THE IMPORTANCE OF DIFFERENT TYPES OF MAGMATISM IN VHMS MINERALIZATION- EVIDENCE FROM THE GEOCHEMISTRY OF THE HOST VOLCANIC-ROCKS TO THE BENAMBRA MASSIVE SULFIDE DEPOSITS, VICTORIA, AUSTRALIA

Citation
Aj. Stolz et al., THE IMPORTANCE OF DIFFERENT TYPES OF MAGMATISM IN VHMS MINERALIZATION- EVIDENCE FROM THE GEOCHEMISTRY OF THE HOST VOLCANIC-ROCKS TO THE BENAMBRA MASSIVE SULFIDE DEPOSITS, VICTORIA, AUSTRALIA, Mineralogy and petrology, 59(3-4), 1997, pp. 251-286
Citations number
96
Categorie Soggetti
Mineralogy,"Geochemitry & Geophysics
Journal title
ISSN journal
09300708
Volume
59
Issue
3-4
Year of publication
1997
Pages
251 - 286
Database
ISI
SICI code
0930-0708(1997)59:3-4<251:TIODTO>2.0.ZU;2-2
Abstract
The Wilga and Currawong Cu-Zn massive sulphide deposits in southeaster n Australia are hosted by a deformed sequence of Upper Silurian basalt ic to rhyolitic volcanic and sedimentary rocks. The syn-volcanic miner alisation occurs immediately above a thick package of rhyolitic volcan ic rocks and volcaniclastic rocks (Thorkidaan Volcanics), and is overl ain by relatively thin intercalated sills, intrusive domes and flows o f basalt, andesite and dacite (Gibson's Folly Formation). The Thorkida an Volcanics have epsilon(Nd(420Ma)) = - 2.2 to - 9.8 and are consider ed to have been derived by partial melting of older crustal rocks, whe reas the basalt-andesite-dacite hangingwall sequence has epsilon((Nd(4 15Ma)) = - 0.5 to + 2.0 suggesting derivation from a relatively undepl eted mantle source. Relatively high-Ti andesitic to dacitic rocks from the Bumble Creek area have epsilon(Nd(415)) = + 5.2 to + 5.9 suggesti ng affinities with Ordovician volcanic rocks elsewhere in the Lachlan Fold Belt. The Thorkidaan Volcanics display a limited silica range (73 to 79 wt.%), but have distinctive minor and trace element variations indicating a substantial fractionation history involving feldspar and several accessory phases. Major and trace element compositions of the basalt-andesite-dacite suite display regular variations consistent wit h a cogenetic relationship by fractional crystallisation. The basaltic rocks mostly have low TiO2 (< 0.8 wt.%) and other chemical characteri stics such as high Zr/Nb and La/Nb which suggest formation in a subduc tion-related setting; probably an embryonic back-are basin developed o n stretched continental lithosphere, or in small pull-apart basins dev eloped adjacent to a transtensional margin. The magmatic history and p aleogeography reflect an extensional tectonic and magmatic cycle compr ising uplift, rhyolitic magmatism from crustal melting, extension, sub sidence, and penetration of a mantle-derived basalt-andesite-dacite su ite up extensional faults to the sea floor. Massive sulphide ores are located exactly at the stratigraphic change from rhyolitic to more maf ic mantle-derived magma types. Consideration of the types of mineralis ation associated with crustal, S-type granitoids, coupled with thermal constraints limiting the capacity of small bodies of silicic magma to initiate and sustain hydrothermal convection cells of reasonable size , suggests that in the absence of coeval mafic magmatism, S-type crust al-derived silicic volcanic packages are likely to be barren of VHMS d eposits. Mineralisation occurs in association with mantle-derived basa lt-andesite-dacite suites that either provide the necessary heat to fa cilitate leaching of the footwall volcanic rocks, or contribute metal- rich hydrothermal solutions during fractional crystallisation, or both .