Synmetamorphic carbon mobility and graphite enrichment in metaturbidites as a precursor to orogenic gold mineralisation, Otago Schist, New Zealand

Henne, Anicia; Craw, Dave

doi:10.1007/s00126-012-0399-2

Cited by 42 publications

(23 citation statements)

References 40 publications

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“…3C; Craw et al 1999;Pitcairn et al 2005;Henne & Craw 2012). Gold is disseminated through these sheared schists, commonly with porphyroblastic textures, although brittle overprints have brecciated and remineralised some such rocks.…”

Section: Macraes Mine Geologymentioning

confidence: 99%

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

Large

Thomas

Craw

et al. 2012

New Zealand Journal of Geology and Geophysics

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110

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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...

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Section: Macraes Mine Geologymentioning

confidence: 99%

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

Large

Thomas

Craw

et al. 2012

New Zealand Journal of Geology and Geophysics

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110

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“…Many of the metamorphic boundaries in the schist belt are now defined by faults ( Fig. 1; Mortimer 2000; Henne and Craw 2012). Miocene basaltic volcanism near the eastern coast was partially controlled by these pre-existing structures, and some of these structures are still channelling surface springs that contain detectable mantle helium emanations ( Fig.…”

Section: General Settingmentioning

confidence: 99%

“…Pyrite and arsenopyrite have overgrown the shear fabric with porphyroblastic replacement textures (Fig. 2b, c), and many of the porphyroblasts were subsequently deformed by progressive shear evolution (Craw 2002;Henne and Craw 2012). Some of the more strongly deformed mineralised shears have abundant (up to 3 wt%) hydrothermal graphite that accompanied sulphide porphyroblast emplacement (Craw 2002;Petrie et al 2005;Pitcairn et al 2005a;Craw et al 2007;Henne and Craw 2012).…”

Section: Mineralisation At Macraes Minementioning

confidence: 99%

“…These sulphide grains have locally overgrown the syn-mineralisation shear fabric via replacement reactions to form porphyroblasts ( Fig. 2; Craw 2002;Henne and Craw 2012). Some pyrite also occurs in thin syn-mineralisation quartz veinlets that locally cut shear fabric but have been deformed by shearing as well (Fig.…”

Section: Helium Data From Quartz and Sulphidesmentioning

confidence: 99%

“…The Torlesse Terrane protoliths, especially the argillites, of the host rocks in the Macraes area (Fig. 1) contain minor but widespread detrital organic material which becomes progressively mature with increasing metamorphic grade (Pitcairn et al 2005a;Henne and Craw 2012). Hence, fluids interacting with these host rocks within the metamorphic pile have apparently inherited noble gas and halogen characteristics from the sedimentary protolith that have subsequently been preserved in the Macraes deposit.…”

Section: Crustal Fluid Signaturementioning

confidence: 99%

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

et al. 2016

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Ar ratios up to 200 times greater than air. Similarly, I/Cl ratios for fluids extracted from mineralised quartz are similar to those of brines from marine sediments that have interacted with organic matter and are ten times higher than typical magmatic/mantle fluids. The Macraes mineralising fluids were compositionally variable, reflecting either mixing of two different crustal fluids in the metasedimentary pile or a single fluid type that has had varying degrees of interaction with the host metasediments. Evidence for additional input of meteoric water is equivocal, but minor meteoric incursion cannot be discounted. The Macraes deposit formed in a metasedimentary belt without associated coeval magmatism, and therefore represents a purely crustal metamorphogenic end member in a spectrum of orogenic hydrothermal processes that can include magmatic and/or mantle fluid input elsewhere in the world. There is no evidence for involvement of minor intercalated metabasic rocks in the Macraes mineralising system. Hydrothermal fluids that formed other, smaller, orogenic deposits in the same metamorphic belt have less pronounced noble gas and halogen evidence for crustal fluid-rock interaction than at Macraes, but these deposits also formed from broadly similar metamorphogenic processes.

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Regional Geological Setting of the Otago Schist

Craw

MacKenzie

2016

SpringerBriefs in World Mineral Deposits

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Synmetamorphic carbon mobility and graphite enrichment in metaturbidites as a precursor to orogenic gold mineralisation, Otago Schist, New Zealand

Cited by 42 publications

References 40 publications

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

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

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

Regional Geological Setting of the Otago Schist

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