Iridium anomalies and fractionated siderophile element patterns in impact ejecta, Brockman Iron Formation, Hamersley Basin, Western Australia: evidence for a major asteroid impact in simatic crustal regions of the early Proterozoic earth

This paper summarises the roost habitat and distribution of the ghost bat, Macroderma gigas (Dobson, 1880), in the Pilbara region of Western Australia, with particular emphasis on natural habitats. The preferred habitat of M. gigas in the Hamersley Ranges appears to be caves beneath bluffs of low rounded hills composed of Marra Mamba geology. Habitats were also found in the larger hills of Brockman Iron Formation in the Hamersley Range, and other formations beneath bluffs composed of Gorge Creek Group geology to the north east. Granite rockpiles are also used in the eastern Pilbara. A summary of Pilbara records from numerous sources is presented, including anecdotal accounts and other new records. This includes a newly discovered maternity site from the Hamersley Ranges, only the third reported from natural cave formations in the region. Threats to M. gigas in the region are highlighted and include disturbances associated with mining and entanglement in barbed wire fences.

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Archean to Paleoproterozoic seawater halogen ratios recorded by fluid inclusions in chert and hydrothermal quartz

American Mineralogist ◽

10.2138/am-2020-7238 ◽

2020 ◽

Vol 105 (9) ◽

pp. 1317-1325

Author(s):

Ray Burgess ◽

Sarah L. Goldsmith ◽

Hirochika Sumino ◽

Jamie D. Gilmour ◽

Bernard Marty ◽

...

Keyword(s):

Western Australia ◽

Fluid Inclusions ◽

Hydrothermal Systems ◽

Iron Formation ◽

Banded Iron Formation ◽

Hydrothermal Quartz ◽

Ocean Salinity ◽

Halide System ◽

Chemical Sediment ◽

Substantial Change

Abstract Past changes in the halogen composition of seawater are anticipated based on the differing behavior of chlorine and bromine that are strongly partitioned into seawater, relative to iodine, which is extremely depleted in modern seawater and enriched in marine sediments due to biological uptake. Here we assess the use of chert, a chemical sediment that precipitated throughout the Precambrian, as a proxy for halide ratios in ancient seawater. We determine a set of criteria that can be used to assess the primary nature of halogens and show that ancient seawater Br/Cl and I/Cl ratios can be resolved in chert samples from the 2.5 Ga Dales Gorge Member of the Brockman Banded Iron Formation, Hamersley Group, Western Australia. The values determined of Br/Cl ~2 × 10-3 M and I/Cl ~30 × 10-6 M are comparable to fluid inclusions in hydrothermal quartz from the 3.5 Ga North Pole area, Pilbara Craton, Western Australia, that were the subject of previous reconstructions of ancient ocean salinity and atmospheric isotopic composition. While the similar Br/Cl and I/Cl values indicate no substantial change in the ocean halide system over the interval 2.5–3.5Ga, compared to modern seawater, the ancient ocean was enriched in Br and I relative to Cl. The I/Cl value is intermediate between bulk Earth (assumed chondritic) and the modern seawater ratio, which can be explained by a smaller organic reservoir because this is the major control on marine iodine at the present day. Br/Cl ratios are about 30% higher than both modern seawater and contemporary seafloor hydrothermal systems, perhaps indicating a stronger mantle buffering of seawater halogens during the Archean.

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Occurrence and genetic implications of hyalophane in manganese-rich iron-formation, Cuyuna Iron Range, Minnesota, USA

Mineralogical Magazine ◽

10.1180/minmag.1994.058.392.04 ◽

1994 ◽

Vol 58 (392) ◽

pp. 387-399 ◽

Cited By ~ 13

Author(s):

Peter L. McSwiggen ◽

G. B. Morey ◽

Jane M. Cleland

Keyword(s):

Depositional Environment ◽

Iron Formation ◽

East Central ◽

Early Proterozoic ◽

Iron Range ◽

The North ◽

History Of ◽

Ferromanganese Deposits ◽

Iron Formations ◽

Chemical Sediments

AbstractThe recent discovery of hyalophane [(K,Ba)Al1−2Si3−2O8] on the North range segment of the Early Proterozoic Cuyuna Iron Range of east-central Minnesota has shed new light on the depositional environment of these rocks. This Ba-feldspar occurs in a 10 m thick interval within the main iron-formation and typically contains between 8 and 26 mol.% celsian (BaAl2Si2O8). Its occurrence in several textural settings suggests that barium was being deposited at various stages in the paragenetic history of the iron-formation. Some of the hyalophane grains occur as the cores of micronodules, which are structurally similar to oolites or oncolites, but mineralogically are very complex. The hyalophane also occurs as rims on core grains of diverse mineral composition and as discrete phases in late crosscutting veins.Hyalophane, like other Ba-silicates, has a very restricted paragenesis. They are associated typically either with sedimentary manganese and ferromanganese deposits, or with Cu-Pb-Zn-Ba deposits. The presence of hyalophane in the Early Proterozoic manganiferous iron ores of east-central Minnesota casts doubt on the historic interpretation of these deposits as typical Superior-type sedimentary iron-formations and instead supports the view that these deposits, at least in part, consist of chemical sediments from a hydrothermal fumarolic system. The suggested involvement of a hydrothermal system is also supported by the occurrence of aegirine within the hyalophane-rich layer, and the occurrence of tourmalinites and Sr-rich baryte veins elsewhere in the Cuyuna North range.

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