boddington industry project, yilgarn craton, w.a

7/28/2019 Boddington Industry Project, Yilgarn Craton, W.A.

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Boddington

Industry

Project, Yilgarn

Craton, W.A.

by

Dale Cameron

a1219014

Mineral Geoscience Project


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ABSTRACT

The Boddington gold mine, situated in the Saddleback greenstone belt, Yilgarn Craton, W.A., is a geologically

complex and highly varied deposit. A variety of genetic models have been invoked in the past to explain the

genesis of the deposit and features observed, including porphyry- and orogenic- models, as well as more recently,

an intrusion-related gold system. Mineralisation occurs as veins, veinlets, shears, lenses and disseminations with

host rocks of diorite, andesite and dacites. Veins and alteration are pervasive and consist of multiple stages of

quartz-sericite, quartz-biotite, quartz-albite and actinolite alteration. Detailed ore mineralogical, petrographic and

mineral-chemical study of representative ore samples from five of the eight domains within the deposit have given

insights into the distribution of precious metals and also provided evidence for the formation of the Boddington

deposit and provide evidence for its genetic evolution. Mineralisation is characterised by a reduced assemblage,

with chalcopyrite and pyrrhotite as the dominant sulphides. Pyrite (often replacing pyrrhotite), sphalerite,

cubanite, cobaltite, arsenopyrite and pentlandite are minor sulphides. Molybdenite is also relatively abundant and

occurs as a major mineral in localised areas throughout the deposit. The study has shown that the deposit also

contains an extremely diverse array of trace minerals that can provide supporting evidence for aspects of ore

genesis. Native gold and electrum are the main gold minerals; LA-ICPMS analysis of pyrite and arsenopyrite

revealed that these minerals are not significant Au-carriers at Boddington. In addition to maldonite, the deposit

contains a suite of Bi-minerals, including native bismuth and a suite of Bi, Bi-Ag, Ag- and Pb-tellurides and

selenides. These minerals are identified both in ore samples and in Cu-concentrates.

Microprobe analysis of Bi-chalcogenides of the tetradymite group shows compositions from across the full range of

the series, demonstrating the multiphase character of the Boddington mineralisation and, specifically, (often

incomplete) overprinting by more oxidising fluids. This dataset also includes several previously unreported and

non-stoichiometric compositions of tetradymite group phases; these may represent unnamed phases, but may also

be disordered at the lattice-scale.

Boddington is clearly not the product of a single ore-forming event but is rather a multiphase system recording

successive overprinting and replacement of minerals, often very localised, and displaying strong lithological

control.


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TABLE OF CONTENTS

CHAPTER 1 INTRODUCTION 3

CHAPTER 2 GEOLOGICAL SETTING 4

REGIONAL GEOLOGY 4

MINERALIZATION 5

STRUCTURAL HISTORY 5

CHAPTER 3 METHODOLOGY 6

OPTICAL MICROSCOPY (UNIVERSITY) 6

OPTICAL MICROSCOPY (ADELAIDE MICROSCOPY) 6SCANNING ELECTRON MICROSCOPY (ADELAIDE MICROSCOPY) 6

CHAPTER 4 PETROGRAPHY RESULTS 7

GANGUE 8

MINERALIZATION 9

OTHER MINERALS 11

CHAPTER 5 - SULPHIDE MINERAL CHEMISTRY 12

CHAPTER 6 LA-ICP-MS MAPPING OF MOLYBDENITE 12

CHAPTER 7 TETRADYMITE GROUP MINERALS 13

CHAPTER 8 DISCUSSION 14

CHAPTER 9 CONCLUSION 15

BIBLIOGRAPHY 18

APPENDIXES 20


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INTRODUCTION

The Boddington Gold deposit, is economically one of Australias largest gold deposits. The world-class Boddington

Cu-Au-Mo deposit has a complex genetic history. The deposit was originally discovered in 1980. The relative

importance of different ore-forming processes during the period 3.0 2.6 Ga is debated, particularly with respect to

the role played by the 26113 Ma Wourahming granite. LA-ICP-MS analysis of trace element concentration in

molybdenite represents a valuable new metallogenetic tool to track mineralising events in deposits with protracted

geologic histories. The Re content and trace-element signatures in molybdenite from diorite and granite show three

distinct populations, attributed to porphyry-style (hundreds of ppm) orogenic- and granite-related systems (


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GEOLOGICAL SETTING

Regional Geology

The regional geology of the Boddington gold mine is situated in the Yilgarn Craton. The Yilgarn Craton iscomposed of volcanic and sedimentary rocks that formed between 3000 and 2600 Ma and are metamorphosed to

upper greenschist/amphibolites facies during an event at 2649 Ma (Nemchin et al. 1993). The Yilgarn Craton

formation scripts a major chapter of tectonic activity on Earth and is interpreted to have fashioned from the

accumulation of a number of crustal terrains including volcanic arcs, back arc basins and micro continents with

ages from 3730 to 2550 Ma (Myers 1993). The period from 2760 to 2620 Ma in the late Archaean marks the

formation of the majority of the greenstone belts in the Yilgarn Craton of W.A. (Nelson 1998). The formation of the

Greenstone belts has been suggested to have formed in ensialic rift environments (Groves et al. 1987). Though,

new information gathered has swayed geologists to recognize the formation of greenstone belts (within the South-

western are of the Yilgarn) to be associated with subduction manners. The chemistry of the Saddleback greenstone

belt (SGB) indicates that the belt formed in an island arc setting (McCuaig et al. 2001). The

Saddleback greenstone belt is an isolated fault-bounded cut of volcanic and intrusive rocks surrounded by

granitoids and gneisses of the Western Gneiss Terrane. The Saddleback Greenstone belt extends for approximately

50km along strike in a north-northwest orientation and has a maximum width of 8 km (Allibone et all., 1998).

The Boddington gold deposit is located within the Archaean SGB, most of the deposit lies in the northern region

within the Yilgarn Block (fig. 1). The deposit is within a considerably faulted, steep-dipping sequence of

metamorphosed pelites, felsic and mafic extrusive and intrusive rocks. The metamorphosed rocks are greenschist

to lower amphibolite facies that have been extensively faulted (Anand 2005).

The primary mineralisation is hosted in 2715-2690 Ma intermediate to felsic intrusive and volcanic rocks diorite,

andesite and dacite. It is a low sulphide gold deposit with an Au-Cu-Mo-W-Bi association (Symons et al. 1990;

McCuaig et al. 2001). Mineralisation at Boddington occurs as shear zones, brittle ductile faulting, veins and

reactivated veins, veinlets, lenses and disseminated ores.


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Mineralization

The primary mineralization, Au is hosted in intermediate to felsic volcanic rocks. It is a low-sulphide system with

intense silification, potassic and calc-silicate alteration. It also includes an association with Au-Cu-Mo-W- (Bi)

(Symmons et al., 1988). Regolith samples indicate that the Au is located in the lower part of the bauxite zone, which

is the middle of the clay saprolite and the base of the saprolite. The major components of the Cu-Mo mineralisation

at Boddington are chalcopyrite, pyrrhotite and molybdenite. Other sulphides identified here are cubanite, pyrite,

sphalerite, stannite, pentlandite, argentopentlandite, a Fe-Ni-thiospinel, Co-mackinawite, bornite, chalcocite and

covellite; native copper was also found in one of the samples with zeolite alteration (RG67). The Au mineralisation

at Boddington is characterised by an Au-Ag-Bi- (Pb)-Te- (Se) mineral association which includes native gold,

electrum, maldonite (Au2Bi), native bismuth, hessite (Ag2Te), altaite (PbTe), intermediate members of the galena-

clausthalite series (PbS-PbSe), naummanite (Ag2Se), abundant Bi-tellurides from the tetradymite group (Cook et al.

2007a) and subordinate Bi-Pb-sulphotellurides from the aleksite series (Cook et al. 2007b).

Structural History

A large portion of Western Australia is composed of Archaean metavolcanic and metasedimentary rocks initially

formed at 3050-2600 Ma (Myers et al. 1993, Duuring et al. 2007). These rocks were originally part of volcanic arcs,

back arc basins and micro continents that amalgamated during a time of tectonic movement and formed the

Yilgarn Craton (Myers 1993). Crustal fragments were aligned and became joined during a period of increased

tectonic activity dated at 2780-2630 Ma (Myers 1993). A significant section of the Yilgarn Craton is comprised of

granitoids and associated greenstone belts that underwent metamorphism. All greenstone belts in W.A. contain

packages of igneous and sedimentary rocks that display similar lithologies and broad-scale structural controls.

Greenstone belts are widely associated with metallic mineral deposits, since the latter form in volcanic arc and back

arc basin environments where faulting can occur (Barley et al. 1990).

The series of deformational events at Boddington is hotly debated. Allibone et al. (1998) propose seven distinct

post-mineralisation deformational events. In this scheme, molybdenite and chalcopyrite veins formed between

2714-2696 Ma before the first deformational event. Widespread metamorphism led to a quartzoligoclasealbite-

clinozoisite-muscovite-biotitechlorite mineralogy and well-defined rock fabric. Sericite-quartz alteration then took

place within shear zones after peak metamorphism. D3 shears dated at post-2675 Ma hold quartz-albite-epidote

alteration and cut through the previous two sets. Dikes were intruded along D4 faults introducing actinolite-


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bearing veins and biotite-clinozoisiteactinolite alteration halos. Further movement on the D4 faults introduced

superimposed mineralisation.

METHODOLOGY

In order to determine the microscopic mineralogy of the Boddington ore body, several different methods of

observation and analysis were undertaken. These included optical microscopy, both in the classroom and using the

technologically advanced microscopes courtesy of Adelaide Microscopy, and Scanning Electron Microscope

analysis (SEM), also within Adelaide Microscopy.

OPTICAL MICROSCOPY (ADELAIDE UNIVERSITY)

Nikon reflective light microscopes were used to examine the microscopic optical properties of polished section

samples. These are equipped with varying magnification options of 2.5x, 5x, 10x and 20x zoom. These microscopes

were used prior to the other methods of microscopic mineral observation as they were designed to give an

overview of what was within the samples rather than for close examination or analysis. Polished sections

approximately 3cm in diameter and 1cm thick were observed. These were derived from samples taken from

various locations around the Prominent Hill mine site, including the open pit.

OPTICAL MICROSCOPY (ADELAIDE MICROSCOPY)

Optical analysis at Adelaide Microscopy was undertaken using a similar Nikon reflective light microscope to those

used at Adelaide University, however these were also equipped with a digital camera and 2.5x, 5x, 10x, 20x and

50x air objectives. These microscopes were used to enhance what was viewed under the microscopes at Adelaide

University due to their higher quality imaging and elevated zooming options. The same polished sections were

used to study the microscopic mineralogy and the digital camera attached to the microscope allowed photographs

of features to be taken and thus examined in closer detail.

SCANNING ELECTRON MICROSCOPY (SEM)

The SEM was performed at Adelaide Microscopy using the Philips XL30 FEG-SEM equipped with an Energy

Dispersive X-ray Spectrometer (EDS). The instrument was operated in back-scatter electron mode at a voltage of


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20eV and ~20 na beam current, using a spot size of 3 or 4. A small sample is placed within the machine to be

analysed. Through this method of examination, the minerals contained within a sample can be identified at an

elemental level. Graphs showing the proportion of elements contained within a spot are produced which helps to

positively identify the minerals. Still images were taken so further analysis could be undertaken.

RESULTS

The sample chosen for the microscopy sections of the practical was sample RG43, this sample was selected because

of the high amount of containing molybdenite, which is a key aspect in the mineralization in the Boddington

region.

The reflected light microscopy results gave vital information on the hot spots and hints on where to find economic

mineralization. At Boddington the mineralisation occurs as disseminations and veinlets throughout several types

of magmatic rocks, including diorite, andesite, rhyodacite, dolerite and granite, all with various degrees of

alteration. All these rocks are deformed and metamorphosed at lower amphibolite facies (biotite-plagioclase

present). Sample RG43 showed heavily deformed molybdenite veinlets surrounded by dark gangue. Most

commonly, the sulphides occur as narrow veinlets (mm-cm size) or as patches and disseminations, a great example

of this is image 1. This image contains deformed molybdenite with chalcopyrite growth in the lamellae of the

molybdenite, it is also backed up in image 2 at a broader scale. Chalcopyrite is often associated with pyrrhotite

and in some cases with molybdenite, an example of this is shown in image 5 with the chalcopyrite and pyrrhotite

intergrowing against the gangue. The latter however is often found in monomineralic patches of cm- to mm-size.

The molybdenite in the granite shows strenuous deformation in the lamellae and is best explained in image 10 and

image 3. The molybdenite in image 10 is completely deformed with no additional mineralogy, the differing in

colour is only due to the local twinning of the sample and should not be mistaken as gangue; however in image 3

there is a tiny amount of gangue (quartz) present. The local gangue of the granite is comprised mostly from

quartz, rutile, epidote (Allanite), feldspars and sphalerite, biotite. All these minerals are found in both granite and

diorite samples. Also common in the two types of rocks is the presence of sphalerite, a dark grey mineral in the

RLM that occurs as small grains where the latter occurs along boundaries mostly between the pyrrhotite and

chalcopyrite.


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The scanning electron microscope was a useful piece of equipment as it presented sub-accurate chemical

compositions of the selected areas in the sample (RG 43). The results showed that what was interpreted in the

reflected light microscopy but other additions in smaller parts of the sample were discovered.

GANGUE:

ALBITE: NaAlSi3O8 or Na1.00.9Ca0.0

The albite in sample RG43 interplayed with the quartz grains amongst the granite. They were commonly found in

veinlets and alterations of quartz-albite.

BIOTITE: K(Mg,Fe)3(AlSi3O10)(F,OH)2

The data for biotite in the granitic sample, as well as the diorite shows minute differences around an intermediate

composition between the Fe-rich (annite) and Mg-rich (phlogopite) end-members. A slightly Fe-richer variety is

present in a granite sample

EPIDOTE GROUP MINERALS (ALLANITE): (Ce,Ca,Y,La)2(Al,Fe+3

)3(SiO4)3(OH)

Data for minerals from the epidote group show the presence of rare earth elements, elements such as Europium

(Eu), Thulium (Tm), Cerium (Ce), Praseodyium (Pr) and Promethium (Pm). These rare earth elements distinguish

the Epidote mineral into Allanite, more specifically an allanite-(Ce), due to the high content of Cerium present

(4.15 Mol%). The epidote was dominantly composed of Silicon Dioxide (51.31 Mol%), Calcium Oxide (16.13 Mol%)

and Aluminium Oxide (13.32 Mol%).

K-FELDSPAR: NaAlSi3O8 CaAl2Si2O8

Plagioclase feldspars from the granite are Na-rich with a narrow compositional variation within the albite.

Likewise, the potassium feldspar is nearby to end-member K-fs presenting a slight difference in all samples. The

compositional spread for the plagioclase feldspars in the diorite is also in the albite-oligoclase range but unlike in

the granite, this variation is observed in each of the analysed samples. The feldspars in sample RG43 grew as a


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disseminated grain in a host of other gangue minerals, mostly quartz. It had no link to the mineralisation of

sulphides.

RUTILE: TiO2

The rutile in sample RG43 was very disseminated. The mineral was found amongst the quartz filled gangue

background and look as the quartz was almost eating away at the rutile, perhaps a sharing of the Oxygen element

in both groups is applicable.

QUARTZ: SiO4

The Quartz created the background of the gangue network and was dominantly the main gangue mineral in

distribution. As this sample (RG 43) is contained within the granite, a granitic composition is obviously noted, eg.

Quartz, biotite, k-feldspar dominant.

MINERALISATION:

MOLYBDENITE: MoS2

The Molybdenite in sample RG43 contains a strange molecular ratio, the usual ratio for molybdenite is Mo=1, S=2,

however in the samples taken during the scanning electron microscopy, the formula was M= 0.6553, S=0.355.

The Molybdenite is the main attraction point of sample RG 43; the presence of molybdenite in the granite is quite

abundant. It is largely associated with the chalcopyrite and pyrrhotite. It displays great morphological variety from

bundles and knots to lamellar aggregates along the foliation or single lamella scattered in the surroundings of the

larger patches. It is mostly coarse in grain texture and is quite heavily deformed in sections (see fig. 10 in RFM).

Great twinning is subdue to the molybdenite and can sometimes be mistaken for gangue if care is not taken.

In molybdenite from both granite and diorite Bi-tellurides from the tetradymite group are abundant, galena and

hessite are also noted. Phases from the aleksite series are present in the granite whereas chalcopyrite, native gold,

electrum, native bismuth and minor altaite are seen in the diorite.


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The molybdenite in the granite displays regular, dense fields of telluride inclusions. In the diorite, however,

chalcopyrite inclusions are regular and dense. In the latter case all other inclusions, including tellurides, have an

irregular distribution and range in abundance from scattered to very dense in certain areas. In both cases the

inclusions are seen preferentially along the molybdenite lamellae with a tendency to coarsening across intervals of

kink deformation or in the axial plane of tight microfolds. Ductile remobilisation of inclusions is also observed

along microshears oblique to the molybdenite lamellae. The thicker chalcopyrite inclusions display pointing edges

indicating they are formed under stress conditions.

CHALCOPYRITE: CuFeS2

Chalcopyrite occurred as both veinlets and disseminations in sample RG43. The sulphide grew within the

molybdenite and formed as veinlets with in the lamellae of deformed molybdenite (image 1 RLM). The

disseminated chalcopyrite occurred still within the molybdenite but as patches that were mostly associated withthe pyrrhotite.

PYRRHOTITE: Fe1-xS

The pyrrhotite was largely associated with chalcopyrite; they grew together in patchy disseminations. In sample

RG43, the pyrrhotite was not found without a deposit of chalcopyrite next to it. This gives a strong relationship

indicator between the Fe bearing minerals. Also common in the two types of rocks is the presence of sphalerite

where the latter is found as small grains at the boundaries between chalcopyrite and pyrrhotite.

CUBANITE:CuFe2S3

Cubanite formed as exsollution lamellae within the disseminated chalcopyrite in sample RG43 and is showing the

same traits mostly throughout the granites. However in the diorite sample RG27, the cubanite occurs as a main

mineral and was contained in a high mafic component mineralogy with cubanite (Cub) dominant and almost fully

replaced chalcopyrite, inclusions of granular cobaltite. It also played host to small Parkerite and cobalite inclusions

that may or may not have further hosted Bismuth-Tellurides.


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SPHALERITE: (Zn,Fe)S

There was minimal sphalerite present in sample RG 43, however a disseminated patch of it is existent. It occurs as inclusions

within the main ore minerals, indicating that Au deposition occurred at the same time as crystallisation of the main sulphides.

Sphalerite is a minor sulphide as is cubanite, which forms exsollutions from chalcopyrite

OTHER IMPORTANT MINERALS NOT FOUND IN RG43:

CHLORITE: (Mg,Fe)3(Si,Al)4O10

Chlorite was not found in the granitic sample, RG43 however in the diorite sample RG27 it most certainly was.There was abundant chlorite in the diorite, in fact there were two differing iron contents, which distinguished two

different styles of the mineral chlorite. One side of the chlorite is characterised by the richness in Fe (94.9 mol%

chamosite, the Fe-end-member). Otherwise, the chamosite component samples ranges from 70.9 to 81.7 mol%. In

the diorite, Mg-richer compositions are noted; Fe/(Fe+Mg+Mn) ranges from 0.38 to 0.78, inferring that both

chamosite and clinochlore (Mg end-member) are present. This suggests two distinct generations, therefore two

different forming conditions.

The two populations in RG27 give temperature estimates at the minimum of the range (219 C) and towards

the upper limit (367 C), further suggesting these two populations were formed under different conditions.

Temperature estimates from individual analyses are also summarised in Figure 3. (Guerin 2011).

GOLD: Au

The granitic sample RG43 contained no gold what so ever, however gold is mostly hosted in the intermediate to

felsic rocks (diorite). The Au mineralisation at Boddington is characterised by an Au-Ag-Bi-(Pb)-Te-(Se) mineralassociation which includes native gold, electrum, maldonite (Au2Bi), native bismuth, hessite (Ag2Te), altaite (PbTe),

intermediate members of the galena-clausthalite series (PbS-PbSe), naummanite (Ag2Se), abundant Bi-tellurides

from the tetradymite group (Cook et al. 2007a) and subordinate Bi-Pb-sulphotellurides from the aleksite series

(Cook et al. 2007b).


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SULPHIDE MINERAL CHEMISTRY

Compositional variation in the sulphides was measured using EPMA data. Results are presented in terms of means

and standard deviation for each sample or distinct populations in a given sample.

Pyrrhotite, Fe1-xS, where 0


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TETRADYMITE GROUP MINERALS

The study of the tetradymite group minerals is relatively new to the geological society, tellurides have been a

struggle to study over the years, as Nigel Cook and co. have quoted; Unlike the Bi-dominant sulfosalts, for which

enormous progress has been achieved in recent years in understanding the structural architecture of numerous

homologous and homeotypic series (e.g., Makovicky 1989, 1997a, b), tellurides and selenides have been difficult to

investigate at the same scale, owing to their invariably smaller grain-size and intergrown character. (Cook et al

2007). Extensive solid-solution has been invoked by Bayliss (1991) and termed disorder. Focusing on tsumoite in

particular, Bayliss argued for interchangeable occupancy of Bi and Te sites, causing extensive (isostructural)

variation in Bi:Te ratio. The compositional range of tsumoite was said to encompass other binary tellurides such as

hedleyite and pilsenite. Such disorder has been since used to account for a wide variety of values of the Bi:Te

ratio observed in natural tellurides of Bi. Dobbe (1993) considered that his suite of compositional data from

Tunaberg, Sweden, supported the idea of a single solid-solution, a tsumoite-type phase (bismuthian tsumoite)

extending between values of the Bi:Te ratio of 1.35 to 2.61, this data from the Canadian Mineralogist is consistent

with the Boddington Tsumoite in that Bi:Te ratios are ranging from 2.73:1 to 2.99:1 with up to 0.0448 K-ratio of

Sulphur being included from mineralogical probe data.

Tellurides of Au, Ag, Bi, Pb and other elements are common trace minerals in gold deposits that span the

magmatic-hydrothermal spectrum, as well as those formed in metamorphic terranes. The association between

tellurides and gold is long recognized, and is most evident from the fact that a small number of deposits, mostly

but not exclusively of epithermal type, contain exploitable Au-(Ag)-tellurides. These are the true Au(Ag)-telluride

deposits in which a significant proportion of the precious metals are carried by the tellurides themselves. Although

the majority of such deposits are epithermal, Au-(Ag) tellurides are also known from many Archaean and

Proterozoic orogenic gold deposits, the giant Golden Mile deposit, W.A. being a prime example.

Insights into mechanisms responsible for telluride deposition have been examined in relatively few instances and

no general model currently exists. A general scheme for telluride deposition via gas condensation is attractive to

explain features of mineralising systems that experienced sustained boiling, but tectonically driven hydrothermal

systems could be as efficient in making an Au-telluride deposit. Depending on temperatures, scavenging of Au by

Bi(Te) melts, or partial melting of a pre-existing ore, may offer alternatives to generate telluride-rich gold ores.

Pressure variation, e.g., by throttling, appears to impact on telluride deposition and distribution in any given

deposit.


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Relatively recently, this strong relationship between Au and Bi has been explained in terms of the generation of

bismuth melts within the ore during metamorphic overprinting (Ciobanu et al. 2006a; 2010). Underlying the

concept of melt generation is the rather low melting point of bismuth (271 C) and the observed tendency for Bi-

droplets to scavenge gold from fluid. Textures between Au- and Bi-minerals suggestive of deposition in the form of

melts have been given in a number of studies (Ciobanu et al. 2005; Ciobanu et al. 2006a; Tooth et al. 2008). Bismuth

is thus considered a powerful scavenger for gold if conditions favour melt generation and Bi-melts have been

proven experimentally to scavenge gold from fluids under saturated in Au (Douglas et al. 2000). Tellurides may

also be a prominent component of such assemblages, indicating that Au- Bi-Te melts can be considered analogous

to simple Au-Bi melts (Ciobanu et al. 2005).

DISCUSSION

The Boddington deposit contains tetradymite-group minerals from across the full compositional range of the series,

as well as probable disordered, non-stoichiometric compositions that do not correspond to named minerals. This

diversity, coupled with strong variation in the associations of these minerals and reaction between phases in the

group, clearly show that the Boddington deposit is not the product of a single ore-forming event, but rather suggests

discrete events during the evolution of the deposit, as well as showing the major role that lithology played during

ore genesis.

While metal abundances in molybdenite from the granite are low and the granite is relatively unmineralised it does

contain similar alteration assemblages and minor chalcopyrite/pyrrhotite mineralisation + molybdenite. Such

evidence does not rule out the granite as a potential metal source but could be explained by late stage crystal

fractionation and metal incompatibility. Metal enrichment from fractionation of granite has been suggested by past

geologists (by Mustard et al. (2006)) to explain mineralisation associated with intrusion-related systems. Bismuth

levels in the molybdenite sample are relatively elevated and could have also assisted with the removal of Au from

the granite as a result of Au scavenging from a fluid under saturated in the Au.Petrographic, mineralogical and geochemical evidence support a three-stage model for Boddington. An early

porphyry event can account for the bulk of the Cu mineralisation, as well as some of the Au and Mo. A subsequent

orogenic-Au event led to shearing and remobilisation of ore components. New constraints on metamorphic

conditions are offered by chlorite and stannite-sphalerite geothermometry and the occurrence of two co-existing


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pyrrhotite species. The granite introduced some Au, Mo and other granitic elements, notably Bi leading to

substantial upgrading of Au grades by Bi-melt scavenging.

LIMITATIONS

The results observed in this report are of limited outcomes. Due to the fact that only a few hand samples were

examined, only one polished section was observed which restricted information only to the granitic component of

the Boddington deposit. Future research would include more polished sections from different areas of the deposit

and around the deposit looking at the bordering minerals and assemblages to tie in with the deposit itself. Laser

ablation data and EPMA data was scavenged from past thesis due to lack of data upload and capabilities.

CONCLUSION

This summary highlights that at Boddington there are at least 2 main ore stages; relating to host diorite and that

formed during emplacement of granite. The similarity in the main sulphide and alteration assemblages, as well asthe reducing character indicate that the metals and the fluids involved in both mineralising stages have either

undergone a similar mechanism of ore deposition or that a strong superposition or obliteration occurred during

granite emplacement.

The Wandoo basement gold mineralisation is therefore interpreted to represent a structurally-controlled, intrusion-

related Au-Cu deposit, paragenetically associated with both ~2700 and 2612 Ma events, with the main stage, higher

grade mineralisation being apparently synchronous with the late, K-rich post-magmatic monzogranite suite.

The dominant ore minerals are chalcopyrite and pyrrhotite. Such an assemblage is typical of an ore formed under

slightly reducing conditions. Sphalerite, native bismuth, various Bi-Te species and electrum occur as inclusions

within the main ore minerals, indicating that Au deposition occurred at the same time as crystallisation of the main

sulphides


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Ore Minerals Occurs as

Chalcopyrite Disseminated

Pyrrhotite Disseminated

Gold Fine grains

Molybdenite Blocky

Cubanite Disseminated within lamellae of Molybdenite

Sphalerite Disseminated fine grains around Fe-sulphides

Gangue Minerals

Albite

Biotite

Epidote

K-Feldspar

Rutile

Quartz


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Mineral Abbreviation

Actinolite Act

Albite Ab

Allanite Aln

Biotite Bt

Bismuth tellurides Bi-tell

Bornite Bn

Calcite Cal

Carbonate Carb

Chalcocite Cc

Chalcopyrite Cp

Chamosite Cham

Clinozoisite Clz

Covellite Co

Cubanite Cub

Electrum El

Epidote Ep

Gold Au

Ilmenite Ilm

K-feldspar KFs

Maldonite Mld

Molybdenite Mo

Muscovite Mu

Native bismuth Bi

Native copper Cu

Native silver Ag

Oligoclase Ol

Pentlandite Pn

Plagiocalse Plag

Pyrite Py

Pyrrhotite Po

Quartz Qz

Rare Earth Minerals REE-min

Rutile Ru

Sphalerite Sph

Stannite Stn

Thiospinel ThioTsumoite Ts

Volynskite Vol


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BIBLIOGRAPHY

ALLIBONE A. H., WINDH J., ETHERIDGE M. A., BURTON D., ANDERSON G., EDWARDS P. W.,

MILLER A., GRAVES C., FANNING C. M. & WYSOCZANSKI R. 1998. Timing relationships and

structural controls on the location of Au-Cu mineralization at the Boddington gold mine, Western

Australia. Economic Geology 93, 245-270.

ANAND R. R. 2005. Boddington gold deposit, Western Australia. CRC LEME, CSIRO Exploration and

Mining

COOK N. J., CIOBANU C. L., WAGNER T. & STANLEY C. J. 2007a. Minerals of the system bi-te-se-s

related to the tetradymite archetype: Review of classification and compositional variation. Canadian

Mineralogist45, 665-708.

GROVES D. I., PHILLIPS G. N., HO S.E., HOUSTOUN S.M. & STANDING C. A. 1987. Craton scale

distribution of Archaean greenstone gold deposits: Predictive capacity of the metamorphic model.

Economic Geology 82, 2045-2058

GUERIN, R. (2010). Petrography, mineralogy and trace element chemistry of Cu-Au-Mo mineralisation

from Central Diorite, Boddington, W.A..Adelaide University

MYERS J. S. 1993. Precambrian history of the west Australian craton and adjacent orogens. Annual

Review of Earth Planetary Sciences, 21, 453-485

NELSON D. R. 1998. Granite-greenstone crust formation on the Archaean Earth: A consequence of two

superimposed processes. Earth and Planetary Science Letters 158, 109-119

NEMCHIN A. A., PIDGEON R. T. & WILDE S.A. 1993. Timing of the late Archaean granulite faciesmetamorphism and the evolution of the southwestern Yilgarn Craton of Western Australia: Evidence

from U-Pb changes of zircons from mafic granulites. Precambrian Research 68(3-4), 307-321


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SYMONS, P.M., Anderson, G., Beard, T.J., Hamilton, L.M., Reynolds, G.D., Robinson, J.M., Staley, R.W.,

1988. The Boddington Gold Deposit. The second international Conference on Prospecting in Arid

Terrain, Perth, Excursion Guidebook, pp 77-84.


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Figure 1: Location of

the Boddington Au +

Cu deposit in the

southwestern

Yilgarn craton,

Western Australia

Figure 2: The series

of structural

deformation

events in theYilgarn Craton

APPENDIXES


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65

43

21

Mo

Chalco

Mo Chalco

Qtz

Sph

Po

Qtz

Chalco

Po

Mo

Chalco

Below: Images taken from Reflected Light Microscopy at Adelaide Microscopy 2012


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11 12

9 10

7 8

Qtz

Ep

Qtz

Rut

Mo

QtzRut

Fldspr

Qtz


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1 2

3 4

5 6

Below: Images taken from Scanning Electron Microscopy at Adelaide Microscopy 2012


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78

9 10

11


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Figure 3: Binary plots of chlorite, showing

the temperature ranges for distinct

populations in diorite and granite.


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Figure 4: Binary plots of total metal

(M) versus S for pyrrhotite in granite

and diorite. (a) Single-phase

pyrrhotite corresponding to Fe7S8 from

granite. (b) Single-phase pyrrhotite

corresponding with Fe7S8-Fe9S10 from

diorite. (c) Distinct compositional

fields for two-phase pyrrhotite

occurring as lamellar exsolutions in

diorite. The phase that is bright on

BSE images forms a field close to FeS(troilite) whereas composition of the

dark phase straddles across Fe10S11-

Fe7S8. (d) Sphalerite-stannite

geothermometry plot showing

formation temperatures from

logXFeS/XZnS ratios in the two minerals.

The cluster of points along the 350 C

calibration line is the most reliable.


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TSUMOITE

Figure 5: Electron Microprobe data of Tsumoite in sample RG43

boddington industry project, yilgarn craton, w.a

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