University of Tasmania
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Structural and geochemical controls on mineralisation at Renison, Tasmania

posted on 2023-05-26, 01:43 authored by Kitto, PA
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."


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