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Abstract:
The Teena Zn-Pb deposit is located in the Carpentaria Zn Province (Australia), which contains some of the largest clastic dominant (CD-type) massive sulfide Zn-Pb deposits in the world. The timing of the main stage of hydrothermal sulfide mineralization in the Teena subbasin is constrained to the midstage of burial diagenesis, during a period of short-lived regional extension. To distinguish hydrothermal alteration from spatially and temporally overlapping burial diagenetic alteration, and to establish the primary controls on hydrothermal mass transfer, it is necessary to evaluate the various foot- and hanging-wall alteration assemblages that formed between early- (eogenesis) and late- (mesogenesis) stage diagenesis. To achieve this, we have statistically evaluated a large lithogeochemistry dataset (n >2,500) and selected a subset (n = 65) of representative samples for detailed mineralogical (X-ray diffraction, illite crystallinity) and petrographic (scanning electron microscopy) analyses; hyperspectral core imaging data were then used to upscale key paragenetic observations.
We show that sulfide mineralization was predated by multiple diagenetic alteration assemblages, including stratiform pyrite, dolomite nodules and cement, disseminated hematite and authigenic K-feldspar. These assemblages formed during eogenesis in multiple subbasins across the broader McArthur Basin and are not part of the synmineralization alteration footprint. Whereas pyrite and dolomite formed primarily from the in situ degradation of organic matter, feldspar authigenesis was the product of K metasomatism that was focused along permeable coarse-grained volcaniclastic sandstone beds within the host-rock sequence. The immature volcaniclastic input is broadly representative of the siliciclastic compositional end member in the subbasin, which formed the protolith for phyllosilicate (illite, phengite, chlorite) formation during burial diagenesis. There is no evidence of extensive phyllosilicate alteration in any of the geochemical, mineralogical (illite crystallinity), or petrographic datasets, despite some evidence of K-feldspar replacement by sphalerite in the Lower and Main mineralized lenses. Rather, the high Zn grades formed via dolomite replacement, which is resolvable from a chemical mass balance analysis and consistent with petrographic observations.
There are significant exploration implications associated with carbonate-replacement sulfide mineralization during mesogenesis: (1) the capacity for secondary porosity generation in the host rock is as important as its sulfate-reducing capacity; (2) hydrothermal mineralization has a short-range cryptic lateral and vertical synmineralization alteration footprint due to acid neutralization by a carbonate-rich protolith; and (3) the distribution and chemistry of premineralization phases (e.g., pyrite, dolomite nodules) cannot be directly related to the mineralization footprint, which is localized to the 4th-order subbasin scale. Future exploration for this deposit style should therefore be focused on identifying units that contain a mixture of organic carbon and carbonate in the protolith, at favorable stratigraphic redox boundaries, and proximal to feeder growth faults.
