Abstract

Integrated three-dimensional structural studies have led to new insights into the geologic controls on the distribution of magmatic nickel sulfide ore shoots associated with metakomatiites at the Archean Kambalda dome, Western Australia. Exploration models emphasize volcanic channels in deep embayments (trough structures) modified by thermal erosion at the base of turbulent lava flows. However, analysis of three-dimensional models of the Kambalda exploration database reveals ore shoot-scale controls consistent with the regional deformation sequence D1 to D4. Ore shoots in D1 settings (e.g., Ken shoot) are transposed into the north-northwest D1 trend and have asymmetric trough structures with reentrant updip margins indicating a sense of movement opposite to that on the Kambalda dome. Ore shoots in D2 settings (e.g., Lunnon shoot) have asymmetric trough structures bound by low-angle thrusts indicating a sense of movement typical of that on the dome, with thrust movement of ore into the talc and carbonated hanging-wall rocks lacking primary igneous features (e.g., McMahon shoot). Ore shoots in D3 settings are offset by normal faults which host gold mineralization at the south end of the Kambalda dome (e.g., Hunt shoot) or by major reverse faults at the north end of the dome (e.g., Otter-Juan shoots). Ore shoots in D4 settings (Fisher shoot) are transposed from the north-northwest trend into the north to north-northeast D4 trend. In contrast, ore shoots associated with shallow, poorly defined trough structures, with or without serpentinized hanging wall showing relict igneous features (e.g., Durkin shoot), indicate preserved volcanic control.

The spectrum of volcanic and structural controls indicates a continuum of Kambalda ore shoots from a volcanic controlled end member (3 shoots in shallow trough structures with relict igneous features) through structurally modified (7 shoots in deep trough structures without relict features) to a structurally controlled end member (1 shoot tectonically emplaced in the hanging wall with deformation features). Shape analysis of the three-dimensional models indicates that the structurally modified and controlled ore shoots have prolate shapes and two of the volcanic controlled ore shoots have oblate shapes. Comparison of the prolate shapes with the shapes of strain indicators reveals the importance of constrictional D1 deformation in elongation of the ore shoots, particularly at the north and south ends of the Kambalda dome. The oblate volcanic-controlled end members reflect low-strain settings adjacent to major fault zones. Comparisons to previous experimental systems suggest that the ore shoots and metakomatiite deformed preferentially by ductile flow. The importance of deformation controls on nickel sulfide distribution at the Kambalda dome means that exploration for Kambalda style mineralization should take into consideration the deformation history of terranes that host ore.

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