Geophysical exploration in the Xi-cheng lead-zinc orefield, Gansu Province, China

Abstract

The Xi-Cheng lead-zinc orefield lies in Gansu Province in north-central China. The orefield consists of several separate deposits and more than 200 orebodies in Middle Devonian schist and limestone. Examples include the Changba (Zn+Pb >3.8 Mt), Lijiagou (Zn+Pb >3.2 Mt), Huangchang (Zn+Pb ~1.0 Mt), Dengjiashan (Zn+Pb ~1.0 Mt) and Bijiashan (Zn+Pb >1.0 Mt) deposits. The deposits occur as stratiform and/or tabular forms with ores mainly comprising massive and banded pyrite, sphalerite and galena. The ore contains >8.5% of Zn+Pb, plus associated Ag, Cd, Ga, and Ge. Self-potential, ground magnetic, induced polarisation, resistivity and mise-a-la-masse surveys were carried out in the early stages of exploration in the late 1960s to early 1970s. This paper presents some examples of such surveys over the Changba, Lijiagou and Huangchang deposits. This early geophysical practice showed that: (1) the self-potential method was cheap and effective in rapid reconnaissance for mineralisation below thin cover, but the interpretation of the results was sometimes complicated by the presence of graphite in the host limestone; (2) the ground magnetics was able to define small but recognisable anomalies over shallow concealed mineralisation that contains a small amount of pyrrhotite and/or magnetite, as is often the case; (3) induced polarisation was very useful for mapping mineralisation hosted in schist, but was less useful in areas of graphite-bearing limestone; (4) attempts to use resistivity surveys for mapping mineralisation were largely unsuccessful; (5) mise-a-la-masse surveys were not only useful for determining the horizontal extent of intersected mineralisation in drillholes, but were also able to indicate the presence of other mineralisation in the vicinity of the intersected mineralisation.