation timing for the ZnPbAg mineralization at Dugald River, as suggested by the ubiquitous truncations of D₂ fabrics by ore mineral assemblages throughout the deposit.     

A pediment survey at middle pinnacle,near broken hill,New South Wales

Extensive and well‐developed pediments and pediplains in western New South Wales have hitherto received less than due attention in the literature. This paper records the details of instrumental survey and of excavation at The Pinnacles, near Broken Hill. It presents observation and analysis of slope, rock‐type, structure, and surficial cover, and discusses the significance of a peripediment which contains multiple soil profiles.     

Early Mesoproterozoic bimodal plutonism in the southeastern Gawler Craton,South Australia

The Curramulka Gabbronorite on Yorke Peninsula, southeastern Gawler Craton has an emplacement age of 1589 ± 5 Ma. This is similar to previously determined ages for Arthurton Granite (1582 ± 7 Ma), Tickera Granite (ca 1600 – 1575 Ma), regional alteration, the Moonta – Wallaroo mineralisation (ca 1585 Ma) and localised deformation (Tiparra Deformation). Mesoproterozoic bimodal plutonism is interpreted to have resulted from mafic underplating, emplacement of mafic magmas during lithospheric attenuation and enhanced high heat flow assisting in melting of the lower crust to form the broadly A-type Arthurton and Tickera Granites. Plutonism either directly or indirectly created advective fluid-flow to form Cu – Au mineralisation in the Moonta – Wallaroo area. The nature and characteristics of Mesoproterozoic mafic bodies on the Gawler Craton are poorly known. The Curramulka Gabbronorite has a continental tholeiitic composition and igneous layering that is partly of cumulus origin but also contains magmatic segregations formed by fractionation. Some of these segregations have provided zircons for dating. This igneous layering is overprinted by two foliations of tectonic origin: the first is interpreted to be coeval with magma emplacement and the second with conjugate shearing accompanied by retrogression.     

Age of granite emplacement in the Norseman region of Western Australia

Ion probe U‐Th‐Pb dating of zircons from the Late Archaean granites of the Norseman region of the southeastern Yilgarn shows the existence of two distinct magmatic episodes. Large regional tonalite and granodiorite plutons were emplaced between 2685 and 2690 Ma, whereas large regional granite, and small tonalite and leucogranite plutons that intrude the greenstones have ages of 2660–2665 Ma. A small body of granite that intrudes the western edge of the greenstones has an inferred emplacement of 2672 ± 7Ma, and contains inherited zircon that is ～2800 Ma. The monzogranite core from a second pluton in a similar structural position also contains ～2800 Ma zircon; this age is similar to published Sm‐Nd and Rb‐Sr whole rock ages for banded gneisses associated with other members of this suite of domal plutons and is interpreted as representing the age of a significant component within the source region for these distinctive rocks.

Available geochemical and isotopic data are interpreted as indicating derivation of both the older granodiorite and younger granite suites through anatexis of pre‐existing crust of broadly andesitic composition, whereas both the domal granites and the small, late tonalite plutons could have been derived by anatexis of heterogeneous material similar to that represented by the banded gneisses.

If regional metamorphism was related to the emplacement of large volumes of felsic magma within the upper crust, as suggested by Binns et al. (1976), then the Norseman area has probably undergone two periods of regional metamorphism of comparable intensity at approximately 2660 and 2685 Ma.