Amethyst quartz deposits occur in some regions of Minas Gerais state, Brazil, related to different geological environments. The most important are associated with hydrothermal veins crossing the ...Espinhaço mountain range, mainly in small and eroded mountains near its east margin, related to the Macaúbas Group. In the border region between the states of Minas Gerais (MG) and Bahia (BA), some of these deposits produce amethyst crystals that become green under heat treatments and are known in the gemmological market as “prasiolite.” The aim of the present study was to provide details of the clear structural control over the mineralized veins. The quartz veins are hosted in quartzites, which have long been the subject of controversy regarding their stratigraphic position. In this study, they are inserted into the Santo Onofre (MG) and Serra de Inhaúma (BA) sequences of unknown ages. These quartzites show a folded subvertical foliation, indicating a posterior deformational phase that had not been observed in rocks from the Macaúbas Group in other regions of MG state. At the Montezuma mine (MG), the main veins are around 0.70-1.10 m thick with steep dips to the NE, perpendicular to the NE-SW foliation; bedding orientation is approximately N-S, dipping to the E. In the Coruja mine area (BA), host quartzites have foliation attitudes varying between N35-45°E/ subvertical, and mineralized veins are around 1 m thick, nearly concordant with the foliation. At Tibério’s mine (BA), the mineralization is related to narrow fractures, less than 30 cm wide, and perpendicular to the host quartzite foliation, with directions/dips around N20°W/80°SW. In described deposits, amethyst veins occur structurally related to axial surfaces in foliation folds.
It has been widely accepted that magmas genetically linked to porphyry (-skarn) Cu (Mo) deposits are commonly oxidized. Recently, using laser ablation-inductively coupled plasma-mass spectrometry ...(LA-ICP-MS) techniques, zircon Ce
4+
/Ce
3+
, Ce
N
/Ce
N
*, Eu
N
/Eu
N
*, and Ce/Nd ratios, and magma ΔFMQ values (departure from the fayalite–magnetite–quartz oxygen buffer) based on zircon trace element compositions, have been used as proxies to quantify magma oxidation state. Here we present the zircon trace element compositions of 13 Mesozoic porphyry (-skarn) Mo deposits in NE China of various sizes to examine the relationship between magma Mo fertility and magma oxidation state. Generally, the studied deposits with > 0.3 Mt Mo have Ce
4+
/Ce
3+
> 100, Ce
N
/Ce
N
* > 100, Ce/Nd > 10, and Eu
N
/Eu
N
* > 0.3, whereas those containing < 0.3 Mt Mo have Ce
4+
/Ce
3+
< 100, Ce
N
/Ce
N
* < 100, Ce/Nd < 10, and Eu
N
/Eu
N
* < 0.3. The calculated magma ΔFMQ values do not show significant correlation with metal tonnage, probably due to the large uncertainties of the estimated ΔFMQ data. Among these proxies, Ce
4+
/Ce
3+
and Ce
N
/Ce
N
* ratios show the strongest correlation with Mo tonnage, followed by Ce/Nd and Eu
N
/Eu
N
*. The above results confirm the previous proposal that zircon Ce and Eu anomalies can represent an intrusion’s oxidation state and indicate that the Mo endowment of magmatic-hydrothermal deposits is positively correlated with the magma oxidation state. Compared with Mo-bearing intrusions, the trends for Cu-bearing intrusions are similar but are more complicated, especially for those deposits with > 10 Mt Cu. The findings in this study can be used to evaluate an intrusion’s potential to produce Mo mineralization.
Precise geochronology is a crucial tool for understanding the genesis of low-temperature hydrothermal mineral deposits, but datable minerals are not always available; e.g., mineralization within ...sedimentary rocks, such as Carlin-type or Carlin-like Au deposits. Here, we demonstrate that hydrothermal apatite associated with Au from the large Nibao deposit in the Yunnan–Guizhou–Guangxi region of South China can be dated to yield the age of Au mineralization. Nibao is a Carlin-like deposit hosted in a Permian carbonate-bearing pyroclastic breccia. The auriferous minerals are predominantly zoned pyrite containing As-, Cu-, and Au-rich rims. Euhedral and subhedral apatite forms a mosaic texture and is intergrown with hydrothermal quartz, sericite, and zoned auriferous pyrite, indicating that the apatite is of hydrothermal origin and was coeval with Au mineralization. Unlike sedimentary and igneous apatite, hydrothermal apatite is relatively depleted in LREEs, enriched in MREEs, and slightly depleted in Eu. Hydrothermal apatite also has high Th/U ratios, negligible U concentrations, and very low concentrations of common Pb. This makes the apatite a potential candidate for ThPb dating. Our SIMS ThPb analyses yield a weighted mean 232Th/208Pb age of 141 ± 3 Ma (N = 23, MSWD = 2.2) for apatite coeval with auriferous pyrite, and this is the best estimate of the timing of Au mineralization. Hydrothermal apatite is also present in similar deposits in the Golden Triangle of South China. Therefore, ThPb dating of hydrothermal apatite potentially allows for the accurate dating of low-temperature hydrothermal deposits in China and worldwide.
•Hydrothermal apatite shows particular REE pattern, Th/U, and O isotopes.•SIMS apatite ThPb age of 141 ± 3 Ma is suggested as Au deposition time in Nibao.•Hydrothermal apatite has the potential of dating the low-temperature Ore deposits.
Nitrogen is the main constituent of the Earth's atmosphere, but its provenance in the Earth's mantle remains uncertain. The relative contribution of primordial nitrogen inherited during the Earth's ...accretion versus that subducted from the Earth's surface is unclear.sup.1-6. Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare .sup.15N.sup.15N isotopologue of N.sub.2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle delta.sup.15N (the fractional difference in .sup.15N/.sup.14N from air), N.sub.2/.sup.36Ar and N.sub.2/.sup.3He. Our results show that negative delta.sup.15N values observed in gases, previously regarded as indicating a mantle origin for nitrogen.sup.7-10, in fact represent dominantly air-derived N.sub.2 that experienced .sup.15N/.sup.14N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the .sup.15N.sup.15N data allow extrapolations that characterize mantle endmember delta.sup.15N, N.sub.2/.sup.36Ar and N.sub.2/.sup.3He values. We show that the Eifel region has slightly increased delta.sup.15N and N.sub.2/.sup.36Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts.sup.11, consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has delta.sup.15N values substantially greater than that of the convective mantle, resembling surface components.sup.12-15, its N.sub.2/.sup.36Ar and N.sub.2/.sup.3He ratios are indistinguishable from those of the convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution of mantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume delta.sup.15N values may both be dominantly primordial features.
The critical metal germanium (Ge) is recovered as a by-product of mining other commodities, such as zinc and thermal coal. We investigated the Ge incorporation mechanism in sphalerite synthesized ...under hydrothermal conditions like those of sediment-hosted Zn-Pb deposits. Sphalerite ± galena ± barite formed via reactions of Ge ± Fe ± Cu ± Ba-bearing brine with calcite and reduced sulfur at 200 °C and water vapor-saturated pressure. The products were examined using backscattered electron (BSE) imaging, electron probe microanalysis (EPMA), electron backscattered diffraction (EBSD), synchrotron X-ray fluorescence (SXRF) and micro-X-ray absorption near-edge structure (μ-XANES). We show that Ge(IV) is incorporated into sphalerite and bonded with reduced sulfur, both in the experimental sphalerite and in natural zinc ore samples from the MacArthur River Zn-Pb-Ag deposits, Australia. Copper K-edge XANES spectra show that copper occurs as Cu(I) in the experimental sphalerite, consistent with previous studies on Cu in natural sphalerite. The experiments reveal that Ge(IV) substitution in sphalerite occurs with and without the presence of other metal ions (e.g., Cu(I)), indicating that Ge(IV) substitution can be accommodated via charge balance by vacancies as well as by coupled substitution in the synthesized sphalerite. Ab initio quantum chemical simulations confirm that sphalerite can readily accommodate Ge, with the crystal structure and average Zn-S, Zn-Zn, S-S distances retained when replacing > 3 mol% of the Zn sites with Ge(IV), Ge(II), Cu(I) or Fe(II), demonstrating the resilience and flexibility of the sphalerite crystal structure. These Ge incorporation mechanisms explain the previous observations of multiple ways of Ge incorporation in natural sphalerite. The study provides experimental and molecular simulation insights for understanding the processes related to the formation and extraction of Ge in zinc ores.
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•Indium resources in China are mainly from Sn polymetallic deposits related to Mesozoic granites.•Breakdown of biotite is a key process for Sn-In mineralization in ...magmatic-hydrothermal system.•Indium is mainly incorporated into sphalerite by the coupled substitution of Cu＋＋In3+↔ 2Zn2+.•Enrichment mechanism of indium in Sn-poor polymetallic deposits need more studies.•Behavior of indium in magmatic-hydrothermal systems requires more attention.
China has abundant indium resources and is the main supplier of refined indium in the world. In this contribution, we systematically summarize the geology and spatial–temporal distribution of indium deposits in China, with emphasis on the genesis and enrichment mechanism of these deposits. The indium resources in China are mainly from Sn-polymetallic deposits related to Mesozoic granites within the southern Great Xing’an Range, southwestern Yangtze Block, western Nanling Range, and their surrounding regions. Moreover, these Mesozoic granitic intrusions are mainly composed of highly fractionated S- or A-type granites with peraluminous characteristics (A/CNK > 1.1) and high volatile concentrations, which are considered to be generated by biotite-dehydration melting in the crust. Because biotite is an important carrier of tin and indium, the breakdown of biotite during partial melting could release tin and indium into magmatic-hydrothermal system, leading to Sn-In mineralization. During the precipitation of indium minerals from ore-forming fluids, it is preferentially incorporated into sphalerite lattice by a coupled substitution of In3+ + Cu+ ↔2Zn2+, causing a decoupling from tin. Therefore, indium resources are dominantly sourced from sphalerite-bearing ores in Sn-polymetallic deposits in China. Recently, indium mineralization has been discovered in Sn-poor polymetallic deposits (e.g., the Qibaoshan Cu-polymetallic deposit); however, the enrichment mechanism of indium in Sn-poor deposits is still unclear. Consequently, we suggest that future studies should focus on the enrichment mechanism of indium in Sn-poor polymetallic deposits and the behavior of indium in magmatic-hydrothermal systems, which would contribute to a better understanding of the coupling and decoupling of tin and indium and their metallogenesis.
Studies have reported that various hydrocarbons and hydrocarbon-degrading bacteria are found in global deep-sea hydrothermal regions. However, little is known about degradation characteristics of ...culturable hydrocarbon-degrading bacteria from these regions. We speculate that these bacteria can be used as resources for the bioremediation of oil pollution. In this study, six oil-degrading consortia were obtained from the hydrothermal region of the Southern Mid-Atlantic Ridge through room-temperature enrichment experiments. The dominant oil-degrading bacteria belonged to Nitratireductor, Pseudonocardia, Brevundimonas and Acinetobacter. More varieties of hydrocarbon-degrading bacteria were obtained from sediments (preserved at 4 °C) near hydrothermal vents. Most strains had the ability to degrade high molecular weight petroleum components. In addition, Pseudonocardia was shown to exhibit a high degradation ability for phytane and pristine for the first time. This study may provide new insights into the community structure and biodiversity of culturable oil-degrading bacteria in deep-sea hydrothermal regions.
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•The oil-degrading bacteria isolated from the sediment sample in NHV exhibited a high-degradation ability for phytane and pristane, with degradation rates higher than 86%.•Erythrobacter, Nitratireductor, Acinetobacter, Pseudonocardia and Brevundimonas were dominant oil-degrading bacteria in sediments of deep-sea hydrothermal region.•The diversity of oil-degrading bacteria in NHV was higher. We obtained more culturable oil-degrading bacteria from NHV stored at 4 °C.
High resolution airborne magnetic and radiometric data were used to map probable sites for polymetallic-magmatic hydrothermal deposits in the study area. The first and second vertical derivatives, ...horizontal gradient, analytic signal and tilt angle derivative were used to enhance magnetic data in order to determine the location of short wavelength anomalies, magnetic sources and geologic boundaries. Also, to understand and detect radiometric anomalies controlling mineralization in host rocks, enhancement operations involving potassium (K), equivalent thorium (Th), equivalent uranium (U) concentrations and ternary imageries were carried out. All these operations enabled the delineation of responses associated with mineralization, lithology and geologic structures. Generally, lineaments trend in the NE-SW, NNE-SSW, NW-SE and E-W directions, and serve as potential pathway for hydrothermal fluid migration and mineralization. Radiometric anomalies from spectrometric imageries are caused by felsic minerals, hydrothermal rock alterations, widespread shales, residual clay, oxides and accessory minerals that constitute regolith of the area. The coincident high magnetic and potassium intensities sites in Anambra Bain (AB), Abakaliki Anticlinorium (AA), Ikom-Mamfe Rift (IMR) and Obudu Plateau (OP) are probable sites for polymetallic-magmatic hydrothermal deposits. Generally, the joint magnetic and radiometric results were able to establish link between lithology, geologic structures and hydrothermal alteration patterns.
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