Abstract

The Lake Harris Komatiite in the central Gawler craton of South Australia is the first documented komatiite outside the West Australian craton and the easternmost occurrence of such primitive ultramafic rocks in Australia. A U-Pb zircon age of ca. 2520 Ga for the komatiitic sequence indicates a previously unknown period of mantle-plume activity in the Late Archean. An integrated program of airborne magnetic surveys, gravity surveys, and core drilling was successful in defining the distribution and volcanic architecture of the komatiitic flows and associated greenstones through an extensive thin cover of Cenozoic alluvial sediments. Surface exposure of the komatiitic rocks is restricted to one small outcrop near Lake Harris. The greenstones form a series of subparallel east-northeast-trending sinuous magnetic highs flanked by large ovoid to elongate magnetic highs and lows that correlate with Archean-Proterozoic granitic bodies associated with province-wide shear systems, similar to the Archean greenstone terranes in the Yilgarn craton of Western Australia. The steeply dipping greenstone sequence was metamorphosed to middle amphibolite facies during the ca. 2440 Ma Sleafordian orogeny and sheared during the ca. 1700 Ma Kimban and ca. 1540 Ma Kararan orogenies. The greenstonesconsist of komatiite cumulates (43-32% MgO, anhydrous), high to low Mgkomatiite (32-18% MgO), komatiitic and tholeiitic basalt (<18% MgO), pyroxenite cumulates, felsic volcanic rocks, minor metasedimentary rocks, pyroclastic rocks, and rare banded iron formation. They extend over 300 km in three subparallel belts that appear to be isoclinally folded around east-northeast axes and tectonically dismembered to the south by the Yerda shearzone. Komatiitic rocks have been confirmed by drilling in all three belts, but the absence of outcrop and structural complexities prevent detailed stratigraphic correlations within and between the belts. The komatiitic rocks display a range of quenched and cumulus textures definedby the different habits of olivine and its alteration products. Trace sulfides (pyrrhotite, chalcopyrite, pentlandite, pyrite, marcasite, polydymite violarite, heazelwoodite, millerite) form very small (0.01-0.2 mm) single-phase disseminated grains and coarser disaggregated grains. Their distribution largely reflects metamorphic and serpentinization processes, with high Ni/S ratios andprobable sulfur loss from the more magnesian parts of the flows. Rare compositepyrrhotite-pentlandite-chalcopyrite blebs (0.1-0.5 mm) characterize some lowMg flows. Locally, there is supergene pyrite-marcasite and nativecopper-bornite-chalcocite(?) assemblage infilling of late low-temperatureserpentine-chlorite veinlets. Thick ponded lava lake and distal composite sheet flow facies have been identified from different parts of the komatiitic sequences. Systematic whole-rock and mineral chemical trends indicate that despite the effects ofrecrystallization and reequilibration during amphibolite-facies metamorphism, the original magmatic geochemical profiles are largely preserved. The whole-rockdata for the Lake Harris Komatiite does not show any obvious Ni depletion duringfractionation but indicate a strong olivine control in dominantly sulfur undersaturated environments. Low sulfur (100-600 ppm S) and high Pd + Pt (5-30ppb) contents, and Ti/Pd ratios of 2 to 4 x 105 for the komatiiticrocks are similar to sulfur-undersaturated Archean komatiites hosting Ni-Cu-PGE deposits, i.e., there is little evidence for sulfur saturation in the sampled komatiites. Identification of a pre-2.5 Ga source of sulfur in the substrate would be a positive indicator of potential sulfur saturation of the lavas elsewhere in the greenstone belt and a possible target for mineralization.
Google map showing geographic bounding box with values North bound -29.0 East bound 138.0 West bound 133.0 South bound -34.0
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Product Type/Sub Type

dataset - External Publication - Scientific Journal Paper

Constraints

license
Creative Commons Attribution 3.0 Australia Licence

IP Owner

Commonwealth of Australia (Geoscience Australia)

Author(s)

Date (publication)

2005

Product Type

dataset

Topic Category

geoscientificInformation

Keywords

External Publication
Scientific Journal Paper
Earth Sciences

Resource Language

English

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utf8

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unclassified

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-29.0
East bound
138.0
West bound
133.0
South bound
-34.0

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Unknown

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Organisation Name
Geoscience Australia
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Canberra
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Australia
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a05f7892-fa9f-7506-e044-00144fdd4fa6

Metadata Standard Name

ANZLIC Metadata Profile: An Australian/New Zealand Profile of AS/NZS ISO 19115:2005, Geographic information - Metadata

Metadata Standard Version

1.1

Metadata Date Stamp

2010-11-16

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unclassified

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pointOfContact
Organisation Name
Geoscience Australia
City
Canberra
Administrative Area
ACT
Postal Code
2601
Country
Australia
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Related Links
For information on acquiring this product,
please contact the Geoscience Australia Sales Centre via:

fax:
+61 2 6249 9960; or
phone:
1800 800 173 (within Australia);
 
+61 2 6249 9966 (outside Australia).

Please note that support hours are 9 am to 5 pm weekdays