Ore deposits are highly anomalous regions of Earth’s crust where the composition has been altered by protracted fluid flow. In such systems, which are demonstrably open, it is attractive to interpret mineral trace element variations in terms of changes in fluid composition. However, as one mineral assemblage reacts to another in the presence of the fluid, the mineral that are breaking down can also give up their trace elements to a more stable host. Many techniques that provide low detection limits with spatial resolutions smaller than a single grain are limited in the total area that can be analysed. We present data from the Maia Mapper, a new XRF mapping instrument capable of analysing 500 mm lengths of drill core with 30 µm resolution and ppm level detection limits. With analysis times of <10 ms per pixel, and long-term X-ray beam stability provided by an Excillum MetalJet source, it is possible to analyse such samples in 20-30 hours. Because the datasets are areal extensive, we show how simple statistical methods can be used to separate multiple populations of mineral compositions and ensure representative analysis in further detailed studies.
We show compositional maps showing variations in pyrite compositions in the Ernest Henry iron oxide-copper-gold deposit from the Mount Isa inlier in Queensland. The data show multiple sub-populations of pyrite (FeS2) with varying Co-As-Ni contents. The patterns are interpreted in terms of reactions between early formed cobaltite (CoAsS) and arsenopyrite (FeAsS) and continued growth of pyrite to incorporate elements from these minerals. A second stage of pyrite is formed after a period of instability and contains more Ni. Imaging of the silicate minerals highlight a potential source for the Ni suggesting the system is not as open as it may first appear.