Can sphalerite be a polarisable mineral? An example from the Century Zn-Pb deposit

Abstract

While sphalerite is an important ore-forming mineral, the difficulties of exploring for this mineral target due to its low petrophysical contrast with gangue material have been well documented. Fortunately, many large zinc deposits also contain dense and conductive secondary sulphide minerals, which are good targets for standard geophysical tools. The low-grade metamorphic, sediment hosted Century deposit of northwest Queensland represents a style of mineralisation rich in zinc, but with relatively few accessory sulphide minerals. Mineralisation is generally stratiform, consisting of fine-grained laminae of sphalerite, pyrite and galena cross cut by veins of sphalerite and galena in the upper layers of the deposit. Pyrite replaces sphalerite as the main sulphide mineral at the base of the deposit. Of the wide range of techniques applied to detect the deposit, including the gravity, magnetic and electromagnetic methods, only the induced polarisation technique shows a clear anomaly. Conventional wisdom would suggest this induced polarisation anomaly was sourced from galena in the upper part of the deposit or pyrite enrichment in the footwall shales. The authors suggest that sphalerite may be a significant contributor to the observed chargeability response. Further study of the electrical properties of this mineral is required to assess the potential for using induced polarisation as direct targeting tool for zinc-rich orebodies with a low content of secondary sulphides.