Alteration of mafic igneous rocks of the southern McArthur Basin: comparison with the Mount Isa region and implications for basin-hosted base metal deposits

Mafic igneous rocks are thought to be an important source of metals for the ca. 1640–1595 Ma sediment-hosted base metal deposits in the Paleo- to Mesoproterozoic Mount Isa – McArthur Basin system of northern Australia. Such rocks are widespread—the voluminous rift-related mafic magmatism at ca. 1790–1775 Ma and ca. 1730–1710 Ma—and show local evidence for intense hydrothermal alteration and metal leaching. To better constrain the nature, degree, and regional and temporal extent of alteration and metal leaching in these rocks, we have undertaken regional sampling of mafic igneous units from available drillcore, for geochemistry, stable isotopes and petrological examination. Sampling focused on magmatism of both ages in the southeastern MacArthur Basin, complementing the extensive pre-existing data for the Mount Isa region. Alteration in the mafic igneous rocks of the southeastern McArthur Basin ranges from mildly to strongly chloritic in the older units to strongly potassic (K-feldspar–chlorite–hematite) in the younger units. The latter alteration is ubiquitous, well developed and characterised by strong K2O enrichment and extreme depletion in CaO and Na2O. Geochemical data show that this intense and pervasive potassic alteration extends to similar-aged mafic rocks in the western Mount Isa region. Metal leaching is present in both alteration types, with strong Cu and Pb depletion in the most chlorite-altered rocks, and Zn and Cu depletion in the potassic alteration. Our oxygen isotope data for these mafic rocks (of both ages) in the southeastern McArthur Basin show a limited range of values (δ18O of 6–10‰) that are negatively correlated with K2O content. Our values are significantly lighter than published data for similar igneous rocks to the west, and indicate either a temperature zonation (ca. 250 °C in the east versus ca. 100 °C in the west; preferred) and/or different fluids. Results from our geochemical forward modelling indicate the requirement for exogenous K2O to produce the observed potassic alteration. The most likely source of this K was saline brines, consistent with the interpreted lacustrine and/or evaporitic environments for much of the McArthur Basin. Timing of alteration is uncertain, and the alteration may have included diagenetic low-temperature local K-rich brines and younger higher-temperature deep basinal brines. The temporal and geographically restricted nature of the potassic alteration, however, suggests restriction of K-rich, bittern evaporitic brine production in the younger and inboard parts of the Mount Isa – McArthur Basin system. Our results provide insights that directly relate to the genesis and exploration of basin-hosted Zn-Pb and Cu-Co mineral systems. They confirm that mafic igneous rocks in the region have lost significant amounts of both Zn and Cu, many times more than required for known deposits. The study also shows that metal leaching was accompanied by magnetite-destructive alteration. Hence, identifying zones of metal leaching may be possible using inversions of geophysical data, which may assist in targeting exploration.
Citation: Champion, D.C., Huston, D.L., Bastrakov, E., Siegel, C., Thorne, J., Gibson, G.M. and Hauser, J., 2020. Alteration of mafic igneous rocks of the southern McArthur Basin: comparison with the Mount Isa region and implications for basin-hosted base metal deposits. In: Czarnota, K., Roach, I., Abbott, S., Haynes, M., Kositcin, N., Ray, A. and Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.

Tags: Alteration, EARTH SCIENCES, EFTF, Exploring for the Future, Geochemistry, Mineral Systems, Published_External
Organization: Geoscience Australia Data