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Atypical Biotite Alteration: Timing and Environmental Factors

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Atypical Biotite Alteration: Timing and Environmental Factors Atypical biotite alteration: timing and environmental factors Keeling1, Zwingmann2, Raven3 & Self3 1 Geological Survey of South Australia 2 Kyoto University, Japan 3 CSIRO Land and Water Department of State Development 1 Kalinjala Shear Zone Eyre Peninsula Donnington Suite Uley Mine Sleaford Bay 20 km Celadonite sites, southern Eyre Peninsula (on Total Magnetic Intensity (TMI) image) ARGA 2018 Wallaroo 2 Uley Graphite Mine Main fault Graphite schist Amphibolite Graphite schist / NAu-1 gneiss NAu-2 Uley Graphite Mine - view easterly, 1996 Generalised regolith section showing patchy nontronite alteration distributed around fault and fracture zones ARGA 2018 Wallaroo 3 Uley brown nontronite: (NAu-2) in altered amphibolite 0.3 m 4 Uley green nontronite: (NAu-1) nontronite after biotite NAu-1 Uley green nontronite - alteration product of biotite in graphite-biotite schist ARGA 2018 Wallaroo 5 Uley Graphite Mine 2012 – celadonite Remnant celadonite in kaolinised amphibolite Celadonite: Fe-rich dioctahedral ‘white mica’ ARGA 2018 Wallaroo 6 Uley Graphite Mine – Geological Section NAu-1 NAu-2 celadonite main fault CMS Nontronite: NAu-1 and NAu-2 – celadonite in plasmic zone 7 ARGA 2018 Wallaroo Sleaford Bay – Clem Cove section Vassallo & Wilson 2002 Graphitic schist with celadonite Key outcrop of Paleoproterozoic Hutchison Group with graphitic units. Section mapped several times from Tilley (1921) to Vassallo and Wilson (2001, 2002) – granulite facies metasediments, highly deformed. ARGA 2018 Wallaroo 8 Sleaford Bay - Clem Cove graphitic schist and gneiss Detail showing celadonite, iron oxide and manganese oxide altered graphite-rich schist ARGA 2018 Wallaroo 9 Sleaford Bay celadonite 200 μm Highly altered graphite-biotite schist: Celadonite forms as veins and replacement of biotite interleaved with flake graphite ARGA 2018 Wallaroo 10 Sleaford Bay celadonite - SEM Celadonite as lath-shaped fibre masses as fracture infill and replacement of biotite interleaved with graphite ARGA 2018 Wallaroo 11 Celadonite: conditions for formation • Nontronite and celadonite form precipitates in basalt, at sites of active sea-floor spreading. Sub-oxic conditions and temperatures up to 90°C. • Submarine ‘weathering’: saponite, celadonite, nontronite form by alteration of volcanic glass in basalt by slow circulating seawater. Largely an oxidation process, high Fe3+ content in celadonite and nontronite, but buffered by reactions to slightly reducing (Velde 2003). Alteration fluid temperatures <30°C. • “Continental meteoric fluids generally too oxidizing and have too low a cation content to favour the genesis of celadonite.” (Odin et al. 1988). • Keeling et al. (2000) concluded that at Uley graphite mine, nontronite and celadonite formed by low temperature hydrothermal activity, later overprinted by deep weathering. • Celadonite was recently described from continental flood basalts - O2 fugacity and fluid composition buffered by basalt groundmass dissolution and celadonite crystallisation (Baker et al. 2012). ARGA 2018 Wallaroo 12 Biotite alteration • Weathering • Biotite biotite interstratified with vermiculite or smectite • vermiculite or smectite (montmorillonite – neutral/alkaline pH) • kaolinite or halloysite and amorphous Fe3+ (hydr)oxide (which converts to goethite (α-Fe3+OOH)). • Hydrothermal (low temperature) • Biotite chlorite, phengitic ‘white mica’, illite, or halloysite / kaolinite • While biotite alteration to Fe-rich smectite (nontronite) is uncommon but feasible, biotite weathering to celadonite is highly unusual. ARGA 2018 Wallaroo 13 Celadonite – K-Ar dating results K Age Error δO18 Site Sample (%) Ma Ma per mill Sleaford 0.2-2 μm 6.69 48.9 1.1 21.9 Bay 3-1 μm 6.94 46.1 1.0 21.8 Uley Mine UGCE1 3.34 20.9 0.7 21.8 UGCE4 3.68 16.7 0.4 21.3 UGCZ 2-5 μm 3.25 15.4 0.6 21.8 ARGA 2018 Wallaroo 14 Timing of celadonite formation – origin? • Celadonite formation during the Cenozoic is more likely a product of weathering – no igneous or hydrothermal activity recorded in the region at this time. • Oxygen isotope results of ~21.8‰ are consistent with lowest temperatures estimated for oceanic basalt alteration (<30°C) (Odin 1988). • Why then celadonite and nontronite after biotite rather than vermiculite / montmorillonite – kaolin – goethite? • Local environmental factors and the presence of graphite? ARGA 2018 Wallaroo 15 Celadonite formation - timing and sea level change Ma Eucla Basin → Gawler Craton Regional S/L Celadonite 0 Formation (Ma) Pliocene 5 Miocene L Garford Fm 10 Uley M Nullarbor 15 Limestone 15.4±0.6 16.7±0.4 E 20 Abrakurrie 20.9±0.7 Limestone 25 Oligocene L Ooldea Barton Sand Sand 30 E 35 Eocene L Wilson 40 Bluff Sleaford Bay M Limestone Pidinga 45 Fm 46.1±1.0 50 48.9±1.1 Hampton Sst E Erosion and weathering ARGA 2018 Wallaroo 16 Southern Eyre Peninsula – flooded to 90 m (Miocene s/l?) Port Lincoln Possibility that the weathering reactions at Uley mine these sites were buffered by groundwater with sea- Sleaford Bay water salinity Neutral to alkaline pH Above sea level Flooded land ARGA 2018 Wallaroo 17 Effect of weathering around graphitic conductors Natural voltaic cell Redox reactions require electron transfer + (e.g. O2 + 4e- + 4H → 2H2O) Oxidised O2 O2 O2 One source: 4Fe2+ → 4Fe3+ + 4e- Weathering / oxidation gives rise to a Red potential difference between the oxidised and reduced zones Cations M+ Conductive bodies (e.g. graphite) facilitate Anions X- the flow of negative charge Graphite (electrons/ions) - gives rise to natural conductor spontaneous potential (SP) negative Ox anomaly – with a relatively reduced environment around the upper zone of the conductor (Sato & Mooney 1980) Reduced ARGA 2018 Wallaroo 18 SP Survey Uley Mine - 2012 Self Potential (SP) anomaly over Uley Graphite orebody -100 mV to -300 mV Uley graphite mine Eastern portion: Uley mine lease Easting [m] 564150 564200 564250 564300 564350 564400 564450 564500 564550 -50 -100 50 m -150 -200 -250 Self potential [mV] potential Self -300 -350 ARGA 2018 Wallaroo 19 Eh-pH stability fields for Fe in water at 25°C Coastal seawater • Self Potential (SP) O2 Water oxidised anomaly of -100 mV to 1000 +++ -300 mV at the Uley Fe H2O 750 graphite mine indicates localised areas of reduced groundwater, sufficient to ) 500 permit Fe2+ to remain in 250 Fe++ Fe(OH)3 (s) solution at neutral to goethite/ slightly alkaline pH of 7-8 millivolts hematite 0 (e.g. seawater buffer). Eh ( Eh H2O Uley SP • Conditions suitable for -250 anomaly H2 crystallisation of nontronite and celadonite -500 Water reduced 25°C 0 2 4 6 8 10 12 14 (Simplified Eh-pH diagram for Fe pH modified from Dill et al. 2010) ARGA 2018 Wallaroo 20 Alteration during bedrock weathering modified by graphite conductor Fe3+ Oxidised Fe3+ Oxidised Fe3+ Oxidised kaolin / 3+ Fe-oxide Fe celadonite/ Fe2+ nontronite Fe2+ Graphite conductor Reduced Reduced Reduced ARGA 2018 Wallaroo 21 Summary • Biotite alteration to nontronite and celadonite is not typical. • Nontronite / celadonite are characteristic of low temperature hydrothermal alteration of oceanic basalt. • Requires a neutral to slightly alkaline, oxidising to reducing environment (i.e. maintain high activity of Fe2+ and Fe3+ ions). • These conditions might be replicated for Fe-rich mineral alteration around a conductive graphite body in saline groundwater, due to anomalous electro-chemical activity that develops in response to weathering. • Possible implications for the role of graphite in low temperature redox reactions leading to precipitation of uranium from groundwater – with particular relevance to unconformity-related uranium deposits. ARGA 2018 Wallaroo 22 References Baker LL, Rember WC, Sprenke KF, Strawn DG 2012. Celadonite in continental flood basalts of the Columbia River Basalt Group. American Mineralogist 97: 1284-1290. Cervini-Silva J, Palacios E, Gómez-Vidales V (in press, 2018). Nontronite as natural source and growth template for (nano)maghemite [y-Fe2O3] and (nano)wüstite [Fe1-xO]. Applied Clay Science. Dill HG, Hansen B, Keck E, Weber B 2010. Cryptomelane: A tool to determine the age and the physical-chemical regime of a Plio-Pleistocene weathering zone in a granitic terrain (Hagendorf, SE Germany). Geomorphology 121: 370-377. Keeling JL, Raven MD, Gates WP 2000. Geology and characterization of two hydrothermal nontronites from weathered metamorphic rocks at the Uley graphite mine, South Australia. Clays and Clay Minerals 48: 537-548. Odin GS, Desprairies A, Fullagar PD, Bellon H, Decarreau A, Frohlich F, Zelvelder M 1988. Nature and geological significance of celadonite. In: Odin GS (ed.) Green marine clays, Developments in Sedimentology 45: 337–398, Elsevier, Amsterdam. Velde B 2003. Green clay minerals. In: MacKenzie FT (ed.) Sediments, Diagenesis, and Sedimentary Rocks p. 309–324, Pergamon, Oxford. 23.
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