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Lewis Ponds, a hybrid carbonate and volcanic-hosted polymetallic massive sulphide deposit, New South Wales, Australia


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Agnew, MW, Large, RR and Bull, SW 2005 , 'Lewis Ponds, a hybrid carbonate and volcanic-hosted polymetallic massive sulphide deposit, New South Wales, Australia' , Mineralium Deposita, vol. 39, no. 8 , pp. 822-844 , doi: 10.1007/s00126-004-0456-6.

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Abstract The Lewis Ponds Zn-Pb-Cu-Ag-Au deposit,
located in the eastern Lachlan Fold Belt, central
western New South Wales, exhibits the characteristics
of both volcanic-hosted massive sulphide and carbonate-
hosted replacement deposits. Two stratabound
massive to disseminated sulphide zones, Main and
Toms, occur in a tightly folded Upper Silurian
sequence of marine felsic volcanic and sedimentary
rocks. They have a combined indicated resource of
5.7 Mt grading 3.5% Zn, 2.0% Pb, 0.19% Cu, 97 g/t
Ag and 1.9 g/t Au. Main Zone is hosted by a thick
unit of poorly sorted mixed provenance breccia,
limestone-clast breccia and quartz crystal-rich sandstone,
whereas Toms Zone occurs in the overlying
siltstone. Pretectonic carbonate-chalcopyrite-pyrite
and quartz-pyrite stringer veins occur in the footwall
porphyritic dacite, south of Toms Zone. Strongly
sheared dolomite-chalcopyrite-pyrrhotite veins directly
underlie the Toms massive sulphide lens. The mineralized
zones consist predominantly of pyrite, sphalerite
and galena. Paragenetically early framboidal, dendritic
and botryoidal pyrite aggregates and tabular pyrrhotite
pseudomorphs of sulphate occur throughout the
breccia and sandstone beds that host Main Zone, but
are rarely preserved in the annealed massive sulphide
in Toms Zone. Main and Toms zones are associated
with a semi-conformable hydrothermal alteration
envelope, characterized by texturally destructive chlorite-,
dolomite- and quartz-rich assemblages. Dolomite,
chlorite, quartz, calcite and sulphides have
selectively replaced breccia and sandstone beds in the
Main Zone host sequence, whereas the underlying
porphyritic dacite is weakly sericite altered. Vuggy and
botryoidal textures resulted from partial dissolution of
the dolomite-altered sedimentary rocks and unimpeded
growth of base metal sulphides, carbonate and quartz
into open cavities. The intense chlorite-rich alteration
assemblage, underlying Toms Zone, grades outward
into a weak pervasive sericite-quartz assemblage with
distance from the massive sulphide lens. Limestone
clasts and hydrothermal dolomite at Lewis Ponds are
enriched in light carbon and oxygen isotopes. The
dolomite yielded delta 13 CVPDB values of -11 to +1 per mil and delta 18O VSMOW values of 6 to 16per mil. Liquid-vapour fluid inclusions in the dolomite have low salinities (1.4-7.7 equiv. wt% NaCl) and homogenization temperatures
(166-232 degrees C for 1,000 m water depth). Dolomitization
probably involved fluid mixing or fluid-rock interactions
between evolved heated seawater and the limestone-bearing facies, prior to and during mineralization.
delta 34 SVCDT values range from 2.0 per mil to 5.0 per mil in the massive sulphide and 3.9 per mil to 7.4 per mil in the footwall carbonate-chalcopyrite-pyrite stringer veins, indicating that the hydrothermal fluid may have contained mamgatic sulphur and a component of partially reduced seawater. The sulphide mineral assemblages at
Lewis Ponds are consistent with moderate to strongly
reduced conditions during diagenesis and mineralization.
Low temperature dolomitization of limestonebearing
facies in the Main Zone host sequence created
secondary porosity and provided a reactive host for
fluid-rock interactions. Main Zone formed by lateral
fluid flow and sub-seafloor replacement of the poorly
sorted breccia and sandstone beds. Base metal
sulphide deposition probably resulted from dissolution
of dolomite, fluid mixing and increased fluid pH.
Pyrite, sphalerite and galena precipitated from a relatively
low temperature, 150-250C hydrothermal fluid.
In contrast, Toms Zone was emplaced into finegrained
sediment at or near the seafloor, above a zone
of focused up-flowing hydrothermal fluids. Copperrich
assemblages were deposited in the Toms Zone
footwall and massive sulphide lenses in Main and
Toms zones as the hydrothermal system intensified.During the D1 deformation, fracture-controlled fluids
within the Lewis Ponds fault zone and adjacent
footwall volcanic succession remobilized sulphides
into syntectonic quartz veins. Lewis Ponds is a rare
example of a synvolcanic sub-seafloor hydrothermal
system developed within fossiliferous limestone-bearing
facies. The close spatial association between limestone,
hydrothermal dolomite, massive sulphide and dacite
provides a basis for new exploration targets elsewhere
in New South Wales.

Item Type: Article
Authors/Creators:Agnew, MW and Large, RR and Bull, SW
Keywords: Volcanic-hosted massive sulphide, Carbonate-hosted replacement, Sulphide textures, Limestone, Lachlan Fold Belt, Hill End Trough, Australia,VHMS, NSW
Journal or Publication Title: Mineralium Deposita
ISSN: 0026-4598
DOI / ID Number: 10.1007/s00126-004-0456-6
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