Yongxin produced 20 t Au at an average grade of 4.1 g/t from quartz-sericite-pyrite veins at the contact between late Carboniferous mylonite and Early Cretaceous volcanic rocks. Two orebodies have been recognized with echelon vein- to lenticular-shapes in horizontal and vertical profiles that strike NE and dip NW at angles of 20–30°. Orebody I is approximately 375 m long and plunges 800 m deep, with thicknesses ranging from 6.8 to 73.4 m, and an average gold grade of 3.92 g/t. Orebody II is approximately parallel to orebody I, 250 m long and plunges 330 m deep, with thicknesses ranging from 5.9 to 18.0 m, and an average gold grade of 3.55 g/t.

The deposit comprises over ten major orebodies which are located within contact and the fracture zones between mylonite and syenogranite. The exposed length of the orebodies is approximately 375 m, and the thickness of the gold orebodies ranges from 6.8 to 73.4 m. These ore bodies are closely constrained by a group of NW- and NE-trending faults and extend ∼1.6 km gradually along the fault. The Yongxin Te-Au deposit exhibits three main types of ore, namely tectonic breccia, altered granitic mylonite, and quartz veins. The breccia ore samples contain angular blocks primarily made up of granite.
The mineralization can be divided into four stages based on mineral assemblages and crosscutting relationships. The first stage, known as the pyrite-quartz stage (I). The second stage, referred to as the quartz-pyrite stage (II), is represented by fine-grained pyrite veinlets that crosscut Py1. The main mineralization stage (III) is characterized by anhedral aggregated pyrite (Py3) that has a number of grain boundaries (GBs) and porosities. The fourth stage (IV) is characterized by carbonate mineralization.

Two ore types are present at Yongxin: mylonitic and hydrothermal breccia, with the latter being predominant. Metallic minerals are mainly pyrite with minor amounts of galena, chalcopyrite, sphalerite, and Au-bearing minerals. Non-metallic minerals include quartz, sericite, calcite, and chlorite. Hydrothermal alteration includes silicification, sericitization, pyritization, carbonatization, and chloritization. Based on mineral assemblages, metasomatic textures, and cross–cutting relationships, five mineralization stages with five pyrite types are recognized at Yongxin. Stage I (milky quartz-pyrite) occurs in mylonite–type ore, characterized by milky quartz and gray sericite veins with low sulfide content dominated by clean euhedral to anhedral PyⅠ and minor chalcopyrite. Stage II (gray to smoky quartz-Au-pyrite) is the main mineralization stage, hosted in hydrothermal breccias, marked by intense silicification and pyritization with abundant coarse porous pyrite (PyⅡ), gray to smoky quartz veinlets, and minor galena, sphalerite, chalcopyrite, and Au–bearing minerals (native Au, electrum, kustelite, calaverite, petzite, and sylvanite), where Au occurs as fracture fillings and inclusions in PyⅡ, quartz, galena, and tellurides. Stage III (gray-black quartz-pyrite) occurs in hydrothermal breccias, consists of gray-black quartz and fine porous subhedral pyrite (PyⅢ). Stage IV (pyrite) is characterized by fine, homogeneous euhedral to anhedral PyⅣ-1 (aggregates surrounding PyⅡ; Fig. 5b,c) and vein-type PyⅣ-2 (cross-cutting mylonitic granite and breccias), with local chalcopyrite exsolution in pyrite veins. Stage V (transparent quartz-calcite) is the latest stage, consisting of drusy transparent quartz or milky calcite veins cutting earlier ore types.

Au-Ag-tellurides, and Bi-tellurides
Deposit model: low sulfidation epithermal gold deposit

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References