Application of Pb isotopes to evaluation of mineral resources:A case study of Yingzuishan gold deposit,Gansu Province

A new method is proposed to predicate the depths, shapes and mineral resources of blind ore deposits based on previous studies of the Pb isotopes tracing and a case study of the Yingzuishan gold deposit in Gansu Province. According to the requirements for prognosis of blind deposits by using Pb isotopes and detailed field geological work on the ground surface, the authors collected representative samples in the peripheral areas of the Yingzuishan gold deposit so as to determine regional background values of Pb isotopes and construct the target indices for ore exploration in these peripheral areas. Meanwhile, based on the requirements for establishment of a profile for chemical prospecting in the ore district by using Pb isotopes, the authors collected representative samples of both rocks and ores for prediction of the depths and reserves of blind ores in this deposit. The major conclusions and working progresses are summarized as follows:(1) Distinct negative anomalies of both V1 and V2 values for the Pb isotope-based chemical prospecting profile at the ground surface were found in both the ore district and its adjacent alteration zones, and the anomaly values are close to the average levels of the earth's crust in the study area. From the 7 profiles for chemical prospecting in the ore district, the V1 and V2 values for the ore deposits show regular decrease from the east to the west.(2) V2 average values decrease from 43.57 at line 122 (above the alteration zone) to 35.67 at line 46, which suggests that the revelation depth of the ore deposit increases regularly from the east to the west. The V2 average values for the ore deposit decrease linearly from 40 at line 116 to 36.62 at line 90, demonstrating that the area to the east of line 116 is the target area for blind ores, the area between lines 90～116 is the middle to upper part of the ore deposit, while the area to the west of line 80 represents the lower part, which is close to the bottom of the ore deposit.(3) From line 46 extending eastward to line 116, the ore bodies exposed at the ground surface show a increasing trend in the extension depth, i.e., from 200 m to 1 049 m. This indicates that the depth of the entire ore body is lower in the east than in the west, symbolizing its eastward inclining trend (in a hidden way).(4) RS, the average hidden coefficient, of the ore district averages 3.68 in general, and the predicted reserves for the ore bodies between line 46 and line 116 are 42.5 tons. Therefore, the Yingzuishan gold deposit is likely to be a large to superlarge gold deposit, implying the existence of huge mineralization potentials and the great value for further exploration and development. Drill holes arranged by Gansu Bureau of Exploration and Development of Geology and Mineral Resources all intersect economic gold ore bodies, showing that the method discussed in this paper is effective in the prognosis of blind deposits.