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Structure, volcanic setting, hydrothermal alteration and genesis of the Thalanga massive sulphide deposit

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Hill, Anthea P (1996) Structure, volcanic setting, hydrothermal alteration and genesis of the Thalanga massive sulphide deposit. PhD thesis, University of Tasmania.

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Abstract

The Thalanga Zn-Pb-Cu-Ag deposit is located at the contact between the rhyolitic volcanics
of the Mount Windsor Volcanics and the overlying dacitic and andesitic volcanic units of the
Trooper Creek Formation in the Cambro-Ordovician Mount Windsor subprovince, northern
Queensland. The sheet-like massive sulphide lenses and enclosing volcanic units are locally
strongly deformed and metamorphosed to upper greenschist facies (T = 485 ± 23°C, P = 2.5 ± 1.5
kbars). Bedding is now subvertical (D2) and the ore horizon has been offset by two generations
of normal faults. Normal faults at a high angle to the stratigraphy separate West and
Central Thalanga from the Vomacka Zone and East Thalanga, whereas younger normal faults
(D3 ), at a low angle to stratigraphy, have repeated and locally structurally thickened the
ore lenses.
The pervasive S2 cleavage has been locally weakly crenulated by S3. Decussate biotite has
overgrown the crenulations, indicating that peak metamorphism at Thalanga was post-S 3 .
The presence of rare kinked chlorite pseudomorphs of biotite is interpreted to indicate that
there was an earlier metamorphic event, which may correlate with regional metamorphism.
The massive sulphides are interpreted to be syn-volcanic in origin because (1) they have been
overprinted by the same generations of tectonic structures as the host stratigraphy, (2) the
rhyolitic volcanics stratigraphically underlying the massive sulphide lenses contain
abundant quartz ± muscovite ± chlorite ± phlogopite ± pyrite assemblages (inferred to be
metamorphosed quartz-sericite ± chlorite ± pyrite alteration), whereas the overlying dacite
lavas contain metamorphic biotite and local epidote-quartz ± albite-rich assemblages, (3)
subvertical pyrite stringer zones (5-40 % pyrite) within the footwall rhyolitic volcanics
intersect the ore horizon at the thickest lenses of massive sulphides, and (4) there is a strong
stratigraphic control on the location of massive sulphides. The ore lenses at Thalanga occur
within and stratigraphically below a poorly-sorted, coarse quartz-bearing, polymict breccia.
The internal organisation of this breccia is consistent with transportation to the site of final
deposition by subaqueous mass-flows. The breccia has a similar distribution to the massive
sulphide lenses and is interpreted to have filled local seafloor depressions. The presence of
massive sulphide clasts within the polymict breccia indicates synchronous volcanism and
mineralisation. Comagmatic, non-vesicular quartz-feldspar porphyry, with peperitic
margins, has locally intruded the polymict breccia.
The main ore minerals are sphalerite, pyrite, chalcopyrite and lesser galena, with minor
magnetite, arsenopyrite and tetrahedrite-tennantite. In most places, the sulphides are
coarsely recrystallised and preserve no evidence of the deformation history. However,
banding in the polymetallic sulphide lenses is interpreted to be tectonic in origin because the alternating pyrite- and sphalerite-rich bands are subparallel to S2. Chalcopyrite, sphalerite
and galena have been remobilised during deformation and now occupy faults and sites of
dilation within the ore horizon, including subhorizontal boudin necks, shallowly-dipping
extension veins and piercement structures at the contact between massive sulphides and the
overlying dacite. Metal zonation at Thalanga is poorly developed due to widespread
sulphide remobilisation, although there is a general zonation from pyrite-chalcopyrite-rich
sulphides at the stratigraphic base of ore lenses, to sphalerite-galena ± barite-rich
sulphides at the top.
The ore lenses at West and parts of Central Thalanga are composed of massive sulphide veins
and disseminations that have cross-cut carbonate (dolomite-calcite) and Mg-rich chlorite
assemblages. The Ti/Zr values of the carbonate- and chlorite-rich assemblage are similar to
those of the underlying footwall rhyolitic volcanics (Ti/Zr = 2-5), and locally to those of the
overlying coarse quartz-bearing polymict breccia (Ti/Zr = 3-10). Carbonate- and chlorite-rich
units are therefore interpreted to have formed by the replacement of formerly glassy
rhyolitic volcanics and the base of the coarse quartz-bearing polymict breccia by cooler Mgand
HCO3- -bearing hydrothermal solutions prior to sulphide deposition. The coarse quartzbearing
polymict breccia may have acted as an impermeable barrier during carbonatechlorite
alteration and trapped the ascending hydrothermal solutions, restricting alteration
to a narrow zone at the top of the footwall rhyolitic volcanics. The lack of extensive
carbonate-chlorite alteration in the Vomacka Zone and East Thalanga may be due to higher
temperature hydrothermal solutions or lack of a capping unit.
The high Mg content of chlorite within the carbonate-chlorite assemblages is consistent with
mixing between cold seawater and warm hydrothermal solutions in porous perlitic rhyolite
lavas or rhyolitic breccia units in the footwall. HCO3- -bearing hydrothermal solutions are
interpreted to have been derived from seawater that had circulated through the rhyolitic
volcanics of the Mount Windsor Volcanics and possibly some magmatic solutions containing
HCO3-.
The 634S values of sulphides (5.6-17.3 %co) are consistent with formation from solutions
containing dissolved igneous sulphur and Cambro-Ordovician seawater that had been
inorganically reduced during hydrothermal convection. The increasing •534S of pyrite towards
the stratigraphic top of footwall is interpreted to be a consequence of progressive mixing
between the hydrothermal solutions and cold seawater. The 6M5 values of barite (27.6-32.4
700) are consistent with a Cambro-Ordovician seawater source of sulphur, and support the
interpretation that barite was deposited on the seafloor.
The abundance of proximal coherent lavas, domes and syn-volcanic intrusions and associated
volcaniclastic facies underlying and overlying Thalartga, and absence of coherent units in thehangingwall along strike (closest is —2 km along strike), is interpreted to indicate that the
massive sulphide lenses formed within a volcanic centre. Local formation of massive
sulphides on the seafloor is inferred to have been interrupted by deposition of the polymict
breccia units. The sulphide lenses are interpreted to have formed predominantly by subseafloor
replacement of the coarse grained bases of polymict breccia units, and local
carbonate- and chlorite-rich alteration assemblages, prior to emplacement of the overlying
dacite lavas. The presence of coarse quartz-bearing polymict breccia and porphyry units, is
interpreted to indicate a long-lived hydrous magma at depth that probably drove
hydrothermal convection and may have released volatiles (including CO2 and potentially
metals) to the ascending hydrothermal solutions.

Item Type: Thesis (PhD)
Keywords: Volcanism, Sulfides, Hydrothermal alteration
Copyright Holders: The Author
Copyright Information:

Copyright 1996 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1998. Includes bibliographical references

Date Deposited: 19 Dec 2014 02:36
Last Modified: 21 Nov 2016 02:21
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