Peak Of Regional Metamorphism Research Articles

Zircon U–Pb–Hf isotope and geochemical constraints on the petrogenesis and tectonic setting of Mesoarchean granitoids from the Carajás province, Amazonian craton, Brazil

Based on U–Pb–Hf isotope and whole-rock geochemical data, this paper investigates the origin and tectonic setting of Mesoarchean granitoids from the Ourilândia do Norte area, Carajás province (northern Brazil) and compares them with other Archean units. The following units were selected: (i) tonalite-trondhjemite-granodiorite (TTG) from the Mogno and Rio Verde suites, represented by a 2.92 Ga tonalitic xenolith [εHf(2.92 Ga) = +2.0 to –0.2] and a 2.92 Ga porphyritic trondhjemite [εHf(2.92 Ga) = +2.3 to –3.5], respectively; (ii) the Ourilândia sanukitoid suite, composed of the 2.92 Ga Arraias granodiorite [εHf(2.92 Ga) = +1.9 to –4.4] and 2.88 Ga Ourilândia tonalite-granodiorite complex [εHf(2.88 Ga) = +3.2 to –1.0]; and (iii) the Boa Sorte granite (Canaã dos Carajás granitic suite), which provides four zircon populations dated at 3.04 Ga, 2.98 Ga, 2.93 Ga and 2.88 Ga. The 2.88 Ga population represents the granitic crystallization age, with εHf(2.88 Ga) values ranging from –0.8 to –4.1. The Paleoarchean geological record from the Carajás province is scarce and restricted to detrital zircons (<3.7 Ga), rare xenocrystals (<3.7 Ga) and crustal model ages (Hf-TDMC = 3.6–3.1 Ga), which indicate Paleoarchean crust extraction from the mantle. The zircon U–Pb–Hf isotope and whole-rock geochemistry data from the Mesoarchean granitoids support the hypothesis that the Carajás province experienced mantle enrichment and progressive crustal reworking during 3.1–2.8 Ga, similar to the Mesoarchean record from the Pilbara and Kaapvaal cratons. A proposed three-stage tectonic model explains the petrogenesis and Hf isotope signatures of the studied granitoids. Regardless of the extent of the Paleoarchean geological record, the available data allow us to postulate that the formation of Paleoarchean crust in the Carajás province involved a long-lived dome-and-keel setting (3.6–3.1 Ga), and that this crust was later recycled, allowing mantle enrichment during Mesoarchean low-angle subduction (3.07–2.92 Ga). Then, a short-lived collision defined by peak regional metamorphism (2.89–2.84 Ga) and associated with crustal thickening and slab breakoff gave rise to mantle- and crust-derived magmas at 2.88 Ga.

Structure and metamorphism of the Cushamen Complex in Sañicó and Collón Curá-Limay rivers confluence: A Devonian metamorphic event related to the subduction stage of the Gondwanan Patagonian orogen

In the North Patagonian Andes and western North Patagonian Massif, the Cushamen Complex is still a matter of interest in order to unravel the Paleozoic evolution of southwestern Gondwana margin. In this contribution we present new petrographic and structural data from the Cushamen Complex cropping out nearby Sañicó town and Collón Curá and Limay rivers confluence together with U–Pb (Q-ICP-MS) detrital zircon ages. In these areas, this metamorphic complex is composed of migmatites, orthogneisses and calcsilicate rocks, with a Late Devonian metamorphic peak age, intruded by Devonian - lower Carboniferous granites. The regional metamorphic peak would also has been accompanied by the development of a pervasive tectonic foliation and associated isoclinal folds. Devonian - Carboniferous granites crosscut the foliation of the migmatites and would have produced a weak contact metamorphism represented by static recrystallization in quartz grains. The regional metamorphic event can be related to the subduction-related Gondwanan orogenic event. At least two more folding phases and related foliations were identified postdating these granitic intrusions, probably related to the collisional event of the Gondwanan orogeny, late Carboniferous - early Permian in age. Evidence of retrograde metamorphism probably related to the previously mentioned foliations and folds are common. Zircons from a mesosome were analyzed by U–Pb Q-ICP-MS yielding a maximum depositional age of 441.79 ± 1.68 Ma (early Silurian/Rhuddanian) indicating a sedimentation for the protholits probably during middle Silurian, age of the pre-orogenic successions of the Gondwanan cycle in Patagonia.

Nova-Bollinger Ni-Cu Sulfide Ore Deposits, Fraser Zone, Western Australia: Petrogenesis of the Host Intrusions

Abstract The Nova-Bollinger Ni-Cu sulfide ore deposit is the first economic Ni-Cu-Co sulfide deposit to have been discovered in the Albany-Fraser orogen in Western Australia. The host rocks are mafic-ultramafic intrusive cumulates subdivided into two connected intrusions, designated Upper and Lower. The Upper Intrusion is bowl-shaped and modally layered with alternating peridotite and norite mesocumulate layers, with a Basal Series of dominantly orthocumulate mafic lithologies. The Lower Intrusion is a much thinner, semiconformable chonolith (flattened tube-shaped intrusion) consisting of mostly unlayered mafic to ultramafic orthocumulates. The Lower Intrusion hosts all the high-grade mineralization and most of the disseminated ores. A distinctive plagioclase-bearing lherzolite containing both orthopyroxene and olivine as cumulus phases is a characteristic of the Lower Intrusion and the Basal Series of the Upper. The intrusions differ slightly in olivine and spinel chemistry, the differences being largely attributable to the more orthocumulate character of the Lower Intrusion. Sector zoning in Cr content of pyroxenes is observed in the Lower Intrusion and in the lower marginal zone of the Upper and is attributed to crystallization under supercooled conditions. Symplectite pyroxene-spinel-amphibole coronas at olivine-plagioclase contacts are ubiquitous and are attributed to near-solidus peritectic reaction between olivine, plagioclase, and liquid during and after high-pressure emplacement, consistent with high Al contents in igneous pyroxenes and estimates of the peak regional metamorphism. Original cumulus olivines had compositions around Fo86 and were variably Ni depleted, interpreted as the result of preintrusion equilibration with sulfide liquid. The Upper and Lower Intrusion rocks represent cumulates from a similar parental magma, a high-Al tholeiite with MgO between 10 and 12%, low TiO2 (0.5–0.6%), and high Al2O3 (14–17%). Modeling using alphaMELTS indicates a primary water content of around 2 wt %. The cumulates of both intrusions were derived via multiple magma pulses of liquid-olivine-sulfide slurries with variable amounts of orthopyroxene emplaced into the deep crust at pressures of around 0.7 GPa during the peak of regional metamorphism. The intrusions developed initially as a bifurcating sill, the lower arm developing into the ore-bearing Lower Intrusion chonolith and the upper arm inflating into the cyclically layered Upper Intrusion.

The extensive hydrocarbon-mediated fixation of hydrothermal gold in the Witwatersrand Basin, South Africa

There is a close spatial relation between high-grade gold mineralization in the Witwatersrand basin and carbonaceous nodules, veins and seams. Hydrocarbons thus may well have been essential in ore genesis. We have sampled four major gold-, uranium- and hydrocarbon-bearing ore horizons, namely the Carbon Leader, Vaal, B and Black reefs, to determine the role of hydrocarbons in the accumulation and hydrothermal fixation of gold. Our multipronged approach included high-resolution scanning and transmission electron microscopy (SEM, TEM), nanotomography with video clips, and geochemical modeling. Post-depositional hydrothermal activity at the peak of regional metamorphism produced an assemblage of quartz, phyllosilicates, brannerite, crandallite, florencite, monazite and gold in all four reefs. The gold, hydrocarbons and associated mineral assemblages are closely related on the micro to nano scale. Gold deposition occurred in interstices, as fracture fillings in detrital minerals, and on the surface of migrated solid hydrocarbon residues. The spherical to elliptical inclusions in the gold consist of an outer pyrobitumen phase and a central void space, partially associated with nanometric gold, uraninite, coffinite and silica. The hydrocarbon-bearing inclusion likely formed by the entrapment of a fossil liquid oil precursor during gold precipitation. The oil was subsequently thermally altered and converted into the final pyrobitumen and gaseous residues. Geochemical calculations to simulate the interaction of an invading hot hydrothermal fluid with the hydrocarbons in the reefs reveal that a very small amount of hydrocarbons will drastically decrease the aqueous solubility of gold and hence cause its instant precipitation.We extend our genetic model for the epigenetic formation of gold in the Witwatersrand. Regional metamorphism promoted the extensive and likely basin-wide circulation of hydrothermal fluids; these were capable of mobilizing substantial amounts of gold. The liquid, gaseous and solid hydrocarbons in the reefs acted as efficient chemical traps for the concentration of gold. Being strong chemical reductants, they caused the rapid precipitation and accumulation of gold on the surface of the fossil oil droplets and already solidified hydrocarbons. The release of the gases from accessible hydrocarbons into the sediments away from their source buffered the redox state of the hydrothermal solutions even at a considerable distance from the pyrobitumen seams and veins, likely resulting in the deposition of gold in the absence of visible hydrocarbons. Although our findings do not explain the ultimate origin and exceptional endowment of gold in the Witwatersrand, we do provide intriguing evidence for the large-scale hydrothermal mobilization, accumulation and fixation of gold mediated by hydrocarbons during post-depositional metamorphism.

Constraints on the nature of metamorphism in the Kalgoorlie gold camp (Yilgarn Craton, Western Australia) and implications for genetic models of gold mineralisation

The timing and nature of metamorphism in the Kalgoorlie gold camp (Yilgarn Craton, Western Australia) are key aspects to genetic models for gold mineralisation. Despite this, existing constraints on metamorphism are equivocal as to whether the Fimiston and Oroya Au–Te lodes formed pre- or post-peak regional metamorphism. Evaluation of an actinolite (S1)–chlorite (S2) crenulation cleavage in the Hannan’s Lake Serpentinite on the western margin of the Golden Mile Super Pit supports a syn-D1 timing for peak metamorphism and a post-peak metamorphic timing for the Fimiston and Oroya Au–Te lodes. These results support a key aspect of metamorphic models for gold mineralisation. If intrusion-related models are correct, they should not be premised on the argument that the Fimiston and Oroya Au–Te lodes are equivalent to other examples of metamorphosed, epithermal-type mineralisation in orogenic belts. KEY POINTS The definition of an S1 cleavage in the Hannan’s Lake Serpentinite by actinolite supports a syn-D1 timing (ca 2685–2680 Ma) for peak regional metamorphism (sub- to lower-greenschist facies) in the Kalgoorlie gold camp. A syn-D1 timing for peak regional metamorphism suggests that the D2 (ca 2675–2665 Ma) Fimiston and Oroya Au–Te lodes and D3 (ca 2650–2640 Ma) Mt Charlotte stockwork Au orebodies developed in post-peak metamorphic settings. A post-peak metamorphic timing for the Fimiston and Oroya Au–Te lodes suggests that these mineralisation types do not represent metamorphosed equivalents of epithermal Au–Te mineralisation.

Petrogenesis of Mesoproterozoic Granites of the Swartoup Hills Region, Kakamas Domain, Namaqua Belt, South Africa

The Swartoup and Polisiehoek plutons in the Swartoup Hills (South Africa) formed during an episode of significant magma emplacement in the Mesoproterozoic Namaqua Sector of the Namaqua Metamorphic Province. They intruded into mid-crustal metasedimentary rocks of the metapelitic Koenap and mafic to carbonate-bearing Bysteek Formations during and shortly after the ∼1,200–1,220 Ma regional metamorphic peak that reached ultrahigh temperatures. Subsequent to pluton emplacement, the crust underwent regional high-temperature deformation during slow near-isobaric cooling. A further episode of pluton emplacement associated with fluid circulation truncated the first-order regional tectonic structures at ∼1,100 Ma. Based on their petrography, the Swartoup pluton is subdivided into leuco-granitoids with biotite as the sole mafic phase, pyroxene granitoids, and garnet-bearing granitoids, which may contain significant biotite. These subgroups display distinctive geochemical variations from one another, and from the Koenap Formation migmatites and the Polisiehoek granites, which are exposed nearby. Incompatible trace element distributions suggest that the Swartoup and Polisiehoek granitoids represent modified A-type granite magma, consistent with derivation from partial melting of quartzo-feldspathic crust. The magmas incorporated significant amounts of juvenile mantle-derived magma (εNd1200 of ∼−5, and LREE-depleted), but do not require older, early to late Paleoproterozoic crust. Particularly close to contacts to the calcic Bysteek Formation, localized contamination of the Swartoup granites by supracrustal carbonates is evident. A relatively pervasive alkali metasomatic effect is manifested strongly in the initial 87Sr/86Sr and LILE profiles of the Polisiehoek granites in particular, as well as in some of the Swartoup pyroxene granitoids, which could be either a symptom of CO2 metasomatism related to the Bysteek Formation carbonates, or to post-magmatic fluid metasomatism, perhaps linked to regional shearing. The comparison of our results with literature data suggests that similar sources, A-type granitic, Meso- to Paleoproterozoic crustal, and enriched mantle, have contributed, in locally differing proportions, to granites in most parts of the Namaqua Sector. Most likely, these plutons were generated during crustal and mantle melting in a long-lived hot continental back-arc environment.

Evolution of structures and hydrothermal alteration in a Palaeoproterozoic supracrustal belt: Constraining paired deformation–fluid flow events in an Fe and Cu–Au prospective terrain in northern Sweden

Abstract. An approximately 90 km long Palaeoproterozoic supracrustal belt in the northwestern Norrbotten ore province (northernmost Sweden) was investigated to characterize its structural components, assess hydrothermal alteration–structural geology correlations, and constrain a paired deformation–fluid flow evolution for the belt. New geological mapping of five key areas (Eustiljåkk, Ekströmsberg, Tjårrojåkka, Kaitum West, and Fjällåsen–Allavaara) indicates two major compressional events (D1 and D2) have affected the belt, with each associated with hydrothermal alteration types typical for iron oxide–apatite and iron oxide Cu–Au systems in the region. Early D1 generated a regionally distributed, penetrative S1 foliation and oblique reverse shear zones that show a southwest-block-up sense of shear that formed in response to NE–SW crustal shortening. Peak regional metamorphism at epidote–amphibolite facies broadly overlaps with this D1 event. Based on overprinting relationships, D1 is associated with regional scapolite ± albite, magnetite + amphibole, and late calcite alteration of mafic rock types. These hydrothermal mineral associations linked to D1 structures may form part of a regionally pervasive evolving fluid flow event but are separated in this study by crosscutting relationships. During D2 deformation, folding of S0–S1 structures generated F2 folds with steeply plunging fold axes in low-strain areas. NNW-trending D1 shear zones experienced reverse dip-slip reactivation and strike-slip-dominated movements along steep, E–W-trending D2 shear zones, producing brittle-plastic structures. Hydrothermal alteration linked to D2 structures is a predominantly potassic–ferroan association comprising K-feldspar ± epidote ± quartz ± biotite ± magnetite ± sericite ± sulfides. Locally, syn- or post-tectonic calcite is the main alteration mineral in D2 shear zones that intersect mafic rocks. Our results highlight the importance of combining structural geology with the study of hydrothermal alterations at regional to belt scales to understand the temporal–spatial relationship between mineralized systems. Based on the mapping results and microstructural investigations as well as a review of earlier tectonic models presented for adjacent areas, we suggest a new structural model for this part of the northern Fennoscandian Shield. The new model emphasizes the importance of reactivation of early structures, and the model harmonizes with tectonic models presented by earlier workers based mainly on petrology of the northern Norrbotten area.

The essence of time – fertile skarn formation in the Variscan Orogenic Belt

Extending from Northern America to Central China the Variscan belt is a Paleozoic Orogen exceptionally well endowed in magmatic-hydrothermal ore deposits, including skarn deposits. Yet, the genesis of fertile skarns and their distinction from barren equivalents in orogenic zones is only poorly constrained. Here, we present innovative U-Pb laser-ablation inductively-coupled-plasma mass-spectrometry geochronology of garnet from different skarns in the Erzgebirge, a classic metallogenic province in central Europe. Garnet ages obtained not only constrain the timing of fertile skarn formation and associated Sn, W, Fe, Zn, Cu and In mineralization, but also clearly distinguish these from barren skarn bodies. We show that barren skarns formed during times of peak regional metamorphism at ∼340 Ma whereas mineralized skarns are temporally associated with late-orogenic magmatism at ∼325-313 Ma as well as post-orogenic magmatism at ∼308-295 Ma. The recognition of discrete mineralization events associated with the largest and economically most important skarn deposits provides valuable insight into the punctuated evolution of magmatic-hydrothermal systems in ancient collisional orogens on a regional scale; this has important implications to direct future mineral exploration.

Ushuaia pluton: Magma diversification, emplacement and relation with regional tectonics in the southernmost Andes

The Ushuaia pluton (UP) and the Ushuaia Peninsula Andesites (UPA) are intrusive units of a Late Cretaceous rear-arc in the Southernmost Andes. We report new LA-ICP-MS zircon ages from ∼71 to 75 Ma for the UP, and of ∼84 Ma for the UPA, which allow to differentiate these two magmatic events; consistent with their distinct petrography and geochemistry. The UP is a composite, incrementally assembled pluton in upper crustal levels. Successive injections of magma batches define two main sections, each in turn constructed by several smaller pulses: an early ultramafic-mafic section consists mainly of hornblende pyroxenite, hornblendite and gabbro/diorite with reverse concentric zonation, and a later intermediate section built up mainly by monzodiorite. Mingling and layering between mafic and intermediate facies along the contact point to simultaneous emplacement at the end of one main pulse and the beginning of the other. Mineralogy and geochemistry suggest the rock sequence in the UP is cogenetic. In part it resulted by in-situ magmatic processes that included crystal-melt fractionation during injection of magma mushes, mingling, local melt segregation and mixing with crustal partial melts. The country rock is affected by penetrative ductile deformation and regional metamorphism developed during mid-Cretaceous Andean main orogenic phase. It also shows an overimposed pluton-margin parallel foliation and a post-kinematic contact metamorphic aureole. On the other hand, the UP has a concentric igneous foliation parallel to pluton margins interpreted as a primary magmatic feature. The above evidences, along with absence of subsolidus fabrics and metamorphism, indicate the pluton post-date peak regional metamorphism and ductile structures in the country rock. The new zircon ages constrain the top age of early Andean deformation, as well as the magmatic gap registered for the end of the Cretaceous, and confirm an emplacement during a recorded compressive regime that transmitted shortening to the foreland.

P-T evolution of metapelites from the Bajgan complex in the Makran accretionary prism, south eastern Iran

The Bajgan Complex, one of the basement constituents of the arc massif in Iranian Makran forms a rugged, deeply incised terrain. The complex consists of pelitic schists with minor psammitic and basic schists, calc silicate rocks, amphibolites, marbles, metavolcanosediments, mafic and felsic intrusives as well as ultramafic rocks. Metapelitic rocks show an amphibolite facies regional metamorphism and contain garnet, biotite, white mica, quartz, albite±rutile±apatite. Thermobarometry of garnet schist yields pressure of more than 9kbar and temperatures between 560 and 675°C. The geothermal gradient obtained for the peak of regional metamorphism is 19°C/km, corresponding to a depth of ca. 31km. Replacement of garnet by chlorite and epidote suggest greenschist facies metamorphism due to a decrease in temperature and pressure through exhumation and retrograde metamorphism (370–450°C and 3–6kbar). The metapelitic rocks followed a ‘clockwise’ P–T path during metamorphism, consistent with thermal decline following tectonic thickening. The formation of medium-pressure metamorphic rocks is related to presence of active subduction of the Neotethys Oceanic lithosphere beneath Eurasia in the Makran.

Structural setting for Canadian Malartic style of gold mineralization in the Pontiac Subprovince, south of the Cadillac Larder Lake Deformation Zone, Québec, Canada

The structural setting of the Pontiac Subprovince in the vicinity of the world-class Canadian Malartic gold deposit has been revisited by combining and reinterpreting airborne geophysical surveys together with a century of structural observations. Felsic-intermediate intrusive bodies are a key component of this deposit. Defining the regional and local favorable structural setting for intrusive rock emplacement within the clastic meta-sedimentary rocks of the Pontiac Group, south of the Cadillac Larder Lake Deformation Zone, may further assist gold exploration in similar tectonic environment.Three structural domains are interpreted in the area based on the geometry of the bedding, folds and structural fabrics related to the three major phases of deformation. During these events four phases of magmatism and one metamorphic episode occurred. The North domain, which hosts the Canadian Malartic deposit, is characterized by highly variable bedding orientations produced by the interference patterns of isoclinal F1 folds overprinted by open to tight, steeply dipping, F2 folds. The bedding in this domain is cut by a penetrative S2 biotite foliation, which possibly built on rheological changes enhanced by metasomatism in the footprint of the Canadian Malartic deposit. By contrast, the Central and South domains display homogeneous bedding orientations cross-cut by a discrete S2 biotite foliation and syn- to late- D2 metamorphic porphyroblasts. In all domains the L2 stretching lineation consistently plunges at about 60 degrees toward the east. D3 is a minor deformation event in the Pontiac Subprovince, which possibly correlates with late dextral transcurrent movement along the Cadillac Larder Lake Deformation Zone.Intrusive bodies were emplaced throughout the first and second deformation events. Phase 1 monzonite, quartz-monzodiorite and granodiorite plutons (ca. 2683–2680Ma) intruded into consolidated Pontiac sedimentary rocks during D1. Phase 2 quartz-monzodiorite bodies (ca. 2679–2676Ma) predominantly formed in the North Domain along F1 fold axial surfaces prior to or at the onset of D2. Phase 3 basic dykes (ca. 2675–2673Ma) cross-cut earlier felsic-intermediate intrusions across all domains and subsequently developed an S2 metamorphic hornblende foliation. Phase 4 magmatism (ca. 2672–2662Ma) is related to the Décelles Batholith S-type granite and pegmatite, which is interpreted to be contemporaneous with the peak of regional metamorphism. The Décelles Batholith may project at depth underneath the Canadian Malartic deposit and could be associated with magmatic-hydrothermal mineralizing fluids in the Canadian Malartic deposit.Gold mineralization at Canadian Malartic is spatially located on the contact of Phase 2 quartz-monzodiorite bodies. The proximity to the Cadillac Larder Lake Deformation Zone combined with the rheological contrast between steeply dipping clastic meta-sedimentary rocks and quartz-monzodiorite intrusions favored the protracted failure of the contacts between these two rock masses, thus forming favorable conduits for hydrothermal fluids (e.g., the Sladen Fault Zone). Such specific rheological behavior is demonstrated by domains of structural complexity, emphasized by the variance of the bedding dip. These structurally complex zones systematically host gold mineralization in the Canadian Malartic district of the Pontiac Subprovince proximal to the Cadillac Larder Lake Deformation Zone.

Timing and conditions of regional metamorphism and crustal shearing in the granulite facies basement of south Namibia: Implications for the crustal evolution of the Namaqualand metamorphic basement in the Mesoproterozoic

Granulite facies basement gneisses from the Grünau area in the Kakamas Domain of the Namaqua-Natal Metamorphic Province in south Namibia show high-grade mineral assemblages, most commonly consisting of garnet, cordierite, sillimanite, alkali feldspar and quartz. Cordierite + hercynitic spinel, and in some places quartz + hercynitic spinel, indicate granulite facies P-T conditions. The peak assemblage equilibrated at 800–850 °C at 4.0–4.5 kbar. Sillimanite pseudomorphs after kyanite1 and late-stage staurolite and kyanite2 indicate that the metamorphic record started and ended within the stability field of kyanite. Monazite in the metamorphic basement gneisses shows a single-phase growth history dated as 1210-1180 Ma, which we interpret as the most likely age of the regional metamorphic peak. This time coincides with the emplacement of granitic plutons in the Grünau region.The ∼10 km wide, NW-SE striking Grünau shear zone crosscuts the metamorphic basement and overprints high-temperature fabrics. In sheared metapelites, the regional metamorphic peak assemblage is largely obliterated, and is replaced by synkinematic biotite2, quartz, alkali feldspar, sillimanite and cordierite or muscovite. In places, gedrite, staurolite, sillimanite and green biotite3 may have formed late- or post-kinematically. The mylonitic mineral assemblage equilibrated at 590–650 °C at 3.5–5.0 kbar, which is similar to a retrograde metamorphic stage in the basement away from the shear zone. Monazite cores in two mylonite samples are similar in texture and age (∼1200 Ma) to monazite in metapelites away from the shear zone. Chemically distinct monazite rims indicate a second growth episode at ∼1130-1120 Ma. This age is interpreted to date the main deformation episode along the Grünau shear zone and the retrograde metamorphic stage seen in the basement.The main episode of ductile shearing along the Grünau shear zone took place 70–80 million years after the thermal peak metamorphism and granite emplacement, and after substantial isobaric cooling of the basement. Metamorphism and regional shearing in the Grünau area can be correlated with the crustal evolution in the Kakamas Domain in South Africa, but not with the timing of metamorphism in the Aus area, 230 km to the NW of Grünau, which is significantly younger.

Geochemical and Spectral Footprint of Metamorphosed and Deformed VMS-style Mineralization in the Quinns District, Yilgarn Craton, Western Australia*

Volcanic-hosted massive sulfide (VMS) occurrences in the Quinns district are hosted predominantly by ca. 2814–2800 Ma banded iron formation (BIF) within a sequence of rhyolite, basalt, and minor siltstone. The VMS occurrences and their surrounding rocks are folded, metamorphosed, and deeply weathered and variably covered by transported regolith. This study uses an integrated petrological and geochemical approach to map gradients in synvolcanic mineral and element abundances, with the aim of understanding the effects of postvolcanic processes on Archean VMS ore. Rhyolite-dominant footwall rocks and the BIF record kilometer-scale gradients in alteration mineral patterns and geochemistry. Rhyolite exposed throughout the district and intersected by drill holes records distal alteration assemblages of quartz-white mica ± chlorite. At the Austin deposit in the western part of the district, abundant talc and anthophyllite with minor cummingtonite and hornblende are associated with mineralization in the BIF, indicating intense magnesium metasomatism. Further away from mineralization, chlorite is the dominant alteration mineral. Adjacent rhyolite is altered to chlorite in proximal zones and white micas in more distal areas. Approaching known VMS prospects in the eastern half of the district, the rhyolite grades into a 2 × 1 km zone of schistose rhyolite with generally dispersed, but locally abundant, coarse-grained andalusite ± kyanite ± garnet rhyolitic schist. This broad Al-rich silicate alteration zone envelops two discrete 1 km × 500 m proximal alteration zones in rhyolite, defined by chlorite ± talc, with minor disseminated magnetite, pyrite, and chalcopyrite. The northernmost proximal alteration zone lies stratigraphically below the BIF-hosted Cu-Zn-rich gossan exposed near the Tasman prospect. The proximal zones are interpreted to be the result of the interaction between synvolcanic, Mg-rich fluids and rhyolite footwall at the time of VMS mineralization and BIF deposition. In contrast, outer zones of andalusite ± kyanite ± garnet suggest the removal of silica and alkali elements and residual concentration of aluminium in rhyolite footwall by acidic synvolcanic hydrothermal fluids prior to peak regional metamorphism. The mapped patterns in alteration minerals mirror bulk rock geochemical gradients in rhyolite; proximal alteration zones are enriched in Cu, Zn, Ag, Au, Bi, Fe2O3, In, MgO, Mo, S, Se, and Te, with local enrichments in As, Cd, MnO, Pb, and V. These rocks are depleted in Ba, K2O, Li, Na2O, and Rb relative to least-altered rhyolite. Rhyolite located up to 100 m from proximal alteration zones is enriched in Ag, Fe2O3, In, K2O, MgO, S, and V. Banded iron formation hosting massive sulfides is altered to talc, anthophyllite, chlorite, and magnetite; these rocks are enriched in Cu, Zn, Pb, Ag, and Sn, with local enrichments in Bi, In, and MgO. Positive Eu anomalies are associated with mineralized BIF in the Austin VMS deposit and contrast with the mostly flat REE slopes for the BIF sampled from less mineralized areas in the district. The kilometer-scale gradients in hydrothermal alteration minerals and bulk rock geochemistry in rhyolite and the BIF are interpreted to be mostly controlled by synvolcanic hydrothermal alteration related to the development of VMS-related hydrothermal fluid pathways. Metamorphism of the synvolcanic alteration was isochemical, apart from localized metasomatism related to later shear zones or fault zones. The defined alteration mineral and geochemical gradients serve as useful tools for detecting fluid alteration pathways related to VMS systems in complexly deformed and metamorphosed districts globally.

Sedimentation, metamorphism and granite generation in a back-arc region: Records from the Ediacaran Nova Venécia Complex (Araçuaí Orogen, Southeastern Brazil)

The Nova Venécia migmatite-granulite-granite Complex (NVC) in the core of the Araçuaí Orogen (AO, 630–480Ma), southeast Brazil, exposes a mid-crustal section with abundant evidence for high-grade metamorphism linked to production, extraction and emplacement of peraluminous melts. Although the AO represents the textbook example of a confined orogen, there is surprisingly a lack of detailed studies on its metamorphic evolution related to widespread granitogenesis occurring from the Neoproterozoic to early Paleozoic. In this study, we combine U–Pb geochronology and metamorphic petrology to constrain the evolution of the NVC migmatitic metasedimentary granulites, from deposition to high-grade metamorphism, and to correlate the metamorphic history of the terrain with the several episodes of granite magmatism (G1–G5) in the AO. The sedimentation of the NVC can be bracketed within a maximum 13My period, between its maximum depositional age at ca. 606Ma and the intrusion of early syn-collisional granitoids at 593Ma. Compilation of available U–Pb data shows that the bulk of the magmatic rocks in the AO (G1+G2 rocks) crystallized contemporaneously over a period of 15 My (ca. 595–570Ma) with a peak at ca. 575Ma. Although it is inferred a protracted period of crustal heating in the AO (from ca. 640–480), U–Pb ages of metamorphic and magmatic zircons and monazites suggest at least two major heat pulses at ca. 593–560 and 523–495Ma. The timing of peak regional metamorphism is constrained from 575 to 560Ma, which temporally overlaps with the crystallization of the youngest granitoids. Phase equilibrium modeling of metasedimentary granulites from three different localities within the NVC, indicates that all areas record similar peak P–T conditions of 750–850°C and 5300–7500bar. This is followed by high temperature retrograde evolution to 640–800°C and 4500–6000bar. A post-collisional thermal event linked to the intrusion of large norite bodies (520–480Ma) is recorded in our metagreywackes (monazite U–Pb) and in granites (monazite and zircon U–Pb) from 523 to 495Ma.

Provenance and metamorphic PT conditions of Cryogenian–Ediacaran metasediments from the Kid metamorphic complex, Sinai, NE Arabian–Nubian Shield: Insights from detrital zircon geochemistry and mineral chemistry

The Malhaq and Um Zariq formations occupy the northern part of the Neoproterozoic Kid metamorphic complex of SE Sinai, NE Arabian–Nubian Shield. This study presents new mineral chemistry data and LA-ICP-MS analyses of the trace element concentrations on zircons separated from metapelites from these formations. The detrital zircons of Um Zariq Formation are more enriched in ΣREE, whereas Malhaq Formation zircons are markedly HREE-enriched with strongly fractionated HREE patterns. The quite differences in the overall slope and size of the Eu and Ce anomalies between REE patterns of the two zircon suites provide a robust indication of different sources. The Ti-in-zircon thermometer has revealed that the zircons separated from Malhaq Formation were crystallized within the 916–1018°C range, while those from Um Zariq Formation exhibit higher range of crystallization temperatures (1084–1154°C). The detrital zircons of Malhaq Formation were derived mainly from mafic source rocks (basalt and dolerite), whereas Um Zariq Formation zircons have varied and more evolved parent rocks. Most of the investigated zircons from both formations are concluded to be unaltered magmatic that were lately crystallized from a high LREE/HREE melt. All the studied detrital zircon grains show typical trace elements features of crustal-derived zircons. All of the Um Zariq Formation and most of Malhaq Formation detrital zircons are geochemically discriminated as continental zircons. Both formation metapelites record similar, overlapping peak metamorphic temperatures (537–602°C and 550–579°C, respectively), and pressures (3.83–4.93kbar and 3.69–4.07kbar, respectively). The geothermal gradient, at the peak metamorphic conditions, was quite high (37–41°C/km) corresponding to metamorphism at burial depth of 14–16km. The peak regional metamorphism of Um Zariq and Malhaq formations is concluded to be generated during extensional regime and thinning of the lithosphere in an island arc setting with heat flow from the underlying arc granitoids.

CO2-rich inclusions in vein gold–copper mineralization of the Sarekoubu–Qiaxia District, southern Altaides, China: Implication for ore genesis

The Sarekoubu–Qiaxia area of the southern Altaides hosts a number of vein gold and copper deposits, in which a great number of carbonic fluid inclusions were found. For many years, the ore genesis of these deposits and the origin of the carbonic inclusions are still unclear. This study attempts to explain where the carbonic inclusions came from and their contribution to gold mineralization. The Sarekoubu gold deposit (SR) and the Qiaxia copper–gold deposit (QX) occur in the lower Devonian Kangbutiebao Formation (D1k22). Two types of quartz veins in these deposits can be distinguished: (1) lentoid or banded quartz veins (QI) parallel to the foliation of metamorphic rocks; and (2) chalcopyrite-bearing quartz veins (QII) cutting across the foliation of metamorphic rocks and banded sulfides.Three main types of fluid inclusions can be found in the quartz veins: CO2–H2O fluid inclusions (Type CW), carbonic inclusions (Type C), and salt-aqueous inclusions (Type W). The CO2-rich fluid inclusions (type CW and type C) occur both in the QI and QII veins. The fluid inclusion assemblage (FIA) method was applied in petrography study and thermometry data analysis. The results show that CO2-rich fluid inclusions have been trapped with the minimum temperatures ranging from 243 to 395°C for the early vein quartz stage (QI), and 230 to 328°C for late polysulfide quartz veins (QII) of the Sarekoubu deposit. For the Qiaxia deposit, the minimum trapping temperatures are from 201 to 382°C for QI, and from 207 to 365°C for QII respectively. The estimated minimum trapping pressures are from 110 to 330MPa which are in agreement with the P–T conditions of the regional metamorphism.The CO2-rich fluids in the vein gold mineralization at the Sarekoubu–Qiaxia area might be produced after peak regional metamorphism during the early Carboniferous to the Permian due to decomposition of marble and calcareous siltstone in the Lower Devonian Kangbutiebao Formation. High density carbonic inclusions in SR are much more abundant than in QX, indicating that regional tectonic stress in SR might be higher than in QX. The hydrothermal alterations are also much more intensive in SR than in QX, which may illustrate loss of water in CO2–H2O fluids in SR during the process of wall rock alterations and leaving CO2-rich fluids behind. The CO2-rich fluids may have played an important role in gold mineralization and modifying the VMS deposits.

New data on the Mesozoic magmatism of the Bechasyn zone, in the Greater Caucasus, and estimation of the age of the regional metamorphism

The results of the local U–Th–Pb isotope dating of accessory zircons from gabbroic rocks of the Khasaut subvolcanic complex are considered. Dikes of gabbroic rocks, which were previously mapped as subvolcanic intrusions in the structure of the Precambrian metamorphic complex, were studied in the Bechasyn zone of the crystallinicum of the Greater Caucasus, in the riverbed of the Khasaut River (the left tributary of the Malka River). The U–Pb age of igneous zircons from these rocks of ~220 Ma does not support the Proterozoic age and is evidence of possible manifestation of Triassic magmatism in the Greater Caucasus. The recent data on the zircon age of the protolith of the Bechasyn complex indicate that some sedimentary rocks are younger than 440 Ma and the peak of regional metamorphism of greenschist facies in the Bechasyn zone, taking the thermochronological corrections into account, probably corresponded to 400–420 Ma.

Exhumation-Driven Devolatilization as a Fluid Source for Orogenic Gold Mineralization at the Damang Deposit, Ghana

Gold mineralization at the Damang deposit is unique among currently known orogenic gold deposits in Ghana, comprising gold hosted within metasediments of the Tarkwaian System and contained in a subhorizontal, extensional quartz vein array that formed during regional compression. The Damang region has an extended paragenesis involving numerous structural, metamorphic, igneous, and metasomatic events. Orogenic gold mineralization occurred late in the geologic paragenesis at Damang, postdating regional metamorphism and an earlier episode of hydrothermal alteration, locally termed ”pink hematite” alteration, associated with the intrusion of mafic sills and dikes. This earlier pink hematite alteration event involves extensive silicification that changed the rheology of the altered rocks and promoted later fracturing. Following peak regional metamorphism at around 2005 Ma, the Damang region underwent a short period of rapid exhumation, as constrained through numerical thermal modeling of existing pressure-temperature-time data. This exhumation triggered the generation of a subhorizontal fracture array that was fed by fluids released through decompression-driven metamorphic devolatilization. The interaction between these fluids and the host rock resulted in precipitation of gold in association with sulfide-carbonate-potassic alteration halos around quartz veins. Such postpeak metamorphic, exhumation-driven, devolatilization is unlikely to be a singular occurrence and represents a potentially important source of fluid for orogenic gold deposits elsewhere in Ghana, West Africa, and globally.

Syn‐metamorphic folding in the Tauern Window, Austria dated by Th–Pb ages from individual allanite porphyroblasts

AbstractHigh‐precision 232Th–208Pb dates have been obtained from allanite porphyroblasts that show unambiguous microstructural relationships to fabrics in a major syn‐metamorphic fold in the SE Tauern Window, Austria. Three porphyroblasts were analysed from a single garnet mica schist from the Peripheral Schieferhülle in the core of the Ankogel Synform, one of a series of folds which developed shortly before the thermal peak of Alpine epidote–amphibolite facies metamorphism: allanite grain 1 provided two analyses with a combined age of 27.7 ± 0.7 Ma; grain 2, which was slightly bent and fractured during crenulation, provided two analyses with a combined age of 27.7 ± 0.4 Ma; a single analysis from grain 3, which overgrew an already crenulated fabric, gave an age of 28.0 ± 1.4 Ma. The five 232Th–208Pb ages agree within error and define an isochron with an age of 27.71 ± 0.36 Ma (95% confidence level; MSWD = 0.46). The results imply that the crenulation event was in progress in a short interval (&lt;1 Ma) c. 28 Ma, and that the Ankogel Synform was forming at this time. The thermal peak of regional metamorphism in the SE Tauern Window was probably attained shortly after 28 Ma, only c. 5 Ma after eclogite facies metamorphism in the central Tauern Window. Metasediment may contain allanite porphyroblasts with clear‐cut microstructural relationships to fabric development and metamorphic crystallization; for such rocks, 232Th–208Pb dating on microsamples offers a powerful geochronological tool.

Recognition of Yanshanian magmatic-hydrothermal gold and polymetallic gold mineralization in the Laowan gold metallogenic belt, Tongbai Mountains: New evidence from structural controls, geochronology and geochemistry

The Laowan metallogenic belt in China is an important metallogenic belt within the Tongbai orogenic belt, and contains the medium-sized Laowan and Shangshanghe gold deposits, the small Huangzhuyuan lead–zinc–silver–gold deposit and some gold and Cu–Pb occurrences. These deposits are hosted in Mesoproterozoic plagioclase amphibolite (or schist) and mica-quartz schist. The gold ores are mainly quartz veins and veinlets and disseminated altered ores. Subordinate ore types include massive sulfides and breccias. The Laowan gold deposit is characterized by three right-stepping en-echelon fracture-controlled alteration zones that dip gently to the south and includes disseminated, sheeted and stockwork ores. These lodes were formed by the interaction of ore-forming fluid with foliated-to laminated cataclasite within the transpressional faults. The Shangshanghe gold deposit is characterized by parallel ore lodes that dip steeply to the north, and includes quartz veins and breccias in addition to ores in altered wallrocks. These lodes were formed by focusing of fluids into transtensional faults. These ore controlling faults displaced early barren quartz veins 10m horizontally with a dextral sense of motion. The ore-hosting structures at the Laowan and Shangshanghe deposits correspond to the P and R-type shears of a brittle dextral strike-slip fault system, respectively, which make angles of about 15° and −15° to the Laowan and Songpa boundary faults. The ore-controlling fault system post-dated formation of a ductile shear zone, and peak regional metamorphism. This precludes a genetic relationship between hydrothermal mineralization and regional metamorphism and ductile shear deformation. These gold deposits are not typical orogenic gold deposits. The metallogenic belt displays district-scale-zoning of Mo→Cu–Pb–Zn–Ag→Au relative to Songpa granite porphyry dike zone, suggesting the mineralization may be closely related to the granite porphyry. Measured δ34S of sulfides and δ18O and δD of fluid inclusion waters in auriferous quartz also are consistent with a magmatic source for sulfur and ore fluids. The similarity of Pb isotope ratios between the ores and Yanshanian granitoids suggests a similar source. As the age (139±3Ma) of granite porphyry obtained by zircon U–Pb isotope overlaps the mineralization age (138±1Ma: Zhang et al., 2008a), the gold and polymetallic metallogenesis of the Laowan gold belt has close spatial, temporal and possibly genetic relationships with Yanshanian high level magmatism.