Low-grade Regional Metamorphism Research Articles

Mobilization of Tl-Hg-As-Sb-(Ag,Cu)-Pb sulfosalt melts during low-grade metamorphism in the Alpi Apuane (Tuscany, Italy)

We report the discovery of an exceptional assemblage of Tl-Hg-As-Sb-(Ag,Cu)-Pb sulfosalts showing textural evidence for their mobilization as melts in the barite–pyrite–iron oxide orebodies of the Monte Arsiccio mine (Alpi Apuane, Tuscany, Italy). The relative abundance of rare thallium sulfosalts (including three new mineral species), their peculiar textural features within the orebodies (e.g., migration along matrix grain boundaries, drop-like internal textures, low interfacial angles between sulfosalts and matrix minerals), and the overall high thallium content in pyrite from the entire mining district (to ∼900 ppm) make the barite–pyrite–iron oxide deposits of the Alpi Apuane a reference locality for studying low-temperature sulfosalt melts in low-grade metamorphic complexes (greenschist facies). Our study reveals how sulfosalt melting during low-grade regional metamorphism controls the redistribution of economically valuable and environmentally critical elements such as thallium in sulfide orebodies containing significant amounts of low-melting-point chalcophile elements.

Pyro-expanded black slate in sculptural art

Black slate transformed through a pyroplastic process named pyro-expansion or exfoliation has been explored in the present work in order to be used as sculptural material. Black slate is a highly fissile, fine grained and organic matter rich rock that is the product of the action of low-grade regional metamorphism on black shale (a sedimentary clay and organic matter rich, also fissile and fine grained rock). Black slate if fired at an adequate firing rate up to the temperature range 1000 ºC-1240 ºC shows great potential for the manufacture of sculptural pieces. The technical possibilities of the shaping or conformation of pyro-expanded black slate have also been studied, including the reactions that take place when two different black slate pieces are closely associated with each other, or when black slate pieces are closely associated to other materials, such as metals and ceramics. These interactions, while associating different materials that react with each other, emphasize the unique characteristics of new sculptural compositions increasing the plastic capacities of the pyro-expanded black slate. Some examples of the associations referred to will be shown, which highlight the close functional relationship between art and science; research involves the approach to new techniques and materials, looking at the development of unique plastic configurations.

Characterization of hydrated silicate-bearing outcrops in Tyrrhena Terra, Mars: Implications to the alteration history of Mars

The Tyrrhena Terra region of Mars is studied with the imaging spectrometers OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) onboard Mars Express and CRISM (Compact Reconnaissance Infrared Spectrometer for Mars) onboard Mars Reconnaissance Orbiter, through the observation of tens of craters that impacted into this part of the martian highlands. The 175 detections of hydrated silicates are reported, mainly associated with ejecta blankets, crater walls and rims, and central up-lifts. Sizes of craters where hydrated silicates are detected are highly variable, diameters range from less than 1km to 42km. We report the presence of zeolites and phyllosilicates like prehnite, Mg-chlorite, Mg-rich smectites and mixed-layer chlorites–smectites and chlorite–vermiculite from comparison of hyperspectral infrared observations with laboratory spectra. These minerals are associated with fresh craters post-dating any aqueous activity. They likely represent ancient hydrated terrains excavated by the crater-forming impacts, and hence reveal the composition of the altered Noachian crust, although crater-related hydrothermal activity may have played a minor role for the largest craters (>20km in diameter). Most detected minerals formed over relatively high temperatures (100–300°C), likely due to aqueous alteration of the Noachian crust by regional low grade metamorphism from the Noachian thermal gradient and/or by extended hydrothermal systems associated with Noachian volcanism and ancient large impact craters. This is in contrast with some other phyllosilicate-bearing regions like Mawrth Vallis where smectites, kaolinites and hydrated silica were mainly identified, pointing to a predominance of surface/shallow sub-surface alteration; and where excavation by impacts played only a minor role. Smooth plains containing hydrated silicates are observed at the boundary between the Noachian altered crust, dissected by fluvial valleys, and the Hesperian unaltered volcanic plains. These plains may correspond to alluvial deposition of eroded material. The highlands of Tyrrhena Terra are therefore particularly well suited for investigating the diversity of hydrated minerals in ancient martian terrains.

Manganese deposits of the Devonian Magnitogorsk palaeovolcanic belt (Southern Urals, Russia)

Recent studies provide an overview on hydrothermal–volcaniclastic–sedimentary manganese deposits from the Magnitigorsk palaeovolcanic belt in Southern Urals, Russia (~100 deposits). This report presents data on the structures of ore bodies, chemical composition, and mineralogy of manganese ores. These deposits are divided into two types with different ore-bearing band structures, chemical and mineral ore compositions, and ore component accumulation conditions. Manganese ores of the type 1 deposit are associated with jasperites. Ores from type 2 deposits are localised within thin-banded jaspers. Type 1 deposits are thought to form during hydrothermal activity in areas where hydrothermal fluids leak into the seafloor. Type 2 deposits are formed within a closed basin as hydrothermal activity decreases away from hydrothermal vents, and typically contain a greater volcaniclastic contribution. Manganese ores comprised more than 70 mineral types and form three groups of mineral assemblages: (a) minerals formed during low grade regional metamorphism (T≈200–260°C, P=2–3kbar) that compose the main part of ore bodies, (b) mineral assemblages in segregatic and metasomatic veins filling late stage tectonic fractures, and (c) an assemblage of subsurface supergene minerals. Most of these minerals (which total approximately 38 minerals) are metamorphic silicates. The formation of metamorphic mineral assemblages is controlled by the presence of organic material (OM) in primary sediments. The conditions under which type 1 deposits formed favour the accumulation of OM. The metamorphism of these sediments is responsible for the formation of oxide–carbonate–silicate (“reduced”) ores, which consist of Mn-silicates and carbonates (such as tephroite, rhodonite, caryopilite, and rhodochrosite), andradite, quartz, hematite, and hausmannite. Insignificant preserved quantities of OM were found in type 2 deposits. During the metamorphic alteration of these sediments, oxide-silicate (“oxidised”) ores were formed with the dominance of braunite.

Caracterização geotermobarométrica dos metabasitos de Cajamar (SP), Grupo São Roque, Cinturão Ribeira

Metamafic orthoderivate rocks, characterized by MORB signatures and paleoproterozoic ages occur in Cajamar region central east part of Sao Paulo State, Brazil. They are associated to philites, metasandstones and metalimestones of Sao Roque Group volcanosedimentary sequence, Southeastern portion of the Ribeira Belt. For the metamafic rocks the assemblage of metamorphic peak is represented by actinolite-albite-clinozoisite-chlorite typical of low-grade regional metamorphism, greenschist facies. To have a good characterization of the metamorphic P and T parameters of metamorphism microprobe analyses of main minerals were performed. Metamorphism did not destroy completely the primary mineralogy of original basalt and microgabbro, and augite crystals are registered in many places, preserved together with metamorphic assemblage, that indicate for the igneous temperatures of crystallization values between 900 and 1100 o C. Textures of mineral equilibrium for the peak paragenesis, are not always present in the studied samples because of the superposition of retrometamorphic events caused by regional shearing process. For this reason it becomes difficult the application of geothermometers and geobarometers. Where the equilibrium is reached, in the occurrences in the south of the area, temperature and pressure determinations indicate values close to 487 ± 42 o C and 8,2 ± 2.0kbar, respectively. Other values for P and T lower than that of the peak were also obtained in central and north portion of the area and they are: 382 o C for T e 5,7kbar for P (point 45) and 341 o C for T and 4,1kbar for P (point 77) and can indicate a metamorphic event of retrometamorphism. As a concluding remark one can say that the exumation process in this area occurred by isothermal decompression

Early Cambrian archaeocyathan limestone blocks in low-grade meta-conglomerate from El Jagüelito Formation (Sierra Grande, Río Negro, Argentina)

Massive grey limestone blocks containing a fairly diverse but poorly preserved archaeocyath fauna were recovered from a meta-conglomerate bed in the El Jaguelito Formation (Sierra Grande area, Eastern North Patagonian Massif, Rio Negro, Argentina). This is the first documented reference of the presence of archaeocyaths in continental Argentina. Seven different taxa were identified, preliminary described and figured. Recrystallization of the skeletons due to regional low-grade metamorphism and deformation of the unit does not allow observation of key detailed features and prevents identification to genera and species. Nevertheless, the specimens studied show general affinities with archaeocyathan assemblages from the Australia-Antarctica palaeobiogeographic province and indicate a middle Early Cambrian (Atdabanian-Botomian) maximum age for the deposition of the El Jaguelito Formation protoliths. The similarities between the North Patagonian Early Paleozoic El Jaguelito Formation and those rocks from Antarctica suggest a geologic and biologic common history of these regions on the same southwest margin of Gondwana during that time.

Mineralogy of metamorphosed manganese deposits of the South Urals

A mineralogical investigation of metamorphosed manganese rocks was carried out at ore deposits related to the Devonian volcanic complexes of the Magnitogorsk paleovolcanic belt of the South Urals. The mineralogical appearance of these rocks is determined by three consecutively formed groups of mineral assemblages: (1) assemblages occupying the main volume of orebodies and formed during low-grade regional metamorphism (T = 200−250°C, P = 2–3 kbar); (2) assemblages of segregated and metasomatic veinlets that fill the systems of late tectonic fractures; and (3) assemblages of near-surface supergene minerals. Sixty-one minerals have been identified in orebodies and crosscutting hydrothermal veinlets. The major minerals are quartz, hematite, hausmannite, braunite, tephroite, andradite, epidote, rhodonite, caryopilite, calcite, and rhodochrosite. The mineral assemblages of metamorphosed manganese rocks (metamanganolites) are characterized. Chemical compositions of braunite, epidote-group minerals, piemontite, pyroxenes, rhodonite, pyroxmangite, and winchite are considered. The bibliography on geology and mineralogy of the South Ural manganese deposits is given.

Eoalpine (Cretaceous) very low- to low-grade metamorphism recorded on the illite-muscovite-rich fraction of metasediments from South Tisia (eastern Mt Papuk, Croatia)

Eoalpine (Cretaceous) very low- to low-grade metamorphism recorded on the illite-muscovite-rich fraction of metasediments from South Tisia (eastern Mt Papuk, Croatia)Eoalpine very low- to low-grade metamorphism related to Cretaceous orogenesis has been investigated in the Slavonian Mts, Croatia. Samples belonging to the Psunj metamorphic complex (PMC), the Radlovac metamorphic complex (RMC) and Permian-Triassic and Triassic sedimentary sequences (PTSS) were studied. The Kübler and Árkai indices of all the analysed samples indicate high-anchizonal to epizonal metamorphism. The degree of Eoalpine metamorphism tends to be constant in all samples implying that the different complexes passed through and recorded the same event. Measurements of illite-white K-micab0-parameter of the RMC samples imply transitional low- to medium-pressure character of the metamorphism. These data together with K-Ar ages (~100-80 Ma) measured on illite-white K-mica rich < 2 μm grain-size fractions point to Late Cretaceous very low- to low-grade regional metamorphism presumably related to the main nappe-forming compressional events in the Pannonian Basin and the Carpathians. The P-T-t (pressure-temperature-time) evolution of the studied area is in good agreement with similar scenarios in the surrounding areas of Tisia, but also in Eastern Alps, Carpathians and Pannonian Basin (ALCAPA).

Development of Framboidal Pyrite During Diagenesis, Low-Grade Regional Metamorphism, and Hydrothermal Alteration

Distribution patterns, microstructural relationships, and compositional variations for framboidal pyrite in the Paleozoic sedimentary host rocks to gold deposits on the northern Carlin trend, Nevada, and at Fosterville in central Victoria, collectively demonstrate that it is not all of synsedimentary-early diagenetic origin. Framboidal pyrite also nucleated in hydrothermal veins formed at several kilometers depth and along dissolution surfaces (stylolites and crenulation cleavages) formed during metamorphism at up to anchizonal grade (i.e., prehnitepumpellyite facies). There are no obvious differences in either the size range or internal morphology of framboids formed in these various settings, suggesting that a single, relatively simple crystallization path may operate over a wide depth range in the upper crust. At Fosterville, anomalously metal-rich (including Au) framboidal pyrite is most abundant adjacent to laminated shear veins that developed by slip along bedding surfaces during folding of the sedimentary host rocks. Microstructural relationships and Pb isotope compositions for the framboidal pyrite, and other similarly distributed, early-formed sulfides, support development in response to the localized, episodic influx of metal-enriched H 2S-bearing hydrothermal fluids during regional deformation between 440 and 400 Ma. These findings may have implications for other similarly deformed metasedimentary successions, where the discovery of Au-bearing framboidal pyrite has recently been cited as evidence for significant Au enrichment during sedimentation. © 2009 Society of Economic Geologists, Inc.

The Bikkulovskoe manganese deposit (South Urals): Geological setting, composition of metalliferous rocks, and formation model

The results of investigation of the Bikkulovskoe manganese deposit confined to volcanosedimentary piles of the Magnitogorsk paleovolcanic belt are presented. The paper characterizes the geological setting of the deposit and mineral-chemical compositions of ores and enclosing rocks (volcanomictic sandstones; ferruginous, manganiferous, and ferruginous-siliceous tuffites; and jasperites). Analysis of the data obtained made it possible to identify four sequential stages of deposit formation: (1) accumulation and diagenesis of ore-bearing sequences (D2–3); (2) burial and low-grade (T = 200–250°C, P = 2 to 3 kbar) regional metamorphism of rocks (D2–3-C1); (3) tectonic deformations of volcanosedimentary piles (C2-P); and (4) hypergenesis and partial denudation of rocks (MZ-CZ). According to the model proposed for the accumulation of ore-bearing rocks, the productive member was formed in a zone of hydrothermal solution outflow to the seafloor surface. Discharge of solutions and precipitation of Fe and Si began below the seafloor surface (rather than above the surface) in the near-bottom sequence of volcanomictic sediments. Upon reaching the seafloor, the impoverished solutions mixed with seawater and gave up metals completely: Fe and Mn were transferred to sediments to make up the ore-bearing bed. Thus, zonal sediments with ferruginous tuffites at the base and manganese ores at the top were formed.

New U–Pb and Rb–Sr constraints on pre-Acadian tectonism in North Wales

A new U–Pb date of 615.2 ± 1.3 (2σ) Ma for the Twt Hill Granite, North Wales, contrasts with an Rb–Sr isochron age of 491 ± 12 (2σ) Ma from the same body. The latter age is thought to result from isotope resetting during regional low-grade metamorphism or fault reactivation. The Rb–Sr age also coincides with the onset of latest Cambrian to Early Tremadoc regression and is taken to reflect tectonic uplift prior to the Arenig overstep at around 478 Ma. It is proposed that this in turn reflects plate-scale processes along the contemporary peri-Gondwanan continental margin.

Monazite begets monazite: evidence for dissolution of detrital monazite and reprecipitation of syntectonic monazite during low-grade regional metamorphism

Back-scattered electron (BSE) imaging and X-ray element mapping of monazite in low-grade metasedimentary rocks from the Paleoproterozoic Stirling Range Formation, southwestern Australia, reveal the presence of distinct, high-Th cores surrounded by low-Th, inclusion-rich rims. Previous geochronology has shown that the monazite cores are older than 1.9 Ga and overlap with the ages of detrital zircon grains (∼3.5–2.0 Ga), consistent with a detrital origin. Many cores have scalloped and embayed surfaces indicating partial dissolution of former detrital grains. Textural evidence links the growth of the monazite rims (∼1.2 Ga) to deformation and regional metamorphism during the Mesoproterozoic Albany-Fraser orogeny. These results indicate that high-Th detrital monazite is unstable under low-grade metamorphic conditions (<400°C) and was partially or completely dissolved. Dissolution was followed by near-instantaneous reprecipitation and the formation of low-Th monazite and ThSiO4. This reaction is likely to operate in other low-grade metasedimentary rocks, resulting in the progressive replacement of detrital monazite by metamorphic monazite during regional prograde metamorphism.

Comment on 40Ar/39Ar age constraints on low-grade metamorphism and cleavage development in the Transvaal Supergroup (central Kaapvaal craton, South Africa): implications for the tectonic setting of the Bushveld Igneous Complex (South African Journal of Geology, 109, 393 410), by Alexandre et al. (2006)

Alexandre et al. (2006) recently presented the results of a detailed structural study of brittle-ductile deformation, which could, as they state, be seemingly coincident with regional low-grade metamorphism of the pre-2.15 Ga Transvaal Supergroup strata between the Vredefort impact structure of ~2.02 Ga age (Kamo et al. , 1996; Gibson et al. , 1997) and the Bushveld Complex that was intruded at ~2.06 Ga (Walraven, 1997; Buick et al. , 2001). Alexandre et al. (2006) also performed six 40Ar/39Ar step-heating dating experiments with synkinematic white mica specimens. The results comprise ages of ~2.15 Ga and 2042.1 ± 2.9 Ma, the latter derived from “plateaus and pseudo-plateaus, as well as cumulative probability statistics” ( ibid ). These authors suggest that “at ~2.04 Ga the rocks of the Transvaal Supergroup in the central [Kaapvaal] craton were a part of a more extensive fold-and-thrust belt, named here [ibid] the Transvaalide fold-and-thrust belt ”. Alexandre et al. (2006) admit the difficulties to explain the ~2.15 Ga phase (although they gave a try), but assign the time at ~2.04 Ga to “orogenic activity at the margins of the craton”. Alexandre et al. (2006) conclude (p.402) that “The age of 2042.1 ± 2.9 Ma is based on unambiguous plateau and pseudo-plateaus ([their] Figure 6), and therefore can be considered a robust and reliable one…”. They proceed to discuss that this age is clearly distinguishable from the ages of the Bushveld magmatic event and the Vredefort impact event, and that a definite event at that time was also supported by other chronological results for rocks from the entire region between the Limpopo Mobile Belt and the Vredefort impact structure. In the light of these authors’ conclusion based on the age of 2042.1 ± 2.9 Ma, it appears reasonable to scrutinize the argon chronological results …

Carbonaceous cherts of the Barberton Greenstone Belt, South Africa: Isotopic, chemical and structural characteristics of individual microstructures

Carbonaceous matter occurring in chert deposits of the 3.4–3.2 Ga old Barberton Greenstone Belt (BGB), South Africa, has experienced low grade regional metamorphism and variable degrees of local hydrothermal alteration. Here a detailed study is presented of in situ analysis of carbonaceous particles by LRS (laser Raman spectroscopy) and SIMS (secondary ion mass spectrometry), reporting degree of structural disorder, carbon isotope ratio and nitrogen-to-carbon ratio. This combination of in situ analytical tools is used to interpret the δ 13C values of only the best preserved carbonaceous remains, enabling the rejection of non-indigenous (unmetamorphosed) material as well as the exclusion of strongly hydrothermally altered carbonaceous particles. Raman spectroscopy confirmed that all carbonaceous cherts studied here have experienced a regional sub- to lower-greenschist facies metamorphic event. Although this identifies these organics as indigenous to the cherts, it is inferred from petrographic observations that hydrothermal alteration has caused small scale migration and re-deposition of organics. This suggest that morphological interpretation of these carbonaceous particles, and in general of putative microfossils or microlaminae in hydrothermally altered early Archean cherts, should be made with caution. A chert in the Hooggenoeg Formation, which is older than and has been hydrothermally altered by a volcanic event 3445 Ma ago, contains strongly altered carbonaceous particles with a uniform N/C-ratio of 0.001 and a range of δ 13C that is shifted from its original value. Cherts of the Kromberg Formation post-date this volcanic event, and contain carbonaceous particles with a N/C-ratio between 0.002 and 0.006. Both the Buck Reef Chert and the Footbridge Chert of the Kromberg Formation have retained fairly well-preserved δ 13C values, with ranges from −34‰ to −24‰ and −40‰ to −32 ‰, respectively. Abiologic reactions associated with hydrothermal serpentinization of ultramafic crust (such as Fischer–Tropsch synthesis) were an unlikely source for carbonaceous material in these cherts. The carbonaceous matter in these cherts has all the characteristics of metamorphosed biologic material.

40Ar/39Ar age constraints on low-grade metamorphism and cleavage development in the Transvaal Supergroup (central Kaapvaal craton, South Africa): implications for the tectonic setting of the Bushveld Igneous Complex

Rocks of the Palaeoproterozoic Transvaal Supergroup, situated between the Bushveld Igneous Complex and the Vredefort Impact Structure in the central parts of the Kaapvaal craton (South Africa), experienced a regional low-grade metamorphism synchronous with brittle–ductile deformation that formed small- to medium-scale folds, cleavages, monoclines and thrusts in the dolomites, slates, and phyllites. Single-grain 40Ar/39Ar step-heating laser probe dating of synkinematic white mica from the phyllites indicates ages of ~2.15 Ga and 2042.1 ± 2.9 Ma (plateaus and pseudo-plateaus, as well as cumulative probability statistics). These ages are clearly distinct from the emplacement age of the Bushveld Igneous Complex (~2.06 Ga) and from the Vredefort impact event (~2.02 Ga), both of which have previously been suggested as possible causes of the deformation. The new data suggest that at ~2.04 Ga, the rocks of the Transvaal Supergroup in the central craton were part of a more extensive fold-and-thrust belt, named here the Transvaalide fold-and-thrust belt. While it is difficult to explain the significance of the ~2.15 Ga ages, the ~2.04 Ga event may be related to orogenic activity along the margins of the Kaapvaal craton at this time.

Testing ore deposit models using in situ U-Pb geochronology of hydrothermal monazite: Paleoproterozoic gold mineralization in northern Australia

The inability to establish absolute ages for gold deposition in the Pine Creek orogen of northern Australia has led to conflicting ore deposit models, ranging from intrusion related, which predict that gold mineralization was synchronous with granite magmatism (ca. 1835–1820 Ma), to orogenic, which place ore deposition nearly 100 m.y. later. Here we present ion microprobe U-Pb geochronology for a mineralized quartz reef from Tom's Gully mine, Mount Bundey, Northern Territory, Australia, and nearby granitic rocks and associated contact aureoles. Isotopic dating of zircon and monazite indicates that intrusion and contact metamorphism occurred ca. 1825 Ma, whereas hydrothermal monazite from the auriferous quartz reef gives a mean 207Pb/206Pb age of 1780 ± 10 Ma, interpreted as the time of gold mineralization. Mineralization therefore postdated intrusion by ∼45 m.y. and preceded a postulated ca. 1740–1730 Ma cratonwide orogenic gold event by ∼50 m.y. Hence, neither the intrusion-related model nor the recently proposed orogenic model is applicable. Combined with a reevaluation of age data from the nearby Goodall gold deposit, our data suggest that mineralization coincides with, and may be related to, an episode of regional low-grade metamorphism, deformation, and fluid circulation (Shoobridge event). Our results demonstrate the importance of high-precision in situ geochronology and detailed petrography for deciphering age relationships in ore deposits, and of testing the veracity of models for ore formation.

Mesozoic evolution of the Tisza Mega-unit

The south-eastern part of the basement of the Pannonian Basin is made up of Variscan crystalline complexes and early Mesozoic formations showing striking affinity with the corresponding formations in the southern margin of the European Plate. This large composite structural unit, which is actually an exotic terrane of European Plate origin, has been named the Tisza Mega-unit. Based upon relevant data of the pre-Tertiary basement of southern Hungary the reconstruction of the position of the Tisza Terrane in the early Alpine evolutionary stages, the process of its separation and break-off from the European Plate, and results of its Eo-Alpine deformations are summarised in the present paper. In the Variscan and early Alpine evolutionary stages the area of the later Tisza Mega-unit was located at the margin of the European Plate. During Variscan orogeny terrane accretion led to intensive deformation and metamorphism in this belt. This was followed by transpressional tectonics and the development of molasse basins in the late and post-Variscan stages, and passive margin evolution after the Neotethys opening in the Middle Triassic. The separation of the Tisza Mega-unit began with incipient continental rifting along the axis of the later Ligurian–Penninic–Vahic oceanic branch in the Late Triassic. The end of terrigenous material deposition in the most external zones, and a coeval change in fossil assemblage, point to the separation of the Tisza Block from the European Plate in the Early Bathonian. Significant rotation of the Tisza Mega-unit and coeval paroxysm of alkaline rift-type basalt volcanism took place in the Early Cretaceous. In the mid-Cretaceous, due to the northward motion of the Adria Block and the related closure of the westernmost Neotethys basin, the extensional regime changed to a compressional one, leading to onset of the nappe stacking and low-grade regional metamorphism within the Tisza microplate. In the foreland of the nappe systems flexural basins came into existence that are characterised by flysch-type sedimentation. In the Early Tertiary the north-eastward motion of the Alcapa and Tisza + Dacia Blocks led to the formation of the present-day heterogeneous basement of the Pannonian Basin.

Origin and significance of aromatic hydrocarbons in giant iron ore deposits of the late Archean Hamersley Basin, Western Australia

Late Archean to earliest Paleoproterozoic shales associated with two giant iron ore deposits in the Hamersley Province, Western Australia, contain traces of solvent extractable saturated and aromatic hydrocarbons. The host rocks belong to the ∼2.5 billion years (Ga) old Mt McRae Shale and Brockman Iron Formation, Hamersley Group, and were collected in mines near Tom Price and Newman (Mt Whaleback). The saturated hydrocarbons in the rock extracts have the composition of highly mature gas condensates. The aromatic fraction predominantly consists of unsubstituted two and three ring hydrocarbons that are demonstrably indigenous and syngenetic based on their unusual pyrolytic composition and the presence of hydrocarbons with similar attributes covalently bound to the kerogen. The bitumen composition is interpreted as recording the interaction of organic matter with hydrothermal fluids or oxidizing solutions overprinted by regional low-grade metamorphism. The organic matter could potentially have recorded flow direction, redox potential and temperature of fluids that formed the iron deposits. Thus, a new organic geochemical approach is suggested for determining hypogene and supergene controls on iron mineralization in the Hamersley Province and related ore deposits.

PGE-BEARING MINERALS IN SILURIAN SEDEX DEPOSITS IN THE POBLET AREA, SOUTHWESTERN CATALONIA, SPAIN

The Lower Llandoverian metasedimentary series of the Prades Mountains, in southwestern Catalonia, Spain, consists of interbedded massive feldspar beds, composed of almost pure anorthite, with chert, phosphate beds, sulfide-rich black shale, massive sulfides and minor calc-silicate beds. The sulfides mainly consist of pyrrhotite with minor chalcopyrite. The metasedimentary rocks are anomalously V- and Cr-rich and contain disseminated minerals bearing precious metals, e.g. , sperrylite and palladian lollingite. On the basis of the geological setting of ore formation and the textural relationships among the minerals, a sedex model is proposed for the primary origin of these deposits and occurrences. However, textural patterns and mineral compositions were modified during several episodes. Firstly, Hercynian deformation and associated regional low-grade metamorphism produced cleavage and small-scale veining in the primary associations. Later, contact metamorphism related to Late Hercynian intrusions annealed the mineral associations. A sulfidation stage of mineralization caused replacement of Pd-bearing lollingite by arsenopyrite. During this process, the arsenopyrite structure was unable to accommodate such high Pd contents and, consequently, Pd was precipitated as PdAs 2 and scarce small grains of native Pd, disseminated along the lollingite – arsenopyrite contact, in association with argentian gold, and tellurides and selenides of Au, Ag, Bi and Pb.

'Mantle plume' type magmatism in the Central Belt of Peninsular Malaysia and its tectonic implications

None of the existing tectonic models of Peninsular Malaysia fully explains the spatial, age or geochemical characteristics of the Central Belt intermediate to basic igneous rocks. The Central Belt granitoids, which lie critically close to the Bentong-Raub Line, have distinct geochemical characteristics. They have very high LIL elements, i.e. Ba and Sr are nearly 1,000 times rock/mantle. The high Ba and Sr values may result from the penetration of the lower lithosphere by a small volume of mantle material that is enriched in those elements attributed to 'mantle plume' type magmatism. Other supporting evidences include the presence of bimodal magmatism, the presence of mafic enclaves of older granitoids in younger granitoids and the new age data indicated that there is a significant time interval (up to between 30 Ma) between the first mafic magmatism and the later felsic magmatism; the Central Belt plutons are post­ orogenic and penecontemporaneus with rapid post-orogenic uplift, and erosion with the development of the Jurassic­ Cretaceous continental deposits of the Central Basin; the presence of thin continental crust beneath the Central Belt; the Benom granite has yielded an initial 87Srj86Sr ratio of 0.7079, which points to an origin in highly enriched lithospheric mantle; and the presence of mafic enclaves is consistent with a mafic lower crust beneath central belt formed by underplating. A model have been proposed that involved the oblique convergence of the two tectonic provinces of Peninsular Malaysia where slab breakoff which is the natural consequence of the attempted subduction of the continental crust is invoked to account for the 'mantle-plume' type magmatism of the Central Belt. The outcome of the slab breakoff is the long linear belt of single plutons characterized by high-K, shoshonitic granitoids with characteristic trace element signatures, specifically high Ba and Sr, which lie critically close to the Bentong-Raub Line. Other features include the bimodal association of mafic and felsic rocks, the low grade regional metamorphism, thinned continental crust, rapid uplift and erosion with the development of extensionaVtranstensional basins.