Research on isotopic dating methods required for exploration of metallic mineral deposits (Project Leader: CHEN Wen)

The research period for the geological mineral survey & evaluation project “Research on Isotopic Dating Method Needed for Exploration of Metallic Ore Deposits” was 2013-2015. The main aim was to specify isotopic dating methods needed for the survey of metallic ore deposits, to undertake instrument debugging, hardware configuration and exploration and optimization of experimental processes, to establish the zircon (U-Th)/He isotopic dating method and the experimental process for superfine mineral Ar-Ar isotopic dating, to perfect sulfide Re-Os isotopic analysis in magmatic metallic mineral deposits and to select typical metallic mineral deposits and tectonic zones for method validation so as to provide technical support for research on formation of mineral deposit, their uplift and denudation. The main results are as follows: 

(1) China’s first single-grain zircon (U-Th)/He dating laboratory was established. 1) We completed the construction of a (U-Th)/He dating laboratory and successfully installed a helium isotope quadrupole mass spectrometer (QMS), the analytical precision of which for ⁴He (helium-4) in zircon samples is above 5‰; 2) We established the experimental process for single-grain zircon (U-Th)/He dating and dated standard samples of the FCT and Sri Lanka zircons. Our (U-Th)/He age for the FCT zircon (recommended age is 28.3±2.6 Ma) is between 25.81 Ma and 30.72 Ma and the weighted average age is 28.18 ± 0.51 Ma; our (U-Th)/He age for the Sri Lanka zircon (recommended age is 470±11 Ma) is between 445.5 Ma and 489.5 Ma and the weighted average value after eliminating outliers is 479.0±8.0 Ma. The analytical errors for zircon (U-Th)/He dating of the above samples are between 2.7 and 4.6 %, which are below 5 %. The ages for two international standard samples tested are both consistent with the reference values, indicating that our laboratory process for zircon (U-Th)/He dating is precise and reliable. 

(2) Remarkable progress was achieved in superfine mineral Ar-Ar dating. 1) We successfully installed and debugged a noble gas mass spectrometer (Helix MC), the key index of which meets the requirements of precise measurement of the content of Ar isotope and the precision of measuring the ⁴⁰Ar peak is better than 0.5 ‰ (the best value may reach 0.05 ‰). The long-term monitoring results for the ⁴⁰Ar/³⁶Ar value of atmospheric argon are mostly between 293 and 297, the average value is 295.69, which is consistent with the Neil value (295.5), indicating that the instrument and experimental process are of reliable precision; 2) We successfully developed an experimental processes of superfine-grain sample making and fractionation for metal ores and rock samples, and mineral separation and check for superfine Neogene potassium-bearing minerals; 3) Using a step-heating Ar experiment to study the Ar release characteristic in superfine minerals, we established the experimental process for Ar extraction from superfine minerals; 4) Using Ar-Ar dating of standard samples from conventional to superfine size, we provisionally determined the lower limit of the size of mineral samples where the nuclear recoil effect obviously influences the dating result. The experimental results indicate that we can still obtain reliable Ar-Ar ages from samples as fine as 300 mesh, which considerably enlarges the fractional range for Ar-Ar dating.

(3) Progress was also achieved in metal sulfide Re-Os dating. 1) By reducing the total procedural blank and increasing Os recovery, the ionization rate and ion current density, we further optimized the chemical preparation process of Re-Os isotope samples and obtained precise analysis of ~10 pg-grade sulfide (such as pyrrhotite), the precision of which may reach 5 ‰; 2) We obtained a Re-Os isochron for a sulfide mineral combination such as pyrite in the Huangshandong Cu-Ni deposit in Xinjiang, the age of which is consistent with a zircon U-Pb age within error, verifying the stability and reliability of this Re-Os dating process. Based on this, we believe that Re-Os isotope dating by selecting minerals formed during the same geological event, with the same cause of formation, and with one or more sulfides symbiotic on a microscale and with homogeneous isotopes within a small range can help use to obtain an isochron age of geological significance. This further expands the objects for Re-Os isotopic dating. 

(4) Research on the hydatogen sediment optical stimulated luminescence (OSL) dating technology. 1) We systematically established the OSL signal strength and equivalent distribution of different types of modern hydatogen sediments in northern China and analyzed the influence of the signal zero returning degree on age precision. The results show that the equivalents of modern sediment residuals with different causes of formation and with different sizes in the northern region, northern China plains and northern China region vary considerably, and the measured data are of great significance to hydatogen sediment OSL dating in northern China; 2) We improved the pre-treatment and testing process of hydatogen sediments and developed a basis and method suitable for the treatment and selection of OSL age data of hydatogen sediments, by which we reduced the dating error to within 5 % and expanded the OSL dating range of hydatogen sediments, making it possible to obtain the age of sediments more than 200,000 years old.

(5) Research progress in method validation and application demonstration. By zircon and apatite (U-Th)/He dating, as well as superfine mineral and normal size mineral Ar-Ar dating, we defined the upper limit of the age of the hydrothermal processed at the Katebaasu Gold Deposit located in the western Tianshan. We also established the uplift and erosion history of the deposit, revealed the thermal evolution history in the middle range of the South Tianshan since the Paleozoic and the uplift and erosion history since the Early Jurassic, determined the metallogenetic epoch and analyzed the genesis for the Wangfeng gold deposit in the South Tianshan. 

Tectonic attributes and evolution of the Paleoproterozoic Jiao–Liao–Ji Belt, eastern North China Craton (Project Leader: MENG En)

Extensive research over a two-year period has yielded the following results:

(1) The Paleoproterozoic metamorphic volcanic rocks in the southern and northern zones of the Liao–Ji Belt are calc-alkaline in composition, and were erupted at an active continental margin and a back-arc basin, respectively, at ~2.19 Ga, and were metamorphosed at 1.90 and 1.85 Ga.

(2) The protoliths of the Paleoproterozoic metamorphic sedimentary rocks in the southern and northern zones were deposited in the same tectonic environment after 2.02 Ga and were metamorphosed at the same time as the metavolcanic rocks.

(3) TTG gneisses formed at ~2.50 Ga in the southeastern Jilin Province.

(4) Paleoproterozoic granitic magmatism occurred in three pulses at 2.17–2.11 Ga, and 2.09–2.08 Ga granites were emplaced in a back-arc environment, whereas 1.90–1.85 Ga granites are syn-collisional.

(5) The metagabbro/diabase of the central Liaodong Peninsula formed in a back-arc basin at 2.15 Ga and was metamorphosed at 1.90 Ga.

Tectonic evolution and metallogenetic potential of important ophiolite belts in China (Project Leader: XIAO Xuchang)

Ophiolite is a remnant of oceanic crust and plays a key role in the identification of paleoplate sutures and reconstructing the evolution of ancient ocean basins. A complete ophiolite suite is composed of peridotite, cumulate gabbro, mafic sheets or dikes, pillow lava, and deep sea sediments. We investigated important ophiolite belts in China, and their features are summarized as follows.

       (1) The Shimian ophiolite in the western Yangtze Block is a tectonic complex which mainly consists of serpentinite, gabbro, basalt, and rodingite. Zircon U–Pb dating using a sensitive high-resolution ion microprobe (SHRIMP) on a gabbro sample yielded an age of 1066 ± 11 Ma, representing the formation age of the Shimian ophiolite. All basalt and gabbro samples are tholeiitic in composition and have normal mid-ocean ridge basalt (N-MORB)-like trace element patterns with negative Nb and Ti anomalies. The low zircon δ¹⁸O (4.6 to 5.6 ‰) and high positive zircon ε_Hf(t) (+10.8 to +18.3) values indicate that these rocks were derived from a depleted mantle source. These features are comparable with those of basaltic rocks in SSZ-type ophiolites.

       (2) The NE Jiangxi ophiolite in the eastern Jiangnan Orogen is a tectonic mélange that mainly consists of individual tectonic blocks comprising pyroxenite, gabbro, basalt, diorite, granite and chert in a matrix of serpentinite or tuffaceous graywacke. The basalts display markedly negative Nb–Ta anomalies and show a geochemical affinity to island-arc basalt (IAB), indicating a slightly enriched MORB-type mantle source that was significantly influenced by subduction-derived fluids and/or melts. SIMS zircon U–Pb dating of gabbros gave ages of 995 ± 22 Ma and 993 ± 12 Ma, which are interpreted as the formation age of the NE Jiangxi ophiolite. Positive zircon ε_Hf(t) (+8.8 to +13.8) values for the gabbros and whole-rock ε_Nd(t) (+5.5 to +6.6) values for the basalts indicate that the NE Jiangxi ophiolite originated from an isotopically homogeneous depleted mantle source. The diversity of MORB- to IAB-like basalts and the presence of Fe–Ti basalts favor formation of the NE Jiangxi ophiolite during the initial rifting phase of an intra-oceanic back-arc basin between an oceanic arc (Huaiyu Terrane) and the continental margin of the Yangtze Block (Jiuling Terrane) at ca. 990 Ma.

       (3) The Nyainqentanglha Group is one of the most important Precambrian rock sequences  in the Tibetan Plateau and is composed of a suite of tectonic slices with different sizes and ages, including metasedimentary, metavolcanic and meta-basic-felsic intrusive rocks. We reported new zircon U-Pb ages and Hf isotopes as well as whole-rock major and trace element compositions for metagabbro from the Nyainqentanglha Group in the Ren Co area, north-central Lhasa terrane, Tibet. LA-ICP-MS and SIMS U-Pb zircon dating yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 930.9±7.3 and 925.2±6.2 Ma, respectively. Zircons of the metagabbro exhibit pudding, fan-shaped or slight zoned textures and high Th/U ratios (0.73–31.6), indicating that they are of magmatic origin, and the above ages represent the tome of crystallizatio of the metagabbro protolith. The metagabbros contain low SiO₂ concentrations (47.83–49.32wt. %) and show sub-alkaline chemical features. Chondrite-normalized rare earth element (REE) and primitive-mantle-normalized trace element multi-element patterns indicate that these rocks can be divided into two groups. The REE and trace element patterns of both groups are similar to those of N-type mid-ocean ridge basalt (N-MORB), but Group A samples have relatively higher REE and high field strength element (HFSE) contents than Group B samples. The high field strength element ratios (Ce/Zr, Th/Yb, Th/La, La/Yb, La/Sm, La/Nb, Zr/Nb, Hf/Nb, and Y/Nb) of both groups are comparable to those of N-MORB. In addition, the metagabbros also display some chemical features of island arc tholeiite (IAT), whereas Group B samples show some subduction affinity in geochemical discrimination diagrams. Furthermore, zircons of the metagabbros have strongly positive ε_Hf(t) values (8.26–13.7) and young zircon Hf model ages (t_DM = 933–1205 Ma), suggesting that the protoliths are derived from a long-term depleted mantle source. These features indicate that the metagabbros are probably remnants of early Neoproterozoic oceanic crust.

Tectonic settings and main mineral deposits in the Tethyan metallogenic domain in southwestern Asia (Project Leader: ZHANG Hongrui)

Located in southwestern Asia, the Zagros orogen is an important segment of the Tethyan metallogenic domain (TMD). It contains numerous large or giant ore deposits, such as Mehdi Abad, Sar Cheshmeh, and Sungun. The main achievements of this project are as follows:

(1) The tectono-magmatic and metallogenic evolution of the Zagros orogen is reviewed. Four major tectonic units are recognized, they are the Zagros fold-and-thrust belt (ZFTB), the Sanandaj–Sirjan zone (SSZ), the Urumieh–Dokhtar magmatic assemblage (UDMA), and the central Iran block (CI). The Zagros was generated during Paleozoic-Mesozoic Tethyan accretionary and Cenozoic continental collisional orogenesis. The Neo-Tethyan oceanic slab was subducted beneath the SSZ during Jurassic and Cretaceous and left a back arc basin in the north of the SSZ where volcanogenic massive sulphide deposits and the Mehdi Abad Pb–Zn deposit formed. A mafic to ultramafic complex hosting Cr mineralization formed in the south of the SSZ. This podiform chromite was emplaced 40 Ma later during obduction of the oceanic crust. At the end of the Eocene to Oligocene, collision between the Arabian and Eurasian continents occurred which caused the northern margin of the Arabian plate to become deformed (ZFTB), and abundant magmatic activity occurred bewteen the SSZ and CI (UDMA). Some adakitic granites hosting Cu mineralization intruded during the early Miocene. The collision also led to Pb-Zn mineralization in the SSZ. The Zagros orogen has been in a post-collisional stage since the middle Miocene.

(2) The main and significant metallogenic belts in the Zagros include the Zagros podiform chromite, the Arasbaran–Kerman porphyry Cu–Mo–Au, and the Takab-Yazd Pb–Zn deposits. Among these, the Takab-Yazd belt is composed of two types of deposits, e.g., MVT-like deposits and volcanogenic massive sulfide (VMS) deposits. 

(3) Synthesizing the architecture and tectonic evolution of collisional orogens within the TMD and comparisons with these collisional orogenic systems led to identification of four basic collision types: orthogonal and asymmetric (e.g., the Tibetan collision), orthogonal and symmetric (Pyrenees), oblique and symmetric (Alpine), and oblique and asymmetric (Zagros). The tectonic evolution of collisional orogens typically includes three major processes: (a) syn-collisional continental convergence, (b) late-collisional transform faulring, and (c) post-collisional crustal extension, each forming distinct types of ore deposits in specific settings.

Division of the Andean metallogenic domain in South America and regional metallogenic structures in northern Chile (Project Leader: ZHU Xiaosan)

The Andean metallogenic belt is the most famous belt worldwide for copper, gold and multi-metal mineralization. There are a several large to giant copper, gold and polymetallic deposits in the Andean belt. We divided the Andean metallogenic domain into three metallogenic provinces, and they are the North, Middle and South Andean metallogenic provinces, based on the basement composition, tectono-magmatic evolution, plate subduction, difference of mineralization and major metallogenic types of the Andes. We also subdivided the 3 metallogenic provinces into 14 metallogenic belts and briefly elaborate on their geological background and metallogenic characteristics.

The Atacama and Domeyko faults systems in northern Chile control tectono-magmatic activities migrating eastward and the types of mineral resources. We processed and interpreted aeromagnetic data from northern Chile with reducing to pole, upward field continuation, the second derivative calculation in a vertical direction, inclination angle calculation and analytical signal amplitude analysis. We discovered the locations and planar distribution characteristics of regional deep faults along the NNE and N-S directions. We also found the main reasons for formation of the tectono-magmatic belts and that there are nearly parallel deep faults distributed from west to east and multiple magmatic activities along these faults. We ascertained the locations of volcanic mechanisms and the relationships between these and with the regional deep faults. We deduced the spatial distribution of mafic-intermediate igneous rocks, felsic igneous rocks, intrusive rocks and sedimentary sequences. We revealed linear positive magnetic anomalies and magnetic anomaly gradient zones based on the slowly varying background negative magnetic anomaly field, which indicates that strong magmatic activity occurred along these regional deep faults, and our survey revealed favorable areas for copper and polymetallic mineralization.

The detailed division of the Andean metallogenic belt provided basic information on further sub-division and summarized regional metallogenic regularities. This study also provided some basic information for further research in geology, structural characteristics and mineral resource prospecting in northern Chile.