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Tongjing Mine, Yinan Au-Cu Mine, Yinan County, Linyi, Shandong, Chinai

Regional Level Types
Tongjing Mine	Mine
Yinan Au-Cu Mine	Mine
Yinan County	County
Linyi	Prefecture-level City
Shandong	Province
China	Country

Gold-copper skarns, associated with epithermal vein-type silver mineralization.
The stratigraphy of the Tongjing ore district primarily consists of the Neoarchaean Taishan group, Neoproterozoic Tumen group, and Cambrian sequences. The Taishan group is overlain by the Tumen group (Tongjiazhuang formation) in an unconformable contact and both are concealed strata. The Tongjiazhuang formation is composed of alternating layers of sandstone and shale with thin-bedded limestone exhibiting localized skarnization. The ore district extensively exposes the Cambrian sequences (Changqing and Jiulong groups) which serve as the primary host rocks for mineralization. The Changqing group (Zhushadong and Mantou formations) and Jiulong group (Zhangxia and Gushan formations) consist mainly of sandstone and shale intercalated with thick-bedded dolomite and (dolomitic) limestone. These strata extend in a NW–SE (300°) direction and dip 10–30° SW. Major structures include NNE-, NW-, and SN-trending faults affected by the Tan–Lu fault. The NNE-trending faults strike 10–40° and incline to the northwest at angles of 62–80° whilst the NW-trending faults strike 300–345° and incline to the southwest at angles of 65–85°. The NNE-trending faults intersect the NW-trending faults, characterized by multi-stage activities, and the magmatic intrusion (e.g., Tongjing intrusive complex) formed at the intersections of these fault sets. The Tongjing complex comprises pyroxene/quartz diorite and hornblende/quartz diorite porphyry, primarily distributed in the eastern part of the ore district and formed during the early Cretaceous (136–126 Ma). The Tongjing deposit consists of six ore segments: Shanzijian, Gongquan, Jinlong, Gongquandong, Duijinshan, and Tonghanzhuang. These mineralized segments are predominantly located within the contact zone between the Tongjing complex and Proterozoic to Cambrian sequences.
Numerous significant ore bodies were discovered within the contact zone where carbonate strata intersect with diorite porphyry, particularly in areas of structural weakness such as unconformities and fracture/slip zones between layers. These ore bodies also occur in the internal and external regions of cis-layered diorite porphyry. The spatial distribution of these ore bodies exhibits a circular pattern surrounding the intrusive rocks. The ore body is lenticular and layered with lengths of 15–240 m along strike and 12–210 m along dip and a thickness of 0.5–25 m. However, it should be noted that individual ore bodies are typically of a limited scale. For example, the No. T001 ore body is characterized by its layered occurrence near the contact zone between diorite porphyry and marl of the Tongjiazhuang group. The ore body strikes nearly E–W and dips at 0–8° to the south. It has a length of 195 m along the strike and 92 m along the dip and a thickness of 0.93–4.61 m. The average grades of Au and Cu in the ore body are 1.64 g/t and 0.35%, respectively.
According to field and petrographic observations, the Tongjing deposit has undergone three stages: (I) thermal metamorphism, (II) prograde alteration, and (III) retrograde alteration. The intrusion of magma caused thermal effects that resulted in the metamorphic recrystallization (I) of impure carbonate units, including shale, sandstone, siltstone, and limestone. This recrystallization process led to the formation of (calc-silicate) hornfels or marble, with minimal to no significant introduction of chemical constituents. The hornfels exhibit an extremely fine-grained texture and primarily consist of minerals such as pyroxene, garnet, and biotite while simultaneously preserving the original sedimentary textures, such as laminations or bedding.

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