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- Cave, Ben J., et al.
(författare)
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A metamorphic mineral source for tungsten in the turbidite-hosted orogenic gold deposits of the Otago Schist, New Zealand
- 2017
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Ingår i: Mineralium Deposita. - : Springer Science and Business Media LLC. - 0026-4598 .- 1432-1866. ; 52:4, s. 515-537
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Tidskriftsartikel (refereegranskat)abstract
- The orogenic gold deposits of the Otago Schist, New Zealand, are enriched in a variety of trace elements including Au, As, Ag, Hg, W and Sb. We combine laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) traverses and images to show that detrital rutile is the most important host mineral for W in the subgreenschist facies rocks. Furthermore, the prograde metamorphic recrystallisation of detrital rutile to titanite releases significant amounts of W (potentially 0.41 g/tonne of rock). Scheelite development closely follows the progression of this W-liberating reaction. Scheelite micrograins form early within the fabric of the rock evolving to locally and regionally sourced scheelite-bearing veins. Scheelite from syn-metamorphic veins at Fiddlers Flat and Lake HAwea shows distinct differences in composition compared with scheelite from late-metamorphic veins at the Macraes Mine, the latter of which is enriched in REEs, Y and Sr. We suggest that the scheelite at Macraes became enriched due to the liberation of these elements during alteration of the Ca-silicate minerals epidote and titanite by the ore-forming fluid. These results are supportive of recent models for orogenic gold mineralisation in the Otago Schist, whereby prograde metamorphic recrystallisation of diagenetic or detrital metal-rich mineral phases (pyrite to pyrrhotite: Au, As, Ag, Hg and Sb; rutile to titanite: W) releases significant amounts of metals into the concurrently developing metamorphic fluids that can be subsequently focussed into regional structures and form significant tungsten-bearing orogenic gold deposits.
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| 5. |
- Craw, Dave, et al.
(författare)
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Contrasting geochemistry of orogenic gold deposits in Yukon, Canada and Otago, New Zealand
- 2015
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Ingår i: Geochemistry. - : Geological Society of London. - 1467-7873 .- 2041-4943. ; 15:2-3, s. 150-166
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Tidskriftsartikel (refereegranskat)abstract
- The Yukon-Tanana Terrane (YTT) of western Yukon Territory in NW Canada and Otago Schist belt (OSB) of South Island, New Zealand share similar geological evolutionary histories as convergent orogenic belts. Both belts host orogenic gold deposits of mainly Jurassic to Early Cretaceous age. Jurassic mineralization in the YTT occurred during convergent orogenesis and stacking of previously-metamorphosed (Palaeozoic) greenschist-amphibolite facies metasediments, metavolcanic rocks, and metagranitoids. Early Cretaceous OSB mineralization occurred in the latter stages of terrane accretion of un-metamorphosed turbidites with minor basaltic rocks. Metamorphism of the OSB turbidites mobilised background levels of Au (0.6-1.3 ppb), As (2-20 ppm), Sb (0.1-1 ppm), and W (< 10 ppm), primarily under greenschist to lower amphibolite facies conditions when diagenetic pyrite (Au c.0.5-2 ppm; As c.500-10000 ppm) transformed to pyrrhotite on a regional scale. In contrast, the previously-metamorphosed YTT rocks had generally low background As contents (1-2 ppm) apart from some As-rich quartzites (up to 100 ppm As). Consequently, there was less As available for orogenic mobilisation, and YTT Au deposits generally have lower concentrations of this pathfinder element compared to the OSB. YTT host rocks, especially metagranitoids, have anomalous levels of Mo (10-300 ppm), and many orogenic deposits contain elevated Mo, locally including molybdenite. OSB turbidites have elevated Mo (2-200 ppm), along with elevated Au and As, in diagenetic pyrite, but this Mo became largely dispersed through the metamorphic pile as metamorphic grade increased and pyrite transformed to pyrrhotite. OSB orogenic deposits have only marginally elevated Mo (c.1 ppm), no molybdenite, and accessory scheelite in these deposits is distinctly Mo-poor. Only minor mobilisation of base metals occurred in these orogenic belts, and orogenic Au deposits contain sparse base metal sulphides. Orogenic deposits in the YTT and OSB differ in that Au (and other associated elements) in many of the orogenic deposits in the YTT was remobilised from relatively local sources (e.g. pre-existing Cu-Mo-Au porphyry or volcanogenic sulphide mineralization) whereas Au in the OSB was mobilised from larger volumes of homogeneous rock at depth.
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- Craw, Dave, et al.
(författare)
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Geochemical signatures of mesothermal Au-mineralized late-metamorphic deformation zones, Otago Schist, New Zealand
- 2007
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Ingår i: Geochemistry: Exploration, Environment, Analysis. ; 7, s. 225–232-
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Forskningsöversikt (populärvet., debatt m.m.)abstract
- Hydrothermal processes along two regional-scale shear zones in theOtago Schist were dominated by structurally controlled fluid flow and mineralization in the host schist, with relatively minor quartz vein formation, and mineralized rocks are only subtly different from unmineralized rocks. Most Au in the shear zones is associated with sulphide minerals (pyrite and arsenopyrite) disseminated through the host schist or along microshears. Minor enrichment of Sb, Mo and Bi (ppm level) is detectable in the Hyde-Macraes Shear Zone (HMSZ). Hydrothermal muscovite is slightly more aluminous (1–2 wt%) than metamorphic muscovite in both shear zones. HMSZ muscovite averages >900 ppm N, in contrast to metamorphic muscovite that averages c. 200 ppm N. In both shear zones, rutile has replaced metamorphic titanite and epidote has altered to carbonate and phyllosilicates, but these reactions were nearly isochemical. Structurally controlled hydrothermal graphite in the HMSZ occurs in microshears (up to 3 wt%, above background <0.2 wt%). Alteration in the Rise & Shine Shear Zone (RSSZ) was accompanied by addition of abundant ankerite. The two shear zones have subtly different geochemical signatures and are not directly genetically related. However, As enrichment is a key exploration target for both shear zones.
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| 7. |
- Menzies, Catriona D., et al.
(författare)
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Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand
- 2018
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 481, s. 305-315
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Tidskriftsartikel (refereegranskat)abstract
- Collisional mountain building influences the global carbon cycle through release of CO2 liberated by metamorphic reactions and promoting mechanical erosion that in turn increases chemical weathering and drawdown of atmospheric CO2. The Southern Alps is a carbonate-poor, siliciclastic mountain belt associated with the active Australian Pacific plate boundary. On-going, rapid tectonic uplift, metamorphism and hydrothermal activity are mobilising carbon. Here we use carbon isotope measurements of hot spring fluids and gases, metamorphic host rocks, and carbonate veins to establish a metamorphic carbon budget. We identify three major sources for CO2 within the Southern Alps: (1) the oxidation of graphite; (2) consumption of calcite by metamorphic reactions at the greenschist-amphibolite fades boundary, and (3) the dissolution of groundmass and vein-hosted calcite. There is only a minor component of mantle CO2 arising on the Alpine Fault. Hot springs have molar HCO3-/Ca2+ similar to 9, which is substantially higher than produced by the dissolution of calcite indicating that deeper metamorphic processes must dominate. The total CO2 flux to the near surface environment in the high uplift region of the Southern Alps is estimated to be similar to 6.4 x 10(8) mol/yr. Approximately 87% of this CO2 is sourced from coupled graphite oxidation (25%) and disseminated calcite decarbonation (62%) reactions during prograde metamorphism. Dissolution of calcite and mantle-derived CO2 contribute 10% and 3% respectively. In carbonate-rich orogens CO2 production is dominated by metamorphic decarbonation of limestones. The CO2 flux to the atmosphere from degassing of hot springs in the Southern Alps is 1.9 to 3.2 x 10(8) mol/yr, which is 30-50% of the flux to the near surface environment. By contrast, the drawdown of CO2 through surficial chemical weathering ranges between 2.7 and 20 x 10(9) mol/yr, at least an order of magnitude greater than the CO2 flux to the atmosphere from this orogenic belt. Thus, siliciclastic mountain belts like the Southern Alps are net sinks for atmospheric CO2, in contrast to orogens involving abundant carbonate rocks, such as the Himalaya, that are net CO2 sources.
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| 8. |
- Pitcairn, Iain K., et al.
(författare)
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Metabasalts as sources of metals in orogenic gold deposits
- 2015
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Ingår i: Mineralium Deposita. - : Springer Science and Business Media LLC. - 0026-4598 .- 1432-1866. ; 50:3, s. 373-390
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Tidskriftsartikel (refereegranskat)abstract
- Although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits, the mobility of Au and related elements (As, Sb, Se, and Hg) from these rocks during alteration and metamorphism is poorly constrained. We investigate the effects of increasing metamorphic grade on the concentrations of Au and related elements in a suite of metabasaltic rocks from the Otago and Alpine Schists, New Zealand. The metabasaltic rocks in the Otago and Alpine Schists are of MORB and WPB affinity and are interpreted to be fragments accreted from subducting oceanic crust. Gold concentrations are systematically lower in the higher metamorphic grade rocks. Average Au concentrations vary little between sub-greenschist (0.9 +/- 0.5 ppb) and upper greenschist facies (1.0 +/- 0.5 ppb), but decrease significantly in amphibolite facies samples (0.21 +/- 0.07 ppb). The amount of Au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in Otago. Arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element. The concentrations of Sb and Hg decrease between sub-greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered MORB protoliths and therefore unaltered oceanic crust cannot be a net source of Sb and Hg in a metamorphic environment. The concentrations of Au, As, Sb, and Hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism. We suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit.
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| 9. |
- Pitcairn, Iain K., et al.
(författare)
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The gold conveyor belt : Large-scale gold mobility in an active orogen
- 2014
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 62, s. 129-142
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Tidskriftsartikel (refereegranskat)abstract
- The Southern Alps of New Zealand are part of an active collisional orogen where metamorphism, hydrothermal fluid flow and the formation of orogenic gold deposits are ongoing. The Southern Alps are forming due to transpressional collision between continental crust fragments on the Pacific and Australian tectonic plates. The plate tectonic rates and geometries, the sources of fluid and broad-scale fluid pathways in the hydrogeological system, and the geochemical compositions of the Torlesse Terrane rock that is being advected through the orogen are well defined so that a mass balance of metal mobility during active orogenic processing in the Southern Alps of New Zealand can be calculated. Advection of a 10 km wide x 5 km deep section of Torlesse rock through the orogen at tectonic rates (0.01 m/yr) that is then metamorphosed up to amphibolite fades causes mobilisation of over 11,27 t Au, 10.1 Mt As, 47,000 t Hg, 560,000 t Sb and 14,000 Mt H2O in 1 Myrs. The masses of elements mobilised at the same rate along the length of the Southern Alps (>200 km) for 5 Myrs would be more than 100 times greater. The metals were mobilised by the metamorphic fluid produced during the orogenic processing of the Torlesse Terrane rocks and the concentrations of Au, As, Hg and Sb in this fluid are calculated to be 0.08, 711, 3, and 40 mg/kg, respectively. The mobilised metals form the orogenic gold deposits that occur in the Southern Alps. Different styles of gold deposits form contemporaneously during the active orogenesis of the Southern Alps, including those with a fluid temperature > rock temperature that may appear to have formed after the peak of metamorphism but are instead just the product hydrothermal fluid mineralising rocks on their retrograde metamorphic path. The mass balance shows that there has been orders of magnitude more metal mobilised in the orogen than resides in the currently known deposits. There is a clear potential for large gold deposits occurring in the yet to be uplifted parts of the Southern Alps if there have been efficient enough fluid focusing and metal precipitation mechanisms occurring under the Southern Alps.
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| 10. |
- Pitcairn, Iain, 1974-, et al.
(författare)
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Sulfide evolution during prograde metamorphism of the Otago and Alpine schists, New Zealand
- 2010
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Ingår i: Canadian Mineralogist. - : Mineralogical Association of Canada. - 0008-4476 .- 1499-1276. ; 48:5, s. 1267-1295
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Tidskriftsartikel (refereegranskat)abstract
- Sulfide minerals in the Otago and Alpine schists, New Zealand,a metasedimentary belt exposed from unmetamorphosed greywackesup to amphibolite facies, underwent systematic changes in abundance,composition and texture during prograde metamorphism. In unmetamorphosedrocks, the most common sulfide mineral is framboidal pyrite,which contains abundant As (up to 14000 ppm), Co (up to 4000ppm), 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 isolatedgrains. Chalcopyrite and sphalerite contain few trace elements,whereas galena contains significant Se (up to 1600 ppm) andlocally abundant Hg (up to 600 ppm). The distribution of thesetrace and minor elements is extremely heterogeneous. In subgreenschist-faciesrocks, pyrrhotite replaces pyrite, and there is a clear texturalchange from framboidal pyrite to composite grains of pyrrhotite,sphalerite, chalcopyrite, galena and cobaltite. Pyrrhotite containsCo (average values 1100 ± 490 ppm), Cu (up to 17000 ppm),and Ni (up to 11000 ppm). Antimony and Hg are above detectionin rare individual grains, but none of the other trace and minorelements sought are detectable. Sphalerite, galena and cobaltiteincrease in proportion in subgreenschist-facies rocks, and alsocontain 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), andSb (up to 1280 ppm in cobaltite and 770 ppm in galena). Pyrite,sphalerite, galena, and cobaltite become less abundant fromsubgreenschist- to amphibolite-facies rocks. In amphibolite-faciesrocks, only pyrrhotite, chalcopyrite and trace amounts of galenaand molybdenite occur, and none of these minerals contain detectablelevels of Ag, Au, As, or Hg. Mass-balance calculations betweensulfide minerals and whole rocks show that sulfides are importanthost minerals for S, Cu, and As, but host a minor proportionof Fe, Zn, and Pb. As pyrite, sphalerite, galena, and cobaltitebecome less abundant at higher metamorphic grade, Pb, Zn andCo are retained in the rock incorporated in other minerals,whereas As, Sb, Hg, Au, and Ag are removed from the rock, mostlikely by metamorphic devolatilization.
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