Andradite Research Articles

Explaining Color Change in Gem-Quality Andradite Garnet

The homomorphic substitution of the garnet group is common in nature. Two rare color-changing andradite garnets are studied in this paper. One color changes from yellowish-green in the presence of daylight to maroon under incandescent light; the other color changes from brownish yellow to brownish red. In this study, conventional gemological instruments, infrared (IR) spectroscopy, ultraviolet–visible–near infrared (UV–Vis–NIR) spectroscopy, Raman spectroscopy, and electron probe microanalysis (EPMA) were used to explore the gemology and coloration mechanisms of color-changing garnets. Experiments revealed that the color-changing gemstones in the study are andradite garnets. There are two transmission windows in the UV–Vis spectrum: the red region (above 650 nm) and the green region (centered at 525 nm). The chemical compositional analysis indicates that the samples are very low in Cr (<1 ppm) and high in Fe2+ (from 2.31 wt.% to 4.66 wt.%). The combined spectra and chemical compositional analysis show that Fe2+ is the main cause of the color change. Based on the IR spectrum (complex water peaks), UV–Vis–NIR spectrum (similar to that of Namibian andradite garnet), and chemical compositional analysis (low Cr content), it is concluded that color-changing andradite may be related to skarn rock genesis.

THE GENESIS OF GARNET-PYROXENE SYENITES (SVIATONOSSITES) OF THE MALOBYSTRINSKY MASSIF (SLYUDYANKA COMPLEX, SOUTH BAIKAL REGION): RESULTS OF GEOCHEMICAL AND ISOTOPIC STUDIES

This paper deals with the study of syenites and sviatonossites (andradite-bearing syenites) of the Malobystrinsky massif of the Slyudyanka complex (South Baikal region, Siberia), and a large monzonite dike similar in age and composition to the rocks of the massif considered. The studied rocks belong to a series of highly ferriferous and metaluminous A-type granitoids (ASI index <1). They are characterized by SiO2 45–65 wt. %, K2O+Na2O up to 12 wt. %, MgO <4 wt. %, TiO2 up to 2.5 wt. %, and Al2O3 up to 17 wt. %. CaO varies in a wide range, from 2.2 to 14.7 wt. %. The rocks are similar to each other in trace element composition and show patterns with troughs for Th-U, Nb-Ta and Ti. Low-amplitude negative Eu anomaly is observed in the distribution spectra of rare earth elements for the entire rock complex. The obtained Sm-Nd age of sviatonossites in the Malobystrinsky massif is 487.1±6.1 Ma (MSWD=0.99). Our results indicate that syenites and monzonites have εNd(t) –1.9…–2.8, at εSr(t) 21–30, and sviatonossites have εNd(t) –3.8…–4.1 at εSr(t) – 26. Model ages TNd(DM) for all rock types are Mesoproterozoic (1.3–1.4 Ga). Based on the chemical and Sr-Nd isotopic composition of the magmatic rocks studied, it can be assumed that they have been generated by partial melting of lower crustal rocks (amphibolites). Crystallization of andradite garnet in syenite magma can occur due to melt contamination with metamorphic host rocks of the Slyudyanka complex.

Genesis and Evolution of the Yolindi Cu-Fe Skarn Deposit in the Biga Peninsula (NW Turkey): Insights from Genetic Relationships with Calc-Alkaline Magmatic Activity

The current work investigates the impact of magmatic fluids and metasomatic processes on the Yolindi Cu-Fe skarn deposit in the Biga Peninsula, Turkey. It traces the stages of skarn evolution, from prograde to retrograde alterations, and investigates findings within a broader geological, mineralogical, and geochemical framework. Additionally, it assesses the evolutionary history of the Yolindi deposit in relation to calc-alkaline magmatic activity in an island-arc environment and compares its mineral compositions and genesis with other global and regional Cu-Fe skarn deposits. The Yolindi Cu-Fe skarn deposit in the Biga Peninsula was formed by the intrusion of Şaroluk quartz monzonite pluton into Upper Paleozoic Torasan Formation rocks such as phyllite, schists, hornfels, marble, and serpentinites. During skarnification, reactions between the magmatic fluids from the Şaroluk quartz monzonite pluton and the Torasan Formation produced skarn minerals associated with metals such as Fe and Cu. Initially, these reactions formed prograde skarn minerals such as augite-rich pyroxenes and andradite garnets with magnetite and pyrite. As the system cooled, these initial minerals underwent retrograde alteration, leading to the formation of minerals such as epidote, actinolite, and chlorite, as well as other copper and iron minerals including chalcopyrite, bornite, secondary magnetite, and specular hematite. Therefore, four main stages influenced the formation of the Yolindi Cu-Fe deposit: metamorphic bimetasomatic, prograde metasomatic, and retrograde metasomatic stages. Later, oxidation and weathering resulted in supergene minerals such as cerussite, malachite, and goethite, which serve as examples of the post-metamorphic stage. The mineralogical shifts, such as the andradite–grossular transition, reflect changing hydrothermal fluid compositions and characteristics due to the addition of meteoric fluids. Importantly, the formation of magnetite after garnet and clinopyroxene during the retrograde stage is evidenced by magnetite crystals within garnet. The mineral associations of the Yolindi Cu-Fe skarn deposit align with the global skarn deposits and specific Turkish skarns (e.g., Ayazmant Fe-Cu and Evciler Cu-Au skarn deposits). The Yolindi Cu-Fe skarn deposit, in association with ore-bearing solutions having magmatic origins, developed in an island-arc setting.

Dating hydrothermal processes related to the formation of the Lagoa Real uranium deposits using in situ U–Pb dating of andradite and titanite

To understand the mechanisms and origin of ore deposits, it is essential to know their age. However, the processes responsible for ore formation do not always leave a geochronological record, with hydrothermal processes often either not affecting zircon or causing only partial resetting. The Lagoa Real uranium province hosts the most important U resource in Brazil. The U mineralization is hosted by Na-rich rocks known as albitite, which also contain variable amounts of garnet, clinopyroxene, biotite, amphibole, magnetite and titanite. These rocks are presumed to be the result of hydrothermal alteration of the Lagoa Real intrusive suite, a group of A-type granitic rocks emplaced between 1740 and 1720 Ma. Despite the economic importance of these rocks, determining the age of metasomatism and U mineralization has been extremely difficult, with previous attempts using a variety of methods and targeting the whole rock, titanite, uraninite, heavy-mineral concentrates and zircon, yielding a wide variety of ages in the range of 1.5 to 0.9 Ga. This study is the first to directly date metasomatic minerals (garnet and titanite) with the in situ U–Pb dating method. Garnet and titanite from an unmineralized sample gave U–Pb ages of ca. 545 Ma and ca. 520 Ma, respectively. U–Pb dating of zircon from two other unmineralized samples yielded ages of 1740–1720 Ma, interpreted as igneous ages. Electron-microprobe dating of uraninite from mineralized samples produced ages of 530–520 Ma. Altogether, these results suggest that both metasomatism and U mineralization occurred as a result of fluids associated with the gravitational collapse phase (ca. 530–480 Ma) of the Araçuaí–West Congo orogen. This study illustrates the utility of U–Pb dating of andradite garnet for determining the age of mineralization related to metasomatism.

In Situ U—Pb Dating and Trace Element Analysis of Garnet in the Tongshanling Cu Polymetallic Deposit, South China

The grossular–andradite garnet is an ideal mineral for indicating the formation age of skarn, which also pretends to constrain skarn processes because of its higher REE (rare earth elements) content. The Tongshanling deposit is a medium-sized reduced skarn Cu–W–Pb–Zn deposit associated with a highly evolved I-type granodiorite intrusion in the Nanling metallogenic belt, South China. Different mineral assemblages, microscopic characteristics, and BSE images distinguish two kinds of garnets in the prograde and retrograde skarn stages. The garnet grains from the prograde skarn stage have a U–Pb isochron age of 165.4 ± 3.8 Ma (MSWD = 0.7) and that from the retrograde skarn stage have a U–Pb isochron age of 159.5 ± 1.7 Ma (MSWD = 1.8), implying that the thermal metamorphism and hydrothermal metasomatism mainly occurred in the middle Upper Jurassic. The total amount of rare earth elements (∑REE+Y) in the garnet gradually decreased and the REE patterns shifted from enriched HREE with Eu negative anomaly to HREE-depleted with Eu positive anomaly. The decreasing U content and increasing Eu anomaly in the retrograde skarn stage indicate a redox environment change from oxidation to reduction. However, garnet from different elevations within the same stage (+90 m, +5 m, −35 m, −200 m, and −400 m) exhibit similar REE patterns, despite weak cooling and significant depressurization processes confirmed by fluid inclusion microthermography. As a result, the REE content and patterns are dominated by the REE species of parent fluids, which are changed over time by symbiotic REE-enriched mineral precipitation and the redox environment, while being slightly affected by the fluid pressure. Grossular garnets, rich in U and REEs, and found in reduced skarn deposits, can constrain chronology and reveal the spatio-temporal zonal characteristics.

The petrogenesis of chemically zoned, phonolitic, Plinian and sub-Plinian eruptions of Somma-Vesuvius, Italy: Role of accessory phase removal, independently filled magma reservoirs with time, and transition from slightly to highly silica undersaturated magmatic series in an ultrapotassic stratovolcano

A mineralogical, major, LA-ICP-MS trace element mineral chemistry and bulk-rock geochemical study of juvenile samples of the Mercato, Avellino, Pompeii and Pollena eruptions, collected in stratigraphically and volcanologically well-characterized sections of the Somma-Vesuvius stratovolcano (Roman Magmatic Province), along with reference data on the 1944 CE and the Pomici di Base eruptions, highlights the compositional variability of bulk-rock and glass from leucite phonotephrite to garnet-bearing phonolite . The latter products have extreme fractionation of trace elements (e.g., La/Yb n = 126, Zr/Y = 89, Zr/Hf =78, Nb/Ta = 40; Th/U = 2.3), very low Sc, V, Y, HREE and very high As, Tl, Cs, Pb, Th and U. The Pomici di Base products, older than the eruptions described above, range from leucite-bearing shoshonites to trachytes , are devoid of garnet and belong to an independent liquid-line-of-descent, having also different fractionation between trace elements (e.g., La/Yb n = 15; Zr/Y = 12.4, Zr/Hf = 50, Nb/Ta = 15.6; Th/U = 3 in the Pomici di Base trachytes). A marked chemical variability of the observed phases is found. The geochemistry of garnet, amphibole , clinopyroxene and other phases shows wide variations of concentrations and elemental ratios (e.g., La/Yb n up to 520 in the sadanagaite coexisting with garnet). The magmatic evolution is dominated by low-pressure, oxidized, nearly closed-system fractional crystallization of clinopyroxene, plagioclase , leucite, ±magnetite, ±biotite, ±olivine and apatite in the transition tephrite-phonotephrite, and of potassic sanidine (±hyalophane), Fe-clinopyroxene, melanite garnet±Fe-amphibole in the transition tephriphonolite-phonolite. Mineralogic and geochemical evidence and modelling points out the existence of independent, zoned magma batches throughout the activity of the stratovolcano, which possibly started to crystallize at similar depths. The Somma-Vesuvius magmas thus evolved in shallow independent reservoirs with respect to those of the neighbouring volcanic complexes (Campi Flegrei, Ischia) as shown, for instance, by the contrasting compositional trends of clinopyroxene and amphibole, and have very limited evidence of crustal contamination (and/or carbonate assimilation). • Pollena, Pompeii, Avellino and Mercato are zoned eruptions with phonolites • The magmas which formed the various eruptions evolved independently • Different garnets dominate HREE, Y and HFSE; amphibole is strongly HREE-poor • The phonolites and their mineral compositions have no tendency to become peralkaline • F, Cl and H 2 O dominate the volatile budget

Sr-Nd Isotopic Composition of Pyroxenes as a Provenance Indicator of a Double-Volcanic Source in Sands of the Ofanto River (Southern Italy)

The sands of the Ofanto River (Southern Italy) include a volcaniclastic component represented by clinopyroxene, melanite garnet, amphibole and subordinate volcanic lithics. In order to infer the provenance of this component, we have conducted a microanalytical and isotopic study on the volcanic minerals of the sands collected in three different sites along the Ofanto River: (1) upstream sector, where only sedimentary rocks are drained; (2) middle course, along a tributary draining only the Monte Vulture volcanic rocks, (3) the mouth of the Ofanto. Moreover, minerals of the beach sands of the Tyrrhenian Cilento coast were analyzed for comparison. The microanalytical study and the Sr-Nd isotopic composition reveal the existence of two populations of pyroxenes characterized by distinct isotopic signatures and indicating a provenance from two different volcanic sources. One is confidently identifiable with Monte Vulture, and is evident in the minerals of the middle course and mouth sands. The pyroxenes of the upstream sector, isotopically similar to those of the Cilento coast, have a Campanian signature. The simultaneous occurrence of amphibole and garnet point to the explosive eruptions of Mount Vesuvius as the most probable sources of these minerals. The absence of Campanian pyroxenes at the mouth of the Ofanto River could be related to the occurrence of dams limiting the transport of solids along the river.

Genesis of the titanian andradite in alkaline rocks and their petrological and mineralization significance: A study example of the garnet from the Fanshan ultramafic-syenitic complex in North China Craton

Genesis of the titanian andradite in alkaline rocks and their petrological and mineralization significance: A study example of the garnet from the Fanshan ultramafic-syenitic complex in North China Craton

Evolution of ultrapotassic volcanism on the Kaapvaal craton: deepening the orangeite versus lamproite debate

Abstract Orangeites are a significant source of diamonds, yet ambiguity surrounds their status among groups of mantle-derived potassic rocks. This study reports mineralogical and geochemical data for a c. 140 Ma orangeite dyke swarm that intersects the Bushveld Complex on the Kaapvaal craton in South Africa. The dykes comprise distinctive petrographic varieties that are linked principally by olivine fractionation, with the most evolved members containing minor amounts of primary carbonate, sanidine and andradite garnet in the groundmass. Although abundant groundmass phlogopite and clinopyroxene have compositions that are similar to those of cratonic lamproites, these phases show notable Ti-depletion, which we consider a hallmark feature of type orangeites from the Kaapvaal craton. Ti-depletion is also characteristic of bulk rock compositions and is associated with strongly depleted Th–U–Nb–Ta contents at high Cs–Rb–Ba–K concentrations. The resultant high large ion lithophile element/high field strength element ratios of orangeites suggest that mantle source enrichment occurred by metasomatic processes in the proximity of ancient subduction zones. The Bushveld-intersecting orangeite dykes have strongly enriched Sr–Nd–Hf isotopic compositions (initial 87 Sr/ 86 Sr = 0.70701–0.70741; ε Nd = −10.6 to −5.8; ε Hf = −14.4 to −2.5), similar to those of other orangeites from across South Africa. Combined with the strong Ti–Nb–Ta depletion, this ubiquitous isotopic feature points to the involvement of ancient metasomatized mantle lithosphere in the origin of Kaapvaal craton orangeites, where K-rich metasomes imparted a ‘fossil’ subduction geochemical signature. Previous geochronology studies identified ancient K-enrichment events within the Kaapvaal cratonic mantle lithosphere, possibly associated with collisional tectonics during the 1.2–1.1 Ga Namaqua–Natal orogeny of the Rodinia supercontinent cycle. It therefore seems permissible that the cratonic mantle root was preconditioned for ultrapotassic magma production by tectonomagmatic events that occurred along convergent plate margins during the Proterozoic. However, reactivation of the K-rich metasomes had to await establishment of an extensional tectonic regime, such as that during the Mesozoic breakup of Gondwana, which was accompanied by widespread (1000 × 750 km) small-volume orangeite volcanism between 200 and 110 Ma. Although similarities exist between orangeites and lamproites, these and other potassic rocks are sufficiently distinct in their compositions such that different magma formation processes must be considered. In addition to new investigations of the geodynamic triggers of K-rich ultramafic magmatism, future research should more stringently evaluate the relative roles of redox effects and volatile components such as H 2 O–CO 2 –F in the petrogeneses of these potentially diamondiferous alkaline rocks.

“Horsetail” Inclusions in the Ural Demantoids: Growth Formations

The term “demantoid”, first proposed in 1856 by the famous Finnish mineralogist Nils von Nordensheld, refers to a highly dispersed yellow-green mineral from the Central Urals placers. In 1874, it was found to be a gem variety of andradite garnet. “Horsetail” inclusions are considered a sign of the Ural type demantoid. Although these inclusions are large (visible to the naked eye), their diagnostics remains debatable: some researchers attribute them to byssolite (amphibole-asbestos), others consider them chrysotile. We investigated the horsetail inclusions in the Ural demantoids through various methods: optical microscopy, scanning electron microscopy (SEM), Raman spectrometry, X-ray powder diffraction, and thermal analysis. In most cases, “horsetail” inclusions in the Ural demantoid were represented by hollow channels and only the outcrops, on the demantoid surface, were occasionally filled with serpentine (established by SEM); in one case, magnetite was observed. Hollow canals were usually collected not in bundles, such as a “horsetail”, but in fans, sometimes curved into cones. The structure of the grains was spheroidal, sectorial, and sometimes had induction surfaces, which, to the periphery of the grain, were replaced by tubular channels assembled in a fan. The specifics of the growth of the “horsetail” inclusions of the demantoid grains can be explained by the decompression conditions that arose when the ultrabasites (a crust-mantle mixture) were squeezed upwards during collision.

Garnet major and trace element evidence of the alteration and mineralizing processes associated with genesis of the Qiaomaishan skarn deposit, Xuancheng ore district, eastern China

The Qiaomaishan deposit is a skarn-type Cu–W deposit located within the Xuancheng ore district of the Middle–Lower Yangtze River Metallogenic Belt of eastern China. The mineralization within the deposit is predominantly located along the contact between a porphyritic granodiorite intrusion and surrounding Carboniferous limestone units. The deposit is associated with a well-developed and zoned skarn system that includes both endoskarn and exoskarn zones. This study presents new garnet major, trace, and rare earth element (REE) geochemical data for these skarn zones, discusses the factors (e.g., crystal chemistry, oxygen fugacity and water/rock interaction) that control the incorporation of trace elements into garnet, and constrains the formation of skarn within the Qiaomaishan deposit. Grossular (Adr21–70Grs29–78; type 1) garnet is generally located within the endoskarn and is light REE (LREE) enriched but heavy REE (HREE) depleted with weakly negative Eu anomalies and higher total REE contents than the exoskarn garnet in the study area. Type 1a grossular garnet is further split from type 1b grossular garnet by the presence of positive Pr and Nd anomalies and high LREE concentrations. Exoskarn garnet has pure andradite compositions and is split into type 2 and 3 garnets, where type 2 andradite garnet has variably positive Eu anomalies and variable total REE contents that are indicative of formation under transitional or oscillating conditions (e.g., pH, oxygen fugacity). In comparison, the type 3 andradite garnet has low total REE contents and positive Eu anomalies. Endoskarn garnet Al2O3 and total REE contents positively correlate, indicating that REE is incorporated into this garnet through coupled YAG-type substitution. The differences between the total REE contents of the type 2 exoskarn and endoskarn garnets may reflect host rock buffering as well as variations in the crystal chemistry of the two garnets present within the different parts of the skarn system (grossular and andradite). In comparison, the differences in the total REE contents of the two types of exoskarn garnet may reflect variations in water–rock ratios and crystal growth rates. The different Eu anomalies and U contents present within the different types of garnets record variations in the pH and oxygen fugacity of the hydrothermal fluid over time. These changes may have promoted the crystallization of magnetite and scheelite and the formation of the tungsten mineralization within the deposit. This study demonstrates the usefulness of garnet in tracking variations in mineralizing conditions and processes and confirms the validity of these minerals as vectors or indicator minerals that can be used in combination with traditional exploration approaches to determine whether a system is mineralized or not during the early stages of exploration.

Coupled hydrogen and fluorine incorporation in garnet: New constraints from FTIR, ERDA, SIMS, and EPMA

Abstract It is well known that some garnet compositions can incorporate hydrogen and/or fluorine at levels up to several wt%. However, accurate measurement of these elements can be difficult at trace to minor concentration levels, so they are frequently ignored in routine chemical analysis. Furthermore, the mechanisms of H incorporation are still under debate, and only one mechanism for F substitution is commonly considered. We employed infrared spectroscopy (FTIR), elastic recoil detection analysis (ERDA), secondary ion mass spectrometry (SIMS), and electron probe microanalysis (EPMA) to measure H and F concentrations and constrain incorporation mechanisms in ten grossular garnets. We also present SIMS data for 11 spessartine and two andradite garnets. Three grossular garnets were measured with ERDA to obtain an infrared integral molar absorption coefficient (εi) for H2O of 13 470 L/(mol·cm2). Grossular H2O and F concentrations range from 0.017 to 0.133 wt% and 0.012 to 0.248 wt%, respectively. Correlations between 16OH and 19F and interpretation of FTIR spectra prompt us to consider various coupled substitutions of H and F for Si, which can explain some high-frequency IR absorption bands that have been attributed previously to “hydrogrossular clusters” (variably sized clusters in which 4H substitute for Si) or to inclusions of hydrous minerals. A strong correlation between 16OH and 19F in spessartine and similar high-frequency IR bands implies a similar role for H-F substitution. Coupled H-F substitution is also probably relevant to some andradite-rich garnets, rare pyrope from the Dora Maira massif, and some synthetic garnets. Improvements in analytical methods for trace to minor H and F open up more possibilities for using these elements to calculate the activities of H2O and F-species in fluids that were in equilibrium with garnet-bearing phase assemblages, as well as constraining the recycling of these elements into the mantle via study of xenoliths.

Anisotropic grossular–andradite garnets: Evidence of two stage skarn evolution from Rudnik, Central Serbia

Anisotropic grossular–andradite garnets: Evidence of two stage skarn evolution from Rudnik, Central Serbia

The prograde-to-retrograde evolution of the Huangshaping skarn deposit (Nanling Range, South China)

Huangshaping is a world-class skarn deposit hosting abundant W–Sn polymetallic mineralization in the Nanling Range, South China. Although the geochemistry of skarn-hosted mineral deposits has been extensively studied, the role of the prograde-to-retrograde stage transition in the enrichment and precipitation of metallic elements in high-temperature systems has received little study to date. Here, we analyzed garnet, scheelite, and cassiterite in high-temperature granite porphyry-related W–Sn polymetallic system of the Huangshaping deposit to investigate these processes. Three generations of garnet (Grt I, Grt II, and Grt III), two generations of scheelite (Sch I and Sch II), and two types of cassiterite (Cst I and Cst II) were distinguished with regard to their mineral associations, microscopic characteristics, and geochemical features. The results show that grossular–andradite garnet formed from Al-rich andradite (Grt I, Al2O3: 8.18 wt%) in the early prograde stage and pure Fe-andradite (Grt III, Al2O3: 0.15 wt%) in the late prograde stage. Mo-rich scheelite (Sch Ia, MoO3: 19.41 wt%) formed in the prograde stage and coexisted with Grt III, sharing the same REE patterns. A shift from HREE enrichment in Grt I to HREE depletion in Grt III reflects progressive uptake of REEs by secondary mineral phases. Grt III has the highest average contents of W (905 ppm), Mo (19 ppm), and Sn (5610 ppm), suggesting an enrichment of metallic elements at the end of the prograde stage. In contrast, Mo-poor scheelite (Sch II, MoO3: 0.63 wt%) co-crystallized with molybdenite and fluorite in the retrograde stage and shares similar REE patterns with the granite porphyry. Skarn mineralization at Huangshaping was a two-step process controlled by metamorphic stage. The prograde stage was characterized by high temperatures and fO2 and a Cl-rich fluid, and it resulted in enrichment of ore-forming elements with minor scheelite precipitation. The retrograde stage was characterized by lower temperatures and fO2 and a F-rich fluid, and it resulted in major precipitation of ore minerals (scheelite, molybdenite, and cassiterite). Dissolution–reprecipitation reactions played an important role in extraction of metallic elements from decomposing anhydrous skarn minerals and formation of ore minerals. A decrease in the high-field-strength element (HFSE) content of cassiterite from proximal skarn (Cst I) to distal skarn (Cst II) indicates declining temperature accompanied by precipitation of fluorite. This study examines the transition from the prograde stage to the retrograde stage in the Huangshaping deposit, and it provides insights into the genesis of other skarn W–Sn polymetallic deposits in the Nanling magmatic–tectonic–metallogenic belt.

Molybdenite Re–Os, titanite and garnet U–Pb dating of the Magushan skarn Cu–Mo deposit, Xuancheng district, Middle–Lower Yangtze River Metallogenic Belt

The Magushan skarn Cu–Mo deposit is a representative example of the skarn mineralization occurring within the Xuancheng ore district of the Middle–Lower Yangtze River Metallogenic Belt of eastern China. The precise age of an ore deposit is important for understanding the timing of mineralization relative to other geological events in a region and to fully place the formation of a mineral deposit within the geological context of other processes that occur within the study area. Here, we present new molybdenite Re–Os and titanite and andradite garnet U–Pb ages for the Magushan deposit and use these data to outline possible approaches for identifying genetic relationships in geologically complex areas. The spatial and paragenetic relationships between the intrusions, alteration, and mineralization within the study area indicates that the formation of the Magushan deposit is genetically associated with the porphyritic granodiorite. However, this is not always the case, as some areas contain complexly zoned plutons with multiple phases of intrusion or mineralization may be distal from or may not have any clear spatial relationship to a pluton. This means that it may not be possible to determine whether the mineralization formed as a result of single or multiple magmatic/hydrothermal events. As such, the approaches presented in this study provide an approach that allows the identification of any geochronological relationships between mineralization and intrusive events in areas more complex than the study area. Previously published zircon U–Pb data for the mineralization-related porphyritic granodiorite in this area yielded an age of 134.2 ± 1.2 Ma (MSWD = 1.4) whereas the Re–Os dating of molybdenite from the study area yielded an isochron age of 137.7 ± 2.5 Ma (MSWD = 0.43). The timing of the mineralizing event in the study area was further examined by the dating of magmatic accessory titanite and skarn-related andradite garnet, yielding U–Pb ages of 136.3 ± 2.5 Ma (MSWD = 3.2) and 135.9 ± 2.7 Ma (MSWD = 2.5), respectively. The dating of magmatic and hydrothermal activity within the Magushan area yields ages around 136 Ma, strongly suggesting that the mineralization in this area formed as a result of the emplacement of the intrusion. The dates presented in this study also provide the first indication of the timing of mineralization within the Xuancheng district. providing evidence of a close genetic relationship between the formation of the mineralization within the Xuancheng district and the Early Cretaceous magmatism that occurred in this area. This in turn suggests that other Early Cretaceous intrusive rocks within this region are likely to be associated with mineralization and should be considered highly prospective for future mineral exploration. This study also indicates that the dating of garnet and titanite can also provide reliable geochronological data and evidence of the timing of mineralization and magmatism, respectively, in areas lacking other dateable minerals (e.g., molybdenite) or where the relationship between mineralization and magmatism is unclear, for example in areas with multiple stages of magmatism, with complexly zoned plutons, and with distal skarn mineralization.

Garnet Geochemistry of Reduced Skarn System: Implications for Fluid Evolution and Skarn Formation of the Zhuxiling W (Mo) Deposit, China

A newly discovered tungsten ore district containing more than 300,000 tons of WO3 in southern Anhui Province has attracted great attention. The Zhuxiling W (Mo) deposit in the district is dominated by skarn tungsten mineralization. This paper conducted in suit EPMA and LA-ICPMS spot and mapping analysis of the skarn mineral garnet to reveal the evolution of fluids, metasomatic dynamics, and formation conditions of skarn. Two generations of garnet have been identified for Zhuxiling W (Mo) skarn: 1) Gt-I generation garnet is isotropic, Al-rich grossular without zoning. As a further subdivision, Gt-IB garnet (Adr19-46Grs49-77 (Sps+Pyr+Alm)4-5) contains significantly high content of Ti and Mn compared with Gt-IA garnet (Adr3-42Grs53-96 (Sps+Pyr+Alm)1-5). 2) Gt-II generation garnet is anisotropic, Fe-rich andradite with oscillatory zoning. Gt-II garnet displays compositional changes with a decrease of Fe and an increase of Mn from proximal skarn (Gt-IIA) to distal skarn (Gt-IIB) with the presence of subcalcic garnet for Gt-IIB type (Sps+Pyr+Alm = 56–68). The presence of pyrrhotite associated with subcalcic garnet indicates a relatively reduced skarn system. Gt-I grossular is overall enriched in Cr, Zr, Y, Nb, and Ta compared with the Gt-II andradite, and both W and Sn strongly favor Fe-rich garnet compared with Al-rich garnet. Gt-IA grossular garnet presents a REE trend with an upward-facing parabola peaking at Pr and Nd in contrast to low and flat HREE, and Gt-IB grossular garnet has a distinct REE pattern with enriched HREE. Gt-IIA andradite garnet displays a right-dipping REE pattern (enriched LREE and depleted HREE) with a prominent positive Eu anomaly (Eu/Eu* = 3.6–15.3). In contrast, Gt-IIB andradite garnet shows depleted LREE and enriched HREE with a weak positive Eu anomaly (Eu/Eu* = 0–6.0). The incorporation and fractionation of REE in garnet are collectively controlled by crystal chemistry and extrinsic factors, such as P–T–X conditions of fluids, fluid/rock ratios, and mineral growth kinetics. Major and trace elements of two generations of garnet combined with optical and textural characteristics suggest that Gt-I Al-rich grossular garnets grow slowly through diffusive metasomatism under a closed system, whereas Gt-II Fe-rich andradite represent rapid growth garnet formed by the infiltration metasomatism of magmatic fluids in an open system. The Mn-rich garnet implies active fluid–rock interaction with Mn-rich dolomitic limestone of the Lantian Group in the district.

Olivine from aillikites in the Tarim large igneous province as a window into mantle metasomatism and multi-stage magma evolution

Abstract Aillikites are carbonate-rich ultramafic lamprophyres, and although they are volumetrically minor components of large igneous province (LIP), these rocks provide important clues to melting and meta-somatism in the deep mantle domain during the initial stages of LIPs. In this study, we investigate the Wajilitag “kimberlites” in the northwestern part of the Tarim LIP that we redefine as hypabyssal aillikites based on the following features: (1) micro-phenocrystic clinopyroxene and Ti-rich andradite garnet occurring in abundance in the carbonate-rich matrix; (2) Cr-spinel exhibiting typical Fe-Ti enrichment trend also known as titanomagnetite trend; and (3) olivine showing dominantly low Mg values (Fo < 90). To constrain the magma source and evolution, the major, minor, and trace element abundance in olivine grains from these rocks were analyzed using electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry. Olivine in the aillikites occurs as two textural types: (1) groundmass olivines, as sub-rounded grains in matrix, and (2) macrocrysts, as euhedral-anhedral crystals in nodules. The groundmass olivines show varying Mg (Fo89–80) with high-Ni (1606–3418 ppm) and Mn (1424–2860 ppm) and low-Ca (571–896 ppm) contents. In contrast, the macrocrysts exhibit a restricted Fo range but a wide range in Ni and Mn. The former occurs as phenocrysts, whereas the latter are cognate cumulates that formed from earlier, evolved aillikite melt. The two olivine populations can be further divided into sub-groups, indicating a multi-stage crystallization history of the aillikite melt. The crystallization temperatures of groundmass olivines and macrocrysts in dunite nodules as computed from the spinel-olivine thermometers are 1005–1136 and 906–1041 °C, respectively. The coupled enrichment of Ca and Ti and lack of correlation between Ni and Sc and Co in the olivine grains suggest a carbonate-silicate metasomatized mantle source. Moreover, the high 100·Mn/Fe (average 1.67) at high Ni (up to 3418 ppm), overlapping with OIB olivine, and the 100·Ni/Mg (~1) of primitive Mg-Ni-rich groundmass olivines suggest a mixed source that involved phlogopite- and carbonate-rich metasomatic veins within mantle peridotite.

In situ LA-ICP-MS study of garnets in the Makeng Fe skarn deposit, eastern China: Fluctuating fluid flow, ore-forming conditions and implication for mineral exploration

Most garnets in Fe-Cu skarn deposits have simple petrographic textures, LREE-enriched geochemical patterns, are enriched in Fe, and are distinguished from garnets of typical W or W-Mo skarn deposits by their extremely low W, Sn, and Mo contents. However, little is known about the garnets in Fe skarn systems that have more complex textures and high concentrations of W, Sn, and Mo. Here we describe diverse textures and LA-ICP-MS data for garnets in the largest Fe skarn deposit (Makeng) in eastern China, and we discuss the dynamics of the mineralizing fluids and the significance of garnet for W-Mo exploration. Garnets from the Makeng deposit fall into four main types, corresponding to four-stage garnet growth based on their textural characteristics and trace element contents: Stage 1, Al-rich relatively homogeneous garnets (Adr51-55) with a portion (Adr17-48) and HREE-enriched patterns and weak negative Eu anomalies; Stage 2, Fe-rich garnet with LREE-enriched or flat patterns and weak negative or positive Eu anomalies, represented by oscillatory zoned rims (Adr50-90) with dodecahedral faces and oscillatory zones (cores Adr54-68, rims Adr65-80) with composite dodecahedral-trapezohedral faces; Stage 3, nearly pure andradite garnets with significantly LREE-enriched patterns and strong positive Eu anomalies, as represented by irregular zones (Adr70-99) in oscillatory zoned rims and relatively homogeneous garnets (Adr95-100) with extremely weak zoning; and Stage 4, Al-rich garnet with HREE-enriched patterns and weak negative or weak positive Eu anomalies, as represented by garnet veinlets (Adr45-77) with weak zoning. These four stages of garnet were formed by periodic fluctuations in the fluid flux which can be explained by a kinetic dispersion model: a low flux responsible for the Al-rich garnet, a high flux responsible for the Fe-rich garnets, the highest flux responsible for the nearly pure andradite garnet, and a low flux responsible for the veins of Al-rich garnet. Relative to garnet in Fe-Cu skarn deposits, all garnet types in the Makeng Fe skarn deposit are rich in trace elements such as W, Sn, and Mo, especially the pure andradite garnet in stage 3, which has the highest contents of W, Sn, and Mo. These elemental enrichments are closely correlated with Fe, LREE/HREE, and fO2 signatures. The W and Mo signatures are identical to those of garnets in typical W or W-Mo skarn deposits, and they are useful geochemical indicators during exploration for W-Mo ore deposits.

Mineralogy of a High-Temperature Skarn, in High CO2 Activity Conditions: The Occurrence from Măgureaua Vaţei (Metaliferi Massif, Apuseni Mountains, Romania)

A shallow-level monzodioritic to quartz-monzodioritic pluton of the Upper Cretaceous age caused contact metamorphism of Tithonic–Kimmeridgian reef limestones at Măgureaua Vaţei (Metaliferi Massif, Apuseni Mountains, Romania). The preserved peak metamorphic assemblage includes gehlenite (Ak 33.64–38.13), monticellite, wollastonite-2M, Ti-poor calcic garnet, and Ca-Tschermak diopside (with up to 11.15 mol.% Ca-Tschermak molecule). From the monzodioritic body to the calcitic marble, the periplutonic zoning can be described as: monzodiorite/agpaitic syenite-like inner endoskarn/wollastonite + perovskite + Ti-poor grossular + Al-rich diopside/wollastonite + Ti-poor grossular + diopside + vesuvianite/gehlenite + wollastonite + Ti poor grossular + Ti-rich grossular (outer endoskarn)/wollastonite + vesuvianite + garnet (inner exoskarn)/wollastonite + Ti-rich garnet + vesuvianite + diopside (outer exoskarn)/calcitic marble. Three generations of Ca garnets could be identified, as follows: (1) Ti-poor grossular (Grs 53.51–81.03 mol.%) in equilibrium with gehlenite; (2) Ti-rich grossular (Grs 51.13–53.47 mol.%, with up to 19.97 mol.% morimotoite in solid solution); and (3) titanian andradite (Grs 32.70–45.85 mol.%), with up to 29.15 mol.% morimotoite in solid solution. An early hydrothermal stage produced retrogression of the peak paragenesis toward vesuvianite, hydroxylellestadite (or Si-substituted apatite), clinochlore, “hibschite” (H4O4-substituted grossular). A late hydrothermal event induced the formation of lizardite, chrysotile, dickite, thaumasite, okenite and tobermorite. A weathering paragenesis includes allophane, C-S-H gels and probably portlandite, unpreserved because of its transformation in aragonite then calcite. Overprints of these late events on the primary zoning are quite limited.

Ore-forming processes and mechanisms of the Hongshan skarn Cu–Mo deposit, Southwest China: Insights from mineral chemistry, fluid inclusions, and stable isotopes

• Ore-forming fluids are of predominating magmatic–hydrothermal origin at Hongshan. • Positive and uniform sulfur isotopes suggest the magmatic sulfur with predominant H 2 S. • Metals mainly precipitated due to the increased pH caused by water-rock interaction, decreasing temperature and decompression boiling. The Hongshan skarn Cu–Mo deposit is located in the southern Yidun terrane, SE Tibet Plateau, with more than 78.7 Mt resources (Cu: 0.64 Mt @ 1.23%, Mo: 5769 t @ 0.03%). The ore deposit was spatially and temporally associated with post-subduction Late Cretaceous monzogranite porphyries. Detailed geological mapping and deep drill-hole loggings reveal the vertical skarn zonation patterns of pyroxene skarn – garnet skarn – magnetite skarn – pyrrhotite-chalcopyrite skarn – garnet skarn – pyroxene skarn away from the marble, which is similar with typical skarn Cu deposit worldwide. Three hydrothermal stages have been recognized at Hongshan. They are characterized by assemblages of prograde skarn (stage 1), retrograde skarn and Cu–Fe–Mo sulfides (stage 2), and Pb–Zn sulfides associated with calcite and quartz (stage 3). Prograde skarns contain mainly andraditic garnet (two series: And 37–82 Gro 17–61 Spe+Alm+Pyr 0–4 and andradite) and pyroxene (Di 64–88 Hd 12–35 ) resulted from the interaction between magmatic-hydrothermal fluids and carbonate wall-rocks. Retrograde skarn mineralogy is controlled by hydrous Mg–Fe-rich silicate minerals, such as tremolites, actinolites, and epidotes. Petrographical and microthermomertic studies on fluid inclusions (FIs) in garnet, epidote, quartz and calcite from the three stages reveal four types of fluid inclusions: vapor CO 2 –Liquid CO 2 –H 2 O (C-type), vapor-rich two-phase inclusions (V-type), liquid-rich inclusions (L-type) and halite (sylvite)-bearing hypersaline inclusions (H-type). The C-type, l -type and V-type FIs within the garnet of stage 1 have homogenization temperatures between 400 and 550 °C, and salinities of 3.9–11.5 wt% NaCl eqv. A boiling fluid inclusion assemblage with coexisting l -type and V-type FIs was defined within the epidote and quartz of stage 2. The fluids of stage 3 are characterized by lower homogenization temperatures of 100–300 °C, developing a fluid inclusion assemblage defined solely by l -type FIs. The wide range of calculated δ 18 O H2O values in garnet (2.0 to 13.1 ‰), magnetite (10.9 to 26.3 ‰), tremolite (15.9 to 16.4 ‰) and sericite (10.5 ‰) further indicate the mixing of δ 18 O-enriched components with magmatic fluids. Sulfur isotope compositions of sulfides have a narrow range of δ 34 S values, ranging from 3.5 to 5.4 ‰, consistent with a magmatic origin and reducing conditions throughout the process of sulfide precipitation. The increased pH caused by water-rock interaction and CO 2 degassing, decreasing temperatures and decompression boiling could be crucial for the extensive ore deposition.