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Abstract

The Mineral Hill "volcanic-hosted massive-sulphide" type copperlead-zinc
deposits are located in central New South Wales, Australia,
in the Tasman Orogenic Zone.
The deposits formed in an Upper Silurian (?) sequence of felsic
volcanics and pyroclastics and overlying siltstones, sandstones,
mudstones, and carbonates which have been affected by normal faulting,
gentle Mid-Devonian (?) upright folding, and lower greenschist-grade
metamorphism.
Mineralization is of two types: (1) discordant near-vertical
cylindrical stockworks delineated by high metal grades, intense alteration,
and a zoned ore-mineral assemblage within felsic lapilli tuffs, vitric
tuffs and ignimbrites, and (2) conformable massive-sulphide lenses (most
of which have been affected by oxidation and leaching during weathering)
and conformable hematitic chert lenses in sediments overlying the pyroclastic
succession.
Massive ore lenses exhibit banded interlayering of ore with chert
and in some areas are highly brecciated. Unoxidized massive-sulphide
mineralization contains a pyrite-sphalerite-galena-quartz-chloritetetrahedrite
assemblage and well-preserved framboidal textures.
Hematitic chert (jasper) lenses and veins from the upper part of
stockworks have colloform textures, chalcedony spherulites, botryoidal
structures, shrinkage (?) cracks, flow structures, and rotated and
displaced bands. These features suggest a gel state existed prior to
the crystallization of these minerals.
The major stockwork zone (the Parkers Hill stockwork) contains
the following zoned alteration and mineralization assemblage:
Upper section - Quartz + chlorite + adularia + bornite + chalcopyrite +
galena + sphalerite ± tetrahedrite ± biotite
Lower section - Quartz + chlorite + sericite + pyrite + chalcopyrite +
galena + sphalerite.
As well as being associated with sulphides in the lower section of
the Parkers Hill stockwork, sericite-chlorite-quartz alteration is
found surrounding the upper section of the stockwork and extends in
decreasing amounts well into non-mineralized pyroclastics throughout
the field.
The hydrothermal alteration of the central Parkers Hill stockwork
is pervasive and represents a pronounced depletion of Ca and Na and
an addition of Fe, Si and Mg. The mole % FeS in sphalerite in the
stockwork varies from less than 1% to 5%. The Mg/Mg+Fe in the chlorites
varies from 0.3 to 0.8; and S 4 S values in chalcopyrite vary from +6.0
to 9.0%o.
Three types of fluid inclusions are found in quartz veins from the
Parkers Hill stockwork: Type I are liquid-filled inclusions;
Type II are two-phase inclusions with small bubbles; and Type III are
vapour-filled inclusions. Homogenization of Type II inclusions gave
temperatures from 79 °to 351 °C with peak values at 160 °C and 250°C; and
salinities determined from freezing studies were 5 to 22 wt.% NaC1
equivalent with the majority of values falling between 9 and 21%.
Type III inclusions indicate the solution was boiling at the time of
entrapment.
Phase diagrams at 250 oC show that the transition in the Parkers
Hill stockwork from pyrite-chalcopyrite to bornite-chalcopyrite can be
produced by an increase in pH and/or a decrease in temperature but that
the concomittant transition from quartz-chlorite-sericite to quartz-chlorite-adularia
indicates an increase in pH.
A mass-transfer model for a boiling system is presented which
follows the theoretical interaction of sixty-eight solution, gas,
and solid species through decreasing temperature, increasing pH, and
changes in the concentration of solutes. The model demonstrates that
the zonation in the Parkers Hill stockwork, and in particular the
transition from sericite to adularia, can be generated by boiling.
The addition of a component of wall-rock reaction to the model
facilitates the precipitation of the assemblage minerals.
It is proposed that the stockwork mineralization and alteration
was formed below the sea floor by a process of boiling and wall-rock
reaction and that the overlying massive zones were precipitated at the
sea water-rock interface during mixing of emerging solutions with
sea water.

Item Type:	Thesis - PhD
Authors/Creators:	Bush, Asahel
Keywords:	Ore deposits, Mines and mineral resources
Copyright Holders:	The Author
Copyright Information:	Copyright 1980 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, 1981. Bibliography: l. 271-295
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