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Structural and geochemical controls on mineralisation at Renison, Tasmania


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Kitto, PA 1994 , 'Structural and geochemical controls on mineralisation at Renison, Tasmania', PhD thesis, University of Tasmania.

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Renison Tin Mine is located at Renison Bell; on the west coast of Tasmania. It is Australia's
largest primary tin producer, with an identified mineral resource totalling 9.5 mt at 1.4% Sn
and an annual production in 1993 of 580,000 t at 1.6% Sn (Thomas & Roberts, 1994). Total
Sn recovery since the commencement of large scale underground mining operations in the
1960's is over 115,000 t. Renison is hosted by subaerial to shallow marine, Late
Precambrian to Early Cambrian dolomitic and clastic sediments of the Early Palaeozoic
Dundas Trough. The deposit occurs on the north-east limb of a broad, south-east plunging
Devonian anticline which constitutes a major fault-bounded horst. Major brittle structures
associated with tin mineralisation at Renison formed during the forceful emplacement of the
Pine Hill Granite, include the Federal-Bassett Fault, Argent Fault, Blow Fault and a series of
east-west trending interconnected Transverse Faults.
The Pine Hill Granite forms a buried 'spine' that connects the Heemskirk and Granite Tor
Batholiths. The Pine Hill granite is classified as ilmenite-series, and is reduced (Fe3+;Fe2+
+ Fe3+ ratio of 0.14), peraluminous, and has corundum normative values between 0.8 and
1 .5. Plots of major, trace and REE analyses of the unaltered Pine Hill Granite show welldeveloped
fractionation trends indicating approximately 60 % Rayleigh fractionation during
crystallisation. Beneath Renison, an apophysis of late stage quartz-feldspar porphyry granite
generated a high temperature boron and fluorine-rich fluid, which caused in-situ
sericitisation, albitisation and tourmalinisation.
Detailed study of kinematic indicators on mineralised faults has revealed four phases of
brittle deformation (Devonian to Tertiary), based on style and relative ages of fault striations.
Initial brittle deformation associated with the forceful emplacement of the Pine Hill Granite
formed the Federal-Bassett Fault, with up to 700m of normal-dextral dip-slip movement,
which allowed magmatic-hydrothermal fluids access to the dolomitic host sequence. A high
temperature oxide-silicate vein stage (qz-asp-cass) formed during this fault movement. Fluid
inclusions associated with this event have· homogenisation temperatures ranging from
>400°C at the base of the fault (3000 m beneath the Devonian palaeosurface), to 300°C
near the top of the mine workings. These early NaCI-KCI-H20 brines had average salinities
between -8 and 12 eq. wt.% NaCI, and fluid pressures of 250 bars (hydrostatic). o180tluid
values of 9 %o are clearly magmatic, consistent with a fluid which ascended and cooled with
the Federal-Bassett Fault before interacting with the wallrocks in the higher mine levels.
o34sfluid values from the oxide-silicate stage are< Groo and indicate the probable source of
sulphur is magmatic.
As the granite-related stress field decayed, a regional Taberraberran-related dextral wrench
reactivated earlier fault structures, and produced a dilational jog in the Federal-Basset Fault. This fault reactivation released a second generation of magmatic-hydrothermal fluids,
that ascended within the Federal-Bassett Fault and infiltrated the overlying dolomite
horizons. Main sulfide stage mineralisation (pyrrhotite+cassiterite-quartz-fluorite-stannitechalcopyrite±
arsenopyrite and minor base metals) produced the stratabound carbonate
replacement orebodies that characterise. the Renison deposit. During this stage of
mineralisation, mineral deposition in the Federal-Bassett Fault occurred over a temperature
range from <350°C, immediately above the Pine Hill Granite, to -200°C at the top of the
mine workings. The deep-level NaCI-KCI-H20-rich magmatic-hydrothermal brines evolved
to CaCI2·MgCI2-NaCI-H20-rich fluids during fluid-rock reactions with carbonates in the
upper mine levels. Salinities averaged between 8 and 12 eq. wt. % NaCI throughout the
sulphide stage. Contoured tin values and homogenisation temperatures from fluid inclusions
clearly outline two high temperature tin-rich dilational jogs on the Federal-Bassett Fault, as
do variations in a34smineral values. a34sfluid values remained constant at -5%o throughout
the sulphide stage, which is consistent with a homogeneous magmatic sulphur source.
Minor uneconomic base metal veins (rhodochrosite-galena-sphalerite-quartz}, associated
with late stage fault reactivations, overprint the earlier vein stages, as do vug-fill carbonatequartz
veins (quartz-carbonate±fluorite±pyrite). The late stage veins were associated with
low temperature (150° to 200°C), bimodal salinity (<2 and -10 eq. wt.% NaCI), NaCI-KCIH20
brines that formed via mixing of contel!lporary meteoric groundwaters with magmatichydrothermal
fluids. a34sfluid values (-5%o) remained unchanged, indicating that magmatic
fluid continued to supply sulphur to the Renison system over a protracted period. The only
fluid inclusion evidence for phase separation at Renison occurs in these late stage veins.
Fluid inclusion results from the oxide-silicate stage, in association with thermodynamic
modelling, provide the following estimates for initial magmatic-hydrothermal fluids at
Renison: -250 bars fluid pressure, -350°C temperature, salinity -12 eq. wt.% NaCI, pH 3.8
to 5.4, I.S = 0.05 molal, log 102 between -32.0 and -33.8, log JH2S between -0.5 and -2.5,
aH3As03 between 1 o-5 and 10-1, aNa+ = 0.1742, aK+ = 0.085, mMg2+ .between 1.6 x 1 o-5
and 6.2 x 10-2, mca2+ between 7.96 x 10·3 ~nd 12.61, mf. between 1.05 x 1o-5 and 4.16 x
1 o-4, and Sn solubility = 20 ppm. Numerical simulations for this Renison-type oxide-silicate
stage fluid predict that boiling, cooling, and mixing with pure water (25°C} are inefficient
depositional mechanisms for precipitating cassiterite. In contrast, fluid-rock interaction
appears to be crucial for cassiterite deposition. Based on numerous simulations of fluid-rock
interaction, reaction with dolomite provides. the closest approximation of the actual oxidesilicate,
sulphide stage and carbonate replacement mineral assemblages. Sn transport in
the Renison-type fluid was dominated by SnCI3" and Sn(OH)2CI2 complexes at 350°C and
logf02 =33.5, allowing the hydrothermal fluid to carry= 20 ppm I.Sn. At lower temperatures,
Sn(OH)2CI2 complexes became dominant. The most effective mechanism for cassiterite deposition, as predicted by numerical modelling, was by redox and pH changes induced by
arsenopyrite deposition and carbonate dissolution, respectively.
In conclusion, distal skarn deposits like Renison, associated with carbonate replacement
mineralisation are intimately related to shallow mesothermal granitoids. The highly
fractionated and reduced ilmenite series granites acted as fertile sources for tin, which is
transported by magmatic-hydrothermal fluids from volatile-rich {B, F, Cl) apophyses in the
roof of the intrusion. Thermal metamorphism and forceful granite emplacement assisted
brittle deformation of the overlying host sequences allowing high temperature (300° to
400°C} acidic ore fluids access to reactive carbonate hosts. Cassiterite deposition is
controlled by fluid-rock interaction associated with redox changes induced by arsenopyrite
deposition, and by increased pH due to carbonate dissolution.

Item Type: Thesis - PhD
Authors/Creators:Kitto, PA
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