Geochemical and sedimentological characterization of the Proterozoic Blue Billy Formation shale-hosted Zn-Pb mineralization; Edmund Basin, Western Australia

[摘要] The early Mesoproterozoic Blue Billy Formation in the Edmund Group (Bangemall Basin) of Western Australia comprises a ~1250m thick sequence of black carbonaceous shales and is similar in age to the prospective McArthur Group in the McArthur Basin (Northern Territory), which hosts the gigantic HYC sediment-hosted Zn-Pb deposit. Furthermore, the regolith overlying the Blue Billy Formation yields the highest geochemical (Zn) anomalies in the Capricorn Orogen. Therefore, the Blue Billy Formation has been considered prospective for sediment-hosted Zn mineralization for some time, but there has been very limited exploration activity in this terrane to date. In 2017 four new diamond drill holes were drilled into the Blue Billy Formation and underlying and overlying carbonate sequences to test strong airborne electromagnetic (AEM) conductive responses and search for sediment-hosted Zn mineralization in pyritic shales, close to the strong surface geochemical anomalies and the major structural feature, the Talga Fault. Low-grade Zn was encountered in the central horizon of the black mudstones of the Blue Billy Formation, with increasing grade toward the NW. These diamond drill cores offer an unparalleled opportunity to study the mineralization in the Blue Billy Formation, characterize the mineral system and assess the prospectivity of the basin. In this study, we combine advanced geochemical and petrographic analytical techniques with cutting-edge geophysical inversion methods to investigate the Blue Billy Project mineral system. For clarity, these have been separated into two separate reports, and this report will focus on the geochemical and petrographic aspects of the study.Super high resolution XRF mapping of fresh drill core using the Maia Mapper system shows that the Blue Billy Formation contains abundant stratiform, laminated Zn (sphalerite) mineralization within black pyritic mudstone sequences that are cross-cut by a later carbonate-pyrite-sphalerite vein system. More detailed investigations using scanning-electron microscopy (SEM) demonstrate that there are at least two paragenetic phases of sphalerite mineralization: 1) a primary nodular phase that overprints early diagenetic pyrite and possibly replaces diagenetic carbonate, and 2) a secondary sphalerite phase that co-precipitates with chlorite during an alteration event, or events. Maia Mapper data of Mn-bearing basal carbonate sequences underlying the Blue Billy Formation show that Mn occurs homogenously in massive carbonates, and appears to be diagenetic in origin due to the presence of Mn-bearing marine pore water fluids. Interbedded carbonate-siltstone sequences display zones of intense amphibole-chlorite alteration at the boundary between the different lithologies. This is strong evidence for either a burial-driven, or (more likely) a deformation driven alteration event throughout this part of the basin.Assessment of the sedimentary sequences across the four drill holes demonstrates that the basin in the Project area was laterally-continuous with no sub-basin development on the scale of the tenement. There is no evidence of syn-sedimentary movement of the pre-existing and regionally-important Talga Fault during deposition of the Blue Billy Formation, which precludes any control on sedimentation or significant fluid input into the sediment during deposition. Later movement of the Talga Fault, however, may have allowed movement of metal-bearing (hydrothermal) fluids causing chlorite alteration-associated Zn mineralization that overprinted earlier primary Zn mineralization from an unknown distal source.

[发布日期] 2018-12-03 [发布机构] CSIRO

[学科分类] 地球科学(综合)