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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. Structure of this Presentation.

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chapman conference august 23 2001

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.

structure of this presentation
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.

slide4
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

number of large gold base metal discoveries w estern world

…. 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

slide6

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

the oil industry shows a similar trend number of giant oil fields by discovery year 1931 94
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

slide8
Modelling of Processes.

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

this paper is concerned with the processes that operate in hydrothermal mineralising systems
This paper is concerned with the processes that operate in hydrothermal mineralising systems.

We are concerned primarily with the mechanics of these processes.

slide10
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.

the fully coupled four fold problem
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

slide12
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.
slide15
Mineralisation in hydrothermal systems arises from one or a combination of the following three fundamental processes:
  • Isothermal fluids/rock reactions,
  • Gradient reactions,
  • Discontinuity reactions.
slide16

The three end-member types of hydrothermal ore bodies.

GRADIENT

DISCONTINUITY

FLUID-ROCK REACTION

slide18

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.

slide19
Thus, for isothermal fluid-rock reactions,
  • The grade is proportional to (cA-cB),
  • The tonnage is proportional to the Darcy fluid velocity, uB.
characteristics of ore bodies with dominant origins through fluid rock reactions
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.
slide23

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.

slide24

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

mineralisation rate with no local chemical reaction
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:
mineralisation rate with local chemical reaction
Mineralisation Rate with Local Chemical Reaction.
  • Mineralisation rate = -fu.grad ce + Ri

Or:

+ Ri

slide27

PROGRESS OF REACTION FRONT IN FLUID ROCK REACTION.

H2O+CO2

K-SPAR+QUARTZ

Quartz+muscovite

Muscovite

Pyrophyllite

K-spar

Quartz

slide28

100 YEARS

5000 YEARS

K-SPAR

MUSCOVITE

PYROPHYLLITE

QUARTZ

many gradients in equilibrium concentrations of metals arise from fluid mixing
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.

slide33

FLUID FOCUSSING IN A NARROW LENS.

Vertical Darcy velocity 8*10-8ms-1.

slide39

CO2

CH4

slide42

Au Precipitation

MAXIMUM RATE OF MINERALISATION:

20 g/tonne/million years

slide43

U Precipitation

MAXIMUM RATE OF MINERALISATION:

100 g/tonne/million years

slide45

Gold

Pyrrhotite

Muscovite

Graphite

Chlorite

Pyrophyllite

slide46

K-FELDSPAR

Redox

vs

pH

Diagram

slide48

K-FELDSPAR

Redox

vs

pH

Diagram

characteristics of ore bodies with dominant origins through gradient reactions
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.
discontinuity reactions
Discontinuity Reactions.
  • Here a change in pressure and/or temperature lead to boiling or to other forms of phase separation; H2S may be bled from the system or salinity may be reduced.
  • This leads to changes in salinity, pH and/or Eh and hence changes in the equilibrium solubility of metals.
slide52

Discontinuity Reactions.

Equilibrium concentration, c

Equilibrium concentration, ce

Mineralisation rate, and hence the grade, proportional to (c-ce).

Tonnage governed by size of dilating region.

slide53

If there is a pore pressure drop in this system there is an extra component to the mineralisation rate:

{

}

u.

Mineralisation Rate = -

G

Many mineralised vein systemsare probably hybrids like A.

Notice, no need for seismicity.

D

F/R

slide55

Higher shear stress

lower shear stress

slide56

Dilation zone arrays are developed at fault tips

Red to Green – volume increase

slide63

Flow stream lines, showing fluid flow and mixing patterns

Permeability structures reflect damage zones (tensile failure) related to individual faults

slide66

3. Areas with greater concentration gradient of important chemical species (e.g. CH4) and areas of chemical reaction front

characteristics of ore bodies with dominant origins through discontinuity reactions
Characteristics of Ore Bodies with Dominant Origins through Discontinuity Reactions.
  • Deposits can be of exceptionally high grade but are of limited spatial extent since the geometry of the system which produces the discontinuity in metal equilibrium solubility, such as a pressure drop, is commonly quite localised spatially.
  • Examples are many vein hosted gold deposits such as Bendigo-Ballarat.
slide69

TOPOGRAPHICALLY DRIVEN FLOW.

LOCALDEVOLATILISATION.

EXTERNALLY DERIVED FLUIDS-DEVOLATILISATION OR IGNEOUS.

HYBRID.

i topographically driven fluid flow
(i) Topographically driven fluid flow.
  • Widely used as important fluid driving mechanism for MVT deposits; eg Grant Garven and co-workers.
  • Undoubtedly an important mechanism but there are aspects we still do not understand.
  • In particular, there seems to be a mass balance and timing problem.
slide71

TRF

Century fluid system supplied by “squeegying” of deep basin aquifers Hydraulic head promoted by meteoric water input (height of orogen approx 2-3km above Century position, giving effective hydraulic head of ~500m-1km?)

Thrust belt to SE outboard of developing Orogen

Century “D1” SE-NW

~1570Ma

Meteoric water drive interrupted by structuring, Century fluid cell declines

TRF

Century “D2” E-W

~1550Ma

Inspired by Oliver (1986) and Garven & Freeze (1984)

fluid flow vectors associated with critical faults and stratigraphic units in the century system

Fluid flow vectors associated with critical faults and stratigraphic units in the Century system

the problem with topographically driven flow
The Problem with Topographically Driven Flow.

Consider the following situation:

B

A

100km

A Darcy flow rate of 1 m y-1 is equivalent to 1 m3 m-2 y-1.

Thus a cubic meter of fluid at A would be transported to B in 105 years.

ii dilatancy driven fluid flow
(ii) Dilatancy driven fluid flow.

Often thought of in terms of “fault valve pumping” (eg., Sibson). However, seismic behaviour is not essential here and any dilatancy associated with deformation, distributed or localised, will lead to a gradient in hydraulic head to drive fluid flow.

iii fluid flow driven by devolatilisation
(iii) Fluid flow driven by devolatilisation.

Typical metamorphic fluxes as recorded by metamorphic petrologists are in the order of 10-9 kg m-2 s-1 ( eg., Connolly).

At porosities and permeabilities to be expected in metamorphic rocks, this corresponds to Darcy velocities of say 10-7 m s-1.

Although such fluids are undoubtedly important in mixing to change pH and Eh, their volumes do not seem large enough to transport significant masses of metal.

two basic mechanisms
TWO BASIC MECHANISMS.
  • HYDROFRACTURE: if the rate of fluid pressure generation rate exceeds the dilatancy rate.
  • POROSITY WAVES: if the dilatancy rate exceeds the rate of fluid pressure generation.
slide77

THE ORIGIN OF POROSITY WAVES.

TENSILE FAILURE

LITHOSTATIC GRADIENT

DEPTH

DEPTH

HYDROSTATIC GRADIENT

PORE COLLAPSE

FLUID PRESSURE

FLUID PRESSURE

slide78

Thus if we look at the various mechanisms for transporting significant masses ofmetals:

Topographic:A problem. Convection: Let us now look.

Devolatilisation: A problem.

Igneous sources: OK.

slide79

(iv) Fluid flow driven by thermal convection.

FLUID FLOW VECTORS

STREAMLINES

TEMPERATURE

slide80

VELOCITY

STREAMLINES

TEMPERATURE

CHARACTERISTICS OF THERMAL CONVECTION SYSTEMS WITH FIXED TOP AND BOTTOM TEMPERATURES.

H2S

SO42-

H+

Galena

Sphalerite

Pyrite

slide81
It has long been claimed (eg., Wood) that convection is impossible in a system with a lithospheric fluid pressure gradient.
  • This is true for a system with impermeable upper and lower boundaries and fixed temperatures at these boundaries.
  • However, for other boundary conditions, this is not true.
  • For example, with fixed pressure and thermal flux boundaries, convection is possible with vertical fluid fluxes.
slide83

Thermal convection in a system with lithospheric pressure gradient.

Upper Boundary: constant pressure and constant thermal flux

Lower Boundary: constant mass and thermal fluxes

Flux due to lithospheric pore pressure gradient.

slide84

Thus, systems such as shown below, deep or shallow in the crust, seem to be one of the few ways of transporting significant masses of metal over extended periods of time.

impermeable base top
Impermeable Base & Top

CO2

Convective temperature

permeable fault impermeable boundaries
Permeable Fault,impermeable boundaries

L

ko

ko

L

10 ko

3L

Flow focussed

in the fault

Convective T &

Vg

slide88

Geological Model

In

GOCAD;

Imported from standard CAD package.

slide94

Coupled FLAC-FASTFLO

Gold bisulphide

Graphite

sketch of program interfaces
Sketch of Program Interfaces

Geological model

(e.g. in Gocad)

Return to Gocad

for

3D visualisation

Control of

simulation via

browser file (XML)

Process 2

(e.g. FastFlo)

Process 1

(e.g. Flac3D)

distribution of metals through time
DISTRIBUTION OF METALS THROUGH TIME.
  • Most gold on Earth was emplaced between 2.8 and 2.6 billion years ago.
  • Most lead-zinc on Earth emplaced 1.7 billion years ago.

WHAT GLOBAL SCALE PROCESSES WERE RESPONSIBLE FOR THESE MINERALISING EVENTS?

outcomes from modelling mantle behaviour have been very significant in the past decade

Outcomes from modelling mantle behaviour have been very significant in the past decade.

These outcomes include:

Convection is dominated by hot upwelling plumes and cold down going sheets.

Temperature increases due to viscous dissipation are important on the edges of slabs and plumes.

further outcomes
Further Outcomes.
  • Stratified mantle convection exists at high Rayleigh Numbers (Ra) and whole mantle convection at low Ra.
  • Progressive cooling of the Earth’s core, together with nonlinear effects associated with radioactive internal heating leads to massive flow instabilities and large increases in thermal flux through the lithosphere.
the future
The Future.
  • Modern seismic tomography of the Earth is defining the details of modern thermal convection patterns.
  • Modelling will enable us to describe past convection systems and define periods of convective instability or periods of gross change in behaviour.
  • Signatures of these changes in the geological record will enable us to identify these effects at the surface and understand why some periods of time and some regions are more endowed than others.
slide107

The Beginning.

Thank you.

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