The role of magmatic fluids in the ~3.48 Ga Dresser Caldera, Pilbara Craton: New insights from the geochemical investigation of hydrothermal alteration

doi:10.1016/j.precamres.2021.106299

Precambrian Research

Volume 362, 15 August 2021, 106299

https://doi.org/10.1016/j.precamres.2021.106299 Get rights and content

Highlights

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~3.48 Ga hydrothermal alteration preserved after 3 subsequent thermal overprints.
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Alteration distribution is spatially controlled by the distance to the hydrothermal veins.
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The underlying magma chamber influenced the composition and pH of the hydrothermal fluids.
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Most of S, Ba and K added in the hydrothermal system was sourced from the underlying magma chamber.
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Barium was transported in the hydrothermal fluids by colloidal silica.

Abstract

Hydrothermal fluids played a key role in establishing the environmental conditions in which ancient stromatolites grew within the North Pole Chert of the ~3.48 Ga Dresser Formation (Pilbara Craton, Western Australia). However, there has been uncertainty as to the physicochemical conditions of the hydrothermal system in relation to (i) the distribution of hydrothermal alteration, (ii) the relative contribution of seawater and/or magmatic volatiles to the hydrothermal fluids, and (iii) the origin of some of the major elements mobilized in the hydrothermal fluids.

This study examines the hydrothermal alteration of the underlying North Star Basalt in order to better understand the nature of the circulating fluids and to determine the processes responsible for the transport and accumulation of metals and metalloids to the near surface environment. Detailed geological mapping reveals a complex distribution of alteration mineral assemblages that is controlled at all stratigraphic depths by the distance to the major fluid pathways that are now represented by hydrothermal silica veins. With increasing distance from the vein margins, alteration assemblages change from argillic (kaolinite–quartz) to phyllic (illite–quartz), and then to propylitic (chlorite–albite–epidote) and actinolitic (actinolite–albite–chlorite–epidote) at more distal positions. Illite Ar–Ar dating of argillic–altered basalt proximal to major hydrothermal veins immediately below the North Pole Chert confirms a syn–depositional hydrothermal origin of alteration, and demonstrates that the mineralogical and chemical features developed through the circulation of hydrothermal fluids were largely preserved after subsequent thermal overprints at 3.25, 3.06, and 2.29 Ga.

The spatial distribution of the alteration mineral assemblages indicates that the fluids circulating in the hydrothermal system were highly acidic (pH < 3) for at least some time during the evolution of the Dresser Caldera. Such highly acidic fluid conditions were likely promoted by the input of magmatic volatile phases, such as HCl, SO₂, H₂S, and F. Bulk geochemical analyses of altered basalts reveal that large amount of metals, including Fe, Mg, Ni, and Zn, were leached from the North Star Basalt during hydrothermal alteration and delivered to the surface. Furthermore, our data indicate that K and Ba were introduced into the hydrothermal system from external reservoir(s). Although the contribution of K–rich seawater cannot be completely discounted, we argue that the bulk of K and Ba was sourced from an underlying magma chamber undergoing fractional crystallization of a melt with TTG–like composition.

Introduction

The 3481 ± 3.6 Ma Dresser Formation of the Warrawoona Group, in the East Pilbara Terrane of the Pilbara Craton, Western Australia, hosts wrinkly laminated, conical, and domical stromatolites that represent the earliest most convincing evidence of life on Earth (Baumgartner et al., 2019, Buick et al., 1981, Dunlop et al., 1978 Van Kranendonk et al., 2008; Van Kranendonk, 2011). The stromatolites occur within the North Pole Chert, which is the lowermost member of the Dresser Formation and comprises hydrothermal−sedimentary rocks with substantial lateral variations of both thickness and sedimentary facies (Djokic et al., 2020, Nijman et al., 1999, Van Kranendonk et al., 2019, Van Kranendonk et al., 2008). Multiple lines of evidence, which include detailed lithostratigraphic mapping of the North Pole Chert (Djokic et al., 2020, Djokic et al., 2017) and structural analysis of the extensive hydrothermal vein network underlying the Dresser Formation (Nijman et al., 1999, Tadbiri and Van Kranendonk, 2020), support the interpretation that the sedimentary rocks of the North Pole Chert were deposited in a tectonically active volcanic caldera setting (Nijman et al., 1999, Van Kranendonk et al., 2019, Van Kranendonk et al., 2008). The deposition of the North Pole Chert varied up–section from deep water marine, through shallow marine to sub–aerial, and back to deep water marine environments (Djokic et al., 2020, Van Kranendonk, 2006, Van Kranendonk et al., 2008) and was strongly influenced by hydrothermal activity, as indicated by the close association between sedimentary strata and hydrothermal silica veins, as well as by the occurrence of siliceous hot spring sinter deposits (Djokic et al., 2017, Nijman et al., 1999, Van Kranendonk, 2006, Van Kranendonk et al., 2008).

Previous studies examined selected features of the hydrothermal alteration that affected the underlying North Star Basalt as a way of elucidating the nature of the hydrothermal fluids that circulated through the Dresser Caldera. Yet these studies provided conflicting interpretations on both the style of alteration and composition of the hydrothermal fluids, leaving a number of unanswered questions pertaining to the distribution of alteration mineral assemblages, the nature of the fluids, and the processes associated with the mobility of metals and metalloids (Brown et al., 2011, Brown et al., 2005, Kitajima et al., 2001, Terabayashi et al., 2003, Van Kranendonk and Pirajno, 2004). In this study, we address these knowledge gaps, and resolve previous discrepancies by combining published results with new field observations, mineralogical and geochemical data, and radiometric dating of argillic alteration. Our results demonstrate that the nature and distribution of the hydrothermal alteration within the Dresser Caldera is analogous to that of high–sulfidation epithermal systems, in which an underlying magma chamber not only represented the heat source for the circulation of the hydrothermal fluids, but actively influenced their pH and composition.

Section snippets

The Dresser Formation

The 3481 ± 3.6 Ma Dresser Formation lies within the lower part of the volcano–sedimentary Warrawoona Group in the North Pole Dome of the East Pilbara Terrane, Pilbara Craton (Fig. 1; Van Kranendonk et al., 2008, Van Kranendonk et al., 2007). The Dresser Formation lies conformably on the ~3490 Ma North Star Basalt and is overlain by the ~3470 Ma Mount Ada Basalt, both of which commonly preserve pillow structures (Fig. 2A) and host minor doleritic intrusions (Fig. 1; Van Kranendonk et al., 2007).

Materials and methods

Geological mapping of an area of ~7 km² (Fig. 3) was undertaken in order to determine the detailed distribution of hydrothermal alteration mineral assemblages in relation to stratigraphic depth beneath the North Pole Chert and in relation to the major hydrothermal veins. Eighty–two basaltic samples were collected during field mapping and subjected to mineralogical examination and geochemical analysis. The samples comprise pillow basalt (n = 55), fine–grained basalt (n = 20), and medium– to

Field observations

Four different alteration assemblages were identified and mapped in in the North Star Basalt underlying the Dresser Formation (Fig. 3). The alteration assemblages are classified using the terminology developed for porphyry Cu and epithermal systems (Gifkins et al., 2005 and references therein), which is similar to that utilized in previous studies on the hydrothermal alteration at the North Pole Dome (Table 1; Brown et al., 2011, Brown et al., 2005, Van Kranendonk and Pirajno, 2004). The most

Hydrothermal alteration distribution

The alteration distribution identified through field mapping of the North Star Basalt is distinctly hydrothermally–related. The distribution of the mineral assemblages is controlled by (i) the distance from the hydrothermal silica veins that transect the North Star Basalt, and by (ii) the stratigraphic depth within the North Star Basalt.

As noted above, the degree of alteration gradually decreases with increasing distance away from the margins of the hydrothermal veins, as reflected by changes

Conclusions and Summary

Field mapping, ⁴⁰Ar/³⁹Ar dating, geochemical and petrographic analyses of the hydrothermally–altered North Star Basalt lend new insights into the hydrothermal system that developed in the Dresser Caldera. Detailed field mapping and XRPD characterization of the hydrothermal alteration reveal a complex distribution pattern that is controlled, at all stratigraphic levels, by a dense network of hydrothermal veins that transect the North Star Basalt. Argillic alteration occurs near the silica veins

CRediT authorship contribution statement

Stefano Caruso: Conceptualization, Methodology, Formal analysis, Investigation, Writing - Original Draft, Visualization. Martin J. Van Kranendonk: Supervision, Funding acquisition, Writing - Review & Editing. Raphael J. Baumgartner: Writing - Review & Editing. Marco L. Fiorentini: Supervision, Funding acquisition, Writing - Review & Editing. Marnie A. Forster: Investigation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We acknowledge the technical and scientific assistance from the Australian Microscopy and Microanalysis Research Facility at the Centre for Microscopy, Characterisation and Analysis of the University of Western Australia. This study was supported by the Australian Research Council through the Discovery Project 180103204 awarded to MVK and MLF. Geoff and Faye Meyers at Normay, and Haoma Mining Pty Ltd are thanked for their generous support during field work. We thanks Frances Westall for the

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Citation Excerpt :
However, laboratory experiments have determined that the incorporation of Li into clays (e.g., smectites and illites) during basalt alteration is limited to fluid temperatures up to 150 °C, whereas at higher temperatures Li is released into the fluids (Seyfried et al., 1998; James et al., 2003). In the Dresser caldera, mineral assemblages and fluid inclusion estimates define a downward-increasing temperature profile from 120 to 150 °C in the upper 50 to 100 m of the system, to temperatures above 300 °C in deeper parts of the complex (Harris et al., 2009; Caruso et al., 2021). This temperature profile traces the threshold for the incorporation of Li into clays to the shallowest portions of the hydrothermal system (Fig. 5).
Voluminous hydrothermal circulation in the ∼3.48 Ga Dresser caldera produced zoned alteration haloes around major fluid pathways. Specifically, in the North Star Basalt (the footwall) hydrothermal alteration decreases with increasing distance from the margins of hydrothermal silica±barite veins, changing from argillic (i.e., kaolinite-quartz) to phyllic (i.e., illite-quartz), and then to either propylitic (i.e., chlorite-albite-epidote) or actinolitic (i.e., actinolite-albite-chlorite-epidote) assemblages at distal positions. This alteration series developed through hydrolysis reactions at decreasing acidic conditions, which promoted variable degrees of mobilization of major and trace elements in the hydrothermal fluids.
In this study, we characterize the B and Li concentrations and the relative stable isotope ratios of the altered North Star Basalt to provide new insights into the hydrothermal processes that generated these complex alteration patterns. In particular, we focus on the magnitude and timing of the inputs from different fluid reservoirs, the mobilization of B and Li in the hydrothermal system of the Dresser caldera and their influence on the near-surface environment where ancient stromatolites were forming.
Actinolitic and propylitic samples have homogeneous δ¹¹B and δ⁷Li values (−17 to −15‰ and +1 to +3‰, respectively) that are associated with lower B and Li concentrations in actinolitic (4 to 11 ppm and 46 to 128 ppm, respectively) relative to propylitic samples (9 to 25 ppm and 176 to 305 ppm for B and Li, respectively). This pattern suggests that at least the propylitic assemblage interacted with alteration fluids of largely magmatic origin, thus excluding the possibility of an early seawater contributions to the hydrothermal system. Conversely, phyllic samples have distinctly higher B contents (57 to 99 ppm) and δ⁷Li values (+7 to +8‰), but have much lower δ¹¹B values (−19 to −28‰) and Li contents (5 to 6 ppm) relative to actinolitic and propylitic samples. We argue that these striking differences are attributable to mineralogical and temperature controls that promoted the preferential removal of ⁶Li during chlorite dissolution and incorporation of ¹⁰B into illite. Furthermore, the highly negative δ¹¹B values in phyllic samples suggest that the additional B incorporated into the altered North Star Basalt originated from the magma chamber underlying the Dresser caldera rather than from seawater or other crustal sources. The most intensely altered (argillic) samples have moderate B and Li contents (2 to 43 ppm and 41 to 71 ppm, respectively) and highly variable δ¹¹B and δ⁷Li values (−24 to −0.4‰ and −10 to +11‰, respectively). These signatures are interpreted to represent the input of ‘external’ fluids into the upper portions of the hydrothermal system during periodic crack-seal events that allowed the influx of restricted amounts of seawater and, possibly, meteoric fluids.
Overall, this study provides a novel non-traditional stable isotope perspective on the evolution of the hydrothermal system of the Dresser caldera that adds richness to our understanding of this complex environment. Both B and Li stable isotopes support a prominent magmatic contribution to the hydrothermal fluid budget, reinforcing the interpretation of a hot-spring origin of the tourmaline-bearing layers associated with the stromatolites of the North Pole Chert.

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The role of magmatic fluids in the ~3.48 Ga Dresser Caldera, Pilbara Craton: New insights from the geochemical investigation of hydrothermal alteration

Highlights

Abstract

Introduction

Section snippets

The Dresser Formation

Materials and methods

Field observations

Hydrothermal alteration distribution

Conclusions and Summary

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgments

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