Government of Western Australia Department of Mines and Petroleum Government of Western Australia Department of Mines and Petroleum Government of Western Australia Department of Mines and Petroleum

Government of Western Australia Department of Mines and Petroleum

Geological Survey of Western Australia

Regolith geochemistry in the Kimberley Science & Conservation Strategy − links to bedrock

Paul Morris

Outline • Kimberley Science and Conservation Strategy

• Geology of the Balanggarra area

• Regolith – landform mapping

• Examples of bedrock lithological variations shown by regolith chemistry

Carson Volcanics

Pentecost Sandstone

Kimberley Science and Conservation Strategy: • State Government multi – agency

initiative to better understand the Kimberley region, including its geological evolution

• GSWA program

Geophysics (roadside gravity and magnetotellurics)

Collection of grid-based regolith samples and targeted rock sampling

Regolith sampling • At conclusion of the KS & CS,

GSWA had completed four regolith sampling programs:

Balanggarra*

Bunuba

Yuriyangem – Taam

Dambimangari

• Total of 1019 samples

• *this talk

Balanggarra

Yuriyangem-Taam

Bunuba

Dambimangari

Balanggarra regolith sampling program Carried out in August − September 2013 with co-operation of traditional owners from Balanggarra country

Balanggarra

Vernon Gerrard and Ambrose Chalarimeri

Kimberley Group

Balanggarra area – regional geology • Gently SE - dipping sequence of essentially

undeformed Paleoproterozoic rocks of the Kimberley Group Quartz – rich siliciclastic sandstones of

the King Leopold, Warton and Pentecost Sandstones

More silt-rich rocks of the Elgee Siltstone

Mafic volcanic rocks of the Carson

Volcanics

• Abundant outcrop

• Thin regolith

North Kimberley

Regolith − landform mapping* • The majority of the Balanggarra project area is

composed of: Thin regolith in areas of outcrop (60 %)

Residual regolith (i.e., formed in situ; 11%)

• Transported regolith is largely colluvium (21%),

which is found close to outcrop => High probability regolith chemistry reflects the composition of the underlying or nearby bedrock

*Nadir de Souza Kovacs

Balanggarra regolith chemistry • Multi-element analysis of the < 10 mm

fraction of regolith for 70 components (oxides, trace elements, LOI, pH and total dissolved solids)

• Statistical treatment of regolith chemical data identifies samples with anomalous element concentrations, shown as and

• Strong link between regolith and rock

chemistry − e.g., most samples with anomalous concentrations of Ni in regolith are found over the Carson Volcanics

1. Lithological variations in the Carson Volcanics revealed by regolith chemistry • As would be expected, the highest

concentrations of elements such as Cr, and the most regolith samples with anomalous Cr concentrations are found over the Carson Volcanics

Lithological variations in the Carson Volcanics revealed by regolith chemistry • As would be expected, the highest

concentrations of elements such as Cr, and the most regolith samples with anomalous Cr concentrations are found over the Carson Volcanics

• Several samples with anomalously high Cr concentrations are found at or near the base of the Carson Volcanics

• Overall, there is a marked increase in the Cr concentration of regolith towards the base of the Carson Volcanics

• Is high Cr reflecting bedrock, or the concentration of resistate minerals such as chromite due to chemical weathering?

• Labile elements such as Mg are removed during the earliest stages of chemical weathering

• Some samples with high MgO in regolith are found near the base of the Carson Volcanics => reflecting bedrock

Behaviour of labile elements

• More than 95% of samples returned Pt levels below detection (1 ppb)

Behaviour of platinum

• More than 95% of samples returned Pt levels below detection (1 ppb)

• But several samples with detectable Pt are found close to the base of the Carson Volcanics

Behaviour of platinum

Behaviour of chalcophile elements (As)

• There is also a high proportion of regolith samples with elevated As concentrations towards the base of the Carson Volcanics

Copper • Copper

mineralization has been recorded at the base of the Carson Volcanics, but there is no clear relationship of Cu in regolith and stratigraphic height

Regolith chemistry and lithological variations in the Carson Volcanics

• As samples with high MgO, as well as Cr and As are found at the base of the Carson Volcanics, regolith chemistry reflects aspects of bedrock composition

• This, and the presence of detectable Pt in samples close to the base of the unit, show that more unfractionated, chalcophile- and platinoid-rich magma was emplaced early in the succession

• Little variation in Cu with stratigraphic height indicates a higher Cu background throughout this unit

2. REE chemistry of regolith in the Carson Volcanics

Does unusual REE chemistry of regolith in the Carson Volcanics result from chemical weathering or other factors?

Chemical weathering of the Carson Volcanics • Early stages of chemical

weathering of the Carson Volcanics results in variable depletion of

labile elements such as Rb, Ba, K, Mg, Na

enrichment in Al, Th, Hf, Zr and Nb

• This is attributed to the

alteration of olivine, pyroxene and feldspar, the development of clay, and the concentration of resistate minerals (e.g., monazite, zircon and rutile)

Normalized to fresh Carson Volcanics average (n = 47)

Regolith in areas of outcrop

Chemical weathering of the Carson Volcanics • Early stages of chemical

weathering of the Carson Volcanics results in variable depletion of

labile elements such as Rb, Ba, K, Mg, Na

enrichment in Al, Th, Hf, Zr and Nb

• This is attributed to the

alteration of olivine, pyroxene and feldspar, the development of clay, and the concentration of resistate minerals (e.g., monazite, zircon and rutile)

Normalized to fresh Carson Volcanics average (n = 47)

Regolith in areas of outcrop

REE are little affected

REE enrichment of 213316

• Regolith sample 213316 is from an area of outcrop

• It shows the same alteration pattern as other regolith samples, apart from anomalously high REE and Zn, and elevated Ba

213316

Normalized to fresh Carson Volcanics average (n = 47)

=> Chemical weathering is inconsistent with elevated REE and Ba

1st vertical derivative RTP

• Several regolith samples with anomalous REE concentrations lie on or close to NE-trending lineaments

• If these are dikes, does 213316 represent regolith from one of these dikes?

• Not Carson Volcanics, but could be:

Hart Dolerite, or

c.500 Ma Antrim Plateau Volcanics

213316

REE-enriched samples, magnetics and regional lineaments

Normalized to fresh Carson Volcanics average (n = 47)

Hart

Antrim

• Both Hart Dolerite and Antrim Plateau Volcanics have similar chemistry to Carson Volcanics on this diagram

• Neither can explain the REE and Ba chemistry of 213316

213316 compared with Hart Dolerite or Antrim Plateau Volcanics*?

(*Hanley and Wingate (2000), AJES, 47, 1029 – 1040)

Hydrothermal alteration? • Elevated REE, Ba and Y

of some Cenozoic basalts from southeastern Victoria have been discussed in relation to hydrothermal activity (Price et al., 1991)*

• Fodor et al. (1987)** – post magmatic hydrothermal alteration of Hawaiian basalts results in secondary REE-enriched minerals

Altered Victorian basalt (Price et al., 1991)

213316

* Chemical Geology, 93, 245 – 265 ** JVGR, 3, 171 – 176

Chemistry of 213316

• Depletion in Rb, K, and Mg is consistent with the early stages of chemical weathering

• Enrichment in REE, Y and Ba is inconsistent with chemical weathering, but could reflect hydrothermal alteration

• If this is the case, then hydrothermal alteration occurred after regolith formation, with hydrothermal fluids channelled along NE-trending structures

3. Identification of heavy mineral concentrations in the Pentecost Sandstone using regolith chemistry

• Most samples with anomalous concentrations of Zr in regolith are found in the Pentecost Sandstone, with Zr concentrations up to 1046 ppm

• These are regolith samples from areas of outcrop

• The samples lie roughly parallel to strike

Concentrations of other high-field strength elements (HFSE)

• The same nine samples have anomalous concentrations of Hf, and

• the sample with the highest concentration of Zr, also has the highest Hf concentration of 25 ppm

Other HFSE

• Samples with anomalous concentrations of other HFSE such as Nb and Ta are not found in any regolith samples from the Pentecost Sandstone

Interpretation of anomalous Zr and Hf in regolith from the Pentecost Sandstone

• Nine samples with anomalous concentrations of Zr and Hf in regolith lie approximately parallel to strike

• HFSE can be enriched by chemical weathering, but this is inconsistent with the concentrations of Nb and Ta

• The association of Zr and Hf reflect zircon control

• The relatively high density of zircon (4.7) relative to rutile (4.2 – 4.3), garnet (3.5 – 4.3), and allanite (3.5 – 4.2) could mean the concentration of zircon reflects both the composition of the source rock and mechanical sorting during deposition of the Pentecost Sandstone

Conclusions • Regolith − landform mapping has shown the high proportion of thin

regolith in areas of outcrop and residual regolith in the north Kimberley

• Regolith chemistry can be used to investigate spatial and stratigraphic variations in bedrock, but …..

• ….. account needs to be taken of even incipient chemical weathering before regolith composition is used as a bedrock proxy