i) geochemical modelling · i) geochemical modelling presented by james cleverley and nick oliver...
TRANSCRIPT
I) Geochemical Modelling
Presented by James Cleverley and Nick Oliver as part of the JCU/EGRU/CRC supported minerals masters program, May 2005
James Cleverley & Nick Oliver
Why do we need geochemistry?Geochemistry helps you to understand the way in which metals (and other goodies) are transported from place A and deposited in place B.
We can attempt to:
• Predict the processes that led to formation of certain mineral deposits.
• Predict possible alteration assemblages and mineral paragenesis.
• Understand which are the most important or effective processes in mineral deposit formation.
• Start to build computer models of mineral deposit formation, and
• Combine fluid-flow, deformation and geochemistry codes to produce a predictive tool for mineral deposit exploration.
Why geochemical modelling?
1) What was the fluid-rock
system that did this?
2) How did all this
happen?
Simulating process 3) What do we
expect to see elsewhere?
Defining inputs
Predictive modelling
•Exploring parameters
•Generating testable questions
?
HH:Red Herring
Fluid
II:Red Herring
Alteration
AA:source fluid Source
GG: Spent fluid alteration FF: spent fluid
CC: other source fluid
BB: modified source fluid
EE:alteration halos
deposit DD
What are concept models
Before computer modelling you need to knowThe problem that you are tackling,
What you hope to achieve and
Understand the limitations and assumptions
Need to build concept modelsGeological observation -> Modelling cartoon
Geochemical models are based on geological concept
What do we know about the system and the theory – GeoKnowHow
Geochemical Model Concepts
Three Broad TypesDifferent fluid-rock processes
Closed System Static Model
Flow-through or Flush Model
Fluid InfiltrationVariations limited by imagination in writing algorithms in HCh
Closed System (Log f/r): K,Fe-Brine & NaCa-Rocks
kfs
bt
ms
mt
hm
qtz
ab
T = 500500ooCC, P = 3500 bars, Rock = NaCavolcanic
Reaction step increasing
Reactor 0 Reactor iFlui
dR
ock
Reactor inFLUSH & FLOWFLUSH & FLOW--THROUGHTHROUGH
INCREASING FLUID THROUGHPUT
bt
kfs
ms
hm
mt
rut
ilm
T = 500500ooCC, P = 3500 bars, Rock = NaCa-felsic volcs
cor
N0Rock
Flui
d so
urce
at
cons
tant
flux
N0
Nx
Nmax
Ix
Time increasing
Distance increasing
FLUID INFILTRATIONFLUID INFILTRATION
III) Introducing Geochemical Modelling & Software
A world of acronyms:
HCh-PIG and ELF
James Cleverley & Nick Oliver
It’s all a matter of equilibrium!
1) 4 Gold + O2(g) + 4 H2S(aq) = 2 H2O + 4 Au(HS)(aq)
2) Log K + LogfO2(g) + 4Log(aH2S) = 4Log(aAuHS)
3) Log K(T = 400, P = 3500) = 7.37
4) (7.37 + -30 + -4)/4 => -8.6
5) 2.2*10-9 moles 0.05 ppm Au
1 2 3 4 5 6 7 8 9 10150
200
250
300
350
400
450
500
550
pH
T (°
C)
jc140209 Mon May 10 2004
Dia
gram
Au+ ,
a [m
ain]
=
10
–7.5
, a
[H2O
] =
1,
rat
io [
SO
4--/H
2S(a
q)]
=
10–4
, a
[H2S
(aq)
] =
10–3
(sp
ecia
tes)
, a
[Fe++
] =
10
–3
Au(HS)2-
AuHS(aq)
AuOH(aq)
Au+
Gold Gold
H 2S(a
q)
HS
- )
A
Stability Diagrams
HCh-what the?
HCh (Yuri Shvarov & Evgeniy Bastrakov)Uses ‘gibbs energy minimization’ to locate the equilibrium point of any system. This is a different approach from log K-type modelling, although the end point should be the same result.
Advantages:Powerful and flexible algorithm generator that can be used to model a much wider range of geological (fluid-rock) scenarios.
Well maintained high PT thermo datatset that will soon be ‘online’ thanks to GA developers.
Ongoing development in user interface and visualisation
T
0
500
1000
P
0
2000
4000
6000
G
10
15
20
25
30
35
XY
Z
Gold + H2S(aq) + HS- = Au(HS)2- + 0.5H2(g)
a e 00 ⏐ 05 p 005 ⏐ Co e ted ce ataGibbs Surface
• Build surface for each reaction we want to consider
• Natural systems will always tend towards the lowest energy configuration
What can we do with HCh?
Assemblage plots of bulk composition over PT, or reacting with different rocks.Fluid mixing, fluid mixing with rock (and increasing mass of rock)Outflow modelsFlow and rock reaction across a P, T or PT gradient (veins!)Fluid infiltration models (pseudo reactive transport), possible time-space plots of fluid through rock
System
Blank
Input
Control
UNITHERM
Gibbs WinGibbs
.RE
1
2
reConv
Geo
logi
cal &
Geo
chem
ical
Info
rmat
ion
.TXT
ProductiveInteractiveGraphics
Other Plotting
FreeGs Web Database
HChSystem
Blank
Input
Control
UNITHERM
GibbsGibbs WinGibbsWinGibbs
.RE
1
2
reConv
Geo
logi
cal &
Geo
chem
ical
Info
rmat
ion
.TXT
ProductiveInteractiveGraphics
ProductiveInteractiveGraphics
Other Plotting
FreeGs Web Database
HCh
The Geochemical Modelling Toolbox
The theoretical dataThe theoretical data
The workhorseThe workhorse
The result viewerThe result viewer
HCh Control File
The HCh control file uses simple algebraic notation to handle the interaction between the input systems and PT.
[*] = [1]+([2]*10^(i-6))Key workflow problem:
Geological/ore deposit process concept Algebraic control algorithm
The control file manipulation will become more straightforward with:
ELF (or daughter of ELF) and,
Control file library
Using “experts” to help??
Control File Algorithms
1
2
550oCAu-HCOS
450oCCu-Brine
450oCHm-bearingvolcanics
Control file algorithms can become quite complex in order to model key fluid-rock interaction conceptsThis example mixes two fluids in the presence of rock and looks at the passage of outflow fluid into wall rockOnce you understand the concepts behind the control file you can conceptualise a large range of ore-forming processes
* * * Primary wave * * *T = 450P = 2500[*] = [1]+(0.1*[5])+(0.1*[8])
General step...T = (i/60)*550+(1-i/60)*450P = 2500[*] = ([1]*(1-(1/60)*i))+(([2]/2)*((1/60)*i))+(0.1*[5])+(0.1*[8])Stop when: i=60
* * * Secondary wave * * *T = 450P = 2500[*] = {A}+(0.1*[5])+(0.1*[8])
General step...T = 450P = 2500[*] = {A}+((0.1*[5])+(0.1*[8]))Stop when: i=60Stop when: N=40
The list of scenarios covers all common models discussed in geochemical modelling handbooks (Bethke, Reed, Borisov & Shvarov…)
IV) Concept Models and the Modelling Approach
Application to Geological Exercise
James Cleverley & Nick Oliver
Why geochemical modelling?
1) What was the fluid-rock
system that did this?
2) How did all this
happen?
Simulating process 3) What do we
expect to see elsewhere?
Defining inputs
Predictive modelling
What do we know?Structural Controls:
Modelling results from previous days?
Structural model
Fluid Drivers:Metamorphic-Igneous History
Rocks:BIF
Basalt
Black Shale
Psammite/Pelite
Two generations of granite
Fluids:??
• Older mineralisation (known resource)• Black Shale-BIF contact• Faults important fluid focusing• Granite is not active at time of mineralisation• Metamorphic fluids
• Younger potential mineralisation• No known/discovered resource!• Granite fluids cutting metamorphic strat• Flow across T-gradient into different liths
Information on Fluids?
1) Early MineralisationFluids sourced from metamorphic rocks
Basalt-greenschist equilibrated fluid – H2O-CO2
Metals from basalt
Require external source fluid?
2) Later MineralisationFluids sourced from oxidised granites
Fluids are oxidised-brines with S and metals
1) Metamorphic fluid and mineralisation
Known mine in BIF/Black Shale location
Metamorphic fluid derived from 400oC carbonated basalts
Flow along faults
Mineralisation at 350oCOnly small amount of cooling
2)Granite fluid and mineralisation
No known mineralisation in exposed areasWhat is potential for undiscovered?
Oxidised granite fluid with metal enrichments in melt (see granite analysis)
Flow is driven by T gradient 450-400 to 200oC (post metamorphic)
Less requirement for fault flow, more rock unit flow
Model Workflow 1: Defining the inputs
RocksBulk composition (XRF)
Need to define things like water and redox
Mineralogy (VolMol converter)
FluidsStandard fluids
Rock equilibrium
Standard fluids + Rocks -> Modified fluid
Rocks and Redox
Na-Granite (balloon pegmatite) with 50g H2O
Vary the Fe2O3/FeO from XFe2O3 1 to 0
650oC 2kb
Plot shows only key minerals
Rocks, LOI and volatiles
Mafic extrusive from OZCHEM database from Laverton District
Use LOI as H2O component
Add 0.006 ppm Au
Add 1 Molal Cl
Add 0.5 m of CO2 to rock in 0.01 m increments
400oC and 2kb
Modelling Workflow 2: Making a fluidRock equilibrium
Rock
HCl(aq)
H2O
?
Rock
Fluid
compare
Convert to 1kg Model Fluidcompute
*Remember: moles and molality (per kg of water)
Modelling Workflow 2: Making a fluidMeasured or estimated
Fluid InclusionsFull or part analysis
General KnowledgeStandard or Reported analysis (geothermal waters, sedimentary brines)
GeoKnowHow
You will need to iterate to a sensible solution composition!Temperature will be an important factor
Rocks in input file (masters2)
Carbonated-Basalt (greenschist)
Mt-Carbonate-Graphite-BIF
Mt-Carbonate-BIF
Black Shale
Albite-Quartz Granite (oxidised)
Fluids in input file (masters2)
Granite Fluid (mt-stable)
H2O-CO2-S-Cl Fluid (metamorphic)Basalt eq. @ 400oC
Qz-Ab-Granite BrineGranite eq. @ 400oC
Generic/Standard Blank Fluid (Na-K-Cl-S)
V) Geochemical Process Modelling
Replicating what we know and predicting what we don’t
James Cleverley & Nick Oliver
VI) Application within Exploration
The geochemical modelling application workflow
James Cleverley & Nick Oliver
HChThe Work Horse
Geological Observation, Geochemistry, Fluid Inclusions
PIGData Vis
Geo Know-how
FreeGsThermo Data
NET Interface
Outflow
Ore BodyDistal Rocks
PT
Gra
dien
t
200200ooC, 1000 barsC, 1000 bars
380380ooC, 2500 barsC, 2500 bars
Fluid Dominated
Rock Dominated
Alteration & Geophysics
po
py
ht
po
mt
po
ht
mt
pyanh
ht
mt mt
g / kg rock15 0 15 0 15 0 15 0 15 0 15a)
K2O
K2O
po
py
anh
pyanh
ht
K2O
anh
K2O K2O K2O
360
340
300
260
Orebody
320
280
0
Ore BodyDistal Rocks
MagnetiteMagnetite
PyrrhotitePyrrhotitePyritePyrite
““Halo” GoldHalo” Gold
PT GradientPT Gradient
Dis
tanc
eD
ista
nce