Honours Thesis Abstract
MAGMATIC DEVELOPMENT OF A MAFIC GILES COMPLEX
INTRUSION AND ITS POTENTIAL FOR MINERALISATION AT
HANGING KNOLL, MUSGRAVE BLOCK, SOUTH AUSTRALIA
Ian Fahey
University of South Australia, SA
2005 AIG Honours Bursary Winner
The gabbroic Hanging Knoll Giles Complex intrusion is located in the northwest corner
of South Australia. The outcrop can be divided into a layered zone at the base which is
separated by aeolian sediments from the upper gabbronorite zone and from the felsic
Birksgate Complex, the interpreted country rock of the intrusion. The layered zone
comprises macro to <1m gabbronorite and pyroxenite cyclic units that become wider
spaced towards the top of the layered zone. Microprobe mineral analysis demonstrates
that gabbronorite cyclic units follow a nearly constant fractionation trend that continues
into the gabbronorite zone. Pyroxenite cyclic units have a constant Mg# until two thirds
up the layered zone where Mg# begins to parallel gabbronorite as both units become
iron-rich following a tholeiitic trend. The lower half to two thirds of the pyroxenite
cyclic units are often orthopyroxene-rich and have a bimodal mineralogy with finer
grains towards the base of the unit. The top of the pyroxenite cyclic units become
clinopyroxene-rich and interstitial plagioclase increases and becomes patchy.
Orthopyroxene-rich pyroxenites represent mixing with a more fractionated resident
magma (potentially has had some felsic crustal contamination which can result in
precipitation of sulphides) which causes the melt to shift into the orthopyroxene stability
field. The more clinopyroxene-rich cyclic units represent an early fractionate of the
new enriched inflow of more mafic magma that has possibly mixed less with resident
magma. Flat and high REE may indicate these clinopyroxene-rich pyroxenites come
from a volumous melt. Whole rock geochemical data indicates an island arc tectonic
environment with enriched LILE and mantle or less than mantle values of HFSE.
Within the cyclic units there is small-scale layering that comprises modal, isomodal and
graded layering. Scour and fill structures that sometimes exist with small-scale layering
demonstrate there has been magmatic flow within the magma chamber. Magmatic flow
may be initially formed by the filling of the chamber with fountains of primitive
magma. Convection then becomes the main process which facilitates the formation of
small-scale layering analogous to the sedimentary environment.
Late stage migration of interstitial melt combined with hydrothermal fluid has formed
irregular anorthosite bodies cross cutting solid cumulates while the pile was still hot.
Disseminated chalcopyrite at the top of a gabbronorite cyclic unit is overlain by a
pyroxenite unit with higher Ni in orthopyroxene may indicate part of the system is
becoming sulphide saturated. With new injections of primitive magma there is greater
chance of mineral rich magma combining with sulphide and concentrating at the base of
pyroxenite layers.