Ore-forming mechanisms and spatio-temporal framework for intrusion-related deposits in NE China

Shu, Qihai (2015) Ore-forming mechanisms and spatio-temporal framework for intrusion-related deposits in NE China. PhD thesis, James Cook University.

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Abstract

Northeastern China is composed of the eastern part of the Central Asian Orogenic Belt and the northeastern margin of the North China Craton. It underwent two major sets of orogenic events, including the pre-Mesozoic amalgamation of several micro-continents and the Mesozoic subduction of the Paleo-Pacific plate. It hosts numerous ore deposits of dominantly porphyry and skarn types, most of which have Mesozoic ages. In this study, both mineralization types were studied, aiming to improve the understanding of ore genesis, hydrothermal evolution, mineralization mechanism, regional metallogeny as well as the geodynamic setting.

Systematic zircon U-Pb and/or molybdenite Re-Os dating on five porphyry deposits (i.e., Aolunhua, Haisugou, Shabutai, Banlashan, and Yangchang) in the northern Xilamulun district indicates that the timing of the magmatism and the Mo mineralization is broadly coeval, mainly at 130-140 Ma. Major and trace element geochemistry reveals the intrusions hosting Mo-only deposits (e.g., Haisugou) have stronger crystal fractionation than intrusions hosting porphyry Cu and Cu-Mo deposits (e.g., Aolunhua), indicating that such a process may have played a role in selective enrichment of Mo. A comparison of zircon Ce/Nd ratios as a proxy for the oxidation state of magmas between mineralized and barren intrusions shows that the mineralized intrusions are associated with more oxidized magmas than the cospatial barren granites, and therefore it is proposed that higher oxygen fugacity may also be important to produce economic Mo mineralization. Whole rock Sr-Nd-Pb and zircon Hf isotopes show that these mineralized granites in Xilamulun are associated with magmas generated from three different source regions (i.e., remelting of old crust material, mixing of old crust material with depleted mantle component, and juvenile mantle-derived magmas). The variation in the origin of the magmas from which the porphyry Mo systems were generated suggests that the composition of magma sources is unlikely to have played a major role in the formation of Mo deposits.

The compilation of existing geochronological data on Mo deposits in NE China, including the newly obtained data from this study, shows that Mesozoic Mo deposits (~250 to 90 Ma) widely occur in this region and are linked to three tectonic-magmatic events: (1) Triassic Mo deposits (250–220 Ma) are mainly distributed along the east-west Xilamulun fault and are related to post-collisional crustal extension following the final closure of the Paleo-Asian ocean; (2) Jurassic to Early Cretaceous Mo mineralization (200–130 Ma) displays a clear younging trend from southeast to northwest and is interpreted to be related to the northwestward flat-slab subduction of the Paleo-Pacific plate beneath the Eurasian continent that started from Early Jurassic (ca. 200 Ma); (3) Cretaceous Mo mineralization (130–90 Ma) shows a distinctly reversed migration trend from northwest to southeast, and can be explained by the coastward migration of slab rollback related lower crust delamination, asthenospheric upwelling and lithospheric thinning.

For skarns, the Baiyinnuo'er Zn-Pb deposit was selected as a representative example for detailed study in this study. It is one of the largest Zn-Pb deposits in China, with 32.74 Mt resources averaging 5.44% Zn, 2.02% Pb and 31.36 g/t Ag. Several phases of igneous rocks, including Permian, Triassic and Early Cretaceous intrusions, are exposed in the mining areas, and among them the Early Cretaceous granites, which intruded into limestone of the early Permian Huanggangliang Formation, are interpreted to be the source of ore, since their Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.25–18.35, ²⁰⁷Pb/²⁰⁴Pb = 15.50–15.56 and ²⁰⁸Pb/²⁰⁴Pb = 38.14–38.32) are highly consistent with the sulfides including sphalerite, galena and chalcopyrite (²⁰⁶Pb/²⁰⁴Pb = 18.23–18.37, ²⁰⁷Pb/²⁰⁴Pb = 15.47–15.62 and ²⁰⁸Pb/²⁰⁴Pb = 37.93–38.44). Sulfur isotope values of the sulfides fall in a narrow δ³⁴S interval of -6.1 to -4.6‰ (mean = -5.4‰, n = 15), suggesting the ore-forming fluid is of magmatic origin.

The deposit formed in three stages: the pre-ore stage (prograde skarn minerals with minor magnetite), the syn-ore stage (sulfides and retrograde skarn minerals including calcite and minor quartz), and the post-ore stage (late veins composed of calcite, quartz, fluorite and chlorite; cutting the above mineral assemblages).The pre-ore stage fluids trapped in pyroxene have higher temperatures (471 ± 31 °C), higher salinity (43.0 ± 3.1 wt. % NaCl eq.), and higher concentrations of Zn (~1.1 wt. %), Pb (~1.7 wt. %), and other elements (e.g., Na, K, Li, As, Rb, Sr, Cs, Ba, Cl and Br) than syn-ore mineralizing fluids (<400 °C, <12 wt. % NaCl eq., ~0.05 wt. % Zn and ~0.03 wt. % Pb). The post-ore fluids are much cooler (<270 °C; averaging ~210°C), with much lower salinity (<5.1 wt. % NaCl eq.), Zn (~38 ppm) and Pb (~19 ppm). Geochemically, the fluids of all paragenetic stages in Baiyinnuo'er are characterized by magmatic signatures based on the element ratios, which are distinctively different from basin brines. The inclusion fluids in pre-ore stage show little variation in composition between ~520 °C and ~420 °C, indicative of a closed cooling system. In contrast, the major components of the syn- and post-ore stage fluids including Cl, Na and K decrease with the temperature dropping from ~350 to <200 °C, indicating a dilution by mixing with groundwater. The metal contents in pre-ore fluid are significantly higher than in syn-ore fluid, but no mineralization occurred. This confirms that the early fluid was, although enriched in metal elements, not responsible for ore precipitation, likely due to its high temperature high salinity nature. The metal deposition was mostly due to mixing with groundwater, which caused temperature decrease and dilution that significantly reduced the metal solubility, thereby promoting metal deposition. The deposition was probably accompanied and facilitated by carbonate dissolution that buffered the acidity generated during the breakdown of Zn (Pb)-Cl complexes and the formation of sulfides. Boiling occurred in both pre-ore and early part of the syn-ore stages, but no evidence indicates that it was related to metal deposition. The current Baiyinnuo'er massive skarns contain both prograde and retrograde minerals (including ore minerals). Paragenetically, they were not formed at the same time, but could be attributed to two (or more) successive pluses of hydrothermal fluids released episodically from residual melts of a progressively downward crystallizing magma. The prograde alteration increased the permeability and porosity, and created sufficient spaces, which was essential for later metal deposition.

Item ID: 52702
Item Type: Thesis (PhD)
Keywords: fluid inclusions; geology; NE China; northeastern China; ore deposits; porphyry Mo deposits; porphyry molybdenum deposits; skarn deposits; Sr–Nd–Pb–Hf isotopes; Xilamulun; zinc-lead deposits; Zn-Pb deposits
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Chapters 2, 3, 5 & 6 (publications) are not available through this repository.

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Shu, Qihai, Lai, Yong, Wang, Chao, Xu, Jiajia and Sun, Yi (2014) Geochronology, geochemistry and Sr–Nd–Hf isotopes of the Haisugou porphyry Mo deposit, northeast China, and their geological significance. Journal of Asian Earth Sciences, 79. pp. 777-791.

Chapter 3: Shu, Qihai, Lai, Yong, Zhou, Yitao, Xu, Jiajia, and Wu, Huaying (2015) Zircon U–Pb geochronology and Sr–Nd–Pb–Hf isotopic constraints on the timing and origin of Mesozoic granitoids hosting the Mo deposits in northern Xilamulun district, NE China. Lithos, 238. pp. 64-75.

Chapter 4: Shu, Qihai, Chang, Zhaoshan, Lai, Yong, Zhou, Yitao, Sun, Yi, and Yan, Cong (2016) Regional metallogeny of Mo-bearing deposits in northeastern China, with new Re-Os Dates of porphyry Mo deposits in the northern Xilamulun district. Economic Geology, 111 (7). pp. 1783-1798.

Chapter 5: Shu, Qihai, Lai, Yong, Sun, Yi, Wang, Chao and Meng, Shu (2013) Ore genesis and hydrothermal evolution of the Baiyinnuo’er zinc-lead skarn deposit northeast China: evidence from isotopes (S, Pb) and fluid inclusions. Economic Geology, 108 (4). pp. 835-860.

Chapter 6: Shu, Qihai, Chang, Zhaoshan, Hammerli, Johannes, Lai, Yong, and Huizenga, Jan (2017) Composition and evolution of fluids forming the Baiyinnuo'er Zn-Pb Skarn Deposit, Northeastern China: insights from laser ablation ICP-MS study of fluid inclusions. Economic Geology, 112. pp. 1441-1460.

Date Deposited: 27 Feb 2018 04:27
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 80%
04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 20%
SEO Codes: 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840108 Zinc Ore Exploration @ 50%
84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840199 Mineral Exploration not elsewhere classified @ 50%
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