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The geology and genesis of the chlorite-carbonate alteration in the footwall of the Hellyer Volcanichosted Massive Sulphide (VHMS) deposit

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Bradley, A (1997) The geology and genesis of the chlorite-carbonate alteration in the footwall of the Hellyer Volcanichosted Massive Sulphide (VHMS) deposit. Honours thesis, University of Tasmania.

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Abstract

The chlorite-carbonate alteration assemblage is an unusual feature in the
footwall of the Hellyer volcanic~hosted massive sulphide deposit, Tasmania.
Carbonate alteration is common to many VHMS deposits however its origin and
characteristics has been rarely studied in detail.
Chlorite-carbonate alteration is defined as texturally diverse dolomite in a
matrix of fine-grained chlorite. This distinctive alteration is found at the top of
the chlorite-alteration zone around the central and northern hydrothermal
discharge sites beneath the deposit. On the ore contact ('contact zone'), the
chlorite-carbonate assemblage occurs as thin discontinuous lenses. A lower
stratiform chlorite-carbonate alteration zone ('lower zone') occurs approximately
40 metres below the contact zone, is more laterally continuous and only occurs
on the west side of the east flank of the ore deposit. Carbonate alteration is
associated with sericite, chlorite and quartz in the contact zone but is primarily
associated with chlorite in the lower zone. Carbonate-chlorite assemblages have
not been observed at greater depths in the footwall alteration pipe, or distal to the
deposit.
The carbonate mineralogy is Fe-dolomite or dolomite. Carbonate textures
consist of large and small spheroids, rhombs, massive carbonate and veinlets.
These textures appear to be associated with the initial porosity and permeability of
the host rock. The various carbonate textures formed synchronously and have
no textural zonation within the contact and lower alteration zones.
Whole-rock geochemical studies indicate major gains in Ca, Mg, and Fe
and losses in Si and Na in the chlorite-carbonate alteration compared to the
unaltered footwall andesite. An absolute mass-gain of 18g/100g is achieved when
converting an andesite to a chorite-carbonate altered rock.
Carbon isotopes (013C) range between +0.31 and +2.80/00 and oxygen isotopes
(0180) range from +10.29 to +18.29%0. Isotope studies indicate that the dolomites
formed from upwelling hydrothermal fluid (modified seawater with a minor
magmatic input) in the lower zone or from the mixing of the hydrothermal fluid
with infiltrating seawater in the contact zone. Carbon isotopic values are
uncharacteristically high at Hellyer compared with other VHMS deposits and
could indicate deep-seated contributions of 013C.
From distribution and textural evidence, it is proposed tha t the dolomite
alteration formed by direct precipitation within chloritised volcaniclastic units
near the seafloor or in shallow sub-seafloor units in the mixiI,lg zone between
upwelling hydrothermal fluids and infiltrating seawater. As the hydrothermal
fluid mixed with infiltrating seawater a pH increase counteracted the affects of
cooling and caused dolomite to precipitate.

Item Type: Thesis (Honours)
Additional 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).

Date Deposited: 12 Aug 2011 02:46
Last Modified: 11 Mar 2016 05:55
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