i) geochemical modelling · i) geochemical modelling presented by james cleverley and nick oliver...

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.

Fluids & Rocks

Fluid ReservoirA

B

C

D

Rock

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

II) Geochemical Concept Models and Modelling

The theory


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 MODELSCLOSED SYSTEM MODELS

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

Fluid Infiltration of initial log w/r = 0

qtz

bt

an

ab

act

cpx

kfs

III) Introducing Geochemical Modelling & Software

A world of acronyms:

HCh-PIG and ELF


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

Geochemical Modelling Tools

• Geochemists Workbench

• HCh

• EQ36NR

• WinGibbs/Starter

• PIG

• ELF

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



Other Plotting

FreeGs Web Database

HCh

The Geochemical Modelling Toolbox

The theoretical dataThe theoretical data

The workhorseThe workhorse

The result viewerThe result viewer

HCh concept

A BInput

A+B

P-T Path

Control

Output

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


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

CO2-Basalt

Mt-Carb-BIF Black Shale Fa

ult

s

350oC2000 bars

400oC2000 bars

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

400oC 2000 bars

200

400300

200

Mt-Carb-BIF

Black Shale

Basalt

Granite

oC 2000 bars

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

650oC

Som

e m

iner

als

mol

%

Mast_a03

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

Increasing CO20.01m increments

Mast_a02

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


Stage 1

CO2-Basalt

Mt-Carb-BIF Black Shale Fa

ult

s

350oC2000 bars

400oC2000 bars

400oC 2000 bars

200

400300

200

Mt-Carb-BIF

Black Shale

Basalt

GraniteStage 2

oC 2000 bars

VI) Application within Exploration

The geochemical modelling application workflow


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

Coupled Modelling

i) geochemical modelling · i) geochemical modelling presented by james cleverley and nick oliver...

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