ABSTRACT Large bolide impacts in the Phanerozoic produced global change identifiable in the postimpact sediments. Aside from a few isolated examples, however, evidence of postimpact change associated with Precambrian impacts is sparse. This study used the Neoarchean Paraburdoo spherule layer as a case study to search for impact-induced change in the sediments above the spherule layer. We found possible minor sedimentary changes that may have been due to either a disturbance by bottom currents or changing diagenetic conditions. Contrary to the trends found with several post–Great Oxidation Event large bolide impacts, we found no evidence of shifts in tectonic regime, sediment weathering and deposition, or paleoenvironment induced by the Paraburdoo spherule layer impact, for which the impactor is estimated to have been approximately three times larger than the Cretaceous–Paleogene bolide. This lack of a clear signal of climatic shift may be due to one or more mechanisms. Either the Paraburdoo spherule layer’s deposition in several-hundred-meter-deep water within the Hamersley Basin of Western Australia was too deep to accumulate and record observable changes, or the Neoarchean’s high-CO 2 atmospheric composition acted as a threshold below which the introduction of more impact-produced gases would not have produced the expected climatic and weathering changes. We also report minor traces of elevated iron and arsenic concentrations in the sediments immediately above the Paraburdoo spherule layer, consistent with trends observed above other distal impact deposits, as well as distinctive layers of hematite nodules bracketing the spherule layer. These geochemical changes may record ocean overturn of the Neoarchean stratified water column, which brought slightly oxygenated waters to depth, consistent with the observation of tsunami deposits in shallower impact deposits and/or heating of the global oceans by tens to hundreds of degrees Celsius in the wake of the Paraburdoo spherule layer impact. Either or both of these mechanisms in addition to impact-induced shallow-water ocean evaporation may also have caused a massive die-off of microbes, which also would have produced a postimpact increase in iron and arsenic concentrations.

Abstract South Flank is a ~1.8-billion-tonne martite-goethite iron deposit located in the Late Archean to Paleoproterozoic central Hamersley province, Pilbara craton, Western Australia—a district containing multiple giant iron deposits. A combination of detailed mapping, high-precision airborne magnetic and gravity gradiometer data, and resource range analysis, followed up by systematic drilling, was used to discover and fully define iron mineralization at South Flank. Exploration was targeted using a deposit-scale model, based on observed geologic controls on martite-goethite deposits in the South Flank district, combined with a systems approach, which identified key processes in the formation of iron mineralization at the camp scale, namely fluid pathways, controlling structures, potential host rocks, and ore preservation beneath detrital cover. Iron mineralization at South Flank is hosted by the Marra Mamba Iron Formation and occurs as a series of strata-bound tabular orebodies over a strike length of 25 km. Individual ore zones are up to 150 m thick and can extend to depths of 300 m. Martite-goethite-ochreous goethite ore is predominantly hosted by N2 and N3 subunits of the Mount Newman Member and is best developed in E-W–trending, upright to N-verging asymmetric synclines and associated low-angle reverse faults, which have caused substantial thickening of host rocks. Primary textures within banded iron formation are largely preserved within ore zones and can control location and grade of iron mineralization. Both unmineralized iron formations and ore zones are overprinted by recent extensive ferricrete, locally termed “hardcap.” Phosphorous, Al 2 O 3 , and volatile contents of ore co-vary with iron, albeit at low absolute abundances, whereas SiO 2 is strongly negatively correlated with Fe, reflecting the transition from iron formation (Fe = 30–35 wt %) to iron ore (Fe = 50–65 wt %). Premineralization host-rock composition is an important control on both ore geochemistry and mineralogy. Martite-goethite-ochreous goethite is the dominant style of iron mineralization in the Hamersley province, in terms of overall tonnage and contained Fe, and is also widely developed in iron formations in the Pilbara and Yilgarn cratons and in other major global iron ore districts (e.g., India and Brazil). In each of these regions, martite-goethite and ochreous goethite are commonly developed as a weathering-related supergene overprint of earlier-formed hypogene hematite mineralization. In contrast, South Flank and other major deposits in the central Hamersley province (e.g., Mining Area C, Hope Downs) show no evidence of hypogene iron mineralization and its commonly associated wall-rock alteration. These iron orebodies are characterized by common structural association with synclines and associated reverse faults, preferential host-rock settings within particular units of the Brockman and Marra Mamba iron formations, simple ore mineralogy and geochemistry, and absence of associated wall-rock alteration. The giant martite-goethite deposits in the Hamersley province, of which South Flank is a type example, potentially represent a distinct deposit style. While some of the geologic characteristics of iron mineralization at South Flank are compatible with a supergene origin, many factors relating to ore genesis are unknown or not adequately constrained, including timing and mechanisms of ore formation.

Spherules from three impact layers ranging in age from 3.24 Ga to 2.49 Ga display petrologic and size diversity. All of these layers represent the distal debris from asteroid impacts in the Archean and early Proterozoic. We examine the petrologic control on the size of these spherules. Though all three spherule layers have different diagenetic histories, some textural properties are consistent in all of the layers and can be used to infer original mineralogy. Spherules that are uniform in composition, with the exception of fine-grained minerals around the rims, are inferred to have been altered from an original glassy composition and are the largest spherules. Spherules that are heterogeneous and contain either pseudomorphs of olivine or contain Ni-chromite are the smallest spherules in all sections. Spherules with plagioclase pseudomorphs tend to be intermediate in size. The larger the impact, the more pronounced is the size segregation of these spherule types. Spherules in the older S3 layer from the Barberton greenstone belt are significantly larger and display a wider range of sizes, and these differences in sizes are related to their petrologic type compared to the younger Dales Gorge and Paraburdoo layers from the Hamersley Basin. The S3 layer also tends to have the largest aggregate bed thickness and Ir content, consistent with this bed resulting from a larger impactor. The Dales Gorge spherule layer contains nonspherical particles, indicative of a ballistic melt, and it is therefore used as a point of comparison with the vapor plume condensates of the S3 and Paraburdoo layers. The Dales Gorge spherules are similar in size to those of the Paraburdoo layer; however, they have a significantly higher percentage of largely glassy spherules, and only ~21% have crystalline pseudomorphs.