Geophysical exploration for gold in Gansu Province, China

Abstract

Gansu Province is in north central China. The Province is rich in mineral resources with more than 100 known deposits, containing a variety of commodities, and of a number of different styles. However, gold reserves in Gansu were insignificant until the mid-1980s when developments in geochemical exploration techniques led to the discovery of over 40 new deposits. These geochemical methods play a key role in gold exploration in Gansu, particularly during reconnaissance exploration. Geophysics tends to be used as a tool to follow-up in prospective areas. The role of geophysics in gold exploration in Gansu can be illustrated using a number of case studies. The deposits described here are at Liba, Huaxi, Liziyuan, Nanjinshan and Xiaoxigong. These deposits are of different types, and this results in different geophysical methods being effective in each case. The Liba and Huaxi deposits occur in sedimentary rocks. They are structurally controlled and sulphides are associated with mineralisation. Ground magnetic surveys are useful for mapping the mineralised structures and stratigraphy. However, the IP method is badly affected by conductive cover. The deposit at Liziyuan is also structure-controlled but occurs in altered granodiorite. Sulphides are again associated with mineralisation. Mineralised bodies within granodiorite are associated with coincident IP anomaly and geochemical anomalies. However, in this case ground magnetic data are ineffective. The Nanjinshan deposit occurs within high-grade quartzitic metasediments and again structural control is evident. Sulphides scattered within quartz veins are associated with the mineralisation. The resistivity method is effective for determining positions and depth extensions of gold-bearing quartz veins. In the Xiaoxigong deposit, borehole geophysical methods were very useful during the evaluation of the deposit. Geophysical logging was able to efficiently identify the mineralised sections. Mise-a-la-masse potential and IP data were able to determine the spatial distribution of mineralisation. Drill hole measurements of potential, using a surface source, helped to find blind mineralisation adjacent to the drillhole.