Evaluating geological complexity and complexity gradients as controls on copper mineralisation, Mt Isa Inlier

Faults and lithological boundaries are pathways for focusing the large volumes of fluid required to form hydrothermal orebodies. The distribution of faults and lithological boundaries as a function of scale can be measured by the geological complexity, quantified by a fractal dimension obtained by box counting, that increases with complexity. Copper mineralisation in the Mt Isa Inlier has well-documented structural and stratigraphic controls, and may therefore have a strong relationship with geological complexity. In this study, a two-dimensional approach is implemented for analysing the relationship between complexity, complexity gradients and copper mineralisation. There is a strong positive relationship between complexity and copper distribution and endowment in both the major lithostratigraphic subdivisions of the inlier, the Eastern and Western Successions. This relationship may suggest that abundant fluid pathways and physico-chemical contrasts are critical factors in copper mineralisation. A weak inverse relationship exists between complexity gradients and copper endowment. At small scales, there is a departure from the fractal relationship between number of boxes containing faults or lithological boundaries and box size, called roll-off. Roll-off is shown to be a function of the detail of mapping. This allows variation in mapping detail to be accounted for in measurements of geological complexity by due consideration of the scale at which roll-off occurs. The results imply that complexity could be used as an exploration tool.