Chen Mao’s research while affiliated with China University of Geosciences and other places

The Pangjiahe deposit is an orogenic gold deposit located on the northwestern margin of the Fengtai Basin and has proven reserves of 38 t Au at an average grade of 6.3 g/t. Phyllite is widely distributed in the mining area with Triassic granite porphyry and diabase dykes intruded in faults crosscutting it. Gold mineralization is mainly hosted in phyllite rocks. The granite porphyry present in the deposit is locally mineralized when it shares the same space with ore-controlling faults. Pyrite always displays zoning textures: Py0 and Py1 are present in the core; rim Py2 replaces and/or overgrows Py0 and Py1; and Py3 comprises the outermost rim. Previously determined in situ δ³⁴S values indicate that Py0, Py1, and Py2 and Py3 are diagenetic, magmatic hydrothermal, and hydrothermal ore stage in origin, respectively. Electron microprobe analysis (EMPA) mapping of the zoned pyrite (especially Py2) shows a negative correlation between sulfur and arsenic, indicating the substitution of sulfur by As¹⁻. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) mapping and spot analysis of Py2 and Py3 reveal two Au and As relationships. In the mineralized phyllite and granite porphyry, Au has strong positive correlations with As, Sb, Cu, and Ag, similar to other arsenian pyrites in hydrothermal ore deposits. However, in altered samples, Au and As display decoupled geochemical behavior, especially in Py2, which has the same levels of As, Sb, Cu, and Ag but 2–5 orders of magnitude lower Au. We think that the ore fluid reacted with altered samples located distal to the ore-controlled fault (distal to the ore fluid center), and the fluid may not have been sufficiently charged with those elements, especially Au. Because As, Sb, Cu, and Ag could maintain at the same level in the ore fluid after precipitation of pyrite enriched in those elements, but Au was sharply depleted. This was also noted in a published fluid inclusion study in Carlin-type gold deposits in Guizhou Province, which has similar zoned pyrite texture. Based on the similar δ³⁴S values (8 ‰ and 10 ‰) and trace element enrichments (Au, As, Sb, Cu, and Ag) between hydrothermal ore-stage pyrite and the Devonian seafloor exhalation sequence in the Fengtai Basin, the source of the gold in the Pangjiahe deposit should be attributed to metamorphism of the Devonian seafloor exhalation sequence during the Triassic orogeny.

In the eastern Hegenshan-Heihe suture zone (HHSZ) of NE China, Cu-Au hydrothermal mineralization at the newly discovered Hongyan deposit is associated with the Shanshenfu alkali-feldspar granite (SAFG). Zircon U-Pb dating showed that the inner phase and outer phase of the SAFG were formed at 298.8 ± 1.0 Ma and 298.5 ± 1.0 Ma, respectively. Whole rock geochemistry suggests that the SAFG can be classified as an A-type granite. Halfnium isotopes and trace elements in zircon suggest that the SAFG has high Ti-in-zircon crystallization temperature (721–990 °C), high magmatic oxygen fugacity and largely positive εHf(t) (from +6.0 to +9.9). We proposed that the SAFG was derived from crustal assimilation and fractional crystallization of juvenile crust metasomatized by subducting oceanic crust. The high oxygen fugacity of the SAFG suggests the chalcophile elements (e.g., Cu, Au) remained in the magma as opposed to the magma source. An arc-related juvenile source favors enrichment of Cu and Au in the resulting magma. Combined, these magmatic characteristics suggest Cu ± Au exploration potential for magmatic-hydrothermal mineralization related to the SAFG, and similar bodies along the HHSZ. The results obtained combined with regional geological background suggest that the Permian A-type granites and related mineralization along the HHSZ were formed in a post-collisional slab break-off process.

In order to study the petrogenesis and tectonic setting of Permian A-type granites and their relationships with hydrothermal mineralization along the Hegenshan-Heihe suture zone (HHSZ) in northeastern China, we select the newly discovered Hongyan Cu-polymetallic deposit in the northeastern part of the HHSZ that develops three stages of mineralization associated with the Shanshenfu alkali-feldspar granite (SAFG). The zircon U-Pb dating and whole rock geochemistry suggest that the SAFG is a typical A-type granite formed in the Early Permian. The zircon Hf isotopes and trace elements suggest that the SAFG has high Ti-in-zircon temperature (721–990℃), high magmatic oxygen fugacity and largely positive εHf(t) (+6.0 to +9.9). Therefore, we propose that the SAFG was derived from the crustal assimilation and fractional crystallization of the charnockitized juvenile crust. The high oxygen fugacity favors the chalcophile elements (e.g., Cu, Au, Ag) of the source region enriched in the fluid phases after magmatic fractional crystallization, consequently facilitating subsequent hydrothermal mineralization, which is also consistent with the characteristics of ore-forming fluids that changed from the initial high temperature, high salinity, high fO2 and CO2-rich magmatic-hydrothermal fluids of stage I to CO2-poor, dilute, and cooling meteoric fluids of stage III. Combined with regional geological background, the Permian A2-type granites along the HHSZ can be formed in post-collisional slab break-off process. In subsequent exploration for hydrothermal deposits along the HHSZ, the Permian A-type granites with arc-related juvenile crustal source and high fO2 have great potential and need more attention.

Shadong deposit is the first large-scale tungsten deposit found in the East Tianshan orogenic belt, and the geologic characteristics of the deposit indicate that the deeply concealed granite body is genetically related with the mineralization. The LA-ICPMS U-Pb age of zircons from the Shadong concealed granite obtained in this research is 239±2.0 Ma, belonging to the Middle Triassic. The whole rock samples are metaluminous to slightly peraluminous (A/CNK=0.95–1.02) with low contents of SiO2 (64.0 wt.%–68.5 wt.%) and low K2O/Na2O ratios (0.73–0.96). The samples reveal enrichment of K, Rb, Th and depletion of Nb, Ta, P, Ti and have a negative slope from La to Lu (LaN/YbN=16.29–36.8) with weak negative Eu anomaly (Eu/Eu*= 0.71–0.82). Initial 87Sr/86Sr ratios of whole rock range of 0.706 59–0.707 75, εNd(t) values range from -1.77 to -2.53 and εHf(t) values of zircon are between 2.54 and 4.90. The lithogeochemistry and Sr-Nd-Hf isotopic characteristics revealed that the concealed granite in Shadong tungsten deposit is I-type granite, and occurs in an intraplate tectonic setting. The magma mixing during intraplating of mantle derived magma intruding into the crust in Indosinian Period is the major formation mechanism of the granite. Of which, the proportion of mantle derived magma ranges from 58% to 60%, and the crustal materials are mainly the metamorphic basement of Xingxingxia Group of Mesoproterozoic Changcheng System, which may provide the main source of ore forming metals of Shadong tungsten deposit.