Universität zu Köln

Kirchenbaur, Maria, Maas, Roland, Ehrig, Kathy ORCID: 0000-0002-5381-9445, Kamenetsky, Vadim S., Strub, Erik ORCID: 0000-0002-4136-2871, Ballhaus, Chris and Muenker, Carsten (2016). Uranium and Sm isotope studies of the supergiant Olympic Dam Cu-Au-U-Ag deposit, South Australia. Geochim. Cosmochim. Acta, 180. S. 15 - 33. OXFORD: PERGAMON-ELSEVIER SCIENCE LTD. ISSN 1872-9533

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Abstract

The Olympic Dam Cu-U-Au-Ag deposit in the Archean-Proterozoic Gawler Craton (South Australia) is a type example of the iron oxide-copper-gold (IOCG) spectrum of deposits and one of the largest Cu-U-Au resources known. Mineralization is hosted in a lithologically and texturally diverse, hematite-rich breccia complex developed within a granite of the 1.59 Ga Gawler Silicic Province. Emerging evidence indicates that both the breccia complex and its metal content developed over similar to 1000 Ma, responding to major tectonic events, e.g., at 1300-1100, 825 and 500 Ma. However, metal sources and exact mechanism/s of ore formation remain poorly known. New high-precision (238) U/U-235 data for a set of 40 whole rock samples representing all major lithological facies of the breccia complex show a narrow range (delta U-238(CRM112a) = -0.56 parts per thousand to +0.04 parts per thousand). At the scale of sampling, there is no correlation of delta U-238 with lithology, degree of alteration or U mineralogy, although ores with U > 5 wt.% have subtly higher delta U-238 values (-0.20 parts per thousand to 0.00) than the majority of samples (<0.7 wt.% U, -0.56 parts per thousand to -0.23 parts per thousand). The new U isotope data are consistent with published data for uraninites from Olympic Dam, and with published results from high-temperature U deposits. They overlap completely with the range of delta U-238 values in granitoids (including the host granite, -0.18 parts per thousand to -0.32 parts per thousand) and with estimates of the upper continental crust in general. This similarity suggests that Olympic Dam delta U-238 values reflects the crustal sources of U, which probably include felsic volcanic rocks and granitoids. The isotopic homogeneity suggests depositional mechanisms that involve minimal isotopic fractionation of U; alternatively, primary fractionation signatures may have been erased during the long history of the U mineralization. High-grade U ores may record isotopic neutron-capture effects related to fissionogenic neutrons. High-precision Sm isotope data for five high-U (>5 wt.% U, U/Sm >> 500) Olympic Dam ores define a neutron capture line, with correlated depletions in Sm-149 (up to similar to 2 epsilon units) and excesses in Sm-150 (up to similar to 4 epsilon units), but fission fragment contributions to Sm are below detection. These observations provide evidence for small-scale neutron-capture effects, with calculated neutron fluences of 10(15) to 10(16) n cm(-2), similar to those observed in several Proterozoic and Phanerozoic U deposits. The apparent lack of fission fragment contributions in Olympic Dam high-grade ores can be explained with an age of U deposition, or re-deposition that is substantially younger than the initial 1.59 Ga age of the oldest IOCG-style mineralization. The results presented here thus (i) suggest uranium sources in common (likely igneous) upper crustal lithologies, (ii) support geochronological evidence for gradual addition of U in several stages over 1000 Ma at elevated temperatures of mineralization, and (iii) do not show the high delta U-238 signatures expected from low-temperature reworking of older low-delta U-238 ores. (C) 2016 Elsevier Ltd. All rights reserved.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Kirchenbaur, Maria UNSPECIFIED UNSPECIFIED UNSPECIFIED Maas, Roland UNSPECIFIED UNSPECIFIED UNSPECIFIED Ehrig, Kathy UNSPECIFIED orcid.org/0000-0002-5381-9445 UNSPECIFIED Kamenetsky, Vadim S. UNSPECIFIED UNSPECIFIED UNSPECIFIED Strub, Erik UNSPECIFIED orcid.org/0000-0002-4136-2871 UNSPECIFIED Ballhaus, Chris UNSPECIFIED UNSPECIFIED UNSPECIFIED Muenker, Carsten UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-278167
DOI:	10.1016/j.gca.2016.01.035
Journal or Publication Title:	Geochim. Cosmochim. Acta
Volume:	180
Page Range:	S. 15 - 33
Date:	2016
Publisher:	PERGAMON-ELSEVIER SCIENCE LTD
Place of Publication:	OXFORD
ISSN:	1872-9533
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language OKLO NATURAL REACTOR; FISSION REACTORS; DECAY CONSTANT; ROXBY DOWNS; FRACTIONATION; IRON; CONSTRAINTS; GD; U-238/U-235; EVOLUTION Multiple languages Geochemistry & Geophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/27816