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Structure and mineralisation of western Tasmania. AMIRA Project P.291A. Report No. 2, October 1994


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Berry, RF, Davidson, GJ, Kitto, PA, White, M and Selley, D 1994 , Structure and mineralisation of western Tasmania. AMIRA Project P.291A. Report No. 2, October 1994.

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The last report for project P291A, Faulting and
mineralisation in western Tasmania, was in May 1994. Since then the project has concentrated on detailed laboratory analysis from the last field season. Two sections have been active. The S isotope study (Davidson G & Kitto PA)has
concentrated on the northern margin of Rosebery and is starting to tackle the problem of what the
signature of hydrothermal circulation is in the CVC.

Sixty-four sulfur isotope analyses were obtained from footwall rocks at the north end of the Rosebery hydrothermal system, using a laser ablation microprobe at the University of Tasmania. Results to date indicate that the Cambrian footwall pyrite within 1 km of B-lens has the same isotopic values as B-lens and its underlying footwall pyrite (d34S = 7.3-17.3 permil, mean =11.5 permil), but beyond this increases distinctly to d34S =11.8-20.2 permil, with a mean of 15.1 permil. Post-cleavage pyrite ranged from 1.1 permil to 3.8 permil (n=4). Black slate pyrite was divided for sampling purposes into vein and disseminated pyrite, which have the same wide isotopic range (d34S= 4.6-19.2 permil), although in the two samples
where both types could be analysed together, vein pyrite was up to 4 permil lighter.

The increase in d34S values of footwall pyrite at
Rosebery indicates that the margins of this Cambrian hydrothermal system contained high concentrations of seawater sulfate-derived sulfur. This sulfur was entrained by shallow convection at the periphery of the main up-flow limb of a deep convective cell below Rosebery. The increase in d34S values occurs further from ore, and is less pronounced, than at Hellyer. These differences are
attributed to the original highly porous character of
the Rosebery footwall, and leads to the general
conclusion that fluid flow in the felsic breccias
(which typify much of the Mount Read Volcanic Belt) must have been more diffuse than in coherent lava sequences such as the Que-Hellyer belt.

The other major activity since May has been the analysis of the stratigraphic correlation within the Mt Read volcanics, aimed at the establishment of a basin geometry. The Mt Read Volcanics have been subdivided into three depositional cycles each with a distinct basin geometry and history. This subdivision has been substantially revised from the version available in the May report with input available from a number of additional sources.
Samples of sandstones have been collect from each
of these cycles. Cycle 2 sandstones are recognisable
from the very large component of Fe oxides, including ilmenite and magnetite. The cycle 3 sandstones have much less Fe oxide and the major external contribution to the heavy minerals is chromite and rounded zircon. These are most abundant in the Animal Creek Greywacke. The heavy mineral suites have potential to aid correlation within the stratigraphy.

Research by CODES students outside the AMlRA project is producing important results relevant to the project. A summary of two of these CODES/AMIRA studies is included here as the work is not yet generally available, and is important to the arguments in this report.

Item Type: Report (Technical Report)
Authors/Creators:Berry, RF and Davidson, GJ and Kitto, PA and White, M and Selley, D
Publisher: CODES/AMIRA
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