Opal Formation at Lightning Ridge

Transcription

1 Opal Formation at Lightning Ridge JJ Watkins NSW Geological Survey Dept of Primary Industries

2 Opal Formation at Lightning Ridge 1. Structures - faults/lineaments 2. Cretaceous palaeochannels 3. Microbes 4. Isotopes Three models for opal formation: 1. Weathering 2. Syntectonic 3. Microbes

3 Weathering Opal formed during an intense weathering event Sandstone is source of silica Passive use of faults as pathways Silica transported in meteoric water to top of claystone ( a permeability barrier) Opal deposited in cavaties (nobbies) and cracks (seam opal)

4 Syntectonic Opal Formation Opal formed during faulting and movement Opal deposited from deeply sourced hydrothermal fluids Fluids ascend along faults Relatively high tempereatures (>100 0 C) and pressure Opal formed rapidly from cooling ( Pecover 1996)

5 Microbe model Behr 1999, Behr et al 2000, Watkins 2002 Large populations of fossil microbes found in opal (aerobic, cold climate) Microbes produce acids and enzymes that alter clay minerals and feldspar to produce silica-hydrosol and opal Low temperature, low pressure surficial process Microbes are same age as opal - Cretaceous Dowell 2002 opal is biogenic and silica from phytoliths - < yrs Colony of Nocardia in nobby-type potch (Behr et al 2000; Watkins 2002)

6 Opal Formation at Lightning Ridge 1. Structures - faults/lineaments 2. Cretaceous palaeochannels 3. Microbes 4. Isotopes Three models for opal formation: 1. Weathering 2. Syntectonic 3. Microbes

7 Regional-scale structures Aster satellite image Lightning Ridge Nine Mile Opal Field Lightning Ridge Coocoran Opal Field Carters Rush Sheepyards Opal Field

8 Regional-scale structures dem, magnetics The Opal Corridor outlying opal fields Regional airborne Total Magnetic Intensity data 9 Mile

9 Local-scale structural features Nine Mile Opal Field Opal Corridor

10 Local-scale structural features Three Mile Opal Field (Photo: S Aracic)

11 Opal Formation at Lightning Ridge 1. Structures - faults/lineaments 2. Cretaceous palaeochannels 3. Microbes 4. Isotopes Three models for opal formation: 1. Weathering 2. Syntectonic 3. Microbes

12 Lightning Ridge sandstone geometry - channel sands (Watkins 1985)

13 Channel sandstone cutting down into claystone

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17 Seismic Refraction Wyoming Opal Field Stacked channel sandstones 0m 6m 12m 18m

18 Pelecypods Fossil reeds in claystone Extensive fauna Dominantly a freshwater setting

19 Lightning Ridge area 110 million years ago A coastal plain

20 The Opal Corridor

21 Lightning Ridge area 110 million years ago. Meander plain structurally controlled

22 The Opal Corridor

23 Opal Formation at Lightning Ridge 1. Structures - faults/lineaments 2. Cretaceous palaeochannels 3. Microbes 4. Isotopes Three models for opal formation: 1. Weathering 2. Syntectonic 3. Microbes

24 Lightning Ridge nobby Claystone Microbes found in opal and in claystone (Behr 1999 ) Opal 1 cm Silica produced at reaction front by progressively breaking down claystone Note: weathering overprints on opal (Webb 1999)

25 Opal filling a cavity a proto chinese hat nobby 1 cm Meniscus indicates low (atmospheric) pressure

26 Microbes and opal are cogenetic 0.3u 20u Small spheres of silica with a similar size arranged in a regular pattern (Jones et al., 1964) Hyphae of Micromonospora grown along planes in nobby

27 Opal Formation at Lightning Ridge 1. Structures - faults/lineaments 2. Cretaceous palaeochannels 3. Microbes 4. Isotopes Three models for opal formation: 1. Weathering 2. Syntectonic 3. Microbes

28 Stable isotopes ( K Dowell 2002) 1 Weathering 2 Source of carbon Ge/Si ( μmol/mol) Arenite Argilllite Black opal Weathering trend (Kurtz et al., 2002) Kaolinite (Kurtz et al., 2002) Diorite (Kurtz et al., 2002) Plagioclase (Kurtz et al., 2002) Lightning Ridge trend δ 13 C(VPDB) Limestone Plant Black opal Plant Matter Atmosphere NSW δ 13 C Phytolith range Al/Si (molar ratio) -40 Phytolith δ 18 O (SMOW) δd (SMOW) 3 Source of water Groundwater 10 BMMW Met Black opal -10 Lightning Ridge Sediment -30 SMOW -50 MWL Mag Sediment Conclusions: 1. Opal not on the weathering trend 2. Carbon in opal is organic 3. Water in opal not from groundwater or hydrothermal waters, not meteoric δ 18 O (SMOW)

29 Opal Formation at Lightning Ridge Conclusions Microbes provide evidence for the conditions of formation of opal Opal formed in a near surface environment Low temperature and low pressure Associated with palaeochannels, billabongs, back swamps Structural setting is important regional and local Stable isotopes opal not on weathering trend - not hydrothermal - support a biogenic origin for opal

30 Prospecting targets: 1. Channel sands, billabongs, backswamps 2. Regional and local structures 3. Thin coaly units

31 Geophysical Techniques trialled at Lightning Ridge Old Goodooga Road: -Resistivity, SIROTEM, ( local faults) Wyoming Opal Field: -Shallow seismic refraction (palaeochennels) Coocoran South: Airborne EM (essentially mapped weathering) HEM data acquisition CDI data presentation - Ground TEM NanoTEM -Downhole logging conductivity (distinguished thick sandstone units) magnetic susceptibility natural gamma

32 Historic Prospect Drilling with Initial Processing First pass ground truthing

33 Detailed geological and geophysical logging Drill Hole 8 - Geological and Geophysical Logs Conductivity (ms/m) m - Gravel/Sand Silcrete Silicified Claystone Drill Hole 7 - Geological and Geophysical Logs Conductivity (ms/m) m - Gravel/Sand Silcrete Silicified Claystone 10m - 20m - 30m - Claystone Interbedded Sandstone/ Claystone Claystone Silty Claystone Sandstone Silty Claystone Claystone electrical conductivity logs distinguish sandstone 10m - 20m - 30m - Claystone Sandstone Sandy Claystone Claystone Sandstone Claystone 40m - Sandstone 40m - Claystone 50m - 50m -

34 Thank You