University of Tasmania
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The cassiterite-sulphide deposits of western Tasmania

posted on 2023-05-26, 05:19 authored by Groves, DI
The cassiterite-sulphide deposits in Western Tasmania are stratiform replacement deposits (Mt. Bischoff; Renison Bell; Cleveland; Mt. Lindsay), fissure deposits with marginal replacement (Federal Lode, Renison Bell; Razorback-Grand Prize), and fissure lodes (Mt. Bischoff), that consist essentially of pyrrhotite with subordinate pyrite, arsenopyrite, chalcopyrite, sphalerite and cassiterite. They are localized in dolomitic horizons of Upper Proterozoic-Lower Cambrian age in faulted, complex anticlinal structures. These have been the loci for intrusion of cupolas and associated dyke swarms of late Devonian granitic rocks, which have been extensively topazized, tourmalinized and/or greisenized. The major and trace element chemistry of these granitic rocks combined with isotopic dating indicates that they probably represent the most highly fractionated portions of penecontemporaneous, high-level, post-kinematic granitic batholiths occurring throughout Western Tasmania. These granitic batholiths may be interconnected at depth. The spatial association of the polyascendant tin mineralization with these granitic rocks, and their associated prejoint tin mineralization, suggests that they are derived from a common source. Around the cassiterite-sulphide deposits are haloes of dispersed Pb-Zn-Ag mineralization, which on the basis of field relationships is considered to be related to the same granitic activity. A uniform distribution of Co and Ni in Fe-sulphides, Cd in sphalerites, Se in sulphides, and Mn in both the cassiterite-sulphide and Pb-Zn-Ag deposits provides empirical support for a single metallogenic province covering Western Tasmania in the late Devonian. In the cassiterite~sulphide deposits, cassiterite and wolframite have generally crystallized before the earliest formed sulphides, which are pyrite and arsenopyrite, followed by pyrrhotite. Chalcopyrite, sphalerite and stannite were formed penecontemporaneously and were followed by tetrahedrite, jamesonite, bismuth and finally galena with associated canfieldite, frackeiite boulangerite and pyrargyrite. At Mt. Bischoff pre-sulphide metasomatism of dolomite at temperatures in excess of 4000 c resulted in formation of tourmaline, quartz, wollastonite, chondrodite and garnet. These minerals were subsequently replaced by sulphides and talc, fluorphlogopite, serpentine, chlorite, Fe-Mn~g carbonates, fluorite and sellaite. Similar alteration has occurred at Renison Bell and Cleveland but the pre-Sulphide phase is limited to the formation of minor tourmaline, topaz and quartz. A mineralogical and isotopic lateral zonation can be demonstrated at Mt. Bischoff. The zonal and paragenetic sequences are essentially parallel. Fluid inclusion studies demonstrate that the zoned sequence is partly a function of declining temperature, with initial deposition of fluorite up to 5800 , decreasing to 2000 C in the marginal zone. It is probable that the temperature, salinity and composition of the ore-forming fluids were constantly changing during mineralization as an initially hot, saline fluid was mixed with cooler, less saline meteoric and connate waters, in conjunction with heat loss to the wall rocks. In general, compositional changes in sulphide systems within the zonal sequence are consistent with cooling of the ore-forming fluid from temperatures as high as 7000C to below 3000 C, with a sympathetic decline in fugacity of sulphur and oxygen. However some anomalies exist between interpretations based on the Fe-S and Fe-Zn-S systems, and the Fe-As-S system. The other cassiterite-sulphide deposits lack a well defined zonation with respect to both mineralogy and isotopic composition of sulphur, but their initial deposition temperatures are comparable or slightly lower than those at Mt. Bischoff. Ore deposition was probably initiated by increasing alkalinity of the ore-forming fluids caused by reaction with dolomitic host rocks.


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