Fang-Fang Hu

Geological and geochronological constraints on the genesis of the giant Tongkuangyu Cu deposit (Palaeoproterozoic), North China Craton

ABSTRACT The Zhongtiao Mountain region is endowed with some of the largest Cu deposits in northern China, among which the Tongkuangyu Cu deposit contains 70% of the total reserves of copper. The orebodies in this deposit are confined to metamorphosed volcanic tuff and monzogranitic porphyry which are enclosed within metasediments. Metamorphism and deformation resulted in intense modification of the deposit, leading to debates surrounding its genesis. In this study, we investigate the lithology, hydrothermal alteration, and mineralization in the deposit. Secondary ion mass spectrometer and laser ablation ICPMS zircon U–Pb dating show that the meta-monzogranitic porphyry was emplaced contemporaneous with the surrounding lithologies at 2180–2190 Ma as a sill, and that the basic volcanic rocks erupted slightly earlier at ~2220 Ma. The Re–Os geochronological data on molybdenite from the deposit constrain the timing of copper mineralization to 2122 ± 12 Ma. Together with published information from previous studies on this region, we infer that during ~2100–2200 Ma the Zhongtiao Mountain region experienced continental or back-arc rifting. We exclude a porphyry copper model for the deposit and favour sediment-hosted stratiform explanation for this deposit.

Origin of the Yinshan epithermal-porphyry Cu–Au–Pb–Zn–Ag deposit, southeastern China: insights from geochemistry, Sr–Nd and zircon U–Pb–Hf–O isotopes

The Yinshan deposit is a large epithermal-porphyry polymetallic deposit, and the timing and petrogenesis of ore-hosting porphyries have been hotly debated. We present new results from geochemical, whole-rock Sr–Nd and zircon U–Pb–Hf–O isotopic investigations. Zircon U–Pb data demonstrate that the quartz porphyry, dacitic porphyry, and quartz dioritic porphyry formed at ˜172.2 ± 0.4 Ma, ˜171.7 ± 0.5 Ma, and ˜170.9 ± 0.3 Ma, respectively. Inherited zircon cores show significant age spreads from ˜730 to ˜1390 Ma. Geochemically, they are high-K calc-alkaline or shoshonitic rocks with arc-like trace element patterns. They have similar whole-rock Nd and zircon Hf isotopic compositions, yet an increasing trend in ϵNd(t) and ϵHf(t) values typifies the suite. Older (inherited) zircons of the three porphyries display Hf compositions comparable to those of the Jiangnan Orogen basement rocks. In situ zircon oxygen isotopic analyses reveal that they have similar oxygen isotopic compositions, which are close to those of mantle zircons. Moreover, a decreasing trend of δ18O values is present. We propose that the ore-related porphyries of the Yinshan deposit were emplaced contemporaneously and derived from partial melting of Neoproterozoic arc-derived mafic (or ultra-mafic) rocks. Modelling suggests that the quartz porphyries, dacitic porphyries, and quartz dioritic porphyries experienced ˜25%, ˜10%, and ˜10% crustal contaminations by Shuangqiaoshan rocks. Our study provides important constraints on mantle–crust interaction in the genesis of polymetallic mineralization associated with Mesozoic magmatism in southeastern China.

Aqueous-carbonic-REE fluids in the giant Bayan Obo deposit, China: implications for REE mineralization

The Bayan Obo REE-Nb-Fe deposit hosts the world’s largest known REE resource. The deposit consists of replacement bodies hosted in dolomite marble made up of magnetite, REE fluorocarbonates, fluorite, aegirine, amphibole, calcite and barite. Three types of fluid inclusions have been recognized: two phase aqueous liquid-apor (L-V), two to three phase CO2 (C), and three phase liquid-vapor-solid (L-V-S) inclusions. Microthermometry measurements indicate that the carbonic phase in C inclusions is nearly pure CO2. During heating experiments, hexagonal or irregular shape daughter minerals in L-V-S inclusions complete dissolution at temperatures of 420–480°C and re-crystallize again at about 400–320°C. These show that daughter minerals in multiphase inclusions in mineralizing veins were crystallized from trapped fluids, and are real daughter minerals. REE-carbonates, halite, sylvite, barite, calcite and pyroxene (?) have been identified on the basis of crystal habit (microscopic and SEM) and EDX analysis. By comparison with Raman spectra of reference REE-carbonate mineral crystals, the hexagonal or irregular shaped daughter minerals in the L-V-S inclusions might be cebaite and bastnaesite. The presence of REE-carbonates as an abundant solid in the ore-forming veins shows that the original ore-forming fluids are very rich in REE, and therefore, have the potential to produce economic REE ores at Bayan Obo.

Late Mesozoic Gold Mineralization in the North China Craton

The North China Craton (NCC) hosts numerous gold deposits and is known as the most gold-productive region in China. These deposits are generally sited in the cratonic margin. Main gold concentration districts include Jiaodong in the eastern margin, Xiaoqinling and Xiong’ershan in the southern margin, Jibei-Jidong, Chifeng-Chaoyang, Ji’nan, and Liaodong in the northern margin, and central Taihangshan in the central Craton. The gold deposits mostly formed within a few million years of the early Cretaceous (130–120 Ma), coeval with widespread occurrences of bimodal magmatism that marked the peak of lithospheric thinning or craton destruction of the NCC. Dehydration of the subducted and stagnant slab in the mantle transition zone has led to continuous hydration and considerable metasomatism of the mantle wedge beneath the NCC. The large-scale gold mineralization in the NCC in the early Cretaceous has genetic relation with craton destruction. The westward subduction of the west Pacific plate (Izanagi) beneath the eastern China continental margin during the Early Cretaceous has been an optimal setting for a large-scale gold mineralization throughout the NCC.

Fluid inclusions evidence for differential exhumation of ultrahigh pressure metamorphic rocks in the Sulu terrane

Differential exhumation was petrologically recognized in ultrahigh pressure metamorphic rocks from the southern and northern parts of the Sulu terrane. While a normal exhumation occurred for eclogites and gneisses in south Sulu, granulite-facies overprinting of ultrahigh pressure metamorphic rocks took place with high retrograde temperatures in north Sulu. A study of fluid inclusions reveals trapping of five type fluid inclusions in high and ultrahigh pressure eclogite minerals and vein quartz in the Sulu terrane. These are A-type N2±CO2 inclusion trapped at high and ultra-high pressure eclogite-facies metamorphic condition, B-type pure-CO2 liquid phase inclusion with higher density trapped during granulite-facies overprinting metamorphism of eclogites, C-type CO2−H2O inclusion and D-type hypersaline inclusion trapped in high pressure eclogite-facies re-crystallization stage, and E-type low salinity H2O inclusion trapped in the latest stage of ultrahigh pressure exhumation (amphibolite-facies retrogression). Identification of crowded-distributing pure-CO2 liquid inclusions with higher density trapped in garnet of eclogites provides an evidence for granulite-facies overprinting metamorphism in the north Sulu terrane.

Insights into the Ore Genesis of the Giant Bayan Obo REE-Nb-Fe Deposit and the Mesoproterozoic Rifting Events in the Northern North China Craton

Bayan Obo ore deposit is the largest rare earth element (REE) resource and the second largest niobium (Nb) resource in the world. The REE enrichment mechanism and genesis of this giant deposit still remains intense debated. The deposit is hosted in the massive dolomite, and nearly one hundred carbonatite dykes occur in the vicinity of the deposit. The carbonatite dykes can be divided into three types from early to late: dolomite, coexisting dolomite–calcite, and calcite type, corresponding to different evolutionary stages of carbonatitic magmatism, and the latter always has higher LREE content. The origin of the ore-hosting dolomite at Bayan Obo has been addressed in various models, ranging from a normal sedimentary carbonate rocks to volcano-sedimentary sequence, and a large carbonatitic intrusion. More geochemical evidences and field interspersed relationship show that the coarse-grained dolomite represents a Mesoproterozoic carbonatite pluton and the fine-grained dolomite resulted from the extensive REE mineralization and modification of the former one. The ore bodies, distributed along an E-W striking belt, occur as large lenses and underwent more intense fluoritization and fenitization with wall rocks. The first episode mineralization is characterized by disseminated mineralization in the dolomite. The second or main-episode is banded or massive mineralization, cut by the third episode consisting of aegirine-rich veins. Various dating methods gave different mineralization ages at Bayan Obo, resulting in long and hot debates. Compilation of available data suggests that the mineralization is rather variable with two peaks at ~1400 and 440 Ma. The early mineralization peak closes in time to the intrusion of the carbonatite dykes. A significant thermal event at ca. 440 Ma resulted in the formation of late-stage veins with coarse crystals of REE minerals. Fluids involving in the REE-Nb-Fe mineralization at Bayan Obo might be REE-F-CO2-NaCl-H2O system. The presence of REE-carbonates as an abundant solid in the ores shows that the original ore-forming fluids are very rich in REE, and therefore, have the potential to produce economic REE ores at Bayan Obo. The Bayan Obo deposit is a product of mantle-derived carbonatitic magmatism at ca. 1400 Ma, which was likely related to the breakup of the supercontinent Columbia. Some remobilization of REE occurred due to subduction of the Palaeo-Asian oceanic plate in the Early Paleozoic, forming weak vein-like mineralization.

Mineralizing age and ore-forming fluid evolution in the Rushan lode gold deposit, Jiaodong Peninsula, eastern China

The Rushan gold deposit in the Jiaodong Peninsula is currently the largest lode gold in China in terms of single vein gold resources. Gold is sited mainly in pyrite- and polymetallic sulfide-quartz vein/veinlet stockworks. Fluid inclusions in the deposit contain C-O-H fluids of three main types, namely CO2-H2O, H2O-rich and CO2. Microthermometric data show that the initial fluids were CO2-dominant (XCO2=0.21∼0.53) with salinities from 4.07∼10.48 wt% NaCl eq., and the homogenization temperatures fall in the range of 290∼369°C. These fluids, modified by fluid/rock reactions with altered wallrocks, were gradually evolved into H2O-riched fluids with minor CO2 (XCO2=0.01∼0.12) and aqueous inclusions in the main ore-forming stage with salinities ranging from 1.23∼12.55 wt% NaCl eq., and the homogenization temperatures range from 170∼324°C. Fluid immiscibility led to the gold precipitation.