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Abstract

Gold is ubiquitous in the unconformity-related uranimum deposits of Australia’s Alligator Rivers uranium field and in some cases constitutes an economic resource, Palladium and platinum accompany gold in significant amounts at the Coronation Hill and Jabiluka deposits. Spatial association of uranium, gold, and the platinum-group elements and textural relationships between gold and uraninite suggest that all these metals together with magnesian chlorite (amesite) were deposited during the same mineralizing event. The most likely ore-transporting solution would have been an oxidized, slightly acidic, chloride-rich brine (or brines), derived from within the mid-Proterozoic hematitic terrestrial clastic cover sequence to the deposits. In addition to carrying ore-forming amounts of U, Au, Pd, and Pt as chloride complexes, such brines are predicted to have been capable of transporting Se and Te as acid oxyanions. The reaction path history of these brines probably involved initial equilibration with atmospheric oxygen and subsequent equilibration with rocks with a high intrinsic oxidation state, namely a hematitic clastic sedimentary sequence (Kombolgie Formation), poor in organic detritus and ferrous iron. It is proposed that this reaction path history is more important in ore formation than interaction with specific, metal-enriched source rocks.

Preliminary calculations of irreversible mass transfer are presented which support the interpretations of previous workers who have suggested that reduction is the principal mechanism of ore deposition. These calculations show that either mixing of an oxidized metal-bearing solution with a reduced CH4-bearing fluid, derived from lower Proterozoic graphitic schist host rocks, or direct interaction of the oxidized metal-bearing solution with lower Proterozoic graphitic and ferrous iron-bearing schistose host rocks could have produced the association of uranium, gold, and platinum-group elements. Some involvement of ferrous iron-bearing phases in the host rocks is suggested by hematitic alteration around the deposits. The predicted sequence of ore and associated gangue mineral deposition is magnesian chlorite and uraninite, followed by native gold (as is actually observed), followed by palladium telluride (kotulskite), palladium sulfide (vysotskite), and finally platinum sulfide (cooperite). Thus, spatial association of uranium, gold, and platinum-group elements deposition with the unconformity reflects a major difference in ambient oxidation state between mid-Proterozoic cover rocks and underlying lower Proterozoic metasediments, coupled with fault permeability. Similar depositional mechanisms are believed to occur in deposits hosted by younger rocks, the Kupferschiefer of Poland being one possible example.

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