Tiebing Liu

Composition and origin of Early Cambrian Tiantaishan phosphorite-Mn carbonate ores, Shaanxi Province, China

Abstract The Tiantaishan phosphorite–Mn carbonate ores occur in the Early Cambrian Tananpo Formation in complexly folded and faulted rocks located in southern Shaanxi Province. About 65×10 6 tonnes of 17% P 2 O 5 ore reserves exist and Mn-ore reserves are about 8.3×10 6 tonnes of +18% Mn. The stratigraphic sequence in ascending order consists of black phyllite, black to gray phosphorite ore, black phyllite, rhodochrostone ore, Mn mixed-carbonates, and dolostone. Data are presented from microprobe mineral chemistry, whole-rock chemistry, stable isotopes of carbonates, X-ray mineralogy, petrographic and SEM observations, and statistical analysis of chemical data. The dominant ore-forming minerals are hydroxy- and carbonate fluorapatite and Ca rhodochrosite, with Mg kutnahorite and dolomite comprising the Mn mixed-carbonate section. Pyrite occurs in all rock types and alabandite (MnS) occurs throughout the rhodochrostone section. The mean P 2 O 5 content of phosphorite is 31% and argillaceous phosphorite is 16%, while the mean MnO content of rhodochrostone ore is 37%. Phosphorite ores are massive, spheroidal, laminated, and banded, while rhodochrostone ores have oolitic, spheroidal, and granular fabrics. The most distinguishing characteristics of the ores are high total organic carbon (TOC) contents (mean 8.4%) in the phosphorite and high P 2 O 5 contents (mean 2.7%) in the rhodochrostone ore. The atypically high TOC contents in the Tiantaishan phosphorite probably result from very strong productivity leading to high sedimentation rates accompanied by weak reworking of sediments; poor utilization of the organic matter by bacteria; and/or partial replacement of bacterial or algal mats by the apatite. The depositional setting of the ores was the margin of an epicontinental seaway created as a direct consequence of global processes that included break-up of a supercontinent, formation of narrow seaways, creation of extensive continental shelves, overturn of stagnant, metal-rich deep-ocean waters, and marine transgression. Water depth increased from deposition of the black phyllite sequence through deposition of the Mn mixed-carbonate sequence, then shallowed again during deposition of the overlying dolostone sequence. Bottom waters were mostly dysoxic to suboxic, but fluctuated from oxic to anoxic. Productivity was high during deposition of the black phyllite sequence, increased during precipitation of phosphorite, and then decreased to moderate levels during precipitation of rhodochrostone ores. Biosilica contributions occur in each lithology, but are greatest in rhodochrostone. Changes in sedimentation were determined by changes in water depth, productivity, upwelling, sea-level change, and ventilation of the depositional basin. The source of the phosphorus was organic matter produced in great quantities during deposition of the black phyllite and phosphorite sequences in a zone of coastal upwelling. Organic matter accumulation was rapid. Globally, Mn was supplied by overturn of stagnant, metal-rich deep-ocean waters, which were redistributed to areas of coastal upwelling and seaways; that process may have been initiated by latest Proterozoic glaciations which would have promoted density stratification and accumulation and storage of metals. Regionally, Mn was supplied by terrigenous input into the shallow seaway and hydrothermal input into the deeper water parts of that seaway. Locally, Mn sources included leaching and transport of metals from the sediment column. Manganese was stored locally in low-oxygen (not anoxic) seawater prior to Mn-ore formation. The source of the carbon in the Mn carbonates and dolostones was predominantly seawater bicarbonate and secondarily CO 2 derived from the oxidation of organic matter in the bacterially mediated diagenetic zone of sulfate reduction.

Structure, Isotopes, and 40Ar/39Ar Dating of the Pengjiakuang Gold Deposit, Mesozoic Jiaolai Basin, Eastern China

The Pengjiakuang gold deposit, located on the northeastern margin of the Mesozoic Jiaolai basin, eastern Shandong Province, China, is controlled by a low-angle detachment fault. Three economic ore bodies and more than 10 mineralized bodies have been proved in recent years. The main ore bodies are made up of gold-bearing breccia, cataclastic rocks, and sulfide-quartz veins in lenses and veins. Gold ores are typically brecciated, veinlet, and disseminated. Lamprophyre dikes are spatially and temporally associated with the gold ore bodies. The isotopic analysis indicates that the δ34S range (6.5-12.89‰) of sulfides in the Pengjiakuang gold deposit is consistent approximately with that of metamorphic rocks, Mesozoic granites, and the Jiaojia and Linglong gold deposits in eastern Shandong. However, there are wide ranges of compositions of δ34S and lead isotopes in the Pengjiakuang ore except for lower 206Pb/204Pb ratios. Analyses of sulfur and lead isotopic compositions suggest that the ores have a multiple origin. Unlike the Jiaojia and Linglong deposits, the oxygen and hydrogen isotopic compositions of fluid inclusions in the Pengjiakuang deposit show a mixed source: dominant meteoric water, and subordinate magmatic water. The Ar-Ar method was adopted to date the Au-bearing quartz. Brecciated ore yields a 40Ar/39Ar age of 118.42 ± 0.25 Ma and an isochron age of about 117.03 ± 0.13 Ma. Veinlet ore yields a 40Ar/39Ar age of 118.70 ± 1.40 Ma and an isochron age of 117.33 ± 0.15 Ma. The results indicate that the Pengjiakuang gold deposit was formed at ∼117.3-118.4 Ma. Biotite from a lamprophyre dike gives an age of 117.49 ± 0.25 Ma and an isochron age of 116.83 ± 0.36 Ma. Clearly, the formation of both lamprophyre dikes and gold mineralization are coeval products of tectonic-magmatic activity. The consistency in metallogenic age between the Pengjiakuang gold deposit, located on the margin of a Mesozoic pull-apart basin, and gold deposits of the northern uplift area in eastern Shandong suggests that both can be ascribed to the same the metallogenic event.

Origin of the early Sinian Minle manganese deposit, Hunan Province, China

Abstract The Minle Mn ores occur at the base of the Early Sinian Minle Formation black shales, on the southeast limb of the Motianling anticline. These rhodochrosite ores are rich in organic carbon and fossil algae and formed in a bay or lagoon on a paleo-continental margin with adjacent island barriers. The salinity of water in the depositional basin was probably brackish. The partly restricted ocean basin was characterized by low-oxygen bottom waters and pore-water pH values ranging from 9 to 11 and neutral to negative Eh values, which produced alkaline and reducing conditions suitable for precipitation of MnCO 3 . Growth of blue algae played a role in concentrating metals, especially Mn, changing the sedimentary environment (photosynthesis prior to deposition and degradation of organic matter during diagenesis), and enhancing the deposition of MnCO 3 during diagenesis. The Mn was mainly derived from the continent, with some portion supplied from submarine volcanic activity and supracrustal deep-seated hydrothermal fluids.

Characteristics of the Devonian Xialei manganese deposit, Guangxi Zhuang Autonomous Region, China

Abstract The Xialei Mn deposit occurs in the Late Devonian (Famenian) Wuzhishan Formation and consists of three ore horizons composed of three ore types: (1) Mn carbonate, (2) Mn silicate–Mn carbonate, and (3) Mn-oxide. Mn silicate–Mn carbonate ore beds consist of Mn carbonates and associated rhodonite, stilpnomelane, actinolite, chlorite, Mn epidote, biotite, and Mn–Fe antigorite. These minerals are mixed with each other and form thin beds, bands, pelletal and pisolitic grains, and horizontal or wavy laminae. Abundant oolitic and pisolitic grains in Mn ore beds show distinct boundaries and nuclei of Mn carbonate and Mn silicate minerals, rarely clastic grains. The ore deposit is divided into three zones: (1) an inner zone composed of Mn carbonate, silicate, and oxide minerals; (2) a transitional zone consisting of Mn carbonate and stilpnomelane; and (3) an outer zone characterized by Mn carbonate. Argillaceous limestone at the base of the ore beds has a mean δ 13 C PDB of about 0.0‰, indicating that the carbon was derived from seawater bicarbonate. The δ 13 C PDB of Mn carbonate ranges from −2.8 to −14.3‰ (mean −7.1‰), indicating that the carbon was derived from both degradation of organic matter and seawater bicarbonate. Ores formed in a trough on a carbonate platform. Primary sedimentary Mn carbonates precipitated from alkaline and negative Eh waters in the diagenetic zone of sulfate reduction, which occurred either during shallow burial or at the seafloor. Oolites and pisolites formed by sedimentary processes on the seafloor. In places, deposits were reworded and transported by gravity-flow processes. The region where these sedimentary-diagenetic Mn deposits formed was intruded by magma, and hydrothermal fluids with dissolved metals leached from volcanic and sedimentary rocks ascended along fractures. The Mn silicate–Mn carbonate ore may have been produced by contact metamorphism of primary carbonate ore and metal sulfides deposited by hydrothermal fluids, which overprinted the primary sedimentary ore.

Genesis of volcanic-hosted gold deposits in West Junggar, NW China

Volcanic rocks and a series of volanic-hosted gold deposits are well-developed in West Junggar, which comprises the Sawuer gold belt in the north and the Hatu gold belt in the centre-south. A volcanic centre existed in the two gold belts, with volcanic lithofacies and fracture systems of volcanic association controlling the formation and distribution of orebodies. Isotopic studies including D, O and S reveal that the ore-forming fluids of three deposits derived mainly from magmatic water, partially from meteoric water; and sulfur derived from mantle beneath the island arc and back-arc. 40Ar-39Ar dating constrains the metallogenic time of the deposits which is close to the age of the hosting rocks. These formed via volcanic activity in the Sawur and Hatu gold belts. Based on these results, with combined consideration of the tectonic setting and geological features, it is possible to propose that the genesis of the gold deposits in west Junggar is volcanogenic late-stage hydrothermal.

Sulfur, helium and argon isotopic features of gold deposits in the Sawuershan region, Xinjiang, NW China

Two gold deposits (Kuoerzhenkuola and Buerkesidai) have been discovered, explored and mined in the Sawuershan district, Xinjiang, NW China. Gold mineralization at Kuoerzhenkuola deposit occurs within Carboniferous andesite and volcanic breccias. Gold mineralization at Buerkesidai deposit occurs within Carboniferous siltstones. δ34S‰ values of pyrite separates of ores and altered andesite from Kuoerzhenkuola deposit variy from 0.25‰ to 1.3‰ and 1.5‰ to 2.44‰, respectively. δ34S‰ values of pyrite separates of ores and altered albite porphyry from Buerkesidai deposit vary from 0.44‰ to 0.84‰ and 1.1‰ to 2.81‰, respectively. He isotope data from fluid inclusions in pyrites formed during mineralization stage of Kuoerzhenkuola and Buerkesidai gold deposits show that the 40Ar/36Ar and 3He/4He ratios of fluid inclusions are respectively in the range of 282–525 and 0.64–9.48 R/Ra, suggesting that mantle fluids may have played an important role in the ore-forming processes. On the basis of S, He and Ar isotope data, we conclude that the ore-forming materials may be derived from the mantle and the ore-forming fluid may be a mixture of mantle-derived fluid with the meteoric water.

Rare-earth element and noble gas studies of Kuoerzhenkuola gold field, Xinjiang, China: A mantle connection for mineralization

The Kuoerzhenkuola gold field is the most important one in the Sawuer gold belt, in north Xinjiang. Rare-earth element studies reveal that the ore-forming fluids of two deposits were derived mainly from the mantle, with a minor component originating from another source. Helium isotope studies of fluid inclusions in pyrites indicate 3He/4He ratios of 0.64 R a to 4.25 R a for the Kuoerzhenkuola and 1.16 R a to 5.08 R a with two exceptions of 8.18 R a and 9.48 R a for the Buerkesidai. The data suggest that the helium of the ore-forming fluids is mainly a mantle helium and partially participation of meteoric water in the mineralization process, with the combined consideration of 40Ar/36Ar ratios. Based on these, we propose that the genesis of the two gold deposits is identical, being volcanogenic late-stage hydrothermal.

Application of the EH4 image system to the detection of blind gold deposits, China

The EH4 image system (Stratagem) can detect depth ranges from meters, to greater than one kilometer. It has been successfully used in exploring for ore deposits in China. Buried gold mineralising belts (or gold orebodies) of the altered rock type can be defined by the resistivity value of less than 150 ©m. Experimental studies show that the EH4 image system is one of the most advanced applications, and is a flexible, portable and effective way of exploring gold ore deposits.