Multistage ore genesis in the New Zealand geosyncline a history of post-metamorphic lode emplacement

110

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...

Section: Low-grade Rocks As Sources For Au and Asmentioning

confidence: 99%

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

Large

Thomas

Craw

et al. 2012

110

“…Uplift and resultant tensional fracturing would have enabled the release of trapped fluids such as those derived from the breakdown of the hydrous mineral phases chlorite and micas during transition to higher grade metamorphic facies at depth. Such a derivation of the hydrothermal fluids in Otago from underlying higher grade schists has been argued by several authors (e.g., Henley et al 1976;McKeag et al 1989;Craw & Norris 1991;Craw 1992). However, the similar oxygen isotope values obtained for the quartz lodes and the country rocks simply show that the hydrothermal fluids equilibrated with the host schists, and cannot distinguish other fluid sources such as deeply circulating meteoric water.…”

Section: Ages Of Metamorphism Mylonite and Hydrothermal Alterationmentioning

confidence: 75%

Mesozoic mesothermal quartz‐gold‐scheelite lodes, Wakamarina, Marlborough, New Zealand

Skinner

Brathwaite

1999

In the Wakamarina valley, Marlborough, ribbonbanded quartz lodes fill tensional faults in Caples Terranederived, Marlborough Schist and Wakamarina Quartzite. The largest of the lodes, the Golden Bar, has a strike length of at least 1.8 km and an average width of 2 m. It fills a steeply dipping normal fault which separates pumpellyite-actinolite fades psammitic schists on the footwall from stilpnomelanepumpellyite-spessartine quartzite (Wakamarina Quartzite) and metabasites on the hanging wall. The lode consists of bands of buck quartz separated by thin chlorite-sericite-rich laminae that have been split off the wall-rock schist.Five deformation events are recognised. The first two are linked with early Jurassic metamorphism, the younger event producing the earliest scheelite, whereas in early Mid Jurassic (illite/sericite K-Ar age of c. 175 Ma), the third event produced late metamorphic transtensional mylonites with minor quartz-scheelite veining. As uplift of the schist belt continued through the brittle-ductile transition zone, the fourth deformation created tensional faults across the mylonites which localised deposition of the quartzscheelite ± gold lodes (Golden Bar and Alfords). Internal lode structures indicate that internal pulsed fluid inflow and ductile vein deformation continued during this deformation. The fifth deformation, again with minor quartz-scheelite veining, created late brittle fracturing and compressional deformation of the lodes and country rocks.Fluid P-T conditions are probably comparable to those in similar mesothermal quartz-scheelite lodes at Glenorchy in Otago (1.5-3.5 kbar and 275-330°C), but co-existing albite and calcic plagioclase in Alfords lode implies that fluid temperatures at Wakamarina may have been locally higher. An extensive zone of sericitic hydrothermal alteration characterised by sericite and K-feldspar (sericite K-Ar age of c. 140 Ma -earliest Cretaceous) and loss of pumpellyite, stilpnomelane, and actinolitic amphibole surrounds the Golden Bar lode. The potassic enrichment, the stability of albite and chlorite in both wall rocks and the lodes, and a low sulphide content indicate that the fluids at Wakamarina had a higher aK + /aH + than in the Otago lodes, and a more neutral pH with a low capacity to carry gold in solution.

“…Definition of fluid source(s) is problematic because of the extensive fluidrock interaction that occurs during passage through host rocks for the deposits (Bierlein & Crowe 2000;Goldfarb et al 2005). Determination of the source of metals is probably more significant than fluid source for mineral exploration, as some authors suggest that distinctive rock types or igneous bodies contribute most metals for gold deposits (Henley et al 1976;Rock & Groves 1988;Spooner 1993;Bierlein et al 2006). Verification of this concept would make the geological environs of these metal-yielding rocks more prospective.…”

Section: Introductionmentioning

confidence: 99%

“…2) (e.g., Paterson 1986;Mortensen et al 2010). In contrast, metabasic rocks have been considered by some workers to be potentially fertile sources for metals in orogenic mineralising systems (Bierlein et al 2006) including the Otago Schist (Henley et al 1976;Bierlein & Craw 2009). …”

Section: Introductionmentioning

confidence: 99%

Lead isotope constraints on the origin of Cenozoic orogenic gold systems in the Southern Alps and northwestern Otago, South Island, New Zealand

Mortensen

Craw

MacKenzie

et al. 2010

Lead isotopic compositions were determined for sulphides from Pliocene-Pleistocene gold-bearing veins in the Alpine Schist and from Miocene gold-bearing veins and vein breccias from the Shotover-Macetown area in the northwest Otago Schist belt. The lead isotopic signatures are consistent with derivation of Pb in the vein minerals predominantly from metasedimentary rocks that underlie the region. Differences in Pb isotopic signatures between deposits are interpreted to result from lateral and vertical lithological variability within the source rock mass. The host rocks also contain metabasic rocks with N-MORB, E-MORB or within-plate basalt chemistry. However, the observed Pb isotopic signatures in the gold-bearing veins preclude incorporation of significant amounts of Pb from the metabasites. The Pb isotopic signatures of lamprophyre dikes that were intruded into the Otago Schist coeval with Miocene gold mineralisation are distinctly more radiogenic than those of the hydrothermal veins. Thus, although the lamprophyre dikes were emplaced into similar extensional structural sites to the gold-bearing veins, there was no genetic relationship between lamprophyre dikes and gold mineralisation.