Pyrite (FeS2) is a common host for gold in various hydrothermal gold deposits, but the close relationship between siderite (FeCO3) and gold is rarely reported. Furthermore, the gold precipitation mechanism is also poorly understood. In the Jiudian gold deposit of the world-class Jiaodong gold province (eastern China), two gold ore stages were identified: pyrite-smoky grey quartz (the first ore stage), quartz-siderite-polymetallic sulfide (the second ore stage). Gold occurs as visible electrum with minor native gold in both ore stages. The latter stage ores have higher gold grade, indicating gold precipitation associated with second-stage siderite is more efficient than that associated with first-stage pyrite. An integration of mineral texture and composition and fluid inclusion evidence demonstrates that gold deposition in the first ore stage was formed by fluid immiscibility, which destabilized the Au (HS)2− complex in the ore-forming fluid. Fluid immiscibility (boiling) also occurred in the second ore stage, resulted from the abrupt drop of pressure and temperature due to the fracturing of the ore-controlling faults during seismic activity and mixing with cold oxidized meteoric water, and then the polymetallic sulfide precipitation was triggered. The presence of CH4 in the hydrothermal fluid of the second ore stage may have also promoted the fluid unmixing. Subsequently, the metasomatic reactions of pre-existing pyrite into marcasite and siderite occurred successively, and further facilitated with fluid immiscibility mutually by building a positive chain circuit. These complex processes may have formed a highly drastic and disequilibrated fluid system, in which the crystallized pyrite is featured by the decoupling of As and Au, which is different from the common early-ore Au-As association. The abnormal hydrothermal physicochemical perturbance may have triggered high-efficient gold precipitation associated with siderite.
The recently discovered Xiaokelehe porphyry Cu-Mo deposit (PCD) is situated in the northern part of the Great Xing'an Range in NE China. At Xiaokelehe, chlorite-illite alteration is characterized by chlorite, illite, disseminated chalcopyrite and pyrite assemblages that are widely developed in the granodiorite porphyry. The illite is altered from plagioclase or K-feldspar, and the chlorite from both igneous and hydrothermal biotite. The chlorite can be divided into three types (Chl1, Chl2 and Chl3): Chl1 is locally developed in the deeper part of the granodiorite porphyry, and is altered from hydrothermal biotite; Chl2 is pervasively distributed in the deeper part of the granodiorite porphyry, and is altered from igneous biotite and associated with abundant disseminated Cu-Fe-sulfide mineralization; Chl3 is widely developed in the shallower part of the granodiorite porphyry, and is altered from igneous biotite. Chl3 is overprinted by phyllic alteration and associated with minor disseminated Cu-Fe-sulfide mineralization. These Xiaokelehe chlorites resemble the chlorites from typical sericite-chlorite-calcic alteration (SCC) in PCDs, and are the first reported SCC-related chlorites in the Great Xing'an Range. Mineral geochemical results show that Chl1 has relatively high FeO, Mn, Ti, Li and Zn contents. Chl2 has relatively high FeO, Mn, Co, Ni, Zn and Sr contents, and Chl3 has relatively high MgO, K and Ni contents. These geochemical differences are likely controlled by temperature (e.g., for Ti), precursor minerals (e.g., for FeO and MgO) and fluid compositions (e.g., for FeO, MgO, Mn, Co and Zn). All Chl1 to Chl3 are classified as trioctahedral chlorite. The Fe-Mg, Tschermark and di-trioctahedral substitution mechanisms may have controlled the chemical differences of the Xiaokelehe chlorites. Empirical thermometer yielded chlorite formation temperatures of 209–314 °C, indicating that the Xiaokelehe chlorites were formed from intermediate-high temperature hydrothermal fluids. Compared with the chlorites from propylitic zones in other PCDs, the Xiaokelehe chlorites have higher FeO, Ti, V, Zn and Ga contents and Fe/(Fe + Mg) ratio, as well as lower MgO, K, Co and Sr contents, which distinguish chlorites in the SCC zone from those in the propylitic zone. Such differences may have also been influenced by temperature, and the protolith and fluid compositions. Compared with typical metamorphic chlorites, the Xiaokelehe chlorites have lower FeO content but higher MgO and Zn contents, of which the Zn difference is likely influenced by the fluid chemistry. We propose that the presence of Chl3 can be used as an effective vector toward mineralized zone. In addition, chlorite Mn and Zn contents in the mineralized zone are clearly higher than those away from it, and thus the chlorite Mn and Zn contents can also be potential exploration vectors at Xiaokelehe.
Widespread regolith cover makes it difficult to identify the anomalous size of their mineralogical or geochemical footprint at the surface in gold exploration. Short wavelength infrared (SWIR) spectroscopy with effective identification of clays and other hydrous minerals shows potential application in gold exploration. The detailed geological and short wavelength infrared (SWIR) spectroscopy mapping was conducted in the newly-discovered orogenic-type Mailong gold deposit (13.06 t @ 6.29 g/t Au), East Kunlun, to determine alteration stage and mineral assemblages and discuss the potential values of SWIR spectral in gold exploration. In the Mailong area, magmatic rocks consist of granodiorite, biotite granodiorite, plagiogranite and mafic dikes, and the gold mineralization shows a close spatial and temporal relationship with mafic dikes. The earliest chloritization (chlorite-quartz) is widely distributed in granodiorite, biotite granodiorite and plagiogranite. The following potassic alteration (hydrothermal K-feldspar and quartz) is developed along high-angle fault/fracture, which cut through granodiorite, biotite granodiorite and plagiogranite. The subsequent sulfide alteration (chlorite-muscovite-sulfide-carbonate) overprints the former potassic and chloritization alteration, and shows spatial association with mafic dikes. Native gold occurs in fracture arsenopyrite or associated with minerals such as pyrite, and chalcopyrite during the early sulfide stage. Supergene alteration is characterized by limonite and malachite. At Mailong, the SWIR data indicate that the most abundant alteration minerals (chlorite, white micas and ankerite) mainly occurred in the northern area. White mica Al-OH absorption occurs at longer wavelengths (Pos2200 = 2202.5 ∼ 2203 nm) and higher Illite crystallinity (IC = 1.2 ∼ 3.1) probably reflecting a higher temperature and pH conduit at the bottom of ZK0001 and ZK0801. The sharp shift of IC values near the bottom reflects the control of fault. This area is also situated at the V-shaped intersections of the three granitic intrusions and structural transection, indicating more of a possible hydrothermal fluid channel than a mineralization center. Low-grade Au mineralization (Au = 0.1 ∼ 0.5 g/t) is mainly distributed within hydrothermal channels with higher Pos2200 (mainly 2022.5 ∼ 2203 nm) and IC values (mainly 1.2 ∼ 3.1), while high-grade Au endowment associated with mafic dikes has low Pos2200 (mainly 2201 ∼ 2202.5 nm) and IC values (mainly 0.43 ∼ 1.2). Meanwhile, an equation of distance = -15*IC + 31 (R2 = 0.47) can be summarized for low-grade orebodies vectoring. Integrating geological mapping and spectral data, target areas for high-grade ore bodies can be outlined using the criteria of white mica, shorter wavelength of Al-OH absorption position (Pos2200 = 2201 ∼ 2202.5 nm), lower IC values (IC = 0.43 ∼ 1.2), and chlorite Fe-OH absorption position Pos2250 (>2247 nm) along with occurrences of mafic dikes. This study demonstrates that utilizing the detailed SWIR identification in conjunction with accurate geological mapping has great potential for greenfield exploration of orogenic gold deposits.
Abstract Ion-adsorption rare earth element (REE) deposits in South China are currently the main source of heavy rare earth elements (HREE). The Gucheng deposit in western Guangdong Province is one example of HREE mineralization hosted in weathered coarse-grained biotite granites (CGBG). Titanite is a common accessory mineral in the CGBG and contains significant amounts of total REE (31 621 to 38 431 ppm), especially HREE (18 906 to 22 249 ppm). Titanite with a U-Pb age of 102.6 ± 1.9 Ma in the CGBG crystallized under relatively high temperatures (722–798 °C), high fH2O, and high fO2 conditions in the late magmatic stage, and has similar Nd isotopic compositions similar to the host CGBG: 143Nd/144Nd = 0.512062 to 0.512125 and εNd(t) = –7.4 to –8.6. Backscattered electron (BSE) imaging and TESCAN integrated mineral analyzer (TIMA) measurements show that titanite in the CGBG has been altered partly to fergusonite-(Y), rutile, calcite, quartz, and fluorite. The hydrothermal fluid responsible for titanite alteration was enriched in CO32− and F, and was probably exsolved from the granitic magma. HREE released from the alteration of titanite were mostly scavenged by fergusonite-(Y) and rutile, which have been further replaced by gadolinite-(Y) and synchysite-(Ce). In addition, gadolinite-(Y) in the alteration assemblages exhibits further alteration and is characterized by elevated PO43− and SO42− contents in the altered parts. These results demonstrate that magmatic titanite in the CGBG underwent complex hydrothermal alteration, with a preferential accumulation of HREE in fergusonite-(Y) and gadolinite-(Y) in the alteration assemblages. Preferential HREE enrichments in magmatic titanite, and its alteration assemblages, are shown to play significant roles in the formation of the Gucheng HREE deposit.
Abstract Several groups of bacteria have complex life cycles involving cellular differentiation and multicellular structures. For example, actinobacteria of the genus Streptomyces form multicellular vegetative hyphae, aerial hyphae, and spores. However, similar life cycles have not yet been described for archaea. Here, we show that several haloarchaea of the family Halobacteriaceae display a life cycle resembling that of Streptomyces bacteria. Strain YIM 93972 (isolated from a salt marsh) undergoes cellular differentiation into mycelia and spores. Other closely related strains are also able to form mycelia, and comparative genomic analyses point to gene signatures (apparent gain or loss of certain genes) that are shared by members of this clade within the Halobacteriaceae . Genomic, transcriptomic and proteomic analyses of non-differentiating mutants suggest that a Cdc48-family ATPase might be involved in cellular differentiation in strain YIM 93972. Additionally, a gene encoding a putative oligopeptide transporter from YIM 93972 can restore the ability to form hyphae in a Streptomyces coelicolor mutant that carries a deletion in a homologous gene cluster ( bldKA-bldKE ), suggesting functional equivalence. We propose strain YIM 93972 as representative of a new species in a new genus within the family Halobacteriaceae , for which the name Actinoarchaeum halophilum gen. nov., sp. nov. is herewith proposed. Our demonstration of a complex life cycle in a group of haloarchaea adds a new dimension to our understanding of the biological diversity and environmental adaptation of archaea.
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