In-situ LA-ICP-MS trace elemental analyses of magnetite: The Bayan Obo Fe-REE-Nb deposit,North China

The Bayan Obo Fe-REE-Nb deposit in northern China is the world's largest light REE deposit, and also contains considerable amounts of iron and niobium metals. Although there are numerous studies on the REE mineralization, the origin of the Fe mineralization is not well known. Laser ablation (LA) ICP-MS is used to obtain trace elements of Fe oxides in order to better understand the process involved in the formation of magnetite and hematite associated with the formation of the giant REE deposit. There are banded, disseminated and massive Fe ores with variable amounts of magnetite and hematite at Bayan Obo. Magnetite and hematite from the same ores show similar REE patterns and have similar Mg, Ti, V, Mn, Co, Ni, Zn, Ga, Sn, and Ba contents, indicating a similar origin. Magnetite grains from the banded ores have Al + Mn and Ti + V contents similar to those of banded iron formations (BIF), whereas those from the disseminated and massive ores have Al + Mn and Ti + V contents similar to those of skarn deposits and other types of magmatic-hydrothermal deposits. Magnetite grains from the banded ores with a major gangue mineral of barite have the highest REE contents and show slight moderate REE enrichment, whereas those from other types of ores show light REE enrichment, indicating two stages of REE mineralization associated with Fe mineralization. The Bayan Obo deposit had multiple sources for Fe and REEs. It is likely that sedimentary carbonates provided original REEs and were metasomatized by REE-rich hydrothermal fluids to form the giant REE deposit.     

Investigation on mineralogy,geochemistry and fluid inclusions of the Goushti hydrothermal magnetite deposit,Fars Province,SW Iran: A comparison with IOCGs

The Goushti iron deposit from Dehbid area located in the Sanandaj-Sirjan metamorphic Belt (SSB), SW Iran is hosted by the Early Mesozoic silicified dolomite. Mineralized zones are lithostructurally controlled and oriented NW-SE parallel to the Zagros Orogenic Belt (ZOB). Magnetite, the major ore mineral, occurs as open space fillings and is accompanied by the secondary mineral phases hematite, goethite and martite. Gangue minerals mainly include quartz, dolomite and K-feldspar are associated with minor hydrosilicates. Calc-silicates such as wollastonite and diopside, minerals typical of skarns, are virtually absent from the ore zones. Fe₂O₃ content in the mineralized zones varies in the range of 38–75 wt%. The concentrations of Au, Cu, P, Ti, Cr and V as well as Co/Ni, Cr/V, (LREE)/(HREE), Eu/Sm and La/Lu values and Eu-Ce anomalies of the studied ores indicate that the Goushti deposit is a hydrothermal magnetite type. The subvolcanic rhyolite and basalt in this area are regarded as the source of iron and heat in the mineralizing system. The fluid inclusion data showed that magnetite deposited from the ore-bearing fluid with salinities 1–7 wt% NaCl equivalent at temperatures of 130–200 °C. A decrease in temperature and pressure, supplemented by fluid mixing, is the major controlling factor in iron oxide precipitation. The field relationships and mineralogical–geochemical characteristics of iron ores indicate that the Goushti hydrothermal deposit could not be classified as a member of the IOCG (Iron Oxide-Copper-Gold) deposits.     

Hydrothermal alteration and ore-forming fluids associated with gold-tellurium mineralization in the Dongping gold deposit,China

The Dongping gold deposit hosted in syenites is one of the largest hydrothermal gold deposits in China and composed of ore veins in the upper parts and altered zones in the lower parts of the ore bodies. Pervasive potassic alteration and silicification overprint the wall rocks of the ore deposit. The alteration minerals include orthoclase, microcline, perthite, quartz, sericite, epidote, calcite, hematite and pyrite, with the quartz, pyrite and hematite assemblages closely associated with gold mineralization. The phases of hydrothermal alteration include: (i) potassic alteration, (ii) potassic alteration - silicification, (iii) silicification - epidotization - hematitization, (iv) silicification - sericitization - pyritization and (v) carbonation. Mass-balance calculations in potassic altered and silicified rocks reveal the gain of K₂O, Na₂O, SiO₂, HFSEs and transition elements (TEs) and the loss of REEs. Most major elements were affected by intense mineral reactions, and the REE patterns of the ore are consistent with those of the syenites. Gold, silver and tellurium show positive correlation and close association with silicification. Fluid inclusion homogenization temperatures in quartz veins range from 154 °C to 382 °C (peak at 275 °C–325 °C), with salinities of 4–9 wt.% NaCl equiv. At temperatures of 325 °C the fluid is estimated to have pH = 3.70–5.86, log fO₂ = − 32.4 to − 28.1, with Au and Te transported as Au (HS)₂⁻ and Te₂^2 − complexes. The ore forming fluids evolved from high pH and fO₂ at moderate temperatures into moderate-low pH, low fO₂ and low temperature conditions. The fineness of the precipitated native gold and the contents of the oxide minerals (e.g., magnetite and hematite) decreased, followed by precipitation of Au- and Ag-bearing tellurides. The hydrothermal system was derived from an alkaline magma and the deposit is defined as an alkaline rock-hosted hydrothermal gold deposit.     

Mineralogy,fluid inclusions,and isotopes of the Cihai iron deposit,eastern Tianshan,NW China: Implication for hydrothermal evolution and genesis of subvolcanic rocks-hosted skarn-type deposits

Most skarn deposits are closely related to granitoids that intruded into carbonate rocks. The Cihai (>100 Mt at 45% Fe) is a deposit with mineral assemblages and hydrothermal features similar to many other typical skarn deposits of the world. However, the iron orebodies of Cihai are mainly hosted within the diabase and not in contact with carbonate rocks. In addition, some magnetite grains exhibit unusual relatively high TiO₂ content. These features are not consistent with the typical skarn iron deposit. Different hydrothermal and/or magmatic processes are being actively investigated for its origin. Because of a lack of systematic studies of geology, mineral compositions, fluid inclusions, and isotopes, the genetic type, ore genesis, and hydrothermal evolution of this deposit are still poorly understood and remain controversial.The skarn mineral assemblages are the alteration products of diabase. Three main paragenetic stages of skarn formation and ore deposition have been recognized based on petrographic observations, which show a prograde skarn stage (garnet-clinopyroxene-disseminated magnetite), a retrograde skarn stage (main iron ore stage, massive magnetite-amphibole-epidote ± ilvaite), and a quartz-sulfide stage (quartz-calcite-pyrite-pyrrhotite-cobaltite).Overall, the compositions of garnet, clinpyroxene, and amphibole are consistent with those of typical skarn Fe deposits worldwide. In the disseminated ores, some magnetite grains exhibit relatively high TiO₂ content (>1 wt.%), which may be inherited from the diabase protoliths. Some distinct chemical zoning in magnetite grains were observed in this study, wherein cores are enriched in Ti, and magnetite rims show a pronounced depletion in Ti. The textural and compositional data of magnetite confirm that the Cihai Fe deposit is of hydrothermal origin, rather than associated with iron rich melts as previously suggested.Fluid inclusions study reveal that, the prograde skarn (garnet and pyroxene) formed from high temperature (520–600 °C), moderate- to high-salinity (8.1–23.1 wt.% NaCl equiv, and >46 wt.% NaCl equiv) fluids. Massive iron ore and retrograde skarn assemblages (amphibole-epidote ± ilvaite) formed under hydrostatic condition after the fracturing of early skarn. Fluids in this stage had lower temperature (220°–456 °C) and salinity (8.4–16.3 wt.% NaCl equiv). Fluid inclusions in quartz-sulfide stage quartz and calcite also record similar conditions, with temperature range from 128° to 367 °C and salinity range from 0.2 to 22.9 wt.% NaCl equiv. Oxygen and hydrogen isotopic data of garnet and quartz suggest that mixing and dilution of early magmatic fluids with external fluids (e.g., meteoric waters) caused a decrease in fluid temperature and salinity in the later stages of the skarn formation and massive iron precipitation. The δ¹⁸O values of magnetite from iron ores vary between 4.1 and 8.5‰, which are similar to values reported in other skarn Fe deposits. Such values are distinct from those of other iron ore deposits such as Kiruna-type and magmatic Fe-Ti-V deposits worldwide. Taken together, these geologic, geochemical, and isotopic data confirm that Cihai is a diabase-hosted skarn deposit related to the granitoids at depth.     

Mineralogy of high-field-strength elements (Y,Nb, REE) in the world-class Vergenoeg fluorite deposit,South Africa

The Vergenoeg fluorite deposit in the Bushveld Complex in South Africa is hosted by a volcanic pipe-like body. The distribution characteristics, composition and formation conditions of high-field-strength element (HFSE)-rich minerals in different lithological units of the deposit were investigated by optical and cathodoluminescence microscopy, scanning electron microscopy, X-ray fluorescence, inductively-coupled plasma mass-spectrometry and electron-probe microanalysis. The Vergenoeg host rocks comprise a diverse silica-undersaturated assemblage of fayalite–magnetite–fluorite with variably subordinate apatite and mineral phases enriched in rare-earth elements (REEs). The Sm–Nd isotope systematics of the fluorite from the various lithological units of the pipe support the model that the HFSE budget of the Vergenoeg pipe was likely derived from a Lebowa-type granitic magma. Isotopically, there is no evidence for other REE sources. Formation of the pipe, including development of the fluorite mineralization, occurred within the same time frame as the emplacement of other magmatic rock units of the Bushveld Complex (Sm–Nd isochron age for fluorite separates: 2040 ± 46 Ma). Hydrothermal alteration is manifested in strongly disturbed Rb–Sr isotope systematics of the Vergenoeg deposit, but did not affect its HFSE and REE budget. Whole-rock chondrite-normalized REE + Y distribution patterns of two types were observed: (i) flat patterns characteristic of magnetite–fluorite unit, gossan, metallurgical-grade fluorite (“metspar”) plugs and siderite lenses, and (ii) U-shaped patterns showing enrichment towards the heaviest REE (Tm–Lu) observed in the fayalite-rich units. Common HFSE minerals are complex Nb-rich oxides (samarskite, fergusonite), REE phosphates and fluorocarbonates. Additionally, fluocerite and REE silicates, whose identification requires further work, were found. Most of the HFSE-rich minerals are spatially associated with Fe-rich phases (e.g., pyrite, magnetite, greenalite and hematite). To a smaller extent, they are found finely disseminated or healing micro-fractures in fluorite. The whole-rock REE + Y distribution patterns of the individual lithological units are mainly controlled by the distribution of Yb-rich and Y-rich xenotime in these rocks. The common occurrence of bastnäsite-(Ce) in the gossan, “metspar” plugs and especially in the rhyolitic carapace at the pipe–wall-rock contact, controls the REE + Y distribution patterns of these rocks. HFSE minerals in the Vergenoeg pipe rocks have formed in several stages. Samarskite and coarse fluorapatite belong to the primary mineral assemblage. Fergusonite and Yb-rich xenotime formed during high- to moderate-temperature hydrothermal activity. Significant remobilization of the HFSE from the early-crystallized minerals (breakdown of fluorapatite and possibly allanite with release of REE + Y) and subsequent partial redistribution of these elements into near surface rocks are inferred. The late-stage assemblages are characterized by the presence of fine-grained REE fluorocarbonates, monazite-(Ce), monazite-(La) and xenotime-(Y).     

Dissolution–reprecipitation process of magnetite from the Chengchao iron deposit: Insights into ore genesis and implication for in-situ chemical analysis of magnetite

Magnetite formed in different environments commonly has distinct assemblages and concentrations of trace elements that can potentially be used as a genetic indicator of this mineral and associated ore deposits. In this paper, we present textural and compositional data of magnetite from the Chengchao iron deposit, Daye district, China to provide a better understanding in the formation mechanism and genesis of the deposit and shed light on analytical protocols for in-situ chemical analysis of hydrothermal magnetite. Magnetite grains from the ore-related granitoid pluton, mineralized endoskarn, magnetite-dominated exoskarn, and vein-type iron ores hosted in marine carbonate intruded by the pluton were examined using scanning electron microscopy and analyzed for major and trace elements using electron microprobe. Back-scattered electron images reveal that primary magnetite from the mineralized skarns and vein-type ores were all partly reequilibrated with late-stage hydrothermal fluids, forming secondary magnetite domains that are featured by abundant porosity and have sharp contact with the primary magnetite. These textures are interpreted as resulting from a dissolution–reprecipitation process of magnetite, which, however, are mostly obscure under optically.Primary magnetite grains from the mineralized endoskarn and vein-type ores contain high SiO₂ (0.92–3.21 wt.%), Al₂O₃ (0.51–2.83 wt.%), and low MgO (0.15–0.67 wt.%), whereas varieties from the exoskarn ores have high MgO (2.76–3.07 wt.%) and low SiO₂ (0.03–0.23 wt.%) and Al₂O₃ (0.54–1.05 wt.%). This compositional contrast indicates that trace-element geochemical composition of magnetite is largely controlled by the compositions of magmatic fluids and host rocks of the ores that have reacted with the fluids. Compared to its precursor mineral, secondary magnetite is significantly depleted in most trace elements, with SiO₂ deceasing from 1.87 to 0.47 wt.% (on average) and Al₂O₃ from 0.89 to 0.08 wt.% in mineralized endoskarn and vein type ores, and MgO from 2.87 to 0.60 wt.% in exoskarn ores. On the contrary, average content of iron is notably increased from 69.2 wt.% to 71.9 wt.% in the secondary magnetite grains. The results suggest that the dissolution–reprecipitation process has been important in significantly removing trace elements from early-stage magnetite to form high-grade, high-quality iron ores in hydrothermal environments. The textural and compositional data confirm that the Chengchao iron deposit is of hydrothermal origin, rather than being crystallized from immiscible iron oxide melts as previously suggested. This study also highlights the importance of textural characterization using various imaging techniques before in-situ chemical analysis of magnetite, as is the case for texturally complicated UTh-bearing accessory minerals that have been widely used for UPb geochronology study.     

Chadormalu Kiruna-type magnetite-apatite deposit,Bafq district,Iran: Insights into hydrothermal alteration and petrogenesis from geochemical,fluid inclusion,and sulfur isotope data

The Chadormalu is one of the largest known iron deposits in the Bafq metallogenic province in the Kashmar-Kerman belt, Central Iran. The deposit is hosted in Precambrian-Cambrian igneous rocks, represented by rhyolite, rhyodacite, granite, diorite, and diabasic dikes, as well as metamorphic rocks consisting of various schists. The host rocks experienced Na (albite), calcic (actinolite), and potassic (K-feldspar and biotite) hydrothermal alteration associated with the formation of magnetite–(apatite) bodies, which are characteristic of iron oxide copper-gold (IOCG) and iron oxide-apatite (IOA) systems. Iron ores, occurring as massive-type and vein-type bodies, consist of three main generations of magnetite, including primary, secondary, and recrystallized, which are chemically different. Apatite occurs as scattered irregular veinlets in various parts of the main massive ore-body, as well as apatite-magnetite veins and disseminated apatite grains in marginal parts of the deposit and in the immediate wall rocks. Minor pyrite occurs as a late phase in the iron ores. Chemical composition of magnetite is representative of an IOA or Kiruna-type deposit, which is consistent with other evidence.Whole rock geochemical data from various host rocks confirm the occurrence of Na, Ca, and K alteration consistent with the formation of albite, actinolite, and K-feldspar, respectively. The geochemical investigation also includes the nature of calc-alkaline igneous rocks, and helps elaborating on the spatial and temporal association, and possible contribution of mafic to felsic magmas to the evolution of ore-bearing hydrothermal fluids.Fluid inclusion studies on apatites from massive- and vein-type ores show a range of homogenization temperatures from 266 to 580 °C and 208–406 °C, and salinities from 0.5 to 10.7 wt.% and 0.3–24.4 wt.% NaCl equiv., respectively. The fluid inclusion data suggest the involvement of evolving fluids, from low salinity-high temperature, to high salinity-low temperature, in the formation of the massive- and vein-type ores, respectively. The δ³⁴S values obtained for pyrite from various parts of the deposit range between +8.9 and +14.4‰ for massive ore and +18.7 to +21.5‰ for vein-type ore. A possible source of sulfur for the ³⁴S-enriched pyrite would be originated from late Precambrian-early Cambrian marine sulfate, or fluids equilibrated with evaporitic sulfates.Field observations, ore mineral and alteration assemblages, coupled with lithogeochemical, fluid inclusion, and sulfur isotopic data suggest that an evolving fluid from magmatic dominated to surficial brine-rich fluid has contributed to the formation of the Chadormalu deposit. In the first stages of mineralization, magmatic derived fluids had a dominant role in the formation of the massive-type ores, whereas a later brine with higher δ³⁴S contributed to the formation of the vein-type ores.     

Mineralogical and geochemical investigations of the Mombi bauxite deposit,Zagros Mountains,Iran

The Mombi bauxite deposit is located in 165 km northwest of Dehdasht city, southwestern Iran. The deposit is situated in the Zagros Simply Fold Belt and developed as discontinuous stratified layers in Upper Cretaceous carbonates (Sarvak Formation). Outcrops of the bauxitic horizons occur in NW-SE trending Bangestan anticline and are situated between the marine neritic limestones of the Ilam and Sarvak Formations. From the bottom to top, the deposit is generally consisting of brown, gray, pink, pisolitic, red, and yellow bauxite horizons. Boehmite, diaspore, kaolinite, and hematite are the major mineral components, while gibbsite, goethite, anatase, rutile, pyrite, chlorite, quartz, as well as feldspar occur to a lesser extent. The Eh–pH conditions during bauxitization in the Mombi bauxite deposit show oxidizing to reducing conditions during the Upper Cretaceous. This feature seems to be general and had a significant effect on the mineral composition of Cretaceous bauxite deposits in the Zagros fold belt. Geochemical data show that Al₂O₃, SiO₂, Fe₂O₃ and TiO₂ are the main components in the bauxite ores at Mombi and immobile elements like Al, Ti, Nb, Zr, Hf, Cr, Ta, Y, and Th were enriched while Rb, Ba, K, Sr, and P were depleted during the bauxitization process. Chondrite-normalized REE pattern in the bauxite ores indicate REE enrichment (ΣREE = 162.8–755.28 ppm, ave. ∼399.36 ppm) relative to argillic limestone (ΣREE = 76.26–84.03 ppm, ave. ∼80.145 ppm) and Sarvak Formation (ΣREE = 40.15 ppm). The REE patterns also reflect enrichment in LREE relative to HREE. Both positive and negative Ce anomalies (0.48–2.0) are observed in the Mombi bauxite horizons. These anomalies are related to the change of oxidation state of Ce (from Ce³⁺ to Ce⁴⁺), ionic potential, and complexation of Ce⁴⁺ with carbonate compounds in the studied horizons. It seems that the variations in the chemistry of ore-forming solutions (e.g., Eh and pH), function of carbonate host rock as a geochemical barrier, and leaching degree of lanthanide-bearing minerals are the most important controlling factors in the distribution and concentration of REEs. Several lines of evidences such as Zr/Hf and Nb/Ta ratios as well as similarity in REE patterns indicate that the underlying marly limestone (Sarvak Formation) could be considered as the source of bauxite horizons. Based on mineralogical and geochemical data, it could be inferred that the Mombi deposit has been formed in a karstic environment during karstification and weathering of the Sarvak limy Formation.     

The Mesoproterozoic Abra polymetallic sedimentary rock-hosted mineral deposit,Edmund Basin,Western Australia

Abra is a blind, sedimentary rock-hosted polymetallic Fe–Pb–Zn–Ba–Cu ± Au ± Ag ± Bi ± W deposit, discovered in 1981, located within the easterly trending Jillawarra rift sub-basin of the Mesoproterozoic Edmund Basin, Capricorn Orogen, Western Australia. The Edmund Basin contains a 4–10 km thick succession of siltstone, sandstone, dolomitic siltstone, and stromatolitic dolomite. The age of the Edmund Group is between 1.66 and 1.46 Ga. The Abra polymetallic deposit is hosted in siltstone, dolostone, sandstone and conglomerate of the Irregully and Kiangi Creek Formations, but the mineralised zones do not extend above an erosion surface marking the change from fluvial to marine facies in the lower part of the Kiangi Creek Formation. The Abra deposit is characterised by a funnel-shaped brecciated zone, interpreted as a feeder pipe, overlain by stratiform–stratabound mineralisation. The stratiform–stratabound mineralisation includes a Red Zone and an underlying Black Zone. The Red Zone is characterised by banded jaspilite, hematite, galena, pyrite, quartz, barite, and siderite. The jaspilite and hematite cause the predominant red colouration. The Black Zone consists of veins and rhythmically banded sulphides, laminated and/or brecciated hematite, magnetite, Fe-rich carbonate and scheelite. In both zones, laminations and bands of sulphide minerals, Fe oxides, barite and quartz commonly exhibit colloform textures. The feeder pipe (Stringer Zone) merges with Black Zone and consists of a stockwork of Fe-carbonate-quartz, barite, pyrite, magnetite and chalcopyrite, exhibiting fluidised and/or jigsaw textures.The Abra mineral system is characterised by several overprinting phases of hydrothermal activity, from several stages of brecciation and fluidisation, barite and sulphide veining to barren low-temperature chalcedonic (epithermal regime) veining. Hydrothermal alteration minerals include multi-stage quartz, chlorite, prehnite, Fe-rich carbonate and albite. Albite (Na metasomatism) is an early alteration phase, whereas Fe-rich carbonate is a late phase. Fluid inclusion studies indicate that the ore fluids had temperatures ranging from 162 to 250 °C, with salinities ranging from 5.8 to about 20 wt.% NaCl. In the course of our studies, microthermometric and Raman microprobe analyses were performed on fluid inclusions in carbonate, quartz and barite grains. Fluid inclusions in quartz show homogenisation temperatures ranging from 150 to 170 °C with calculated salinities of between 3.7 and 13.8 wt.% NaCl.The sulphur isotopic system shows δ³⁴S values ranging from 19.4 to 26.6‰ for sulphides and from 37.4 to 41.9‰ for barite (Vogt and Stumpfl, 1987, Austen, 2007). Sulphur isotope thermometry between sulphides and sulphide–barite pairs yields values ranging from 219 to 336 °C (Austen, 2007).Galena samples were analysed for Pb isotope ratios, which have been compared with previous Pb isotopic data. The new Pb isotope systematics show model ages of 1650–1628 Ma, consistent with the formation of the host Edmund Basin.Re–Os dating of euhedral pyrite from the Black Zone yielded an age of ~ 1255 Ma. This age corresponds to the 1320–1170 Ma Mutherbukin tectonic event in the Gascoyne Complex. This event is manifested primarily along a WNW-trending structural corridor of amphibolite facies rocks, about 250 km to the northwest of the Abra area. It is possible that the Re–Os age represents a younger re-activation event of an earlier SEDEX style system with a possible age range of 1640–1590 Ma.A genetic model for Abra is proposed based on the above data. The model involves two end-members ore-forming stages: the first is the formation of the SEDEX style mineral systems, followed by a second multi-phase stage during which there was repeated re-working of the mineral system, guided by seismic activity along major regional faults.     

Geochemical and mineralogical characteristics of weathered ore in the Dalucao REE deposit,Mianning–Dechang REE Belt,western Sichuan Province,southwestern China

The Dalucao deposit in western Sichuan Province, southwest China, is one of the largest and most extensive rare earth element (REE) deposits in the Himalayan Mianning–Dechang REE Belt. Moreover, this is the only deposit identified in the southern part of the belt. The deposit contains the No. 1, 2, and 3 orebodies. The No. 1 and 3 orebodies are hosted in two breccia pipes within syenite–carbonatite rocks that intrude a Proterozoic quartz–diorite pluton. Both breccia pipes have elliptical horizontal cross-sections at the surface, being 200–400 m long, 180–200 m wide, and extending to > 450 m depth. The No. 1 and No. 3 orebodies have total thicknesses of 55–175 m and 14–58 m, respectively. REE mineralization is associated with four brecciation events that are recorded in both pipes. The ore grades in the No. 1 and 3 orebodies are similar, with the rocks containing 1.0–4.5% rare earth oxides (REOs). The No. 1 orebody is characterized by a mineral assemblage comprising fluorite + barite + celestite + bastnäsite (i.e., Type I), whereas the No. 3 orebody is characterized by an assemblage comprising fluorite + celestite + pyrite + muscovite + bastnäsite + strontianite (i.e., Type II). Significant amounts of weathered high-grade REE ore (up to 60 wt.% of the rock mass) is mainly present in the No. 1 orebody. This is the main ore-type targeted for exploration within the Dalucao deposit, but is rarely present in other deposits in the Mianning–Dechang REE Belt.Faulting and cryptoexplosive breccia events, possibly linked to movement on the Panxi Fault, were more common in the No. 1 orebody than in the No. 3 orebody. This facilitated the introduction of ore-forming hydrothermal fluids and provided space for the precipitation of REE minerals. Based on the present results, we infer that the Dalucao deposit was the product of multiple stages of ore formation. REE minerals formed in envelopes around, or fractures within, quartz, fluorite, calcite, barite, and celestite in the brecciated ores. The main REE minerals were deposited from hydrothermal fluids within cryptoexplosive breccia, followed by weathering that increased the ore grade. Petrographic studies and X-ray powder diffraction (XRD) analyses indicate that the weathered ore contains 5–60% REE minerals (including bastnäsite, parisite, and monazite), together with gangue (quartz, barite, celestite, and fluorite), large amount of clay minerals (smectite, illite, kaolinite, and sepiolite), and relict igneous minerals (quartz, albite, and K-feldspar). The weathered samples are strongly enriched in La (up to 92,390 ppm), Ce (up to 103,500 ppm), Pr (up to 8006 ppm), and Nd (up to 16,690 ppm) compared with the unweathered brecciated ores. Conversely, Sr concentrations are significantly more enriched in the brecciated ores (up to 256,500 ppm) than in the weathered ores (generally less than 2671 ppm with one exception of 37,850 ppm) due to less celestite. Calcite is largely absent from the weathered ores (except one sample with up to 30% mode), which contrasts with the brecciated ores that contain up to 75% calcite. The effects of weathering, oxidation, loss of ions, and hydration on the brecciated ores led to the refertilization of the REEs and an increase in the grade of the ore deposit.     

Iron isotope fractionation during skarn-type alteration: Implications for metal source in the Han-Xing iron skarn deposit

The Han-Xing iron mineralization in the central North China Craton is a typical Fe skarn deposit associated with altered diorites. Here we report the Fe isotopic compositions of whole rocks and mineral separates from this deposit with a view to evaluate the Fe isotope fractionation during the formation of Fe skarn deposit, and to constrain the metal source. The Fe isotopes show a large variation both in whole rocks and mineral separates. Altered diorites show a wide range in δ⁵⁶Fe values (− 0.07‰ to + 0.21‰ relative to the Fe isotope standard IRMM-014) which positively correlate with their TFe₂O₃/TiO₂ ratios (Fe₂O₃ and FeO calculated as TFe₂O₃). The positive correlation indicates that heavy Fe isotopes were preferentially leached from diorites during the skarn-type alteration. Among the metallic minerals, pyrite and pyrrhotite are isotopically heavier (+ 0.12‰ to + 0.48‰) than the magnetite (+ 0.07‰ to + 0.21‰). Fe isotope fractionation between mineral pairs demonstrates that magnetite did not attain Fe isotopic equilibrium with pyrite and pyrrhotite, whereas pyrite and pyrrhotite might have attained isotopic equilibrium. Petrological observations and major element data also suggest that iron was leached from the diorites during the skarn-type alteration. If the leached iron provides the main Fe budget of the Han-Xing Fe skarn deposit, magnetite in ores would be isotopically heavier than the unaltered diorite. However, our results are in contrast with the magnetite being isotopically lighter than the unaltered diorite. This suggests that the major Fe source of the Han-Xing Fe skarn deposit is not from the leaching of diorites, and might be from magmatic fluid which is isotopically lighter than the silicate melt. Our data demonstrate that Fe isotopes can be used as important tracers in deciphering the metal source of Fe skarn deposits.     

In-situ LA-ICP-MS trace elemental analyses of magnetite: Cu-(Au,Fe) deposits in the Khetri copper belt in Rajasthan Province,NW India

Magnetite is common in many ore deposits and their host rocks, and is useful for petrogenetic studies. In the Khetri copper belt in Rajasthan Province, NW India, there are several Cu-(Au, Fe) deposits associated with extensive Cu ± Fe ± Au ± Ag ± Co ± REE ± U mineralization hosted in phyllites, schists and quartzites of the Paleoproterozoic Delhi Supergroup. Ore bodies of these deposits comprise dominantly disseminated and vein-type Cu-sulfide ores composed of chalcopyrite, pyrite, and pyrrhotite intergrown with minor magnetite. There are also Fe-oxide ores with minor or no Cu-sulfides, which are locally overprinted by the mineral assemblage of the Cu-sulfide ores. In addition to the Fe-oxide and Cu-sulfide ores, the protolith of the Delhi Supergroup includes banded iron formations (BIFs) with original magnetite preserved (i.e. magnetite-quartzites) and their sheared counterparts. In the sheared magnetite-quartzites, their magnetite and quartz are mobilized and redistributed to magnetite and quartz bands. Trace elemental compositions of magnetite from these types of ores/rocks were obtained by LA-ICP-MS. The dataset indicates that different types of magnetite have distinct concentrations of Ti, Al, Mg, Mn, V, Cr, Co, Ni, Zn, Cu, P, Ge and Ga, which are correlated to their forming environments. Magnetite grains in magnetite-quartzites have relatively high Al (800–8000 ppm), Ti (150–900 ppm) and V (300–600 ppm) contents compared to those of BIFs in other regions such as the Yilgarn Craton, Western Australia and Labrador, Canada. The high Al, Ti and V contents can be explained by precipitation of the magnetite from relatively reduced, Al–Ti-rich water possibly involving hotter, seafloor hydrothermal fluids derived from submarine mafic volcanic rocks. Magnetite in sheared magnetite-quartzites is generally irregular and re-crystallized, and has Ni, Mn, Al, Cu and P contents lower than the magnetite from the unsheared counterparts, suggesting that the shearing-related mobilization is able to extract these elements from original magnetite. However, elevated contents of Ti, V, Co, Cr, Ge and Mg of the magnetite in the sheared magnetite-quartzites can be ascribed to involvement of external hydrothermal fluids during the shearing, consistent with occurrence of some hydrothermal minerals in the samples.Compositions of magnetite from the Fe-oxide and Cu-sulfide ores are interpreted to be controlled mainly by fluid compositions and/or oxygen fugacity (fO₂). Other potential controlling factors such as temperature, fluid–rock interaction and co-precipitating minerals have very limited impacts. Magnetite in the Cu-sulfide ores has higher V but lower Ni contents than that of the Fe-oxide ores, likely indicating its precipitation from relatively reduced, evolved fluids. However, it is also indicated that the two types of magnetite do not show large distinctions in terms of concentrations of most elements, suggesting that they may have precipitated from a common, evolving fluid. We propose a combination of Ge versus Ti/Al and Cr versus Co/Ni co-variation plots to discriminate different types of magnetite from the Khetri copper belt. Our work agrees well with previous studies that compositions of magnetite can be potentially useful for provenance studies, but also highlights that discrimination schemes would be more meaningful for deposits in a certain region if fluid/water chemistry and specific formation conditions reflected in compositions of magnetite are clearly understood.     

In-situ LA-ICP-MS trace elemental analyses of magnetite and Re–Os dating of pyrite: The Tianhu hydrothermally remobilized sedimentary Fe deposit,NW China

The Tianhu Fe deposit (> 104 Mt at 42% TFe) in the Eastern Tianshan (NW China) is hosted in the schist, quartzite, marble, and amphibolite of the Neoproterozoic Tianhu Group. The deposit consists of disseminated, banded and massive ores. Metallic minerals are dominantly magnetite and pyrite, with minor titanite, pyrrhotite, chalcopyrite, and sphalerite. Gangue minerals include dolomite with minor forsterite, diopside, apatite, biotite, chlorite, tourmaline, tremolite, talc, calcite, and magnesite. Pyrite separates from ores have 10.7 to 54.7 ppb Re and 0.033 to 0.175 ppb common Os. Those from the massive ores have a model 1 isochron age of 535 ± 36 Ma (2σ), in agreement with the isochron age (528 ± 18 Ma) of pyrite from the banded ores by regression of seven Re–Os analyses. The Re–Os age of ~ 530 Ma reflects the timing of a hydrothermal event that remobilized the Tianhu deposit. Magnetite has Mg, Al, Ti, V, Mn, Zn, and Ga contents ranging from ~ 5 to 3500 ppm and Cr, Co, Ni, and Sn contents ranging from ~ 1 to 200 ppm. Most magnetite grains have Ca + Al + Mn and Ti + V contents similar to those of the banded iron formation (BIF). Some grains have elevated Ti and V contents, indicating that that magnetite was formed by sedimentary process and overprinted by hydrothermal activity. Pyrite has δ³⁴S_CDT values from − 9.23 to 10.96‰, indicating that the sulfur was reduced from the marine sulfates either by bacterial or thermochemical processes. Pyrite has relatively high Co (~ 346 to 3274 ppm) but low Ni (~ 5.6 to 35.4 ppm) with Co/Ni ratios ranging from ~ 10 to 270, indicating remobilization from a volcanic–hydrothermal fluid. Therefore, the Tianhu Fe deposit was originally a sedimentary type deposit but was overprinted by a hydrothermal event related to volcanic activity.     

The post-collisional Cihai iron skarn deposit,eastern Tianshan,Xinjiang, China

The Cihai iron skarn deposit is located in the southern part of the eastern Tianshan, Xinjiang, northwestern China. The major iron orebodies are banded and nearly parallel to each other. The iron ores are hosted in an early diabase dike and in skarn. Post-ore diabase dikes cut the iron ores and their hosting diabase. Hydrothermal activity can be divided into four stages based on geological and petrographic observations: initial K–Na alteration (stage I), skarn-minor magnetite event (II), retrograde skarn-magnetite main ore event (III), and quartz–calcite–sulfide veining (IV). Zircon U–Pb dating yields ages of 286.5 ± 1.8 Ma for early diabase and 275.8 ± 2.2 Ma for post-ore diabase dikes. Amphibole separated from massive magnetite ore gives a ⁴⁰Ar–³⁹Ar plateau age of 281.9 ± 2.2 Ma and is the time of ore formation. Formation of the Cihai iron deposit is closely related to post-collisional magmatism and associated Cu–Ni–Au polymetallic mineralization in the eastern Tianshan.     

Magnetite geochemistry of the Longqiao and Tieshan Fe–(Cu) deposits in the Middle-Lower Yangtze River Belt: Implications for deposit type and ore genesis

Magnetite is a common mineral in many ore deposits and their host rocks, and contains a wide range of trace elements (e.g., Ti, V, Mg, Cr, Mn, Ca, Al, Ni, Ga, Sn) that can be used for deposit type fingerprinting. In this study, we present new magnetite geochemical data for the Longqiao Fe deposit (Luzong ore district) and Tieshan Fe–(Cu) deposit (Edong ore district), which are important magmatic-hydrothermal deposits in eastern China.Textural features, mineral assemblages and paragenesis of the Longqiao and Tieshan ore samples have suggested the presence of two main mineralization periods (sedimentary and hydrothermal) at Longqiao, among which the hydrothermal period comprises four stages (skarn, magnetite, sulfide and carbonate); whilst the Tieshan Fe–(Cu) deposit comprises four mineralization stages (skarn, magnetite, quartz-sulfide and carbonate).Magnetite from the Longqiao and Tieshan deposits has different geochemistry, and can be clearly discriminated by the Sn vs. Ga, Ni vs. Cr, Ga vs. Al, Ni vs. Al, V vs. Ti, and Al vs. Mg diagrams. Such difference may be applied to distinguish other typical skarn (Tieshan) and multi-origin hydrothermal (Longqiao) deposits in the MLYRB. The fluid–rock interactions, influence of the co-crystallizing minerals and other physicochemical parameters, such as temperature and fO₂, may have altogether controlled the magnetite trace element contents of both deposits. The Tieshan deposit may have had higher degree of fO₂, but lower fluid–rock interactions and ore-forming temperature than the Longqiao deposit. The TiO₂–Al₂O₃–(MgO + MnO) and (Ca + Al + Mn) vs. (Ti + V) magnetite discrimination diagrams show that the Longqiao Fe deposit has both sedimentary and hydrothermal features, whereas the Tieshan Fe–(Cu) deposit is skarn-type and was likely formed via hydrothermal metasomatism, consistent with the ore characteristics observed.     

Geology and genesis of Kafang Cu–Sn deposit,Gejiu district,SW China

Kafang is one of the main ore deposits in the world-class Gejiu polymetallic tin district, SW China. There are three main mineralization types in the Kafang deposit, i.e., skarn Cu–Sn ores, stratiform Cu ores hosted by basalt and stratiform Cu–Sn ores hosted by carbonate. The skarn mainly consists of garnet and pyroxene, and retrograde altered rocks. These retrograde altered rocks are superimposed on the skarn and are composed of actinolite, chlorite, epidote and phlogopite. Major ore minerals are chalcopyrite, pyrrhotite, cassiterite, pyrite and scheelite. Sulfur and Pb isotopic components hint that the sources of different types of mineralization are distinctive, and indicate that the skarn ore mainly originated from granitic magma, whereas the basalt-hosted Cu ores mainly derived from basalt. Microthermometry results of fluid inclusions display a gradual change during the ore-forming process. The homogenization temperature of different types of inclusions continuously decreases from early to late mineralization stages. The salinities and freezing temperatures exhibit similar evolutionary tendencies with the T _{homogenization}, while the densities of the different types keep constant, the majority being less than 1. Oxygen and hydrogen isotopic values (δ¹⁸O and δD) of the hydrothermal fluids fall within ranges of 3.1 to 7.7‰ with an average of 6.15‰, calculated at the corresponding homogenization temperature, and − 73 and − 98‰ with an average of − 86.5‰, respectively. Microthermometry data and H–O isotopes indicate that the ore-forming fluid of the Kafang deposit is mainly derived from magma in the early stage and a mixture of meteoric and magmatic water in late stage. Molybdenite Re–Os age of the skarn type mineralization is 83.4 ± 2.1 Ma, and the stratiform ores hosted by basalt is 84.2 ± 7.3 Ma, which are consistent with the LA-ICP-MS zircon age of the Xinshan granite intrusion (83.1 ± 0.4 Ma). The evidence listed above reflects the fact that different ore styles in the Kafang deposit belong to the same mineralization system.     

Mineralogical and geochemical studies of brecciated ores in the Dalucao REE deposit,Sichuan Province,southwestern China

The Dalucao deposit, located in western Sichuan Province, southwestern China, in the western part of the Yangtze Craton, is one of the largest and most extensive rare earth element (REE) deposits in the Himalayan Mianning–Dechang REE belt. Moreover, the Dalucao deposit is the only deposit identified in the southern part of the belt. The Dalucao deposit contains the No. 1, 2, and 3 orebodies; the No. 1 and 3 orebodies are both hosted in two breccia pipes, located in syenite–carbonatite host rocks. Both pipes have elliptical cross-sections at the surface, with long-axis diameters of 200–400 m and short-axis diameters of 180–200 m; the pipes extend downwards for > 450 m. No. 1 and No. 3 have total thickness varying between 55 and 175 m and 14 to 58 m respectively. The REE mineralization is associated with four brecciation events, which are recorded in each of the pipes. The ore grades in the No. 1 and 3 orebodies are similar, and consist of 1.0%–4.5% rare earth oxides (REOs). The No. 1 orebody is characterized by a Type I mineral assemblage (fluorite + barite + celestite + bastnäsite), whereas the No. 3 orebody is characterized by a Type II assemblage (fluorite + celestite + pyrite + muscovite + bastnäsite + strontianite). Argon (⁴⁰Ar/³⁹Ar) dating of hydrothermal muscovite intergrown with REE minerals in typical ores from the No. 1 and 3 orebodies yielded similar ages of 12.69 ± 0.13 and 12.23 ± 0.21 Ma, respectively, which suggest that both mineral assemblages formed coevally, rather than in paragenetic stages. Both ages are also similar to the timing of intrusion of the syenite–carbonatite complex (12.13 ± 0.19 Ma). The ore-mineral assemblages occur in breccias, veinlets, and in narrow veins. The ore veinlets, which usually show a transition to mineralized breccia or brecciated ores, are commonly enveloped by narrow veins and stringer zones with comparable mineral assemblages. The brecciated ores form 95% of the volume of the deposit, whereas brecciated ores are only a minor constituent of other deposits in the Mianning–Dechang REE belt. The carbonatite in the syenite–carbonatite complexes contains high concentrations of S (0.07–2.32 wt.%), Sr (16,500–20,700 ppm), Ba (3600–8400 ppm), and light REEs (LREE) (2848–10,768 ppm), but is depleted in high-field-strength elements (HFSE) (Nb, Ta, P, Zr, Hf, and Ti). The syenite is moderately enriched in large-ion lithophile elements (LILE), Sr (155–277 ppm), and Ba (440–755 ppm). The mineralized, altered, and fresh syenites and carbonatites exhibit similar trace element compositions and REE patterns. Brecciation events, and the Dalucao Fault and its secondary faults around the deposit, contributed to the REE mineralization by facilitating the circulation of ore-forming fluids and providing space for REE precipitation. Some hydrothermal veins composed of coarse-grained fluorite and quartz are distributed in the syenite–carbonatite complex. The oxygen isotope compositions of ore-forming fluids in equilibrium with quartz at 215 °C are − 4.95‰ to − 7.45‰, and the hydrogen isotope compositions of fluid inclusions in coarse-grained quartz are − 88.4‰ to − 105.1‰. The syenite–carbonatite complex and carbonatite are main contributors to the mineralization in the geological occurrence. Thus, the main components of the ore-forming fluids were magmatic water, meteoric water, and CO₂ derived from the decarbonation of carbonatite. According to the petrographic studies, bastnäsite mineralization developed during later stages of hydrothermal evolution and overprinted the formation of the brecciated fluorite–quartz hydrothermal veins. As low-temperature isotope exchange between carbonates of the carbonatite and water-rich magmatic fluids will lead to positive shifts in δ¹⁸O values of the carbonates, C–O isotopic compositions from the bulk primary carbonatite to hydrothermal calcite and bastnäsite changed (δ¹⁸O_V-SMOW from 8.0‰ to 11.6‰, and δ¹³C _V-PDB from − 6.1 to − 8.7‰). According to the chemical composition of syenite and carbonatite, REE chloride species are the primary complexes for the transport of the REEs in the hydrothermal fluids, and the presence of bastnäsite and parisite means the REE were precipitated as fluorocarbonates. High contents of Sr, Ba and S in the syenite–carbonatite complex led to the deposition of large amount of barite and celestite.     

Geological and geochemical characteristics in the paleo-weathering crust sedimentary type REE deposits,western Guizhou,China

A supergene REE deposit closely interrelated with the weathering of the Emeishan basalt formation was produced in the Xuanwei formation, the overlying stratum of the late Permian Emeishan basalt formation in West Guizhou, China. The host strata consist primarily of offwhite kaolinite clay rock and/or grayish black carbonaceous shale. Mineralogical analyses reveal that kaolinites are the major minerals in REE ores with small amounts of smectite, illite, boehmite, hornblende, pyrophyllite, calcite, dolomite and/or iron-bearing minerals, with a certain proportion of feldspar, quartz crystal debris and noncrystal debris. Geochemical analyses reveal high enrichment of trace elements like Cu, Nb, Ta, Zr and Hf. The host strata feature considerable lithological variability, close interrelation of the REE grade with the lithology and uneven spatial distribution of the REE ores, which are mostly found in Lufang, Maojiaping and Zhangsigou profiles of Weining County and can be as thick as 20 m. Of the five stratigraphic profiles, 48% have their whole-rock ∑REE higher than 1000 ppm. The REE in this framework consists primarily of ion adsorbed phases and REE-rich residual independent mineral phases. Comprehensive analyses suggest that the source may not only include the Emeishan basalt, but the intermediate acid volcanic rocks evolved from the Emeishan basalt in the later periods; the hydrothermal alteration subsequently imposed on the host strata might have boosted the mineralization of the rare earth. The preliminary genetic model should have been: the denudation product from the weathering of the parent rock was migrated to the sea-continental margin at the continent side carrying huge quantities of REE with it and was preserved by the quick marine transgression. The host strata consist primarily of kaolinite clay rock and/or carbonaceous shale, which are so far believed to be a sedimentary type REE deposit closely interrelated with weathering effect.     

A new genetic model for the Triassic Yangyang iron-oxide–apatite deposit,South Korea: Constraints from in situ U–Pb and trace element analyses of accessory minerals

The Yangyang iron-oxide–apatite deposit in South Korea has undergone multiple episodes of igneous activity, deformation, hydrothermal alteration, and iron-oxide–apatite (IOA) mineralization. The iron orebodies occur as concordant- to discordant-layered lenticular or massive magnetite and/or magnetite–pyrite ores. The iron mineralization occurs along a N–S-trending shear zone within the Yangyang syenite, which experienced both early ductile and later brittle deformations. Alteration was caused mainly by the injection of hydrothermal fluid through the shear zone, leading to Fe–P mineralization. We recognize multiple stages of alteration in the Yangyang deposit, based on a paragenesis that is defined by distinct mineral assemblages including Na–Ca–K alteration phases (e.g., albite, diopside, actinolite, and biotite) and accessory minerals containing high field strength elements (e.g., apatite, sphene, allanite, and monazite). The alteration around the magnetite ore body shows an evolutionary trend from Ca (–Na) alteration, through K to phyllic alterations. The Fe–P mineralization is associated with the Ca–K and K alteration products. The iron orebodies are hosted by deformed and altered syenite, which intruded the Paleoproterozoic gneiss complexes at 233 ± 1 Ma (SHRIMP U–Pb zircon age) in a post-collisional tectonic setting. LA-ICP-MS U–Pb dating of REE-rich sphene and apatite from the iron ores and alteration products yields Fe mineralization ages of 216 ± 3 Ma (sphene) and 212 ± 13 Ma (apatite). This is the first time, which IOA-type mineralization in the Korean Peninsula was dated as Triassic age related to post-collisional magmatism within the Gyeonggi Massif, South Korea. The U–Pb system was subsequently reset (208 ± 3 Ma–sphene and 151 ± 13 Ma–apatite) by Jurassic and Cretaceous magmatism. This unique geological evolution was responsible for Mesozoic metal enrichment and remobilization into suitable structural traps in the Yangyang district.     

REE redistribution during hydrothermal alteration of ores of the Kalahari Manganese Deposit

The Kalahari Manganese Deposit (KMD) is the largest land-based manganese deposit, hosting approximately 80% of the world's known, mineable manganese resources. The deposit, located near Kuruman in the Northern Cape Province of South Africa, is one of five erosional relics of the Paleoproterozoic (ca. 2.2 Ga) Hotazel Formation, with sedimentary manganese ores occurring as up to 50 m thick beds interbedded with banded iron-formation (BIF) and hematite lutite.The study focuses on the manganese ores of the Nchwaning–Gloria mining area of the northern KMD. In this area, pronounced mineralogical and major element alteration was imparted on the sedimentary manganese ores by a structurally-controlled hydrothermal fluid flow event. Most notable effects of hydrothermal alteration are the decomposition and leaching of Ca- and Mg-carbonate, and marked residual enrichment of manganese. On the basis of mineral assemblage, grade, texture and geochemical characteristics, three ore types were distinguished in the studied sample set, classified into least altered (LA), partially altered (PA) and advanced altered (AA) types. Advanced altered ores may be further classified into five different types, based on mineral assemblages that contain hausmannite and/or braunite as significant minerals. The rare earth element (REE) geochemistry of these fundamental ore types was studied in detail, to document REE mobility during hydrothermal alteration.Total REE concentrations in LA ores were found to be very low (14–22 ppm) and remarkably uniform, within the range typically observed for BIF. Hydrothermal alteration results in residual enrichment and a much larger scatter in REE contents. A small Ce anomaly observed in the protolith remains similar in magnitude when observed in PAAS-normalised REE plots. The data define, however, a power trend in the (Ce/Ce*) vs (Pr/Pr*) diagram. Such behaviour is interpreted in terms of a conservative system that was predominantly protolith-buffered. Local remobilisation of REE during hydrothermal alteration is attributed to the dissolution of diagenetic apatite and redistribution of hydrothermal trace minerals, including neoformed apatite, monazite and cerianite.