Gersdorffite with pentlandite, violarite, pyrrhotite, and pyrite, northwest Nelson, 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 (Fe_0.21Co_0.14Ni_0.65)As_1.02S_0.96 (quartz veins) and (Fe_0.19Co_0.05Ni_0.76)As_1.04S_0.96 (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 (Fe_0.16Co_0.07Ni_0.77) (As_0.98Bi_0.02)S_1.00) 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 (Fe_0.16Co_0.21Ni_0.63)As_0.99S_0.95 and (Fe_0.18Co_0.33Ni_0.73) As_1.01S_0.97 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 <100°C (below that of associated silicate/carbonate assemblages of 320–490°C), ‐33 to ‐30 logƒO₂, c. ‐1 to ‐1.3 logSS, and between c. ‐0.9 and ‐0.11 logƒS 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 >380°C, the mss subsequently exsolving pentlandite/pyrrhotite at <230°C, and formation of low‐nickel pyrite at <150°C.

Keywords:

• gersdorffite
• violarite
• pentlandite
• pyrrhotite
• pyrite
• analyses
• temperatures, ƒO₂, ƒS
• hydrothermal fluids
• talc‐carbonate alteration, greenschist‐amphibolite facies metamorphism
• northwest Nelson
• New Zealand