4. Cherts – study of petrography, trace elements and triple oxygen isotopes with implications on the
4.2 Sample description and petrographic study
4.2.4 Archean cherts
4.2.4.1 Dresser Formation, Pilbara Craton, Western Australia (PilBC)
The Warrawoona Group in the Pilbara Craton, Western Australia, is ca. 18 m thick succession deposited over ~150 Ma (e.g., Van Kranendonk et al. 2002 and references therein) and situated in the eastern part of the Pilbara craton.
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Figure 16 Generalized stratigraphic column of the Pilbara Supergroup, W. Australia (after Hickman and Van Kranendonk, 2012).
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Dominantly low-grade (prehnite–pumpellyite to greenschist facies) metabasalts
with interbedded felsic volcanic horizons (dominantly tuffs, some flows) and numerous chert beds (Hickman 1983; Van Kranendonk et al. 2002) make up the Warrawoona Group. It is the oldest group of the rock successions in the craton and is subdivided into four subgroups. The samples in this study come from different horizons within the Warrawoona Group.
The Dresser Fmn. is the uppermost formation of the second oldest subgroup within the
Warrawoona - the Talga Talga subgroup (Hickman and van Kranendonk, 2012). This formation contains bedded cherts with interbedded pillow basalts (Van Kranendonk 2000). Within the chert horizons interlayered stratiform barite, and lesser amounts of jaspilitic chert, carbonate–
chert laminates, and clastic sedimentary rocks (Buick & Dunlop 1990; Nijman et al. 1999; Van Kranendonk 2004) are found. The barite-chert intercalated units are associated with cross-cutting silica-barite veins which appear to be feeding these bedded horizons.
At least two alternative interpretations of the depositional environment of the Dresser Fmn. have been proposed. Locally observable dessication cracks and other evidences indicate shallow water deposition involving chertification of earlier carbonates or gypsum (Buick and Dunlop, 1990).
Alternatively, the cross-cutting silica-barite feeder veins, apparently syndepositional with the bedded chert-barite units, indicate a primary hydrothermal origin (Nijman et al., 1999; Runnegar et al. 2001; Van Kranendonk 2004).
This formation has been attributed an age of ca. 3490 Ma based on Pb-Pb dating on barite (Thorpe et al., 1992).
Note: The sample group PilBC comprises bedded cherts from the Pilbara Craton, which also includes the sample from Marble Bar Chert Member described in the section after this.
4.2.4.1.1 4 of 03-08-85 (W2)
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This sample was collected in vicinity of the locality described in Walter et al. (1980). This study described a 30 km long and 40 m thick chert-barite unit bearing metasediments at North Pole, within the Warrawoona Group. The sample W2 is a black bedded chert, containing numerous quartz veins in thin-section and few clay pockets in a finely crystallized matrix. It is possibly of hydrothermal origin.
4.2.4.1.2 UENO 2.2012 or DRESS-12613-10a (DUC-1 and DUC-2)
Approximate location of sample is 21°09.04.46' S, 119°26.09.00' E. The sample is emplaced in the field as a thick chert vein cutting across the intercalated chert and greenstone beds of the Dresser Fmn., apparently feeding into the bedded chert and probably of hydrothermal origin. DUC-1 and DUC-2 were drilled out from the same sample, spaced ~10 cm from each other. The sample was provided by Joachim Reitner.
4.2.4.2 Marble Bar Chert, Duffer Formation, Pilbara Craton, W. Australia (PilBC)
The Marble Bar Chert Member is the youngest member of the Coongan Subgroup within the Warrawoona Group. The Marble Bar is a 50- to 80-m thick sequence of beautifully bedded multicoloured interlayered cherts. Three distinct coloured interlayered (centimeters to tens of centimeters layering) varieties of thinly bedded red jasper, milky white chert and blue-black chert (Hickman and Van Kranendonk, 2012) are present (Fig. 23). The succession
is metamorphosed to lower greenschist facies (Collins and Van Kranendonk, 1999). Individual beds are up to 10 m or more continuous along the strike. Different amounts of very fine-grained iron oxide impurities, with some subtle microscopic grain-size variations leads to the different colourations.
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U-Pb zircon dating closely constrains the age of Duffer Fmn. from 3471 ± 5 Ma to 3465 ± 3 Ma (Thorpe et al., 1992). Sm/Nd ages of 3200 ± 300 Ma for red (or black) and white banded cherts and 2500 ± 200 Ma for yellow-grey cherts associated with quartz veins were obtained (Minami et al., 1995).
4.2.4.2.1 Marble 14613 13 (MBC)
This particular sample is from the white chert layers. This has been grouped with another bedded chert, sample W2, from the Dresser Formation; together they form sample group PilBC (bedded cherts from Pilbara Craton). This particular sample was provided by Joachim Reitner.
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Figure 17 Field photographs of the Marble Bar Chert, Warrawoona group, W. Australia. (Source:
internet) Interlayered red, white and blue-black chert is visible.
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4.2.4.3 Mt. Ada Basalt, Pilbara Craton, Western Australia
Overlying a disconformity terminating the lower Talga subgroup is the Mt Ada Basalt formation.
It is located within the Coongan Subgroup. The Mount Ada Basalt and coeval Duffer Formation have an age range of 3474–3463 Ma (Van Kranendonk et al., 2002). The Mount Ada Basalt consists of pillowed metabasalts with chert and calcite in the inter-pillow spaces.
4.2.4.3.1 Mt Ada 13613 6F (PL-2)
This sample comprises cryptocrystalline silica, in the form of white agate (PL-2), along with coarser quartz and calcite in adjacent zones, all encased together within the interspaces of pillow basalt. The sample was provided by Joachim Reitner.
Figure 18 Photograph of hand-specimen from Mt Ada Basalt, Pilbara Craton, W. Australia. The sample comprises chert/quartz, i.e., the PL-2 sample along with calcite within the interpillow spaces of the pillow basalt.
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Figure 19 Photomicrographs of sample 4 of 03-08-85 (W2) from the Dresser Fmn., Pilbara Craton, Australia. (A) and (B) are PPL and CPL images offering a general view of the sample composed of cross-cutting quartz veins of different thicknesses within a groundmass of micro- and crypto-crystalline quartz (a). The veins contain mesoquartz crystals (b), some of which show undulose extinction. The thicker vein at the lower left section of the figure is filled with fibrous quartz crystals (c), referred to as ‘tigers eye’. The veins are relatively younger than the silicification event and are of different generations relative to each other as well. Panels (C) and (D) are PPL and CPL images showing a thick quartz grain cutting across a relatively older thin quartz vein. The quartz grains present within the thicker vein show undulose extinction.
4.2.4.4 Swartkoppie Formation/Mendon Formation, Barberton Greenstone Belt, South Africa (BBC)
The early Archean Barberton greenstone belt is a well-preserved, generally weakly
metamorphosed sequence of supracrustal volcanic and sedimentary rocks. It forms a ca. 15 km thick supracrustal sequence.
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Figure 20 Generalized stratigraphic column of Onverwacht (Onv.) Group and geologic map of Barberton greenstone belt (BGB). Modified from Lowe and Byerly (1999). Ages are from Byerly et al.
(1996) and Dann (2000). Percentages to right of column indicate approximate proportions of komatiitic volcaniclastic layers within each major interflow chert unit on the west limb of the Onverwacht anticline (O.A.) and interbedded within komatiitic flow sequences in the Weltevreden (Welt.) Formation (after Stiegler et al., 2008).
The Mendon Formation, composed of interbedded komatiitic lavas and cherty metasedimentary rocks, is a relatively newly defined stratigraphic unit at the top of the Onverwacht Group (Byerly, 1996 and references therein) (within Geluk Subgroup).
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This formation is 3298 Ma old based on U-Pb and Pb-Pb dating of zircons within ash layers in the cherty stromatolitic metasediment Metamorphic grade of the Formation is lower Greenschist to Amphibolite facies (Viljoen and Viljoen, 1969).
4.2.4.4.1 PRG 200 (Z1)
Sample was collected from 22km NE of Barberton. The sample is a black chert containing fine grained organic material and preserved microbial mat laminations. The latter is indicative of subsequent but relatively early silica impregnation.
4.2.4.4.2 PRG 197 (Z2)
Sample collected from 11km NE of Barberton. Sample is a fine grained black chert.
4.2.4.4.3 PRG 194 (Z3)
Sample collected from 10 km NE of Barberton. The sample is a black and gray chert with visibly well preserved stromatolitic laminations, indicating early silicification.
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Figure 21 Photomicrographs of sample PPRG 200 (Z1), from Swartkoppie Fmn., Barberton
Greenstone Belt, South Africa. The age of the chert is 3298 Ma (Viljoen and Viljoen, 1969). (A) Plane polarised light (PPL) image of sample shows well preserved microbial laminations and dark organic matter, which indicate relatively early silicification. Cross-cutting veins, of various thicknesses, are seen all over. These belong to later and possibly different generations, post the silicification event.
The dark circles are an artefact of sample preparation. (B) Cross polarised light (CPL) image of A. A matrix of recrystallized microcrystalline quartz (individual crystal size < 20 µm) (a) with crenulated crystal boundaries and undulose extinction, mesocrystalline quartz (individual crystal size > 20 µm but otherwise optically similar to microcrystalline variety) (b) and cryptocrystalline quartz (crystal size not determinable with normal optical microscope) (d) is seen. Microcrystalline quartz is the dominant component of majority of cherts. The veins are composed of recrystallized macroqz (c) or cc or both?. C. Magnified view (see scale bar) of recrystallized micro and mesocrystalline quartz matrix, a major constituent of cherts. D. Quartz vein cutting through the microquartz matrix.
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4.2.4.5 Other samples from Barberton Greenstone Belt, South Africa
4.2.4.5.1 K-1113This is a nodular chert sample in siderite bed.
4.2.4.5.2 K-1100
The sample is a quartz stalactite, 3560 Ma old.
4.2.4.5.3 BIF-1
Banded iron formation of white chert layers alternating with black magnetite and red haematite bands. The sample was provided by Joachim Reitner (Georg-August-Universität-Göttingen, Göttingen).
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Figure 22 Small piece (~3 cm chip is a part of a bigger hand-specimen which was later
4.4.1.1.1 Phanerozoic nodular cherts (PNC)
The PNC cherts, samples M1, M7, M6 and AR, have almost identical PAAS normalized REE+Y patterns, as well as REE+Y concentrations (Figure 23), with small to negligible LREE depletions
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