New Zealand Journal of Geology and Geophysics abstracts
Gersdorffite with pentlandite, violarite, pyrrhotite, and pyrite, northwest
Nelson, New Zealand
R. H. GRAPES
School of Earth Sciences
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand
G. A. CHALLIS
Institute of Geological & Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand
Abstract Gersdorffite (NiAsS), only reported in New Zealand
in 1887 from the Richmond Hill area, has been found in two further localities
in northwest Nelson. Gersdorffite has been relocated in a quartz-veined
ophicarbonate at Parapara River near Richmond Hill, and has average
compositions of (Fe0.21Co0.14Ni0.65)As1.02S0.96 (quartz veins) and
(Fe0.19Co0.05Ni0.76)As1.04S0.96 (ophicarbonate host). It is associated with
nickeliferous pyrrhotite (1.93-4.54 wt% Ni), pyrite (0.47-3.73 wt%
Ni), and violarite. Another ophicarbonate occurrence at Calphurnia Creek
contains gersdorffite (Fe0.16Co0.07Ni0.77) (As0.98Bi0.02)S1.00) with up to
2.8 wt% bismuth, associated with cobaltian violarite (2.78-4.83 wt%
Co) and unusually nickel-rich pentlandite (42.99-43.29 wt% Ni) with a
Ni:Fe ratio of nearly 2:1. A further occurrence, a fuchsite quartzite at
Contact Creek, contains cobalt-rich gersdorffite (7.68-12.00 wt% Co) with
average compositions of (Fe0.16Co0.21Ni0.63)As0.99S0.95 and
(Fe0.18Co0.33Ni0.73) As1.01S0.97 together with composite grains of pyrrhotite
(0.74-1.47 wt% Ni) - low cobalt-pentlandite (0.90-1.71 wt% Co), and
minor pyrite. The Ni-As-Fe sulphides at Parapara River and Calphurnia Creek are
products of talc- carbonate facies alteration of ultramafic rock during early
Paleozoic (Pre-Baton Event) regional greenschist facies metamorphism, with
temperatures of sulphidation ranging between c. 300 and <100deg.C
(below that of associated silicate/carbonate assemblages of 320-490deg.C), -33
to -30 log[[florin]]O2, c. -1 to -1.3 log[[Sigma]]S, and between c. -0.9 and
-0.11 log[[florin]]S in the fluid phase; the presence of gersdorffite requires
the introduction of As +/- Bi from a carbonate-bearing fluid. At Contact Creek,
arsenic was introduced into the fuchsite quartzite by hydrothermal fluids from
the nearby Early Cretaceous Onahau Granite that also transported nickel and
cobalt scavenged from a lens of antigorite-talc-magnesite within the pluton.
Temperatures of gersdorffite and an initial metal solid solution (mss)
formation at Contact Creek were between c. 500 and >380deg.C, the mss
subsequently exsolving pentlandite/pyrrhotite at <230deg.C, and formation of
low-nickel pyrite at <150deg.C.
Keywords gersdorffite; violarite; pentlandite; pyrrhotite;
pyrite; analyses; temperatures, [[florin]]O2, [[florin]]S; hydrothermal fluids;
talc-carbonate alteration, greenschist-amphibolite facies metamorphism;
northwest Nelson; New Zealand
New Zealand Journal of Geology and Geophysics, 1999, Vol. 42: 189-204
0028-8306/99/4202-0189 $7.00/0 (c) The Royal Society of New Zealand
1999
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