AbstractPalaeozoic granitoids in the Chinese Altai are important for understanding the evolution of the Central Asian Orogenic Belt (CAOB). The Xiaodonggou granitic intrusion, situated in the Chinese Altai (southern CAOB), is composed of two intrusive phases, medium-grained granite intruded by porphyritic granite. Zircon LA-ICP-MS U–Pb analyses of medium-grained granite and porphyritic granite yield ages of 409 ± 2 Ma and 400 ± 1 Ma, respectively, indicating that these formed in Early Devonian time. Medium-grained granite and porphyritic granite have similar geochemical features and Nd–Hf isotopic compositions. Arc-like geochemical characteristics (e.g. enrichment of LILEs and negative anomalies of Nb, Ta, Ti, and P) show that both phases are volcanic arc granites (VAGs). Geochemical and isotopic characteristics suggest that these magmas originated from melting older crust. Based on their near-zero or negative εNd(t) values (−1.4to 0) and positive εHf(t) values (+1.4 to +7.8), together with Nd model ages of 1.15–1.26 Ga and zircon Hf model ages of 0.90–1.30 Ga, we suggest that the Xiaodonggou granites were derived from a mixture of juvenile and old crustal components. Some other Devonian granitic intrusions were recently identified in the Chinese Altai with ages between 416 and 375 Ma. These Devonian granites have similar geochemical characteristics and petrogenesis as Xiaodonggou granites. The formation of these Devonian granites was in response to subduction processes, suggesting that Chinese Altai was an active continental margin in Early Devonian time.KEYWORDS: GeochemistrygeochronologypetrogenesisXiaodonggou granitesChinese Altaiactive continental margin AcknowledgementsWe are grateful to Professor Robert J. Stern for providing valuable comments and suggestions, which considerably improved this manuscript. We thank two anonymous reviewers for their constructive comments.Additional informationFundingThis research was jointly supported by the Ministry of Land and Resources Public Welfare Industry Special Funds for Scientific Research Project [grant number 201211073]; the National Basic Research Programme of China [No. 2012CB416803]; NSFC [No. 41372062].
ABSTRACTPalaeozoic granitoids in the Chinese Altai are important for understanding the evolution of the Central Asian Orogenic Belt (CAOB). The Xiaodonggou granitic intrusion, situated in the Chinese Altai (southern CAOB), is composed of two intrusive phases, medium-grained granite intruded by porphyritic granite. Zircon LA-ICP-MS U–Pb analyses of medium-grained granite and porphyritic granite yield ages of 409 ± 2 Ma and 400 ± 1 Ma, respectively, indicating that these formed in Early Devonian time. Medium-grained granite and porphyritic granite have similar geochemical features and Nd–Hf isotopic compositions. Arc-like geochemical characteristics (e.g. enrichment of LILEs and negative anomalies of Nb, Ta, Ti, and P) show that both phases are volcanic arc granites (VAGs). Geochemical and isotopic characteristics suggest that these magmas originated from melting older crust. Based on their near-zero or negative εNd(t) values (−1.4to 0) and positive εHf(t) values (+1.4 to +7.8), together with Nd model ages of 1.15–1.26 Ga and zircon Hf model ages of 0.90–1.30 Ga, we suggest that the Xiaodonggou granites were derived from a mixture of juvenile and old crustal components. Some other Devonian granitic intrusions were recently identified in the Chinese Altai with ages between 416 and 375 Ma. These Devonian granites have similar geochemical characteristics and petrogenesis as Xiaodonggou granites. The formation of these Devonian granites was in response to subduction processes, suggesting that Chinese Altai was an active continental margin in Early Devonian time.
Abstract The Tianshan orogenic belt hosts several world-class gold deposits and is one of the largest gold provinces on Earth. The Katbasu Au-Cu deposit in the Chinese Western Tianshan is hosted in a granite intrusion. Previous researchers have shown that the main gold ores formed much later than the ore-hosting granite. However, the formation age of Cu mineralization and its possible link to Au mineralization remain poorly understood. This paper reports detailed mineralogical studies, combined with zircon U-Pb, in situ hydrothermal monazite as well as rutile U-Pb ages to constrain the timing of Cu mineralization and its possible link to Au mineralization. The two main ore types in the Katbasu deposit include Cu-Au ores with pyrite-chalcopyrite veins that crosscut the granite and Au ores with massive pyrite and quartz as the main minerals. The Cu-Au ores are spatially associated with diorite that intruded the granite, and they are overprinted by massive gold ores. Detailed mineralogical studies show that chalcopyrite is the main Cu-bearing mineral in the Cu-Au ores, and it is closely associated with some native gold, monazite, and rutile. Secondary ion mass spectrometer (SIMS) U-Pb dating of zircon grains from the ore-hosting granite and mafic enclave yielded concordant ages of 354.1 ± 1.6 and 355.8 ± 1.7 Ma, respectively. The diorite that intruded the granite has a zircon U-Pb age of 352.0 ± 3.2 Ma. The trace element compositions of the monazite suggest they were formed by hydrothermal fluids rather than inherited from the ore-hosting granite. Hydrothermal monazite coexisting with chalcopyrite and native gold yielded a concordant age of 348.7 ± 2.3 Ma, and the W-rich hydrothermal rutile grains associated with the chalcopyrite yielded a U-Pb age of 345 ± 27 Ma, indicating an early Cu-Au mineralization event prior to the major Au mineralization (ca. 323–311 Ma). The formation time of early Cu-Au mineralization is consistent with the emplacement age of the diorite and may be of magmatic-hydrothermal origin, whereas the main Au has no genetic associations with magmatic rocks in the ore district and may belong to the orogenic type. Monazite geochronology provided a more reliable age constraint than rutile in the Katbasu Au-Cu deposit, and we suggest hydrothermal monazite has advantages over rutile in dating the complex mineralization ages of gold deposits.
The northern Xinjiang region is one of the most significant iron metallogenic provinces in China. Iron deposits are found mainly within three regions: the Altay, western Tianshan, and eastern Tianshan orogenic belts. Previous studies have elaborated on the genesis of Fe deposits in the Altay orogenic belt and western Tianshan. However, the geological characteristics and mineralization history of iron deposits in the eastern Tianshan are still poorly understood. In this paper I describe the geological characteristics of iron deposits in the eastern Tianshan, and discuss their genetic types as well as metallogenic-tectonic settings. Iron deposits are preferentially distributed in central and southern parts of the eastern Tianshan. The known iron deposits in the eastern Tianshan show characteristics of magmatic Fe–Ti–V (e.g., Weiya and Niumaoquan), sedimentary-metamorphic type (e.g., Tianhu), and iron skarn (e.g., Hongyuntan). In addition to the abovementioned iron deposits, many iron deposits in the eastern Tianshan are hosted in submarine volcanic rocks with well-developed skarn mineral assemblages. Their geological characteristics and magnetite compositions suggest that they may belong to distal skarns. SIMS zircon U–Pb analyses suggest that the Fe–Ti oxide ores from Niumaoquan and Weiya deposits were formed at 307.7 ± 1.3 Ma and 242.7 ± 1.9 Ma, respectively. Combined with available isotopic age data, the timing of Fe mineralization in the eastern Tianshan can be divided into four broad intervals: Early Ordovician–Early Silurian (476–438 Ma), Carboniferous (335–303 Ma), Early Permian (295–282 Ma), and Triassic (ca. 243 Ma). Each of these episodes corresponds to a period of subduction, post-collision, and intraplate tectonics during the Paleozoic and Mesozoic time.
Abstract Iron isotope fractionation in hydrothermal systems is a useful diagnostic tool for tracing ore-forming processes. Here, we report on the Fe isotopic compositions of a suite of hydrothermal minerals from ores (pyrite, pyrrhotite, and quartz) from the Wulong gold deposit, Liaodong Peninsula, East China. Pyrites from quartz sulfide ores show a δ56Fe (56Fe/54Fe in the sample relative to IRMM-14) range from +0.11 ± 0.03‰ to +0.78 ± 0.03‰ (2SD), and pyrrhotites from the same vein are isotopically lighter than the pyrites, varying between −0.85 ± 0.01‰ and −0.07 ± 0.00‰. This result is consistent with theoretical predictions of equilibrium fractionation and published mineral compositions. For the first time, to our knowledge, we report the Fe isotopes of hydrothermal quartz that records the isotopic compositions of the ore-forming fluids. Two quartz separates in the quartz-sulfide vein yield δ56Fe values of −0.02 ± 0.02‰ and +0.07 ± 0.07‰. Our Fe isotope fractionation calculations show that pyrrhotite with light Fe isotopes crystallized first from the ore-forming fluids, which indicates a relatively reduced condition for the initial ore-forming fluids. Then the remaining fluids with heavy Fe isotopes precipitated pyrites with positive δ56Fe values, and their mineral crystallization sequence records an increase of oxygen fugacity during mineralization. Gold deposits in Wulong and Jiaodong share many similar geological characteristics. The pyrites from the Wulong deposit have higher δ34S values (+0.5 to +4.1‰) than the pyrrhotites (–1.2 to +1.2‰). Pyrites from the Jiaodong gold deposits show a wide range of both positive and negative δ56Fe values as well as high δ34S values, whereas those from the Wulong deposit have a relatively narrow range of positive δ56Fe values and near-zero δ34S values. The differences in Fe isotopes may be due to early precipitation of pyrrhotite with light Fe isotopes under a relatively low oxygen fugacity environment in the Wulong deposit, resulting in pyrite precipitated from the remaining fluids with heavy isotopes. The sulfur isotope variations between Wulong and Jiaodong gold deposits reflect differences in their source regions rather than oxygen fugacities. In addition, we have compiled Fe isotopic compositions of pyrites and pyrrhotites from different types of ore deposits to investigate their Fe isotopic behavior in magmatic-hydrothermal systems. Pyrite grains show a wide range of δ56Fe values. Positive pyrite δ 56Fe values reflect an equilibrium isotope effect, whereas negative pyrite δ56Fe values may be due to kinetic isotope effects or to a mixture of sedimentary host rocks. Pyrrhotite grains show similar negative δ56Fe values, and they have a strong influence on the Fe isotope systematics in magmatic and hydrothermal systems. Our data show that Fe isotopes can be used to trace precipitation orders of pyrite and pyrrhotite and oxygen fugacity evolution in relatively reduced hydrothermal deposits.
Apatite is a common accessory mineral in iron oxide-apatite (IOA), magmatic Fe-Ti oxide, and metamorphosed sedimentary Fe deposits. Apatite chronology and oxygen isotopes in the IOA deposits have been extensively studied, however, such information in the metamorphosed sedimentary Fe deposits remains poorly understood. Here, we report detailed trace elements, oxygen isotopes, and U-Pb chronology of apatite from the Tianhu metamorphosed sedimentary Fe deposit in the eastern Tianshan, NW China. To our knowledge, this is the first report on the in situ oxygen isotopes of apatite from the metamorphosed sedimentary Fe deposits. Cathodoluminescence imaging of the apatite grains from the Tianhu massive Fe ores show a homogeneous texture, suggesting that they were formed by a single event rather than multiple superimposed events. These apatite grains contain abundant fluid inclusions, and show relatively high δ18O values of 7.9 ‰-14.0 ‰ and low Th and U contents, which are consistent with their metamorphic hydrothermal origins. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) dating of the studied apatite grains yielded a U-Pb age of 348.5 ± 18.5 Ma, indicating that the metamorphic enrichment of the Tianhu metamorphosed sedimentary Fe deposit occurred considerably later than the Ordovician ore-hosting rocks. Combined with previous studies, we suggest that the metamorphic massive ores of the Tianhu metamorphosed sedimentary Fe deposit formed in an orogenic setting. Our study highlights that a combination of trace elements, oxygen isotopes, and U-Pb chronology of apatite can be used as powerful tools to constrain the genesis of metamorphosed sedimentary Fe deposits.
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