Primary Mineral Assemblage Research Articles

Mineral chemical and geochronological constraints on the age and provenance of the eastern Circum-Rhodope Belt low-grade metasedimentary rocks, NE Greece

In north-eastern Greece the mid-greenschist facies Makri Unit and the anchizonal Melia Formation belong to the eastern Circum-Rhodope Belt that forms the uppermost tectonostratigraphic unit of the Rhodope metamorphic nappe pile. The two metasedimentary successions had different source areas, although they now lie in close proximity in the Rhodope Massif. The U–Pb isotopic ages of detrital zircons from a metasandstone of the Makri Unit analysed using LA–SF–ICP–MS and SHRIMP-II gave age clusters at ca. 310–290 Ma and at ca. 240 Ma for magmatic zircons, which may have been derived from Carboniferous–Permian basement rocks of the Thracia Terrane (Lower Tectonic Unit of the Rhodope Massif) that subsequently underwent Triassic rifting. The youngest detrital zircon grains found so far indicate that the metasedimentary succession of the Makri Unit, or at least parts of it, cannot be older than Late Triassic. By contrast, clastic sedimentary rocks of the Melia Formation contain the primary detrital mineral assemblage of epidote, zoisite, garnet, and phengitic mica, which is absent in the Makri Unit, and clearly points to metamorphic rocks being the major source for these sediments. U–Pb analyses of detrital zircons gave a prominent age cluster at ca. 315–285 Ma for magmatic zircons. Inherited cores indicate the involvement of Pan-African and Late Ordovician–Early Silurian crustal sources during Late Carboniferous–Early Permian igneous event(s). Moreover, U–Pb detrital zircon geochronology indicates that the Melia Formation cannot be older than latest Middle Jurassic. We suggest that the Melia Formation was deposited in front of a metamorphic nappe pile with Rhodopean affinities in Tithonian or Cretaceous times. Both the Makri Unit and the Melia Formation have been tectonically juxtaposed from different sources to their present location during Balkan and Alpine orogenic processes.

Vapor-condensed phase processes in the early solar system

Abstract– Equilibrium thermodynamic calculations of the sequence of condensation of phases from a cooling gas of solar composition at total pressures thought to have prevailed in the inner part of the solar nebula successfully predict the primary mineral assemblages of refractory inclusions in CM2 and CV3 chondrites. Many refractory inclusions in CM2 chondrites contain a relatively SiO2-poor assemblage (spinel, hibonite, grossite, perovskite, corundum) that represents a high-temperature stage of condensation, and some may be pristine condensates that escaped later melting. Compact Type A and Type B refractory inclusions, consisting of spinel, melilite, perovskite, Ca-rich clinopyroxene ± anorthite, in CV3 chondrites are more SiO2-rich and equilibrated with the solar nebular gas at a slightly lower temperature. Textures of many of these objects indicate that they underwent melting after condensation, crystallizing into the same phase assemblage as their precursors. The Ti3+/Ti4+ ratio of their pyroxene indicates that this process occurred in a gas whose oxygen fugacity () was approximately 8.5 log units below that of the iron-wüstite buffer, making them the only objects in chondrites known to have formed in a system whose composition was close to that of the sun. Relative to CI chondrites, these inclusions are uniformly enriched in a group of elements (e.g., Ca, REE, Zr, Ta, Ir) that are chemically diverse except for their high condensation temperatures in a system of solar composition. The enrichment factor, 17.5, can be interpreted to mean that these objects represent either the first 5.7 wt% of the condensable matter to condense during nebular cooling or the residue after vaporization of 94.3% of a CI chondrite precursor. The Mg and Si isotopic compositions of Types A and B inclusions are mass-fractionated by up to 10 and 4 ‰/amu, respectively. When interpreted in terms of Rayleigh fractionation during evaporation of Mg and Si from the inclusions while they were molten, the isotopic compositions imply that up to 60% of the Mg and up to 25% of the Si were evaporated, and that approximately 80% of the enrichment in refractory (CaO+Al2O3) relative to more volatile (MgO+SiO2) in the average inclusion is due to initial condensation and approximately 20% due to subsequent evaporation. The mineralogical composition, including the Ti3+/Ti4+ ratio of the pyroxene, in Inti, a particle sampled from Comet Wild 2 by the Stardust spacecraft, is nearly identical to that of a Type B inclusion, indicating that comets contain not only the lowest-temperature condensates in the form of ices but the highest-temperature condensates as well. The FeO/(FeO+MgO) ratios of olivine and pyroxene in the matrix and chondrules of carbonaceous and ordinary chondrites are too high to be made in a system of solar composition, requiring s only 1 or 2 log units below iron-wüstite, more than 105 times higher than that of a solar gas. Various ways have been devised to generate cosmic gases sufficiently oxidizing to stabilize significant FeO in olivine at temperatures above those where Fe-Mg interdiffusion in olivine ceases. One is by vertical settling of dust toward the nebular midplane, enriching a region in dust relative to gas. Because dust is enriched in oxygen compared to carbon and hydrogen relative to solar composition, a higher results from total vaporization of the region, but the factor by which theoretical models have so far enriched the dust is 10 times too low. Another is by transporting icy bodies from the outer part of the nebula into the hot, inner part where vaporization of water ice occurs. Not only does this method fail to make the needed by a factor of 30–1000 but it also ignores simultaneous evaporation of carbon-bearing ices that would make the even lower.

Sedimentation on the inner shelf of the East China Sea: Magnetic properties, diagenesis and paleoclimate implications

High-resolution magnetic analysis was performed on the 19.22 m core MD06-3040 (27°43.3663´N, 121°46.8822´E; 46 m water depth) from the East China Sea (ECS) inner continental shelf, which was recovered during the IMAGES XIV MD155-Marco Polo 2 cruise of the R/V Marion Dufresne (IPEV). Magnetic grains are mainly transported to the inner shelf from erosion on mainland China, delivered by the Yangtze River. Nine 14C AMS dates indicate that the core spans about the last 10.4 ka, with an average sedimentation rate of nearly 2 mm/yr. The primary magnetic mineral assemblage of the uppermost sediments contains magnetite, with some contributions from hematite and greigite. Two main stepwise changes are observed in the magnetic mineral content, due to post-depositional reductive diagenesis. The first occurs from the top of the core to 3.5 m and is characterized by reduction of both the magnetite and hematite contents. The second occurs at 8.45–9 m and is characterized by a loss of fine-grained ferrimagnetic material. Variations in the magnetic mineral content of this core are related to geochemical and redox environment changes in the ECS, through the oxic–suboxic–sulfidic–methanic zones from top to bottom. Despite significant reduction of a large amount of the ferrimagnetic mineral content due to diagenesis, intervals characterized by lower magnetic mineral concentrations and higher coercivities are still identified at 0.45–0.6, 1.5–1.7, 2.9–3.1, 4.4–4.7, 5.3–5.7, 5.9–6.1 and 8.2–8.4 ka. These intervals are in phase, within dating errors, these intervals are in phase with weak summer monsoon events, which indicate that a relationship between magnetic results and paleoclimatic events is partially preserved.

Gold in the Brunswick No. 12 volcanogenic massive sulfide deposit, Bathurst Mining Camp, Canada: Evidence from bulk ore analysis and laser ablation ICP─MS data on sulfide phases

The 329-Mt Brunswick No. 12 volcanogenic massive sulfide deposit (total resource of 163 Mt at 10.4% Zn, 4.2% Pb, 0.34% Cu, and 115 g/t Ag) is hosted within a Middle Ordovician bimodal volcanic and sedimentary sequence. Massive sulfides are for the most part syngenetic, and the bulk of the sulfide ore occurs as a Zn–Pb-rich banded sulfide facies that forms an intimate relationship with a laterally extensive Algoma-type iron formation and defines the Brunswick Horizon. Zone refining of stratiform sulfides is considered to have resulted in the development of a large replacement-style Cu-rich basal sulfide facies, which is generally confined between the banded sulfide facies and an underlying stringer sulfide zone. Complex polyphase deformation and associated lower- to upper-greenschist facies regional metamorphism is responsible for the present geometry of the deposit. Textural modification has resulted in a general increase in grain size through the development of pyrite and arsenopyrite porphyroblasts, which tend to overprint primary mineral assemblages. Despite the heterogeneous ductile deformation, primary features have locally been preserved, such as fine-grained colloform pyrite and base and precious metal zonation within the Main Zone. Base metal and trace element abundances in massive sulfides from the Brunswick No. 12 deposit indicate two distinct geochemical associations. The basal sulfide facies, characterized by a proximal high-temperature hydrothermal signature (Cu–Co–Bi–Se), contains generally low Au contents averaging 0.39 ppm (n = 34). Conversely, Au is enriched in the banded sulfide facies, averaging 1.1 ppm Au (n = 21), and is associated with an exhalative suite of elements (Zn–Pb–As–Sb–Ag–Sn). Finely laminated sulfide lenses hosted by iron formation at the north end of the Main Zone are further enriched in Au, averaging 1.7 ppm (n = 41) and ranging up to 8.2 ppm. Laser ablation inductively coupled plasma-mass spectrometry (ICP-MS) analyses of pyrite (n = 97) from the north end of the Main Zone average 2.6 ppm Au and range from the detection limit (0.015 ppm) to 21 ppm. Overall, these analyses reveal a distinct Au–Sb–As–Ag–Hg–Mn association within pyrite grains. Gold is strongly enriched in large pseudo-primary masses of pyrite that exhibit relict banding and fine-grained cores; smaller euhedral pyrite porphyroblasts, and euhedral rims of metamorphic origin surrounding the pyrite masses, contain much less Au, Sb, Ag, As, and Sn. Arsenopyrite, occurring chiefly as late porphyroblasts, contains less Au, averaging 1.0 ppm and ranging from the detection limit (0.027 ppm) to 6.9 ppm. Depth profiles for single-spot laser ablation ICP-MS analyses of pyrite and arsenopyrite display uniform values of Au and an absence of discrete microscopic inclusions of Au-bearing minerals, which is consistent with chemically bonded Au in the sulfide structure. The pervasive correlation of Au with Sn in the Zn–Pb-rich banded sulfide facies suggests similar hydrothermal behavior during the waxing stages of deposition on the seafloor. Under high temperature (>350oC) and moderate- to low-pH conditions, Au and Sn in hydrothermal fluids would be transported as chlorocomplexes. An abrupt decrease in temperature and aH2S, accompanied by an increase in fO2 and pH during mixing with seawater, would lead to the simultaneous destabilization of both Au and Sn chlorocomplexes. The enrichment of Au in fine-grained laminated sulfides on the periphery of the deposit, accompanied by sporadic occurrences of barite and Fe-poor sphalerite, supports lower hydrothermal fluid temperatures analogous to white smoker activity on the flanks of a large volcanogenic massive sulfide system. In lower temperature (<350oC) and mildly acidic hydrothermal fluids, Au would be transported by thiocomplexes, which exhibit multifunctional (retrograde–prograde) solubility and a capacity to mobilize Au to the outer parts of the sulfide mound. The sluggish nature of this low-temperature venting together with larger variations in ambient fO2 could lead to a sharp enrichment of Au towards the stratigraphic hanging wall of massive sulfide deposits.

Geology of the western Arunachal Himalaya in parts of Tawang and west Kameng districts, Arunachal Pradesh

The Western Arunachal Himalaya comprises migmatitic gneisses of the Sela Group intruded by Tertiary leucogranite (tourmaline, muscovite and K-feldspar bearing), metasedimentaries of Lumla and Dirang Formations and granite gneiss of the Bomdila Group. The litho units of Sela Group are completely migmatised with distinct leuco, melano and mesosomes (Neosome). The mineral assemblages indicate a sedimentary protolith (para gneiss). Concordant metabasic bodies are observed (rarely discordant) indicating older dykes/sills. The migmatites of Sela Group are of Significantly higher grade than the tectonically underlying Dirang Formation, characterized by kyanite ± biotite± staurolite ± sillimanite assemblages. The tourmaline bearing pegmatitic High Himalayan leucogranite crops out in the higher altitudes of the High Himalayan Sequence. The primary mineral assemblages recorded are tourmaline + muscovite + biotite + garnet apart from quartz and K-feldspar. This intrudes the migmatite of Sela Group parallel as well as oblique to earlier gneissosity as lens shaped bodies. The Sela's are thrust over the metasedimentary rocks of Dirang Formation. Earlier workers marked this thrust as the Main Central Thrust (MCT). During the present studies the whole Dirang Formation was found to be a high strained zone with many shear planes, almost all the lithounits are sheared with prolific graphite/carbon emplacement along the permeable shear planes. The Dirang Formation comprises quartzite, kyanite ± staurolite + garnet ± biotite schist, crystalline limestone and carbon phyllite. Basic and ultrabasic rocks intrude these lithounits. The Bomdila Gneiss, which forms a part of the Lesser Himalayan Sequence, is a large body of granite orthogneiss. The primary mineral assemblage includes quartz, K-feldspar, biotite and garnet. Though the gneisses have an intrusive relationship with the Dirang metasediments the contact is marked by a thrust. Metamorphic grade increases from the base of Dirang to the top of the Sela Group. The base of the Dirang Formation is characterized by biotite + garnet assemblages followed by kyanite + staurolite at the top the Dirang Formation. The base of the Sela Group is marked by kyanite ± garnet ± biotite followed by sillimanite ± biotite at the top of Sela Group. Two phases of deformations are recorded from Sela and Dirang rocks with type III interference pattern folds.

Aliphatic hydrocarbons in metasomatized gabbroic xenoliths from Hyblean diatremes (Sicily): Genesis in a serpentinite hydrothermal system

Many tholeiite gabbro xenoliths from the Hyblean tuff-breccia deposits (Sicily, southern Italy) present mineralogical and geochemical evidence for hydrothermal alteration at different temperatures and water/rock ratios. In some cases, the primary mineral assemblage has been entirely replaced by Na-rich alkali feldspar, chlorite/smectite interlayers, zeolites, aegirine–augite, titanite, zircon etc. Hence the chemical composition of such metasomatic rocks displays larger amounts of volatiles, alkalis, Zr, Hf, U, Th and lower Ca, Mg, Fe with respect to the original gabbro. Five hydrothermally altered gabbroic xenoliths were selected for thermal decrepitation and bulk gas analyses by quadrupole mass spectrometry. All the samples analyzed display the same Electron Impact-Direct Pyrolysis Mass spectra (EI-DPMS). These show a series of peaks differing by 14 mass units due to loss of methylene groups (–CH 2), by a fragmentation process typical of saturated aliphatic and aliphatic–aromatic hydrocarbons. In addition, Fourier Transform Infrared (FT-IR) spectra of the samples present several bands typical of vibration frequencies of aliphatic hydrocarbons. The high-molecular-weight hydrocarbons observed probably originated from Fischer–Tropsch-type (FT-t) synthesis in the high temperature section of a serpentinite-hosted hydrothermal system. This suggestion may lend support to the recent hypothesis regarding the original oceanic nature of the Hyblean lithospheric basement.

Variscan lamprophyres in the Lower Penninic domain (Central Alps): age and tectonic significance

Abstract Lamprophyre dykes have been recently discovered in blocks of gneiss embedded in a calcschist formation of wildflysch type that forms the top of the Mesozoic-Tertiary metasedimentary cover of the Antigorio nappe (the Teggiolo zone) in the Val Bavona (Lower Penninic, NW Ticino, Switzerland). The presence of the lamprophyres gives a clue to the possible source of these blocks. Similar dykes occur in the N part of the Maggia nappe where they are intruded into the Matorello granite and the surrounding gneisses. We studied these lamprophyres at two localities in the Teggiolo zone (Tamierpass and Lago del Zött) and at one locality in the Maggia nappe (Laghetti). Detailed mineralogical and geochemical investigations confirm their great similarity, particularly between the Tamier and Laghetti dykes. They all recrystallized during Alpine metamorphism under amphibolite facies conditions and lost their primary mineral assemblages and textures. The chemistry reveals a calc-alkaline affinity, a limited differentiation range, features of mineral accumulation and intense remobilization in some cases. The lamprophyres are characterized by a high mg# and relatively low contents in REE and other incompatible elements. In situ SHRIMP and LA-ICPMS U-Pb zircon dating yielded ages of 284.8 ± 1.7 Ma (Tamier), 290.0 ± 1.3 Ma (Zött) and 290.5 ± 3.7 Ma (Laghetti). These ages are compatible with the general late- to post-Variscan magmatic evolution of the Helvetic and Lower Penninic domains. The lamprophyres are considered as melts derived from the lithospheric Variscan mantle, variously hybridized and differentiated at the contact with crustal material during late- to post-orogenic extension. These lamprophyres are chemically distinct from earlier lamprophyres of Visean age, emplaced together with their associated granites in transcurrent fault zones during the Variscan orogenic compression. The similarity of these different dykes suggests that the front of the Maggia nappe is a likely source of the gneissic blocks embedded in the calcschists at the top of the Teggiolo zone. They would have been provided by the advancing Maggia nappe during its thrusting over the Antigorio nappe and simultaneous closure of the Teggiolo sedimentary basin.

Low-temperature hydrothermal alteration of intra-caldera tuffs, Miocene Tejeda caldera, Gran Canaria, Canary Islands

The Miocene Tejeda caldera on Gran Canaria erupted ~ 20 rhyolite–trachyte ignimbrites (Mogán Group 14–13.3 Ma), followed by ~ 20 phonolitic lava flows and ignimbrites (Fataga Group 13–8.5 Ma). Upper-Mogán tuffs have been severely altered immediately within the caldera margin, whereas extra-caldera Mogán ignimbrites, and overlying Fataga units, are apparently unaltered. The altered intra-caldera samples contain minerals characteristic of secondary fluid–rock interaction (clays, zeolites, adularia), and relics of the primary mineral assemblage identified in unaltered ignimbrites (K-feldspar, plagioclase, pyroxene, amphibole, and groundmass quartz). Major and trace-element data indicate that Si, Na, K, Pb, Sr, and Rb, were strongly mobilized during fluid–rock interaction, whereas Ti, Zr, and Nb behaved in a more refractory manner, experiencing only minor mobilization. The δ 18O values of the altered intra-caldera tuffs are significantly higher than in unaltered extra-caldera ignimbrites, consistent with an overall low-temperature alteration environment. Unaltered extra-caldera ignimbrites have δD values between − 110‰ and − 173‰, which may reflect Rayleigh-type magma degassing and/or post-depositional vapour release. The δD values of the altered intra-caldera tuffs range from − 52‰ to − 131‰, with ambient meteoric water at the alteration site estimated at ca. − 15‰. Interaction and equilibration of the intra-caldera tuffs with ambient meteoric water at low temperature can only account for whole-rock δD values of around − 45‰, given that ∆D clay–water is ca. − 30‰ at 100 °C, and decreases in magnitude at higher temperatures. All altered tuff samples have δD values that are substantially lower than − 45‰, indicating interaction with a meteoric water source with a δD value more negative than − 15‰, which may have been produced in low-temperature steam fumaroles. Supported by numerical modeling, our Gran Canaria data reflect the near-surface, epithermal part of a larger, fault-controlled hydrothermal system associated with the emplacement of the high-level Fataga magma chamber system. In this near-surface environment, fluid temperatures probably did not exceed 200–250 °C.

Banded iron formations of the calciphyre-metabasite-gneiss association in the East European Craton

The paper presents characteristics of the least studied iron formations of the East European Craton (Archean banded iron formations of the calciphyre-metabasite-gneiss association), a typical member of granulite complexes of the Ukrainian Shield, Belarussian-Baltic region, and Voronezh crystalline massif. They are mainly composed of diverse metasedimentary rocks: aluminous gneisses; silicate-magnetite, magnetite, and barren quartzites; eulysites; calciphyres; and marbles associated with metavolcanic rocks. Data on chemical compositions of the metasedimentary rocks are summarized for the first time and their possible primary mineral composition has been reconstructed using the MINLITH software. It is shown that they could be formed from a lithogenetic series of sediments linked by gradual transitions and geochemical commonness of sediments: from fine-grained terrigenous insufficiently mature sediments to chemogenic sediments depleted in terrigenous material (ferruginous-siliceous, ferruginous-siliceous-carbonate, siliceous-carbonate, and carbonate sediments). The inferred primary mineral assemblage indicates sedimentation in the central parts of large paleobasins in a reducing environment characterized by deficit of oxygen and excess of carbon dioxide. Lithological specifics of the banded iron formations in different regions presumably reflect different distances of sedimentation zones from submarine hydrothermal discharge sites and sources of terrigenous material. The banded iron formations at the present-day erosion section of basement represent metamorphosed fragments of the lateral-facies zoning of rocks of the Archean sedimentary basins (or a single basin) of the East European Craton. Unlike other Early Precambrian banded iron formations of the East European Craton, rocks of the calciphyre-metabasite-gneiss association are marked by a high Mn content.

CO2 Injection into Saline Carbonate Aquifer Formations II: Comparison of Numerical Simulations to Experiments

Sequestration of carbon dioxide in geological formations is an alternative way of managing extra carbon. Although there are a number of mathematical modeling studies related to this subject, experimental studies are limited and most studies focus on injection into sandstone reservoirs as opposed to carbonate ones. This study describes a fully coupled geochemical compositional equation-of-state compositional simulator (STARS) for the simulation of CO2 storage in saline aquifers. STARS models physical phenomena including (1) thermodynamics of sub- and supercritical CO2, and PVT properties of mixtures of CO2 with other fluids, including (saline) water; (2) fluid mechanics of single and multiphase flow when CO2 is injected into aquifers; (3) coupled hydrochemical effects due to interactions between CO2, reservoir fluids, and primary mineral assemblages; and (4) coupled hydromechanical effects, such as porosity and permeability change due to the aforementioned blocking of pores by carbonate particles and increased fluid pressures from CO2 injection. Matching computerized tomography monitored laboratory experiments showed the uses of the simulation model. In the simulations dissolution and deposition of calcite as well as adsorption of CO2 that showed the migration of CO2 and the dissociation of CO2 into HCO3 and its subsequent conversion into carbonate minerals were considered. It was observed that solubility and hydrodynamic storage of CO2 is larger compared to mineral trapping.

Rare-Metal Pegmatites from the Southeastern Desert, Egypt: Geology, Geochemistry, and Petrogenesis

The pegmatite province of the Southeastern Desert (SED) is part of a district that extends from eastern Egypt to the Red Sea (and covers an area of approximatly 120 km2). Rare-metal pegmatites within the SED are divided into three major types: (1) unzoned, Sn-mineralized bodies; (2) zoned Li-, Nb-, Ta-, and Be-bearing bodies; and (3) pegmatites containing colored, gem-quality tourmaline. Rb-Sr isotope data reflect a crustal origin for the rare-metal pegmatites and indicate that the original SED magma was generated during the regional metamorphic peak predating intrusion of posttectonic leucogranites. These bodies developed an early border zone consisting of coarse to very coarse-muscovite + quartz + alkali feldspar, followed by an intermediate zone of dominantly quartz + feldspar + muscovite rock. Garnet, tourmaline, beryl, galena, pyrite, amblygonite, apatite, and monazite are rare accessories in both zoned and unzoned pegmatites. Cassiterite occurs in replacement zones and along fractures in albite + quartz + muscovite-rich portions. The highest concentrations of coarse-grained cassiterite occur in irregular greisenized zones that consist dominantly of micaceous aggregates of green Li-rich muscovite, quartz, and albite. Different metasomatic post-solidification alterations include sodic and postassic metasomatism, greisenization and tourmalinization. The pegmatite-generating granites are metaluminous, showing a differentiation trend from coarsegrained, unfractionated, intermediate plagioclase-rich granite toward a highly fractionated, fine- to medium-grained, locally albite-rich rock. Economically important ore minerals were introduced by rare metal-bearing fluids—either primarily or during the breakdown of the primary mineral assemblages. These include niobiumtantalum oxides, Sn-oxides (cassiterite), Li-silicates (petalite, spodumene, euctyptite, and pollucite), Li-phosphates (amblygonite, montebrasite, and lithopilite), and minor REE minerals (Hfzircon, monazite, xenotime, thorian, loparite, and yttrio-fluorite). Pollucite is typically associated with spodumene, petalite, amblygonite, quartz, and feldspar. Primary pollucite has Si/Al ratios of 2.53 to 2.65 and CRK (Cs + Rb + K/Cs + Rb + K + Na + Ca + Li) of 79.5 to 82.2. Thorian loparite is essentially a member of the loparite (NaLREETi2O6)-lueshite (NaNbO3)-ThTi2O6-ThNb4O12 quaternary system with low or negligible contents of other end-members. The mineral compositionally evolves from niobian loparite to niobian thorian and thorian; loparite gave rise to ceriobetafite and belyankinite with high ThO2 content. Thorian loparite is metamict or partly metamict, and upon heating regains a structure close to that of synthetic loparite, NaLaTi2O6.

The LaRonde Penna Au-Rich Volcanogenic Massive Sulfide Deposit, Abitibi Greenstone Belt, Quebec: Part I. Geology and Geochronology

The LaRonde Penna Au-rich volcanogenic massive sulfide (VMS) deposit is the largest Au deposit currently mined in Canada (58.8 Mt at 4.31 g/t, containing 8.1 Moz of Au). It is part of the Doyon-Bousquet-LaRonde mining camp located in the eastern part of the Blake River Group of the Abitibi greenstone belt which is host to several of the world’s most important, present and past, Au-rich VMS deposits (e.g., Horne, Quemont, Bousquet, Bousquet 2-Dumagami). The LaRonde Penna deposit consists of massive to semimassive sulfide lenses (Au-Zn-Ag-Cu-Pb), stacked in the upper part of a steeply dipping, south-facing homoclinal volcanic sequence composed of extensive tholeiitic basaltic flows (Hebecourt Formation) overlain by tholeiitic to transitional, mafic to intermediate, effusive and volcaniclastic units at the base (lower member of the Bousquet Formation) and transitional to calc-alkaline, intermediate to felsic, effusive and intrusive rocks on top (upper member of the Bousquet Formation). The mafic to felsic volcanism of the Hebecourt Formation and of the lower member of the Bousquet Formation formed an extensive submarine basement or platform on which the intermediate to felsic rocks of the upper member of the Bousquet Formation were emplaced at restricted submarine eruptive centers or as shallow composite intrusive complexes. The submarine felsic volcanic rocks of the upper member of the Bousquet Formation are characterized by dacitic to rhyodacitic autoclastic (flow breccia) deposits that are cut and overlain by rhyodacitic and rhyolitic domes and/or partly extrusive cryptodomes and by intermediate to mafic sills and dikes. This volcanic architecture is thought to have been responsible for internal variations in ore and alteration styles, not only from one lens to another, but also along a single mineralized horizon or lens. In the upper part of the mine, the 20 North lens comprises a transposed pyrite-chalcopyrite (Au-Cu) stockwork (20N Au zone) overlain by a pyrite-sphalerite-galena-chalcopyrite-pyrrhotite (Zn-Ag-Pb) massive sulfide lens (20N Zn zone). The latter was formed, at least in part, by replacement of footwall rhyodacitic autoclastic deposits emplaced within a subbasin located between two rhyolite domes or cryptodomes. The 20N Zn zone tapers with depth in the mine and gives way to the 20N Au zone. At depth in the mine, the 20N Au zone consists of semimassive sulfides (Au-rich pyrite and chalcopyrite) enclosed by a large aluminous alteration halo on the margin of a large rhyolitic dome or cryptodome. U-Pb zircon geochronology gives ages of 2698.3 ± 0.8 and 2697.8 ± 1 Ma for the footwall and hanging-wall units of the 20 North lens, respectively. Thus, the formation of the 20 North lens was coeval with other VMS deposits in the Bousquet Formation and in the uppermost units of the Blake River Group. Although deformation and metamorphism have affected the primary mineral assemblages and the original geometry of the deposit, these events were not responsible for the different auriferous ore zones and alteration at LaRonde Penna. Studies of the LaRonde Penna deposit show that the hydrothermal system evolved in time and space from near-neutral seawater-dominated hydrothermal fluids, responsible for Au-Cu-Zn-Ag-Pb mineralization, to highly acidic fluids with possible direct magmatic contributions, responsible for Au ± Cu-rich ore and aluminous alteration. The different ore types and alteration reflect the evolving local volcanic setting described in this study.

Rare metal-bearing pegmatites from the Southeastern Desert of Egypt: Geology, geochemical characteristics, and petrogenesis

The pegmatite province of the Southeastern Desert (SED) is part of a pegmatite district that extends from Egypt (extends to 1200 km2). Rare metal pegmatites are divided into (1) unzoned, Sn-mineralized; (2) zoned Li, Nb, Ta and Be-bearing; and (3) pegmatites and pegmatites containing colored, gem-quality tourmaline. The Rb/Sr data reflect a crustal origin for the rare metal pegmatites and indicate that the original SED magma was generated during the peak of regional metamorphism and predates the intrusion of post-tectonic leucogranites. These bodies developed an early border zone consisting of coarse to very coarse muscovite+quartz+alkali feldspar, followed by an intermediate zone of dominant quartz+feldspar+muscovite rock. Garnet, tourmaline, beryl, galena, pyrite, amblygonite, apatite and monazite are rare accessories in both zones. Cassiterite tends to concentrate in replacement zones and along fractures in albite+quartz+muscovite-rich portions. The highest concentrations of cassiterite occur in irregular greisenized zones which consist dominantly of micaceous aggregates of green Li-rich muscovite, quartz, albite and coarse-grained cassiterite. The different metasomatic post-solidification alterations include sodic and potassic metasomatism, greisenization and tourmalinization. Geochemically, the pegmatite-generating granites have a metaluminous composition, showing a differentiation trend from coarse-grained, unfractionated plagioclase-rich granite towards highly fractionated fine-to medium-grained, local albite-rich rock. Economically important ore minerals introduced by volatile-rich, rare metal-bearing fluids, either primarily or during the breakdown of the primary mineral assemblages, are niobium-tantalum oxides, Sn-oxides (cassiterite), Li-silicates (petalite, spodumene, euctyptite, and pollucite), Li-phosphates (amblygonite, montebrasite and lithopilite) and minor REE-minerals (Hf-zircon, monazite, xenotime, thorian, loparite and yttrio-fluorite). The pollucite is typically associated with spodumene, petalite, amblygonite, quartz and feldspar. The primary pollucite has Si/Al (at) ratios of 2.53–2.65 and CRK of 79.5–82.2. Thorian loparite is essentially a member of the loparite (NaLREETi2O6)-lueshite (NaNbO3)-ThTi2O6-ThNb4O12 quaternary system with low or negligible contents of other end-member compositions. The mineral compositionally evolved from niobian loparite to niobian thorian and thorian loparite gave rise to ceriobetafite and belyankinite with high ThO2 contents. Thorian loparite is metamict or partly metamict and upon heating regains a structure close to that of synthetic loparite NaLaTi2O6.

Identification of chemical sedimentary protoliths using iron isotopes in the > 3750 Ma Nuvvuagittuq supracrustal belt, Canada

An Eoarchean supracrustal belt dated at ca. 3750 Ma was recently identified in the Innuksuac Complex, northern Québec (Canada). Rocks from the Nuvvuagittuq locality include mafic and ultramafic amphibolites, quartz–biotite and pelitic schists, orthogneisses, and banded quartz–magnetite–amphibole/pyroxene rocks of probable chemical sedimentary origin. The purported metasediments are enriched in the heavy isotopes of Fe by approximately 0.3‰/amu relative to IRMM-014. They also have high Fe/Ti ratios, up to 100× that of associated amphibolite units. These signatures demonstrate that quartz–magnetite–amphibole/pyroxene rocks from Nuvvuagittuq are chemical sediments (e.g. banded iron-formations, BIFs) formed by precipitation of dissolved ferrous iron in a marine setting. All units were metamorphosed to upper amphibolite facies, which partly homogenized Fe isotopes. Variable Fe isotope compositions of bulk quartz–magnetite rocks are interpreted to reflect binary mixing between primary oxides and carbonates. Mixing relationships with major element chemistry (Ca/Fe, Mg/Fe, and Mn/Fe) are used to estimate the Fe isotope composition of the primary Fe-oxide phase (0.3 to 0.4‰/amu) and the chemistry of the carbonate (siderite and ankerite). Iron isotopes can thus be used to constrain the primary mineralogy of Fe-rich chemical sedimentary precipitates before metamorphism. The possible presence of siderite in the primary mineral assemblage supports deposition under high PCO 2. We developed an isotope distillation model that includes two possible abiotic oxidation paths, homogeneous and heterogeneous. The isotopic composition of Fe in the precursor phase of magnetite in BIFs can be explained by partial oxidation through oxygenic or anoxygenic photosynthesis of Fe from a hydrothermal source.

Trace element fractionation in deep subduction zones inferred from a lawsonite-eclogite xenolith from the Colorado Plateau

A coesite-bearing, lawsonite-eclogite xenolith from the Colorado Plateau, interpreted as a fragment of the subducted Farallon plate, is used to characterize trace element behavior in subducted oceanic crust. The xenolith consists of almandine-rich garnet, omphacite, lawsonite, phengite, rutile, pyrite and zircon as the primary mineral assemblage. Garnet crystals are extremely zoned with respect to their Mn contents, with core to rim variation from ∼ 1.4 to ∼ 0.2 wt.%. The euhedral zoning feature of garnet crystals and its included mineral assemblages suggest that the garnet continued to grow in the coesite stability field during prograde lawsonite eclogite facies metamorphism. In the lawsonite-eclogite xenoliths, garnet dominates the heavy rare earth elements (HREE), and lawsonite dominates both light rare earth elements (LREE) and Sr inventories. Combining the mineralogical and petrographic observations with precise spatial resolution ion microprobe analyses (< 15 μm) of zoned garnet as well as lawsonite inclusions in garnet, we investigated trace element fractionation in coesite stability field during lawsonite eclogite facies metamorphism. Garnet shows progressive HREE depletion from core to rim, suggesting that HREE, which once partitioned into garnet crystal, would not be involved in postdated metamorphic reactions due to the high partition coefficients of HREE into garnet. Lawsonite inclusions in garnet, which represent lower metamorphic condition relative to lawsonite in the matrix, have LREE concentrations ∼ 10 times lower than those of matrix lawsonite. On the contrary, the concentration of Sr in the included lawsonite is (< 20 relative %) lower than that of the matrix lawsonite. Based on constraints from metamorphic history recorded in the prograde-zoned garnet and mass balance among all constituent minerals in the lawsonite-eclogite xenolith, this contrasting feature for Sr and LREE of lawsonite is most plausibly explained by the hypothesis that allanite coexisting with included lawsonite might have decomposed during prograde metamorphism. The LREE released from the decomposing allanite would have been incorporated into lawsonite crystals. Consequently, REE and Sr could be retained in subducting oceanic crust even in the coesite stability field, if the slab is sufficiently cold enough to pass though the lawsonite eclogite facies.

Mineralogical and geochemical study of element mobility at the sulfide-rich Excelsior waste rock dump from the polymetallic Zn–Pb–(Ag–Bi–Cu) deposit, Cerro de Pasco, Peru

We present a mineralogical and geochemical study of the sulfide-rich waste rock dump Excelsior (size: 94 ha) originating from the Cerro de Pasco mine (altitude: 4300 m a.s.l.), Central Andes, Peru. The aims of this study were: to characterise (1) the secondary mineral assemblage and (2) the acid mine drainage (AMD) from this waste rock dump. (3) This information was used to create a model of the element transport in and out of the waste rock dump under the highly variable mountain climate with high precipitation during the wet season and high evaporation during the dry season. The main ore minerals found in the polymetallic Pb–Zn–(Ag–Bi–Cu) deposit are pyrite, sphalerite, galena and enargite with minor finds of tennantite, covellite and cerrusite. Gangue was dominantly quartz ± hematite ± siderite ± muscovite. The waste rocks had a high potential of acid generation with low neutralization potential (estimated > 60 wt.% pyrite, < 5 wt.% calcite/dolomite) with an already acidic environment (average paste – pH 2.8). Gypsum, different types of jarosite and a variety of Fe-sulfates (e.g. melanterite (Fe(SO 4)·7H 2O) and rozenite (Fe(SO 4)·4H 2O)) were the dominant secondary minerals. Less frequent secondary minerals were: Fe(III)-hydroxides (schwertmannite (Fe 16O 16(OH) 12(SO 4) 2) , fibroferrite (Fe(SO 4)(OH)·5H 2O)), and Mg-, Mn- and Zn-sulfates (e.g. starkeyite (Mg(SO 4)·4H 2O), mallardite (Mn(SO 4)·7H 2O), goslarite (Zn(SO 4)·7H 2O), respectively). For the primary mineral assemblage, X-ray fluorescence analyses displayed average concentrations of the heavy metals: Cu 0.1 wt.%, Zn 1.1 wt.%, Pb 1.2 wt.% and Cd 40 mg/kg and of the metalloid As 0.15 wt.%. In secondary minerals an average enrichment of Cu: 0.8 wt.%, Zn: 2.9 wt.%, As: 0.27 wt.% and Cd: 71 mg/kg was observed. Effects of rain events on the waste rocks were simulated by water-leach tests of (1) solid samples from the top, representing waste rocks leached by infiltrating rainwater and (2) solid samples around the base, representing waste rocks affected by outcropping AMD generated in the waste rock dump. The leachates showed stored acidity (pH 1.2–5.6 of the leachates) and high solubility of secondary minerals (average electrical conductivity: 9.7 mS/cm and average concentrations for Fe: 928 mg/L, Zn: 315 mg/L, Cu: 7.5 mg/L, As: 4.1 mg/L, Cd: 1.59 mg/L, and Pb: 0.31 mg/L). The AMD output at springs at the base of the waste rock dump is chemically similar to the leachates (acidic pH (2.78–5.10), oxidizing E h (319–684 mV)), but higher charged with maximum concentrations for Fe: 5640 mg/L, Zn: 3000 mg/L, Cu: 161 mg/L, As: 8.0 mg/L, Cd: 6.6 mg/L, and with higher electrical conductivity (19–26 mS/cm). The data suggest that AMD formation is strongly controlled by the local climate. During the dry winter season, high evaporation of outcropping pore solutions and subsequently precipitation of efflorescent salts resulted in a heavy metal-enrichment at the base of the Excelsior waste rock dump. During the wet summer season, rain events caused the dissolution of most efflorescent salts, removed the enrichment at the base and resulted in a washout of acid solutions rich in Fe, Mn, Zn, Cu, Cd, As and S.

Low-temperature hydrothermal alteration of intra-caldera tuffs, Miocene Tejeda caldera, Gran Canaria, Canary Islands

ARTICLE I NFO The Miocene Tejeda caldera on Gran Canaria erupted ~20 rhyolite-trachyte ignimbrites (Mogan Group 14- 13.3 Ma), followed by ~20 phonolitic lava flows and ignimbrites (Fataga Group 13-8.5 Ma). Upper-Mogan tuffs have been severelyaltered immediately within the caldera margin, whereas extra-caldera Mogan ignimbrites, and overlying Fataga units, are apparently unaltered. The altered intra-caldera samples contain minerals characteristic of secondary fluid-rock interaction (clays, zeolites, adularia), and relics of the primary mineral assemblage identified in unaltered ignimbrites (K-feldspar,plagioclase, pyroxene, amphibole, and groundmass quartz). Major and trace-element data indicate that Si, Na, K, Pb, Sr, and Rb, were strongly mobilized during fluid-rock interaction, whereas Ti, Zr, and Nb behaved in a more refractory manner, experiencing only minor mobilization. The δ 18 O values of the altered intra-caldera tuffs are significantly higher than in unaltered extra- caldera ignimbrites, consistent with an overall low-temperature alteration environment. Unaltered extra- caldera ignimbrites have δD values between −110 ‰ and −173‰, which may reflect Rayleigh-type magma degassing and/or post-depositional vapour release. The δD values of the altered intra-caldera tuffs range from −52‰ to −131‰, with ambient meteoric water at the alteration site estimated at ca. −15‰. Interaction and equilibration of the intra-caldera tuffs with ambient meteoric water at low temperature can only account for whole-rock δD values of around −45‰, given that ΔDclay-water is ca. −30‰ at 100 °C, and decreases in

Exhumation History of a Garnet Pyroxenite-bearing Mantle Section from a Continent-Ocean Transition (Northern Apennine Ophiolites, Italy)

Garnet clinopyroxenite and garnet websterite layers occur locally within mantle peridotite bodies from the External Liguride Jurassic ophiolites (Northern Apennines, Italy). These ophiolites were derived from an ocean–continent transition similar to the presentday western Iberian margin. The garnet clinopyroxenites are mafic rocks with a primary mineral assemblage of pyrope-rich garnet þ sodic Al-augite (Na2O 2.5 wt %, Al2O3 12.5 wt %), with accessory graphite, Fe–Ni sulphides and rutile. Decompression caused Na-rich plagioclase (An50–45) exsolution in clinopyroxene porphyroclasts and extensive development of symplectites composed of secondary orthopyroxene þ plagioclase (An85–72) þ Al-spinel ± clinopyroxene ± ilmenite at the interface between garnet and primary clinopyroxene. Further decompression is recorded by the development of an olivine þ plagioclase-bearing assemblage, locally under syn-kinematic conditions, at the expense of two-pyroxenes þ Al-spinel. Mg-rich garnet has been also found in the websterite layers, which are commonly characterized by the occurrence of symplectites made of orthopyroxene þ Al-spinel ± clinopyroxene. The enclosing peridotites are Ti-amphibole-bearing lherzolites with a fertile geochemical signature and a widespread plagioclase-facies mylonitic foliation, which preserve in places a spinel tectonite fabric. Lu–Hf and Sm–Nd mineral isochrons (220 ± 13 Ma and 186.0 ± 1.8 Ma, respectively) have been obtained from a garnet clinopyroxenite layer and interpreted as cooling ages. Geothermobarometric estimates for the high-pressure equilibration have yielded T 1100 C and P 2.8 GPa. The early decompression was associated with moderate cooling, corresponding to T 950 , and development of a spinel tectonite fabric in the lherzolites. Further decompression associated with plagioclase–olivine growth in both peridotites and pyroxenites was nearly isothermal. The shallow evolution occurred under a brittle regime and led to the superposition of hornblende to serpentine veining stages. The garnet pyroxenitebearing mantle from the External Liguride ophiolites represents a rare tectonic sampling of deep levels of subcontinental lithosphere exhumed in an oceanic setting. The exhumation was probably accomplished through a two-step process that started during Late Palaeozoic continental extension. The low-pressure portion of the exhumation path, probably including also the plagioclase mylonitic shear zones, was related to the Mesozoic (Triassic to Jurassic) rifting that led to continental break-up. In Jurassic times, the studied mantle sequence became involved in an extensional detachment process that resulted in sea-floor denudation.

Geology, mineralogy and stable isotope geochemistry of the Kabwe carbonate-hosted Pb–Zn deposit, Central Zambia

The carbonate-hosted Kabwe Pb–Zn deposit, Central Zambia, has produced at least 2.6 Mt of Zn and Pb metal as well as minor amounts of V, Cd, Ag and Cu. The deposit consists of four main epigenetic, pipe-like orebodies, structurally controlled along NE–SW faults. Sphalerite, galena, pyrite, minor chalcopyrite, and accessory Ge-sulphides of briartite and renierite constitute the primary ore mineral assemblage. Cores of massive sulphide orebodies are surrounded by oxide zones of silicate ore (willemite) and mineralized jasperoid that consists largely of quartz, willemite, cerussite, smithsonite, goethite and hematite, as well as numerous other secondary minerals, including vanadates, phosphates and carbonates of Zn, Pb, V and Cu. Galena, sphalerite and pyrite from the Pb–Zn rich massive orebodies have homogeneous, negative sulphur isotope ratios with mean δ 34S CDT permil (‰) values of − 17.75 ± 0.28 (1 σ), − 16.54 ± 0.0.27 and − 15.82 ± 0.25, respectively. The Zn-rich and Pb-poor No. 2 orebody shows slightly heavier ratios of − 11.70 ± 0.5‰ δ 34S for sphalerite and of − 11.91 ± 0.71‰ δ 34S for pyrite. The negative sulphur isotope ratios are considered to be typical of sedimentary sulphides produced through bacterial reduction of seawater sulphate and suggest a sedimentary source for the sulphur. Carbon and oxygen isotope ratios of the host dolomite have mean δ 13C PDB and δ 18O SMOW values of 2.89‰ and 27.68‰, respectively, which are typical of marine carbonates. The oxygen isotope ratios of dolomite correlate negatively to the SiO 2 content introduced during silicification of the host dolomite. The depletion in 18O in dolomite indicates high temperature fluid/rock interaction, involving a silica- and 18O-rich hydrothermal solution. Two types of secondary fluid inclusions in dolomite, both of which are thought to be related to ore deposition, indicate temperatures of ore deposition in the range of 257 to 385 and 98 to 178 °C, respectively. The high temperature fluid inclusions contain liquid + vapour + solid phases and have salinities of 15 to 31 eq. wt.% NaCl, whereas the low temperature inclusions consist of liquid + vapour with a salinity of 11.5 eq. wt.% NaCl. Fluid transport may have been caused by tectonic movements associated with the early stages of the Pan-African Lufilian orogeny, whereas ore deposition within favourable structures occurred due to changes in pressure, temperature and pH in the ore solution during metasomatic replacement of the host dolomite. The termination of the Kabwe orebodies at the Mine Club fault zone and observed deformation textures of the ore sulphides as well as analysis of joint structures in the host dolomite, indicate that ore emplacement occurred prior to the latest deformation phase of the Neoproterozoic Lufilian orogeny.

Metamorphism and Chromite in Serpentinized and Carbonate-Silica-Altered Peridotites of the Paleoproterozoic Outokumpu-Jormua Ophiolite Belt, eastern Finland

The 1.95 Ga Outokumpu-Jormua ophiolite belt of eastern Finland contains numerous mafic-ultramafic, predominantly peridotitic bodies which, despite amphibolite-facies metamorphism and pervasive deformation, retain compelling evidence of a residual mantle origin. These rocks therefore currently represent the oldest documented examples of exhumed mantle lithosphere, so information concerning their primary igneous mineral assemblages and textures and chemical and isotopic characteristics is of considerable scientific value. Although several earlier studies have argued for the preservation of primary mineral assemblages, field and petrographic evidence presented here show that the protolith peridotites had already experienced pervasive low-T serpentinization prior to Svecofennian orogenic deformation, during which they were progressively deserpentinized via antigorite metaserpentinites to olivine-talc-anthophyllite-enstatite-bearing metaperidotites. evidence is also presented to show that the premetamorphic serpentinization event was closely followed by extensive low-T (< 250°C) metasomatic alteration of the marginal parts of the ultramafic bodies to carbonate-silica rocks which, during the subsequent prograde metamorphism, were converted to the distinctive chromite-bearing carbonate-skarn-quartz rocks comprising the Outokumpu rock assemblage. Because these quartz rocks are intimately associated with the Outokumpu Cu-Co-Zn-Ni deposits and have generally been regarded as metamorphosed siliceous seafloor exhalative deposits, the revised interpretation presented here has important implications for ore formation as well. equilibrium mineral assemblages in the interior parts of the ultramafic bodies (low XCO2) define four regional metamorphic zones, expressed as an east to west increase in the peak dehydration temperatures from 500° to 775°C, at 3-5 kbar. Large ultramafic bodies commonly show core to margin zoning from talc via anthophyllite- to enstatite-bearing assemblages, reflecting synmetamorphic core-margin gradients in XCO2, attributed to infiltration of CO2 released by decarbonation reactions in previously formed talc-carbonate and carbonate-silica alteration zones. The only primary igneous phase identified in this study was chromite, which occurs as scattered relict cores within large altered grains. These are relatively common in metaserpentinites, but also occur in metaperidotites and even in carbonate-skarn-quartz rocks. even though Mg, Fe2+, and Zn abundances in these relict chromites may have been somewhat modified, absolute concentrations and ratios of Cr and Al appear to be essentially unmodified. The lobate to amoeboid morphology and observed Cr# range (0.41-0.67) of the best-preserved grains are more consistent with residual depleted lherzolite to harzburgite textures than a cumulate origin. However, during regional metamorphism, most mantle chromites were either pervasively altered to ferrian chromites and Cr-magnetites or (where CO2 and S fugacities were high, such as within smaller ultramafic bodies, or at the margins of larger bodies) to high-Cr (Cr2O3 = 50-70 wt%; Cr# = 0.7-1.0) chromites; with further increase in metamorphic grade, the latter typically recrystallized to mostly spongy/chessboard-textured grains. This interpretation contrasts with previously held views, where high-Cr chromites were considered as residual chromites in ultradepleted residual peridotites. Our study demonstrates that Cr-spinel textures and compositions in amphibolite-facies ultramafic rocks may, to a large degree, be influenced by the metamorphic and metasomatic history of the enclosing host rocks. Clearly, valid application of Cr-spinel as a petrotectonic indicator requires, at least for medium and higher-grade ultramafic rocks, a thorough understanding of the metamorphic and hydrothermal history of the host rocks. Interpretation of the quartz rocks of the Outokumpu assemblage as silicified peridotites demands a reappraisal of the widely accepted concept of the Outokumpu-type sulfide ores as a type example of Precambrian ophiolite—related seafloor hydrothermal sulfide deposits. As an alternative, we tentatively propose a syntectonic hydrothermal origin.