Introduction
Smith Island, part of the South Shetland Islands, is composed of a metamorphic complex consisting of polydeformed rocks that have experienced high-pressure/low-temperature metamorphism in the frame of the blueschist/amphibolite facies (Trouw et al. Reference Trouw, Simoes and Valladares1998). Their mineralogy, structural deformation history, protoliths and ages have been the subject of study by many researchers (see Bastias et al. Reference Bastias, Chew, Villanueva, Riley, Manfroi and Trevisan2023). The exposed rocks are dominated by green and blue schists, metachert, grey phyllite, marble and calc-silicate rocks, and they are interpreted as part of a subduction complex composed of oceanic crustal material mixed with arc-derived sediments (Trouw et al. Reference Trouw, Simoes and Valladares1998, Chen et al. Reference Chen, Wei, Zhao, Liu, Hu and Zhang2021). Bastias et al. (Reference Bastias, Spikings, Riley, Chew, Grunow and Ulianov2022) proposed that the exhumation of these rocks might have been linked to renewed subduction activity during the Cretaceous.
During the 33rd Bulgarian Antarctic Expedition on 10 January and 12 February 2025, preliminary geological studies were carried out by the Research Vessel RSV 421 in the area of Cape Smith at the north-eastern end of Smith Island (62°52′44″S, 62°19′13″W; Fig. 1a). This Short Note aims to present preliminary data on the discovered and to-date unexplored copper-rich weathering crust over greenschists from this area of Smith Island.
a. Geological map of Smith Island (Bastias et al. Reference Bastias, Chew, Villanueva, Riley, Manfroi and Trevisan2023, fig. 4). b. General view of the investigated outcrops (sampling site highlighted with a red circle). c. Studied rock samples. d. Microscopic photo image of amphibole (Amph) schist, parallel-polarized light). e. Microscopic image of greenschist (Ep-Chl), cross-polarized light. f. Backscattered electron images of the analysed ore minerals pyrite (Py) and chalcopyrite (Chp). g. Powder X-ray diffraction trace of the weathering crust. h. Average chemical analysis of the weathering crust. Chl = chlorite; Ep = epidote; Px = pyroxene.
Materials and methods
The study has been carried out on representative rock samples and blue-greenish weathering crust covering some of the rocks (Fig. 1b,c). They were analysed using optical microscopy (Nikon polarizing microscope, OLYMPUS SZ61), scanning electron microscopy (SEM) in backscattered electron mode (JEOL JSM-6010PLUS/LA and JEOL JSM-6390) for determination of the mineral composition, X-ray fluorescence (XRF) chemical analysis using a portable Bruker analyser and powder X-ray diffraction (XRD) analysis (Advance D8).
Results
The studied samples from the locality are represented by two metabasic rocks: amphibole schists and greenschists (epidote-chlorite). They are fine-grained, with well-exposed fine foliation and crosscut by chlorite containing quartz veins in places. The amphibole schists consist of pyroxene with light pink pleochroism (augite), amphibole (violet to light blue in colour, pleochroitic and with a chemical composition defined as glaucophane), lawsonite, epidote, chlorite mineral, titanite, albite (Ab99.4–98.6%), quartz and ore mineral (pyrite; Fig. 1d). The second type of metabasic rocks - greenschists - consist mainly of epidote (up to 40–45%), chlorite (20%), amphibole (greenish actinolite and glaucophane), pyroxene (relic), white mica, titanite, albite, quartz and pyrite (Fig. 1e). Here, chalcopyrite as individual grains or as inclusions in pyrite was detected (Fig. 1f). The rocks are characterized by well-developed foliation with the formation of micro-boudinage textures around relic pyroxene crystals. The identified mineral composition classifies the rocks as part of a high-pressure/low-temperature metamorphic complex within the blueschist facies of regional metamorphism.
The studied blue-greenish weathering crust over the greenschist indicates that it consists of a heterogeneous mixture of secondary minerals, including sulphates, phosphate and silicate phases (Fig. 1g). Chrysocolla and langite are the main copper-bearing phases appearing as fine-grained, granular aggregates and clusters, indicating copper enrichment under oxidizing surface conditions. The formation of the Cu secondary minerals is probably due to oxidation of the copper sulphides (chalcopyrite and covellite) in the metamorphic rock samples detected (registered) by SEM and XRD analyses. The semi-quantitative chemical analyses of the blue-greenish crust samples conducted with a portable XRF analyser reveal elevated concentrations of Al2O3, SiO2, P2O5, Fe2O3 and especially Cu (Fig. 1h). The observed complexity of the chemical and mineralogical content suggests a prolonged evolution of the weathering crust under dynamic oxidative conditions, probably including biochemical processes.
Conclusion
This study identifies the presence of copper ore minerals, specifically chalcopyrite and covellite, associated with pyrite in high-pressure metamorphic rocks from Smith Island. These rocks, known as greenschists (consisting primarily of epidote and chlorite), are interlayered with amphibole (glaucophane)-pyroxene schists. The investigated blue-greenish weathering crusts that developed on the surfaces of the greenschists contain copper minerals such as chrysocolla, langite, phosphate and aluminosilicate phases. The formation of this secondary mineral association is a product of advanced chemical and probably biochemical processes. These copper-enriched weathering crusts in the studied area of Smith Island have been discovered for the first time. The obtained results provide new insights into surface processes and their potential connection to ore mineralization in the region. This study emphasizes the significance of continued research to assess the mineralization potential in the region.
Acknowledgements
The authors thank Captain Radko Muevski and the crew of the Bulgarian RSV 421 for the logistical support. This research is part of the National Polar Program ‘From Pole to Pole’ under the auspices of the Ministry of Education and Science.
Competing interests
The authors declare none.
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