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The Lady Loretta formation : sedimentology and stratiform sediment-hosted base metal mineralisation

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Dunster, John N (1997) The Lady Loretta formation : sedimentology and stratiform sediment-hosted base metal mineralisation. PhD thesis, University of Tasmania.

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Abstract

The Palaeoproterozoic Lady Loretta Formation of the Mount Isa Basin, northwestern
Queensland, was deposited during a series of fluctuations in relative sealevel that constitute
an overall regression. The interpreted sedimentary environments range from sub-wavebase
to supratidal. Carbonaceous lagoonal sediments host economic stratiform base metal
mineralisation.
Previous tectono-sedimentary models for the Lady Loretta Formation that invoked a
rift-setting and interpreted a regional "basal breccia" as evidence of syn-sedimentary uplift
and erosion are disputed. This study interprets the setting as a ramp/shelf with no evidence
of large-scale syn-sedimentary tectonic activity. The "basal breccia" is interpreted as a
duricrust that has no palaeoenvironmental implications for the depositional setting.
Sedimentary structures, microbialites (stromatolites), evaporite pseudomorphs and
the arrangement of interpreted depositional environments indicate fluctuations in water
depth, tidal influence and storm activity during the deposition of the Lady Loretta Formation.
Shale, laminated to massive argillaceous dolostone and fine grained sandstone containing
hummocky and swaley cross-stratification are interpreted as the deepest-water facies
present. Shallow marine carbonates now consist of variably silicified dolostone and include
microbialites that range from prone mat to biostromes and bioherms of domal, digitate and
columnar forms with significant synoptic relief. Ooid shoals developed in areas of shallow
agitated water. Widespread mixed carbonate/siliciclastics and carbonates in the northern
outcrops of the Lady Loretta Formation appear to be cyclic and developed as a facies
mosaic. They are interpreted as peritidal on the basis of bipolar-bimodal palaeocurrent
directions and the prevalence of flaser to lenticular bedded units, tidal rhythmites,
interference wave ripples and herringbone cross-stratification with reactivation surfaces.
Storm deposits such as imbricated plate breccias and gutter casts are common.
Widespread casts and moulds of halite and pseudomorphs of discoidal gypsum, enterolithic
anhydrite and cauliflower cherts are interpreted to represent an evaporitic overprint produced
during regression when a marine sabkha developed locally. Associated sedimentary
features include desiccation cracks, synaeresis cracks, washout rills, scour pits and
wrinklemarks. Highly carbonaceous pyritic shale, variably diagenetically altered Fe- and Mn-rich
carbonates and dolomitic siltstone were deposited in isolated areas, laterally and
stratigraphically associated with tidal and possible subaerial facies. These pyritic facies
occur at several localities in the Lady Loretta Formation, one of which hosts the Lady Loretta
ore body consisting of 8.3 Mt of 12% Zn-Pb-Ag combined. The ore and its host rocks contain
a variety of microbialites, including silicified or pyritised prone mat and low-relief, small
diameter, elongate and inclined digitate forms. Parts of the host sequence also contain wave
ripples, crossbeds with bipolar palaeocurrent directions and a possible sulphate evaporite
overprint. Sulphur isotope data for pyrite and base metal sulphides are interpreted to
indicate a closed system with abundant microbial sulphate reduction. Collectively, this is interpreted to indicate that the host rocks were deposited in a restricted lagoon developed
within a regional tidal-flat environment.
Similar potential host lithologies are recognisable elsewhere in the formation by their
sedimentary features, the high Mn and Fe content of the carbonate and the abundant
bedded pyrite interpreted to have originally been prone microbial mat. Theoretically, the
distribution of such lagoonal facies can be predicted by sequence stratigraphy. On a rimmed
shelf, extensive lagoonal facies can develop during a lowstand as the barrier becomes
exposed by falling sealevel. Smaller perched lagoons may develop on the tidal flat of an
unrimmed shelf. In highstand systems tracts on a rimmed shelf, microbialite growth may
keep pace with rising sealevel to create large lagoons during minor regressions. Subwavebase
shales that correspond to maximum flooding surfaces also have the potential to
host base metal mineralisation in the Lady Loretta Formation.
Previously proposed genetic mineralisation models for the Lady Loretta ore body
have been revised based on the new interpretation of the deposition setting. The classic
SEDEX model that relies on exhalation into deep anoxic water confined within a graben is
untenable in the shallow lagoonal setting proposed herein. A new model in which
mineralisation is interpreted to have formed in unconsolidated sediments in the shallow
subsurface is based on geochemical, isotopic and textural data. It is also consistent with a
lagoonal setting where carbonaceous and pyritic sediments would act as reductants.
Alternatively, a late epigenetic model involving the long distance migration of brines and
mineralisation by replacement of, or void-fill in, consolidated rock is supported by the
difference between the SHRIMP U-Pb zircon age of a tuff in the footwall of the host
sediments (1647±4 Ma) and the Pb isotopic model age of the mineralisation (within range of
1600- 1570 Ma).
Comparison of the geochemistry of the ore sequence with other carbonaceous
pyritic packages elsewhere in the formation suggests that geochemical exploration should
be directed towards the direct detection of metals and pathfinder elements such as Ba,
Cd, Hg and TI, and the use of alteration indices and carbon and oxygen stable isotopes to
map haloes.

Item Type: Thesis (PhD)
Keywords: Sedimentary structures, Sedimentary structures, Ore deposits, Ore deposits, Geology, Structural, Geology, Structural
Copyright Holders: The Author
Copyright Information:

Copyright 1997 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: 09 Dec 2014 00:03
Last Modified: 11 Mar 2016 05:55
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