Variable Source: Pb isotopes reveal basement influence on mineralisation at 1590 Ma

Ben NicolsonGeological Survey of South Australia - Discovery Day 2016

Background

Pb isotopes in isotope geochemistry

Two principal uses

• First used historically to determine the ages of rocks and

minerals, and still the case (geochronology)

• Secondly, in petrogenetic studies to identify geological

processes and sources (isotope geology)

2

Radiogenic isotopes provide a means to sample the

deep interior of the earth, which otherwise can only

be studied indirectly using geophysics

Background

Isotope geology and Pb-isotope

evolution (growth curves)

Average Growth Curves:

Indicated by studies on large populations from

similar geological settings; e.g. Doe & Zartman,

1979; and Plumbotectonics model (at left);

Zartman & Doe, 1981)

3Plumbotectonics model lead-isotope evolution curves (Zartman & Doe, 1981)

206Pb/204Pb

20

7P

b/2

06P

b

A = mantle

B = orogene

C = upper crust

D = lower crust

D

A

BC

CB

AD

20

8P

b/2

06P

b

206Pb/204Pb

4

Pb isotopes: Variable Source

Mixing of crustal and mantle reservoirs into the orogene source reservoirAfter Doe and Zartman (1981).

In assessing origin, we expect that several trends will become

apparent (no single curve is sufficient in an area of such

complexity) that all represent different lead reservoirs

ab

c

d

5

Regional Geology & Mineral OccurrencesNumerous

mineral

prospects that

contain Pb

~100 Pb-Zn-Ag

occurrences

Clusters along

southern

margin of GRV

and in Cleve

uplands

Au & Cu association??

NuytsDomain

Cleve-Coulta Domain

6

Regional Geology & GeochronologyLarge number

of dated sites

on Eyre Pen.

Selected: ~70 Represent:

•Mesoarchean

granites

•Sleaford

Complex

•Kimban-aged

intrusions

•Hiltaba Suite

•Gawler Range

Volcanics

NuytsDomain

Cleve-Coulta Domain

7

Nd Data (TDM)

Wide range in ages calculated from Nd values

Based on previous studies, variations in Pb ratios tend to follow similar trends

K-spar sample medium: comparing like samples

NuytsDomain

Cleve-Coulta Domain

The source of metals

- previous studies

8Department of State Development

Dominated by Upper Crustal Source

Lower Crust

Mantle

Orogene

Roache (1996)

20

7P

b /

20

4P

b

206Pb / 204Pb

Focussed on signatures of

mineralisation

~250 analyses from Menninnie Dam

~50 analyses from other Pb prospect

Roache 1996:

• Gawler Range Volcanics / Hiltaba Granites

• Hutchison Group

Numerous additional sources possible include:

Kimban-aged intrusions

Sleaford basement (reworked in the Kimban)

Limited work on feldspars regionallyWork was site-specific relating mineralisation to local sources

Gawler Craton Pb isotopes – recognised as not fitting to general curves readilySomething different about the Galwer Craton evolution??

Comprehensive review of geochemical

data on regional geochronological

samples shortlisted 23 granite or felsic

gneiss samples for analysis

Considerations:

Granites with highest Pb contents

Lowest U contents

Presence of K-feldspar*

Minimal alteration**

9

Isotope Geology: determining our sourcesFirst two points are straight forward

• * K-spar isn’t always present, or preserved intact.

Many samples contain significant perthite lamellae

(intergrown Na-spar).

• ** The region has undergone at least two major

orogenies and one major volcano-plutonic event.

Metasomatic alteration, regional isochemical

calcsilicate formation are particularly strong effects of

regional metamorphism. Disturbance to older rocks is

widespread, in some cases causing severe problems

with interpretation of data in this study.

• Despite attempts to sample unaltered rocks, many

samples have disturbed ratios and are unusable,

such that an iterative sampling & analysis approach

to obtain usable samples is required

• This trend towards greater disturbance correlates

with increasing age, however,

there have been a few

surprises!

10

1.1 Ga

1.6 Ga

2.0 Ga

1.6 Ga Note: Age points in 0.1 Ga increments

ResultsPreliminary interpTwo distinct sources:

11

Results

1.1 Ga

1.6 Ga

1.6 Ga Note: Age points in 0.1 Ga increments

Preliminary interpTwo distinct sources:

1) Upper curveDefine a source with a 238U/204Pb (µ) valueestimated to be ~13.7 (very high)Reflecting strongly on the Kimban OrogenyEven completely rehomogenising Pb in some rocks of Neoarchean age

2.0 Ga

12

1.1 Ga

1.6 Ga

2.0 Ga

1.6 Ga Note: Age points in 0.1 Ga increments

ResultsPreliminary interpTwo distinct sources:

2) A more juvenile source, which closely follows the S-K curve, and indicates a source with a 238U/204Pb (µ) value closer to 10 (less clear at this stage), but includes some Hiltaba Granites

13

1.6 Ga

1.1 Ga

2.0 Ga

Mineralisation trends follow the feldspars:Menninnie TS, intersects upper curve at an age of ~1590 Ma!Other mineral zones appear to follow the more juvenile curve (possibly Paris)Others are mixed

Results - Mineralisation

Note: Advanced argillic alteration (alunite) Ar-Ar, dated @ 1580 Ma, spatially related to GRV (Nicolson et al, in review).

14

Still early days, but:Origin of Pb at Menninnie fits with pattern of upper crustal (supracrustal) to Orogene source (Hutchison Group & Kimban-aged granites of Peter Pan Supersuite)Our contention is that in Eyre Peninsula, Hiltaba-GRV contributed mainly heat, but little PbParis – more complex, very limited data mixed source including GRV-Hiltaba, (nearby alteration at Nankivel Hill constrains mineralisation to

Conclusions

No Pb-isotope ratios (galena) to suggest pre-1590 Ma mineralisation was identified.Anecdotally, this may be a function of preservation. Why?Geobarometry indicates basement rocks of the modern-day surface were formed at ~550 MPa. This is equivalent to a depth of ~17 km. This is a ductile environment and exceeds the depth at which hydrothermal deposits form (optimally, they form at 0-15 km depth)•Quartz veining (apart from scattered veins associated with radiogenic occurrences, an

indication of hydrothermal activity) is notably absent on central Eyre Peninsula

No mineralised samples

Mineralisation, tailing towards increasingly radiogenic ratios

~1580 Ma?)

15

Further Work• The Gawler Craton is a large area; what applies in one area may not apply elsewhere• Highly variable degrees of metamorphism are preserved across the craton• Work commenced on an area containing elements of the Archean origin of the Craton

which has seen two very significant orogenic cycles, and generation of a SLIP• All of these events likely produced mineralisation• Other parts of the craton, which clearly demonstrate a different history are most likely

going to be prospective for different styles of mineralisation (i.e. areas with a strong juvenile component are more prospective for magmatic-hydrothermal deposits including VHMS and porphyry-style deposits.

• Continued regional assessment of Pb isotopes will help resolve the origin of lead (and by inference) other metals.

• May be a useful, predictive tool to apply to areas of limited exposure and without known mineralisation as a tool to assess mineral potential.

Thankyou!

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