Lecture M1: Mineral Systems Overview Why a MS · PDF fileMineral Systems versus Petroleum Systems ... What is the structural geology? ... Faults do not show strong

Mineral Systems

5 Questions Overview

1

predictive mineral discovery*Cooperative Research Centre

A legacy for mineral exploration science

2



A. Gradient inhydraulicpotential

B. Permeability

C. Solubilitysensitivity to P, T, C

D. Spatialgradient of P, T, C

E. Time (duration)

Key Parameter

is reflected in

ExplorationMineral System

scale-dependent translation

5 Questions1. Geodynamics2. Architecture3. Fluid

reservoirs4. Flow drivers &

pathways5. Deposition

Terrain Selection

Area Selection

Drill Targeting

Slide after: A. Barnicoat

Mineral Systems

Defined as:

‘all geological factors that control the generation and preservation of mineral deposits’

(Wyborn et al 1994)

3



Mineral Systems versus Petroleum Systems

• Petroleum Systems introduced the concept of source-transport-trap

– Source only of petroleum required

– Change of phase in source region

– Buoyancy the major driver

– Trapping is purely mechanical or hydrodynamic

• Mineral Systems differ

– Source of various ore metals and a range of fluids

– Wide range of flow drivers

– Change of phase on deposition

– Trapping (of fluid) highly undesirable

4




5



5 Questions

1. Geodynamics

2. Architecture

3. Fluid reservoirs

4. Flow drivers & pathways

5. Deposition

The Where Question

Where is the next ore body?

Inputs from:

Data Compilation

Data Collection

Modelling Simulation

The Why Question

Why is the ore body there?

Mineral Systems Workflow

‘Classic’ deposit types

6



Groves et al 2005

Classic vs. Systems approach

7



DATA SETS

DATA MINING

CONSTRUCTION OF 3D AND 4D

IMAGESDRILL HERE

THE CLASSICAL APPROACH TO MINERAL

EXPLORATION

“Classical” Approach

INTERROGATION OF IMAGES

DATA SETS

DATA MINING

CONSTRUCTION OF 3D AND 4D

IMAGES

GEOLOGICAL INVERSION

COMPUTATIONAL MODELLING

Coupled mechanics-thermal transport-fluid flow-

reaction/transport/diffusion chemistry

GEOLOGICAL INVERSION

DRILL HERE

COLLECT DIFFERENT OR MORE

DATA

INTERROGATE THE DATA

SETS DIFFERENTLY

THE SYSTEMS APPROACH TO MINERAL

EXPLORATION

“Classical” Approach

New paradigm approach

B. Hobbs

provide a calculable framework• test hypotheses of ore formation in a particular location• generate new hypotheses• increase confidence • cut exploration risk and the time before a discovery is made.

4 Mineral Systems?

MagmaticPGE, Ni etc. in intrusions, etc, Diamonds

Magmatic-HydrothermalVHMS, ‘Lode’/slate belt gold, Porphyry & epithermal, IOCG, Intrusion-related Au, W, Sn, Witwatersrand, Cobar (?), Carlin (?)

Basinal‘Sedex’, MVT, Irish type, sedimentary Cu, Unconformity U

SurficialSupergene Au, Ni, etc, bauxite

8



All this is detail in the depositional environment

9



A. Gradient inhydraulicpotential

B. Permeability

C. Solubilitysensitivity to P, T, C

D. Spatialgradient of P, T, C

E. Time (duration)

Key Parameter

is reflected in

ExplorationMineral System

scale-dependent translation

5 Questions1. Geodynamics2. Architecture3. Fluid

reservoirs4. Flow drivers &

pathways5. Deposition

Terrain Selection

Area Selection

Drill Targeting


Deposition

10



Rate of deposition =

Velocity of

transport medium

.

Gradient in

carrying capacity

Examples:• Heavy mineral deposition controlled by flow rate and

entrainment capacity (proportional to velocity2)• Magmatic deposits controlled by magma supply rate and

changes in temperature, magma composition, etc. causing deposition

• Residual deposits (e.g. bauxites) where dissolution and removal of gangue leads to ore formation

• Hydrothermal systems, where fluid flow rate and changes in P, T or chemistry lead to deposition

5 Processes – basic relationship

11



Key parameters:

• Gradient in hydraulic potential

• Permeability

• Solubility sensitivity to P, T, C

• Spatial gradient of P, T, C

• Time (duration)

dtcccp

pcT

TcPgA

rr

r

eee ... ⎟⎟⎠

⎞⎜⎜⎝

⎛∇

∂∂

+∇∂∂

+∇∂∂

∇= ∑∫ µκρ

3 sets of geological inputs

1) Palaeogeography: feeds into most of the critical factors

– Describes distribution of (emergent) topography and hydrocarbon generation potential, both potential sources of hydraulic gradient.

– Controls distribution of facies and diagenesis that control permeability distribution in sedimentary sequences.

– Describes potential source regions for meteoric fluids (emergence again) and brines in marginal marine areas: key control on solubility sensitivity.

– Allows identification of stable areas where P & t gradients could have been stable for long periods.

12



Barnicoat 2008


2) Magmatism: also plays a major role in many critical factors

– Source of fluids and temperature distributions that may create hydraulic gradients.

– Driver for fracture generation and hence a control on permeability.

– Creates spatial gradients in temperature (and potentially chemistry too).

– Act as a fluid source the nature of which will depend on the magma’s origin: key control on solubility sensitivity.

– Repetitive magmatism will lead to long-lived hydrothermal systems.

13



Barnicoat 2008


3) (Structural) Architecture

– Controls the distribution of dilation sites that play an important role in developing hydraulic gradients.

– Defines most of the high-permeability domains in the crust

– Helps to define pressure gradients, and plays a role in facilitating fluid mixing.

– Repeated failure on structures (including reactivation of deeper features) allows prolonged fluid movement an/or multiple deposition/mixing/etc. events.

14



Barnicoat 2008

Example – Cyprus VHMS Deposits

For over 2000 years, Cu has been mined in Cyprus

Hosted in mafic rocks of the Troodos ophiolite

Answer the Five Questions about these mineral systems

15



Cyprus - Introduction

Late Cretaceous(92Ma) Troodosophiolite withyounger cover

Massive sulphidedeposits mined forpyrite (for H2SO4) Cu& Zn

Now only supergene Cu active

Fe-Mn umbers minedfor pigment (active)

16



DykesLavas

Plutonics

UmbersVMS deposits

From Pritchard & Maliotis, 1998

Cyprus - Skouriotissa Mine

Phoenix Pit – supergene Cuenrichment

– Heap leach and SX-EW plant

Primary massive sulphidesmined by Romans (smeltingslag far right)

17



Geodynamics

What is the P, T history?

What geochronological data exists?

What metamorphic and alteration assemblages exist?

Timing?

18



Geodynamic History 1

Bonninitic affinities of lavas in the ophiolite implies a back-arcorigin

Ophiolite overlain in the west by rocks containing calc-alkaline volcanic debris

Modern-day analogue of setting is the Lau basin

19



Africa

Emplaced ophiolites in Syria & Turkey

Troodosmicroplate

From Robertson et al., 1991

Geodynamic History 2

Sheeted dykes form much of the Troodos Mountains – extreme extension with magmatism

Lavas ponded against a fault scarp:syn-extensional magmatism

20



Trace of

dykes

Lava flow

Fault trace

Architecture

How big is the system?

Does the system involve the entire crust or just a sedimentarybasin within the crust?

What is the stratigraphy ? Strength contrasts, permeabilitiesat time of mineralisation and chemistry (reduced or oxidised)of the rock units?

What is the structural geology?

What is the chemistry of igneous intrusions?

21



Architecture 1

Classic (definitive?) ophiolite succession

Complete sequence:Mantle successionPlutonicsSheeted dykesLavasUmbersChertsCarbonate cover

22



Stockworkmineralisa-

tion

Massive Sulphide deposits

Architecture 2

VMS deposits (and associated umbers) located At the top of the succession (dominantly)

Within the lavas (in places)

23



DykesLavas

Plutonics

UmbersVMS deposits

Architecture 3Fossil evidenceindicates formation onsea floor

Left: tube worm preserved in pyrite; Kambia

Right: gastropods preserved in pyrite; Kinousa & Memi

Syn-magmatic oreFormation demonstratedby common burial bylavas

24



Little et al., 1999

Fluid reservoirs

What was the chemical nature of the fluid or fluids responsiblefor mineralisation?

What was the Eh and pH of the fluids?

What was the salinity?

What rocks did these fluids come into contact with to definetheir isotopic and chemical signatures?

25



Fluid Reservoirs 1Fluid sources available areseawater and magmas

Stable isotope dataFluids in equilibrium with country rocks trend between magmatic & seawater fields

Fluids in equilibrium with stockworks plot with seawater

Oxygen isotope profile throughthe ophiolite reveals changingtemperatures of equilibration byusing calculated water-rockfractionation factors

26



δ18O ‰

δD‰

Troodoscountry rocks

Seawater

Primary magmatic waters

Troodosstockworks

Low-T alteration

Fresh magmatic rocks

High-T alteration

Fluid Reservoirs 287Sr/86Sr shows large spreadbetween seawater andmagmatic values in lavas

Faults do not show strongseawater influence

Seawater recharge of systemwas general and not localiseddown faults

27



Troodos fresh glass ~0.7035

Cretaceous seawater ~ 0.7074

Fluid flow drivers and pathways

Fluid flow in porous rocks requires a gradient in hydraulicpotential (!!):

Topographic relief

Changes of fluid pressure created by deformation induceddilation, compaction is a subset of this process

Convection induced by thermal or chemical buoyancy

Changes in fluid pressure generated by chemical reactions

Pressure gradients generated by high-pressure fluids being Released from intrusions

28



Fluid Flow – Drivers & Pathways 1

Whole rock d18O isotope patterns linkreaction zone at base of dykes withorebodiesd18O pattern defines upflow zoneover epidosites driven by heat fromunderlying magma chamber

29



Figures from Schiffman & Smith, 1988


Epidosites (epidote-quartz± chlorite) in basal sheeted dykes

Bands of epidosite parallel dyke margins regardless of joints or other dykes

30



Richardson et al., 1987


Scanning electron micrograph of epidosite

(combined back scatter andcathodoluminescence)

Note infilled porosity, created byalteration reaction forming denser

mineralphases than protolith

Alteration reaction thus leads to increased

porosity and permeability, furtherfocussing fluid flow

31



Epidote

Chlorite

Pore infilled with quartz. Note varying image brightness outlining in white a euhedral and hence open-space-filling zone.

Deposition

What was the process involved in precipitation of the mineralassemblage in the ore deposit?

Fluid-rock reaction?

Fluid mixing?

Boiling?

Pressure drop?

Migration down a temperature gradient?

32



Deposition 1

Pb isotopes indicate the metal inore deposits sourced from localoceanic crust

33



206Pb/204Pb

207 P

b/20

4 Pb

18 19 19.518.515.5

15.6

15.7

Indian Ocean MORB

SeawaterTroodossediments

Sulphides

Troodos glasses

Epidosites

Epidosites and the slightly lessaltered chl-ep-qtz rocks have lost~90% Cu and 50% Zn comparedto background lavas & dykes

Metals sourced from deepportion of dyke complex

Cu ppm

Zn p

pm

Unmineralised lavas & dykesChl-qtz-epidote rockEpidosites

20 40 60 80 100 120

20

40

60

80

100

Deposition 2

Total S data showsthat S has beenredistributed

d34S values revealthe S above deepplutonics to be amixture of Magmaticand seawater S

Epidosites have lowlevels of S

34



Alt, 1994

Primary ranges

Total S ppm δ34S

Epidosites

Deposition 3

Umbers blanket top oflavas, infilling half-grabentopography

Originate by suspension fall-out of oxidised veryfine grained sulphideformed at black smokervents

35



Umber-altered lava contact

Overall Model for Systems

36



From Bickle & Teagle, 1992

Source of metals and some sulphur

Source of water and some sulphur

Focused zone of fluid upflow

Distributed of fluid downflow

Source of heat to drive fluid flow

Implications for Exploration

If we didn’t know about Cyprus-type VHMS deposits:

Proximity to magmatic heat sources essential

Architecture with faults to focus fluid upflow

Marine environment needed to provide fluids and sulphur

Appropriate metal source needed close to magmatic heat source

37





ReferencesWyborn, L. A. I., Heinrich, C.A., and Jaques, A.L. (1994). "Australian Proterozoic

Mineral Systems: Essential Ingredients and Mappable Criteia." Australasian Institute of Mining and Metallurgy Publication Series 5/94: 109-115.

Barnicoat, A. C. (2008). The Mineral Systems approach of the pmd*CRC. New Perspectives: The foundations and future of Australian exploration. Abstracts for the June 2008 pmd*CRC Conference., Perth, Geoscience Australia Record 2008/09.

Groves, D. I., Condie, K.C., Goldfarb, R.J., Hronsky, J.M.A., and Vielreicher, R.M. (2005). "100th Anniversary Special Paper: Secular Changes in Global Tectonic Processes and Their Influence on the Temporal Distribution of Gold-Bearing Mineral Deposits." Economic Geology 100: 203–224.

39



Lecture M1: Mineral Systems Overview Why a MS · PDF fileMineral Systems versus Petroleum Systems ... What is the structural geology? ... Faults do not show strong

Documents