Hydrothermal Overprint Research Articles

Genetic classification, distribution and ore genesis of major PGE, Co and Cr deposits in China: A critical review

Major platinum-group element (PGE), cobalt (Co) and chromium (Cr) deposits in China are commonly associated with mafic-ultramafic rocks regardless magmatic or hydrothermal origins. Nearly all PGE and half of Co resources in China are from magmatic Ni-Cu-(PGE) sulfide deposits hosted in mafic-ultramafic intrusions. Major resources of Cr are from podiform chromite deposits associated with the mantle sequences of ophiolites, including the Luobusha chromite deposit in Tibet and the Sartohay chromite deposit in Xinjiang. However, there is a lack of large PGE and chromite deposits related to layered mafic-ultramafic intrusions. Sedimentary rock-hosted stratiform Cu deposits and Ni-Co laterites are two dominant types of Co-bearing deposits among terrestrial Co resources and are sporadically distributed in China, although these are insignificant. Platinum-group elements from three Ni-Cu-(PGE) sulfide-bearing mafic-ultramafic intrusions that formed within-plate settings account for 74% of the total PGE reserves of China. Among these, the Jinbaoshan and Yangliuping intrusions are parts of the Emeishan large igneous province (LIP) in SW China, and the Jinchuan intrusion emplaced in a rifting margin of the southern North China Craton. The Jinbaoshan intrusion hosts the only PGE-dominated deposit in China, but has not yet been mined due to the difficulty for access. In the Jinchuan and Yangliuping Ni-Cu deposits, PGEs are by-products. The Jinchuan intrusion is by far the largest PGE producer in China. Cobalt resources are from deposits associated with mafic-ultramafic rocks in China, including Ni-Cu sulfide deposits, Ni-Co laterites, volcanogenic massive sulfide (VMS) deposits and magmatic Fe-Ti oxide deposits, which make up 41% of total Co reserves of China. Major Ni-Cu sulfide deposits in both within-plate and convergent margin settings contain economic Co resources, such as the Jinchuan intrusion and those in the Emeishan LIP, and the Xiarihamu intrusion in the East Kunlun orogenic belt, the Jianchaling intrusion in a Neoproterozoic subduction zone in the northern margin of the Yangtze block and those in the central Asian orogenic belt. Ni-Co laterites that contain economic Co resource have been only discovered locally in Hannan, Yunnan and Taiwan. The Derni Cu-Co deposit in the A’nyemaqen suture zone of the East Kunlun orogenic belt is a special type of VMS deposits closely associated with ophiolite suites, which is also very rare worldwide. Magmatic Fe-Ti oxide deposits in the Emeishan LIP and hydrothermal iron oxide deposits in the East Tianshan orogenic belt have likely potential Co resources. Podiform chromite deposits in China occur in ophiolites, such as the Sartohay, Hegenshan and Suolunshan ophiolites in the central Asian orogenic belt, the Dadao Erji and Yushi ophiolites in the Qilian-Qinling orogenic belt, the Dongqiao ophiolite in the Bangong-Nujiang suture zone and Luobusha ophiolite in the Yarlung-Zangbo suture zone. Understanding the mechanisms of PGE enrichment in major Ni-Cu-(PGE) sulfide deposits such as the Jinchuan and Jinbaoshan intrusions has been a focus of studies in the past. The dissolution and re-precipitation processes that may lead to the enrichment of PGE and Co during hydrothermal overprints on magmatic Ni-Cu sulfide deposits have gained popularity. The origin of podiform chromite deposits is generally thought to be triggered by melt-mantle interaction and melt mixing. But it appears that such a traditional thought is challenged by the discoveries of various high-pressure and hydrous minerals that are trapped in chromite. Given PGE, Co and Cr are all strategic and critical metals to China, it is very important to conduct new exploration projects near active mines, such as those in the Emeishan LIP and the Jinchuan intrusion. However, additional experimental and mineralogical studies on the enrichment processes of PGE, Co and Cr are desired to make breakthroughs in order to better understand the behaviors of PGE, Co and Cr in magmatic-hydrothermal systems. Main issues that are worthy to be closely examined in the future include (1) micro- to nano-scale occurrence of PGE, (2) enrichment mechanism and occurrence of Co in mafic-ultramafic rocks and related ore deposits, and (3) mobilization and enrichment processes of Cr in the formation of podiform chromite deposits.

High-Grade Magmatic Platinum Group Element-Cu(-Ni) Sulfide Mineralization Associated with the Rathbun Offset Dike of the Sudbury Igneous Complex (Ontario, Canada)

Abstract Quartz dioritic impact melt dikes around the 1.85 Ga Sudbury Igneous Complex, locally referred to as offset dikes, are well endowed with respect to Ni-Cu-platinum group elements (PGE). However, only those dikes proximal (<6 km) to the main mass of the Sudbury Complex are mineralized at an economic grade and, in places, host world-class deposits. We report on a new discovery of such heavily mineralized offset dike at Rathbun Lake, about 15 km east of the currently known extent of the Sudbury Igneous Complex. There, a segment of amphibole quartz diorite is exposed at the contact between Huronian metasedimentary rocks and gabbro of the 2.22 Ga Nipissing Suite, xenoliths of which are abundant throughout the diorite and record textural evidence of partial melting. The mafic inclusion-bearing quartz diorite is the host of the Rathbun Lake showing, a small but high-grade PGE-Cu(-Ni) sulfide occurrence of hitherto controversial origin. A detailed petrographic and mineralogical characterization of this occurrence revealed a two-stage mineralization history. Disseminated to semimassive (net-textured) chalcopyrite ± loop-textured pentlandite ± magnetite containing Pd-bismuthotellurides and, more rarely, sperrylite and native gold—all of which are closely associated with base metal sulfides—are interpreted as magmatic. The semimassive sulfide averages ~40 g/t Pd + Pt + Au at a Cu/(Cu + Ni) of >0.9 and a Pd/Ir of >100,000. Mineralogy, ore textures, and mantle-normalized PGE + Au patterns match a specific type of Cu-rich mineralization in the Sudbury Igneous Complex known as footwall mineralization. By analogy with these footwall deposits, the occurrence is interpreted as having formed by downward percolation of a highly fractionated sulfide melt toward the bottom of a now largely eroded offset dike. The magmatic paragenesis was hydrothermally overprinted at lower greenschist-facies conditions to pyrite-chalcopyrite-violarite ± covellite ± millerite. This involved also local remobilization into pyrite-chalcopyrite veinlets and the liberation of precious metal minerals from their sulfide hosts. In contrast to base metal sulfides, most precious metal minerals were resistant to hydrothermal alteration, although corrosion of some grains is noted as well as their truncation by chlorite and epidote. Micron-scale X-ray mapping revealed a progressive replacement of magmatic Pd-Bi-Te minerals, where in contact with hydrous silicates, by Sb- and Hg-bearing Pd minerals such as temagamite, Pd3HgTe3. The timing and nature of this hydrothermal overprint remains uncertain, but a connection to later regional metamorphism and faulting seems most plausible. Our finding of magmatic PGE-base metal sulfide at Rathbun Lake suggests a new subtype of distal offset dike-hosted mineralization in an area so far not known for offset dikes. It opens up new opportunities in the search for unconventional ore deposits around the Sudbury impact structure and improves our understanding on the distribution of impact melt-derived dikes around complex craters.

Evolution of brines and CO2-rich fluids and hydrothermal overprinting in the genesis of the Borrachudo copper deposit, Carajás Province

The Carajás Province is one of the world’s major domains of iron oxide-copper-gold (IOCG) deposits, which formed during the Neoarchean (ca. 2.70–2.68 Ga and ca. 2.57 Ga). The province also hosts other copper-gold deposits of Paleoproterozoic age (ca. 1.88 Ga). The Borrachudo copper deposit is located about 6 km southwest of the world-class Cristalino IOCG deposit. Porphyritic rhyolite and amygdaloidal andesite of the ca. 2.76–2.74 Ga Grão Pará Group, formed in a volcano-plutonic event during the Neoarchean, are the main host to the Borrachudo deposit. Copper mineralization was related to two distinct hydrothermal events. The first, controlled by the development of ductile structures, was likely linked to the Carajás ca. 2.70 Ga IOCG system. It evolved from sodic alteration, silicification, and iron metasomatism through to calcic and potassic alteration. Chalcopyrite strings along foliation, breccias, and massive ore represent early copper mineralization. During the early hydrothermal event, aqueous (165° to >400 °C; 0.23 to >38.32 wt% NaCl equiv.), aqueous-carbonic and carbonic fluid inclusions (TmCO2 = −60.1 to −59.4 °C; CO2 density of 0.82 to 0.97 g/cm3) were trapped in quartz formed during the silicification. Immiscibility of a magmatic fluid or heterogeneous trapping of brines and CO2-bearing fluids, followed by mixing with diluted colder fluids, are suggested. The late event, controlled by brittle structures, records recurrence of hydrothermal alteration stages, such as silicification, sodic alteration, and potassic alteration, as well as chlorite and carbonate alteration. The copper mineralization is represented by chalcopyrite in branched breccia pipes and veins with minor magnetite, pyrite, siegenite, cobaltite, galena, anglesite, uraninite, and thorite. Titanite in veins of albite-chalcopyrite-apatite yielded an LA-ICP-MS U-Pb titanite age of 2011 ± 6.8 Ma. During the late hydrothermal event, aqueous fluids (140 to >320 °C; 6.7 to 40 wt% NaCl equiv.) reflected a more advanced mixing process involving diluted external fluids. The Borrachudo deposit points to the recurrence of multiple hydrothermal events responsible for the copper metallogenesis, which were intrinsically controlled by the magmatic and tectonic evolution of the Carajás Province.

Geology, geochronology, and fluid inclusion studies of the Xiaorequanzi volcanogenic massive sulphide Cu–Zn deposit in the East Tianshan Terrane, China

The Xiaorequanzi Cu–Zn deposit is in the westernmost part of East Tianshan Terrane in northwestern China. The deposit is unique in the region being a volcanogenic massive sulphide (VMS) deposit located near a zone (or belt) containing giant late Paleozoic porphyry Cu deposits. Aiming to better understand the genesis of the mineral deposits in the terrane and their tectonic setting, we report our findings of detailed studies on fluid inclusion microthermometry, Re–Os dating of chalcopyrite from the massive ore, and U–Pb dating of zircons from the host volcanic rocks. There are two sulphide stages with early pyrite succeeded by chalcopyrite–sphalerite, which are hydrothermally overprinted and supergene enriched. The hydrothermal overprinting is characterised by quartz–sulphide veins crossed by carbonate-rich quartz veins. Quartz from the chalcopyrite–sphalerite stage is characterised by primary fluid inclusions containing H2O–NaCl(–CO2) and homogenise at 228–392 °C with a salinity of 2.2–13.3 wt.% NaCl equiv. Secondary fluid inclusions related to the hydrothermal overprinting homogenise at 170–205 °C with a salinity of 2.7–12.1 wt.% NaCl equiv. Fluid inclusions in the quartz–sulphide stage of the hydrothermal overprinting contain H2O–NaCl with homogenisation temperatures of 164–281 °C and salinities in ranging from 2.9 to 12.4 wt.% NaCl equiv. Fluid inclusion in the quartz–calcite stage contain H2O–NaCl with homogenisation temperatures of 122–204 °C with salinities of 1.4–12.4 wt.% NaCl equiv. These characteristics are like those of the secondary fluid inclusions in the VMS mineralisation. Combining these findings with H–O isotopic data from previous studies, we propose that the primary mineralising fluid is magmatic in origin. Tuff hosting the mineralisation yields a SHRIMP U–Pb zircon age of 352 ± 5 Ma, which is interpreted as the age of the tuff, and a porphyritic felsite dyke intruding the tuff yields a SHRIMP U–Pb zircon date of 345 ± 6 Ma, interpreted as the emplacement age of the dyke. Chalcopyrite from the main orebody at Xiaorequanzi yields a Re–Os isochron age of 336 ± 13 Ma with an initial 187Os/188Os ratio of 0.25 ± 0.55 (MSWD = 12). Given that the VMS deposit is a syngenetic deposit, we regard the upper ca. 349 Ma limit of the Re–Os date as the approximate age of the chalcopyrite. The three dates are the same within error, and the upper limit of the Re–Os date of ca. 349 is taken as the age of the volcanic, dyke, and mineralisation. The volcanic rocks around the Xiaorequanzi deposit have been previously classified as calc–alkaline to high-K calc–alkaline enriched in large-ion lithophile elements and depleted in high-field-strength elements, which are characteristics indicative of a forearc setting. It is suggested that VMS mineralisation formed in a forearc setting related to the north-directed subduction of the Palaeo-Kangguer or North Tianshan oceanic plates.

Formation of the Mantoverde iron oxide-copper-gold (IOCG) deposit, Chile: insights from Fe and O stable isotopes and comparisons with iron oxide-apatite (IOA) deposits

The Mantoverde iron oxide-copper-gold (IOCG) deposit, Chile, contains hundreds of millions of tonnes (Mt) of mineable iron oxide and copper sulfide ore. While there is an agreement that mineralization at Mantoverde was caused by hydrothermal fluid(s), there is a lack of consensus for the role(s) that non-magmatic vs. magmatic fluid(s) played during the evolution of the mineralized system. In order to overcome the hydrothermal overprint at Mantoverde, which is known to disturb most conventional stable isotope systems (e.g., oxygen), we report the first δ56Fe and δ18O pairs for early-stage magnetite and late-stage hematite that provide information on the source reservoir of the hydrothermal fluids. Magnetite δ56Fe values range from 0.46 ± 0.04 to 0.58 ± 0.02‰ and average 0.51 ± 0.16‰ (n = 10; 2σ). Three hematite δ56Fe values were measured to be 0.34 ± 0.10, 0.42 ± 0.09, and 0.46 ± 0.06. Magnetite δ18O values range from 0.69 ± 0.04 to 4.61 ± 0.05‰ and average 2.99 ± 2.70‰ (n = 9; 2σ). Hematite δ18O values range from − 1.36 ± 0.05 to 5.57 ± 0.05‰ and average 0.10 ± 5.38‰ (n = 6; 2σ). These new δ56Fe and δ18O values fingerprint a magmatic-hydrothermal fluid as the predominant ore-forming fluid responsible for mineralization in the Mantoverde system.

Axinit a doprovodné minerály z lokality Jezuitský rybník východně od Golčova Jeníkova (moldanubikum, Česká republika)

A new occurrence of axinite at the locality Jezuitský rybník near Sirákovice (ENE from Golčův Jeníkov), situated in rocks of the Variegated (Drosendorf) Series (Moldanubian Zone of the Bohemian Massif), is a nice example of contaminated pegmatite in a Ca-skarn with intense superimposed hydrothermal overprint. Axinite [axinite-(Fe) to axinite-(Mg)] forms young hydrothermal infill of pocket/fissure in pegmatite cutting a brecciated Ca-skarn. The hydrothermal assemblage includes amphibole II (actinolite to ferro-actinolite), albite, K-feldspar II, chlorite, epidote (locally containing 0.20 - 0.30 apfu REE), muscovite and Al,F-enriched titanite (with up to 2 % SnO2) passing exceptionally to unnamed CaAlFSiO4. Quartz, plagioclase (andesine), K-feldspar I and amphibole I (mostly K-rich or even potassian ferro-pargasite to ferro-tschermakite) originated in magmatic stage associated with intrusion of externally derived pegmatite melt. Sporadic garnet (grossular-rich almandine) represents relics of mineral assemblage of the host skarn. Dominance of Nd among REE in the REE-rich epidote is explained in terms of chemical fractionation of REE, probably caused by the presence of strong REE-complexing ligands (F-, OH- and/or CO32-) in aqueous fluids enriched in MREE/HREE due to alteration of garnet. With regard to the presence of B, Cr and elevated XMg in some hydrothermal phases compared to the older Fe-Mg minerals, we suggest circulation of fluids affecting host rocks as well as additional rock types.

Desulphurisation, chromite alteration, and bulk rock PGE redistribution in massive chromitite due to hydrothermal overprint of the Panton Intrusion, east Kimberley, Western Australia

The Central and Western Zones of the Halls Creek Orogen (HCO) of the East Kimberley region of Western Australia host various Paleoproterozoic layered mafic–ultramafic intrusions. The 1856 ± 2 Ma layered mafic–ultramafic Panton Intrusion of the Central Zone contains a reef-type platinum-group element (PGE) mineralisation associated with metamorphosed chromitite horizons. Here we investigate the compositional variation of chromite within the A chromitite layer, or Main Chromitite Layer (MCL), base metal sulphides (BMS), and the associated PGE distribution. Electron-microprobe analysis (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data reveal complex chemical zoning of the chromite reflecting post-magmatic sub-solidus re-equilibration and fluid–rock interaction during metamorphism of the host rocks. The sulphur isotopic composition of the BMS (δ34S: −1.67 to 2.89‰) suggests negligible contamination of the parental melt by crustal sulphur and thus favours BMS formation due to fractional crystallisation. Furthermore, pentlandite compositions indicate the highest ƒS2 in the centre of the MCL, which coincides with a considerably heavier sulphur isotope composition compared to pentlandite in the surrounding ultramafic host rock. Alteration of the BMS during amphibolite facies hydration of the Panton Intrusion resulted in the formation of secondary bornite and digenite from the chalcopyrite as well as desulphurisation of pyrrhotite and pentlandite to secondary magnetite. The bulk rock PGE concentrations are controlled by the distribution of platinum-group minerals (PGM) due to the low PGE concentrations in BMS. PGM are present as inclusions in chromite, suggesting that they formed under sulphur-undersaturated conditions. Additional PGM are present within the interstitial serpentine-chlorite matrix suggesting that they precipitated during fluid-rock interaction and hydration of the silicate matrix. Likely sources for the remobilised PGE include PGM inclusions in altered rim zones of magmatic chromite and PGE released due to desulphurisation of BMS despite their low PGE concentrations.

Formation of the Vergenoeg F–Fe–REE Deposit (South Africa) by Accumulation from a Ferroan Silicic Magma

AbstractSituated in the centre of the Paleoproterozoic Bushveld Large Igneous Province (LIP) of South Africa the Vergenoeg F–Fe–REE deposit is one of the largest, but at the same time most unusual, fluorite deposits on Earth. In situ major and trace element analyses of fayalite, magnetite, ilmenite, fluorapatite, fluorite and allanite from fayalite-rich rocks are combined with oxygen isotope data for fayalite, magnetite and ilmenite to unravel the complex evolution of the deposit. Textural and compositional characterization of the fayalite-rich rocks supports a magmatic formation as cumulates and an intense late hydrothermal overprint. Fayalite accumulated together with minor Ti-rich magnetite, ilmenite, fluorapatite and allanite from a highly evolved, H2O-poor felsic melt at low oxygen fugacity. Chondrite-normalized rare earth element (REE) patterns of fayalite and the recalculated parental melts, using fayalite–rhyolite partition coefficients, exhibit positive trends with strong enrichment of the heavy REE (HREE) relative to the light REE (LREE). Apart from the LREE depletion the patterns are similar to those of highly fractionated high-silica REE rhyolites that often occur in siliceous LIPs. We attribute the LREE depletion to crystallization of accessory allanite, the main host of the LREE in the cumulates. Chondrite-normalized REE patterns of the parental melt prior to fayalite accumulation, recalculated using allanite–rhyolite partition coefficients, resemble the composition of the rhyolites of the Rooiberg Group and therefore document a petrogenetic link to the Bushveld LIP. High δ18O values of fayalite (up to ≈7·4 ‰) are consistent with its crystallization in a rhyolitic melt that has formed by extensive fractionation from basic melts of the Rustenburg Layer Suite, the mafic member of the Bushveld LIP. Primary fluorite crystallized together with rare quartz, and a second generation of fayalite, magnetite and ilmenite from rare intercumulus melt in interstices between cumulate fayalite. Textural and mineral compositional data, as well as the generally negative δ18O values of magnetite (–2·9 to 0 ‰), are in agreement with the main magnetite–fluorite ore formation in Vergenoeg being related to a hydrothermal overprint, which was responsible for further F and Fe enrichments of the rocks. Fluorine-rich fluids, released from the crystallizing granites of the felsic member of the Bushveld LIP (Lebowa Granite Suite), caused the extensive alteration of fayalite to bowlingite and its replacement by Ti-poor magnetite and quartz. The hydrothermal overprint was associated with the widespread formation of secondary fluorite and minor fluorapatite. Our new petrogenetic model for the Vergenoeg deposit, as constrained from the primary fayalite cumulates, implies that the formation of the Vergenoeg deposit was directly linked to the evolution of the Bushveld LIP.

Petrographic characterization of Archaean impact spherule layers from Fairview Gold Mine, northern Barberton Greenstone Belt, South Africa

The oldest known evidence of large impacts in Early Earth history are Archaean to Palaeoproterozoic impact spherule layers of the Barberton Greenstone Belt in South Africa and the Pilbara craton in Western Australia. Four drill core sections (BH5901, BH5907, BH5911 and BH5949) containing spherule layers were intersected in exploration drilling at Fairview Gold Mine in the northern Barberton Greenstone Belt. Here, we provide a first detailed mineralogical study of these new spherule layer intersections. The spherule layers are densely packed with sand-sized, spherical to ovoid, i.e., in part strongly deformed, “beads”. They generally have morphological and textural features similar to other 3.4–3.2 Ga old spherule layers of the Barberton Greenstone Belt. The Fairview spherule layers are stratigraphically positioned at the contact between the silicified volcanic tuff of the uppermost unit of the Onverwacht Group and the lowermost siliciclastic rocks of the Fig Tree Group, like the S2 and S4 layers in the southern, and S3 layer in the northern, domain of the greenstone belt. Micro-XRF scanning allowed (i) to distinguish spherule layers from their host rocks by their major element distribution, and (ii) to characterize secondary alteration and micro-deformation features in the SL and host rocks. Ni–Cr spinel, the only primary mineral in evidence in these sections, is not distributed uniformly along a spherule layer intersection, and occurrences can not be stratigraphically correlated between the four layer intersections either. Spinel crystals vary with respect to Ni, Cr, Fe and Zn contents and respective zonation patterns for these elements. Zinc abundances are likely related to post-depositional hydrothermal overprint. The results of this investigation comprising geological, petrographic, semiquantitative micro-X-ray fluorescence spectrometry, and quantitative electron microprobe analysis on spinel support a complex and heterogeneous genetic process that must have taken place during spherule formation in an impact vapor plume and in post-depositional times. The Fairview spherule layers are thought to possibly represent the product of a single impact event. They represent fallout into deep-water conditions, as they lack evidence for reworking or sorting by current or wave action. The heterogeneous Ni–Cr spinel concentrations are similar to what has been reported for spinel occurrences in the S3 layer, but considering the complex tectonic overprint on this lithostratigraphic sequence at the changeover from the Onverwacht to the Fig Tree Group it is not possible to conclude whether or not these spherule layers represent the same impact event.

Geochemistry and geochronology of zircons from granite-hosted gold mineralization in the Jiaodong Peninsula, North China: Implications for ore genesis

Large-scale quart-vein and structurally-controlled gold mineralization is intensely developed in the Jiaodong Peninsula, North China Craton and is typified by the Linglong and Jiaojia deposits, respectively. In this work, zircons from typical gold-bearing ores of the Sanshandao Northern Sea (SNS), Jiuqu (JQ) and Canzhuang (CZ) gold deposits were compared in terms of texture, U–Pb age, trace element content and Hf isotopic signatures in order to discern whether zircon from ore samples can be used as an indicator of ore genesis and grade. Most studied zircons are magmatic in origin with a hydrothermal overprint (i.e., zircons with core-rim assemblages, fractured-dissolution textures and hydrothermal activity-related fluid/mineral inclusions). Three zircon age clusters were differentiated: ~120 Ma, ~150 Ma and >2000 Ma. The ~120 Ma zircons show evidence of hydrothermal alteration (dark in cathodoluminescent images with hydrothermal rim textures) and are characterized by high LREE, P, Ti and εHf(t) (−21 to −15) and young TDMC (2100–2500 Ma). This suggests an interaction between the mantle and crust at the magma source, and that a P/Ti-rich, Al-poor, high temperature, high fO2, wet late-stage residual granitic melt and intense hydrothermal alteration played an important role in the formation of gold mineralization. The ~150 Ma zircons have high Y, Nb, Ta and U contents, low εHf(t) (−26 to −20) and old TDMC (2300–3000 Ma) ages, and HREE-enriched patterns with pronounced Ce and Eu anomalies. These features indicate a reduced, cool and Al-rich magma which did not favor gold mineralization, even though it was highly evolved. This also implies that the degree of magma differentiation is not the key factor that led to gold mineralization in the Jiaodong gold province. The >2000 Ma zircons are primarily limited to gold ores with high Pb contents, HREE-enriched patterns and variable Ce and Eu anomalies, indicating that they were inherited from the Paleoproterozoic (2000–2200 Ma) basement. In addition, the SNS high-grade ores yielded zircon with the greatest U-Pb age range (mainly 110–180 Ma) and the most pronounced hydrothermal overprinting, indicating that large-scale mineralization resulted from multi-stage granitic intrusion, with intense fluid dissolution and metasomatism. This research highlights that correlations between zircon age populations, Hf isotope signatures, internal structures and trace element patterns can provide insights into ore genesis and fluid evolution in gold mineralization systems.

Nb–V-enriched sövites of the northeastern and eastern part of the Amba Dongar carbonatite ring dike, India – A reflection of post-emplacement hydrothermal overprint?

Wakefieldite-(Ce,La) and vanadinite in coarse-grained calciocarbonatites (sövites) are for the first time reported from the northeastern part of the worldwide largest fluorite deposit at the Amba Dongar carbonatite ring dike, India. Sövite in this part of the carbonatite ring dike is rich in pyrochlore, calcite and magnetite. Pyrochlore makes up almost 50% of some sövite samples and shows core-to-rim compositional changes. The core of pyrochlore consists of primary fluorcalciopyrochlore with high F and Na contents while the margins gained elevated amounts of Pb, La and Ce with the associated loss of F and Na due to circulation of hydrothermal solutions. The presence of wakefieldite-(Ce,La) and vanadinite points to an exceptionally high V abundance in hydrothermal solutions formed towards the end of the carbonatite magma activity. This investigation thus opens new promising areas for Nb and REE prospection in the eastern part of the Amba Dongar carbonatite body.

Whole-rock and zircon geochemistry of the Xiaoliugou granites, North Qilian Orogen (NW China): Implications for tectonic setting, magma evolution and W–Mo mineralization

Ordovician W–Mo polymetallic mineralization-related granites are widely developed in the western section of the North Qilian Orogen, NW China. In order to shed light on the tectonic setting, magma evolution and genesis of mineralization in the orogeny, highly evolved granitic rocks (monzogranites) from the Qibao and Qiqing deposits in the Xiaoliugou ore field were studied. Typical of subduction-related A-type granites, the Xiaoliugou granitic rocks are enriched in SiO2, Rb, Th, U, Ta, Nd, Zr and Hf but relatively depleted in Ba, Sr, P and Eu. The variable initial whole rock 87Sr/86Sr ratios (0.62937–0.70543) and wide range of zircon εHf(t) values (−18 to 6) suggesting mantle–crust interaction and mixing of depleted mantle and deep crust in the source area. Whole-rock elemental and Rb–Sr isotopic geochemistry suggest slow cooling, significant fractional crystallization, weak wall-rock assimilation/contamination and intense fluid differentiation/metasomatism during the genesis of the Xiaoliugou granitic rocks. Zircon U–Pb dating yielded three age clusters: Precambrian (>540 Ma) (n = 32), Ordovician (~450 Ma) (n = 40), and Cretaceous (~135 Ma) (n = 18). Zircon internal textures indicate a range of sources including residual/detrital zircons from the magma source region and local sediments, magmatic and fluid-altered zircons, and late stage hydrothermal zircons. Zircon trace element and Lu–Hf isotopic compositions suggest fluid metasomatism during granite emplacement in the Ordovician and overprinting by a more evolved, oxidized fluid in the Cretaceous. Involvement of a late, high-temperature, highly fractionated, F-rich fluid could have enhanced W–Mo mineralization. Consequently, a two-stage genetic model is proposed for the Xiaoliugou granitic-minerogenic system involving primary mineralization in the Ordovician (~450 Ma), followed by a Cretaceous magmatic hydrothermal overprint (~135 Ma).

A Silicocarbonatitic Melt and Spinel-Bearing Dunite of Crustal Origin at the Parker Phlogopite Mine, Notre-Dame-du-Laus, Quebec, Canada

The Parker phlogopite mine, located near Notre-Dame-du-Laus, Quebec, 74 km north of Ottawa, is well known among mineral collectors for its centimetric euhedral crystals of black spinel. Among the dozens of phlogopite mines active in the early 1900s in the Mont-Laurier–Bancroft corridor in the Central Metasedimentary Belt of the Grenville Province, the Parker mine is exceptional because of the association of forsterite + spinel with phlogopite. Euhedral crystals of these minerals are found “frozen” in a carbonate matrix. The carbonate dike and segregations are associated with spinel-rich dunite that contains accessory diopside, phlogopite, and pargasite, as well as ilmenite and apatite. The interstitial melt crystallized to calcite + dolomite. Hematite appeared as flakes in the melt owing to net loss of hydrogen, and the spinel underwent oxidation-induced exsolution. Our spinel crystal entrapped a domain of carbonate during growth. It also entrapped globules of boundary-layer melt that crystallized to a carbonate + sulfate + phosphate + silicate + oxide assemblage. Such globules, where present in the cumulate, are more pristine than in the coarse crystal of spinel, i.e., less affected by a hydrothermal overprint. We contend that the carbonate melt ultimately formed by the hydrous melting of marble, as supported by oxygen-isotope data on all major minerals. Melting occurred 1140 million years ago, at a time of tectonic relaxation following the Shawinigan compressive stresses.

Fluid-zircon interaction during low-temperature hydrothermal processes: Implications for the genesis of the Banxi antimony deposit, South China

Fluid-zircon interaction in hydrothermal systems may provide key information on fluid characterization and ore precipitation. In this study, in situ analyses of Hf isotopes in zircon, combined with U–Pb ages and major and trace element data, provide new insights into fluid genesis at the Banxi antimony deposit. The zircons separated from typical quartz-stibnite ores and altered wall rocks have undergone fluid modification to different degrees, showing hydrothermal overprints in reflected/transmitted light, back scattered electron (BSE) and cathodoluminescence (CL) images and variable concentrations of Si, Zr, Hf, U, Th, Pb, Ti, Nb, Ta, P, F, Y and rare earth elements. They possess identical U–Pb ages to the Banxi Group and underlying Lengjiaxi Group clastic rocks but distinct 176Hf/177Hf, 176Lu/177Hf and 176Yb/177Hf ratios, indicating the possible changing of Lu–Hf systematics during the hydrothermal alteration. The evidence suggests a contribution of ore material from the Banxi Group and a deeper fluid source, derived from the Lengjiaxi Group and the crystalline basement beneath it. A fluid mixing model is proposed wherein the deep-sourced, high radiogenic Hf fluid mixed with circulating near-surface meteoric component-dominated fluid, triggering antimony precipitation. The U–Pb chronometer was not reset during this low-temperature mineralization event whereas the Lu–Hf systematics in the fluid-altered zircons show evidence of lattice breakdown of the old zircons during fluid mixing, coupled with external input of highly radiogenic Hf from the crystalline basement. This research highlights the potential to use zircon structure and geochemistry in tracing complex fluid evolutionary processes.

Multiple-stage tungsten mineralization in the Silurian Jiepai W skarn deposit, South China: Insights from cathodoluminescence images, trace elements, and fluid inclusions of scheelite

The Jiepai deposit, located in the northeastern part of the Miao’ershan-Yuechengling batholith, South China, is characterized by predominant skarn mineralization that formed near the contact between the Silurian biotite granites and the Upper Cambrian carbonate rocks. Jiepai scheelites are recognized as two main generations based on their occurrences, cathodoluminescence (CL) signatures, and REE systematics. Prograde skarn-hosted scheelite I has uniformly dark CL characteristics and LREE-enriched patterns. Retrograde skarn-hosted scheelite IIa is homogeneously dark under CL, and has a slightly MREE-enriched pattern, whereas quartz vein-hosted scheelite IIb is delineated by fine oscillatory CL zones and a relatively flat REE pattern. Additionally, the CL images reveal that both scheelite I and II were modified by later-stage hydrothermal overprints, which occur as ubiquitous overgrowth rims and cutting veins with bright CL signatures, low REE, but similar REE patterns to their precursors. The compositional variations and fluid inclusions in Jiepai scheelites identify episodic pulses of hydrothermal fluids with different properties. The ore-forming fluids with high Nb/Ta, but low Nb, Ta and REY in the early retrograde stage are featured by moderate to high temperature and low to moderate salinity. Under relatively oxidizing conditions, they interacted with the host rock and underwent boiling to precipitate scheelite I. By contrast, the fluids with higher Nb, Ta and REY, but lower Nb/Ta in the late retrograde stage have slightly lower temperature and higher salinity. We therefore speculate that scheelite II may deposit from later magmatic fluids that have experienced continued boiling in more reducing environment.

Crystal chemistry of titanite from the Roxby Downs Granite, South Australia: insights into petrogenesis, subsolidus evolution and hydrothermal alteration

Titanite textures and chemistry have been investigated from the Roxby Downs Granite, host to the Olympic Dam Cu–U–Au–Ag deposit, South Australia. Three textural subtypes of titanite are documented: primary magmatic (cores and rims); deuteric; and hydrothermal (low T recrystallisation). Magmatic cores are defined by enrichment in LREE (~ 3 wt%), Nb (up to 1 wt%) and Zr relative to rims, which typically contain < 1 wt% LREE and Nb, as well as greater concentrations of Al, Ca, Fe and F. Deuteric titanite occurs as overgrowths on pre-existing titanite and other magmatic accessory minerals (magnetite and ilmenite), and is depleted in HFSE compared to magmatic rims, showing geochemical trends consistent with substitution of Ca2+ + Ti4+ ↔ REE3+ + (Al, Fe)3+. Hydrothermal titanite forms as a low-temperature hydrothermal overprint on primary titanite as well as an alteration product of chloritised phlogopite. Applying Zr-in-titanite geothermometry, three temperature ranges are obtained for titanite crystallisation: magmatic cores ~ 765 to 780 °C; rims ~ 705 to 740 °C; and deuteric ~ 680 to 690 °C. Titanite breakdown is a ubiquitous feature of the Roxby Downs Granite, and occurs through interaction with CO2- and F-rich fluids, forming pseudomorphs characterised by the presence of REE-fluorocarbonates, which are subsequently overprinted by REE-phosphates with increased proximity to the Olympic Dam Breccia Complex. This change is related to interaction with fluids containing appreciable PO42− liberated from local dissolution of fluorapatite. Such observations are consistent with and linked to later/retrograde stages in the formation of the Olympic Dam deposit.

Distinguishing Ore Deposit Type and Barren Sedimentary Pyrite Using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry Trace Element Data and Statistical Analysis of Large Data Sets

Faced with ongoing depletion of near-surface ore deposits, geologists are increasingly required to explore for deep deposits or those lying beneath surface cover. The result is increased drilling costs and a need to maximize the value of the drill hole samples collected. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of pyrite is one tool that is showing promise in deep exploration. Since the trace element content of pyrite approximates the composition of the fluid from which it precipitated and the crystallization mechanism, the trace element characteristics can be used to predict the type of deposit with which a pyritic sample is associated. This possibility, however, is complicated by overlapping trace element abundances for many deposit types. The solution lies with simultaneous comparison of multiple trace elements through rigorous statistical analysis. Specifically, we used LA-ICP-MS pyrite trace element data and Random Forests, an ensemble machine learning supervised classifier, to distinguish barren sedimentary pyrite and five ore deposit categories: iron oxide copper-gold (IOCG), orogenic Au, porphyry Cu, sedimentary exhalative (SEDEX), and volcanic-hosted massive sulfide (VHMS) deposits. The preferred classifier utilizes in situ Co, Ni, Cu, Zn, As, Mo, Ag, Sb, Te, Tl, and Pb measurements to train the Random Forests. Testing of the Random Forests classifier using additional data from the same deposits and sedimentary basins (test data set) yielded an overall accuracy of 91.4% (94.9% for IOCG, 78.8% for orogenic Au, 81.1% for porphyry Cu, 93.6% for SEDEX, 97.2% for sedimentary pyrite, 91.8% for VHMS). Similarly, testing of the Random Forests classifier using data from deposits and sedimentary basins that did not have analyses in the training data set yielded an overall accuracy of 88.0% (81.4% for orogenic Au, 95.5% for SEDEX, 90.0% for sedimentary pyrite, 73.9% for VHMS; insufficient data was available to perform a blind test on porphyry Cu and IOCG). The performance of the classifier was further improved by instituting criteria (at least 40% of total votes from the Random Forests needed for a conclusive identification) to remove uncertain or inconclusive classifications, increasing the classifier’s accuracy to 94.5% for the test data (94.6% for IOCG, 85.8% for orogenic Au, 87.8% for porphyry Cu, 95.4% for SEDEX, 98.5% for sedimentary pyrite, 94.6% for VHMS) and 93.9% for the blind test data (85.5% for orogenic Au, 96.9% for SEDEX, 96.7% for sedimentary pyrite, 84.6% for VHMS). The Random Forests classification models for pyrite trace element data can be used as a predictive modeling tool in greenfield terrains by providing an accurate indication of ore deposit type. This advance will assist mineral explorers by allowing early implementation of predictive ore deposit models when prospecting for ore deposits. Furthermore, the ability of the classifier to accurately identify pyrite of sedimentary origin will allow researchers interested in paleoenvironmental conditions of ancient oceans to effectively screen prospective samples that are affected by a hydrothermal overprint.

A new record of late Ediacaran acritarchs from La providencia group (Tandilia System, Argentina) and its biostratigraphical significance

A well preserved assemblage of organic-walled microfossils is described from the Alicia and Cerro Negro formations of the La Providencia Group. The microbiota is dominated by Leiosphaeridia species (L. minutissima, L. tenuissima, L. crassa and L. jacutica) and comprises colonial sphaeroids (Synsphaeridium sp.) and filamentous microfossils of the genus Siphonophycus. The assemblage is assigned to the Late Ediacaran Leiosphere Palynoflora (LELP), which is consistent with the occurrence of the discoidal fossil Aspidella (<565 Ma) in the overlying unit. The distribution of microfossils within the Alicia Formation is mainly controlled by the shallowing upward trend and thus, environmental factors. This is indicated by the abundance and size of filaments, which tend to occur near to the paleoshore in shallow water settings. Furthermore, large leiospheres also occur in shallower facies together with filaments, in accordance with existing models for acritarch ecology and distribution in the Proterozoic marine successions. The absence of acanthomorphic acritarchs is thus interpreted as a real evolutionary trend, and not as a result of preservational or environmental biases. Regarding the preservation of microfossils, a higher Thermal Alteration Index for the Alicia Formation, compared with older and younger units, is interpreted as a result of hydrothermal overprint and/or contact metamorphism by basic dikes.

Dating multiply overprinted granites: The effect of protracted magmatism and fluid flow on dating systems (zircon U-Pb: SHRIMP/SIMS, LA-ICP-MS, CA-ID-TIMS; and Rb–Sr, Ar–Ar) – Granites from the Western Erzgebirge (Bohemian Massif, Germany)

The Variscan granites from the Western Erzgebirge were repeatedly dated by various methods, but no consensus has been reached about their exact intrusion ages. This study presents a multi-dating approach for the four largest intrusions from the Western Erzgebirge (Aue-Schwarzenberg, Bergen, Eibenstock, Kirchberg). We analysed several samples from each pluton/suite with zircon U–Pb CA-ID-TIMS (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) to obtain robust temporal information on their age and tempo of intrusion. These data enable us for the first time to define three intrusive episodes of 1–2 Ma each, separated by quiet periods of several Ma. The Aue-Schwarzenberg suite represents the oldest granites that intruded at ~323–322 Ma followed by the granites from Bergen and Kirchberg 2–4 Ma later. The highly evolved ore-bearing granites from the Eibenstock pluton intruded after a time lag of ~5 Ma at ~315–314 Ma. The new data show that there is a resolvable age difference between the two known granite groups. Granite group 2 (also assigned as younger igneous complex, represented by the Eibenstock pluton) is ≥5 Ma younger than granite group 1 (assigned as older igneous complex, represented by granites from Aue-Schwarzenberg, Bergen and Kirchberg).Protracted magmatism and late-/post-magmatic fluid flow partly reset the U–Pb system of these granites to variable degrees, making a precise and accurate dating of their intrusion ages challenging. Pb loss in zircons is often combined with high common Pb (Pbc). SHRIMP/SIMS (sensitive high mass resolution ion microprobe/secondary ion mass spectrometry) and LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) on non-CA zircons document that Pb loss and high Pbc is quite variable within zircon grains and may be located in micro-fractures. We demonstrate that chemical abrasion (CA) clearly minimizes or removes both Pb loss and Pbc. Results from prior LA-ICP-MS and SHRIMP dating on non-CA zircons from the same samples considerably helped the interpretation of the CA-ID-TIMS data when Pb loss was not completely erased by CA. In such cases we often had to choose the oldest analyses for mean age calculation in contrast to the common practice of the CA-ID-TIMS community to choose the youngest dates.Rb–Sr and Ar–Ar dating systems revealed age differences between the older group and the younger ore-bearing granites albeit with diverging absolute ages. Most Ar–Ar ages are identical with CA-ID-TIMS ages and would imply rapid cooling. However, samples from the older group have excess Ar that could have led to too old ages. In contrast, Rb–Sr ages for the older granites are 0–7 Ma younger than their intrusions. Fluid induced alteration led to the formation of Li-mica, fluorite and cassiterite (greisenization). For the youngest granite (Eibenstock), Li-mica was used to date the first greisenization. Samples without visible hydrothermal overprint yielded identical Ar–Ar and Rb–Sr ages as severely greisenized samples. This implies re-equilibration due to the hydrothermal overprint for all Ar–Ar and Rb–Sr ages from the Eibenstock pluton. According to Ar–Ar dating, the first ore formation (~315 Ma) is coeval with the CA-ID-TIMS intrusion age of the Eibenstock granite while it is delayed by ~6 (±3) Ma according to Rb–Sr dating (308 ± 3 Ma).

Mineralogical characterization and origin of uranium mineralization in Witwatersrand-like metaconglomerate of the Moeda Formation, Quadrilátero Ferrífero, Brazil

The Moeda Formation is the basal unit of the Minas Supergroup in the Quadrilátero Ferrífero, Brazil, and has quartz-pebble metaconglomerate that hosts Au–U mineralization, commonly assumed to be detrital in origin. The Au–U mineralization occurs immediately above an unconformity with the underlying Archean Rio das Velhas greenstone sequence. In this work, we focus on the Moeda metaconglomerate in its U-bearing portions in the Gandarela syncline: the southern sector, near the syncline axis; and the western limb, near the Palmital gold deposit. The metaconglomerate has mica seams that host zircon, xenotime, rutile, nodules of organic matter and sulfide minerals. The sulfide assemblage includes four generations of pyrite along with Fe–Ni–Cu–Co–Zn–Pb–Mo sulfides. A Sb–As sulfosalt assemblage of gersdorffite, stibnite and ullmannite is late in relation to pyrite and other sulfides. Gold is argentiferous with minor Hg. Uranium minerals include uraninite, coffinite, brannerite and uranyl arsenates. An early generation of uraninite and brannerite occurs as inclusions in organic-matter nodules, mainly in the southern sector. A late generation of uraninite formed due to a low-temperature hydrothermal overprint. Coffinite and late-stage brannerite are products of hydrothermal alteration of zircon and rutile, respectively. Uranyl arsenates have Ni as the prevalent cation; they formed as replacement of arsenic minerals, such as gersdorffite, by oxidized fluids containing U. Microstructural data and mineral chemistry favor a hydrothermal hypothesis analogous to an unconformity-related setting, rather than a detrital, placer-deposit model for the occurrence of U in the Moeda metaconglomerate.