Redbed diagenesis links district-scale mineralising fluids to Cu source rock in the Polaris Zn-Pb(-Cu) district, Arctic Canada

https://www.sciencedirect.com/science/article/pii/S0169136824000490?via%3Dihub

A technical paper co-authored by Jordan Mathieu, who works at Storm Copper Project for AW1 as a drill core logging geologist on-site.


Geologists Chaneil Wallace (left) and Jordan Mathieu (right) working on Storm drill core

The Storm copper deposit formed due to the interplay of several factors, specifically the interaction between mineralising fluids and the Aston Formation. Here's a breakdown of the key points from the technical paper:

Fluid History:

1. Pre-ore Stage: A reduced fluid, sourced from the basement rocks, circulated during the Ellesmerian orogeny. This fluid bleached the Aston Formation and potentially liberated metals like copper through this process. The Ellesmerian orogeny refers to a period of mountain-building and tectonic activity that occurred during the Paleozoic era, specifically in the Arctic region. This orogeny resulted in the formation of mountain ranges and significant geological changes in the Ellesmere Island area, which is located in the Canadian Arctic Archipelago.
2. Main-ore Stage: A warm, oxidised, and meteoric-sourced fluid post-dated the Ellesmerian orogeny. This fluid transported copper and other metals to the Storm area, where they were deposited as sulphides. This fluid coincides with the reddening event observed in the Aston Formation.
3. Post-ore Stage: A later, low-temperature fluid further altered the deposit.

Connection to Aston Formation:

• The diagenetic history of the Aston Formation, marked by reddening (oxidation) followed by bleaching (reduction), mirrors the fluid history of the Storm deposit.
• Copper adsorbed onto oxyhydroxides during the reddening event would have been released back into the fluid during subsequent bleaching, providing a potential source for the metals in the deposit.
• The timing and isotopic composition of the main-ore fluid in Storm aligns with the fluid responsible for reddening the Aston Formation.

Key Takeaways:

• The Storm deposit formed from meteoric fluids interacting with the copper-enriched Aston Formation.
• The reddening-bleaching cycle within the formation played a crucial role in liberating and mobilizing copper.
• This process suggests that extensive residence time in the source rock isn't necessary for generating copper-rich fluids, potentially challenging traditional models.

________________________________


Extracts from the Technical paper (Below)




Fig. 1. (A) Simplified map of Canada; outlined area is enlarged in (B). (B) Part of Canada’s Arctic archipelago, showing location of Polaris district (shaded rectangle), Mesoproterozoic Bylot basins (dark blue; after Jackson and Iannelli, 1981), past-producing mines, and the Ellesmerian orogenic front (dashed line). Rectangle over Somerset Island is enlarged in (C). (C) Geological map of northern Somerset Island (after Stewart and Kerr, 1984) showing location of Polaris-district base-metal showings (Seal and Storm) along a possible reactivated Mesoproterozoic basement structure (Aston-Batty line; Dewing et al., 2007b). Proterozoic strata, including the Aston Formation, extend at least some distance under Paleozoic strata. Map colours correspond to those in (D). White rectangle outlines area shown in (E). (D) Simplified bedrock stratigraphy of Somerset Island (modified from Tuke et al., 1966, Dixon, 1974, Miall and Kerr, 1980, Stewart, 1987) showing position of the Storm copper deposit (star) above Proterozoic Aston Formation redbeds and below Silurian-Devonian Peel Sound Formation redbeds, and typical thicknesses of the main stratigraphic units. (E) Simplified local geology map (after Stewart and Kerr, 1984) of area outlined in (C) showing sample locations along a transect through the Aston Formation.

Figure 15: Proposed (A) paragenetic sequence and (B) burial history of the Aston Formation, highlighting Paleozoic diagenetic processes that coincide with paragenetic sequence previously documented at Storm copper (Mathieu et al., 2018) and Polaris (Reid et al., 2013a, Mathieu et al., 2022).




Fig.16. Paragenetic events experienced by Aston Formation (numbered in Fig. 15). (1) After sandstone deposition, subsurface alteration of mafic and other unstable grains (e.g., K-feldspar) released Fe and Si into formation water to allow for the first hematisation (Hem1), and dyke emplacement provided heat for Qz1 cement precipitation. (2) A reduced fluid (Paleozoic) bleached parts of the sandstone, removing most Hem1 from around grains, and precipitated M1 in pore space. (3) Pressure-solution during burial to depth >2 km (Paleozoic) produced concavo-convex and sutured grain contacts, both typically lacking Hem1. (4) Hematisation 2 (Paleozoic) enclosed sutured grains and interacted with local alteration of K-feldspar to M2. Iron sourced from surrounding rocks (basement or dykes) was introduced with circulating (eventual) Cu-mineralising fluid. (5) Qz2 cement precipitated from a heated meteoric fluid (Paleozoic) that supplied externally derived Si occluded most of the remaining porosity.


Relationship to Storm copper mineralising fluids

The mineral paragenesis of the Storm Cu deposit is divided into pre-ore, main-ore, and post-ore stages (Fig. 15A; Mathieu et al., 2018). Pre-ore mineralisation (minor sphalerite, chalcopyrite, and dolomite) resulted from reduced fluids that had equilibrated with basement (-derived) rocks and transported metals to the site of precipitation during the onset of the Ellesmerian orogeny. This reduced fluid would coincide temporally with the reducing fluid that bleached the Aston Formation at approximately the time of maximum burial (i.e., during the Ellesmerian orogeny). Metals can be liberated along with Fe by the bleaching process (Parnell et al., 2021) and could explain the trace element geochemistry (e.g., elevated Co) of pre-ore sulphides at Storm (Mathieu et al., 2018). This process is similar to relationships documented in the Irish Midlands, where bleached redbeds are the suggested source of trace elements (e.g., Co, Ni, and Cu) in sulphides (Wilkinson et al., 2005). The reduced fluid responsible for bleaching the Aston Formation may have contributed to the small amount of pre-ore Storm Cu mineralisation. The main-ore stage at Storm (chalcopyrite, calcite, dolomite, quartz, pyrite) is associated with a warm (150 °C), post (or late)-Ellesmerian (i.e., maximum burial), oxidised, meteoric-sourced fluid that transported Cu (and other base metals) to the Storm area, where it was reduced on site (Mathieu et al., 2018). This fluid had a calculated isotopic composition in the range of −4 to 4 ‰SMOW (Mathieu et al., 2018), which indicates a meteoric origin and is similar to that of both the post-Ellesmerian fluid recorded throughout the Polaris district (Mathieu et al., 2018, Mathieu et al., 2022) and the Qz2-precipitating fluid in this study (Fig. 14). The introduction of an oxidised meteoric fluid after circulation of a reduced fluid and maximum burial is similar to the diagenetic history of the Aston Formation [i.e., reduced (bleaching) fluid circulation was followed by oxidised (reddening) fluid circulation]. Copper adsorbed onto the oxyhydroxide would have been released into the fluid during the recrystallisation of the oxyhydroxide to hematite during the reddening process by oxidised meteoric fluid (Brown, 2005, Brown, 2009, Brown, 2014), which agrees with the fluid history of the Storm copper deposit. A late Paleozoic, low-latitude meteoric water responsible for quartz cementation (Qz2) and reddening in the Aston Formation agrees with fluid characteristics and age estimates for the main-ore stage at Storm copper (Mathieu et al., 2018) and with the paleolatitude (0-30°) in which sedimentary-rock-hosted stratiform Cu deposits typically form (Hitzman et al., 2005, Hitzman et al., 2010). The Aston Formation’s diagenetic fluid history is also shared with paragenetic elements of the Polaris district, and links Storm Cu to the regional mineralising event. Mineralised Zn (±Pb) and Cu showings throughout the district record a main-ore to post-ore transition from a reduced formation fluid being replaced by a low- to mid-latitude, meteoric-derived, oxidised hydrothermal (140–180 °C) fluid that was topographically driven by the Ellesmerian orogeny (Mathieu et al., 2022). The timing of the Polaris district post-ore fluid coincides with Storm copper’s main-ore fluid and agrees with the timing of the Qz2 cement in the Aston Formation. Circulation of hot mineralising fluids in carbonate-rock-hosted Zn-Pb (MVT) deposits in the Illinois basin gradually occluded source rock pores with quartz cement during (or shortly after) precipitation of the deposits (Hyodo et al., 2014). A similar situation could have taken place in the Storm copper area, in that the mineralising fluid probably circulated through the Aston Formation sandstone while initially reddening and transporting Cu (Brown, 2009), but eventually precipitated quartz cement in the Aston Formation sandstone shortly afterwards. This would agree with the observation that some early Qz2 (i.e., closest to grain margins) incorporates hematite micro-particles (Fig. 6J-L), suggesting that the fluid responsible for both reddening the Aston Formation and mineralisation at Storm copper also eventually precipitated Qz2 cement. The diagenetic history and associated fluid characteristics (temperature and stable isotope values) of Aston Formation quartz cements documented in this study are compatible with events and geochemical characteristics of ore-stage calcite and quartz in the overlying Storm copper deposit (and throughout the Polaris district), which indicates that the Aston Formation redbeds were plausibly the metal source for the Storm deposit. Importantly, the time-frame over which metals were liberated from redbeds by heated, meteoric-derived fluid driven by orogenic uplift and then reprecipitated in the Storm Cu deposits was probably short, all taking place in the Late Devonian to (possibly) Early Carboniferous. This suggests that lengthy residence times in subsurface source rock are not necessary for generating Cu-rich fluids in the redbed system, in agreement with the model suggested by Brown, 2005, Brown, 2009.

Conclusions

The geographically limited exposure area of a Proterozoic redbed unit (Aston Formation) coincides with the location of a regionally anomalous carbonate-hosted Cu deposit (Storm) in the Polaris Zn district in Nunavut (Arctic Canada). Based on field, petrographic, and geochemical evidence, the diagenetic history of the Aston Formation redbed succession records multiple fluid circulation events since the formation’s deposition in the Proterozoic. Precipitation of early hematite grain-coatings (Hem1) and early quartz cement (Qz1) took place in the Proterozoic under shallow burial conditions, from an oxidised meteoric fluid. A mid-Paleozoic reducing fluid then bleached (reduced) some of the early hematite, and may be related to the reduced fluid responsible for pre-ore mineralisation at Storm (warm, high salinity, seawater-derived fluid) and main-stage ore formation elsewhere in the Polaris Zn district. Then, under deeper burial (>2 km), extensive pressure-solution of sandstone quartz particles occurred. The most important fluid migration event, in terms of porosity loss and Cu transfer, took place after maximum burial, in the mid- to late Paleozoic, with the circulation of low-latitude meteoric water. This heated, oxidised fluid produced a second generation of hematite grain-coatings in the Aston Formation (Hem2), but also liberated Cu, which migrated a short distance and was reduced at the Storm copper deposit. It also yielded the final paragenetic phase in the Aston Formation, a pore-occluding quartz cement (Qz2) that formed from externally sourced silica. The presence of an anomalous Cu deposit (Storm) in a Zn-Pb district can therefore be understood through its connection to the diagenetic history of a geographically limited nearby metal-sourcing unit. This study demonstrates that metal liberation by diagenetic reddening (oxidation) of subsurface siliciclastic units (the redbed model) is a geochemically and geologically viable source of Cu in sedimentary-rock-hosted Cu deposits, and in the Storm Cu showing system in particular. Together with previous geochemical studies (Mathieu et al., 2018, Mathieu et al., 2022) this study shows that the regional Polaris carbonate-hosted Zn mineralisation and the localised Storm Cu mineralisation, although seemingly dissimilar, are functionally and temporally linked: the two distinct mineralising systems developed in quick succession in the Late Devonian, as a result of topographic fluid mobilisation caused by development of the Ellesmerian orogen, and are related to part of the diagenetic history of a redbed unit that probably served as the Cu source for the Storm Cu showing. This study also suggests that the diagenetic processes encapsulated in the redbed model for Cu deposits can take place on a geologically short time-scale, from heated fluids, and without requiring lengthy residence time in the source rock. The Aston Formation’s Proterozoic depositional age and environment are much older than the mid-Paleozoic reddening and Cu-mineralising episode, indicating that redbed-type Cu mineralising events need not be geodynamically related to the tectonic setting under which the Cu-providing sediment was originally deposited. Hematisation can happen up to a billion years after sedimentation, and in most redbeds was probably not geochemically linked to sedimentary depositional conditions. Therefore, it is important to understand the paragenetic timing of diagenetic events prior to characterising the paleoredox conditions of sedimentary depositional environments.