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

Abstract

Framboidal diagenetic pyrite occurs in low-grade turbidites on the northeast margin of the Otago Schist. Laser ablation inductively coupled plasma mass spectrometry (ICP-MS) analyses of these framboids show that the main metals present, in addition to Fe, are As, Pb, Zn, Te, Ag and Mo; Au is present at between 0.5 and 2 ppm. Minor Au and As anomalies also occur sporadically in the turbidite matrix. Porphyroblastic pyrite at the nearby Macraes mine contains micron-scale native gold and solid solution gold up to 30 ppm. These pyrite porphyroblasts are also enriched in As, W, Ni, Co and Bi. Lead, Zn, Te and Mo contents are distinctly lower than in the low-grade framboids. These results show that low-grade turbidites were a fertile source for Au and As for orogenic gold deposits such as Macraes. The pyrite to pyrrhotite transition, which occurred mainly under greenschist facies conditions, was probably the most effective As and Au mobilisation reaction during Mesozoic metamorphism.Keywords: pyrite; framboid; gold; arsenic; tungsten; orogenic; Macraes mine IntroductionProgressive metamorphism of sedimentary rocks has been shown to mobilise elements that, when concentrated, make up orogenic gold deposits (Pitcairn et al. 2006(Pitcairn et al. , 2010Large et al. 2007Large et al. , 2009Large et al. , 2011. These elements, including gold, were present in trace concentrations in the original sedimentary rocks, and were released from those rocks in metamorphic fluids that are generated by metamorphic dehydration reactions (Pitcairn et al. 2006(Pitcairn et al. , 2010Large et al. 2011). 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). Pyrite is an important constituent of the mineralised rocks at the Macraes mine and hosts a significant proportion of the gold (Craw et al. 1999). Hence, pyrite is a key mineral throughout the metamorphicÁ hydrothermal mineralisation process in the Otago Schist, at the source of metals in low-grade rocks, during metamorphic mobilisation of metals from those rocks, and at the site of deposition under greenschist facies conditions.In this study, we focus on the trace element compositions of diagenetic pyrite in low-grade rocks, and on ore pyrite in the Mac...

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

“…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. However, one such set of historic mines, at Macraes ( Figure 138 1), has been developed into a world-class deposit, with more than 9 million ounces gold the Macraes gold mine, as well as mineralized graphitic sheared rocks, were collected from 165 the Golden Bar pit, a satellite pit of the main Macraes mine described by Large et al (2012).…”

Gold in Devono-Carboniferous red beds of northern Britain

“…The spheroids more commonly contain selenides (Spinks et al 2014). Both pyrite in black shales (Large et al 2011(Large et al , 2012 and the reduction spheroids in red beds contain diagenetic enrichments of gold, although the host mineralogy is different. Much of the gold in pyrite is in solid solution, rather than in particulate form as found in the Millport spheroids.…”

Noble gases fingerprint a metasedimentary fluid source in the Macraes orogenic gold deposit, New Zealand

“…Noble gases were extracted from similar material from this locality. c Au and As contents of Macraes mineralised pyrite (dots) in comparison to pyrite in the host Torlesse Terrane protolith (after Large et al 2012). d Regional cross-section through the Glenorchy area (Fig.…”

“…a Regional cross-section through the Macraes deposit and the nearby Nenthorn goldfield. b Incident light image of an auriferous pyrite porphyroblast in Macraes (Golden Bar pit) mineralised sheared schist (after Large et al 2012). Noble gases were extracted from similar material from this locality.…”

“…1; Large et al 2012). Two samples of quartz were taken from a well-mineralised shear-parallel veined zone that is up to 2-m thick.…”