Tectonic setting and mineralisation potential of the Cowley Ophiolite Complex, north Queensland

Northeast Queensland contains multiple slices of mafic–ultramafic units, strung out along regional faults that mark major tectonic boundaries. One such complex is the Cowley Ophiolite Complex, which is situated along the Russell-Mulgrave Fault. The Cowley Ophiolite Complex is a differentiated mafic–ultramafic complex composed of gabbro, chlorite schist, anthophyllite schist and serpentinite. We interpret that the alteration assemblages observed throughout the Cowley Ophiolite Complex reflect an amphibolite facies metasomatic overprint within a supra-subduction zone setting. This interpretation is consistent with geochemical discrimination of the gabbro, and chromite grains from the anthophyllite schist. The Cowley Ophiolite Complex records a higher metamorphic grade than the surrounding Hodgkinson Formation, and we interpret this to reflect the allochthonous structural setting of the complex, positioned along an ancient subduction margin. This subduction margin is represented today by the Russell-Mulgrave Fault. Metasomatism and emplacement of the complex probably pre-dated deposition of the Mossman Orogen’s active margin successions. Our interpretation of an active subduction complex, which pre-dated the formation of the Mossman Orogen, suggests that the Russell-Mulgrave Fault is a Paleozoic, continental suture zone. The Cowley Ophiolite Complex presents little indication of economic mineralisation; however, much of the complex remains unexplored

A primitive mantle source for the Neoarchean mafic rocks from the Tanzania Craton

Mafic rocks comprising tholeiitic pillow basalt, dolerite and minor gabbro form the basal stratigraphic unit in the ca. 2.8 to 2.6 Ga Geita Greenstone Belt situated in the NW Tanzania Craton. They outcrop mainly along the southern margin of the belt, and are at least 50 million years older than the supracrustal assemblages against which they have been juxtaposed. Geochemical analyses indicate that parts of the assemblage approach high Mg-tholeiite (more than 8 wt.% MgO). This suite of samples has a restricted compositional range suggesting derivation from a chemically homogenous reservoir. Trace element modeling suggests that the mafic rocks were derived by partial melting within the spinel peridotite field from a source rock with a primitive mantle composition. That is, trace elements maintain primitive mantle ratios (Zr/Hf = 32–35, Ti/Zr = 107–147), producing flat REE and HFSE profiles [(La/Yb)pm = 0.9–1.3], with abundances of 3–10 times primitive mantle and with minor negative anomalies of Nb [(Nb/La)pm = 0.6–0.8] and Th [(Th/La)pm = 0.6–0.9]. Initial isotope compositions (ɛNd) range from 1.6 to 2.9 at 2.8 Ga and plot below the depleted mantle line suggesting derivation from a more enriched source compared to present day MORB mantle. The trace element composition and Nd isotopic ratios are similar to the mafic rocks outcropping ∼50 km south. The mafic rocks outcropping in the Geita area were erupted through oceanic crust over a short time period, between ~2830 and ~2820 Ma; are compositionally homogenous, contain little to no associated terrigenous sediments, and their trace element composition and short emplacement time resemble oceanic plateau basalts. They have been interpreted to be derived from a plume head with a primitive mantle composition

Short-wavelength infrared spectroscopy as a tool for characterising hydrothermal alteration at the Geita Hill gold deposit, Tanzania

Geita Hill is a world-class gold deposit located in north-western Tanzania and hosted within an ironstone-dominated sedimentary package, intruded by diorite dykes and sills. The host rocks were metamorphosed to greenschist facies and show a complex deformation history comprising early ductile, and late brittle-ductile events. The regional metamorphic assemblage at the deposit is characterised by Bt + Chl + Act + Kfs ± Phg ± Mt ± Po ± Py. The gold-related alteration overprints the regional metamorphism, and manifests as a series of silicification and sulfidation fronts, and/or microfracture and vein networks. Gold is closely associated with secondary pyrite, and occurs as free-Au and gold tellurides. The mineralized vein/microfracture networks contain Bt and Kfs as primary accessory minerals. The mineralising alteration is overprinted by barren, multiphase quartz-carbonate and carbonate-chlorite veins, characterised by the assemblage Ca + Sd + Chl ± Qtz ± Py ± Ba. The close association between gold and biotite in the mineralized vein/microfracture networks and the scarcity of retrograde chlorite makes the Geita Hill deposit ideal to test the change of the biotite short-wave infrared (SWIR) spectral response with the proximity to the gold alteration. SWIR spectra were collected from three well-characterised drill holes that intercepted the gold mineralization and the results were compared to the gold grades. The SWIR data shows that there is a good correlation between the biotite spectral response and the gold grades. The position of the 2250 nm biotite absorption feature is changing systematically as a function of the ore proximity indicating that SWIR can be used to trace gold mineralization and has the potential to be a powerful exploration tool if used in conjunction with well characterised mineral paragenesis

A magmatic copper and fluid source for the sediment-hosted Mount Isa deposit

The world class Mount Isa deposit is a unique, sediment-hosted, copper deposit with no known equivalent around the world and a controversial origin. We report δ65Cu values (n = 90) from chalcopyrite grains collected systematically across the entire deposit. The δ65Cu shows a unimodal distribution with limited variability (min = −0.87 ‰; max = 0.88 ‰) and an average value (+0.13 ‰) comparable to average igneous rocks. In general, the δ65Cu values in chalcopyrite are lower near major structures and heavier further away, consistent with equilibrium fractionation with distance from the fluid source. The range in δ65Cu of chalcopyrite from the Mount Isa deposit is less variable compared to sedimentary copper, VMS and porphyry/epithermal deposits, but similar to Michigan deposits; meanwhile, average δ65Cu at Mt. Isa is distinctly higher than sedimentary copper deposits, but similar to VMS, porphyry/epithermal and Michigan deposits. These data suggest that, from a copper isotope perspective, the Mount Isa deposit is clearly different from sedimentary copper deposits and more like VMS, porphyry copper/epithermal and Michigan style deposits. The average δ65Cu (+0.13 ‰) is almost identical to the average δ65Cu (+0.14 ‰) from Proterozoic basalts and suggests that copper was sourced from the underlying mafic rocks; the limited fractionation and the normal distribution of the δ65Cu indicate a very effective leaching mechanism and transport by a hot fluid from which chalcopyrite precipitated without significant fractionation of copper isotopes

The petrogenesis of the Neoarchean Kukuluma Intrusive Complex, NW Tanzania

The Kukuluma Intrusive Complex (KIC) is a late Archean igneous complex, dominated by monzonite and diorite with subordinated granodiorite. The monzonite and the diorite suites have low silica content (SiO2 ≤ 62 wt%), moderate Mg# (Mg#average = 49), high Sr/Y (Sr/Yaverage = 79) and high La/Yb (La/Ybaverage = 56) ratios, and strongly fractionated (Lan/Ybn = 9–69) REE patterns. Their moderate Ni (Niaverage = 50 ppm), Cr (Craverage = 85 ppm), variable Cr/Ni ratio (0.65–3.56) and low TiO2 (TiO2average = 0.5 wt%) indicate little to no interaction with the peridotitic mantle. For most major elements (Al2O3, FeOt, Na2O, TiO2 and P2O5) the monzonite and the diorite suites display subparallel trends for the same SiO2 content indicating they represent distinct melts. Intrusions belonging to the diorite suite have high Na2O (Na2Oaverage = 4.2 wt%), Dy/Ybn (Dy/Ybn-average = 1.6), a positive Sr anomaly and uncorrelated Nb/La and Zr/Sm ratios suggesting derivation from partial melting of garnet-bearing amphibolite. Intrusions belonging to the monzonite suite have higher Na2O (Na2Oaverage = 5.61 wt%), Dy/Ybn (Dy/Ybn-average = 2.21), a negative Sr anomaly and correlated Nb/La and Zr/Sm ratios consistent with derivation from partial melting of eclogite with residual rutile. Small variations in the Th/U ratio and near chondritic/MORB average values (Th/Umonzonite = 3.65; Th/Udiorite = 2.92) are inconsistent with a subducting slab signature, and it is proposed that the monzonite and the diorite suites of the KIC formed by partial melting of garnet-bearing, lower mafic crust of an oceanic plateau. The granodiorite suite has lower Mg# (Mg#average = 41), moderately fractionated REE, low Sr/Y (Sr/Yaverage = 20), La/Yb (La/Ybaverage = 15), Dy/Ybn (DyYbn-average = 1.24) and small negative Eu anomalies suggesting derivation from partial melting of amphibolite and plagioclase fractionation. Near-MORB Th/U (Th/Uaverage = 2.92) and Zr/Sm (Zr/Smaverage = 30.21) ratios are consistent with intracrustal melting of amphibolite. Archean rocks with an “adakitic” geochemical signature have been used to argue in favour of a plate tectonics regime in the Archean. The results presented here suggest that tectonic models for the Tanzania Craton, which invoke a subduction-related setting for all greenstone belts may need revision

Connecting deformation, metamorphism and granite emplacement:\ud an example from the contact aureole of the Mooselookmeguntic pluton

The rocks in the contact aureole of the Mooselookmeguntic pluton were multiply deformed and metamorphosed during the Acadian orogeny. The metamorphism occurred at low pressure and high temperature with isograds following the pluton\ud contours. A maximum temperature of ~ 600°C at around 4 Kbrs has been determined from rocks from the lower sillimanite zone. The widespread assemblage Grt+St+And suggests that the pressure must have been in the 3-4 Kbr interval while the temperature varied with the pluton proximity. The\ud sporadic appearance of cordierite porphyroblasts towards the SE indicates slight decompression and possibly the beginning of exhumation. Most porphyroblasts have been variably prograde or retrograde pseudomorphed. The nature of\ud pseudomorphism occurred as a function of pluton proximity while the distribution of pseudomorphs was strictly controlled by the structural fabric. Pseudomorphs have formed only in rocks with steeply dipping foliation trending at high angle to the pluton. Measurement of the foliation inflexion/intersection axes (FIAs) in garnet and staurolite porphyroblasts has revealed that at least five\ud successive periods of growth of these phases have occurred. A succession of 5 different FIA sets for garnet and staurolite porphyroblasts have been interpreted using samples that preserve changes in FIAs trend from core to rim and trend successively NNW-SSE, ESE-WNW, E-W, ENE-WSW and NESW. Staurolite cores with NW–SE trending FIAs are\ud succeeded by N-S, E-W and ENE-WSW rims. N-S trending FIA cores and pre-FIAs are succeeded by NE-SW rims. E-W trending FIAs cores are succeeded by ENE-WSW medians/rims and NE-SW rims. ENE-WSW trending cores are succeeded by NE-SW trending rims. The inclusion trail pitches have been\ud measured in 3D from all staurolite porphyroblasts\ud using vertically oriented thin sections. Most staurolite\ud porphyroblasts overgrew steeply dipping foliations\ud that have similar trends to the FIAs. Monazite dating of staurolite porphyroblasts from each FIA set revealed a continuous period of growth of this phase from ~ 420 Ma to less than 350 Ma. Individual age peaks have been determined at 408±10 Ma for FIA set 2, at 388.0±8.8 Ma for FIA set 3, at 372.1±5.5 Ma for FIA set 4 and at 352.7±4.2 Ma for FIA set 5. Although, the age of FIA set 1 could not be measured\ud it is assumed to have formed between 430 and 415 my. The peaks for each FIA set correlate very well with regional pluton ages suggesting that there must have been a direct connection between deformation, metamorphism and pluton emplacement

The problem, significance and implications for metamorphism of 60 million years of multiple phases of staurolite growth

Microstructural measurements of FIAs in staurolite reveal at least 3 periods of growth in the Proterozoic Colorado Front Range and 5 in the Paleozoic Western Maine. Dated monazite inclusions in staurolite have an absolute age of 1760±12 Ma (FIA 1), 1720±7 Ma (FIA 2), 1682±18 Ma (FIA 3) in Colorado, and 408±10 Ma (FIA 2), 388±8 Ma (FIA 3), 372±6 Ma (FIA 4), 352±4 Ma (FIA 5) in Maine, supporting the multiple periods of deformation and metamorphism indicated by the FIA succession in each region. Multiple phases of growth by similar reactions in the same as well as in diverse adjacent rocks in both regions suggest that PT and X are not the only factors controlling the commencement and cessation of metamorphic reactions. The FIAs preserved by the staurolite porphyroblasts indicate that the local partitioning of deformation at the scale of a porphyroblast was the eventual controlling factor on whether or not the staurolite forming reactions took place

A deformation partitioning approach to resolving the sequence of fold events and the orientations in which they formed across multiply deformed large-scale regions

Regional distributions of axial plane trends retain information on the orientation in which successive generations formed because multi-scale partitioning results in most orogenic belts preserving subsequently undeformed portions of all large-scale folds. At depths greater than not, vert, similar10 km within orogens, successions of regional folds are accompanied by the sequential development of crenulation hinges in pelites, which are commonly overgrown early during their development by successive generations of porphyroblasts. Consequently, the original trends of the axial planes of these folds are preserved within the distribution of foliation inflection/intersection axes within porphyroblasts (FIAs). Peaks in the distribution of FIA trends in western Maine predominantly coincide with peaks in the distribution of trends of the axial planes of macroscopic and regional folds. The WNW–ESE (not, vert, similar420 Ma), N–S (408 ± 10 Ma), W–E (388 ± 9 Ma), WSW–ENE (372 ± 5 Ma), SW–NE (353 ± 4 Ma) succession of FIA peaks defines the sequence of folds and accords with map scale overprinting relationships. This quantitative approach to interpreting fold successions in multiply deformed terrains resolves timing where overprinting criteria are rare, uncertain or obliterated by younger events in portions of the orogen. Significantly, lengthy detailed histories of structural development can be extracted from a small area containing porphyroblastic rocks and applied to very large-scale regions. Highlights • Zones of porphyroblastic rock preserve the effects of the numerous regional deformation events. • FIAs are a product of folding events and their trends are controlled by folds with steep axial planes. • FIA trends directly reflect folds that formed with steeply dipping axial planes. • Regional fold trends have peaks coincident or within 10° of a succession of FIA peaks. • FIAs provide the regional successions of folding preserving the orientations in which they formed

Preservation of deep Himalayan PT conditions that formed during multiple events in garnet cores: mylonitization produces erroneous results for rims

The Kathmandu Thrust Sheet, which overlies the Lesser Himalayas along the southern part of the Main Central Thrust (MCT) and forms the leading edge of the Higher Himalayan crystalline rocks, is folded at a regional scale by the Gorkha-Kathmandu fold couplet in Central Nepal. Garnet porphyroblasts lying close to the MCT within this thrust sheet preserve structural and metamorphic history that predates mylonitization during thrust emplacement. The succession of five FIA sets preserved within these porphyroblasts formed due to changes in the direction of India's motion relative to Asia after they collided. The intersection of Fe, Ca and Mn isopleths for garnet cores reveals that FIA sets 1, 2, 3, 4 and 5 nucleated respectively at 6.2 kbar and 515 °C, 6–7 kbar and 545–550 °C, 6.6 kbar and 530 °C, 5.6–6.2 kbar and 525–550 °C and 6.8–6.9 kbar and 520–560 °C. The average PT mode of thermocalc, which relies on equilibrium being achieved between the garnet rims and the matrix, gives pressures around 11 kbar that do not accord with the lengthy succession of lower core pressures. The many foliations in the matrix, which formed during top to the south thrusting plus subsequent deformations that eventually led to these rocks reaching the surface, truncate all foliations preserved within the porphyroblasts that are defined by inclusion trails. This has resulted in the garnet rims not being in equilibrium with the matrix and the anomalously high pressures. The garnet rims may have been affected by slow dissolution and solution transfer over the period of time that the matrix was deforming plastically at high strain rates as the rocks were uplifted. The assumption of equilibrium between garnet rims and surrounding silicates used by various rim geothermobarometric methods does not hold for these rocks

Polymetamorphism accompanied switching in horizontal shortening during Isan Orogeny: example from the Eastern Fold Belt, Mount Isa Inlier, Australia

Mesoproterozoic low-P/high-T volcano-sedimentary rocks of the Soldiers Cap Group, the southeastern corner of Mount Isa Inlier, record a complex polymetamorphic history that accompanied four periods of bulk horizontal shortening directed NE–SW, N–S, W–E and NW–SE during the long-lived Isan Orogeny (~ 1650–1500 Ma). Low-P/high-T metamorphism (M₁) prevailed during a period of NE–SW bulk horizontal shortening as indicated by the early growth of cordierite porphyroblasts, which entrap monazite grains that gave an average age of 1649 ± 12 Ma. This was followed by medium-P/high-T (M₂) and high-P/high-T (M₃) metamorphisms that accompanied periods of N–S and W–E bulk horizontal shortening, respectively. Growth of first generation garnet, andalusite and staurolite porphyroblasts having an average age of 1645 ± 7 Ma identify the former, whereas growth of the 1591 ± 10 Ma second generation garnet, staurolite and andalusite porphyroblasts together with fewer kyanite porphyroblasts characterizes the latter.\ud \ud The sediments of the Soldiers Cap Group were deposited in a tectonic setting that has the characteristics of an intra-continental rift basin. The upper limit of the age of sedimentation is constrained by detrital zircons at 1654 ± 4 Ma suggesting that tectonism and metamorphism were either active during the final stage of the basin filling or immediate after deposition.\ud \ud Introduction of mafic dykes and sills at different stratigraphic levels over a wide time span was the major source of heat