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Geochemical characterisation of the Golden Mile stratigraphy and

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Geochemical characterisation of the Golden Mile stratigraphy and

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									predictive mineral discovery CRC Conference                                Barossa Valley 1-3 June 2004




Geochemical characterisation of the Golden Mile
 stratigraphy and implications for the structural
                   architecture
               Louis Gauthier1, Steffen Hagemann1 and Francois Robert2
1
 pmd*CRC, Centre for Global Metallogeny, School of Earth and Geographical Sciences,
                      University of Western Australia, Crawley, WA 6009
                                   2
                                    Barrick Gold Corporation
                                    gauth101@segs.uwa.edu.au



A synthesis of an extensive major and trace element geochemical database allows a better
understanding of the internal stratigraphy of the Paringa Basalt and Golden Mile Dolerite, which
host the Golden Mile deposit. A better comprehension of the host stratigraphy is a critical
element to define the structural architecture of the Golden Mile Deposit. Major and trace
element geochemistry of the Paringa Basalt and Golden Mile Dolerite are used in three different
ways to define the structural Architecture: (1) Consistent geochemical fractionation trends within
the Paringa Basalt and individual units of the layered Golden Mile Dolerite are used as younging
indicators, (2) Major faults are documented by offsets in the fractionation trend of the Paringa
Basalt, and (3) Early syn-volcanic faults are interpreted by abrupt changes in thickness and
composition of the Paringa Basalt and Golden Mile Dolerite.

The Paringa Basalt sequence consists of a 400 to 700m thick accumulation of basaltic flows
grading from a high magnesium basalt (>10wt% MgO, anhydrous basis) with ubiquitous
variolitic and local spinifex textures at the base, to a gradually more fractionated tholeiitic basalt
(3 to 10 wt% MgO). The upper portion of the Paringa Basalt is characterised by pillow and flow
breccia textures with a general increase of interflow sedimentary rocks. The Paringa Basalt
displays a gradual fractionation trend with increasingly more evolved compositions towards the
stratigraphic top, characterised by increasing Zr, TiO2, Al2O3, V, Fe2O3 and decreasing MgO, Ni,
Cr. The top 50 to 100m section of the Paringa Basalt sequence consists of a high iron tholeiite
in sharp contact with the underlying Tholeiitic Basalt. It is characterised by a high content of
TiO2 (1.6 to 1.8 wt%), Fe2O3 (14 to 16 wt%), Zr (100 to 130 ppm), P2O5 (0.2 wt%) and low MgO
(4 to 5 wt%). The High Iron Tholeiite also displays a normal fractionation trend.

The Golden Mile Dolerite (GMD) is a differentiated layered gabbroic sill of approximately 700m
in thickness. The intrusion has been subdivided in 10 units based on petrographic and
geochemical characteristics. Unit 1 and 10 form the basal and upper chilled margins,
respectively. Unit 2 and 3, the basal cumulate units, display very high Cr, Ni, MgO with a
decreasing trend upward into the intrusion. Unit 4 and 5 consist of medium grained sub-ophitic
textured gabbro with a generally flat geochemical pattern. Units 6, 7 and 8 are the magnetite-
rich units (10-15 wt% magnetite). Unit 6 displays very strong enrichment in elements with strong
partition coefficient into magnetite, such as V, Cr, Ni and Cu. These elements are in turn
strongly depleted in units 7 and 8. Units 6 and 7 display gradual enrichment trends in Zr, TiO2
and Fe2O3 towards Unit 8. Unit 8 the granophyric unit, is characterised by high SiO2, Zr, TiO2,
P2O5 and Fe2O3. Unit 9 displays a gradual fractionation trend characterised by higher Zr, V, TiO2
and lower Cr, Ni, MgO towards the contact with Unit 8.

The GMD displays important lateral variations in terms of thickness, internal magmatic layering
and geochemistry. The GMD in the east-dipping limb of the upright NW trending Kalgoorlie

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predictive mineral discovery CRC Conference                                Barossa Valley 1-3 June 2004



Anticline is significantly thinner (200m) than the west-dipping limb (700m) and displays a
gradual fractionation trend without the formation of magmatic layering. The fractionation trend
indicates that the dolerite is eastward younging, which is consistent with the interpretation of this
thinner dolerite being on the east-dipping limb of the Kalgoorlie Anticline. South of the Eastern
Lode System, the GMD is slightly thinner (500m) and displays less well developed internal
magmatic layering with significantly less abundant magnetite in Units 6, 7 and 8. This transition
is gradual and not associated with the Adelaide Fault.




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