HR: 14:20h
AN: V33F-03    [Abstracts]
TI: Higher Melt Fractions are Generated in Metamorphosed Hydrothermally Altered Rocks: Significance to Ore Genesis
AU: * Tomkins, A G
EM: Andy.Tomkins@sci.monash.edu.au
AF: School of Geosciences Monash University, P.O. Box 28E, Melbourne, VIC 3800, Australia
AU: Weinberg, R F
EM: Roberto.Weinberg@sci.monash.edu.au
AF: School of Geosciences Monash University, P.O. Box 28E, Melbourne, VIC 3800, Australia
AU: McFarlane, C R
EM: Chris.McFarlane@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
AB: Felsic magma-related ore deposits are Earth's largest source of Cu and Mo, and a major source of Au. Despite this, there have been few field-based studies of processes that affect metal distribution in magma source regions. Based on geochemical studies at ore deposits, it has been suggested that in magmatic arcs, magmas and metal may be derived from subducted oceanic slab, overlying metasomatized mantle wedge, or from the lower crust during anatexis. Intrusion-related Au systems form distally to arcs and are typically associated with reduced granitic magmas thought to be largely derived from partial melting of crustal rocks, casting into question the absolute need for oceanic slab or mantle involvement. Even within arcs, evolution of ore producing magmas may involve mixing between mantle- and crust-derived magmas. It is therefore crucial to understand processes that influence the metal content of these crust-derived magmas. We have compared melting of alteration zones in metamorphosed gold deposits with that of unaltered rocks of the same protolith using the computer program THERMOCALC to examine the relative contributions of metal-rich and metal-poor source regions to crust-derived magmas. Potassium enrichment, caused by pre-metamorphic hydrothermal alteration, is found to stabilize a higher proportion of muscovite and biotite at high metamorphic grades than is possible in unaltered rocks. Because these micas contain water, they control the melt fraction generated through dehydration melting. Therefore, in the vicinity of metamorphosed gold mineralization, the greatest volume of felsic melt is generated in the more metal- and sulfur-rich rocks. Sulfides and Au dissolve, and are physically incorporated into the resulting felsic melt, which thereby becomes metal- and S-enriched. Since the presence of melt significantly weakens rocks, strain preferentially partitions into these melt-rich mineralized zones promoting melt segregation there. As strain increases, high-melt domains become shear zones that expand and link up with other incipient shear zones, forming low pressure channelways for migrating melts. In this way, mineralized zones become part of a regional network of magma pathways through the crust, capable of continuously enriching migrating magmas in metal.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1037 Magma genesis and partial melting (3619)
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting