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
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
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
.