CHAPMAN CONFERENCE, AUGUST 23, 2001.

1. THE FUTURE OF EXPLORATION.Bruce Hobbs, Alison Ord, John Walshe, Hans Muhlhaus, Yanhua Zhang, Chongbin Zhao and Reem Freij Ayoub. CSIRO Exploration and Mining, Perth, Australia. CHAPMAN CONFERENCE, AUGUST 23, 2001.

2. Structure of this Presentation. (a) The Problem – We need a new paradigm for exploration. (b) A process oriented classification of hydrothermal mineralising systems. Isothermal fluid/rock reactions. Gradient reactions. Discontinuity reactions. (c) Processes driving fluid flow. (d) The modelling software environment. (e) Some topics for the future. Self-organisation The role of mantle dynamics.

3. (a) The Problem – We Need a New Paradigm.

4. The Exploration Industry Has Performed PoorlyNumber of Large Gold + Base Metal Discoveries :Western World No of Discoveries per year Based on discoveries with in-situ value >US$1B The Discovery Rate has remained flat over the last 30 years Sources : WMC Global Deposit Data Base May 01 Metals Economic Group 2000 From WMC Page

5. …. but expenditures have approximately tripled over the same period Number of Large Gold + Base Metal Discoveries :Western World Exploration Expenditure (excl Mine Site) June 2001 US$B No of Discoveries per year $4 $3 $2 $1 $0 Sources : WMC Global Deposit Data Base May 01 Metals Economic Group 2000 Note : For years prior to 1990 assumes Minesite exploration makes up 20% of total expenditures Page

6. Average in 1990s US$300m Average in 1950s & 60sUS$100m Average in 1970s US$70m …. Resulting in a Tripling in the Cost per DiscoveryAverage Cost per Gold + Base Metal Discovery : Western World Cost per Large Discovery : June 2001 US$m NPV of Voisey’s Bay = US$617m BNP Paribas 2001 Average Discovery Costs have tripled in the last 30 years Sources : WMC Global Deposit Data Base May 01 Metals Economic Group 2000 Based on discoveries with in-situ value >US$1B Page

7. The Oil Industry Shows a Similar Trend Number of Giant Oil Fields : by Discovery Year 1931-94 Number of Discoveries per Year 117 Giant defined as >500 million barrels The number of giant oil discoveries has dropped significantly in the last 20 years Source : Petroconsultants 1998

8. Modelling of Processes. as a tool to aid thinking and explore a range of “what-if” questions before and during an expensive exploration program.

9. This paper is concerned with the processes that operate in hydrothermal mineralising systems. We are concerned primarily with the mechanics of these processes.

10. The term mechanics is used to mean the science involved in understanding the behaviour of a fluid saturated porous solid subjected to: • a general stress state, • gradients in pore fluid pressure, hydraulic head and temperature, and within which chemical dissolution, transport and reactions may occur.

11. THE FULLY COUPLED FOUR-FOLD PROBLEM. GENERAL STRESS STATE, sij; EFFECTIVE STRESS INFLUENCED BY CHANGES IN PORE PRESSURE FLOWTHROUGHOF CHEMICALLY REACTIVE SPECIES POROSITY AND PERMEABILITY LINKED TO CHEMICAL REACTIONS DEFORMATION LINMKED TO CHANGES IN POROSITY AND PERMEABILITY GRADIENTS IN TEMPERATURE, HYDRAULIC HEAD AND CONCENTRATIONS OF CHEMICAL SPECIES

12. In general there are strong feedback mechanisms associated with these processes, so that each process has an influence upon the others, and part of our goal is to take these feedback processes into account in a quantitative manner.

13. The four-fold, fully coupled system involved in mineralising systems.

14. (b) A Process Oriented Classification of Hydrothermal Mineralising Systems.

15. Mineralisation in hydrothermal systems arises from one or a combination of the following three fundamental processes: • Isothermal fluids/rock reactions, • Gradient reactions, • Discontinuity reactions.

16. The three end-member types of hydrothermal ore bodies. GRADIENT DISCONTINUITY FLUID-ROCK REACTION

17. (i) Isothermal fluid-rock reactions. G D F/R

18. Isothermal Fluid Rock Reactions. Reaction Front Rock type A Rock type B Darcy velocity uB Darcy velocity uA Equilibrium concentration cA Equilibrium concentration cB Mineralisation rate proportional to (cA – cB). Reaction front velocity proportional to uB.

19. Thus, for isothermal fluid-rock reactions, • The grade is proportional to (cA-cB), • The tonnage is proportional to the Darcy fluid velocity, uB.

20. Characteristics of Ore Bodies with Dominant Origins through Fluid/Rock Reactions. • Isothermal fluid/rock reactions are essentially replacement processes where the extent of the ore body is controlled by the magnitude of the Darcy fluid velocity. • Such ore bodies can be of exceptionally high grade but are patchy in their development since very small changes in permeability can lead to fluid focussing leaving low permeability rocks barren. • Examples are the Irish Pb/Zn and Hamersley Fe deposits.

22. (ii) Gradient Reactions. G F/R D

23. Mineralisation due to fluid flow down a gradient in equilibrium concentration. Rate of mineralisation = -fu.grad ce Darcy Fluid Velocity, u Porosity, f. Porosity, f. Porosity, f. Gradient of equilibrium concentration, grad ce.

24. Mineralisation due to fluid flow obliquely across a gradient in equilibrium concentration. 100 ppb 10 ppb Darcy Fluid Velocity, u Darcy Fluid Velocity, u Gradient of equilibrium concentration, grad ce. 1 ppb Porosity, f. Rate of mineralisation = -fu.grad ce

25. Mineralisation Rate with no Local Chemical Reaction. • Mineralisation rate = -fu.grad ce • Since ce = f ( T, p, cr ), • The Chain Rule of differentiation gives us:

26. Mineralisation Rate with Local Chemical Reaction. • Mineralisation rate = -fu.grad ce + Ri Or: + Ri

27. PROGRESS OF REACTION FRONT IN FLUID ROCK REACTION. H2O+CO2 K-SPAR+QUARTZ Quartz+muscovite Muscovite Pyrophyllite K-spar Quartz

28. 100 YEARS 5000 YEARS K-SPAR MUSCOVITE PYROPHYLLITE QUARTZ

29. Many gradients in equilibrium concentrations of metals arise from fluid mixing. An important mechanism that assists fluid mixing is the focussing of fluid flow into regions which have high permeability relative to their surroundings.

30. Fluid focussing into high permeabilitylenses.

31. For long, thin lenses with high permeability contrasts the focussing effect is dramatic.

32. Concentration gradients are also dramatically changed by focussing.

33. FLUID FOCUSSING IN A NARROW LENS. Vertical Darcy velocity 8*10-8ms-1.

34. STREAMLINES IN A NARROW LENS.

35. GOLD GRADES IN A NARROW LENS.

37. Reef X Section Geology

38. Permeability Structure

39. CO2 CH4

40. CH4 Concentration

41. CO2 Concentration

42. Au Precipitation MAXIMUM RATE OF MINERALISATION: 20 g/tonne/million years

43. U Precipitation MAXIMUM RATE OF MINERALISATION: 100 g/tonne/million years

44. Aqueous species at 0.025 million years

45. Gold Pyrrhotite Muscovite Graphite Chlorite Pyrophyllite

46. K-FELDSPAR Redox vs pH Diagram

47. Witwatersrand - Basin Scale Alteration

48. K-FELDSPAR Redox vs pH Diagram

49. Characteristics of Ore Bodies with Dominant Origins through Gradient Reactions. • Deposits are characteristically of high tonnage but rarely are of bonanza grades. • Since grade is proportional to Darcy fluid velocity, mineralisation can be quite extensive. • Examples are Witwatersrand gold and much of the gold mineralisation in the Yilgarn of WA.

50. (iii) Discontinuity Reactions. G F/R D