Geordie Mark

Synthesis of the proterozoic evolution of the Mt Isa Inlier

By virtue of its large area of exposure of different crustal levels, and preservation of a protracted (∼400 million years) Palaeoproterozoic to Mesoproterozoic tectonic evolution, the Mt Isa Inlier is an excellent natural laboratory to study Proterozoic tectonic processes. The inlier preserves evidence of intracontinental basin development, plutonism, low-pressure metamorphism, orogenesis at different crustal levels, and crustal-scale metasomatism. In addition, the Mt Isa Inlier is endowed with a variety of ore deposits, including the Mt Isa Pb – Zn – Ag and Cu deposits, Century Zn – Pb – Ag deposit, Cannington Ag – Pb – Zn deposit, and the Osborne and Ernest Henry iron oxide Cu – Au deposits. Basement rocks were deformed and metamorphosed during the ca 1900 – 1870 Ma Barramundi Orogeny and intruded by the granitic rocks of the ca 1850 Ma Kalkadoon and Ewen Batholiths and their coeval Leichhardt Volcanics. Three stacked and superimposed superbasins evolved between ca 1800 and ca 1595 Ma. These basins evolved in an environment characterised by elevated heat flow and transient episodes of magmatism and basin inversion in an inferred continental backarc setting. The ca 1600 – 1500 Ma Isan Orogeny probably records two phases of orogenesis. The first phase (ca 1600 – 1570 Ma) involved approximately north – south to northwest – southeast shortening in which a northwest-vergent fold-thrust belt evolved in the Eastern Fold Belt and localised basin inversion occurred in the Western Fold Belt. The second phase (ca 1550 – 1500) involved thick-skinned deformation in the Eastern and Western Fold Belts, characterised by upright folding, reverse faulting, and dextral wrenching. Voluminous granites were emplaced throughout the Eastern Fold Belt between ca 1550 and 1500 Ma. Exhumation and cooling of the crustal pile following the Isan Orogeny were related to crustal extension and widespread erosion in eastern and southern Australia. Subtle reactivation of faults within the inlier following the Isan Orogeny records the distal effects of Mesoproterozoic to Neoproterozoic breakup events and orogenesis in central Australia.

Quantitative, high sensitivity, high resolution, nuclear microprobe imaging of fluids, melts and minerals

Samples of fluids and melts trapped and preserved as inclusions in growing minerals or healed fractures provide unique windows on a range of geological processes from mantle melting and metasomatism through to economic ore formation and remobilization. Recent advances in nuclear microprobe (NMP) technology at the CSIRO provide powerful tools for the study of these inclusions and associated mineral assemblages. These tools include a new NMP designed for high resolution and high sensitivity, PIXE analytical methods for quantitative imaging and analysis, and simultaneous PIGE imaging. The quantitative imaging and analysis methods are based on the dynamic analysis approach, which generates a fast matrix transform for projection of list-mode PIXE data onto pure elemental images. Recent advances provide rapid pixel-by-pixel correction for matrix and absorption effects in different (mineral) compositions across the image area to yield true quantitative images. These methods are combined in a software package called GeoPIXE II.

Magma migration, folding, and disaggregation of migmatites in the Karakoram Shear Zone, Ladakh, NW India

Effi cient extraction of granitic magma from crustal sources requires the development of an extensive permeable network of melt-bearing channels during deformation. We investigate rocks that have undergone deformation and melting within the Karakoram Shear Zone of Ladakh, NW India, in which leucosome distribution is inferred to record the permeable network for magma extraction. Delicate structures preserved in these rocks record the development of this permeable magma network and its subsequent destruction to form a mobile mass of melt and solids, resulting from the interplay between folding and magma migration. During folding, magma migrated from rock pores into layer-parallel and axial-planar sheets, forming a stromatic migmatite or metatexite with two communicating sets of sheets, intersecting parallel to the fold axis. Once the network was developed, folding and stretching was eased by magma migration and slip along axial planar magma sheets. Folding and magma migration led to layer disaggregation, transposition, and the formation of a diatexite where rock coherency and banding were destroyed. A number of structures developed during this process such as cuspate fold hinges, disharmonic folds, truncated layering, shear along axial planar leucosomes, and fl ow drag and disruption of melanosomes. In this system, magma migration was an integral part of deformation and assisted the folding and stretching of metatexites, while folding gave rise to a magma sheet network, now preserved as leucosomes, as well as the pressure gradients that drove magma migration and the breakup of the metatexite. Thus, metatexite folding increased melt interconnectivity, while magma mobility increased strain rate and released differential stresses.

Magma ponding in the Karakoram shear zone, Ladakh, NW India

Granitic melt migration and pluton emplacement are commonly closely associated with transcurrent shear zones. The processes that link granites to shear zones are not yet fully understood. The dextraltranspressive Karakoram shear zone in Ladakh, NW India, exposes anatectic rocks where synkinematic melt migration and ponding at kilometer scale were controlled by competency contrasts. Metasedimentary rocks and a dominantly granodioritic calcalkaline intrusion underwent fl uid-present partial melting at upper-amphibolite facies to produce leucogranite sheets and irregular intrusive masses dated at 21–14 Ma. Leucogranitic magmas ponded in the lowpressure strain shadow of the competent grano dioritic calc-alkaline pluton, giving rise to (a) migmatitic rocks that are pervaded by irregular leucogranite intrusions at a scale of meters or tens of meters, and (b) the growth of the Tangtse pluton, a kilometer-scale sheeted complex. Thus, magmas accumulated during shearing and anatexis in a low-pressure strain shadow within the Karakoram shear zone. This magma provided a readily available magma source that could have been tapped to feed larger plutons at shallower levels by modifi cations in the pressure distribution accompanying changes in shear zone geometry and kinematics. We conclude that shear zones tapping anatectic regions act as magma pumps, creating and destroying magma traps at depth as they evolve, and leading to incremental magma addition to upper-crustal plutons.

Magmatic–hydrothermal albite–actinolite–apatite-rich rocks from the Cloncurry district, NW Queensland, Australia

Abstract Albite-, quartz-, actinolite-, apatite-rich rocks with accessory titanite form a carapace that caps a small dome-like intrusion of Roxmere pluton, and provide evidence of the accumulation and release of magmatic fluids that may have contributed to regionally extensive Na–Ca alteration in the Proterozoic Cloncurry district, Australia. The Roxmere pluton was emplaced after the peak amphibolite facies metamorphism, and into psammitic metasedimentary rocks that are stratigraphically younger and overlie the calc-silicate-rich Mary Kathleen Group. Na–Ca hydrothermal assemblages closely spatially related to the Roxmere pluton suggest that Na–Ca alteration, in this case, is inconsistent with excepted up-temperature metamorphic fluid circulation models. The carapace rocks comprise two variants, aplite and pegmatite, both of which were affected by alteration and brecciation. Aplitic material predominates and the carapace has a brecciated top, and a distinct textural zonation, from an aplite-rich, pegmatite-poor base, to a relatively aplite-poor/pegmatite-rich top. This zonation is interpreted to represent the progressive accumulation of volatile phases toward the top of the carapace, which culminated in fluid overpressuring and brecciation of the granitoid stock. The convoluted and ptygmatic habit of the aplitic and pegmatitic material within the carapace is similar in nature to the comb quartz unidirectional solidification textures that are developed in the roof zones of some porphyry stocks. These complex textures are interpreted to have formed from a single phase of fluid saturation. The Roxmere pluton, or a deeper equivalent, has a Na-, Mg- and Ca-rich composition and a clinopyroxene-, amphibole-rich mineralogy that is consistent with the formation of the contained albite-, actinolite-, apatite-rich phases. However, the source of the rocks within the carapace is equivocal given that their contact relationships are obscured. The δ 18 O composition of minerals within the carapace has the following ranges: albite, 7.1–8.7‰; actinolite, 8.0–8.5‰; and quartz, 8.9–11.4‰. Albite and quartz δ 18 O equilibria suggest that these minerals in the pegmatitic and aplitic components formed at temperatures of ∼510°C to 540°C. The calculated δ 18 O composition of a fluid in equilibrium with these minerals (∼6.0‰ to 8.0‰), at temperatures between 450°C and 550°C, is consistent with a magmatic origin. These estimated temperatures and δ 18 O fluid composition are comparable to estimates for regional Na–Ca alteration. In contrast, δD analysis of actinolite (−136 to −150‰) within the carapace demonstrates that these fluids have a significantly lower δD signature compared to Na–Ca alteration assemblages associated with regional alteration (−70‰ to −90‰). The low values in the carapace could either be interpreted as being produced from meteoric fluids or from degassed magmatic fluids, although a meteoric fluid model is deemed improbable as meteoric fluids in the district have a documented δD composition of ca. 0.0‰. As a consequence, an open-system degassed magmatic–hydrothermal fluid origin is considered to be the most plausible explanation for the low isotopic signatures, and is also consistent with the complex textural relationships described within the carapace.

Albitite formation by selective pervasive sodic alteration of tonalite plutons in the Cloncurry district, Queensland

Tonalites along the southern margin of the Mt Angelay igneous complex, Cloncurry district, northwest Queensland, intrude amphibolite facies metasediment and metadolerite and pre‐date the intrusion of post‐1540 Ma potassic granitoids. These tonalites are overprinted by multiple phases of sodic‐rich alteration and preserve a systematic textural variation from tonalite to albitite with increasing intensity of albitic alteration. Albite‐magnetite occurs as grain boundary alteration, microscopic intergranular veinlets, or as cross‐cutting veins. This alteration assemblage is similar to the early sodic alteration observed at many Fe‐(Cu‐Au) deposits in the district and provides a favourable host for later fracturing and brecciation. Mass‐balance calculations indicate that during alteration Na, U, Th (± Zr) were added to altered tonalite, while K, Fe, Si, Ca, Cl, Zn, Rb, Co, Cr, Sr and H2O were released. The fluids that produced early albite and magnetite, and mobilised quartz during alteration, were saline (> 2...

Nd isotope and petrogenetic constraints for the origin of the Mount Angelay igneous complex: implications for the origin of intrusions in the Cloncurry district, NE Australia

Abstract The Mesoproterozoic Mount Angelay igneous complex contained intrusions that were emplaced into amphibolite facies metasedimentary rocks during two periods of ∼1550 and post-1540 Ma magmatism. Sm–Nd isotopic analysis together with mineralogical and chemical considerations suggest that the intrusions were produced from a Paleoproterozoic crustal source with a T 2 model age ∼2200 Ma. On geochemical and petrological grounds, the ∼1550 Ma trondhjemitic intrusions are interpreted to have been produced by melting of amphibolite under garnet-stable conditions (>8–10 kbar). The late-syn to post-peak metamorphic timing of these intrusions suggested that they were associated with the tectono-thermal event that produced regional peak metamorphic mineral assemblages. The post-1540 Ma intrusions are K-rich and consist of two groups of synchronously emplaced intrusions, (1) a high-K monzodiorite and monzogranite suite that range between 51 and 77 wt.% SiO 2 ; and (2) a high-K, Na-enriched hornblende monzonite. The chemistry and mineralogy of these intrusions suggested that they were derived via plagioclase-stable and garnet-unstable melting ( 2 O, P 2 O 5 , Cr, V and Zn compared with slightly younger high-K monzonite, which is interpreted to have formed via one of two mechanisms, (1) melting of a low-K amphibole- and plagioclase-rich source; or (2) melting of residual material that produces a potassic and incompatible element-rich melt. These magmas likely contained mantle-derived material, particularly the K-rich intrusions of mafic composition. The heat required for the production of post-1540 Ma intrusions appears to have been generated by the intrusion of high-T, mantle-derived, mafic material into the crust (∼25–30 km; ∼8–10 kbar). This model is consistent with the synchronous emplacement of mafic and felsic magma and the lack of a consanguineous regional metamorphic association, and suggests high-T, high-degree partial melting in localised pockets within fertile source regions in the crust. An increase in Sm–Nd model source age and decrease in eNd with increasing SiO 2 in the K-rich intrusions suggests the incorporation of juvenile material in the more mafic rocks. The origin of this component is unknown, but it may represent either the incorporation of mantle-derived material during melting, or the partial melting of crust with a younger mafic component. On a district scale, the >30 million year period over which the K-rich post-1540 Ma intrusions were emplaced suggested that mantle-derived material continued to be injected into the crust. A mantle component to these rocks, and the global distribution of Proterozoic intrusions with similar geochemical affinities, strongly suggests a world-wide period of mantle-induced crustal melting at that time. The dominant Paleoproterozoic isotopic composition of most of these intrusions suggests melting of similarly composed and matured source rocks.

Insights into the genesis and diversity of epigenetic Cu – Au mineralisation in the Cloncurry district, Mt Isa Inlier, northwest Queensland

The Proterozoic rocks of the Cloncurry district preserve the effects of some of the worlds largest hydrothermal systems associated with extensive albitisation, brecciation and Na – Ca alteration. These hydrothermal systems are broadly coeval with magmatism, and also host numerous structurally controlled Fe oxide and Cu – Au deposits (ca 1.60 Ga, 1.55 – 1.50 Ga). Fluid-inclusion, stable-isotope, and geochemical data from Cu – Au deposits indicate that the ore-forming fluids were high-T (>300 – 500°C), highly saline (>26 – 70 wt % NaClequiv), typically CO2-bearing, and are mainly considered to be sourced by crystallising intrusions with contributions from other fluid sources and/or host rocks. Fe oxide and Cu – Au mineralisation in the district exhibit a range of interrelationships based upon the metal endowment, relative timing of Fe oxides and sulfides, and Cu:Au ratio. These interrelationships may be divided into four categories: (i) barren magnetite and/or hematite ironstones; (ii) Fe oxide-hosted Cu – Au mineralisation, where relatively Au-rich ore associated with pyrite and hematite overprints older magnetite-rich rocks; (iii) Fe oxide Cu – Au mineralisation, where both Fe oxides and Cu – Au mineralisation are cogenetically deposited; and (iv) Fe oxide-poor Cu – Au mineralisation, where relative Cu-rich mineralisation is associated with pyrrhotite and rare magnetite, and is hosted in relatively reduced rocks such as carbonaceous metasedimentary rocks. These categories reflect variations in fluid redox, f S, aFe, and temperature, as well as host-rock composition. The spectrum from Cu-rich to Au-rich mineralisation is a common phenomenon in Fe oxide – Cu – Au districts and predominantly reflects an increase in the redox of the ore-forming system. The apparent relationship between pH and metal solubility at different redox conditions suggests that Cu – Au mineralisation occurred as a result of decreasing fluid acidity by wall-rock reaction at the site of ore deposition, or potentially by mixing of fluids of different acidity. Fluid mixing provides an effective means to produce high-grade ore deposits via changing pH, cooling, and dilution in hydrothermal systems involving little wall-rock interaction.

Highly selective partial melting of pelitic gneiss at Cannington, Cloncurry district, Queensland

Up to 70% partial melting in some layers of pelitic gneiss at Cannington contrasts with an absence of melting in adjacent pelitic units. Mass‐balance calculations suggest the melted and unmelted units were broadly similar in their initial composition, except for K2O, Al2O3 and SiO2. This difference is interpreted as reflecting more abundant muscovite and less quartz in the intervals that underwent extensive melting. Products of the partial melting are quartzo‐feldspathic veins varying in thickness from a few millimetres to many centimetres. The thinner veinlets tend to be concordant to gneissic layering, whereas the thicker veins are more discordant. This highly selective melting may explain why melt products such as pegmatites might be broadly stratigraphically controlled in parts of some high‐grade terrains.

Fluid mixing versus unmixing as an ore-forming process in theCloncurry Fe-oxide-CuAu District, NW Queensland, Australia: evidence from fluid inclusions

Abstract Fluid mixing and/or unmixing (including boiling) are thought to be important mechanisms of mineralisation in copper-golddeposits. Detailed fluid-inclusion studies of regional sodic (-calcic) alteration and local mineralisation in the Cloncurry Fe-oxide-Cu Au District, NW Queensland, suggest that both fluid mixing and unmixing occurred in these giant mineralised hydrothermal systems. In some cases, the primary character of coexisting multisolid, hypersaline brine inclusions and CO 2 - or vapour-rich inclusions, the latter crosscut by late Ca- and Na-rich fluid inclusions, indicate that fluid mixing probably occurred subsequent to fluid unmixing and finally resulted in Cu Au mineralisation. However, the relationship between hypersaline brines and CO 2 , which was believed to result from an unmixing of a magma-derived H 2 O CO 2 NaCl ± CaCl 2 fluid (see [Miner. Depos. 36 (2001) 93] and references therein), is rather complex as some hypersaline brine inclusions obviously predate CO 2 inclusions.