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Geology and genesis of the mammoth Cu deposit, Mount Isa Inlier, Australia


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Clark, Darry J.(Darryl James) 2003 , 'Geology and genesis of the mammoth Cu deposit, Mount Isa Inlier, Australia', PhD thesis, University of Tasmania.

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Of the numerous structurally-controlled, sediment-hosted Cu deposits throughout the
Proterozoic Western Fold Belt of the Mt lsa lnlier, the two most significant are the giant Mt
Isa Cu deposit (255 Mt @ 3.3% Cu) and the Mammoth deposit (16.8 Mt @ 3.4% Cu). The
deposits are hosted in deformed Proterozoic sediments that range in age from 1740 to 1652
Ma. Copper mineralisation is hosted in breccias and veins, which appear to be fault
controlled and is interpreted as occurring approx. 150 Ma after deposition of the host rocks
during regional deformation (1590 to 1500 Ma; Isar) Orogeny). The Mammoth deposit is
located approximately 115 km to the north of Mt lsa, and copper mineralisation is hosted
within the Palaeoproterozoic Whitworth Quartzite of the Myally Subgroup. Understanding
the local geological and geochemical controls at Mammoth will assist in the development of
a genetic model and exploration targeting of similar deposits in ancient, structurally
deformed, sedimentary basins in Australia and elsewhere in the world.
Structural studies using orientation and kinematic data from mineralised veins and bounding
faults have provided a new understanding of the mechanisms that localised Cu
mineralisation at the Mammoth deposit. The Mammoth deposit formed during E-W
transpression localised at the intersection of the Mammoth and Portal Faults during periods
of high fluid pressure (Pf), subsequent to slip episodes, when the principle stresses were
sub-equal (0.3 :+.- cs2 al ). Dilation was achieved when fluid pressure (PO exceeded the tensile
strength (T) of the host rock as indicated: 1) by the irregular development of syndeformational
quartz-Cu sulfide extensional veins; and 2) by the regular development of syndeformational
quartz-Cu sulfide extensional veins along bedding planes when a i a3 . The Mammoth deposit has 6 major ore lenses that extend over 1,100 metres vertically and
have a strike extent of 100 to 300 m and widths from 10 to 75m. Within these lenses, Cu
mineralisation is hosted in veins and breccias adjacent to the intersection of the Mammoth
and Portal Faults. A new textural classification scheme, consisting of five categories was
used to describe the variations of brecciation and fracturing of the Whitworth Quartzite during
hypogene mineralisation. The grade of Cu mineralisation in these units is variable, but is
high around zones of intense brittle deformation in the breccia units reflecting zones of
enhanced permeability. Wall rock alteration, replacement and cementation of the breccia
units by the hypogene mineral assemblage took place after and/or during the fragmentation.
Hypogene Cu mineralisation consists of chalcopyrite, bornite and chalcocite. Supergene
chalcocite (± covellite) has variably overprinted and enriched the hypogene assemblage.
Hypogene and supergene chalcocite within the deposit are texturally and paragenetically
distinct. A four stage paragenetic sequence has been defined across the Mammoth ore zones: Stage I) hypogene pyrite and quartz; Stage II) hypogene chalcopyrite, bornite,
chalcocite and carrollite (trace) + chlorite, illite and quartz; Stage III) supergene hematite,
chalcocite, covellite and wittichenite, and Stage IV) supergene kaolinite, chalcocite, covellite
and wittichenite. Hypogene Cu sulfides are broadly zoned within the Mammoth ore zones
from hangingwall to footwall and up dip. This zonation is: 1) Stage ll hypogene chalcocite ±
bornite; 2) Stage II bornite ± hypogene chalcocite and 3) Stage II chalcopyrite ± bornite.
Microanalytical trace element geochemistry of sulfide minerals from the Mammoth deposit
confirmed paragenetic interpretations. These investigations involved the development of a
new analytical technique to obtain high quality, sub-ppm, quantitative data from the in situ
microanalysis of sulfide minerals using laser ablation (LA) ICP-MS analysis. Pyrite,
chalcopyrite, bornite and both supergene and hypogene chalcocite were analysed. Stage III
supergene chalcocite is enriched in Bi and depleted in both Ag, Co, Sb, and As compared to
Stage ll hypogene chalcocite. No differences were detected in trace element composition
between disseminated and veined styles of Stage I pyrite. Stage I pyrite has elevated
contents of As, Co, Ni, Sb, and Pb compared to chalcopyrite, bornite and chalcocite. Bornite
is anomalously enriched in Bi compared to the other sulfides minerals The processes of meteoric and supergene alteration have led to complex geochemical
dispersion patterns overprinting geochemical patterns developed during hypogene
mineralisation. Whole-rock geochemistry analysis across the deposit revealed a distinct
metal zonation. Elevated Co, Ni, CaO and P205 contents form a narrow halo (4_ 2 to 5m) -
around the hypogene ore zones at Mammoth. Along the ore equivalent structural horizon
Zn, Co, and Ni are enriched distal (100 to 200m) to the ore zones. In contrast, Cu, Bi, As
and Fe, initially at or below detection, all increase with proximity to the ore zones.
Mineralised structures that have undergone supergene modification also have a similar suite
of trace elements (Cu, Pb, Zn, Ni, Bi, Sb, and As). Their level of enrichment is significantly
lower than their un-oxidised equivalent but more enriched than the enclosing host rocks.
These suites of trace elements form potential vectors to differentiate mineralised and unmineralised
structures and can be used to identify Mammoth style mineralisation in surface
or near surface leached structures.
Sulfur, 0 - D and Pb isotopic investigations allow characterisation of the ore fluids and
identification of potential metal source reservoirs. Lead isotopic ratios from Mammoth ore
samples lie along an isochron that originates from the Eastern Creek Volcanics at 1540 Ma,
indicating that the Eastern Creek Volcanics were the source of Pb, and by inference Cu,
contained in the Mammoth deposit. The 6 34S values of Stage I pyrite (- 15 to - 7 %o; mean =
— 12 %o) and Stage II Cu and Cu-Fe sulfides (— 19 to — 1 %o; mean = - 8 °/00) supports the
conclusion that the Eastern Creek Volcanics were the source of metals and sulfur as they
represent the only probable source of light sulfur in the Western Fold Belt. The calculated
8180 and 6D fluid composition in equilibrium with Stage I quartz and Stage ll chlorite yielded
a range of — 10 to -2 Too 6 180 and —2 to + 11 Too 8 180 and —41 to —56 %o 8D respectively.
These values suggest the fluid had two end members: 1) dominantly meteoric fluid and 2)
fluid originating from metamorphic processes.
Geological, isotopic, paragenetic, geochemical and structural data suggest the Mammoth Cu
deposit formed synchronous with regional deformation when channelised flow of hot salineoxidised
Cu-enriched fluid generated by metamorphic devolatilisation, leached Cu and S
from the Eastern Creek Volcanics, and was focussed along the Esperanza, Mammoth and
Mammoth Extended Faults. Upon reaching the Whitworth Quartzite, a build up of fluid
pressure combined with the existing regional stress field, created the appropriate conditions
for brittle failure in the quartzite forming dilational sites for sulfide precipitation. The hot
metal and sulfur-bearing fluid mixed with a deep circulating methane-bearing hydrothermal
fluid originally of meteoric origin. Changes in the physicochemical conditions upon this
mixing destabilise the metal chlorine complexes resulting in sulfide precipitation. All of these
attributes cumulatively broaden the classification of the metamorphogenic style of Cu
deposit. Most notable of which is that the Mammoth deposit provides evidence that
metamorphogenic Cu deposits can be hosted in host rocks other than meta-carbonates.
This new model for the genesis of the Mammoth deposit has developed new criteria for the
exploration of similar resources in the Western Fold Belt; including the world class Mt Ise Cu
deposit. The exploration strategy currently in use throughout the Western Fold Belt of the Mt
Ise Idler has traditionally focused on sites where structural geometries potentially gave rise
to dilatational fault jogs during deformation. This research provides evidence that structural
geometries that had the potential to create zones of E-W compression and/or N-S extension
should now be included in exploration targeting.

Item Type: Thesis - PhD
Authors/Creators:Clark, Darry J.(Darryl James)
Keywords: Copper mines and mining, Copper, Geology
Copyright Holders: The Author
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Copyright 2003 the Author - The University is continuing to endeavour to trace the copyright
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