B. Krapez

The Sholl Shear Zone, West Pilbara: evidence for a domain boundary structure from integrated tectonostratigraphic analyses, SHRIMP UPb dating and isotopic and geochemical data of granitoids

Abstract The Pilbara Block provides a record of Archaean continental growth involving the tectonic accretion of outboard island-arcs and collisions with other continental-scale fragments. This record of continental growth is balanced by breakup and strike-slip dismemberment of the continent. New SHRIMP Uue5f8Pb in zircon ages and Smue5f8Nd data provide evidence in the West Pilbara which demonstrates that subduction-related and tectonic-accretion processes at the western margin of that ancestral continent between 3.15-2.78 Ga were coeval with, and genetically related to, crustal-scale tectonics and basin formation inboard of that margin. The tectonic division of the West Pilbara is defined by integrated tectonic analyses, geochronology, geochemistry and isotopic analyses. Geochronological studies clearly indicate that the western Pilbara comparises two domains with different recorded geohistories, whereas geochemistry and isotopic systematics reflect the changing tectonic regimes through time. In combination, these studies allow the development of a reconstruction of the relative positions of the domains through time on the western margin of the Pilbara Block. The supracrustal rocks of the northern Roebourne Lithotectonic Complex (Domain 6 in a Pilbarawide scheme) were formed in an island arc setting, facing an ocean to the north-west, prior to 3260 Ma, the time of emplacement of voluminous granitoids into the complex. In contrast, the supracrustal rocks of the southern Sholl Belt (Pilbara Domain 5) were formed in a back-arc setting behind a north-west-facing arc between 3125 and 3112 Ma, with more-or-less synchronous granite emplacement at about 3115 Ma. The two domains were tectonically juxtaposed, between 2991 and 2925 Ma, by the Sholl Shear Zone, a largely sinistral shear zone, with subsequent volcanism in both domains to about 2925 Ma. The Roebourne Lithotectonic Complex (Domain 6) is interpreted to be an allochthonous terrane, which formed north-east relative to its present position, but indigenous to the Pilbara Block rather than an exotic terrane. The East Pilbara is interpreted to have acted as a cratonic hinterland during the convergent margin tectonics that affected the two West Pilbara domains.

Late Archean euxinic conditions before the rise of atmospheric oxygen

Life on Earth is thought to have coevolved with the chemistry of the oceans and atmosphere, and the shift from an anoxic to an oxic world across the Archean-Proterozoic boundary represents a fundamental step in this process. In order to understand the relative influence of biological and geological factors on this transition, we must constrain key variables in seawater chemistry before the Great Oxidation Event (ca. 2500 Ma). We present a multielement (C-S-Fe-Mo) biogeochemical study of ca. 2662 Ma shales from the Hamersley Province in Western Australia. Our data reveal a sustained episode of Fe-limited pyrite formation under an anoxic and sulfidic (euxinic) water column. This is the oldest known occurrence of euxinia in Earth9s history and challenges the paradigm of persistently Fe-rich Archean oceans. Bulk trace metal chemistry and preservation of strong mass-independent S isotope fractionations in sedimentary pyrites indicate that ocean euxinia was possible prior to oxidative weathering, suggesting that sulfidic waters may have been common throughout the Archean Eon. C-S-Fe systematics suggest that oxygenic photosynthesis was the primary source of organic carbon in the basin, and the absence of Mo enrichments highlights a potential link between inefficient nitrogen fixation and the delayed arrival of the Great Oxidation Event.

Major orogenic gold episode associated with Cordilleran-style tectonics related to the assembly of Paleoproterozoic Australia?

New in situ sensitive high-resolution ion-microprobe (SHRIMP) U-Pb analyses of hydrothermal phosphates associated with orogenic gold mineralization in the Paleoproterozoic Ashburton and Pine Creek gold provinces of northern Australia provide ages of ca. 1740 and ca. 1730 Ma, respectively. Argon-argon analyses of gold-related hydrothermal mica from the Tanami gold province of northern Australia provide ages ca. 1730 Ma. It is important to note that late orogenic events across the western half of Australia coincide with gold metallogenesis across this time interval, in several widely separated provinces. Thus, this orogenic gold episode is interpreted to relate to tectonic events during the amalgamation of various continental blocks to form Paleoproterozoic Australia. It is potentially Earths best-preserved record of orogenic gold formation during a major early Precambrian continental assembly event.

Archaean oil migration in the Witwatersrand Basin of South Africa

Abstract: The Late Archaean Witwatersrand Supergroup of South Africa hosts the largest known gold-uranium-pyrite ore deposits. Oil preserved in fluid inclusions in quartz grains in siliciclastic sedimentary rocks of that supergroup implies that hydrocarbon generation and migration occurred during the Archaean, and may have been involved in mineralization processes. Through reference to Phanerozoic analogues, oil-bearing fluid inclusions entrapped in healed microfractures in detrital quartz grains and in early syntaxial quartz-overgrowths imply, that the onset of oil migration coincided with early to intermediate stages of burial, while intra-granular porosity was still preserved. Multiple generations of oil migration are indicated by: (i) oil inclusions within early diagenetic cements at different levels in the stratigraphic succession; (ii) more than one type of oil in entrapment sites; (iii) oil entrapment in multiple stages of the quartz paragenetic sequence. Oil generation and migration are considered to have occurred throughout, and for some considerable time after, development of the Witwatersrand Basin, consistent with progressive burial and kerogen maturation in more than one tectonic regime. Oil-bearing fluid inclusions within detrital sandstone fragments suggest that oil migration also occurred in a sedimentary succession on the Kaapvaal Craton prior to 2.9u2009Ga. Oil in the Witwatersrand Supergroup was most likely derived from multiple source areas, with the principal source probably being shales within the lower Witwatersrand Supergroup. The hydrocarbon migration history of the basin has important implications for understanding the textural relationship between gold, bituminized oil and uraninite in the giant gold-uranium-pyrite ore deposits.