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

Transcription

1 Recent Advances in Genetic Models for Sediment-Hosted Stratiform Copper (SSC) Deposits Alex C. Brown École Polytechnique de Montréal (Ret.) Cu Cu Cu Cu Cu SIMEXMIN Ouro Prieto, Brazil, May 2012

2 What are SSCs (Sediment-hosted Stratiform Copper)? An excellent example - see Coppercap Mountain, NWT, Canad Cu Cu Cu Cu Cu 4 % Copper and continues for many kilometres (unfortunately, only 1 metre thick)

3 Principal SSCs Worldwide (Not many examples, but they can be very large) -German Dongchuan (Yunnan)

4 Principal SSCs Worldwide (Not many examples, but they can be very large) -German Dongchuan (Yunnan) Underlined = SSC producer Economic SSCs: Several metres thick, Red lettering = Super-giant SSC producers tens of kilometres long (or Km 2 in area) with1 to 6% Cu (+ Co or Ag, Au )

5 Grade-Tonnage Plot for SSCs The Giants and Super- Giants Good Grades (after Kirkham, 1995) Good Tonnages

6 Structure of this presentation Part 1: Part 2: Major steps forward over 50 years More precise recent information on the transport and sourcing of copper First.. a rapid review of Part 1

7 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). Bartholomé (1958), White (1960), Brown (1965) and many others Note: Ultimate source of copper was uncertain at this time.

8 Summary of Copper Transport & Deposition (Deposit- Scale) Upper limit of mineralization Cupriferous Zone Py ±Ga Sph Cp Bn Cc Zoned Sulfides (and Metals) Unmineralized Pyritic Zone Ore-grade beds Greybeds (reduced) (adapted from concepts in Bartholomé (1958), White (1960), Brown (1965), etc.) Redoxcline Influx of Low-temperature Copper Chloride Complexes Redbeds (oxidized) Based largely on (1) sulfide zoning and sulfide replacement textures, and (2) copper solubility as chloride complexes, e.g., CuCl 3 2-

9 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

10 1960s Major steps forward over the years 3. Copper is suitably soluble in oxidized low-t brines Helgeson, Brown 4. SSCs post-date atmospheric oxidation (< 2.4 Ga) Ref and others

11 640 ppm Cu 6.4 ppm Cu 1968 Good copper solubilities within the hematite stability field after Brown (1968, 1971); designed for the White Pine SSC

12 Solubility of Copper for low temperatures and high salinities Cu-chloride complexing gives Cu solubilities of >10 to 100 ppm Eh (V) Conditions favourable for significant copper solubilities in redbeds ph 1976 A more complete and accurate diagram (from Brown, 2003; modified after Rose, 1976, 1989)

13 1960s Major steps forward over the years 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

14 Major steps forward over the years 1970s 5. Global association of SSCs with evaporites formed at low paleolatitudes Kirkham, Hitzman Recall: brines are needed to transport Cu

15 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

16 Major steps forward over the years s 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.

17 Sourcing copper (after Walker, 1967, 1989) O 2 -rich Meteoric Water Reddening in progress + Cu leaching Downstream flow to form SSCs ) Note: Fresh meteoric water may assimilate evaporitic brine from surface, or dissolve subsurface evaporites, to become a brine.

18 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)

19 Let us look at this Walker diagram again... O 2 -rich Meteoric Water Reddening in progress + Cu leaching Downstream flow to form SSCs ) Note: Meteoric water is essential (for diagenesis, for O 2 ) and Meteroic water is topography-driven

20 Topography-driven Meteoric Water (asymmetric basin, other highland recharges not shown) SSC O 2 -rich O 2 -rich after Brown (2005, 2009)

21 Sources of copper Recent Advances a) Rift redbeds (and volcanics) reddened by infiltrating O 2 -rich meteoric water (which evolves into a brine by assimilation of evaporite salts) Walker b) Deeper basement rocks, where redbeds are insufficient Cathles, Blundel, Wedepohl & Rentzsch, Hitzman... (the latter is attractive for seismogenic or structurally controlled solutions from basement)

22 Recent Advances 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

23 Topography-driven Meteoric Water (asymmetric basin, other highland recharges not shown) SSC O 2 -rich O 2 -rich after Brown (2005, 2009)

24 But first.. The Source of O 2 -rich Meteoric Water is? Natural Eh-pH conditions Oxygenated (Atmospheric) Oxidizing, slightly acidic conditions of Meteoric Water versus Reducing conditions of Deep Ground Water De-oxygenated (Deep, nonatmospheric) environments From Garrels (1960) (an old story!)

25 Solubility of Copper for low temperatures and high salinities Cu-chloride complexes ( >10 to 100 ppm Cu ) Eh (V) 0 Conditions favourable for significant copper solubilities in redbeds Now, overlay Garrels, Rose and Brown diagrams Rose (1976, 1989) and Brown (2003) ph 25

26 And for a more complete geochemical story.. 1) O 2 -rich meteoric recharge water Evolution of Meteroic Water from O 2 -rich to O 2 -poor 2) Progressive loss of O 2 due to reddening 3) Release and transport of Cu 4) Deposition of Cu under reduced conditions. Brown (2005)

27 Topography-driven evolved meteoric water model (deep-basin flow added) SSC O 2 -rich Basin-fill reddening/cu leaching Evaporite Assimilation Basement reddening & Cu-leaching Dashed red arrow added if basin-fill is an inadequate Cu source after Brown (2009, 2011)

28 Recent Advances 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).

29 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. Recent Advances 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)

30 The Deep Basement Source concept? (my interpretation of descriptions) Greybeds SSC Redbeds Cu-brine Deep Source Brine becomes oxidizing by equilibration with hematite of redbeds (problem here: Redbeds essentially not pre-ore, but syn-ore) Highly reducing conditions (equilibrated with ferrous iron ) (problem here: Cu is not soluble)

31 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.

32 Deep brines remain too reduced to carry copper Ferrous-ferric boundary Brown (2009)

33 Recent Advances 2 nd 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.

34 Two problems Recent Advances 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 topographydriven evolved meteoric water model

35 Topography-driven evolved meteoric water model (chemical aspects) 1) O 2 -rich meteoric recharge water 2) Progressive loss of O 2 due to reddening 3) Release and transport of Cu 4) Deposition of Cu under reduced conditions Brown (2009)

36 Topography-driven evolved meteoric water model (rift basin-scale aspects) SSC O 2 -rich Basin-fill reddening/cu leaching Evaporite Assimilation Basement reddening & Cu-leaching Dashed red arrow added if basin-fill is inadequate Cu source after Brown (2009, 2011)

37 Conclusion Recent Advances 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).

38 The End! Obrigado!