Updated: 07 September 2005

Interactions between Crust and Mantle - Abstract 3

The Metallogenic Potential of Australian Proterozoic Granites, Record 2001/12

Lesley A.I. Wyborn, Murray S. Hazell, Laigee M. Bell, Suzanne M. Edgecombe

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Interactions through time and the relationship to Ore deposit types
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Interactions between Crust and Mantle through time and the relationship to the evolution of Australian Ore deposit types

Compilations of the distribution of ore deposits types in time clearly show them to form three broad groups that are coincident with the Archaean, Proterozoic and Phanerozoic eons (e.g., Meyer 1981). As many deposits are related to the intrusion/ extrusion of igneous rocks, the changes in dominant ore deposit types appear to parallel temporal changes in the dominant type of igneous rocks in response to mantle cooling. Some examples are:

The Archaean mantle was at least 100°C hotter than the present mantle as is reflected in the abundance of komatiites derived from deeper parts of the mantle. Hence the Archaean dominance of komatiite-hosted Ni.
The Archaean lacks the significant volumes of the high-T, oxidised I-(granodioritic) types that are usually associated with Cu/Au mineralisation in the Proterozoic. Thus ironstone hosted Cu-Au deposits are absent.
Some early Palaeoproterozoic mafic igneous rocks are Mg and Si-enriched, suggesting that mantle melts remained hotter than at present and were derived from shallower levels than in the Archaean. These compositions would promote the formation of layered intrusions within the crust (with associated deposits of Ni, Cr, Pt) and would also result in underplating of the crust which is so prevalent in the Proterozoic.
Late Archaean/Palaeoproterozoic granites have higher K, Th and U values than granites of other ages. (However, an oxygenated atmosphere is required to free U allowing unconformity-style U deposits to form.)
Widespread underplating in the Proterozoic could result in more effective conductive heating of the lower crust, whilst the high K, Th and U values would increase radiogenic heat outputs. Both factors would cause higher crustal geothermal gradients, and therefore high temperatures at shallow crustal levels, which in turn would facilitate the generation of the large size of some Proterozoic hydrothermal deposits.
The physical size of igneous suites is generally larger in the Precambrian than in the Phanerozoic, suggesting also that the related thermal anomalies that influenced basin formation were vast. In the Palaeoproterozoic some basin formation resulted in extensive sag phases characterised by evaporitic sequences that were an important source of ligands (such as Cl^-) for transport of U and base metals.
Australian Proterozoic VMS are rare as fractionated volcanics rarely coincide with subaqueous sediments.
I-(granodioritic) types dominate Australian Proterozoic/early Palaeozoic felsic melts and the majority are Sr-depleted, Y-undepleted, possibly signifying high crustal geothermal gradients. Related deposits are mostly hosted in country rock, possibly because the granites are too felsic and/or intrusions are too deep.
S-type granites or I-types that fractionate to more peraluminous compositions are more prominent in the Palaeozoic, this may explain the greater abundance of Sn mineralisation during this era in Australia.
Magma types change notably in the Phanerozoic and more are clearly related to activity at plate margins.
Genuine shoshonites with associated Cu/Au mineralisation made their first appearance in the Ordovician.
Porphyry-style mineralisation is more common in the Phanerozoic, where it is hosted within I-(tonalitic) or M-types above subduction zones. Most melts are Sr-undepleted signifying lower geothermal gradients.
Ophiolite-related deposits (Cu-pyrite deposits, podiform Cr deposits) are more common in the Phanerozoic.
Thus the changing thermal regime of the crust and mantle through time exerts a significant control on the dominance of different magma and ore deposit types through time. Although the Archaean was characterised by highest mantle temperatures, crustal geothermal gradients were probably lower possibly due to a thicker lithosphere. Mantle temperatures were intermediate in the Proterozoic, whereas crustal geothermal gradients may have been higher perhaps due to lithospheric thinning, thus allowing mantle melting at shallower levels than in the Archaean. Because of the hotter thermal structure of the continental lithosphere in the Precambrian, deformation was substantially less partitioned into narrowly defined plate boundaries than at present (Etheridge and Wall 1994). Hence in the Precambrian intraplate deformation was more widespread resulting in more opportunities for fluid migration (and hence some larger ore deposits!). The Phanerozoic represents a transition to a dominance of igneous activity and deformation at plate margins driven by a cooler mantle, explaining the increase of ore deposits that characterise the plate margins, viz., porphyry Au/Cu and ophiolite-related deposits.

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References

Etheridge, M.A. and Wall, V.J. 1994. Tectonic and structural evolution of the Australian Proterozoic. Geological Society of Australia, Abstracts, 37, 102-103.

Meyer, C. 1981. Ore-forming processes in geologic history. Economic Geology, 75th Anniversary Volume, 6-41.