Low-temperature Hydrothermal Fluids Research Articles

The use of mercury vs. cerium in recognising seismically induced fluid overprints on weathered profiles

Reconnaissance whole-rock chemical analyses for Hg and Ce in weathered Mn-oxide-rich rocks from the Quadrilátero Ferrífero, Minas Gerais, Brazil, show a positive linear correlation. In contrast, manganiferous duricrusts from lateritic profiles in south-western Amazonia, Brazil, have no Hg–Ce correlation. Considering the presence fault-related rock fabrics in the former, such as slickenside striae, breccias and veinlets, and the Hg sensitivity to seismic activity, as well as the adsorption of aqueous Ce onto Mn-oxide precipitates, it is proposed that the positive Hg–Ce correlation can be used as an indicator of seismically induced, low-temperature hydrothermal fluid flow in the weathering zone.

Depositional age and tectonic environment of the Gouap banded iron formations from the Nyong group, SW Cameroon: Insights from isotopic, geochemical and geochronological studies of drillcore samples

The discovery of the Gouap banded iron formations (BIFs)-hosted iron mineralization in the northwestern of the Nyong Group (Ntem Complex) in southwestern Cameroon provides unique insights into the geology of this region. In this contribution, we firstly report detailed study of geochemistry, isotopic and geochronology of well preserved samples of the Gouap BIFs collected from diamond drillcores. The Gouap BIFs consist mainly of amphibole BIFs and amphibole–pyrite BIFs characterized by dominant Fe2O3 ​+ ​SiO2 contents and variable contents of CaO, MgO and SO3, consistent with the presence of amphibole, chlorite, epidote and pyrite, formed during amphibolite facies metamorphism and overprinted hydrothermal event. The amphibole–pyrite BIFs are typically enriched in trace and rare earth elements (REE) compared to the amphibole BIFs, suggesting the influence of detrital materials as well as secondary hydrothermal alteration. The Post Archean Australian Shale (PAAS)-normalized REE–Y profiles of the Gouap BIFs display positive La, Eu anomalies, weak negative Ce anomalies, indicating a mixture of low-temperature hydrothermal fluids and relatively oxic conditions probably under relative shallow seawater.We present here the first isotopic data of BIFs within the Ntem Complex. The δ30SiNBS28 values of the quartz from the Gouap BIFs vary from −1.5‰ to −0.3‰ and from −0.8‰ to −0.9‰ for the amphibole BIFs and amphibole–pyrite BIFs, respectively. The quartz has δ18OV-SMOW values of 6.8‰–9.5‰ (amphibole BIFs) and 9.2‰–10.6‰ (amphibole–pyrite BIFs). The magnetite from the Gouap BIFs shows δ18O values ranging from −3.5‰ to −1.8‰ and from −3‰ to −1.7‰ for the amphibole BIFs and amphibole–pyrite BIFs, respectively. Moreover, the pyrite grains in the amphibole–pyrite BIFs display δ34S values of 1.1‰–1.8‰. All isotopic data of the Gouap BIFs confirm that they might have precipitated from low-temperature hydrothermal fluids with detrital input distant from the volcanic activity. According to their geochemical and isotopic characteristics, we propose that the Gouap BIFs belong to the Superior type.In situ U–Pb zircon dating of BIFs was conducted to assess the BIF depositional age based on strong evidence of zircon in thin section. The Gouap BIFs were probably deposited at 2422 ​± ​50 ​Ma in a region where sediments extended from continental shelf to deep-water environments along craton margins like the Cauê Formation of the Minas Supergroup, Brazil. The studied BIFs have experienced regional hydrothermal activity and metamorphism at 2089 ​± ​8.3 ​Ma during the Eburnean–Transamazonian orogeny. These findings suggest a physical continuity between the protocratonic masses of both São Francisco and Congo continents in the Rhyacian Period.

Minerals in Manganese Deposits of Belyaevsky Volcano, the Sea of Japan

The mineral composition of ferromanganese crusts from Belyaevsky Volcano in the Central Basin of the Sea of Japan has been studied. The crusts have diverse structures and a complex polymineral composition. Along with the previously known todorokite, birnessite, manganite, pyrolusite, and vernadite, we determined rhodochrosite, quenselite, manganosite, hetaerolite, manganotantalite, and tungomelane for the first time. The group of accessory minerals includes magnetite, titanomagnetite, hematite, titanohematite, pyrite, pyrrhotite, galena, monazite, cerianite, and baddeleyite. Copper gold was recognized for the first time in manganese breccia. The origin of manganese minerals was caused by low-temperature hydrothermal fluids, which supplied metals and defined local conditions and intermittent growth of manganese crusts. Sapropel organic matter of siliceous–clayey carbonaceous sediments in the Cenozoic sequences of the deep basin could be a source of metals and the reason for the suboxidizing environments of crystallization of some manganese (manganite, todorokite, and birnessite) and accessory (intermetallics, sulfides, and native elements) minerals.

Stratigraphy, petrology, and geochemistry of a neoproterozoic banded iron sequence in the El-Dabbah Group, central Eastern Desert, Egypt

The El-Dabbah Group of the Nubian Shield, Central Eastern Desert, Egypt, contains a Neoproterozoic banded iron sequence within volcaniclastic rocks deposited in an island arc setting. The group contains a volcaniclastic sequence of relatively undeformed lower-greenschist-facies rocks that are unconformably overlain by terrestrial sedimentary strata of the Hamamat Group. The group is > 7000 m thick and contains three formations (the Lower, Middle, and Upper El-Dabbah formations), which include massive metavolcanic rocks, pillow lavas, well-bedded volcaniclastic rocks, black and greenish shale and banded iron sequences. There is no evidence of glaciation, such as diamictites or cap-carbonate beds, in the three formations. The Middle El-Dabbah Formation (2000 m thick) contains well-preserved iron sequences within the volcaniclastic rocks. Most of the iron sequences comprise beds that are a few meters thick. The iron sequence rocks have low contents of Al, Ti, K, and Na, and low Al/(Al + Fe + Mn + Na + K + Ca) ratios, which suggest there was no continental input. Heavy rare earth element enrichment and the absence of Eu anomalies indicate these rocks formed from low-temperature hydrothermal fluids or in a distal setting from a hydrothermal vent system. δ13C values of the black shales are ca. −22‰, suggesting the organic matter was derived from cyanobacterial activity. Given that several black shale units were precipitated below the iron sequences, the cyanobacterial activity might have produced the oxygen at the ocean surface that led to iron oxidation and deposition. Based on the stratigraphic and geochemical characteristics of the El-Dabbah Group, we propose the iron sequences were deposited in a small isolated basin within the central rift zone of an oceanic island arc. The relationship between the El-Dabbah Group and Sturtian glaciation remains unclear.

Complete Genome Sequence of Halomonas hydrothermalis Strain Slthf2, a Halophilic Bacterium Isolated from a Deep-Sea Hydrothermal-Vent Environment.

Halomonas hydrothermalis strain Slthf2 is a Gram-negative bacterium isolated from low-temperature hydrothermal fluids in South Pacific Ocean vent fields located at 2,580-m depth. Here, we report the complete genome sequence of this strain, which has a genome size of 4.12 Mb, with a GC content of 53.2%.

Geochemistry of skarn and porphyry deposits in relation to epithermal mineralization in the Arasbaran metallogenic zone, NE Tabriz, Iran

The Arasbaran metallogenic zone in northern Iran is part of the Alborz–Azerbaijan magmatic zone, which developed along the southern margin of Eurasia during the Early Mesozoic–Late Cenozoic. This region hosts precious and base metal mineralization, including porphyry, skarn, and epithermal copper, molybdenum, and gold deposits. Rare earth element variations across all the deposits are similar, indicating a similar source for these elements. The north-west trending belt comprising the Nabijan to the Sonajil deposits consistently shows chiefly alkaline conditions of formation. Fluid inclusion studies indicate that both high and low temperature hydrothermal fluids participated in the formation of all of the deposits. The mineralization age decreases from north to south and east to west and, although metal zonation is complex, the Cu–Au association post-dated the Cu–Mo mineralization reflecting that the ore fluid evolved in terms of both cooling and chemical changes due to fluid–fluid and fluid–rock interactions. In this region most deposits record a concentric zonation, with the centres preserving porphyry and skarn deposits and deposits becoming progressively epithermal toward the outer parts of the mineralizing system. According to this, the mineralization age decreases from the porphyry and skarn deposits to the epithermal deposits. The homogenization temperature and salinity both decrease from the centre to the outer zone. The pattern of homogenization temperature zonation, which is concordant with salinity zonation, suggests that fluids migrated up-dip and towards the margins of the zonation system.

Aqueous alteration mapping in Rishabdev ultramafic complex using imaging spectroscopy

Hyperspectral remote sensing/imaging spectroscopy has enabled precise identification and mapping of hydrothermal alteration mineral assemblages based on diagnostic absorption features of minerals. In the present study, we use Airborne Visible InfraRed Imaging Spectrometer-Next Generation (AVIRIS-NG) datasets acquired over Rishabdev ultramafic suite to derive surficial mineral map using least square based spectral shape matching in wavelength range of diagnostic absorption features of minerals. Resulting mineral map revealed presence of hydrothermally altered serpentine group of minerals and associated alteration products (talc and dolomite) along with clays and phyllosilicates. Mineral maps are validated using field spectral measurements and published geological map. Involvement of low temperature (<350 °C) hydrothermal fluid in serpentinization of ultramafic rocks in the region is inferred from analysis of deepest absorption features of muscovites at 2.20 μm, spectral abundance of lizardite and absence of prenhite-pumpyllite facies mineral assemblages. Talc was found to be the most common alteration product of serpentines followed by dolomites. Intense alteration of serpentines to talc along the fracture zone is attributed to the circulation of carbon dioxide rich hydrothermal fluids along these conduits. Kaolinite and halloysite are primarily associated with granites and are the result of hydrothermal alteration of plagioclase feldspar in granites while muscovite and illites are generally associated with phyllites and quartzites . The study demonstrates the potential of imaging spectroscopy for mapping hydrothermal alteration mineral assemblages in ultramafic complex.

Ediacaran-Cambrian far-field hydrothermal event in the southeast of the São Francisco Craton, Brazil: Evidence from zircon U-Pb SHRIMP, trace elements, Lu-Hf and oxygen isotopes

Iron formation-bearing sequences from the Guanhães Tectonic Block and Southern Espinhaço Belt of the Southeastern São Francisco Craton, were affected by the Ediacaran-Cambrian Brasilian Orogeny, when the Guanhães Block was upthrusted onto Statherian metasedimentary units of the Lower Espinhaço Supergroup. The iron formation contain a number of hydrothermal high-grade iron ore bodies (e.g. Horto-Baratinha, Cuité, Morro Escuro and Morro do Pilar deposits). Three different types of zircons were studied in order to characterize the temporal and geochemical evolution of the late-orogenic, hydrothermal event: detrital zircons (Lower Espinhaço Supergroup – Orosirian-Statherian maximum depositional age), zircons from the gneissic basement (Archean Guanhães Complex), and zircons from metagranitoid (Statherian Borrachudos Suite). All populations include zircons with <5 μm to 50 μm wide areas (mainly rims) that are brighter in BSE images. These zones are much younger than the rest of the grains (U-Pb SHRIMP ages from 492 Ma to 554 Ma) and display higher U contents and lower Th/U ratios. Zircons from pegmatites that crosscut the Paleoproterozoic sequence show similar ages, as well as high U contents and low Th/U ratios. Some of the pegmatite zircons contain inherited cores from the Borrachudos Suite. A general ZrO2, SiO2 and Th depletion and U, Pb, Hf, Mg, Ca, Fe, Yb and P enrichment was observed in the zircon rims in comparison with their cores. Lower εHf(t) values and higher 176Hf/177Hf(t) ratios were obtained in the zircon rims relative to the unaltered cores. Higher δ18O values (above 7‰) were found in the rims of the detrital and pegmatite zircons, being approximately 1‰ higher than the cores, indicating low-temperature water-rock interactions. Uplift and juxtaposition of Neoarchean gneissic rocks of the eastern São Francisco Craton, Statherian granites and metamorphic supracrustal units occurred during the late to mainly post-collisional stages of the Brasilian Orogeny (560–490 Ma). This uplift and decompression promoted partial melting of the Borrachudos Suite, generating pegmatite intrusions and a magmatic-hydrothermal fluid that affected the entire sequence. Zircons from metasedimentary rocks, granite and gneiss underwent recrystallization generally at sub-solidus conditions during interaction with the moderate- to low-temperature hydrothermal fluid. A concentric porous fabric is present in some grains and registers the fluid pathways during crystal alteration and recrystallization. This recrystallization process had a large impact on the chemical and isotopic compositions of the zircons. The pegmatite zircons represent new anatectic, zircon growth or anatectic overgrowth on older cores in the presence of fluid-saturated melts.

Distribution of trace elements in willemite from the Belgium non-sulphide deposits

Samples of willemite (Zn2SiO4) mineralization from the historical non-sulphide Zn–Pb deposits of La Calamine (eastern Belgium) have been recovered from collections of the Geological Survey of Belgium. Textural and chemical analyses are used to evaluate the critical element distribution (Ge, In, Ga) and deportment. willemite occurs as a variety of types that continuously formed between the protore stage (sulphides) and the late supergene stage (carbonates and hydrated phases). Different types of willemite may be distinguished on the basis of their shape and zoning characteristics, supporting a polyphase non-sulphide mineralization after the protore stage. This is also marked by a significant change of major-element compositions in the late generation of willemite. Laserablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) measurements of minor and trace elements also reveal a strong variability between the different willemite types, although no straightforward relation with the shape of willemite crystals or their zoning is demonstrated. Among the trace elements, we note anomalous high contents of P, Cd, As, Pb, Ag and Sb, the three latter ones being related to tiny galena inclusions. While Ga and In contents are very low (less than 4 ppm) or below detection limits, respectively, significant Ge contents up to 250 ppm were measured. Such contents are consistent with values reported from willemite mineralization throughout the world. The concentrations measured in willemite are similar to those in sphalerite (averaging 250 ppm), supporting a role as precursor for sphalerite. However, the supergene origin of willemite in Belgian deposits is controversial and the influence of low-temperature hydrothermal fluids for willemite precipitation cannot be ruled out. This also questions the origin of Ge further incorporated in zinc silicates.

New isotope constraints on the Mg oceanic budget point to cryptic modern dolomite formation

The oceanic magnesium budget is important to our understanding of Earth’s carbon cycle, because similar processes control both (e.g., weathering, volcanism, and carbonate precipitation). However, dolomite sedimentation and low-temperature hydrothermal circulation remain enigmatic oceanic Mg sinks. In recent years, magnesium isotopes (δ26Mg) have provided new constraints on the Mg cycle, but the lack of data for the low-temperature hydrothermal isotope fractionation has hindered this approach. Here we present new δ26Mg data for low-temperature hydrothermal fluids, demonstrating preferential 26Mg incorporation into the oceanic crust, on average by εsolid-fluid ≈ 1.6‰. These new data, along with the constant seawater δ26Mg over the past ~20 Myr, require a significant dolomitic sink (estimated to be 1.5–2.9 Tmol yr−1; 40–60% of the oceanic Mg outputs). This estimate argues strongly against the conventional view that dolomite formation has been negligible in the Neogene and points to the existence of significant hidden dolomite formation.

The Lavrion Pb-Zn-Ag–Rich Vein and Breccia Detachment-Related Deposits (Greece): Involvement of Evaporated Seawater and Meteoric Fluids During Postorogenic Exhumation

AbstractThe formation of ore deposits in the Lavrion Pb-Zn-Ag district was associated with Miocene detachment that accommodated orogenic collapse and exhumation of high-grade nappes across the ductile-brittle transition. This district consists of (1) low-grade porphyry Mo style, (2) Cu-Fe skarn, (3) high-temperature carbonate replacement Pb-Zn-Ag, and (4) vein and breccia Pb-Zn-Ag mineralization. The vein and breccia mineralization locally contains high-grade silver in base metal sulfides that are cemented by fluorite and carbonate gangue. The rare earth element contents of these gangue minerals, chondrite-normalized patterns, and fluid inclusion studies suggest that they precipitated from a low-temperature hydrothermal fluid. Primary and pseudosecondary fluid inclusions in fluorite and calcite are characterized by a wide range of homogenization temperatures (92°–207°C) and salinities of up to 17.1 wt % NaCl equiv. Secondary fluid inclusions only represent &lt;5 vol % of the total fluid trapped. Fluids extracted from inclusions in fluorite have values of δD = –82.1 to –47.7‰ (Vienna-standard mean ocean water [V-SMOW]) and δ18O = –10.4 to –5.1‰ (V-SMOW). These data and low ratios of Cl/Br measured by crush-leach analyses for fluids in fluorite (102–315) and calcite (162–188) are compatible with the ore fluid being the result of mixing of meteoric water with evaporated seawater. These data suggest that fluids leading to the deposition of late Pb-Zn-Ag–rich vein- and breccia-style mineralization in Lavrion were related to circulation of mixed evaporated seawater and meteoric fluids that was enhanced by brittle deformation. This contrasts with the fluids of magmatic origin related to the formation of low-grade porphyry Mo, Cu-Fe skarn, and high-temperature carbonate replacement deposits spatially related to the Plaka granodiorite.

Concentrations and modes of occurrence of some potentially valuable and toxic elements in the No. 5 coal from the Yanzishan Mine, Datong Coalfield, Shanxi Province, China

In this study, the mineralogical compositions, concentrations and occurrence modes of the elements in the No.5 coal from the Yanzishan Mine were investigated, using X-ray powder diffraction (XRD), X-ray fluorescence spectrometry (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and scanning electron microscopy equipped with an energy-dispersive X-ray spectrometer (SEM-EDS). The No. 5 coal is characterized by high ash yield content (mean 32.30%) and low sulfur content (mean 0.47%). The minerals are mainly composed of kaolinite, quartz and pyrite, with a small proportion of illite, apatite, siderite, sphalerite and Ti-oxide. Compared with the Chinese coals, the No. 5 coal has much higher percentages of SiO2 (mean 16.22%) and Al2O3 (mean 14.57%) than other major oxides. Compared with the world hard coals and Chinese coals, the No. 5 coal is enriched in some potentially valuable (Al2O3, Li, Ge, Zr, and Hf) and toxic (Be, As, Se, Cd, and Pb) elements. Lithium and Hf are mainly associated with clay minerals; Be, Se, and Zr mainly have both inorganically and organically affinities; As mainly occurs as a sulfide form in pyrite; Cd is incorporated in clay minerals; and Pb has an affinity for both clay minerals and organic sulfur. All the coal samples are characterized by weakly positive or no pronounced Eu anomalies, indicating that the rare earth elements and yttrium (REY) likewise originated from granite of the Yinshan Oldland together with injection of the low temperature hydrothermal fluids.

Not all Neoproterozoic iron formations are glaciogenic: Sturtian-aged non-Rapitan exhalative iron formations from the Arabian–Nubian Shield

Neoproterozoic iron formations are exposed in the Wadi Hamama area (Egypt) in the northwestern part of the Arabian–Nubian Shield. Mafic and felsic volcanic and volcaniclastic rocks of an intra-oceanic island-arc setting host multiple, thin iron-formation units. Major element compositions of the iron formation confirm a low detrital input, whereas the rare-earth elements and Y data suggest deposition related to an influx of low-temperature hydrothermal fluids. Unlike most Neoproterozoic banded iron formations, but similar to other iron-formation occurrences from the Arabian–Nubian Shield, the Nd isotopic compositions of the Wadi Hamama iron formations are predominantly mantle-like. SIMS U–Pb zircon ages of the host volcaniclastic units indicate that the age of iron-formation deposition is ca. 695 Ma, which is within the Sturtian epoch that is presumed to be a glacial event of global extent. Nevertheless, there is no robust evidence of any influence of Sturtian glaciation in the Arabian–Nubian Shield. Our results rather suggest that the iron formations in the area may have formed as low-temperature exhalites on the floor of an island-arc basin. The iron formations were deposited during periods of volcanic quiescence, with metals having been derived during low-temperature pervasive hydrothermal alteration of volcanic and volcaniclastic rocks exposed at the seafloor–seawater interface. Precipitation took place due to mixing of metal-bearing hydrothermal fluids and cold, oxygenated seawater. There is no need to invoke possible effects of global glaciation to explain the origin of the Sturtian-aged iron formations in the shield. Our study thus suggests that not all Neoproterozoic iron formations are necessarily linked to glacial events as the Hamama deposit represents a non-Rapitan exhalative iron formation.

Geochemical and Mineralogical Characteristics of the Middle Jurassic Coals from the Tongjialiang Mine in the Northern Datong Coalfield, Shanxi Province, China

There is limited information available on the minerals and elements present in the Jurassic coals from Datong Coalfield. This paper investigates the geochemical and mineralogical characteristics of the Middle Jurassic coals from the Tongjialiang Mine using X-ray powder diffraction (XRD), X-ray fluorescence spectrometry (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and scanning electron microscopy in combination with energy-dispersive X-ray spectrometry (SEM-EDS). No.12 coal is a low-medium volatile bituminous coal and is characterized by low ash yield content, low moisture content, and ultra-low sulfur content. Compared with Chinese coals, the Tongjialiang coals have slightly higher average percentages of MgO and P2O5, and lower average percentages of the other major oxides, including SiO2, TiO2, Al2O3, Fe2O3, CaO, MnO, Na2O, and K2O. Compared with the World hard coals, Be, Cr, Co, Ni, Ge, Sn, Ta, and W are slightly enriched in the Tongjialiang coals. The concentrations of Li, F, Sc, V, Cu, Ga, Se, Sr, Zr, Nb, Hf, Pb, Th, and U are close to the average values of the world’s hard coals. The minerals in No.12 coal mainly include quartz, kaolinite, siderite, and ankerite, along with smaller amounts of pyrite, illite, calcite, and rutile. The formation of syngenetic siderite in No.12 coal is related to the weathering of biotite in the gneiss of the Yinshan Upland. The modes of occurrence of ankerite indicate that the coals may be affected by the injection of low temperature hydrothermal fluids. It is noteworthy that a portion of epigenetic ankerite may be a product of metasomatism between syngenetic siderite and the epigenetic Fe-Mg-Ca rich hydrothermal fluids. The ratios of Al2O3/TiO2, REY (rare earth elements and yittrium) enrichment patterns, the modes of occurrence of siderite and ankerite, as well as the enriched lithophile and siderophile elements indicate that the No.12 coal may have originated from the Yinshan Upland and may also have been influenced by low temperature hydrothermal fluids that might have circulated in the coal basin.

Assessing Thallium Elemental Systematics and Isotope Ratio Variations in Porphyry Ore Systems: A Case Study of the Bingham Canyon District

The Bingham Canyon porphyry deposit is one of the world’s largest Cu-Mo-Au resources. Elevated concentrations of thallium (Tl) compared to average continental crust have been found in some brecciated and igneous samples in this area, which likely result from mobilization of Tl by relatively low temperature hydrothermal fluids. The Tl-enrichment at Bingham Canyon therefore provides an opportunity to investigate if Tl isotope ratios reflect hydrothermal enrichment and whether there are systematic Tl isotope fractionations that could provide an exploration tool. We present a reconnaissance study of nineteen samples spanning a range of lithologies from the Bingham district which were analysed for their Tl content and Tl isotope ratios, reported as parts per ten thousand (ε205Tl) relative to the NIST SRM997 international standard. The range of ε205Tl reported in this study (−16.4 to +7.2) is the largest observed in a hydrothermal ore deposit to date. Unbrecciated samples collected relatively proximal to the Bingham Canyon porphyry system have ε205Tl of −4.2 to +0.9, similar to observations in a previous study of porphyry deposits. This relatively narrow range suggests that high-temperature (&gt;300 °C) hydrothermal alteration does not result in significant Tl isotope fractionation. However, two samples ~3–4 km away from Bingham Canyon have higher ε205Tl values (+1.3 and +7.2), and samples from more distal (~7 km) disseminated gold deposits at Melco and Barneys Canyon display an even wider range in ε205Tl (−16.4 to +6.0). The observation of large positive and negative excursions in ε205Tl relative to the mantle value (ε205Tl = −2.0 ± 1.0) contrasts with previous investigations of hydrothermal systems. Samples displaying the most extreme positive and negative ε205Tl values also contain elevated concentrations of Tl-Sb-As. Furthermore, with the exception of one sample, all of the Tl isotopic anomalies occur in hydrothermal breccia samples. This suggests that ε205Tl excursions are most extreme during the migration of low-temperature hydrothermal fluids potentially related to sediment-hosted gold mineralization. Future investigation to determine the host phase(s) for Tl in breccias displaying both chalcophile element enrichment and ε205Tl excursions can potentially provide new information about hydrothermal fluid composition and could be used to locate sites for future porphyry exploration.

Petrological and geochemical characteristics of REE mineralization in the A-type French Creek Granite, New Zealand

The French Creek Granite, New Zealand, is an alkaline intrusion with enrichment in rare earth elements (REE). Petrography and whole-rock geochemical signatures demonstrate that the c. 82–84-Ma French Creek Granite is a composite granitoid, dominated by a ferroan, weakly peraluminous biotite alkali feldspar granite to syenogranite, with subordinate quartz-alkali feldspar (QAF) syenite. Maximum total REE + Y contents are higher in the granite and felsic dikes relative to the marginal QAF syenite, cogenetic mafic Hohonu Dike Swarm, and pegmatites. Our findings suggest that REE and high-field-strength element enrichment in the unaltered French Creek Granite is a function of partial melting from an enriched HIMU-like lithospheric mantle source, minor crustal assimilation, and extensive magmatic differentiation. Primary (magmatic) REE enrichment is defined by the occurrence of allanite, zircon, apatite, fergusonite, monazite, perrierite, and loparite, which are commonly associated with interstitial biotite. French Creek Granite samples from a phyllic-sericitic alteration zone in the Eastern Hohonu River are strongly elevated in REE relative to unaltered French Creek Granite, indicating remobilization and secondary REE enrichment by hydrothermal fluids. The REE are hosted in bastnasite group minerals, monazite, xenotime, and zircon. Quartz protuberances, dilatational microfractures, and dike emplacement indicate that this alteration is structurally controlled. The δ13C and δ18O values of secondary carbonate veinlets are consistent with mixed low-temperature (~ 250–260 °C) magmatic hydrothermal fluids containing mantle-derived carbon. These fluids were likely part of a late-stage porphyry-type system operating during the same mantle degassing and extensional episode that was associated with the emplacement of the French Creek Granite and lamprophyric dikes of the Hohonu Dike Swarm, and initial Tasman Sea spreading. Results from this study provide an important insight into the complex processes responsible for REE enrichment in alkaline igneous systems.

Dissolved organic carbon compounds in deep-sea hydrothermal vent fluids from the East Pacific Rise at 9°50′N

Deep-sea hydrothermal vents are unique ecosystems that may release chemically distinct dissolved organic matter to the deep ocean. Here, we describe the composition and concentrations of polar dissolved organic compounds observed in low and high temperature hydrothermal vent fluids at 9°50′N on the East Pacific Rise. The concentration of dissolved organic carbon was 46 µM in the low temperature hydrothermal fluids and 14 µM in the high temperature hydrothermal fluids. In the low temperature vent fluids, quantifiable dissolved organic compounds were dominated by water-soluble vitamins and amino acids. Derivatives of benzoic acid and the organic sulfur compound 2,3-dihydroxypropane-1-sulfonate (DHPS) were also present in low and high temperature hydrothermal fluids. The low temperature vent fluids contain organic compounds that are central to biological processes, suggesting that they are a by-product of biological activity in the subseafloor. These compounds may fuel heterotrophic and other metabolic processes at deep-sea hydrothermal vents and beyond.

The genesis of the Hehuashan Pb–Zn deposit and implications for the Pb–Zn prospectivity of the Tongling district, Middle–Lower Yangtze River Metallogenic Belt, Anhui Province, China

The Hehuashan lead–zinc deposit is located in the central Tongling Cu–Au–Mo mining district of eastern China and consists of stratabound ore bodies that are generally hosted by dolostone and limestone units of the Lower Triassic Lower Nanlinghu Formation. The deposit contains two types of sphalerite, namely tan to brown fine-grained (∼40 μm) type I sphalerite that is hosted by a dolomite, calcite, quartz and sericite gangue assemblage and colorless to brown coarse-grained (∼2 mm) type II sphalerite hosted within clasts in calcite-cemented breccias or within dolostone-hosted veins. Plotting these sphalerites in a Fe vs. Cd diagram indicates they have a similar composition as those within typical Mississippi Valley-type (MVT) deposits, suggesting that the Hehuashan deposit may have an MVT affinity. In addition, in-situ sulfur stable isotopic analysis of fine-grained (−7.8‰ to +3.0‰) and coarse-grained (+3.4‰ to +9.7‰) sphalerite indicates that the sulfur in the hydrothermal fluids that formed the deposit was derived from the country rocks within the Hehuashan area. Fluid inclusions within calcite and fluorite from the Hehuashan deposit have low homogenization temperatures (mean of 126 °C) and moderate salinities (mean of 7.1 wt% NaCl equivalent) that demonstrate the deposit formed from low temperature and moderate salinity hydrothermal fluids. This suggests that the Hehuashan deposit was almost certainly not formed from magmatic-hydrothermal fluids but instead was formed as a result of the mixing of basinal brines and meteoric water, again consistent with an MVT origin for this deposit. In summary, the Hehuashan lead–zinc deposit is an MVT deposit, suggesting that the Tongling district of Anhui Province, China is prospective for MVT mineralization and that the Triassic dolostones and limestones in this area represent high priority exploration targets for this style of mineralization.

Geology and geochemical constrains on the origin and depositional setting of the Kpwa–Atog Boga banded iron formations (BIFs), northwestern Congo craton, southern Cameroon

The Kpwa–Atog Boga (KAB) area lies within the Paleoproterozoic Nyong unit which defines the northwestern edge of the Archean Congo craton in southern Cameroon. Rock types in the area are mainly comprises of garnet gneiss associated with banded iron formations (BIFs). The BIFs and its host rocks were subjected to amphibolite facies metamorphism. The Rare Earth Elements (REE) patterns and the discrimination ratios (e.g., Al2O3/TiO2 Cr/Th Th/Sc La/Sc) of the garnet gneiss suggest that they are metasedimentary rocks derived from a felsic parent sources that have undergone intensive weathering. The KAB BIFs exhibit significant concentration of TFe2O3 + SiO2 (>93 wt%), and lower Al2O3 and TiO2 contents and high field strength elements (e.g., Hf, Nb, Ta, and Zr). This involves little contamination with continental detritus at their depositional environment. The Post Archean Australian Shale (PAAS)-normalized REE patterns of the KAB BIFs show enrichment in heavy REE and the depletion in light REE with positive La anomalies, which imply that these BIFs precipitated in ambient seawater. Moreover, the contribution of low-temperature hydrothermal fluids during the deposition of the studied BIFs were highlighted through their high superchondritic Y/Ho ratio (mean: 31.48), weak positive Eu/Eu∗(SN) ratio (mean: 1.83) and the low contents of transition metals. This suggests that the geochemical characteristics of the KAB BIFs and those of the Paleoproterozoic superior-type BIFs are almost similar. In summary, low-temperature hydrothermal fluids mixed with seawater contributed to the deposition of the KAB BIFs on the continental margin. This depositional setting similar to its host rocks is consistent with the Paleoproterozoic geological history recorded within the Nyong unit. This preliminary study reveals that the KAB BIFs are associated with low grade gold (<5 ppb), which could constitute further prospective mineralization besides the iron ore in the KAB area.

Mineralogical and Geochemical Compositions of the Lopingian Coals in the Zhongliangshan Coalfield, Southwestern China

The mineralogical and geochemical compositions of the Lopingian coals from an exploratory drill core (ZK4-1) in the Zhongliangshan Coalfield, southwestern China are reported in this paper. The Zhongliangshan coals are medium volatile bituminous in rank (random vitrinite reflectance, average 1.38%), characterized by a medium-ash yield (26.84%) and high sulfur content (3.38%). Minerals in the Zhongliangshan coals are mainly composed of clay assemblages (kaolinite, the illite/smectite mixed layer (I/S) and chamosite), pyrite, quartz, carbonate minerals (calcite, marcasite, ankerite, and dolomite), and anatase, followed by rutile, jarosite, natrojarosite, bassanite, gypsum and K-feldspar, with traces of apatite, rhabdophane and barite. Compared with the average concentrations of the world hard coals, some trace elements including Li, V, Co, Cu, Se, Y, Zr, Nb, rare earth elements (REE), Cd, Ta, Hf and Hg, are enriched in the Zhongliangshan coals. The modes of occurrence of chamosite, barite, rhabdophane, quartz and calcite in the Zhongliangshan coals indicate that the coals have probably been affected by the injection of low-temperature hydrothermal fluids. Based on the concentrations of Sc, V, Cr, Co, Ni, Cu and Zn, the ratios of Al2O3/TiO2 and the upper continental crust-normalized rare earth element and yttrium (REY) distribution patterns of the Zhongliangshan coals, the dominant sediment source regions are the Leshan–Longnvsi Uplift, Hannan Upland, and Dabashan Uplift, with a small proportion of terrigenous materials from the Kangdian Upland. The K7 and the upper portion of K1 coals have the potential as raw materials for the recovery of REY.