New Zealand Journal of Geology and Geophysics abstracts
Telescoped porphyry-style and epithermal veins and alteration at the
central Maratoto valley prospect, Hauraki Goldfield, New Zealand
Mark P. Simpson*
Jeffrey L. Mauk
Geology Department
The University of Auckland
Private Bag 92 019
Auckland, New Zealand
*email: ma.simpson@auckland.ac.nz
Ross G. Kendrick
Harrison Grierson Consultants Limited
P.O. Box 5760
Auckland, New Zealand
Abstract At the central Maratoto valley prospect, southern
Coromandel Peninsula, New Zealand, andesite flows and dacite breccias host rare
porphyry-style quartz veins that are telescoped by widespread epithermal veins
and alteration. Early porphyry-style quartz veins, which lack selvages of porphyry-style
alteration, host hypersaline fluid inclusions that contain several translucent
daughter crystals, including halite and sylvite. Overprinting epithermal veins
and alteration are divided into two stages. Main-stage epithermal alteration
and veins are characterised by the successive deposition of pyrite, quartz,
and ankerite-dolomite veinlets coupled with intense alteration of the wall rock
to quartz, illite, interlayer illite-smectite (≤10% smectite), chlorite,
pyrite, ankerite, and dolomite. Late-stage epithermal veins and alteration are
characterised by the formation of calcite and siderite veinlets, coupled with
overprinting of the wall rocks by both these minerals.Multiphase fluid inclusions
in a porphyry-style quartz vein formed at temperatures >400°C and trapped
hypersaline magmatic fluid. Lower temperature secondary liquid-rich inclusions
in the porphyry-style quartz vein homogenise between 283 and 329°C and trapped
a dilute fluid with <1.8 wt% NaCl equivalent. Inclusions in later epithermal
quartz and calcite veins homogenise between 240 and 280°C (av. 260°C)
and trapped a dilute fluid with apparent salinities of <2.9 wt% NaCl equivalent.
Based on homogenisation and salinity data, secondary inclusions in porphyry-style
quartz veins may have formed 700-950 m deeper than telescoping epithermal veins.Main-stage
epithermal ankerite and dolomite have δ18O(VSMOW) values of
13.5-18.1‰, whereas late-stage epithermal calcite has δ18O(VSMOW)
values of 3.1-5.1‰. Calculated isotopic compositions for the fluid in equilibrium
with ankerite-dolomite and calcite at 260°C, averages 6 and -3‰, respectively.
The enriched value for main-stage ankerite-dolomite suggests formation from
waters that underwent significant water-rock exchange, whereas isotopically
lighter water that formed late-stage calcite underwent little water-rock interaction.We
propose a three-stage model to explain telescoped veins and alteration styles
at the central Maratoto valley prospect area. Porphyry-style quartz veins were
the first to form from hot hypersaline multi-cation magmatic fluids. These are
telescoped by later widespread epithermal veins and alteration following descent
of the paleowater table possibly due to rapid erosion or sector collapse of
a volcanic edifice. Main-stage epithermal alteration and deposition produced
quartz, chlorite, illite, interlayered illite-smectite, pyrite, and isotopically
heavy ankerite-dolomite from deeply circulating upwelling alkali chloride waters.
Late-stage collapse of the hydrothermal system resulted in the formation of
overprinting calcite and siderite from isotopically lighter descending marginal
steam-heated CO2-rich waters.
Keywords central Maratoto valley; Coromandel Volcanic
Zone; porphyry deposit; epithermal deposit; telescoping; hydrothermal alteration;
carbonates; fluid inclusions; oxygen and carbon isotopes
G03001; Online publication date 25 February 2004
Received 23 January 2003; accepted 10 November 2003
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47: 39-56
0028-8306/04/4701-0039 © The Royal Society of New Zealand 2004
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