Variations in carbonate mineralogy and mineral chemistry of the Griquatown and upper Kuruman Iron Formations

and their possible controls

G.A. Ballantyne (B.Sc. Geology and Chemistry)Supervisor: Professor H. Tsikos

Rhodes UniversityDepartment of Geology

PRIMOR

Background• Mn increases stratigraphically upwards in BIFs of

the Transvaal Supergroup (Ghaap Group) (Rafuza, 2015)• Hosts of manganese are exclusively carbonate

minerals such as ankerite Ca(Fe, Mg,Mn)(CO3)2 and siderite (Fe(Mg, Mn)CO3) .• BIFs in general are very Mn poor. Thus an

increase of Mn in BIF is potentially very significant with respect to atmosphere-ocean oxygenation and the formation of manganese deposits around the Great Oxidation Event.

The Transvaal Supergroup Stratigraphy2.39 Ga

2.45 Ga

2.56 Ga

2.22 Ga

u/c (?)

u/c

POST

MA

SBU

RG

GR

OU

PG

HA

AP

GR

OU

P

Hotazel Fm(BIF + Mn)

Ongeluk Fm (andesite)

Mooidraai Fm (carbonate)

Makganyene Fm (diamictite)

Griquatown &Kuruman Fms (BIF)

Cambellrand Subgroup (carbonate)

500m

Log of Lo Core

Transition to Kuruman

Griquatown Fm

Kuruman Fm

Aims• Examine the distribution of siderite and ankerite in the selected

samples;• Do the two carbonates coexist on a fine scale at all?• When they do, are they associated with similar non-carbonate

mineralogy?• What are their textural relationships, if/when they coexist?• Are they chemically comparable in every instance (e.g. do they

both show predictable Mg/Mn ratio variation across stratigraphy). • Ultimately, I wish to constrain whether: • The two carbonates are entirely diagenetic or possibly primary in

origin;• If primary, whether an active carbon cycle was in operation during

the formation of BIF in the Palaeoproterozoic ocean.

Methods

Primarily a petrographic study which I made use of the following:• Transmitted light microscopy• EPMA• XRF• Trace elements (ICP-MS)• XRF and trace elements analysed from powdered

bulk rock.

MineralogyMineral Group Mineral

Carbonates Ankerite

SideriteCalcite

Oxides MagnetiteHematite

Silicates Greenalite

MinnesotaiteStilpnomelane

RiebeckiteChert (Quartz)

Sulphides Pyrite

Rbk

Mag

Cb

Cal

Stp

Mag

Hem

Cb

Qtz

MagStp

Rbk

Chert

Cb

Qtz

Cb

Chert

Results 15 20 25 30 35 40 45 50 55110

160

210

260

310

360

Fe2O3(wt%)

Dept

h (m

)

0 1 2 3 4 5 6110

160

210

260

310

360

Mn3O4(wt%)

Dept

h (m

)

1 2 3 4 5 6 7 8 9110

160

210

260

310

360

MgO(wt%)

Dept

h (m

)

0 2 4 6 8 10 12 14 16 18110

160

210

260

310

360

CaO(wt%)

Dept

h (m

)

Bulk geochemistry - REE

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu0.01

0.10

1.00

10.00

PAAS-Normalised REE diagram

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1110

160

210

260

310

360

Ank (Mg + Mn)/Fe vs depth(m)De

pth

(m)

0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26110

160

210

260

310

360

Sid (Mg+Mn)/Fe vs depth(m)

Dept

h (m

)

0 0.1 0.2110

160

210

260

310

360

Mn/LOI vs depth(m)

Dept

h (m

)

Overview of suboxic Fe-Mn diagenesis• If the carbonates form entirely via diagenetic processes, Mn & Fe

will enter the carbonate structure when they are reduced by

bacteria in the sediment that are able to utilize manganese and

iron oxides.

• Mn will be reduced first because thermodynamically it is the

better e- acceptor making it a more favourable species for

bacteria.

• Diagenetic model predicts that if Mn increases in carbonates

there would be a subsequent decrease in Fe in carbonates i.e. Mn

and Fe should anticorrelate.

• However, the data of this study suggests differently with Mg and

Mn summed versus Fe to be the actual anticorrelation.

14 16 18 20 22 24 266

7

8

9

10

11

12

13

14

15

R² = 0.974900056206716

FeO(wt%)

MgO

+ M

nO(w

t%)

Ankerite

14 16 18 20 22 24 260

1

2

3

4

5

R² = 0.13132713911823

FeO(wt%)

MnO

(wt%

)

Conclusion• A new mechanism of carbonate deposition should be adopted

in modelling of carbonate fraction within Kuruman and

Griquatown IFs.

• Primary precipitation of carbonates out of the water column

seems like a plausible alternative as opposed to a diagenetic

mechanism.

• Precipitation of carbonates out of the water column would

then have recorded a unique chemical signal in terms of the

stratified water column in which they formed and would then

be incorporated into the sediment on arrival from the water

column.

ReferencesBau, M. and Dulski, P. (1999) Comparing yttrium and rare earths in

hydrothermal fluids from the Mid-Atlantic Ridge: implications for Y and REE behaviour during near-vent mixing and for the Y/Ho ratio of Proterozoic seawater. Chem. Geol. 155, 77–90.

Rafuza, S. (2015) – Carbonate Petrography and Geochemistry of BIF of the Transvaal Supergroup: evaluating the potential of Iron Carbonates as proxies for Palaeoproterozoic Ocean Chemistry. MSc. Thesis. Rhodes University, Grahamstown, South Africa.

Tsikos, H. (2015) – Sedimentary Iron Deposits. Lecture slides. Rhodes University, Grahamstown, South Africa