Hydrothermal Mineral Assemblages Research Articles

Mineralization process of the Changjiang uranium orefield in South China: Constraints from pitchblende geochemistry

The Changjiang uranium (U) orefield in northern Guangdong (South China) contains several important granite-related uranium deposits, including the Mianhuakeng deposit. The hydrothermal evolution and mineralization mechanism of the deposit remain unclear, and hence in this study we analyzed the mineral compositional variations of pitchblende from different elevation (−300 to −50 m) in the orebody at Mianhuakeng deposit. The results indicate that with decreasing depth, the hydrothermal mineral assemblage changes from a reducing one (pitchblende, pyrite, and chlorite) to a moderately oxidizing one (pitchblende, coffinite, hematite, and goethite). The contents of U, Sr, ∑REE, U/Th, and LREE/HREE ratios in the pitchblende decrease (whereas the Cu-Pb-Zn-Ni-Co-Sc-Rb contents increase) progressively with decreasing depth. Also, Ce anomaly (Ce/Ce*) changes from positive to negative, and Eu anomaly (Eu/Eu*) becomes more negative with decreasing depth. We interpreted such vertical variations to be caused by the ascent of deep, mid-to-high temperature, highly-oxidizing ore-forming fluid, which was reduced by early-stage reducing minerals in the wallrock, a process that also formed the mineralization at Changjiang. In addition, the pitchblende REE distribution patterns and Eu/Eu* at depth (−300 m) resemble those of the Zhuguang pluton, indicating that the ore-forming materials were originated from the U-bearing granite wallrocks. Therefore, the Ce/Ce* and Eu/Eu* variations of pitchblende can be used to guide deep uranium ore exploration.

LA–ICP–MS U–Pb geochronology and geological significance of two types of monazite in the Xinjiazui gold deposit, northwestern margin of Yangtze Block, China

The newly discovered Xinjiazui gold deposit situates on the northwestern margin of the Yangtze Block, China. Previously, the absolute age of gold metallogenesis in this area has yet to be well defined due to the lack of suitable dating minerals, significantly hindering the study of ore genesis and metallogenesis of gold deposits. This study presents high precision in-situ laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–MS) U–Pb ages of monazite to address this issue. Two types of monazite have been found in this deposit, with Type 1 developed in the sedimentary diagenetic stage and Type 2 formed in the ore-forming stage. Type 1 monazite collected from carbonaceous slate is symbiotic with framboidal pyrite in a skeletal texture. It exhibits a high total amount of rare earth elements (REE) and low Y, Th, and U contents, indicating a low-grade metamorphic origin. In contrast, Type 2 monazite from the auriferous quartz vein is intergrown with hydrothermal mineral assemblages including free gold, Au-bearing pyrite and arsenopyrite, ankerite and sericite, suggesting a hydrothermal origin. The monazite (Type 1) LA–ICP–MS 206Pb/238U age is 460.8 ± 7.0 Ma, representing the sedimentary age of the host rock. A combination of lithologic associations and geochemical characteristics indicates that the Ordovician strata (O2ch) are important surrounding rocks in the Xinjiazui gold deposit. Additionally, the forming ages of monazite symbiotic with free gold in the auriferous quartz–sulfide veins range from 206.4 to 207.4 Ma, representing the gold mineralization age of the Xinjiazui deposit. Field geological observation shows that the vertical vein wall of columnar quartz crystals grows inward in the auriferous quartz–sulfide vein geodes and the euhedral granular sulfides develop in geodes. Therefore, this study proposes that the tectonic system changed from compression to extension in the late orogenic period with the closure of Paleo-Tethys and gold mineralization occurred in the Back-Longmenshan tectonic belt.

Influence of Low-Temperature Hydrothermal Events and Basement Fault System on Low-Resistivity Shale Reservoirs: A Case Study from the Upper Ordovician to Lower Silurian in the Sichuan Basin, SW China

High graphitization is responsible for low-resistivity shale development with poor reservoir quality. This paper provides an explanation of organic matter graphitization and determines the impact of high graphite content on low-resistivity shale reservoir quality at the Wufeng-Longmaxi Formation in the Southern Sichuan Basin. Fine veins are frequently developed at shale samples with Ro > 3.5%, graphitized organic matter > 25%, and resistivity < 5 Ω•m, which are dominated by three mineral assemblages: brunsvigite, barite-hyalophane-barium feldspar-potassium feldspar-anhydrite, and calcite-ankerite. These filling minerals are characterized by an Eu positive anomaly and high Ba, Fe, and Mn contents, suggesting that low-resistivity shale was modified by magmatic-related low-temperature hydrothermal fluid. Temperature measurements of brine inclusions and a semi-empirical geothermometer of chlorite show that low-temperature hydrothermal fluid experienced the chlorite stage (150–180 °C), the low-sulfidation stage (120–150 °C), and the low-temperature calcitization stage. Paleozoic fault systems and late Permian hydrothermal activities associated with the Emeishan mantle plume control the graphitization of low-resistivity shale. The water formation and seawater infiltrated into the deep crust along the Paleozoic basement faults under gravity, developing alkaline hot brine through mantle plume heating and then causing a water-rock reaction with basement rocks. They migrated upward along deep and large Paleozoic faults through convective thermal circulation in the Tiangongtang area, the Shuanglong-Luochang area, and the Xuyong area. Cation exchange and redox reactions occurred during the interaction between high-temperature hydrothermal fluid and cool wall rocks. The migration of alkaline hot brine via the Wufeng-Longmaxi shale introduced a subsequent water-rock reaction, resulting in the development of hydrothermal mineral assemblages that intricately filled fractures. It increased formation temperature and enhanced thermal maturity and graphitization of organic matter at the Wufeng-Longmaxi low-resistivity shale, resulting in a wide distribution of low-resistivity shale at the Changning Block.

Genesis of the Kiruna-type Nixintage iron deposit, Chinese Western Tianshan, NW China: Constrains of ore geology, geochemistry and geochronology

The Nixintage iron deposit is located in the central segment of the Awulale metallogenic belt, Chinese Western Tianshan. Iron ores are mainly hosted by andesitic tuff and andesite of the Carboniferous Dahalajunshan Formation. The paragenetic sequence includes a magmatic magnetite-apatite stage (I); a magmatic–hydrothermal stage (II), and three substages can be divided, including magnetite-albite substage (II-1), magnetite-chlorite-pyrite substage (II-2), magnetite-quartz-calcite substage (II-3); and a late hydrothermal quartz-calcite-sulfides stage (III). Anhedral, fine-grained magnetite associated with apatite in brecciated and matrix ores formed in stage I with relatively variable compositions, most probably indicate the fast crystallization under a disequilibrium condition. The high-grade magnetite orebodies mainly formed in the stage II, are featured by widespread hydrothermal mineral assemblages associated with the ores. Magnetite formed in this stage has medium-to-high concentrations of V, low-to-medium concentrations of Ti, Cr, relative depletion of Mg, Ni, K, and these data mainly plot in the Kiruna and porphyry fields of discriminant diagrams that comparable to magnetite from typical Kiruna-type iron deposits. Both stages I and II magnetites show similar wide δ18O ranges and with peak values at 6.0–10.0 ‰, heavier than typical Kiruna-type deposits, while could be attributed to the relatively low crystallization temperature inferred by microthermometric data of a previous study. The δ34S values of pyrite formed in different stages range between 0.1 and 2.4 ‰, comparable to the sulfur isotopic composition of the mantle-derived magma. These features indicate that stage I iron mineralization was generated by the extrusion of iron phosphorous oxide melts separated from the parental silicate magma. Subsequent deuteric fluids exsolved from basaltic to andesitic melts during the late stage of volcanism, generated magmatic–hydrothermal iron mineralization in stage II, and the continuous evolution of the deuteric fluids formed stage III hydrothermal veins. In summary, integrated evidence suggests the Nixintage is a Kiruna-type iron deposit.Zircon U-Pb ages of ore-hosting andesite and andesitic tuff are 317.6–314.4 Ma, and timing for iron mineralization was inferred at the same time, i.e., ca. 315 Ma. Previously published petro-geochemical data combined with our geochronological data of these rocks, suggest the ore-related volcanism in Nixintage probably originated from the partial melting of the mantle wedge modified by subducted fluids under a Late Carboniferous continental arc setting. Consequently, a genetic model for the studied deposit could be created. Furthermore, the southeast of the Nixintage nearer to the vent might have potential for prospecting large Kiruna-type iron deposits.

Ultramafic-hosted volcanogenic massive sulfide deposits from Cuban ophiolites

Ultramafic-hosted volcanogenic massive sulfide deposits (UM-VMS) located in the Havana-Matanzas ophiolite (Cuba) are the only known example of this type of mineralization in the Caribbean realm. UM-VMS from Havana-Matanzas are enriched in Cu, Ni, Co, Au, and Ag. The mineralization consists of massive sulfide bodies mostly composed of pyrrhotite and hosted by serpentinized upper mantle peridotites. Chemical composition of unaltered cores in Cr-spinel grains found within the massive sulfide mineralization and in the peridotite host indicates formation in the fore-arc region of the Greater Antilles volcanic arc. A first stage of serpentinization probably took place prior to the sulfide mineralization event. The UM-VMS mineralization formed by the near-complete replacement of the silicate assemblage of partially serpentinized peridotites underneath the seafloor. The sequence of sulfide mineralization has been divided into two stages. The first stage is characterized by a very reduced hydrothermal mineral assemblage consisting of pyrrhotite, Co–Ni–Fe diarsenides, chalcopyrite, Co-rich pentlandite, and electrum. In the second stage, pyrite and Co–Ni–Fe sulfarsenides partially replaced pyrrhotite and diarsenides, respectively, under a more oxidizing regime during the advanced stages of ongoing serpentinization. The proposed conceptual genetic model presented here can be useful for future exploration targeting this type of deposit in the Caribbean region and elsewhere.

Genesis of the Dingjiashan and Fengyan Zn-Pb polymetallic deposits in central Fujian, SE China: Evidence from magnetite geochemistry

Central Fujian is an important polymetallic (Zn-Pb-Ag-Cu-Mo-Au) mineral province in southeastern (SE) China, yet its mineralization is still disputed between Proterozoic volcanogenic massive sulfide (VMS) or Mesozoic skarn-type. The Dingjiashan and Fengyan are two representative of them, with the ores mainly hosted in the Meso-Neoproterozoic Longbeixi Formation marble and schists. For the magnetite from Dingjiashan and Fengyan, their hydrothermal mineral assemblage and their low V, Al, Cr, Ni, Ga, and HFSEs contents (cf. igneous magnetite) are all typical of hydrothermal origin. LA-ICP-MS time-resolved depth profile and inter-element correlation suggest that: 1) Mn, Ti, V, Ni, and Cr are mainly present in solid solution; 2) Na, Mg, Al, K, Si, and Ca occur both as solid solution and the micro-silicate inclusions and/or halite-bearing fluid inclusions; and 3) Zn, Co, Cu, Ag, Pb, and Bi are intimately associated with sulfide micro-inclusions. For both Dingjiashan and Fengyan magnetite, the Ti, V, Al, Ga, and Mn contents are temperature-controlled, whilst Ti, V, Cr, and Sn contents are redox-controlled. Meanwhile, the Ca and Mg contents are controlled by fluid-rock interactions, and the Mn enrichment is inherited from the fluid composition. Coexisting mineral phases have likely exerted major geochemical control on the magnetite samples. We compiled a large published geochemical dataset on magnetite from well-studied VMS and skarn deposits worldwide. Compared with VMS-hosted magnetite, skarn-hosted magnetite has relatively high median contents for Ca, Al, Ge, Sn, Mn, and Mg, and low median contents for Si, Ga, Ti, V, Ni, and Cr. Overall, the Dingjiashan and Fengyan magnetite samples have similar trace element compositions (esp. Mn and V) to those of skarn-hosted (rather than VMS-hosted) magnetite. Magnetite geochemistry thus suggests that both the Dingjiashan and Fengyan are best classified as stratabound, distal Zn-Pb skarn deposits. Our findings highlight the effectiveness of magnetite trace element geochemistry in differentiating ore deposit types, which contributes to regional Zn-Pb ore exploration and research.

Petrological and micrometer-scale geochemical constraints on chert origins in the Dengying Formation, Yangtze Block, South China: Implications for Late Ediacaran hydrothermal activity and tectonic setting

The Ediacaran–Cambrian (E–C) transition was a key interval in geological history. Chert was widely distributed in the Yangtze Block at the end of the Ediacaran and may have recorded important information about the coevolution of earth systems in this period. Five lithofacies of chert were identified in the northern margin of the Yangtze Platform. Breccias with crustiform chert were formed by hydrothermal fracturing of dolomite to form breccias in vents, which were immediately immersed and crusted by a silicon-rich fluid. This silicon-rich fluid was deposited directly onto the inner walls of the conduits to form chert linings. The silicon-rich fluid filled the water-fractured fissures to form spider-veinlet chert and then mixed with residual seawater in primary pores of the bedding to form void-filling cherts. Finally, the dolomite far away from the conduits was replaced by the silicon-rich fluid, forming chert bands. In the vents, there are typical hydrothermal mineral assemblages of quartz, pyrite, barite, Fe–Mn metal microparticles, and saddle dolomite and these assemblages contain abnormally high-temperature inclusions. The rare-earth element (REE-Y) patterns of void-filling chert, crustiform chert, and saddle dolomite show that they originated from normal seawater influenced by a low-temperature hydrothermal fluid, silicon-rich hydrothermal fluid, and high-temperature hydrothermal fluid, respectively. The trace element ratios and Si isotope contents of chert also show that deposition was mainly caused by hydrothermal solutions. All these data indicate that the various types of cherts at the top of the Dengying Formation near the synsedimentary fault zone constitute typical siliceous sinter. There was massive upward migration of silicon-rich hydrothermal fluids from depth to the seafloor during the E–C transition, which resulted in abnormally silicon-rich seawater and a widespread distribution of various cherts in the Yangtze Block. The identification of siliceous sinter in synsedimentary faults indicates an intense extension of the northern margin or even of the entire Yangtze Block during the E–C transition. This may be related to the combined effects of slab pull on the western side and rollback on the northeastern side of the Yangtze Block during the convergence of Gondwana.

In-situ U–Pb geochronology of Ti-bearing andradite as a practical tool for linking skarn alteration and Pb–Zn mineralization: A case study of the Mengya’a deposit, tibet

Ti-bearing andradite (Ti-Grt) occurs in a variety of geological environments, such as Si-unsaturated igneous rocks, high-temperature metamorphic rocks, and contact-metasomatized skarns. This type of garnet remains stable throughout a wide range of pressure-temperature conditions. Abundant skarn Pb–Zn deposits, where the ore-related pluton is not exposed, have been discovered along the eastern Nyainqentanglha metallogenic belt, western China. However, the mineralization age of these skarn Pb–Zn deposits remains controversial. Therefore, in this study, petrography, geochemistry, and in-situ U–Pb geochronology of the Ti-Grt are investigated together with the Re–Os geochronology of the molybdenite in the large Mengya’a Pb–Zn deposit. The potential of Ti-Grt dating is explored and the age relationships between the skarn alteration and the Pb–Zn mineralization are constrained. The results show Ti-Grt samples from the Mengya’a deposit are brown-black in color with TiO2-rich (mean of 3.47 wt%) features. Laser ablation inductively coupled plasma mass spectrometry dating showed that these Ti-Grts have a U–Pb age range of 54.9–53.8 Ma, which is consistent with the ages of two adjacent molybdenite Re–Os models (53.36–52.86 Ma). This indicates that the skarn alteration is temporally associated with a Pb–Zn mineralization event during the Indian-Asian main-collisional orogeny. The positive linear Ti vs. U correlation of the Ti-Grts and other compiled data indicate that Ti-bearing garnets may have high contents of U. The incorporation of U and Ti in garnet is largely controlled by the crystal chemistry of garnet and the components of hydrothermal fluids in a nearly closed system. Thus, this study confirms the reliability and accuracy of in-situ U–Pb dating of Ti-Grt and emphasizes that Ti-bearing garnets can become an invaluable geochronological tool for magmatic, metamorphic, and hydrothermal mineral assemblages.

Application of the capillary method in micro X-ray diffractometry (µ-XRD): A useful technique for the characterization of small amounts of clay minerals

AbstractThe laboratory micro X-ray diffraction (μ-XRD) technique is a suitable method to study minerals in-situ in whole-rock specimens without any sample preparation or in polished thin sections, and even in small amounts in powdered form. The micro X-ray diffraction method uses the conventional, closed-tube X-ray generator, but modifications were needed in the diffraction column, sample holder and detector in order to achieve μ-XRD capability.In this paper, we present a case study of the capillary method used in µ-XRD on hydrothermal clay mineral assemblages that formed in the Velence Mts (Hungary). The capillary method in µ-XRD has many advantages in the investigation of small amounts of clay minerals: (1) easy and rapid preparation of randomly oriented, powdered samples; (2) rapid measurements; (3) accurate diffraction patterns. By using the capillary method, the formation of preferred orientation can be eliminated; thus the (hkl) reflection of the clay minerals can be precisely measured. Illite polytype quantification and the investigation of (060) reflection of clay minerals can be used satisfactorily in µ-XRD.Hydrothermal clay mineral assemblages are indicative of temperature and pH. Their examination can determine the physicochemical parameters of the hydrothermal fluids that interacted with the host granite in the Velence Mts. The analyzed hydrothermal clay minerals from the western part of the mountains suggest lower temperatures (150–200 °C) and intermediate pH conditions. In contrast, the clay mineral assemblages' characteristics for the eastern part of the mountains indicate more intense argillization and higher temperatures (∼220 °C) and intermediate pH conditions.

Chapter 5 Scottish mineral Geological Conservation Review sites – Hydrothermal veins and mineral assemblages

This fifth paper on Scottish mineral Geological Conservation Review (GCR) sites details those of highest conservation value for hydrothermal veins and associated mineral assemblages. The GCR sites are both large and small reflecting variations in past economic mining and extraction history. During the long mining histories, and after, the sites have yielded new mineral species and a new metal. From the largest ore field for lead, zinc and copper 80 or so minerals have been recognised, including nine new species, one of which was established as the first sulphite, scotlandite. Elsewhere gold and silver were also extracted. For some veins the country rocks were analysed to determine whether they provided the vein elements for primary and secondary minerals, and sometimes dated. Two sites exhibit polymetallic mineralisation, i.e., five elements in the mineral assemblages. In one site veins in close proximity to a sanukitoid pluton received signature elements from the intrusion. Veins in various areas formed in faults, fractures and shears which became conduits for ground waters reacting with vein minerals. Oxygen in the water reacted with lead, zinc and copper minerals and in the presence of CO2 and SO4 supergene mineral assemblages formed in appropriate alkalinities. It is therefore possible to establish paragenetic sequences and calculate stability fields for specific assemblages. Alkalinities and formation temperatures may also be obtained through fluid inclusions studies and isotope analysis.

Petrogenesis of Zr–Nb (REE) carbonatites from the Arbarastakh complex (Aldan Shield, Russia): Mineralogy and inclusion data

The Arbarastakh Neoproterozoic ultramafic carbonatite complex is located in the southwestern part of the Siberian Craton (Aldan Shield) and contains ore-bearing Zr–Nb (REE) carbonatites and phoscorites. Carbonatites are mainly represented by calcite and silicocarbonatite varieties. The primary minerals composing the carbonatites are calcite and dolomite, as well as phlogopite, clinopyroxene, fluorapatite, amphibole, fluorite, K-feldspar and feldspathoids. Olivine (forsterite), Ti-magnetite, apatite, phlogopite, calcite, dolomite and the minor spinel group minerals form the primary phoscorites. The ore-bearing Zr–Nb mineral assemblages of the phoscorites and carbonatites include accessory zircon, zirconolite, perovskite, pyrochlore and baddeleyite. The Ba–Sr–REE hydrothermal mineralisation consists of ancylite-(Ce), bastnaesite-(Ce) and burbankite, as well as barite-celestite, strontianite, barytocalcite, and rare Cu–Fe sulphides. The silicocarbonatites and carbonatites formed in multiple stages from a single alkaline Ca–Na–K–silicocarbonatite melt, while the phoscorites are products of differentiation of the carbonatitic melt and were crystallised from an Fe-rich phosphate–carbonate melt at temperatures of more than 720 °C. The silicate–phosphate–carbonate melts were responsible for the Zr–Nb mineralisation of the carbonatites at temperatures of more than 540–575 °C; the hydrothermal REE-bearing mineral assemblages crystallised from saline (60–70 wt%) carbonatitic fluids of Na–Ca–Mg–F–carbonate composition at a minimum temperature range of 350–300 °C. The Ca–Sr-carbonate as well as the Na–hydro–carbonate fluids were responsible for the Ba–Sr–REE mineralisation of the phoscorites at ~500–480 and 450–430 °C.

Microbial Sulfur Isotope Fractionation in the Chicxulub Hydrothermal System.

Target lithologies and post-impact hydrothermal mineral assemblages in a new 1.3 km deep core from the peak ring of the Chicxulub impact crater indicate sulfate reduction was a potential energy source for a microbial ecosystem (Kring et al., 2020). That sulfate was metabolized is confirmed here by microscopic pyrite framboids with δ34S values of -5 to -35 ‰ and ΔSsulfate-sulfide values between pyrite and source sulfate of 25 to 54 ‰, which are indicative of biologic fractionation rather than inorganic fractionation processes. These data indicate the Chicxulub impact crater and its hydrothermal system hosted a subsurface microbial community in porous permeable niches within the crater's peak ring.

Hydrothermal mineral assemblages of calcite and dolomite\u2013analcime\u2013pyrite in Permian lacustrine Lucaogou mudstones, eastern Junggar Basin, Northwest China

The eastern Junggar Basin, controlled by continental extension (rift), was deposited by lacustrine dominated sediments during middle Permian Lucaogou period. An unusual porphyritic-like texture was observed in Lucaogou/Pingdiquan dark fine-grained organic-rich sediments in two sub-tectonic units in the basin. The “phenocrysts” are composed of two types of mineral assemblages. The first is a coarse euhedral calcite assemblage in the Jimusar Sag, and the second consists of dolomite, analcime, and pyrite in the Shishugou Sag. The lithological and mineralogical features indicate a hydrothermal origin for these phenocryst-like minerals. The chondrite-normalized rare earth element patterns show flat or positive Ce anomalies and negative Eu anomalies, which reflect a suboxic to anoxic, off-axis site from the center of the fault system, where the temperature of the hydrothermal fluid might be less than 250 °C. The high ratios of BaN/LaN at 1.6–65.5, strongly positive Sr anomalies at Sr/Sr* = 5.54–39.9, and relatively low 87Sr/86Sr isotopes at 0.705002–0.705776 in the coarse calcite suggest an origin of mixed sources of lake water, underlying biogenetic sediments, and deep magmatic water. However, the low 87Sr/ 86Sr ratios of 0.705321–0.705968 in the dolomite and δ34SV-CDT of 10.8‰–12.3‰ in the pyrite indicate that water–underlying-rock interaction and the abiotic thermochemical sulfate reduction of lake water or organic matter might have participated together resulting in the precipitation of the dolomite–analcime–pyrite assemblages in the Shishugou Sag.

Spectral Alteration Zonation Based on Close Range HySpex-320 m Imaging Spectroscopy: A Case Study in the Gongchangling High-Grade Iron Ore Deposit, Liaoning Province, NE China

Research on wall rock alteration is of great importance to the understanding and exploration of ore deposits. The microscopic changes of the same mineral in different alteration zones can provide information about the migration and enrichment of ore-forming elements. In this paper, a typical profile of a high-grade iron ore body in Gongchangling iron deposit was investigated and sampled. The samples were checked by polarized microscopy, and alterations zonation were delineated according to the hydrothermal mineral assemblages and paragenesis. Moreover, hyperspectral images of wall rocks from each alteration zone were obtained by Norsk Elektro Optikk (NEO) HySpex-320 m imaging system. A customer Interactive Data Language (IDL) software package was used to process the images, and spectral features were extracted from the selected samples. The results indicate that spectral characteristics manifest obviously regular variations; i.e., from proximal to distal for the high-grade iron ore body, the wavelengths at ca. 1200 nm of chlorite and garnet, which accounts for most of the hydrothermal alteration minerals, become longer, and the absorption depths gradually smaller. The spectral features at 1200 nm of chlorite and garnet are always caused by the crystal field effect of Fe2+; therefore, the wavelength variations indicate the increase of Fe2+ and a reduced environment, which can provide more detailed information about the metallogeny and water–rock interaction. Since the hyperspectral features of the altered rocks can disclose unique mineralogical and structural information, the conventional classification of alteration zonation should be combined with the spectral feature, i.e., spectral alteration zonation, which is of great help to the understanding of the forming conditions of wall rock alteration and also the high-grade iron ore bodies.

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.

Hydrothermal Mineral Assemblages at 71° N of the Mid-Atlantic Ridge (First Results)

The article discusses the preliminary results of plume and bottom sediment studies of the Trollveggen hydrothermal vent field based on data from cruise 68 of the R/V Akademik Mstislav Keldysh. The hydrothermal vent field is located east of the axial zone of the slow-spreading Mohn Ridge near the Jan Mayen hotspot at a depth of about 550 m (71°18′ N, Norwegian–Greenland Basin). The hydrothermal vent field plume was characterized by a weak distribution in the water column; temperature, density, and salinity anomalies; a moderate methane concentration; and a low concentration of suspended particulate matter near the bottom. The enrichment of bottom sediments in barium, strontium, and some sulfide-forming elements (zinc, lead, copper, and molybdenum) was shown. Two mineral assemblages of hydrothermally modified bottom sediments were revealed: pyrite and barite–marcasite. The temperature of hydrothermal fluids was established by thermal and cryometric studies of gas–liquid fluid inclusions in barite (128–260°С); the FeS–ZnS equilibrium diagram of sulfide minerals was also used (130–290°С). Our data were close to direct fluid temperature measurement data [28]. We compared the hydrothermal mineralization of the Trollveggen vent field and earlier studied fields of the Mid-Atlantic Ridge located near the Azores hotspot. As a result, we confirmed the influence of ocean depth and PT conditions on the formation of hydrothermal deposits.

Hydrothermal Alteration Associated with Vein-Type Sulphide Mineralization at Lappadata Prospect, South Sulawesi, Indonesia: A Preliminary Study

In Lappadata area, Bone Regency, South Sulawesi Province, Indonesia, indication of vein-type sulphide mineralization is occurred. This paper describes a recent study of the prospect which focused on hydrothermal alteration associated with the mineralization. In this study, alteration and mineralization samples have been collected selectively and representatively from the field and then analyzed in laboratory using petrography, ore microscopy, and X-ray diffraction methods. The study resulted that host rocks of the mineralization are porphyritic basalt and limestone members of the Early Eocene to Late Oligocene Salo Kalupang Formation. Hydrothermal alteration mineral assemblages include quartz, chlorite, muscovite, sericite, calcite, actinolite, epidote, illite, kaolinite, and pyrite, which generally zoned from distal-propylitic mainly characterized by chlorite, calcite and epidote to proximal-argillic characterized by sericite and illite. The mineralization occurred in quartz vein and disseminated with identified hypogene ores of sphalerite, chalcopyrite, galena, and pyrite as well as supergene ores of covellite, chalcocite, and malachite. Study of the hydrothermal alteration stability temperatures suggested that the mineralization was formed in temperature range of about 200 to 300°C, from a near-neutral pH hydrothermal fluid. The whole characteristics of hydrothermal alteration, ore and gangue minerals, mineralization types, as well as range of formation temperature and composition of the responsible hydrothermal fluid indicated that the mineralization in Lappadata prospect is an epithermal type.

Landsat-8, Advanced Spaceborne Thermal Emission and Reflection Radiometer, and WorldView-3 Multispectral Satellite Imagery for Prospecting Copper-Gold Mineralization in the Northeastern Inglefield Mobile Belt (IMB), Northwest Greenland

Several regions in the High Arctic still lingered poorly explored for a variety of mineralization types because of harsh climate environments and remoteness. Inglefield Land is an ice-free region in northwest Greenland that contains copper-gold mineralization associated with hydrothermal alteration mineral assemblages. In this study, Landsat-8, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), and WorldView-3 multispectral remote sensing data were used for hydrothermal alteration mapping and mineral prospecting in the Inglefield Land at regional, local, and district scales. Directed principal components analysis (DPCA) technique was applied to map iron oxide/hydroxide, Al/Fe-OH, Mg-Fe-OH minerals, silicification (Si-OH), and SiO2 mineral groups using specialized band ratios of the multispectral datasets. For extracting reference spectra directly from the Landsat-8, ASTER, and WorldView-3 (WV-3) images to generate fraction images of end-member minerals, the automated spectral hourglass (ASH) approach was implemented. Linear spectral unmixing (LSU) algorithm was thereafter used to produce a mineral map of fractional images. Furthermore, adaptive coherence estimator (ACE) algorithm was applied to visible and near-infrared and shortwave infrared (VINR + SWIR) bands of ASTER using laboratory reflectance spectra extracted from the USGS spectral library for verifying the presence of mineral spectral signatures. Results indicate that the boundaries between the Franklinian sedimentary successions and the Etah metamorphic and meta-igneous complex, the orthogneiss in the northeastern part of the Cu-Au mineralization belt adjacent to Dallas Bugt, and the southern part of the Cu-Au mineralization belt nearby Marshall Bugt show high content of iron oxides/hydroxides and Si-OH/SiO2 mineral groups, which warrant high potential for Cu-Au prospecting. A high spatial distribution of hematite/jarosite, chalcedony/opal, and chlorite/epidote/biotite were identified with the documented Cu-Au occurrences in central and southwestern sectors of the Cu-Au mineralization belt. The calculation of confusion matrix and Kappa Coefficient proved appropriate overall accuracy and good rate of agreement for alteration mineral mapping. This investigation accomplished the application of multispectral/multi-sensor satellite imagery as a valuable and economical tool for reconnaissance stages of systematic mineral exploration projects in remote and inaccessible metallogenic provinces around the world, particularly in the High Arctic regions.

Geochronology and geochemistry of the Dianfang gold deposit, western Henan Province, central China: Implications for mineral exploration

The Dianfang gold deposit, located in the Xiong’ershan–Waifangshan region within the southern margin of the North China Craton, consists of eastern and western ore blocks, which are hosted in the Dianfang breccia pipe and Paleoproterozoic volcanic rocks, respectively. The eastern ore block, which occurs within the southern and hanging wall margins of the Dianfang breccia pipe and in fractures in the rhyolites of the Jidanping Formation, consists of rhyolite-hosted and breccia-hosted disseminated gold mineralization structurally controlled by ENE-striking faults. The western ore block contains gold-bearing quartz-sulfide veins and minor disseminated rhyolite-hosted gold mineralization that occurs in the annular fractures around the Hougou breccia pipe. The two ore blocks have long been researched and prospected as two different genetic types. Detailed fieldwork and microscopic observations reveal that the eastern and western ore blocks contain the same hydrothermal alteration and mineral assemblages, which include K-feldspar, quartz, sericite, chlorite, epidote, carbonate, pyrite, galena, sphalerite, marcasite and chalcopyrite, as well as minor magnetite, anatase, cervelleite, matildite, and bismuthinite. Gold occurs as electrum inclusions in pyrite or along microfractures in sulfides and quartz. Zircon U–Pb geochronology of the granite porphyry around the Hougou breccia pipe constrains the timing of breccia pipe formation to 145.9 Ma which is consistent with the formation age of the Dianfang breccia pipe (145.8 Ma), indicating that the two breccia pipes formed coevally at the end of the late Jurassic, rather than during the Paleoproterozoic, as was suggested by previous studies. The hydrothermal sulfides from the main orebody yield a Rb–Sr isochron age of 119.5 ± 1.8 Ma, suggesting that the eastern and western ore blocks formed coevally at the end of the early Cretaceous. In situ mineral trace element data, as well as sulfur and lead isotope data suggest that the eastern and western ore blocks were formed by a common ore-forming fluid that evolved from a magmatic-hydrothermal system that was genetically related to a hidden late Mesozoic granite. Pre-existing fractures and breccia then served as ideal conduits for fluid flow, accumulation, and gold deposition. Consequently, we propose that the ENE-striking faults developed in and around the breccia pipes and the annular brittle faults around the breccia pipes are good targets for future gold exploration at the Dianfang gold deposit. Based on these results and data from other gold deposits of similar ages around the margins of the North China Craton, we argue that the gold deposits in Xiong’ershan region formed under conditions of lithospheric extension from the late Jurassic to the early Cretaceous, which resulted from lithospheric thinning due to the westward subduction of the Pacific Plate.

Geology and Genesis of Xianglushan Fe-Cu Orefield in Sichuan (SW China): Evidence from C-O-S-Pb Isotopes

The Proterozoic Xianglushan Fe-Cu orefield (western Sichuan) is located in the Huili–Dongchuan ore belt on the southwestern margin of the Yangtze Block in SW China. The orefield has experienced complex magmatotectonic activities, and hosts a wide variety of Fe oxide-(Cu-Au) deposits. At Xianglushan, the orebodies are made of stratabound magnetite–hematite orebodies superimposed by vein-type chalcopyrite mineralization. The stratabound Fe orebodies are hosted mainly in the Proterozoic volcanic-sedimentary rocks of the lower Yinmin Formation, whilst the Cu vein or stockwork ores were mainly emplaced into the upper part of the footwall volcanic rocks and the lower part of the Fe orebodies. We divided the alteration/mineralization and their fluid inclusions (FIs) into the (I) sodic-calcic alteration, (II) potassic-silicic-sericite alteration and Fe-(Cu) mineralization, and (III) carbonate alteration and Cu mineralization stages. Stage II FIs are mainly two-phase (vapor-liquid), and are featured by medium temperature (348 to 379 °C) and high salinity (21.8 to 22.9 wt % NaCl eqv). Their generally negative calcite δ13C (−4.1‰ to −3.1‰) and δ18OH2O (12.2‰ to 15.3‰) values reveal that the Stage II ore fluids have had a seawater-magmatic fluid mixed source. Late Stage II pyrite has δ34S (−3.3‰ to 13.7‰), 206Pb/204Pb (17.663 to 18.982), 207Pb/204Pb (15.498 to 15.824) and 208Pb/204Pb (37.784 to 38.985), suggesting that the ore-forming materials were derived from dominantly upper crustal source. Stage III FIs are also mainly two-phase (vapor-liquid), and are featured by lower temperature (206 to 267 °C) and salinity (19.0 to 22.5 wt % NaCl eqv) than their Stage II counterparts. The Stage III ore fluids were also likely derived from a meteoric–magmatic mixed source with greater magmatic influence, as indicated by the generally negative calcite δ13C (−6.9‰ to −4.6‰) and δ18OH2O (6.3‰ to 9.2‰) values. Similar to the late Stage II pyrite, the Stage III chalcopyrite δ34S (−4.6‰ to 5.2‰) and Pb isotopes (206Pb/204Pb = 18.198 to 18.987; 207Pb/204Pb = 15.534 to 15.876; and 208Pb/204Pb = 37.685 to 39.476) also suggest a crustal ore-forming material source. Therefore, we suggest that the Fe-(Cu) and Cu mineralization at Xianglushan had similar ore fluid and material sources, although the magmatic influence increased in the later stage. This resembles many Fe-(Cu) deposits in the Huili–Dongchuan ore belt. We conclude that the Xianglushan Fe-Cu deposits are both similar to and different from typical iron-oxide copper gold (IOCG) deposits in terms of alteration styles and hydrothermal mineral assemblages, and are thus best classified as IOCG-like deposits hosted in submarine volcanic-sedimentary rocks. Considering the Mesoproterozoic regional tectonics in the southwestern Yangtze Block, we propose that the Xianglushan ore formation occurred in an intra-/back-arc inversion setting, possibly related to the closure of the Anning ocean basin.