Sulfide Evolution During Prograde Metamorphism of the Otago and Alpine Schists, New Zealand

Abstract

Sulfide minerals in the Otago and Alpine schists, New Zealand, a metasedimentary belt exposed from unmetamorphosed greywackes up to amphibolite facies, underwent systematic changes in abundance, composition and texture during prograde metamorphism. In unmetamorphosed rocks, the most common sulfide mineral is framboidal pyrite, which contains abundant As (up to 14000 ppm), Co (up to 4000 ppm), Cu (up to 14000 ppm), Ni (up to 1100 ppm) and, locally, Ag (up to 270 ppm), Au (up to 90 ppm), and Sb (up to 240 ppm). Chalcopyrite, sphalerite, and galena also occur as isolated grains. Chalcopyrite and sphalerite contain few trace elements, whereas galena contains significant Se (up to 1600 ppm) and locally abundant Hg (up to 600 ppm). The distribution of these trace and minor elements is extremely heterogeneous. In subgreenschist-facies rocks, pyrrhotite replaces pyrite, and there is a clear textural change from framboidal pyrite to composite grains of pyrrhotite, sphalerite, chalcopyrite, galena and cobaltite. Pyrrhotite contains Co (average values 1100 ± 490 ppm), Cu (up to 17000 ppm), and Ni (up to 11000 ppm). Antimony and Hg are above detection in rare individual grains, but none of the other trace and minor elements sought are detectable. Sphalerite, galena and cobaltite increase in proportion in subgreenschist-facies rocks, and also contain higher concentrations of Ag (up to 1480 ppm in galena), Au (up to 230 ppm in galena and 110 ppm in cobaltite), As (38 ± 6 wt. % in cobaltite), Co (26 ± 4 wt. % in cobaltite), Hg (up to 4500 ppm in galena and 1100 ppm in sphalerite), and Sb (up to 1280 ppm in cobaltite and 770 ppm in galena). Pyrite, sphalerite, galena, and cobaltite become less abundant from subgreenschist-to amphibolite-facies rocks. In amphibolite-facies rocks, only pyrrhotite, chalcopyrite and trace amounts of galena and molybdenite occur, and none of these minerals contain detectable levels of Ag, Au, As, or Hg. Mass-balance calculations between sulfide minerals and whole rocks show that sulfides are important host minerals for S, Cu, and As, but host a minor proportion of Fe, Zn, and Pb. As pyrite, sphalerite, galena, and cobaltite become less abundant at higher metamorphic grade, Pb, Zn and Co are retained in the rock incorporated in other minerals, whereas As, Sb, Hg, Au, and Ag are removed from the rock, most likely by metamorphic devolatilization.

Diagenetic pyrite as a source for metals in orogenic gold deposits, Otago Schist, New Zealand

“…In the Otago Schist of southern New Zealand, trace metals were released from the metasedimentary pile over a wide range of metamorphic conditions, from subgreenschist facies to amphibolite facies, but primarily within the greenschist facies (Pitcairn et al 2010). The Macraes mine, a world-class orogenic gold deposit, formed under greenschist facies conditions in the same metamorphic pile, with mineralisation driven by metamorphic processes (Craw et al 1999;Craw 2002).…”

“…At least some of these trace metals can be concentrated in the sediments by diagenetic processes on the sea floor, particularly into diagenetic pyrite (Large et al 2007(Large et al , 2009. Hence, recrystallisation and decomposition of this diagenetic pyrite during prograde metamorphism are potentially fertile sources for gold and other trace elements during orogenesis (Pitcairn et al 2006;Scott et al 2009;Large et al 2011).In the Otago Schist of southern New Zealand, trace metals were released from the metasedimentary pile over a wide range of metamorphic conditions, from subgreenschist facies to amphibolite facies, but primarily within the greenschist facies (Pitcairn et al 2010). The Macraes mine, a world-class orogenic gold deposit, formed under greenschist facies conditions in the same metamorphic pile, with mineralisation driven by metamorphic processes (Craw et al 1999;Craw 2002).…”

Raman characterization of carbonaceous material in the Macraes orogenic gold deposit and metasedimentary host rocks, New Zealand

“…These elements were initially weakly enriched compared to the average crustal 124 concentrations, on a regional scale, in diagenetic pyrite in the prehnite-pumpellyite facies 125 protoliths, and prograde metamorphic recrystallization of that pyrite facilitated mobilization 126 of Au and As Pitcairn et al, 2006Pitcairn et al, , 2010Pitcairn et al, , 2014. Minor intercalated 127 metabasic layers in the metamorphic pile did not contribute significantly to the amount of 128 mobilized Au, and were local sinks for mobilized As (Pitcairn et al, 2014 All of the orogenic systems depicted in Figure 1 have been mined historically at some stage, 137 but production has been small.…”

“…The rocks become 108 progressively more recrystallized and foliated through this increase in metamorphic grade, 109 and the schists in the core of the belt have been pervasively recrystallized with several 110 generations of folding and foliation development (Mortimer, 1993). Amphibolite facies rocks 111 underlie the core upper greenschist facies rocks, and these have been tectonically exposed The consistent protolith composition that persists across the whole schist belt has enabled 118 geochemical comparison of metal and metalloid contents at different metamorphic grades, in 119 order to quantify metamorphogenic mobilization of these elements (Pitcairn et al, 2006(Pitcairn et al, , 2010 2014). Gold and arsenic, in particular, have been demonstrably mobilized on a regional scale 121 during metamorphism, with prominent decreases in contents of these elements in the 122 transition from the lower greenschist facies to amphibolite facies (Pitcairn et al, 2006(Pitcairn et al, , 2014.…”

Minéralisations et déformation à proximité de la faille de Davidson, Abitibi, Canada /

“…De plus, les environnements géologiques restent très variables au sein d'une même province, que ce soit les styles de minéralisation, les lithologies présentes, les altérations hydrothermales et le style structural (Robert et Poulsen 2010). Enfin la source de l'or n'est pas encore établie, la théorie d'un héritage sédimentaire local s'opposant actuellement à un apport par des fluides hydrothermaux d'une source profonde, voire mantellique (Tomkins, 2010, Pitcairn et al, 2010et Thomas et al, 2010. La profondeur de mise en place est délimitée entre les zones de sub-surfaces et la zone de transition ductile-fragile (Hagemann et al 1992 Yilgarn situé en Australie (Boulter et al, 1987et Mueller et al, 1987 (Kenworthy et Hagemann, 2007), mais également parfois des failles majeures issues d'un milieu fragile (Hagemann et al, 1992).…”