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Volatile and precious metal geochemistry of the Mount Isa ores and their host rocks

McGoldrick, PJ 1986 , 'Volatile and precious metal geochemistry of the Mount Isa ores and their host rocks', PhD thesis, University of Tasmania.

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Geochemical and petrographic investigations of Pb-Zn-Ag
mineralization ( 12 orebody) and Cu-Co mineralization (1100 orebody)
from Mount Isa were undertaken. Over one hundred and twenty
carefully selected samples were analyzed for major and minor
elements and for some or all of the following volatile metals: Au,
Ag, Cd, As, Sb, Se, Bi, Co and Tl.
A strong Tl enrichment is observed in (pyritic) unmineralized
lateral equivalents of 12 orebody for several kilometers to the
north of the mine sequence. The Se and As contents, S/Se ratios and
S isotope relationships in the Pb-Zn ores and their host pyritic
shales preclude a magmatic or deep-seated hydrothermal S source.
The data suggest that sulfide S in the Urquhart Shales was derived
from reduction of a ''seawater"/evaporitic/pore water sulfate source.
Lateral variations in the thickness of mineralized intervals, the
nature of the sulfide-gangue textures in the ores, the pervasive K
and Tl enrichment in the host rocks and other chemical features of
the Pb-Zn ores indicate that much of the Mount Isa mineralization
formed epigenetically within the unconsolidated Urquhart Shales.
The Pb-Zn-Ag ores contain very little Au and it is argued that this
feature is best explained by the hydrothermal solutions that formed
the Pb-Zn ores being cool (<<200°C) and moderately oxidized.
The "silica dolomite" (the host to all the Mount Isa Cu
mineralization) formed from "normal" Urquhart Shale as a result of
intense fault-related hydrothermal activity (Perkins, 1984). The
alteration has silicified the shales adjacent to the fault, and
dolomite, phyllosilicates and "immobile" elements liberated during
the silicification have been re-deposited at higher levels up-dip in
the silica dolomite bodies. For the most part primary sulfide
textures have not been preserved.
It is argued that the distribution of several elements (notably Co,
Bi, As, Fe and S) in 1100 orebody and its location down-dip from a
strongly pyritic section of Urquhart Shale are good evidence that
stratiform Co (and Cu) mineralization was present in pyritic
Urquhart Shales prior to formation of the silica dolomite. Chemical
and {sotopic evidence suggests that the Cu mineralization had a similar s-~ource and formed from similar solutions to the Pb-Zn-Ag
A new co-genetic model for the Mount Isa Cu and Pb-Zn-Ag deposits in
which the mineralization formed from cool oxidized solutions in the
upper few meters of the unconsolidated Urquhart Shales is presented.
The metal-bearing solutions were expelled from their source rocks
(oxidized clastic sediments lower in the Mount Isa Group) during the
course of normal basin compaction and dewatering. Base metal
sulfides were fixed by sulfate reduction processes occuring in the
diagenetic environment of the Urquhart Shales. Weathered mafic
volcanic detritus may have been an important component of the source.

Item Type: Thesis - PhD
Authors/Creators:McGoldrick, PJ
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