Surface gravity and magnetic responses of mineralization, Mt. Isa, northwest Queensland, Australia

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Abstract
Surface gravity and magnetic surveys around the giant copper‐silver‐lead‐zinc mine at Mt. Isa in northwest Queensland have shown that each type of mineralization presents characteristic responses which are anomalous within the perspective of the host rocks. The responses reflect the entire mineralization‐alteration package and not simply the economic ore. Copper mineralization and associated alteration products, including “silica dolomite,” are not significantly magnetized, but the mineralization across a depth range of 1000 m produces a positive gravity anomaly of more than 2.5 mGal. The related alteration products are associated with large negative anomalies of up to 8 mGal in both hanging wall and footwall host rocks. Silver‐lead‐zinc mineralization is moderately magnetic (about 0.001 cgs). It is also denser and shallower and generates larger gravity anomalies. Negative secondary alteration effects, while significant, are equivalent in magnitude to the mineralization effect. Sites mineralized comparably to Mt. Isa would be located by semiregional surveys with a station spacing of 200 to 400 m and survey precision of better than 0.4 mGal, although there are limits to resolution of individual parts of the mineralization.

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