Accessory Mineral Assemblages Research Articles

Weathering co-mineralization of placer type ilmenite and ion-adsorption type rare earth elements in Guangxi, China: Nature, origin and exploration implications

Weathering co-mineralization of placer type ilmenite and ion-adsorption type rare earth elements in Guangxi, China: Nature, origin and exploration implications

Physical, chemical, and microbial feedbacks controlling brine geochemistry and lake morphology in polyextreme salar environments

Despite the harsh environmental conditions in the world's oldest and driest desert, some salt flat or 'salar' environments in the Atacama Desert host standing bodies of water known as saline lakes. Evaporite minerals deposited within saline lakes result from the equilibrium of environmental, sedimentological, and biogeochemical processes that occur in the salar; consequently, these minerals are sensitive records of human activities and ecological, evolutionary, and geological changes. The objective of this study was to evaluate feedbacks between physical, chemical, and microbial processes that culminate in distinct trends in brine chemistry, saline lake morphology, and associated evaporite sediments. Using samples from the Puquios of the Salar de Llamara, Atacama Desert, northern Chile, an analysis of spatial gradients and vertical stratification of lake elemental chemistry and mineral saturation indices were integrated with a comprehensive analysis of lake morphology, including depth, slope gradient, substrate type, and mineralogy. Lake waters ranged from saline to hypersaline, and exhibited normal, well mixed and inverse stratification patterns, and results suggest a correlation with lake morphology in the Salar de Llamara. Saline to hypersaline lakes (>150 mS/cm) with stratified brines tended to have crystalline substrate and deep (>35 cm) and steep-sided lake morphologies, while unstratified lakes with lower electrical conductivity (<90 mS/cm and microbial substrates had gentle slopes and characteristically shallow depths (<30 cm). Differences in minor element chemistry (Mn and Sr) between saline lakes were observed on scales of meters to kilometers, and result in different accessory mineral assemblages. Quantification of the physical, chemical, and microbial feedbacks that produce the observed heterogeneity in these ecosystems provides key insight into the geochemical composition and lake morphology of saline lakes in extreme environments around the world.

REE Minerals as Geochemical Proxies of Late-Tertiary Alkalic Silicate ± Carbonatite Intrusions Beneath Carpathian Back-Arc Basin

The accessory mineral assemblage (AMA) of igneous cumulate xenoliths in volcanoclastic deposits and lava flows in the Carpathian back-arc basin testifies to the composition of intrusive complexes sampled by Upper Miocene-Pliocene basalt volcanoes. The magmatic reservoir beneath Pinciná maar is composed of gabbro, moderately alkalic to alkali-calcic syenite, and calcic orthopyroxene granite (pincinite). The intrusive complex beneath the wider area around Fiľakovo and Hajnáčka maars contains mafic cumulates, alkalic syenite, carbonatite, and calc-alkalic granite. Both reservoirs originated during the basaltic magma underplating, differentiation, and interaction with the surrounding mantle and crust. The AMA of syenites is characterized by yttrialite-Y, britholite-Y, britholite-Ce, chevkinite-Ce, monazite-Ce, and rhabdophane(?). Baddeleyite and REE-zirconolite are typical of alkalic syenite associated with carbonatite. Pyrochlore, columbite-Mn, and Ca-niobates occur in calc-alkalic granites with strong peralkalic affinity. Nb-rutile, niobian ilmenite, and fergusonite-Y are crystallized from mildly alkalic syenite and calc-alkalic granite. Zircons with increased Hf/Zr and Th/U ratios occur in all felsic-to-intermediate rock-types. If rock fragments are absent in the volcanic ejecta, the composition of the sub-volcanic reservoir can be reconstructed from the specific AMA and zircon xenocrysts–xenolith relics disintegrated during the basaltic magma fragmentation and explosion.

The Haidbach deposit in the Central Tauern Window, Eastern Alps, Austria: a metamorphosed orthomagmatic Ni-Cu-Co-PGE mineralization in the Polymetallic Ore District Venediger Nappe System – Hollersbach Complex

Abstract Cu-Ni-Co-PGE mineralization occurs at Haidbachgraben in the Early Palaeozoic, Subpenninic Hollersbach Complex of the Central Tauern Window, Austria. Massive sulfide ore formed from sulfide melt segregated from silicate melt during intrusion of pyroxenite into magmatic rocks formed in an MORB-type environment. Relics of magmatic minerals include chromian spinel and polyphase sulfide droplets composed of pyrrhotite, chalcopyrite and pentlandite preserved in recrystallized pyrite. Both ore and host rocks were multiply deformed and metamorphosed, leading to hornblendite carrying the ore, enveloped by chlorite-epidote schist. Conditions of – likely Variscan – amphibolite facies metamorphism are documented by relict pargasitic cores in hornblende and actinolite-tremolite, and by ternary sulfarsenide compositions in the Co-Ni-Fe solid solution series that are the most common accessory minerals found in the sulfide ore. Pyrrhotite, pentlandite, chalcopyrite and pyrite are the major sulfide minerals. Chalcopyrite is Cd-rich and retains a high-temperature magmatic signature. High Co/Sb and moderate Se/As ratios in pyrite also point to a magmatic environment of mineralization. The accessory mineral assemblage of small grain size (mostly &lt;10 µm) comprises native Au-Ag alloy and petzite as Au-Ag minerals, sperrylite, a variety of Pd tellurides and bismuthotellurides with elevated Sb, irarsite, and Re sulfides such as tarkianite and a Pb-Re sulfide. In addition, minor molybdenite, bournonite, scheelite and selenides have been identified. Two precious metal assemblages are present in individual samples: (1) hessite associated with Pd tellurides, often accompanied by sphalerite and chalcopyrite; (2) tarkianite forming euhedral inclusions in pyrite. Sperrylite and Au-Ag native alloys are present throughout and were also detected in silicate matrix. Most of the precious metal-bearing phases must have formed during recrystallization of base metal sulfides after the magmatic, and probably during later metamorphic events terminating in the Neoalpine Tauern crystallization.

Microfaunal Signals and Paleoenvironmental Reconstruction Sediments in Well Z-1, Offshore Dahomey Basin, South-Western Nigeria

Fifty ditch cutting rock samples from well Z-1, OPL 310 offshore Dahomey basin, south western Nigeria were analyzed for their microfaunal and lithofacies content for the purpose of reconstructing the environment of deposition. Standard techniques of foraminifera slide processing and analysis was followed for the recovery of foraminifera while the gamma ray log complemented the rock samples for the lithofacies analysis. The lithological analysis revealed two lithofacies units in a generally fining upward sequence. The basal sandstone unit is characteristically milky white to brownish, coarse-pebbly grained, sub-angular to round and poorly to well sort with intercalation of shale. This unit is overlain by light to dark grey, moderately hard and non-fissile shale/mudstone sequence with intercalation of sand. Accessory mineral assemblage present in the formations includes mica flakes, glauconite pellets, carbonaceous detritus and ferruginous materials. The basal sandstone unit belong to the Oshosun Formation while the upper shaly unit is typical of Afowo Formation. Microfaunal study showed good recovery of abundant and well diversified planktic and benthic foraminiferal species. Forty-two (42) planktic, sixty-five (65) benthic calcareous and one benthonic arenaceous foraminiferal species were recovered. Micropaleontologically, Paleoenvironmental deductions were based primarily on the assemblage, abundance and diversity of benthic foraminiferal species and presence or absence of planktic foraminifera. Accessory mineral presence also aided the interpretations. Integration of lithological and micropaleontological synthesis enhanced the delineation of two environmental subzones over the analyzed interval, the outer neritic and the upper bathyal depositional settings corresponding to Afowo and Oshosun Formation respectively. A lowstand prograding wedge which is a good exploration target offshore was recognized between intervals 3400 ft to 3500 ft. In conclusion, the rock succession studied, penetrated Afowo and Oshosun Formations, and were deposited in an environment ranging from outer neritic to upper bathyal settings.

Authigenesis at the Urals Massive Sulfide Deposits: Insight from Pyrite Nodules Hosted in Ore Diagenites

The pyrite nodules from ore diagenites of the Urals massive sulfide deposits associated with various background sedimentary rocks are studied using optical and electron microscopy and LA-ICP-MS analysis. The nodules are found in sulfide–black shale, sulfide–carbonate–hyaloclastite, and sulfide–serpentinite diagenites of the Saf’yanovskoe, Talgan, and Dergamysh deposits, respectively. The nodules consist of the core made up of early diagenetic fine-crystalline (grained) pyrite and the rim (±intermediate zone) composed of late diagenetic coarse-crystalline pyrite. The nodules are replaced by authigenic sphalerite, chalcopyrite, galena, and fahlores (Saf’yanovskoe), sphalerite, chalcopyrite and galena (Talgan), and pyrrhotite and chalcopyrite (Dergamysh). They exhibit specific accessory mineral assemblages with dominant galena and fahlores, various tellurides and Co–Ni sulfoarsenides in sulfide-black shale, sulfide–hyaloclastite–carbonate, and sulfide-serpentinite diagenites, respectively. The core of nodules is enriched in trace elements in contrast to the rim. The nodules from sulfide–black shale diagenites are enriched in most trace elements due to their effective sorption by associated organic-rich sediments. The nodules from sulfide–carbonate–hyaloclastite diagenites are rich in elements sourced from seawater, hyaloclastites and copper–zinc ore clasts. The nodules from sulfide–serpentinite diagenites are rich in Co and Ni, which are typical trace elements of ultramafic rocks and primary ores from the deposit.

Bauxite formation on Tertiary sediments in the coastal plain of Suriname

Abstract Lateritic bauxites in the coastal lowlands of Suriname form part of a belt along the northern margin of the Guiana Shield that has long been one of the world's major bauxite producing regions. The Surinamese deposits, many of which with an extensive mining history, originated on Tertiary siliciclastic sediments and were mostly buried under a layer of young sediments. The bauxite-bearing sequences are generally topped with an iron-rich layer largely made up of hematite and goethite. It covers a gibbsite-rich bauxite horizon that passes downward into a kaolinitic bottom section containing anatase and zircon as main accessory minerals. Weathering profiles across formerly mined deposits were analyzed for geochemical and mineralogical properties aimed at exploring compositional diversity, underlying controls of bauxite-formation and the nature of precursor sediments. Studied profiles in different parts of the coastal plain reveal overall similarities between individual deposits in showing significant depletion of Si, K, Na, Mg and Ca and strong, primarily residual, relative enrichment of Al, Ti, Zr, Nb, Hf, Ta and Th. In detail, however, there are distinct differences in major and trace-element signatures, accessory mineral assemblages, facies distribution and provenance of the terrigenous precursor sediments. Enrichments in high field-strength elements and heavy rare earth elements are largely attributable to accumulation of heavy minerals like zircon in the precursor. Petrological and trace-element evidence does not support a direct genetic relationship between bauxite and the underlying saprolitic clays. The complex petrologic characteristics and compositional heterogeneity of the coastal-plain deposits can essentially be explained by element fractionation, primarily through selective leaching, in combination with relative and absolute enrichment processes, erosion and reworking during two-stage, polycyclic bauxitization of a heterogeneous precursor.

Accessory minerals and evolution of tin-bearing S-type granites in the western segment of the Gemeric Unit (Western Carpathians)

Abstract The S-type accessory mineral assemblage of zircon, monazite-(Ce), fluorapatite and tourmaline in the cupolas of Permian granites of the Gemeric Unit underwent compositional changes and increased variability and volume due to intensive volatile flux. The extended S-type accessory mineral assemblage in the apical parts of the granite resulted in the formation of rare-metal granites from in-situ differentiation and includes abundant tourmaline, zircon, fluorapatite, monazite-(Ce), Nb–Ta–W minerals (Nb–Ta rutile, ferrocolumbite, manganocolumbite, ixiolite, Nb–Ta ferberite, hübnerite), cassiterite, topaz, molybdenite, arsenopyrite and aluminophosphates. The rare-metal granites from cupolas in the western segment of the Gemeric Unit represent the topaz–zinnwaldite granites, albitites and greisens. Zircon in these evolved rare-metal Li–F granite cupolas shows a larger xenotime-(Y) component and heterogeneous morphology compared to zircons from deeper porphyritic biotite granites. The zircon Zr/Hfwt ratio in deeper rooted porphyritic granite varies from 29 to 45, where in the differentiated upper granites an increase in Hf content results in a Zr/Hfwt ratio of 5. The cheralite component in monazite from porphyritic granites usually does not exceed 12 mol. %, however, highly evolved upper rare-metal granites have monazites with 14 to 20 mol. % and sometimes &gt; 40 mol. % of cheralite. In granite cupolas, pure secondary fluorapatite is generated by exsolution of P from P-rich alkali feldspar and high P and F contents may stabilize aluminophosphates. The biotite granites contain scattered schorlitic tourmaline, while textural late-magmatic tourmaline is more alkali deficient with lower Ca content. The differentiated granites contain also nodular and dendritic tourmaline aggregations. The product of crystallization of volatile-enriched granite cupolas are not only variable in their accessory mineral assemblage that captures high field strength elements, but also in numerous veins in country rocks that often contain cassiterite and tourmaline. Volatile flux is documented by the tetrad effect via patterns of chondrite normalized REEs (T1,3 value 1.46). In situ differentiation and tectonic activity caused multiple intrusive events of fluid-rich magmas rich in incompatible elements, resulting in the formation of rare-metal phases in granite roofs. The emplacement of volatile-enriched magmas into upper crustal conditions was followed by deeper rooted porphyritic magma portion undergoing second boiling and re-melting to form porphyritic granite or granite-porphyry during its ascent.

REY‐Th‐U Dynamics in the Critical Zone: Combined Influence of Reactive Bedrock Accessory Minerals, Authigenic Phases, and Hydrological Sorting (Mule Hole Watershed, South India)

AbstractThe sources of REY‐Th‐U and long‐term mass balance were assessed in the gneissic tropical forested Critical Zone Observatory of Mule Hole, India. The study relies on the characterization of the solid compartments (bedrock, soils, streambed and suspended sediments), on batch leaching experiments of the parent gneiss and on extractions of cation‐exchangeable and iron‐related pools of selected soil samples. The REY‐Th‐U primary reservoir is controlled by monazite, xenotime, thorite, allanite, bastnesite, titanite, apatite, and, to a lesser extent, by zircon. This accessory mineral assemblage was profoundly modified by episodes of metamorphism and hydrothermal activity. Allanite, bastnesite, titanite, and apatite are prone to break down at incipient weathering stage while monazite, xenotime and zircon are resistant minerals. On a long‐term basis, REY are roughly inert in the immature saprolite and depleted in the soil cover and are strongly redistributed among secondary phases. LREE and HREY are mainly controlled by poorly crystallized secondary iron oxides and oxyhydroxides, and, to a lesser extent, by secondary phosphates (e.g., rhabdophane). Th, inert in the saprolite, is controlled by well‐crystallized secondary iron oxides and oxyhydroxides and likely by secondary thorianite (ThO2). REY and Th are not easily mobilized during surface processes such as chelation by organic matter and uptake by plants. REY and Th are, by far, exported as suspended sediments associated with iron oxides. The U export by groundwater and suspended sediments dominates over those of streambed sediments and stream.

Magmatic evolution and W-Sn-U-Nb-Ta mineralization of the Mesozoic Jiulongnao granitic complex, Nanling Range, South China

Abstract South China is famous for its large-scale mineralization genetically related to widespread Mesozoic granites. The Jiulongnao complex is one of the largest intrusions in southern Jiangxi Province, and is closely associated with W-dominated polymetallic mineralization. The complex comprises four intrusive phases (I-IV). Phase I consists of medium- to coarse-grained, biotite granite, whereas medium- to coarse-grained, porphyritic, biotite granite and muscovite-bearing granite comprise Phase II. Phase III is composed of medium- to fine-grained, porphyritic, biotite granite and Phase IV consists of medium- to fine-grained, garnet- and biotite-bearing granite. These intrusive phases have LA-ICP-MS zircon U-Pb ages of 160.9 ± 0.6 Ma, 158.6 ± 0.7 Ma, 157.0 ± 1.5 Ma and 154.1 ± 1.2 Ma, respectively. All of the granites are enriched in Rb, Th, U, Ta and Pb, and significantly depleted in Eu, Ba, Nb, Sr, P and Ti. They have eHf(t) and eNd(t) values ranging from −17.9 to −8.2, and −10.7 to −9.8, respectively, with two-stage Hf model ages of 1.7–2.3 Ga. They are S-type granites derived by partial melting of Paleo-Proterozoic metasedimentary rocks. The four phases have TZr of 746–760 °C, 712–802 °C, 798–810 °C, and 648–731 °C, respectively. Oxygen fugacities of Phases I to III display a decreasing trend but Phase IV has much higher oxygen fugacity in its early stage, which decreased in the late stage. The four phases have different accessory mineral assemblages: Phase I is characterized by an assemblage of zircon, apatite, fergusonite, thorite and uraninite, whereas Phase II is rich in thorite, uraninite, REE-bearing minerals (e.g., monazite, fergusonite and xenotime) and fluorite. Monazite is the main accessory mineral in Phase III, whereas Phase IV contains a wide range of minerals, including thorite, uraninite, Nb-Ta-bearing minerals (e.g., fergusonite and pyrochlore), cassiterite and rutile. These mineral assemblages, together with other geological and geochemical features, suggest that W mineralization was genetically related to Phases I, II and IV, whereas U mineralization was most extensive in Phase II. Sn, Nb and Ta mineralization was associated with the highly fractionated of magmas of Phase IV.

A source-depleted Early Jurassic granitic pluton from South China: Implication to the Mesozoic juvenile accretion of the South China crust

Abstract Source-depleted granites were rarely reported in South China. Hereby we identified such a granitic pluton, the Tiandong pluton, at Northeastern Guangdong province in Southeastern (SE) China. Whole-rock Sr-Nd and zircon Hf isotopes of the Tiandong granites both revealed obviously depleted source signatures, with initial isotopic values of initial 87Sr/86Sr = 0.7032–0.7040, eNd(t) = 1.1–1.5, and eHf(t) = 6–13, respectively. Zircon U-Pb dating implied the granite was intruded in Early Jurassic (188 Ma). The dominant minerals of the Tiandong granite consist of K-feldspar, plagioclase, quartz and biotite, with accessory mineral assemblage of apatite + zircon + magnetite. Based on the mineralogy and the depleted isotopic signature, the granites chemically show I-type affinity such as low Zr + Nb + Ce + Y (131.6 to 212.2), 104 × Ga/Al (2.12–2.27), A/CNK values

Mineral characterization as a tool in the implementation of geometallurgy into industrial mineral mining

Industrial minerals play an important role in the Norwegian mining industry. The presented research focuses on defining marble deposit variability in order to evaluate parameters that can potentially be related to downstream process performance. Two types of marble raw material (K2 and K5) from the Verdalskalk open pit, used for precipitated calcium carbonate production (PCC) were tested for possible differences within texture, grain boundaries shape, grain size, accessory mineral assemblage. Additionally, surface hardness was measured using the Proceq Equotip 3 D device. K5 type was found to be finer-grained compared to K2. The presence of quartz was more pronounced in K2 type material, which possessed higher surface hardness values and presented higher variation of those.

Variscan metagranitoids in the central Tauern Window (Eastern Alps, Austria) and their role in the formation of the Felbertal scheelite deposit

Abstract The W mineralised Early Carboniferous orthogneisses (K1 and K3 orthogneiss) in the Felbertal scheelite deposit represent a chemically evolved metagranitoid series. Some of its characteristics are high concentrations of F (

Geology and mineralogy of the Alakha spodumene granite porphyry deposit, Gorny Altai, Russia

The Alakha lithium–tantalum deposit in the southern Altai, Russia, is represented by a stock of spodumene-bearing granite porphyry localized in the Kalba–Narym–Koktogai lithium–tantalum rare-metal granitic belt, unique in extent (more than 1000 km). This belt is a part of the Altai accretionary–collisional system. Judging from forecasting, the Alakha deposit can be regarded as an uneroded proxy of a pegmatite body both in dimensions and mean Li2O and Ta2O5 contents (0.98 wt % and 114 ppm, respectively); however, the oregenerating potential of this deposit remains insufficiently studied and had not yet been claimed. In this paper, we attempt to fill this gap with a detailed mineralogical study, which allows us to provide insights into the crystallization of Li-bearing high-silicic magma and redistribution of components during magmatic and postmagmatic processes. Accessory mineral assemblages in muscovite–spodumene–K-feldspar granite porphyry and muscovite albitite—the main petrographic rock varieties of the Alakha stock—turned out to be almost identical. A significant similarity in the chemistry of major rock-forming minerals is established for spodumene granite porphyry of the Alakha stock and spodumene pegmatites from large deposits, which makes it possible to suggest that they are close in the petrogenetic mechanism of their formation. The mineral assemblages of muscovite albitite in the apical portion of the Alakha stock are connected by gradual transition with those of spodumene granite porphyry. Such a transition is caused by postmagmatic metasomatic alteration of the latter.

The chevkinite group: underestimated accessory phases from a wide range of parageneses

Abstract Chevkinite-group minerals are widespread in a very wide range of igneous and metamorphic parageneses, forming important components of accessory mineral assemblages. Their presence in a rock may be difficult to establish by standard optical techniques, which has contributed to their importance being underestimated; a combination of SEM and EMPA is recommended here. Currently, there are eleven IMAapproved members of the group but undoubtedly several more will be described in the near future. There is considerable compositional variation in the group, which can be expressed as: REE + M2+ C + M3+ C = Ca2+ A + Sr + Ti4+ C + Zr4+ C where A and C are structural sites. Chevkinite-group minerals strongly fractionate geochemically coherent pairs, such as LREE-HREE, Nb-Ta, Zr-Hf and Th-U, and thus play a critical role in geochemical modelling.

Origin of high Sr/Y magmas from the northern Taihang Mountains: Implications for Mesozoic porphyry copper mineralization in the North China Craton

Abstract A number of porphyry Cu deposits have been described from east China which occur in association with Mesozoic high Sr/Y rocks within the continental interior rather than in an arc setting. However, the origin of these high Sr/Y rocks remains controversial. In this study we report precise zircon U–Pb age, as well as major-trace element and Sr–Nd–Pb isotope compositions from the Mujicun Cu mineralized porphyries in the northern Taihang orogen of eastern North China Craton (NCC). LA-ICP-MS zircon U–Pb dating yields an emplacement age of 143 ± 2 Ma, identical to the molybdenite Re–Os isochron ages of 142.5 ± 1.4 Ma for this intrusion. Like most of the Mesozoic adakitic rocks from the eastern NCC, the ore-bearing porphyries and associated volcanic lavas from northern Taihang orogen are rich in large ion lithophile elements and light REE, and have highly differentiated REE patterns. The porphyries and associated volcanic lavas have Sr–Nd–Pb isotopic compositions showing EM1-like isotopic signatures. Such geochemical and isotopic features confirm that the parental magma for these rocks originated from melting of an enriched sub-continental lithospheric mantle source. In comparison to the associated lavas, the ore-bearing porphyries have pronounced low Fe 2 O 3 T , TiO2 and P2O5 contents, and middle-heavy REE (and Y) and Zr concentrations, indicating fractional crystallization of amphibole with the observed accessory mineral assemblage such as Fe–Ti oxides, titanite, zircon and apatite. On the other hand, most of the porphyries exhibit relatively high Al2O3, Ba and Sr concentrations and pronounced positive Eu anomalies, excluding significant plagioclase fractionation due to suppression of the high water content in the magmas. The presence of the contemporary amphibole cumulates regionally exposed in the study area strongly support significant amphibole fractionation during the formation of the Mujicun porphyries. Thus, fractionation of a water-saturated magma is proposed as a plausible mechanism through which the high Sr/Y magmas possessing a high potential to generate porphyry Cu deposits formed. The identification of high Sr/Y magmas associated with porphyry Cu systems during the craton destruction of the NCC has important implications in formulating strategies for prospecting porphyry Cu deposits.

Petrogenesis of basalts for Sangxiu Formation in the central segment from Tethyan Himalayas: Plume-lithosphere interaction

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 125-7

The Geology of the Rakkurijarvi Cu-(Au) Prospect, Norrbotten: A New Iron Oxide-Copper-Gold Deposit in Northern Sweden

The Rakkurijarvi prospect consists of a group of mineralized magnetite and lithic breccias within the ca. 2.05- to 1.90-Ga Proterozoic supracrustal sequence of the Kiruna district, northern Sweden. Potentially economic grades of Cu and Au, largely in the form of chalcopyrite and other sulfide assemblages, are hosted in brecciated magnetite and metavolcanic rocks. The extent of the mineralization is currently open, both downdip and along strike. The deposit was discovered through an integrated geophysical and geochemical program focused on iron oxide-copper-gold (IOCG)–style mineralization. It is hosted by brecciated greenschist facies metavolcanic rocks within and adjacent to an east-northeast–trending shear zone. The dominant characteristics of the deposit are consistent with the IOCG class and include magnetite and lithic breccias hosted in a metavolcanic sequence, with matrices of albite, actinolite, and calcite surrounded by halos of sodic (albitescapolite) and potassic (scapolite-K-feldspar-biotite) alteration. A distinctive accessory mineral assemblage includes apatite, titanite, and allanite. The paragenesis and textural evolution of the deposit includes early Narich alteration accompanying massive magnetite alteration. The Na-rich alteration is overprinted by potassic alteration (also associated with magnetite), although the paragenesis is complex and multiple generations of both sodic and potassic alteration are recognized. Alteration of lithic clasts to magnetite confirms a metasomatic origin, as opposed to an orthomagmatic origin, for the magnetite mineralization. Re-Os analyses of two separates of molybdenite intergrown with magnetite, interpreted as cogenetic with the sulfide assemblage, yield mineralization ages of 1853 ± 6 and 1862 ± 6 Ma. Reconnaissance bulk-rock chemistry of the host volcanic rocks is consistent with an intermediate volcanic protolith, but much of the original character of the rocks is masked by albitization and incipient iron, sodic, and potassic alteration. The data also indicate significant element mobility during metasomatism and, in particular, the addition of Ti to the rock mass in biotite and as titanite. The compositions of secondary minerals are consistent with alteration and mineralization caused by highly saline fluids of relatively low F activity. The stable isotope characteristics of calcite, with δ18OSMOW ranging from 9.43 to 19.89 per mil and δ13CPDB ranging from –11.69 to +4.88 per mil, suggest that the fluids of the calcite and sulfide stage were derived from a magmatic source but had interacted extensively with local sedimentary and volcanic rocks.

A survey of accessory mineral assemblages in peralkaline and more aluminous A-type granites of the southeast coastal area of China

AbstractAn extensive belt of A-type granite exists along the southeast coast of China. The granites are divided into peralkaline and more aluminous subgroups which differ in mineral assemblages, mineral compositions and textures. In the peralkaline subgroup, primary magmatic Th-rich zircon is typically overgrown by Th-poor zircon containing thorite micro-inclusions. REE minerals in this subgroup are dominated by allanite-(Ce), chevkinite-(Ce), titanite and pyrochlore. Fe-Ti oxides are titanian magnetite and Mn-rich ilmenite. In contrast, in the more aluminous subgroup rocks, zircon is weakly zoned and exhibits very low Th but relatively high U contents. The REE minerals are dominated by Th-rich monazite-(Ce). Titanium-poor magnetite, pyrophanite and rutile are the major Fe-Ti oxides. These occurrences indicate that peralkaline magmas favour the formation of REE silicates, whereas magmas with higher alumina saturation stabilize REE phosphates. Peralkaline granites crystallized at temperatures 50–100°C greater than the more aluminous granites, but under lower oxidation conditions. These differences in formation conditions of the two A-type granite subgroups, deduced by accessory mineralcharacteristics, are inferred to be related to magma derivation at different crustal levels, with peralkaline magma deriving from a deeper crustal level with more mantle input.

The Gothara plagiogranite: evidence for oceanic magmatism in a non-ophiolitic association, North Khetri Copper Belt, Rajasthan, India?

Abstract The Gothara granitoid, a small pluton in the vicinity of Khetri town, is one of several granitoid bodies that intrude the Palaeo-Mesoproterozoic Delhi Supergroup of the Khetri Copper Belt (KCB) in north Rajasthan, India. Detailed petrological and geochemical investigations characterise the Gothara granitoid as a plagiogranite. It exhibits profuse granophyric intergrowth between quartz and chessboard albite, besides having the low Al2O3, extremely low K2O, Rb, Ba, and high Na2O that are characteristic of plagiogranites, and matches some typical oceanic plagiogranites of the world in totality. However, unlike the other known plagiogranites, it is mineralogically and chemically exceptionally homogeneous, almost free from alteration and occurs in a non-ophiolitic setting. The field relationships of the Gothara plagiogranite, particularly its dual and overlapping relationship with the mafic magmatic rocks, its accessory mineral assemblage, I-type characteristics and the major, trace and REE geochemistry, all point towards its fractionation-related petrogenesis from a mafic magma of mantle origin in an oceanic ridge tectonic regime. This discovery is of importance not only for the petrotectonic evolution of the Khetri Copper Belt but also for the petrogenesis of plagiogranites elsewhere in the world.