Recent Advances in Genetic Models for Sediment-Hosted Stratiform Copper (SSC) Deposits

Alex C. Brown École Polytechnique de Montréal (Ret.)

acbrown@polymtl.ca

SIMEXMIN Ouro Prieto, Brazil, May 2012

Cu Cu Cu

Cu

Cu

What are SSCs (Sediment-hosted Stratiform Copper) ?

An excellent example - see Coppercap Mountain, NWT, Canada

4 % Copper and continues for many kilometres (unfortunately, only 1 metre thick)

Cu Cu Cu

Cu

Cu

Principal SSCs Worldwide

(Not many examples, but they can be very large)

Dongchuan

(Yunnan)

-German

Principal SSCs Worldwide

(Not many examples, but they can be very large)

Dongchuan

(Yunnan)

Underlined = SSC producer

Red lettering = Super-giant SSC producers

-German

Economic SSCs: Several metres thick, tens of kilometres long (or Km2 in area) with1 to 6% Cu (+ Co or Ag, Au… )

Good Grades

Good Tonnages

Grade-Tonnage Plot for SSCs

(after Kirkham, 1995)

The Giants and Super- Giants

Structure of this presentation

Part 1: Major steps forward over 50 years Part 2: More precise recent information on the transport and sourcing of copper

First.. a rapid review of Part 1

Part 1: Major steps forward over the years

1960s 1. Diagenetic overprint model, based on a) replacement of syndiagenetic pyrite, b) upward zoning of sulfides.

Cu apparently entered host greybeds (reduced) (black shales, ssts, carbonates) from coarse-grained footwall redbeds (oxidized).

Note: Ultimate source of copper was uncertain at this time.

Bartholomé (1958), White (1960), Brown (1965) … and many others

Unmineralized Pyritic Zone

Cupriferous

Zone

Zoned Sulfides (and Metals) Upper limit of

mineralization

Influx of Low-temperature

Copper Chloride Complexes

Redbeds

(oxidized)

Ore-grade beds

Greybeds

(reduced)

Py

±Ga

Sph

Cp

Bn

Cc Redoxcline

(adapted from concepts in Bartholomé (1958), White (1960), Brown (1965), etc.)

Summary of Copper Transport & Deposition (Deposit- Scale)

Based largely on (1) sulfide zoning and sulfide replacement textures, and (2) copper solubility as chloride complexes, e.g., CuCl3

2-

Major steps forward over the years

1960s

2. Pyrite is syndiagenetic,

with typical sedimentary S isotopic

signature (broad and mostly negative).

Baas Becking, Berner, and others

Major steps forward over the years

1960s

3. Copper is suitably soluble in oxidized

low-T brines

4. SSCs post-date atmospheric

oxidation (< 2.4 Ga)

Helgeson, Brown

Ref… and others

after Brown (1968, 1971); designed for the White Pine SSC

64

0 p

pm

Cu

6.4

pp

m C

u

Good copper solubilities within the hematite

stability field

1968

3 5 7 9 11

pH

0.8

0.4

Eh(V)

0

-0.4

Conditions favourable for significant copper solubilities in redbedsCu-chloride

complexing gives Cu solubilities of >10 to 100 ppm

Solubility of Copper for low temperatures and high salinities

A more complete and accurate diagram

1976…

(from Brown, 2003; modified after Rose, 1976, 1989)

Major steps forward over the years

1960s

3. Copper is suitably soluble in oxidized low-T brines

4. SSCs post-date atmospheric oxidation (< 2.4 Ga)

Helgeson, Brown

Meyer, Cloud, and others

Major steps forward over the years

1970s

5. Global association of SSCs with evaporites formed at low paleolatitudes

Recall: brines are needed to transport Cu

Kirkham, Hitzman

Major steps forward over the years

1980s

6. Global association with intracontinental

rifts and rift volcanics, and

perhaps anomalous mantle heat.

Relates SSCs to coarse-grained

footwall redbeds (± volcanics ± basement)

as sources of copper.

Jowett

Major steps forward over the years

1960-1980s

7. Research on Intracontinental rift redbeds: Diagenetically reddened, with copper released simultaneously (from labile minerals , e.g., mafics, feldspars) and carried by a moderately oxygen-rich brine. A multi-million year long reddening and leaching process.

Walker et al.

Note: Fresh meteoric water may assimilate evaporitic brine from surface, or dissolve subsurface evaporites, to become a brine.

)

Sourcing copper (after Walker, 1967, 1989)

O2-rich Meteoric Water

+ Cu leaching Reddening in progress

Downstream flow to form SSCs

And now… Part 2: Recent Advances

Note: Some important concepts

1. Deep-basin waters tend to be warm, dense, saline and reducing… and difficult to move (see petroleum basins).

2. Highland recharge may move deep brines

(Topography-driven,

Gravity-driven)

Garven, Leach (MVTs) Brown (SSCs)

Note: Meteoric water is essential (for diagenesis, for O2) and Meteroic water is topography-driven

)

Let us look at this Walker diagram again...

O2-rich Meteoric Water

+ Cu leaching Reddening in progress

Downstream flow to form SSCs

Topography-driven Meteoric Water (asymmetric basin, other highland recharges not shown)

after Brown (2005, 2009)

SSC

O2-rich

O2-rich

Recent Advances

Sources of copper

a) Rift redbeds (and volcanics) reddened by infiltrating O2-rich meteoric water (which evolves into a brine by assimilation of evaporite salts) Walker

b) Deeper basement rocks, where redbeds are insufficient

(the latter is attractive for seismogenic or structurally controlled solutions from basement)

Cathles, Blundel, Wedepohl & Rentzsch, Hitzman . . .

The above are consistent with two diagrams

1) Eh-pH diagrams showing where moderately oxidizing

water may originate… (from meteoric water)

2) Rift-basin profile showing meteoric water

a) transforms into a brine

b) loses oxygen by reddening of first-cycle basin sediments (and volcanics)

c) leaches copper from the reddening basin fill (and basement if necessary)

d) deposits copper as SSC-type mineralization

Recent Advances

Topography-driven Meteoric Water (asymmetric basin, other highland recharges not shown)

after Brown (2005, 2009)

SSC

O2-rich

O2-rich

Oxygenated

(Atmospheric)

De-oxygenated

(Deep, non-

atmospheric) environments

Oxidizing, slightly acidic conditions of Meteoric Water

Reducing conditions of Deep Ground Water

Natural Eh-pH conditions

From Garrels (1960)

(an old story!)

But first.. The Source of O2-rich Meteoric Water is ?

versus

25 3 5 7 9 11

pH

0.8

0.4

Eh(V)

0

-0.4

Conditions favourable for significant copper solubilities in redbeds

Cu-chloride complexes ( >10 to 100 ppm Cu )

Rose (1976, 1989) and Brown (2003)

Solubility of Copper for low temperatures and high salinities

Now, overlay Garrels, Rose and Brown diagrams…

And for a more complete geochemical story.. 1) O2-rich meteoric recharge water 2) Progressive loss of O2

due to reddening 3) Release and transport of Cu 4) Deposition of Cu under reduced conditions .

Brown (2005)

Evolution of Meteroic Water

from O2-rich to O2-poor

Topography-driven evolved meteoric water model (deep-basin flow added)

after Brown (2009, 2011)

SSC

Basement reddening & Cu-leaching

O2-rich

Basin-fill reddening/Cu leaching

Evaporite Assimilation

Dashed red arrow added if basin-fill is an inadequate Cu source

• Now, recall the 1960s:

1) Cu entered greybeds from footwall redbeds.

2) Hematitic pigment of redbeds suggested that Eh-pH

conditions would be oxidizing and therefore suitable for copper transport.

3) But Walker showed independently that first-cycle redbeds give up Cu during long-term diagenetic reddening,

i.e., redbeds did not exist as redbeds until oxidized by meteoric

water (accompanied by the simultaneous release of copper).

Recent Advances

Numerous suggestions have been made that Cu can be mobilized from various deep basin basement environments Cathles, Blundel, Hitzman, Wedepohl & Rentzsch,…

Those are works in progress, from my perspective.

They suggest that Cu-brines become oxidizing by equilibration with footwall redbeds (this part is doubtful – see below) and then the Cu-brine form SSC deposits by the conventional influx into basal greybeds (this part is ok)

Recent Advances

The Deep Basement Source concept? (my interpretation of descriptions)

Cu-brine

Redbeds

Deep Source

Greybeds

SSC

Highly reducing conditions (equilibrated

with ferrous iron ) (problem here: Cu is not soluble)

Brine becomes oxidizing by equilibration with hematite of redbeds (problem here: Redbeds essentially not pre-ore, but syn-ore)

Two problems:

1) Initially ferrous-iron equilibrated brine cannot become moderately oxidizing, because remnant ferrous iron in basement and redbeds will hold Eh at the ferrous-ferric iron boundary.

Recent Advances

Brown (2009)

Deep brines remain too reduced to

carry copper

Ferrous-ferric boundary

2nd problem:

1) Initially ferrous-iron equilibrated brine cannot become moderately oxidizing , because remnant ferrous iron in basement and redbeds will hold Eh at the ferrous-ferric iron boundary.

2) First-cycle rift redbeds do not exist until oxidized by meteoric water.

Recent Advances

Two problems

1) Initially ferrous-iron equilibrated brine cannot become moderately oxidizing , because remnant ferrous iron in basement and redbeds will hold Eh at the ferrous-ferric iron boundary.

2) First-cycle rift redbeds do not exist until oxidized by meteoric water.

These are not problems for the topography-

driven evolved meteoric water model

Recent Advances

1) O2-rich meteoric recharge water 2) Progressive loss of O2

due to reddening 3) Release and transport of Cu 4) Deposition of Cu under reduced conditions

Brown (2009)

Topography-driven evolved meteoric water model (chemical aspects)

Topography-driven evolved meteoric water model (rift basin-scale aspects)

after Brown (2009, 2011)

SSC

Basement reddening & Cu-leaching

O2-rich

Basin-fill reddening/Cu leaching

Evaporite Assimilation

Dashed red arrow added if basin-fill is inadequate Cu source

Conclusion

SSCs (like MVTs) have a multi-stage origins, including a necessary tectonic setting

1) occur in intracontinental rift basins (extensional)

2) require post-rift first-cycle erosional debris of elevated rift-margins to provide copper source

3) require rift-margin, topography-driven, meteoric recharge water to oxidize and alter footwall, and to leach and transport copper

4) require reduced greybeds for copper deposition (common in marginal marine and lacustrine basins).

Recent Advances

The End ! Obrigado!