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The geology and geochemistry of granitoids in the Childara region, West Gawler craton, South Australia : implications for the Proterozoic tectonic history of the Western Gawler craton, and development of lode-style gold mineralisation at Tunkilla

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Ferris, Gary M.(Gary Michael) (2001) The geology and geochemistry of granitoids in the Childara region, West Gawler craton, South Australia : implications for the Proterozoic tectonic history of the Western Gawler craton, and development of lode-style gold mineralisation at Tunkilla. Research Master thesis, University of Tasmania.

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Abstract

The western Gawler Craton comprises poorly outcropping basement units which has
hampered the tectonic understanding of the area. Recent high-resolution
aeromagnetic data has provided an insight into the basement composition and allowed
preliminary tectonic interpretation. The Gawler Craton is here divided into tectonic
subdomains based on structural/metamorphic and aeromagnetic signature, and records
a dominantly Proterozoic structural history. Regional mapping on CHILDARA 1:250
000 and reconnaissance sampling on FOWLER, STREAKY BAY, and NUYTS, was
designed to determine the geochemistry of the basement lithologies, and attempt to
construct a geological history of the western Gawler Craton, building on recent
geochronology and structural/metamorphic work within the Fowler Subdomain to the
west. This understanding is further used to constrain the age and controls on gold and
base-metal mineralising events. Previous solid geology interpretations of the western
Gawler Craton showed an Archaean basement to much of the area, however, this
study has shown no Archaean exists within the Nuyts Subdomain.
Four main periods of Proterozoic granitoid plutonism over a 130 My period (-1690 —
1560 Ma) are recognised on CHILDARA. Recent geochronology has shown that
major structures post-date the Kimban Orogeny (1845 — 1695 Ma), hence deformation
and associated magmatism on the western and northern Gawler Craton are now
attributed to the Kararan Orogeny (1690 — 1540 Ma).
The 1690-1670 Ma Tunkillia Suite is restricted to the eastern Nuyts Subdomain and
scattered plutons within the Christie, Fowler and Cleve Subdomains. The Tunkillia
Suite is felsic (67 — 76.5 wt% Si02 values) with Na/K <1, has high Rb/Sr, variable
HREE enrichment and moderate to strong negative Eu anomalies. On CHILDARA,
the Tunkillia Suite crops out within the Yarlbrinda Shear Zone, and hosts the
Tunkillia and Nuckulla Hill gold prospects.
The 1630-1608 Ma St Peter Suite is restricted to the Nuyts Subdomain. Within the
Kalanbi and Rocky Point-Cape Beaufort areas, these rocks generally resemble
magmatic arc-related batholiths. The St Peter Suite is characterised by an expanded
SiO2 range (47-76 wt%), is sodic (Na/K >1), shows high Sr, K/Rb and Sr/Y and
relatively low K20, Rb, Rb/Sr, Th, U (ie: low heat producing granites), REE, and Nb.
The St Peter Suite is LREE enriched, HREE and Y-depleted, with little or no Eu
anomaly. These geochemical characteristics are similar to modern magmatic arc rocks
and are consistent with formation within a subduction related environment, with
magmas produced from partial melting of hydrous basalt at depths within the garnet
stability field. The St Peter Suite is considered to have formed by fractionation of arc-type
magmas and/or partial melting of arc-related intrusions, or mafic under-plating
within a subduction related continental margin setting.
The Hiltaba Suite (1595-1575 Ma) is an extensive suite of felsic granites (>70 wt %
Si02) characterised by low Na/K (<1), high Rb/Sr and variable FREE enrichment,
with moderate to strong negative Eu anomalies. The comagmatic Glyde Hill
Volcanics correlate with the Gawler Range Volcanics, and comprise a series of
dominantly felsic lavas and ignimbrites. The Hiltaba Suite/Gawler Range Volcanics
generally do not show an arc affinity. They record a major shift in tectonic
environment from a dominantly compressional environment during the St Peter Suite
magmatic event, to an intracontinental extensional environment, but the initial source
of heat may be related to earlier subduction (ie: back-arc extension origin).
The 1560 Ma S-type Munjeela Granite represents a late stage melt and is related to a
period of extension and basin formation to the west of the Koonibba Fault Zone.
The most likely tectonic model that accounts for these geochemical features is as
follows. Continental collision occurred between the Fowler Subdomain and the proto-Yilgarn
Craton to the west, initiating southward dipping subduction to produce the arc
related St Peter Suite magmatism. However, this change appears to have been
unsustainable and subsequent intraplate extension, possibly together with basaltic
underplating of the continental crust, produced the voluminous Hiltaba Suite/Gawler
Ranges Volcanics.
These geological events produced lode gold-style mineralisation within the
Yarlbrinda Shear Zone. The Yarlbrinda Shear Zone is a major north-south trending
shear zone located on eastern CHILDARA, which hosts the Tunkillia and Nuckulla
Hill gold prospects. The host granites are extensively altered, with mineralisation
associated with zones of intense sericite alteration. K-Ar dating of sericite from
Tunkillia recorded a cooling age of —1600 Ma, suggesting a relationship to the Hiltaba
Suite/Gawler Range Volcanic magmatic event. Two main vein generations are
defined at Tunkillia, with mineralisation located within narrow quartz veins (V1). V1
veins comprise quartz and sulphides and combine to form steeply dipping lenses
ranging in thickness from cm-scale up to 1 m, which are sub-continuous along strike.
Pyrite is the main sulphide with accessory galena and chalcopyrite. V2 veins
comprise late, crosscutting calcite veins, which are barren.
Primary fluid inclusions hosted by V1 veins are typically two-phase at room
temperature containing an aqueous phase and vapour. Preliminary fluid inclusion data
suggests that gold deposition took place over a temperature range of 200 0 - 300° C
from a low salinity fluid (≤4 wt% NaCl equivalent). Sulphur isotope analysis shows
the source of sulphur is magmatic (-2.23 to 3.19), and lead isotope data indicate that
mineralisation is consistent with an origin by fluid release from Hiltaba Suite granites,
however, the Pb-isotopic composition of other granitoid suites has not been
characterised.
The Yarlbrinda Shear Zone shows evidence for an early phase of dextral strike-slip
deformation, which is expressed by a regional foliation and shallow plunging
stretching lineations. Kinematic indicators including S-C fabrics and rotated
porphyroclasts show a dextral sense of shear. Later deformation comprised dip-slip
movement, which produced steeply plunging stretching lineations and rotated
porphyroclasts, showing a west-side-up sense of movement. Mineralisation at
Tunkillia appears to be localised at the junction of the regional foliation and steeply
plunging lineations, and is interpreted as late in the structural history. Late stage
brittle structures related to the locking up of the Yarlbrinda and Yerda Shear Zones
crosscut the shear zone and are unrelated to mineralisation. The Yarlbrinda Shear
Zone is part of a large-scale west-directed transpressional duplex that developed on a
regional restraining bend of the shear system.
U-Pb zircon geochronology shows that deformation within the Yerda Shear Zone was
taking place at -1592 Ma, but deformation within the Yarlbrinda Shear Zone must
have ceased by -1580 Ma when the Kondoolka Batholith was intruded. The extent of
sericite alteration requires a voluminous influx of low pH fluids, with the Hiltaba
Suite the most likely source. This fluid mixed with low salinity metamorphic fluids
which leached Pb and S from the granites at depth.

Item Type: Thesis (Research Master)
Keywords: Geology, Geochemistry, Geology, Stratigraphic, Gold ores, Ore deposits
Copyright Holders: The Author
Copyright Information:

Copyright 2001 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:

Maps in pocket inside back cover. Thesis (M.Sc.Exp.Geosc)--University of Tasmania, 2002. Includes bibliographical references

Date Deposited: 09 Dec 2014 00:07
Last Modified: 11 Mar 2016 05:54
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