New Zealand Journal of Geology & Geophysics abstracts
Contrasting hydrothermal alteration mineralogy and geochemistry in
the auriferous Rise & Shine Shear Zone, Otago, New Zealand
D. J. MacKenzie
D. Craw
Geology Department
University of Otago
PO Box 56
Dunedin 9054, New Zealand
dave.craw@stonebow.otago.ac.nz
Abstract The Rise & Shine Shear Zone is a c. 50
m thick zone of hydrothermally altered schist that has been sheared
subparallel to the pervasive foliation during hydrothermal alteration.
The shear zone is locally auriferous, and has some structural
similarities to the Hyde-Macraes Shear Zone that hosts the currently
active Macraes mine. Shearing was accompanied by recrystallisation of
metamorphic phengite to hydrothermal muscovite, alteration of
metamorphic titanite to rutile, and partial decomposition of
metamorphic albite and calcite. Metamorphic chlorite has been almost
completely decomposed to form hydrothermal ankerite (ferroan dolomite
and magnesian siderite), and this alteration reaction yielded minor
quartz for silicification of sheared schist and localised formation of
millimetre-scale quartz veinlets. Hydrothermal ankerite occurs
principally in fractures in quartz-feldspar segregations adjacent to
micaceous shears. Chalcopyrite and sphalerite accompany this ankerite,
but there has been no Cu or Zn enrichment. Gold occurs in sheared
schist that contains elevated As (up to 10 000 ppm) in arsenopyrite.
Hydrothermal arsenopyrite and pyrite have replaced some schist and have
been controlled by foliation-parallel shear development. Antimony
enrichment (up to 8 ppm) mimics As enrichment in altered schist. There
has been no Sr enrichment associated with ankeritic carbonation of the
schist like that observed in some nearby Otago Schist auriferous vein
systems.
The shear-related alteration is postdated by an
entirely different style of alteration that accompanied upright folding
and vein formation. This later alteration stage was dominated by
formation of hydrothermal albite, chlorite, and calcite. Hence, the
later alteration reactions were essentially reversals of those of the
earlier stage. Differences in alteration assemblages are a result of
differences in fluid composition during alteration, but the reasons for
this, and the exact fluid composition parameters, remain unconstrained.
Keywords ankerite; gold; Otago Schist; Macraes mine;
arsenic; strontium; structure
G06023; Online publication date 4 April 2007; Received 18 July 2006;
accepted 27 February 2007
New Zealand Journal of Geology & Geophysics, 2007, Vol. 50:
67–79
0028–8306/07/5002–0067 © The Royal Society of New Zealand 2007
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