Geology,mineralogy, and geochemistry of magnetite-associated Au mineralization of the ultramafic–basalt greenstone hosted Crusader Complex,Agnew Gold Camp,Eastern Yilgarn Craton,Western Australia; a Late Archean intrusion-related Au deposit?

The Crusader Complex, part of the Agnew gold camp of the Lawlers Anticline of the Yilgarn Craton, Western Australia, is located close to or along the contact between the Lawlers Basalt and Agnew Ultramafics units. Au mineralization within the four orebodies that form the Crusader Complex is dominated by very pure Au, containing less than 1 wt.% Ag, with Au variably associated with scheelite, Bi-tellurides and minor chalcopyrite within a magnetite and titanite gangue assemblage. Hydrothermal alteration associated with this style of mineralization is characterized by increasing concentrations of Mo, Be, Li, Sn and Fe and depletions in Na, Cu, Ba, Pb, Mn, Zn, Si, and K relative to protolith concentrations; these enrichments are more typical in orebodies associated with felsic intrusive-related mineralizing systems rather than the more well-known orogenic Au deposits found elsewhere within the Lawlers Anticline (e.g. at Waroonga) and within the greater Yilgarn Craton.We propose that flexures of the contact between the Lawlers Basalt and Agnew Ultramafic units acted as conduits for Au-bearing felsic intrusive-derived fluids and generated structural traps that enhanced fluid flow. The mineralizing fluids that formed the Crusader deposits were derived from the Lawlers granitoid pluton that intruded into the study area. Enhanced fluid flow promoted interaction between hydrothermal fluids and the reactive mafic–ultramafic rock sequence, augmenting the amount of Au that was precipitated during formation of the orebodies at Crusader. The magnetite-dominated quartz- and sulfide-poor intrusion-related mineralization at Crusader contrasts sharply with other late Archean intrusion-related deposits of the Yilgarn Craton that are usually sulfide- and/or quartz-rich. This may in turn suggest that the Crusader deposit represents a new class of under-explored intrusion-derived deposits, possibly opening new mineral exploration opportunities for the Agnew region, and potentially the wider Eastern Goldfields Superterrane. Enrichments in Mo and Sn and significant depletions in Cu suggest that other parts of the Lawlers batholith may also be prospective for base metal mineralization.Integration of stratigraphic interpretation with the identification of key structural fluid pathways and the presence of felsic intrusive bodies, as presented in this study, enables the delineation of the key elements that underlie mineralization at the Crusader Complex. We propose that these key elements provide vital information for future gold exploration models that can be used within other Archean terranes and within the Eastern Yilgarn Craton in particular.     

Towards a volcanic–structural balance: relative importance of volcanism, folding, and remobilisation of nickel sulphides at the Perseverance Ni–Cu–(PGE) deposit, Western Australia

Perseverance is a world-class, komatiite-hosted nickel sulphide deposit situated in the well-endowed Leinster nickel camp of the Agnew–Wiluna greenstone belt, Western Australia. The mine stratigraphy at Perseverance trends north-northwest (NNW), dips steeply to the west, and is overturned. Stratigraphic footwall units lie along the western margin of the Perseverance Ultramafic Complex (PUC). The PUC comprises a basal nickel sulphide-bearing orthocumulate- to mesocumulate-textured komatiite that is overlain by a thicker, nickel sulphide-poor, dunite lens. Hanging wall rocks include rhyodacite that is texturally and compositionally similar to footwall volcanic rocks. These rocks separate the PUC from a second sequence of nickeliferous, E-facing, spinifex-textured komatiite units (i.e. the East Perseverance komatiite). Past workers argue for a conformable stratigraphic contact between the PUC and the East Perseverance komatiite and conclude that the PUC is extrusive. This study, however, clearly demonstrates that these komatiite sequences are discordant, implying that the PUC may have intruded rhyodacite country rock as a sill with subsequent structural juxtaposition against the East Perseverance komatiite. Early N–S shortening associated with the regional D_I deformation event (corresponding to the local D_P1 to D_P3 events at Perseverance) resulted in the heterogeneous partitioning of strain along the margins of the competent dunite. A mylonite developed in the more ductile footwall rocks along the footwall margin of the PUC, while isoclinal F₃ folds, such as the Hanging wall limb and Felsic Nose folds, formed in low-mean stress domains along the fringes of the elongated dunite lens. Strata-bound massive and disseminated nickel sulphides were passively fold thickened in hinge areas of isoclinal folds, whereas basal massive sulphides lubricated fold limbs and promoted thrust movement along shallowly dipping lithological contacts. Massive sulphides were physically remobilised up to 20 m from their primary footwall position into deposit-scale fold hinges to form the 1A and Felsic Nose orebodies. First-order controls on the geometry of the Perseverance deposit include the thermomechanical erosion of footwall rocks and the channelling of the mineralised komatiitic magma. Second- or third-order controls are several postvolcanic deformation events, which resulted in the progressive folding and shearing of the footwall contact, as well as the passive fold thickening of massive and disseminated sulphide orebodies. Massive sulphides were physically remobilised into multiple generations of fold hinges and shear zones. Important implications for near-mine exploration in the Leinster camp include identifying nickeliferous komatiite units, defining their three-dimensional geometry, and targeting fold hinge areas. Fold plunge directions and stretching lineations are indicators of potential plunge directions of massive sulphide orebodies.     

Geochemistry of mafic–ultramafic magmatism in the Western Ghats belt (Kudremukh greenstone belt), western Dharwar Craton,India: implications for mantle sources and geodynamic setting

Western Ghats Belt of western Dharwar Craton is dominated by metavolcanic rocks (komatiites, high-magnesium basalts (HMBs), basalts, boninites) with occasional metagabbros. This rock-suite has undergone post-magmatic alteration processes corresponding to greenschist- to lower-amphibolite facies conditions. Komatiites are Al-depleted, characterized by lower Al₂O₃/TiO₂ and high CaO/Al₂O₃. Their trace element distribution patterns suggest most of the primary geochemical compositions are preserved with minor influence of post-magmatic alteration processes and negligible crustal contamination. Chemical characteristics of Al-depleted komatiites imply their derivation from deeper upper mantle with/without garnet involvement. HMBs and basalts are differentiated based on their magnesium content. Basalts and occasionally associated gabbroic sills have similar geochemical characteristics. HMB are characterized by light rare earth element (LREE) enrichment, with significant Nb–Ta and Zr negative anomalies. Basalts and associated gabbros display tholeiitic affinity, with LREE-enriched to slightly fractionated heavy rare earth element (HREE) patterns. Boninites are distinctive in conjunction of low abundances of incompatible elements with respect to the studied komatiites. Chondrite-normalized REE patterns of boninites show relative enrichment in LREE and HREE with respect to MREE. Prominent island arc signatures are evident in HMB, basalts, boninites, and gabbros in terms of their Nb–Ta and Zr–Hf negative anomalies, LREE enrichment and HFSE depletion. It is suggested that these HMB–basalts (associated gabbros)–boninites are the products of arc magmatism. Their REE chemistry attests to a gradual transition in melting depth varying between spinel and garnet stability field in an arc regime. The close spatial association but contrasting elemental characteristics of komatiites and HMB–basalts–boninites can be explained by a plume-arc model, in which the ~3.0 Ga komatiites are considered to be the products of plume volcanism in an oceanic setting, while the HMB, basalts, boninites, and associated gabbros were emplaced in a continental margin setting around 2.8–2.7 Ga.     

Zirconolite and xenotime U–Pb age constraints on the emplacement of the Golden Mile Dolerite sill and gold mineralization at the Mt Charlotte mine,Eastern Goldfields Province,Yilgarn Craton,Western Australia

In situ SHRIMP U–Pb dating of magmatic zirconolite (CaZrTi₂O₇) in the Golden Mile Dolerite from the Mt Charlotte gold deposit (Yilgarn Craton, Australia) has yielded the first robust emplacement age (2,680 ± 9 Ma) for the principle host-rock of gold mineralization in the Kalgoorlie district. In contrast, co-magmatic zircon gave ages from ~2.68 Ga to ~2.17 Ga, reflecting isotopic resetting of high-U and -Th crystals. In situ SHRIMP analysis of hydrothermal xenotime (YPO₄), which co-exists with gold in alteration pyrite, provided a Pb/Pb isochron age of 2,655 ± 13 Ma. This date indicates that the youngest deposit in the Kalgoorlie district (Mt Charlotte) formed at ~2.65 Ga, and provides a new minimum age for the structurally older Golden Mile deposit. Our results indicate that gold mineralization at Mt Charlotte is ~50 million years older than indicated by recent ⁴⁰Ar/³⁹Ar dating and places new constraints on the timing of late-stage regional faulting (D₄) in the province.     

Oxidized and reduced mineral assemblages in greenstone belt rocks of the St. Ives gold camp, Western Australia: vectors to high-grade ore bodies in Archaean gold deposits?

Hydrothermal sulfide–oxide–gold mineral assemblages in gold deposits in the Archaean St. Ives gold camp in Western Australia indicate extremely variable redox conditions during hydrothermal alteration and gold mineralization in space and time. Reduced alteration assemblages (pyrrhotite–pyrite) occur in deposits in the southwest of the camp (e.g., Argo, Junction deposits) and moderately to strongly oxidized assemblages (magnetite–pyrite, hematite–pyrite) occur in deposits in the Central Corridor in the northeast (e.g., North Orchin, Revenge deposits). Reduced mineral assemblages flank the Central Corridor of oxidized deposits and, locally, cut across it along E–W trending faults. Oxidized mineral assemblages in the Central Corridor are focused on gravity lows which are interpreted to reflect abundant felsic porphyritic intrusions at about 1,000 m below present surface. Hydrothermal magnetite predates and is synchronous with early phases of gold-associated albite–carbonate–pyrite–biotite–chlorite hydrothermal alteration. Later-stage, gold-associated pyrite is in equilibrium with hematite. The spatial distribution and temporal sequence of iron sulfides and oxides with gold indicate the presence of at least two spatially restricted but broadly synchronous hydrothermal fluids with contrasting redox states. Sulfur isotope constraints support the argument that the different mineral assemblages reflect differences in redox conditions. The δ ³⁴S values for pyrite for the St. Ives gold camp range between −8.4‰ and +5.1‰ with the negative values occurring in oxidized magnetite-rich domains and slightly negative or positive values occurring in reduced, pyrrhotitic domains. Preliminary spatial and paragenetic analysis of the distribution of iron sulfides and oxides in the St. Ives camp suggests that gold grades are highest where the redox state of the hydrothermal alteration assemblages switches from relatively reduced pyrrhotite–pyrite to relatively oxidized magnetite–pyrite and hematite–pyrite both in space and time. Gold deposition is inferred to have occurred where fluids of contrasting redox state mixed.     

Hydrothermal H?gbomite Associated with Vanadiferous-Titaniferous (V-Ti) Bearing Magnetite Bands in Bhakatarhalli Chromite Mine, Nuggihalli Greenstone Belt, Western Dharwar Craton, Karnataka, India

Hogbomite,a rare exotic mineral,is found to be associated with the vanadiferous-titaniferous （V-Ti） bearing magnetite bands at Bhakatarhalli,Nuggihaifi greenstone belt,western Dharwar Craton,India.We report on a second occurrence of hogbomite from the Dharwar craton in Karnataka,which is the sixth documented occurrence of this mineral from India.We evaluate the chemical characteristics of hogbomite and associated Fe-Ti-minerals in an attempt to identify its formation as a primary hydrothermal mineral in a metamorphosed magnetite layer.We report here the presence of hogbomite as a complex oxide of Fe,Mg,Al and Ti with accessory of Zn,V and Sn.Petrographic studies suggest the （V-Ti） bearing magnetite （Mt） contain spinel,hogbomite,chlorite,martite,ilmenite （Ⅱ） and minor amounts of diaspore.The hogbomite displays euhedral to subhedral textures,and is up to 250 μm along the grain boundaries of magnetite and ilmenite.In the samples studied,hogbomite is prismatic,irregular and elongated in shape.The genesis of hogbomite in veins between magnetite and ilmenite implies its precipitation from fluids without involving complicated reactions.Several models were proposed for the formation of hogbomite; however,the subject is still debatable.     

Fred’s Flow (Canada) and Murphy Well (Australia): thick komatiitic lava flows with contrasting compositions,emplacement mechanisms and water contents

Two Archaean komatiitic flows, Fred’s Flow in Canada and the Murphy Well Flow in Australia, have similar thicknesses (120 and 160 m) but very different compositions and internal structures. Their contrasting differentiation profiles are keys to determine the cooling and crystallization mechanisms that operated during the eruption of Archaean ultramafic lavas. Fred’s Flow is the type example of a thick komatiitic basalt flow. It is strongly differentiated and consists of a succession of layers with contrasting textures and compositions. The layering is readily explained by the accumulation of olivine and pyroxene in a lower cumulate layer and by evolution of the liquid composition during downward growth of spinifex-textured rocks within the upper crust. The magmas that erupted to form Fred’s Flow had variable compositions, ranging from 12 to 20 wt% MgO, and phenocryst contents from 0 to 20 vol%. The flow was emplaced by two pulses. A first ~20-m-thick pulse was followed by another more voluminous but less magnesian pulse that inflated the flow to its present 120 m thickness. Following the second pulse, the flow crystallized in a closed system and differentiated into cumulates containing 30–38 wt% MgO and a residual gabbroic layer with only 6 wt% MgO. The Murphy Well Flow, in contrast, has a remarkably uniform composition throughout. It comprises a 20-m-thick upper layer of fine-grained dendritic olivine and 2–5 vol% amygdales, a 110–120 m intermediate layer of olivine porphyry and a 20–30 m basal layer of olivine orthocumulate. Throughout the flow, MgO contents vary little, from only 30 to 33 wt%, except for the slightly more magnesian basal layer (38–40 wt%). The uniform composition of the flow and dendritic olivine habits in the upper 20 m point to rapid cooling of a highly magnesian liquid with a composition like that of the bulk of the flow. Under equilibrium conditions, this liquid should have crystallized olivine with the composition Fo94.9, but the most magnesian composition measured by electron microprobe in samples from the flow is Fo92.9. To explain these features, we propose that the parental liquid contained around 32 wt% MgO and 3 wt% H₂O. This liquid degassed during the eruption, creating a supercooled liquid that solidified quickly and crystallized olivine with non-equilibrium textures and compositions.     

Geology, mineralogy, and chemistry of the M’zila bentonitic clay deposit (Mostaganem, NW Algeria)

Bentonitic clay deposit of M’zila locates in the North border of the upstream portion of the Chélif basin “Bas Chélif” down to 35 km NE of Mostaganem. It consists of 14 layers (most recent) 1 to 14 (oldest) forming the North and South flanks of an E–W Syncline. These layers are enclosed in the median blue marls formation of the Upper Miocene of the “Bas Chélif” basin lying unconformably on the ante thrust sheet sandstone formation. This median member contains metric cinerites thick continuous levels and could beings originally bentonitic clay region of M’zila. The cinerites are the product of explosive type in the volcanic eruption, usually deposited in the marine environment. They are formed by a more or less rich feldspar and biotite volcanic glass dust. Bentonitic clay layers have average thicknesses vary between 2 and 6 m. They are distributed by centimeter to decimeter levels of soft sandstone, sandy, and rich black mica. Sandstone is greywacke clay and carbonated cemented, matrix consisting of quartz, feldspars, biotite as major minerals, zircon, and glauconite as accessory minerals. They deposited in a marine, reduced, and confined environment. Mineralogical composition of bentonitic clay region of M’zila is as follows: (1) Clay minerals are represented by the montmorillonite with a small amount of kaolinite. (2) Non-clay minerals are composed mainly of quartz, albite, biotite, calcite, heulandite, chlorite, dolomite, and gypsum. Infrared spectroscopy shows that bentonitic clay has highlighted the presence of the disorderly a high temperature silica tridymite. Geochemistry of the major elements of M’zila bentonitic clay shows high a percentage of SiO₂ and Al₂O₃. Chemical analysis shows that the 10, 12, 14 B 7, 4 A, and 4 clay layers are ferric clay; 11 and 3 layers are magnesian clay. Particle size analysis by sedimentation shows bentonitic clays are accrued mainly clay fractions to silt. Bentonitic clay deposit of M’zila has probably volcanic origin and deposited in a marine, reduced, and confined environment.     

Apparent conflicting Roadian–Wordian (middle Permian) CA-IDTIMS and palynology ages from the Canning Basin,Western Australia

U–Pb dating of zircons from thin middle Permian tuffs in the Canning Basin of Western Australia by chemical abrasion-isotope dilution thermal ionisation mass spectrometry reveals a conflict with the established spore-pollen zonation. Normally, the first appearance datum of Dulhuntyispora granulata across the continent lies stratigraphically above assemblages assigned to the Microbaculispora villosa Zone. However, the youngest tuffs within non-marine facies from the M. villosa Zone in Pittston SD-1, drilled in the southwest of the Canning Basin, yielded an age of 267.04 ± 0.14 Ma, which is 1.7 million years younger than tuffs associated with the D. granulata Zone in marginal-marine facies from core holes 350–400 km to the northeast. The apparent conflict in ages is possibly due to the non-marine depositional environment having wielded a strong local influence on the palynoflora along the edge of this basin. Although the present information indicates an age 2.5 million years younger than the 266.6 Ma age previously suggested for the top of the M. villosa Zone, revisions to the ages of Roadian–Wordian spore-pollen zones are not considered justifiable without further supporting evidence. Furthermore, considerable care is needed when comparing palynological assemblages from significantly differing facies. Two basaltic sills (43.5 m and 20 m thick) immediately below the tuffaceous beds in Pittston SD-1 are coincidental, as Ar–Ar dating indicates a Late Triassic age for the intrusions.     

Zircon U–Pb and Lu–Hf systematics of the major terranes of the Western Superior Craton,Canada: Mantle-crust interaction and mechanism(s) of craton formation

Paleo-to Neoarchean granitoid gneisses (ca. 3.30 to 2.49 Ga) are well preserved in the Western Superior Craton. Protoliths of these gneisses are mainly I-type granitoids characterized by high Sr/Y and La/Yb ratios and low Mg#, consistent with Archean tonalite-trondhjemite-granodiorites. Zircons from granitoid gneisses commonly contain three growth phases: inherited cores (zircon I), magmatic rims (zircon II) and outer rims that have undergone Pb-loss (zircon III). The 3.12 Ga to 2.86 Ga zircon I represent early crustal material, that was captured in younger zircons; zircon II preserve crustal re-working and younger crustal additions that are constrained between 2.85 to 2.72 and 2.69 to 2.65 Ga.Zircon II contains both positive and negative εHf(t) values (−6.3 to +8.1), with both depleted-mantle and older crustal signatures. Half of the magmatic rims (II) are characterized by depleted mantle signatures with positive εHf(t) values representing juvenile crust-forming events, whereas the other half are characterized by recycled crustal signatures with negative εHf(t) values. εHf(t) results show that the North Caribou and the Island Lake terranes and the northern Uchi domain are isotopically more enriched than the southern Uchi, English River, Wabigoon and Winnipeg River terranes, suggesting the northern Uchi margin represents a major terrane boundary.Based on mass balance calculations, large volumes of juvenile material at circa 3.0 Ga mixed with smaller amounts of older crust. The vast majority of the granites were derived from a source with about 50% mantle material during the peak crust formation events after 2.8 Ga. The decline in the volume of felsic magmatism in the later Archean is coeval with a reduced supply of both heat and material from depleted mantle sources. Combined with previously published geochemical, geochronological and isotopic data, this suggests an evolution in felsic magma sources consistent with crustal thickening.     

Retrograde mineral and fluid evolution in high-pressure metapelites (Schistes lustrés unit, Western Alps)

Fluid inclusions have been analysed in successive generations of syn-metamorphic segregations within low-grade, high-pressure, low-temperature (HP–LT) metapelites from the Western Alps. Fluid composition was then compared to mass transfer deduced from outcrop-scale retrograde mineral reactions. Two types of quartz segregations (veins) occur in the `Schistes lustrés' unit: early blueschist-facies carpholite-bearing veins (BS) and retrograde greenschist-facies chlorite-bearing veins (GS). Fluid inclusions in both types of segregations are aqueous (no trace of dissolved gases such as CO₂, CH₄, N₂), with significant differences in density and composition (salinity). BS fluids are moderately saline fluids (average 9.1 wt% eq. NaCl) characterized by a chronological trend towards more dilute composition (from 15 down to 0 wt% eq. NaCl), whereas GS fluids have a very constant salinity of ∼3.7 wt% eq. NaCl. Both types of inclusions were continuously reset to lower densities along the retrograde path, until a temperature of ∼300 °C. Mass-balance calculations, together with fluid inclusion data, suggest that GS fluids result from the mixing between two fluid sources: one initial, early metamorphic, moderately saline HP fluid and a second nearly pure water fluid provided by the breakdown of carpholite. Estimates of the amount of water released by carpholite breakdown result in a dilution of the interstitial fluid phase (from 10 to 2.5–4 wt% eq. NaCl) consistent with the actual shift of the fluid composition. Alkali elements required for the formation of the GS chlorite + phengite assemblage after carpholite could be locally provided by HP phengite. This is taken as an indirect evidence that, during the generation of both BS and GS fluids, mixing with externally derived fluids may have been very limited. The location, amount and constant composition of the less saline GS fluids appear to be related to an interconnected porosity at the time of inclusion formation. Received: 19 October 1998 / Accepted: 19 July 2000     

The Serra Pelada Au-PGE deposit, Serra dos Carajás (Pará State, Brazil): geological and geochemical indications for a composite mineralising process

The Serra Pelada Au-PGE-rich deposit is located in the Serra dos Carajás, a leading mining area in Brazil. This region is characterised by a complex geological and structural framework and is affected by deep lateritisation which has lasted for more than 70 Ma. The Serra Pelada deposit is emplaced in a late-Archean low-grade metasedimentary sequence (Rio Fresco/Águas Claras Formation) which is host to other gold deposits in the region (Igarapé Bahia, Águas Claras). The Rio Fresco/Águas Claras sequence was deposited in tectonic basins developed on Archean basement and Au-bearing greenstone terranes which were intruded by PGE-rich layered mafic complexes (e.g. Luanga). The Serra Pelada mineralisation is located along a regional, complex system of strike-slip faults (Cinzento-Carajás systems) which were active during the late Archean to early Proterozoic. The mineralisation appears to be concentrated along a faulted hinge zone of a fold. Ore zone rock facies are dominated by low-grade ferruginous to carbonaceous metasiltstones and minor sandstones, locally brecciated and cemented by quartz (-sulphide) stockwork. Supergene alteration led to partial to total transformation into friable aggregates of kaolinite, Fe oxide-hydroxides, silica and secondary phosphate-sulphates even at depths exceeding 200 m. Precious metals are exceptionally enriched, with up to more than 1,000 ppm Au+PGE in some peculiar ferruginous-graphitic zones locally called "hidrotermalito". Geochemistry shows complex patterns of major and trace elements, particularly rare-earth elements (REE), in mineralised vs. nonmineralised samples. These patterns are interpreted in terms of variable degree of superposition of hydrothermal and supergene alteration. Precious metals show progressive increase from samples with hydrothermal imprint to samples with supergene imprint. The geological evolution of the Carajás region and the characteristics of mineralisation at Serra Pelada may suggest a composite mineralising process: hydrothermal activity (by fluids likely originated from granitoids) was followed by supergene alteration during long-lasting lateritisation to develop extreme precious metal enrichments in a geological context probably already anomalous for Au and PGE.     

(Re)Connecting spatial literacy with children’s geographies: GPS,Google Earth and children’s everyday lives

This article sets out an agenda for research that (re)connects research on children’s geographies and childhood studies with studies of spatial literacy. Research on children’s environmental cognition and, latterly, spatial literacy, has been artificially and problematically separated from the majority of research in childhood studies. Our fundamental aim in this article is to argue for – and to evidence – greater attention to how spatial literacy and children’s everyday lives are embedded in one another. To support our broader call for a synthetic research agenda, we draw on some more focussed, qualitative empirical material taken from a large-scale project about children’s mobilities and everyday lives in newly-built urban communities. Our analysis focuses upon children’s interpretations of Global Positioning Systems (GPS) tracks of their mobilities, set against a background of Google Earth imagery. In doing so, we showcase one suite of ways in which research on environmental cognition and children’s geographies might proceed together. We demonstrate that children not only displayed analytical skills (for instance, in relation to scaling effects and pattern recognition) but that many also exercised higher-level, critical analysis, especially in relation to errors on Google Earth outputs. Simultaneously, we interrogate the recursive articulation of a range of qualitative indicators of spatial literacy, with children’s everyday mobilities, routines, emotions and memories. The paper analyses how new conceptual languages and technologies being propounded by spatial literacy scholars could afford a more enriched understanding of key contemporary concerns for children’s geographers, and, recursively, what spatial literacy scholars might gain from engaging with (especially qualitative) research prompted by those concerns.     

Hydrothermal alteration,fluid inclusions and stable isotope systematics of the Alvo 118 iron oxide–copper–gold deposit,Carajás Mineral Province (Brazil): Implications for ore genesis

The Alvo 118 iron oxide–copper–gold (IOCG) deposit (170 Mt at 1.0 wt.% Cu, 0.3 g/t Au) lies in the southern sector of the Itacaúnas Shear Belt, Carajás Mineral Province, along a WNW–ESE-striking, 60-km-long shear zone, close to the contact of the ~2.76-Ga metavolcano-sedimentary Itacaiúnas Supergroup and the basement (~3.0 Ga Xingu Complex). The Alvo 118 deposit is hosted by mafic and felsic metavolcanic rocks and crosscutting granitoid and gabbro intrusions that have been subjected to the following hydrothermal alteration sequence towards the ore zones: (1) poorly developed sodic alteration (albite and scapolite); (2) potassic alteration (biotite or K-feldspar) accompanied by magnetite formation and silicification; (3) widespread, pervasive chlorite alteration spatially associated with quartz–carbonate–sulphide infill ore breccia and vein stockworks; and (4) local post-ore quartz–sericite alteration. The ore assemblage is dominated by chalcopyrite (~60%), bornite (~10%), hematite (~20%), magnetite (10%) and subordinate chalcocite, native gold, Au–Ag tellurides, galena, cassiterite, F-rich apatite, xenotime, monazite, britholite-(Y) and a gadolinite-group mineral. Fluid inclusion studies in quartz point to a fluid regime composed of two distinct fluid types that may have probably coexisted within the timeframe of the Cu–Au mineralizing episode: a hot (>200°C) saline (32.8‰ to 40.6 wt.% NaCl eq.) solution, represented by salt-bearing aqueous inclusions, and a lower temperature (<200°C), low to intermediate salinity (<15 wt.% NaCl eq.) aqueous fluid defined by two-phase (L_H2O + V_H2O) fluid inclusions. This trend is very similar to those defined for other IOCG systems of the Carajás Mineral Province. δ ¹⁸O_H2O values in equilibrium with calcite (−1.0‰ to 7.5‰ at 277°C to 344°C) overlap the lower range for primary magmatic waters, but the more ¹⁸O-depleted values also point to the involvement of externally derived fluids, possibly of meteoric origin. Furthermore, sulphide δ ³⁴S values (5.1‰ to 6.3‰), together with available boron isotope and Cl/Br–Na/Cl data provide evidence for a significant component of residual evaporative fluids (e.g., bittern fluids generated by seawater evaporation) in this scenario that, together with magma-derived brines, would be the main sources of the highly saline fluids involved in the formation Alvo 118 IOCG deposit. The restricted high temperature sodic alteration, the pervasive overprinting of the potassic alteration minerals by chlorite proximal to the ore zones, ore breccias with open-space filling textures in brittle structures, microthermometric and stable isotope data indicate, collectively, that the Alvo 118 IOCG system developed at structurally high levels and may be considered the shallower representative of the IOCG systems of the CMP.     

Late Cretaceous extension overprinting a steep belt in the Northern Calcareous Alps (Schesaplana, R?tikon, Switzerland and Austria)

The Triassic to Cretaceous sediment succession of the Lechtal Nappe in the western part of the Northern Calcareous Alps (NCA) has been deformed into large-scale folds and crosscut by thrust and extensional faults during Late Cretaceous (Eoalpine) and Tertiary orogenic processes. The following sequence of deformation is developed from overprinting relations in the field: (D1) NW-vergent folds related to thrusting; (D2) N–S shortening leading to east–west-trending folds and to the formation of a steep belt (Arlberg Steep Zone) along the southern border of the NCA; (D3) E–W to NE–SW extension and vertical shortening, leading to low-angle normal faulting and recumbent “collapse folds” like the Wildberg Syncline. D1 and D2 are Cretaceous in age and predate the Eocene emplacement of the Austroalpine on the Penninic Nappes along the Austroalpine basal thrust; the same is probably true for D3. Finally, the basal thrust was deformed by folds related to out-of-sequence thrusting. These results suggest that the NCA were at least partly in a state of extension during the sedimentation of the Gosau Group in the Late Cretaceous.