Sector Zoning Research Articles

Monazite-(Ce) and Xenotime-(Y) From An Lct, NYF Tertiary Pegmatite Field: Evidence From A Regional Study In the Central Alps (Italy and Switzerland)

Abstract Monazite-(Ce) and xenotime-(Y) from tertiary pegmatites of the central Alps (Italy and Switzerland) were studied by EMPA and SCXRD analytical methods. The REE-phosphate minerals analyzed are from LCT and NYF pegmatite dikes of the Central Alps, which includes three main areas: Vigezzo (LCT and NYF from the Vigezzo valley), Bodengo (NYF from the Leggia valley and LCT from the Cama valley), and Codera (LCT from Codera and Chiavenna valleys and Alpe Rossaccio). SEM-BSE images of monazite-(Ce) show primary growth zoning, sector zoning, and patchy zoning. SEM-BSE images of xenotime-(Y) show faint patchy-type zoning and spongiform texture. Both REE-phosphates evidence secondary mineral replacements that include thorianite, thorite, and uraninite. EMPA compositions show that the monazite-(Ce) is characterized by variable amounts of huttonite (ThSiO 4 ) and cheralite [Ca(Th,U)(PO 4 )] molecules. SCXRD analyses shows that the unit-cell volume of monazite-(Ce) correlates with its chemical composition; the b cell parameter shows an evident relationship with the huttonite molecule. The unit-cell volume of xenotime-(Y) is well correlated to its chemical composition. Xenotime-(Y) related to the NYF family shows a significant increase in yttrium and a lower U + HREE content. The Tertiary pegmatite field of the central Alps is related to the Masino-Bregaglia intrusion, the San Fedelino granite, and younger magmatic pulses generated by the migration of the metamorphic thermal Lepontine Dome towards the west. The presence of poikiloblastic thorite, thorianite, and uraninite in monazite and xenotime can be induced to form from alkali-bearing fluids via coupled dissolution–reprecipitation, under moderate to high-grade and post-peak Alpine metamorphic conditions.

Evidence for Archean inheritance in the pre-Panafrican crust of Central Cameroon: Insight from zircon internal structure and LA-MC-ICP-MS U[sbnd]Pb ages

The main geological feature of Central Cameroon is the wide spread occurrence of granitoids emplaced in close association with transcurrent regional shear zones. The basement of this vast domain is a Paleoproterozoic ortho-and para-derivative formation, which has been intensely reworked, together with subsequent intrusions and sediments, during the Panafrican orogenesis in the Neoproterozoic. As consequence, the area underwent pervasive metamorphism and intense deformation. This makes it difficult to distinguish between Panafrican metasediments or syntectonic plutonites and their respective basement. Our study presents zircon features (CL-BSE-SE) and in-situ U–Th–Pb LA-MC-ICP-MS geochronology of a meta-sedimentary pyroxene-amphibole-bearing gneiss of the Méiganga area in Central Cameroon. Based on the Internal structures of the zircon four characteristic zonation patterns can be deciphered: 1) cores with magmatic oscillatory zonation 2) zircons with oscillatory or sector zonation, 3) zircons with sector zoning or blurred zoning, and 4) narrow bright un-zoned rims. These groups suggest that the rock experienced a number of geological events. Considering this zircon characteristic, the U–Th–Pb data allow to distinguish four ages: 2116 ± 57 Ma, consistent with ages from the Paleoproterozoic West Central African Belt; 2551 ± 33 Ma which marks a late Neoarchean magmatic event; 2721 ± 27 Ma related to a Neoarchean magmatic even in Central Cameroon, similar to one found in the Congo Craton. A zircon core gives ages around 2925 Ma which provides some evidence of the presence of the Mesoarchean basement prior to the Neoarchean magmatism. A weighted average of lower intercepts ages gives a value of 821 ± 50 Ma, representing the age of later metamorphism event. The various characteristic group and related ages reflect not only the complexity of the history of the pyroxene amphibole gneiss, but also show that the meta-sediment has at least three zircon contributing sources. It is likely that erosion, transport and deposition took place between 2116 and 821 Ma. Geochemical data show that the REE, Y, Yb, Sr/Y of some samples are similar to the known Archean craton formations (depletion in REE, Y ≤ 10 ppm, Yb ≤ 1 ppm, Sr/Y ≥ 30). These characteristics are known as specific for the Archean TTG (Tonalite—Trondhjemite—Granodiorite). It means that: i) Archean TTG contribute significantly to the detritus of the sedimentary basin, ii) The depositional basin and the source rock were close and the detritus was immature.Our results show that the Pre-Panafrican history of central Cameroon includes Meso- to Neo-Archean crustal accretion and associated magmatism prior to the Paleoproterozoic event of the West Central African Belt. In respect to this new insight, any evolutionary reconstruction of the area should integrate the presence of Archean crust.

High-alumina basalts from the Bogda Mountains suggest an arc setting for Chinese Northern Tianshan during the Late Carboniferous

Considerable debate persists as to the tectonic setting of the Tianshan Orogen during the Late Paleozoic, with active subduction system and intraplate large igneous provinces as two dominant schools. With aims of providing constraints on this issue, geochronological and geochemical analyses have been carried out on the Late Carboniferous high-Al basaltic lava (HAB) from the Bogda Mountains. These lavas, in conformable contact with the felsic rocks, belong to the Upper Carboniferous Liushugou Group. Zircon SHRIMP U–Pb dating of two felsic ignimbrites further suggest that they were mainly erupted during 315–319Ma. The Bogda basaltic lava is classified as HAB given their high Al contents >16% and their chemical resemblance to those from modern arcs such as Aleutian and Kamchatka. They are characterized by strong enrichment in large ion lithophile elements (LILE), strong negative Nb–Ta and Ti anomalies, and distinct positive Pb anomalies. Hence, they are significantly different from the mantle plume-related basalts, as exemplified by those from Siberian, Emeishan, and Tarim large igneous provinces. Instead, their MORB-like Nd–Hf–Pb isotopes and arc-like trace elements indicate that the Bogda HABs may have been generated from a mantle wedge metasomatized by sediment-derived melts. The sector and oscillatory zoning in clinopyroxene phenocrysts in the Bogda HABs is attributable to rapid dynamic crystallization during magma ascent. High Al content is due to delayed plagioclase nucleation likely by the high crystallization pressure rather than water content. Collectively, our data lend support to an island arc environment during the Late Paleozoic, probably related to southward subduction of the Paleo-Tianshan Ocean.

HIGH-PRECISION U-Pb ZIRCON-BADDELEYITE DATING OF THE J-M REEF PLATINUM GROUP ELEMENT DEPOSIT IN THE STILLWATER COMPLEX, MONTANA (USA)

The Stillwater Complex, Montana (USA), is one of the world’s largest layered intrusions and hosts the Johns-Manville (J-M) Reef, a world-class platinum group element (PGE) deposit. In this study, zircon and baddeleyite were successfully extracted from three mineralized samples of the J-M Reef. The samples, which were collected over 15 km of strike length of the deposit, provide high-precision crystallization ages for the reef and the Stillwater Complex. All samples are coarse grained to pegmatitic and range in composition from olivine-bearing gabbronorite to anorthosite and troctolite. Irregular (anhedral) morphology and prominent sector zoning characterize interstitial zircon. U-Pb crystallization zircon ages determined by chemical abrasion thermal ionization mass spectrometry (CA-TIMS) for the three samples are indistinguishable within uncertainty: Frog Pond adit (2709.11 ± 0.56 Ma), East Boulder mine (2709.28 ± 0.32 Ma), and West Fork area (2709.00 ± 0.45 Ma). A U-Pb baddeleyite date from the Frog Pond adit sample (2708.85 ± 0.46 Ma) is identical within uncertainty with the zircon dates. Collectively, the results illustrate that the J-M Reef is an intrusion-wide time marker that formed at ca. 2709 Ma during crystallization of the Lower Banded series of the Stillwater Complex. The recognition that zircon can be successfully extracted from mafic-ultramafic rocks associated with magmatic ore deposits provides new opportunities for testing the timing of mineralization and duration of mineralization processes in layered intrusions worldwide.

Origin and geochemistry of agates in Permian volcanic rocks of the Sub-Erzgebirge basin, Saxony (Germany)

Mineralogical and geochemical investigations of agates from Permian volcanic rocks of the Sub-Erzgebirge basin (Saxony, Germany) were made to constrain the genesis and characteristics of these spectacular forms of silica in acidic volcanic host rocks. Samples from the main agate occurrences of Chemnitz, Hohenstein-Ernstthal, St. Egidien and Zwickau were selected for detailed analyses.The results of the study show that agate formation can be related to volcanic activity (Rochlitz ignimbrite) and the subsequent alteration of the volcanic rocks. Most agates originate from the infill of silica into cavities of lithophysae (high-temperature crystallization domains), which formed during cooling of welded ignimbrite. Agate formation temperatures of probably >150°C were calculated from fluid inclusion and oxygen isotope studies, which indicate that the mobilization and accumulation of silica started already during a late phase of or soon after the volcanic activity.The trace-element composition of chalcedony and macrocrystalline quartz in agates is different from that of quartz from magmatic or metamorphic rocks and pegmatites. Elements of the volcanic rock matrix (Al, Ca, Fe, Na, K) were released during the alteration processes and accumulated in the SiO2 matrix of the agates. Extraordinary high contents of Ge (>90ppm), B (46ppm) and U (>18ppm) were also detected, which can exceed the Clark concentration and sometimes the element concentration in the surrounding host rocks. In addition, chondrite-normalized REE distribution patterns of the agates show strong negative Eu-anomalies, enriched HREE as well as tetrad effects. These patterns are different from the REE patterns of the volcanic host rocks and point to fractionation processes during agate formation. The specific geochemical features indicate interactions of the host rocks with magmatic volatile fluids and heated meteoric water, and transport of SiO2 and other elements both in aqueous solution and via stable fluorine (and chlorine) compounds such as SiF4, BF3, GeF4, and UO2F2.The results of cathodoluminescence (CL) microscopy and spectroscopy revealed a microstructure of the agates that is similar for all occurrences. Characteristic features are irregular internal textures and sector zoning in quartz as well as luminescence colours and spectra, respectively, which are caused by a typical high defect density (oxygen vacancies, silanol groups). According to these results, the formation of agates can be explained by crystallization via an amorphous silica precursor under non-equilibrium conditions.

Correlations and zoning patterns of phosphorus and chromium in olivine from H chondrites and the LL chondrite Semarkona

AbstractPhosphorus zoning is observed in olivines in high‐FeO (type IIA) chondrules in H chondrites over the entire range of petrologic grades: H3.1–H6. Features in P concentrations such as oscillatory and sector zoning, and high P cores are present in olivines that are otherwise unzoned in the divalent cations. Aluminum concentrations are low and not significantly associated with P zoning in chondrule olivines. In highly unequilibrated H chondrites, phosphorus zoning is generally positively correlated with Cr. Atomic Cr:P in olivine is roughly 1:1 (3:1 for one zone in one olivine in RC 075), consistent with Cr3+ charge‐balancing P5+ substituting for Si4+. Normal igneous zonation involving the dominant chrome species Cr2+ was observed only in the LL3.0 chondrite Semarkona. In more equilibrated chondrites (H3.5–H3.8), Cr spatially correlated with P is occasionally observed but it is diffuse relative to the P zones. In H4–H6 chondrites, P‐correlated Cr is absent. One signature of higher metamorphic grades (≥H3.8) is the presence of near matrix olivines that are devoid of P oscillatory zoning. The restriction to relatively high metamorphic grade and to grains near the chondrule–matrix interface suggests that this is a response to metasomatic processes. We also observed P‐enriched halos near the chondrule–matrix interface in H3.3–H3.8 chondrites, likely reflecting the loss of P and Ca from mesostasis and precipitation of Ca phosphate near the chondrule surface. These halos are absent in equilibrated chondrites due to coarsening of the phosphate and in unequilibrated chondrites due to low degrees of metasomatism. Olivines in type IA chondrules show none of the P‐zoning ubiquitous in type IIA chondrules or terrestrial igneous olivines, likely reflecting sequestration of P in reduced form within metallic alloys and sulfides during melting of type IA chondrules.

Colombian Trapiche Emeralds: Recent Advances in Understanding Their Formation

Colombia is the traditional source of the world’s finest emeralds, including the famed trapiche crystals, with their distinctive texture resembling a wheel with six spokes. This gemological curiosity, found exclusively in the black shales of the country’s western emerald zone, is linked to the peculiar structural geology of the deposits. The study presents a review and update on Colombian trapiche emeralds, followed by a three-dimensional examination of the crystals combined with spectroscopic and chemical analyses. The proposed formation model incorporates the structural geology of the deposits with the formation of trapiche and non-trapiche emeralds. The fluid accumulation at the faults’ tip in the black shales leads to maximum fluid overpressure and sudden decompression and formation of the emerald-bearing vein system. The authors show that trapiche emerald growth starts at the beginning of the decompression that is responsible for local supersaturation of the fluid. The hydrothermal fluid comes in contact with the black shale matrix, favoring the formation of emerald seed crystals. During the growth of these seeds, textural sector zoning occurs, sometimes associated with chemical sector zoning, along with displacement of the matrix. Displacement growth occurs because the emeralds continue their growth, pushing the matrix material away from the growing faces. An overgrowth, generally of gem quality, can form after decompression, surrounding the core, the arms, and the dendrites, restoring the emeralds’ euhedral habit.

MAGMATIC Au MINERALIZATION AT THE BILIHE Au DEPOSIT, CHINA

Most known primary Au deposits are produced by hydrothermal processes. In this paper, we report Au mineralization of magmatic origin in the Bilihe deposit, China (8.5 Mt averaging 2.9 g/t Au). At Bilihe, most (>70%) native Au (no detectable Ag) grains are euhedral or subspherical in shape, and occur as trails in dendritic quartz phenocrysts and comb-layered quartz in a moderately reduced, highly fractionated diorite-granite intrusion. The hosting quartz typically has a dendritic core (Q1) and a rim (Q2), with Q1 having concentric zoning and sector zoning in cathodoluminescence (CL) images. Cathodoluminescence petrography and crystallographic modeling reveal that most of the Au trails occur along intersecting crystallographic planes of the host quartz, indicating simultaneous precipitation of both Au and quartz. Abundant melt inclusions are present in Q2 with >950°C homogenization temperatures. Minor Au grains also occur in melt inclusions in quartz. In rare cases, necking of Au melt inclusions is present. Neither Q1 nor Q2 contain primary fluid inclusions; only secondary fluid inclusions were found in healed cracks. The above observations indicate a direct magmatic (quartz phenocryst phase) origin for the Au. This defines a new type of Au deposit, and thereby opens new potential for Au exploration. The magmatic origin of Au at Bilihe also implies that enrichment of Au may occur in a source melt prior to volatile escape, which would enhance the possibility of forming a magmatic-hydrothermal Au deposit.

Anomalous Cr-Rich Zones In Sector-Zoned Clinopyroxene Macrocrysts In Gabbro, Mont Royal, Montreal, Quebec, Canada

We document unusual zones of Cr enrichment (up to 0.9 wt.% Cr 2 O 3 ) in sector-zoned clinopyroxene crystals from gabbro sampled near the outer contact of the Mont Royal intrusive complex, of Early Cretaceous age, located in the city of Montreal. Mont Royal is one of the Monteregian Hills, but by no means the best-known mineralogically. We describe the compositional variation in macrocrysts of Ti-rich clinopyroxene by means of element-distribution maps and a detailed 0.37 mm traverse across the contact between a relatively homogeneous core and a mantle showing sector and oscillatory zoning. A Cr- and Mg-enriched zone appears in that interval and is also recorded in the coeval ferri-kaersutite. Both elements, being highly compatible, are not expected to build up in a boundary layer. After a period of slow growth in a hotter part of the reservoir, the macrocrysts seem to have migrated near the outer contact, where more rapid loss of heat and degassing contributed to the development of sector and oscillatory zoning. Such zoning was accompanied by an overall rise in f (O 2 ). The anomalous Cr-Mg-enriched zones seem more likely attributable to local cooling and oxidation of the magma than to an episode of replenishment in the magma reservoir.

Strontium incorporation during calcite growth: Implications for chemical mapping using friction force microscopy

Sr partitioning on calcite crystals growing from Ca2+–Sr2+–CO32− solutions was studied by means of friction force microscopy (FFM). Experiments were performed with various Sr2+(aq)/Ca2+(aq) concentration ratios and total Sr2+(aq) concentration in order to examine conditions under which Sr-calcite growth is self-limiting (e.g., the so-called “template effect”) and also to investigate continuous Sr-calcite growth, where spiral growth predominates and Sr incorporation is sector-dependent. In these latter experiments, the goal was to evaluate the utility of friction force microscopy to discriminate sector zoning. Results from the experiments show that friction increases with the incorporation of Sr into the growing calcite layers. The maximum increase in friction was measured at high Sr2+(aq)/Ca2+(aq), although a quantitative link between a specific amount of increase in friction to a specific amount of Sr incorporation was not possible to determine due to experimental uncertainties. Nevertheless, it was possible to establish that no change in friction is detectable when Sr incorporation yields a solid composition of Sr0.05Ca0.95CO3. Friction was found to increase during growth of several layers in an incremental fashion. The increase can be linked either to an incremental increase in Sr content in the newly formed calcite, controlled by the thermodynamics of the strained layers necessitated by the substitution of larger Sr cations into the calcite, or to the incremental increase in layer thickness which in turn leads to increases in the probe-surface contact area. No difference in friction could be observed between acute and obtuse sectors under any of the experimental conditions, which was primarily due to the limits of the friction measurement sensitivity.

Sector Zoning In Uraninite

A uraninite crystal from the Roode pegmatite in southern Norway displays clear sector zoning mostly defined by the main U-Th substitution, with sharp changes in chemical compositions between the cubic and the octahedral sectors; a weaker concentric zoning defined by the same chemical variations is also present. The {100} sectors incorporate slightly more U whereas the {111} sectors incorporate slightly more Th, Y, and REEs. The cubic sectors contain on average ca. 77.0 wt.% UO2, ca. 6.6 wt.% ThO2, ca. 0.6 wt.% ΣREE2O3, and ca. 0.45 wt.% Y2O3; the octahedral sectors contain on average ca. 73.8 wt.% UO2, ca. 9.1 wt.% ThO2, ca. 0.8 wt.% ΣREE2O3, and ca. 0.55 wt.% Y2O3. The development of sector zoning in uraninite is possibly due to the interplay between the difference in effective ionic radius between U and Th and the difference in protosite density between the {100} and {111} sectors.

Galactic cosmic ray intensity simulation with spatial and temporal dependence of fluctuations of the helioshperic magnetic field

A solution to the two-dimensional transport equation describing the modulation of galactic cosmic rays (GCRs) in the heliosphere in the 11-year cycle is presented. A feature of the solution is that the diffusion coefficient which depends on the index of the power spectrum of heliospheric magnetic field fluctuations is chosen to be different inside sector zones and outside them. This allows us to consider the connection between the diffusion coefficient and the tilt angle of the heliospheric current sheet, and to improve the agreement between the observed and calculated values of GCR intensity.

Rare earth elements in apatite as a monitor of magmatic and metasomatic processes: The Ilímaussaq complex, South Greenland

Textural and compositional variations of apatite from the plutonic Ilímaussaq complex in South Greenland and its surrounding country rocks track magmatic and metasomatic processes. Detailed back-scattered electron (BSE) imaging reveals various types of apatite textures including (i) growth zonation (concentric, oscillatory as well as sector zonation) formed during magmatic differentiation, (ii) resorption and overgrowth textures due to fluid/melt induced metasomatic overprint of precursor apatite and (iii) replacement textures indicating the destabilization of apatite in favor of monazite.In the least evolved rocks of the Ilímaussaq complex, apatite incorporates rare earth elements and Y (REY) mainly via the coupled substitution Ca2++P5+=REY3++Si4+. In the more evolved peralkaline rocks and in metasomatically overprinted rocks, however, the coupled substitution 2 Ca2+=REY3++Na+ becomes increasingly relevant, and apatite incorporates successively more LREE compared to HREE as shown by increasing (La/Gd)N and (Gd/Yb)N ratios. Similarly, at the contact between the Ilímaussaq rocks and the granitic country rocks, the metasomatic effect of the emplacement of the Ilímaussaq melts is preserved in partly resorbed precursor apatite, which is overgrown by REY-enriched apatite with higher (La/Gd)N and (Gd/Yb)N ratios compared to apatite from granites more distant from the contact.This study shows, that apatite textures and compositions are suitable to track both primary magmatic and later fluid-present metasomatic processes. The incorporation of REY in apatite is not only dependent on the amount of REY present but also depends largely on Na activity in the apatite-precipitating melts and fluids.

Polymagmatic processes at monogenetic volcanoes: insights from Baossi monogenetic lava flows, Adamawa Plateau, Cameroon Volcanic Line

Abstract This paper presents principally the textural and geothermometric evidence of polymagmatic activity at a monogenetic volcano along the Cameroon Volcanic Line (CVL) in a bid to contribute to the global understanding of how such volume-limited basaltic magmatic systems operate. Preliminary geochemical data presented show that the lavas are alkali basalt and basanite (SiO2: 40.42 – 47.59 wt%, MgO: 7.61 – 9.13 wt%) and are petrographically indistinguishable. They outcrop as low mounds of columnar basalts associated with sparse pyroclastic materials. The lavas are fine porphyritic with phenocrysts of olivine and clinopyroxene set in a plagioclase microlite-dominated hypocrystalline groundmass. Olivine has two distinct populations: grains that are anhedral to glomerocrystic with spinel and plagioclase inclusions and usually have resorbed margins (olivine 1); and grains that are euhedral, sometimes skeletal, with a thin and well preserved rim (olivine 2). Similarly two clinopyroxene crystal populations are recognizable: clinopyroxene grains that are resorbed, dismembered, with sieve-textured cores and irregular core-rim margins (clinopyroxene 1); and euhedral clinopyroxene grains partially enclosing olivine phenocrysts with hour glass and sector zoning (clinopyroxene 2). These textural features suggest that the eruption was caused by an influx of a fresh batch of magma (that crystallized olivine 2 and clinopyroxene 2) into a fractionated crystal mush (olivine 1 and clinopyroxene 1) in the chamber. The crystal mush was re-heated by the intruding basaltic magma resulting in a broad pre-eruption liquidus temperature of 1040 – 1156oC calculated using mineral chemistry of equilibrium olivine-clinopyroxene pairs. Keywords: Cameroon volcanic line, geothermometry, monogenetic volcano, olivine-clinopyroxene pairs

Constraints on Hf and Zr mobility in high-sulfidation epithermal systems: formation of kosnarite, KZr2(PO4)3, in the Chaquicocha gold deposit, Yanacocha district, Peru

We report the first occurrence of Hf-rich kosnarite [K(Hf,Zr)2(PO4)3], space group R-3c, Z = 6, in the giant Chaquicocha high-sulfidation epithermal gold deposit in the Yanacocha mining district, Peru. Kosnarite crystals are small (<100 μm) and occur in 2–3-mm-thick veins that cut intensively silicified rocks. The paragenesis includes a first stage of As-free pyrite and quartz (plus gratonite and rutile), followed by trace metal-rich pyrite [(Fe,As,Pb,Au)S2] and secondary Fe sulfates. Kosnarite is associated with quartz and is clearly late within the paragenetic sequence. Electron microprobe analyses (EMPA) of kosnarite show relatively high concentrations of HfO2 and Rb2O (7.61 and 1.05 wt.%, respectively). The re-calculated chemical formulas of kosnarite vary from KΣ1.00(Zr1.93Na0.01Hf0.01Mn0.01)Σ1.96(P3.04O4)Σ3 to (K0.92Rb0.05Na0.03)Σ1.00(Zr1.81Hf0.19)Σ2.00 [(P2.98Si0.02As0.01)Σ3.01O4]Σ3, where Hf and Rb are most likely incorporated according to a coupled substitution of Hf4+ + Rb+ ⇔ Zr4+ + K+. Back-scattered electron (BSE) images and elemental mapping of kosnarite reveal that Hf and Rb are enriched in 2–10-μm-wide oscillatory and/or sector zones. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observations of such zones reveal a pattern of alternating, 5–50-nm-thick, Hf-rich and Zr-rich nanozones. These high-resolution observations indicate that the incorporation of Hf does not appear to cause significant distortion in the kosnarite structure. Semiquantitative TEM-energy-dispersive X-ray spectrometry (EDS) analyses of the nano-layers show up to 22 wt.% of HfO2, which corresponds to 31 mol% of the hypothetical, KHf2(PO4)3, end-member. The presence of kosnarite in the advanced argillic alteration zone at Yanacocha is indicative of Hf and Zr mobility under highly acidic conditions and points towards an unforeseen role of phosphates as sinks of Zr and Hf in high-sulfidation epithermal environments. Finally, potentially new geochronological applications of highly insoluble vein kosnarite, including Rb-Sr dating, may provide further age constraints in pervasively altered areas where other isotopic systems might have been reset.

Syn-collisional lower continental crust anatexis in the Neoproterozoic Socorro-Guaxupé Nappe System, southern Brasília Orogen, Brazil: Constraints from zircon U–Pb dating, Sr–Nd–Hf signatures and whole-rock geochemistry

The southern Brasília Orogen occurs along the southernmost border of the São Francisco Craton. This Orogen is structured as a pile of Ediacaran syn-metamorphic thicked-skinned nappes that diachronically migrate toward the cratonic margin. A magmatic arc environment is represented by the Socorro-Guaxupé Nappe (SGN), which is a thick segment of partially molten lower to middle continental crust. The SGN comprises three main units: a lower granulitic, an intermediate diatexitic and an upper metatexitic. High-temperature metamorphic peak was reached at ∼625Ma, when large volumes of high-K diatexitic rocks were formed. Charnockitic and mangeritic magmatic rocks (São José do Rio Pardo suite) and in situ leucosomes of mafic composition are inferred to have formed under hornblende-dehydration melting conditions (i.e. T≥850°C). Granitic diatexites (Pinhal-type) were also formed, and zircon thermometry indicates T≥850°C. Elemental and isotopic geochemistry indicate that these diatexites and in situ leucosomes derived from melting of lower to middle continental crust. TIMS zircon U–Pb ages are presented for the lower and middle units. U–Pb dating coupled to Lu–Hf analysis (LA-MC-ICP-MS) was performed on charno-enderbitic in situ leucosomes from metatexitic rocks that also resulted from hornblende-dehydration melting. Two different zircon typologies were recognized. The first comprises bipyramidal-prismatic grains with oscillatory zoning and high luminescence, sometimes preserving low-luminescence 670Ma cores. The high-luminescence grains show 19 concordant ages at 621±16Ma. The second typology comprises isometric or soccer-ball-type grains. These grains show sector zoning and a concordia age of 608±4Ma. Th/U ratios from soccer-ball grains vary from 1.253 to 2.107, whereas prismatic grains vary from 0.118 to 1.774. Initial ɛHf signatures between −13 and −21 in both typologies provide evidence of a reworked crustal reservoir. The older prismatic grains may be related to the onset of metamorphic peak (∼625Ma), whereas the isometric ones provide evidence of long-lived high-T conditions until ∼610Ma.

High-Sulfidation Epithermal Pyrite-Hosted Au (Ag-Cu) Ore Formation by Condensed Magmatic Vapors on Sangihe Island, Indonesia

Although gold in high-sulfidation epithermal deposits generally occurs as the native metal or electrum, in some deposits, a significant proportion of the gold is hosted in pyrite. Here we use a combination of petrography, whole-rock geochemistry, pyrite chemistry, crystallography, and phase stability relationships to determine how gold was transported and incorporated into pyrite in two relatively young high-sulfidation epithermal deposits, where the gold occurs almost exclusively in solid solution or as nanoparticles in pyrite. The genetically related Bawone and Binebase Au (Cu-Ag) deposits, located 1 km apart on the volcanic island of Sangihe, northeastern Indonesia, are hosted by andesitic volcaniclastic rocks that were altered to a proximal advanced argillic association of quartz + pyrite (py I) + pyrophyllite + natroalunite + alunite + dickite + kaolinite and a more distal intermediate argillic association of quartz + pyrite (py I) + kaolinite + dickite + illite. The economic mineralization takes the form of multiple generations of auriferous pyrite, the first of which, pyrite I (py I), developed during advanced argillic alteration. Mass balance calculations show that all elements were mobile with the exception of Nb, Ti, some rare earth elements, and possibly Al. The highest gold concentration is in pyrite II (py II), which occurs in veins that cut pyrite I. This drusy variety of pyrite is characterized by complex growth and sector zoning, and contains as much as 6.0 wt % Cu. The elevated Cu concentrations correlate positively with Au and As concentrations, whereas the Ag concentration correlates strongly with Au but not Cu. Later barite-enargite mineralization exploited py II veins and vugs, and significant concentrations of Ag and Au are hosted by enargite, although the Au concentration in enargite is lower than in py II or py I. A model is presented in which the fluid responsible for advanced argillic and intermediate argillic alteration and associated stage 1 gold mineralization was a condensed magmatic vapor derived from an oxidized magma. The gold and other metals were transported as hydrated species that ascended through the volcanic pile via fractures and zones of enhanced permeability to a depth between 900 and 1300 m, where the vapor condensed at a temperature between 250 and 340°C to form an acidic liquid with a pH of ~2.5; f o2 ranged up to four log units above the hematite-magnetite buffer. Interaction of this liquid with the host andesites caused advanced argillic and intermediate argillic alteration, including sulfidation of mafic minerals to form py I. During crystallization of py I, Au, Cu and Ag were adsorbed onto the surface of the pyrite and deposited as nanoparticles, or were incorporated in the pyrite structure. Adsorption of Au, Cu, and Ag from the condensed vapor reached a peak during the crystallization of vein-hosted py II, and the uptake of Ag and minor Au continued during later crystallization of enargite. From the distribution of metals among growth and sector zones in py II, incorporation of gold and other metals appears to have been maximized when physicochemical conditions were relatively stable. This is in contrast to the requirement for native gold precipitation, namely that physicochemical gradients be steep to ensure supersaturation of gold in the ore fluid.

Cobaltite-rich mineralization in the iron skarn deposit of Traversella (Western Alps, Italy)

AbstractCobaltite-rich mineralization from the iron skarns of the Traversella magnetite mine (Western Alps, Italy) was studied by reflected-light microscopy, scanning electron microscopy and electron microprobe analysis. Cobaltite is found in carbonate-chlorite-rich rocks at the margins of the main magnetite masses, where it forms disseminations and metasomatic veinlets that postdate the formation of magnetite. The paragenesis includes cobaltite (± arsenopyrite), bismuthinite, pyrrhotite and/or pyrite, chalcopyrite, carbonates, talc, chlorite and native gold, and is indicative of a low-sulfidation environment. The sulfarsenides show oscillatory and sector zoning, which indicates disequilibrium during crystal growth. Compositional variations are mainly due to variations in the Co/Fe ratio of arsenopyrite and in either the Co/Fe or the Ni/(Fe + Co) ratios of the coexisting cobaltite. The Ni contents are low to very low in the cobaltites (&lt;2.4 wt.%) and very low in the arsenopyrites (&lt;0.16 wt.%). The As/S molar ratios in the cobaltites are highly variable (0.59−1.00) and show a broad negative correlation with the Fe contents. The formation of cobaltite is related to circulation of relatively low-temperature (&lt;∼300°C), (Co,As,Bi)-rich fluids during the retrograde sulfidation stage which followed the formation of magnetite. The apparent restriction of cobaltite (+ bismuthinite ± arsenopyrite) to the margins of the main magnetite columns may reflect the establishment of thermochemical gradients around the main direction of infiltration of the retrograde metasomatic fluids.

New Perspectives on the Bishop Tuff from Zircon Textures, Ages and Trace Elements

We present zircon textural, trace element and U-Pb age data obtained by secondaryion mass spectrometry (SIMS) (SHRIMPRG: sensitive high-resolution ion microprobe, reverse geometry) from 15 stratigraphically controlled Bishop Tuff samples and two Glass Mountain (GM) lava samples (domes OD and YA). Bishop zircon textures divide into four suites: (a) dominant sector-zoned grains, with (b) subordinate grains showing bright rims [lower U, Th, rare earth elements (REE)] in CL imaging, (c) sparse GM-type grains (texturally similar to zircons from GM dome YA) and (d) sparse Mesozoic xenocrysts from Sierran granitoid country rocks. All Bishop zircons from suites (a)-(c) combined have a weighted mean age of 777.9±2.2 ka (95% confidence) and a tail back to ~845 ka. Our eruption age estimate using the weighted mean of 166 rim ages of 766.6±3.1ka (95% confidence) is identical within uncertainty to published estimates from isotope-dilution thermal ionization mass spectrometry (IDTIMS) (767.1±0.9 ka, 2σ) and Ar/Ar (767.4±2.2 ka, 2σ) techniques, the latter using the 28.172Ma age for the Fish Canyon sanidine standard. We estimate also an eruption age for GM dome YA of 862±23 ka (95% confidence), significantly older than the currently accepted 790±20 ka K-Ar age. The oldest zircon cores from late-erupted Bishop material (including those with GM-type textures) have a weighted mean age of 838.5±8.8 ka (95% confidence), implying that the Bishop Tuff system was active for only ~80 kyr, and had effectively no temporal overlap with the GM system. Trace element variations in Bishop zircons are influenced strongly for many elements by sector zoning, producing up to 3× concentration differences between sides and tips within the same growth zone. Contrasting trends in molar (Sc+Y+REE)/P ratios between sides and tips indicate contrasting mechanisms of substitution in different sectors of the same crystal. Concentrations of Ti in tips are double those in the sides of crystals, hindering applicability of theTi-in-zircon thermometer, in addition to variations inherent to the 0.15-0.67 range in values proposed for aTiO.The bright-rim portions of grains are inferred to have crystallized from the same magma as that which generated the bright rims seen under cathodoluminescence or back-scattered electron imaging on quartz and feldspar, respectively. This less evolved, slightly hotter magma invaded the deeper parts of the chamber represented in the late-erupted northern units possibly up to ~10 kyr prior to eruption, but invaded shallower levels only very shortly before eruption, as shown by our textural information and previously proposed from the sharp delineation of quartz bright rims. By obtaining a large number of analyses from zircon separates that systematically cover the entire Bishop Tuff eruption sequence we can produce an eruption age estimate using SIMS to the same precision and accuracy as ID-TIMS and Ar/Ar techniques.

Heishan mafic–ultramafic rocks in the Qimantag area of Eastern Kunlun, NW China: Remnants of an early Paleozoic incipient island arc

The Heishan mafic–ultramafic complex in the Qimantag Mountains of the Eastern Kunlun Orogen, consists of olivine pyroxenites, pyroxenites, gabbros, and massive basalts. Zircons from the gabbros have magmatic sector zoning, with Th/U ratios of 0.5–2.3, and yield a SHRIMP age of 486±6Ma (MSWD=0.31), which is taken as the time of magma crystallization. Spinel, pyroxene, and plagioclase mineral compositions indicate that the rocks formed in an island arc environment. The cumulates have low (La/Yb)N ratios (0.6–0.8) and depleted LREE patterns, high abundances of Cr (3031–1185ppm), Co (119–50ppm), and Ni (953–291ppm), and εNd(t) values of about +8.0. The massive basalts have somewhat higher contents of TiO2 (1.0%–1.3%), Na2O (2.6%–2.8%), total REE and Mg#s (55–63) than those of the cumulates. The (La/Yb)N ratios of the basalts range from 0.8 to 1.2 and the chondrite-normalized REE patterns are flat. εNd(t) values of the basalts range from +7.7 to +9.4. The compositions of the cumulates and volcanic rocks, especially their similar Nd isotopes, suggest that the rocks were derived from similar depleted mantle sources. The primary magmas were probably generated in an initial arc environment. We conclude that the Heishan mafic–ultramafic complex in the Qimantag area preserves a record of Proto-Tethyan oceanic lithosphere formed in the early Paleozoic. It possibly marks a suture between the Qaidam and North Kunlun terranes in the western segment of the Eastern Kunlun Orogen.