predictive mineral discovery*cooperative Research Centre A legacy for mineral exploration science Mineral Systems Q3 Fluid reservoirs

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1 Mineral Systems Q3 Fluid reservoirs 1

2 Key Parameter Mineral System Exploration is reflected in scale-dependent translation A. Gradient in hydraulic potential B. Permeability C. Solubility sensitivity to P, T, C D. Spatial gradient of P, T, C E. Time (duration) 5 Questions 1. Geodynamics 2. Architecture 3. Fluid reservoirs 4. Flow drivers & pathways 5. Deposition Terrain Selection Area Selection Drill Targeting Slide after: A. Barnicoat 2

3 Liebscher&Heinrich

4 Potential fluid sources (basins?) Meteoric water Bittern Sea water Evaporites Basin Metamorphic fluid Magmatic fluids Mantle fluid 4

5 Meteoric fluids Surface derived needs rainfall and topography wide range of chemistries anions: sulphate, bicarbonate, chloride or acetate Sulphate: most important in shallow groundwaters Bicarbonate: increases with depth Acetate: may dominate in groundwaters at T C Meteoric waters may be (largely) unmodified, may contribute to basinal waters or to magmatic fluids. 5

6 Meteoric fluids SMOW 6

7 Basinal fluids formation water water present in pores and fractures immediately prior to drilling Connate water water trapped with the sediment and subsequently unmodified (meteoric or seawater) modified meteoric water Many basinal waters are of mixed origin with marine, meteoric and bittern components 7

8 Basinal fluids - origins Meteoric origin recent not recent 8

9 Basinal fluids Chemistry major cations Na, K, Ca and Mg increase in general with increasing salinity (total dissolved solids) The concentration of many cations is controlled by reaction with mineral phases including carbonates, feldspar, illite and chlorite SiO2 is controlled by equilibration with metastable silica polymorphs such as opal at low temperatures, and with quartz at temperatures above about 80 C ph increases with increasing salinity, a function of fluid rock interaction as exemplified by the reaction NaAl 3 Si 3 O 10 (OH) 2 + 6SiO 2 + 2NaCl = 3NaAlSi 3 O 8 + 2H + + 2Cl - Chloride is the dominant anion in waters of seawater salinity and above 9

10 Basinal fluids - salinity Salinity sources Dissolution of evaporites Descent of bittern brines from surface 10

11 Metamorphic fluids Metamorphic fluids have mixed origin Devolatilisation External fluids reacting with metamorphic rocks Devolatilisation During heating, volatiles released Constant temperature leads to no fluid generation Decrease in temperature leads to resorption of residual fluid by retrogression 11

12 Metamorphic fluids Devolatilisation upflow & fluid focussing increase in permeability 12

13 Metamorphic fluids tholeiitic basalt high-ca granlite 13

14 Magmatic fluids Porphyry: H 2 O-NaCl Intrusion-related: CO 2 -H 2 O-NaCl IOCG: NaCl-CO 2 -H 2 O Orogenic Au: HCO 2 O- 2 -H CO 2 O-NaCl 2 14

15 Magmatic fluids Ore deposits associated with magmatic-hydrothermal fluids: Porphyry Cu, Au and Mo deposits C C (A) High-sulphidation epithermal deposits associated with B B porphyries C Intrusion-related gold and other metal deposits, linked to reduced intrusions (B) Iron oxide copper gold A A B C (IOCG) deposits (C) 15

16 Magmatic fluids volatiles Porphyries are characterised by H2O-NaCl fluids that are high in sulphur Intrusion-related systems are associated with CO2-H2O fluids with limited NaCl contents; sulphur contents of the deposits and by inference the fluids are relatively low. IOCG systems are dominated by NaCl-rich melts and CO2-rich fluids. Sulphur contents of the fluids are relatively low. 16

17 Magmatic fluids volatiles 17

18 Mantle fluids Possible source of reduced components Subduction-related processes CO 2 used to be often considered to be mantle derived Mantle redox more variable than initially envisaged - Possible source of (very) reduced fluid 18

19 40 Ar/ 36 Ar Constraints On Crust/Mantle Sources Mantle Chlorine (HCl, NaCl) Kendrick et al.,

20 Fluid reservoirs 3 end-member fluids: 1) ambient crustal aqueous fluid, with low concentrations of salt and volatiles 2) magmatic, dominated by CO2 and SO2 3) deep-earth, highly reduced CH4 - N2 - H2 - fluid - acid volatiles (H2S, HCl ±HF), - noble gases (Ne and Ar) of mantle origin, - possibly metal hydrides (e.g. NaH, MgH2, AlH3 and SiH4) - and probably Au Metal Province 10km 100 km 1000 km Melts & CO 2 -SO 2 fluids H 2 flux (H, Na, N, C, Cl, metals) Fluid 1 Aqueous domain Fluid 2 seal Fluid 3 20

21 400 C Oxidized log fo hm mt CO 2 aq CH 4 aq -0.1 Chlorite + Magnetite -0.3 Tourmaline mt -0.5 am -0.7 Increasing calcic component in albite G Albite + Qtz Albite G Epidote Biotite + Mt mt py Amphibole + Biotite Pyrite Anhydrite saturation am log KCl/NaCl Talc Saturation po -1.7 hm HSO4 H 2 S aq Po 1 3 H 2 S aq po -1.9 am HS - Amphibole am py py am Amphibole a m Ca feldspar ph > ~12 21

22 Fluid1: H 2 0 ± NaCl ± CO 2 ± H 2 S Muscovite Tourmaline Acid Aqueous Paragonite Fe-chlorite, magnetite, quartz, carbonate Qtz, epidote, biotite, magnetite, anhydrite Phengite, hematite Aqueous, neutral Oxidized - Reduced Biotite, amphibole, albite, quartz, carbonate, magnetite Anhydrite,tremolite Biotite K-feldspar Anhydrite Carbonate Oxidized Alkaline Best Au grades Reduced Alkaline Pyrite, pyrrhotite, Albite, quartz Albite, Talc, Amphibole Ca-feldspar Fluid 2: CO 2 -SO 2 volatiles; potassic Anhydrous Volatiles Fluid 3: H 2,H 2 S, CH 4, N 2, HCl, HF sodic 22

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