Archean Gold in the Lindsays deposit, Yilgarn: Hyperspectral and Geochemical Fingerprinting of associated Structures and Alteration Patterns

Abstract

The Lindsays gold deposit in the Coolgardie Goldfield (Western Australia) is of Archaean age and hosted in basalt and dolerite rocks, which experienced amphibolite facies grade metamorphism. Gold occurrence is associated with various mineral assemblages. Understanding of the timing of gold deposition and the associated mineral assemblage is of major importance in exploration for new Archaean gold deposits in this area.

This research applies an integrated approach of infrared reflectance spectroscopy, wholerock geochemistry and (detailed) petrography to unravel the mineral systems’ history. The mineralogy along the major structures in the Lindsays Deposit, e.g. the peak metamorphic Lindsays Fault and the retrograde Departure Fault, were analysed using the PIMA IITM and ASDTM infrared reflectance field spectrometers.

Geochemistry showed that the currently observed high grade metamorphic mineral assemblage is mainly controlled by lithology. Biotite occurrence is limited to a shallow, basaltic unit (mafic unit 1). An intruded dolerite sill at depth (mafic unit 2) and older basalt (mafic unit 3) do not contain pervasive biotite alteration. The distribution of the retrograde mineral assemblage, defined by the presence of prehnite, is structure controlled and occurs in structures that formed both during peak and retrograde metamorphism. The dimensions of the retrograde alteration zones exceed the visually identified damage zones of high grade and retrograde metamorphic deformation structures.

Gold mineralisation in the Lindsays Deposit is found as free gold, hosted by high grade metamorphic extensional quartz-calcite veins, or as gold inclusions and rims surrounding arsenopyrites, that formed during high grade metamorphic conditions. Gold mineralisation is strongly associated to the development the formation of peak metamorphic structures. This paper shows that infrared spectroscopy is an excellent tool for detection of deformation structures by mapping prehnite abundance. Therefore infrared spectroscopy can be used as a vector towards gold mineralised structures in the Coolgardie area.