Deep signatures of a world-class archaean gold system
|Location||International Geological Congress,oslo 2008|
|Author||Blewett, Richard; Goleby, Bruce; Henson, Paul; Champion, David; Roy, Indrajit|
|Holding Date||17 September 2008|
Our knowledge of the crustal architecture and therefore deep signatures of the world-class gold and nickel mineral systems of the Yilgarn Craton has increased greatly over the last decade. Much of this advance is due to the increased collection of a wide range of geophysical and geochemical/isotopic datasets that provide different views of the varying crustal components, and the nature of the boundaries between them. These data range in scales from:
1. lithospheric-scale studies — using distant earthquakes as sources to obtain information on the entire craton down to depths in excess of 350 km, through;
2. regional-scale studies — providing information at the province scale and down to depths of 30-40km.
Receiver Function and broad-band teleseismic results map significant variations in crustal and upper mantle velocities across the Yilgarn Craton. The results are also providing information on potential velocity differences between the mineralized Kalgoorlie Terrane and the less mineralized Youanmi Terrane. An anomalous high velocity body (>4.8 km/s shear wave) occurs at around 100-120 km depth, and has steps and edges that correspond with mantle-derived magmas and the main gold camps. The fast velocity layer may represent mantle alteration, delaminated eclogite layer, or fossil oceanic slab.
Deep crustal seismic reflection studies have provided excellent 2D crustal architecture information in the eastern part of the craton, which, in turn, have provided information on the mineral systems of the Eastern Yilgarn. These seismic data have imaged numerous shear zones and are critical in defining the gross 3D architecture of the crust.
Maps of "crustal age" across the craton have been derived from ~250 Sm-Nd isotopic analyses of granites of all ages (~3.0 to 2.6 Ga). The granites have a significant to dominant crustal component as evidenced by Nd model ages being 200-500 Ma older than crystallisation ages. Importantly, the Nd model age map constructed from the isotopic data shows strong domainal zonations, which appear to reflect crustal age distribution within the Yilgarn Craton. Major changes in the isotopic domains correspond closely with known terrane boundaries in the Yilgarn Craton and, these, and more subtle inter-terrane isotopic variations, are being used as a first-order area selection tool for gold, nickel and base metal exploration.
Magnetotelluric (MT) data also map the main boundaries of the Kalgoorlie Terrane. These data reveal that the prospective Kalgoorlie Terrane is special (in MT space), and is characterised by a highly conductive (graphite-bearing?) upper crust (along strike from the Super Pit) and a dome or "upwelling" of conductive mantle beneath.
Together, these data provide first-order tools for understanding the physical and chemical properties of the components of the Yilgarn Craton. These insights provide first-order architectural understanding, geodynamic clues, and constraints on why the Yilgarn is so well endowed.