Download Structural and mineralogical characteristics of the Menzies

predictive mineral discovery CRC Conference
Barossa Valley 1-3 June 2004
Structural and mineralogical characteristics of
the Menzies-Boorara Shear Zone, Yilgarn Craton
– implications for variations in the gold potential
of a major fault system
Anthony Morey, Frank Bierlein and Roberto Weinberg
pmd*CRC, School of Geosciences, Monash University, Melbourne, VIC 3800
[email protected]
Abstract
Gold mineralisation from five deposits within the Menzies-Boorara Shear Zone (MBSZ), Eastern
Goldfields Province, Western Australia, is characterised by several similar features. These
features have been used to constrain the regional Au mineralisation processes.
The MBSZ is a wide zone of intense shearing constrained to a narrow NNW-trending corridor
bounded by granitic batholiths (Figure 1). Gold deposits within the MBSZ are geologically similar
to the world-class gold deposits associated with the Boulder-Lefroy Shear Zone (BLSZ),
however, significant variations also occur. These variations include: (i) the relative thickness of
the greenstone package; (ii) the presence of nearby granites; (iii) a potentially different shear
zone evolution; (iv) gold mineralisation being relatively late and (v) an apparent lack of Tebearing minerals within the mineralised assemblages analysed from the MBSZ. These
defferences may be used to help understand why the MBSZ is not as well endowed as the
BLSZ. This paper presents preliminary results from field- and laboratory-based studies and
provides some regional implications for variations in the gold endowment within the Eastern
Goldfields Province of the Yilgarn Craton.
Introduction
To date, the MBSZ zone has yielded <100 t of gold, whereas the nearby Boulder-Lefroy Shear
Zone (BLSZ) hosts the giant Golden Mile (>2500t), and two other world-class (>100 t) Au
deposits (Hagemann and Cassidy, 2001). By assessing the structural relationships,
petrogenesis and alteration characteristics of key gold deposits within the MBSZ, this project
aims to understand the nature of gold mineralisation within the MBSZ and to then discuss
genetic links between these and the more heavily endowed deposits associated with the BLSZ.
This will potentially help define target areas for further exploration within the MBSZ, and
elsewhere within the Eastern Goldfields Province.
It has been established that major lode-gold mineralisation within the Eastern Goldfield
Province, is associated with second- to third-order faults, which splay off more regional, and
lesser mineralised first-order faults and shear zones (e.g. Groves and Phillips, 1987; Eisenlohr
et al., 1989). As a >100 km-long, major crustal discontinuity, the MBSZ is a primary shear zone,
and its connecting, more-mineralised splay fault is the BLSZ. By studying the nature of gold
mineralisation from the MBSZ, it is also possible to more quantitatively understand why there is
such variation in gold endowment between primary and their higher-order connecting faults.
To characterise mineralisation within the MBSZ, five representative deposits – selected along its
entire extent – have been chosen. From north to south, these deposits are; Yunndaga, New
Boddington, Broad Arrow, Paddington and Golden Ridge (Figure 1).
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Figure 2: Photograph of Broad Arrow; a
mineralised gabbro is bounded by less competent
sedimentary rocks
Figure 1: Geological map of the southern section
of the Eastern Goldfields Province, showing the
major shear zones and rock units. Modified after
Witt (1993).
Gold deposits of the Menzies-Boorara Shear Zone – defining characteristics
Field and structural relationships
All of the deposits investigated are situated within the NNW-trending shear zone (Figure 1). This
is reflected at the mine scale as the geometry of all the major rock units dip steeply to the west
and rock contacts are controlled by NNW-trending reverse faults and associated folds and
foliations (Figures 2 and 3). Gold mineralisation is always hosted within a relatively competent,
lesser-strained rock unit - except New Boddington, where deformation has developed more
uniformly throughout all of the host rock sequences. In accordance with Witt (1993), the
relatively more competent and more mineralised units vary in rock type from: mafic basalts,
dolerites and gabbros (Yunndaga, New Boddington, Paddington and Broad Arrow), through to a
felsic porphyry intrusive/dyke at Golden Ridge. All of these mineralised units are <100m thick.
Within the bounding sedimentary rock sequences, deformation has been accommodated
through dip-slip and strike-slip faults, shears, and folded structures. A representative pit
photograph is shown in Figure 2. North to NW-trending (Yunndaga, Broad Arrow) and oblique
(New Boddington, Paddington) strike-slip faults occur, however, reverse faulting and folding with
associated dip-slip lineations dominate the structure of the highly strained bounding
sedimentary units (Figure 3). This suggests that the evolution of the MBSZ was dominated by
reverse faulting and folding (D2 of Swager, 1997) and strike-slip (D3) deformation did not
significantly occur within this shear zone.
Nature of gold mineralisation
Gold within the MBSZ is predominantly associated with structurally-controlled lodes. There are
numerous unmineralised veins at each locality but only the major Au-bearing structures are
discussed below.
Veins bearing sulphides and gold are commonly planar and post-date the major NNW-trending
shear foliation, as they cross-cut, and are not sheared by this foliation. At all localities, lode
structures are observed as repetitive vein arrays that are 10’s of metres in length. At Paddington
and Golden Ridge, steeply-dipping continuous lodes also exist, at Paddington over 1 km in
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length. At New Boddington, Paddington and Golden Ridge the repetitive lode structures dip
shallowly to moderately, whereas at Yunndaga and Broad Arrow similar structures have
moderate to steep dips.
The sulphide and gold assemblage occurs within the wall rock directly at, or proximal to the lode
- wall rock interface. The lode mineralogy consists of quartz or quartz-carbonate composite
veins. Centimetre-scale alteration haloes are also identified, and are characterised by carbonate
– chlorite – silica ± sericite ± epidote hydrothermal alteration assemblages.
Fluid infiltration related to sulphide and gold deposition is complex and evidence exists for both
singular and multiple events. Arsenopyrite precipitated relatively earlier, and was then
overprinted by gold (Figure 4). It is as yet unconfirmed if these deposition events are genetically
related, or if they represent independent influxes of hydrothermal fluids.
Figure 4: SEM backscatter image of a fractured
arsenopyrite grain, with gold infilling these fractures
Figure 3: Upright, NNW-trending fold from Yunndaga
Comparisons with the Boulder-Lefroy Shear Zone
From field- and preliminary laboratory-based studies, gold mineralisation within the MBSZ is
predominantly lode-related, and for efficient gold ore formation, Au-bearing fluids must be
localized within a relatively competent rock unit. This allows for brittle-style fracturing, dilation,
fluid focusing and Au mineralization (Weinberg et al., in press). This competent unit does not
have to be a particular rock type, as mafic to felsic igneous rocks are all potential hosts to Au
mineralisation. The importance of rheological contrasts is best exemplified by New Boddington,
where the lack of a significant contrast explains its low gold endowment (0.4 t Au; Witt, 1993).
Mineralised competent units within the MBSZ are also significantly narrower than similar
mineralised rocks associated with the BLSZ, and this could further explain the lower endowment
of the MBSZ. The need for a competency contrast, and limited width competent units within the
MBSZ, could possibly account for its variable Au endowment.
The major structures from the five gold deposits investigated imply that D2-style deformation
was the dominant regional event, and that D3 sinistral strike-slip faults were not significant within
the MBSZ. D3 deformation structures have been recognised from the BLSZ, and this implies
that both shear zones have had a different tectonic evolution.
Our investigations show that Au endowment within the MBSZ is always late in relation to
regional shear zone development, whereas the mineralisation events associated with the BLSZ
are both early (i.e. Fimiston lodes) as well as late (i.e. Mt. Charlotte-type; Bateman et al., 2001).
Moreover, Au-bearing telluride minerals are an important mineralogical association within the
Boulder-Lefroy Shear Zone (Witt, 1993), however, telluride-bearing minerals are yet to be
observed within Au deposits from the MBSZ. These results suggest differences in Au-bearing
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fluid compositions, sources and timing relationships may have affected the gold endowment of
the two shear zones.
Acknowledgements
The continuing support and input from Placer Dome, WMC, AngloGold and Harmony personnel
has been greatly appreciated. Robert Douglass and Rachelle Pierson are also thanked for thin
section and rock sample preparation.
References
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