Chrysocolla from the Okrzeszyn (Sudetes, Poland)

A b s t r a c t. Secondary copper mineralization from an old quarry located in the Okrzeszyn village (Intra-Sudetic Depression) was analysed using X-ray powder diffraction (PXRD) and scanning electron microscope coupled with energy dispersive X-ray spectroscopy (SEM-EDS) methods. About 80 samples were collected from a horizontal channel, filled with clay and blue-green secondary vein mineralization, located on the western side, about five meters below top of the quarry. Secondary Cu mineralization is poorly developed. The only recognized mineral among gathered samples - chrysocolla - is relatively pure and contains small admixtures of Ca, Fe, K, Mg or fine intergrowths of other mineral phases containing these elements. The studied chrysocolla is partially amorphous, and the reflections position on the diffraction pattern closely matches the chrysocolla reference data published in JCPDS and ICDD databases.

Key words: chrysocolla, secondary minerals, Permian. rhyolite, Sudetes

1. Introduction

Supergene alterations of ore minerals usually take place during hydrothermal and weathering processes. In both cases new mineral phases are formed and their chemical composition is highly dependent of surrounding rocks type and chemistry of primary minerals. Crystallization of secondary minerals is also affected by pH – temperature factors which control their stability.
An increasing interest in secondary copper minerals research, combined with modern analytical methods led to significant discoveries in last years. Several secondary Cu-phases were described from the mines of the Lubin-Polkowice region (Fore-Sudetic Monocline), including rare species, i.e. juangodoyite and botallackite (Kruszewski et al. 2020; Siuda et al. 2017). The mineralization is related to recent weathering processes of primary minerals embedded in sedimentary rocks, mainly dolomites, limestones and black shales. Similar research on recently forming minerals was conducted in the abandoned Cu-As-Au mine in Radzimowice, where several secondary Cu-bearing phases were recognized (Siuda, Kruszewski 2013). Other places with well developed supergene mineralization are Miedzianka-Ciechanowice and Kletno polymetallic deposits. Current detailed studies of post-mining dump material from these locations caused numerous discoveries of Cu secondary phases (Gil et al. 2020, Siuda, Gołębiowska 2011). An argument for the sense of secondary minerals research is a discovery of borzęckiite - a new mineral approved by IMA, which was found in the Miedzianka dump material (Siuda et al. 2022). Less significant examples of Cu-mineralization are known from sedimentary rocks in Miedziana Góra and Miedzianka near Kielce (Swęd et al. 2015, Król, Urban 2003), Czarnów (Mochnacka et al. 2009), Lena - Konrad - Nowy Kościół deposits (Maciejak, Maciejak 2016), Przygórze (Kruszewski et al. 2019), Zalas (Gołębiowska et al. 2006) and Stary Lesieniec (Mederski et al. 2021). The last two locations are particulary interesting because of mineralization embedded within volcanic rocks.
Several Cu-bearing minerals have been described from the Okrzeszyn area: chalcocite, bornite, chalcopyrite and malachite (Lis, Sylwestrzak, 1986). In this paper, copper mineralization from the Okrzeszyn was studied again, and a new occurrence of chrysocolla as another secondary product of hydrothermal activity is described.

2. Location and geology

The explored old rhyolite quarry (the 'newer' one is ~200 m west) is located in the northern part of Okrzeszyn village, on the western slope of unnamed hill, about 6 km south of Chełmsko Śląskie and about 1.5 km east of Poland - Czech Republic border (Fig. 1).
Fig. 1. Simplified geological map of the Intra-Sudetic Basin (without Cenozoic deposits) with neighbouring towns and location of the quarry with studied mineralization. Based on Adwankiewicz et al. 2003, modified.

It is situated in the western part of the Intra-Sudetic Basin - the structure ~35 km x 60 km large enclosed in the NE margin of the Bohemian Massif (Awdankiewicz 1998). The Intra-Sudetic Basin is composed of Permo-Carboniferous molasse succession of variable thickness, represented by sedimentary and volcanogenic rocks. Their formation is connected to three-stage volcanic activity. In the central part the succession is overlain by Triassic and Cretaceous deposits (Awdankiewicz 1998).
The rhyolites of Okrzeszyn area belong to the Góry Krucze rock unit, as a part of Słupiec Formation. The Góry Krucze extrusion is strongly eroded, up to several hundred meters thick and consists of different rhyolite varieties. It formed as a result of early Permian effusive volcanism. The extrusion is underlain by lithified sediments of the Słupiec Formation and overlain by the Radków Formation deposits (Awdankiewicz et al. 2003). According to citied authors, the complex of ~25 m thickness, exposed in the Okrzeszyn quarry (Fig. 2), is composed of two different parts. The lower part belong to the Słupiec Formation and is represented by rhyolites with secondary copper mineralization (subject of this study), occurring in several varieties and flow folds. The upper, a few meters thick, sedimentary part belonging to Radków formation, consists of rhyolitic breccias, mudstones and sandstones. These rocks occur only locally due to activity of the quarry in the past.
Fig. 2. A photo of southern part of the quarry with well visible foliation of massive rhyolites and horizontal channels, probably man-made.

3. Methods

Unweathered mineral samples, collected directly from veins, without any host rock fragments, were macroscopically selected for SEM-EDS and PXRD analysis. One powdered sample for PXRD was analysed using Panalytical X'PertPro diffractometer with PIXcel ultrafast detector, CuKα radiation and 0.03 2θ increment size (5° 2θ - 97° 2θ), located in the Laboratory of Nanostructures - Institute of High Pressure Physics PAS. The element compositions of selected two samples were studied by Bruker Quantax 400 EDS coupled with ZEISS Ultra Plus Scanning Electron Microscope located in the Laboratory of Nanostructures - Institute of High Pressure Physics PAS. Spraying the samples with graphite and 15kV of voltage was used. For more reliable results, two analysis per sample were carried out in selected micro-areas.

4. Results

During fieldwork in 2022/23 on the western slope of an old Okrzeszyn quarry, within massive rhyilites, veinlets, crusts and single nodules of green-blue mineralization occuring locally 'in situ' were found. Traces of mineralization were found also on some loose rock fragments at the bottom of the quarry. Tens of samples were collected from a 20 cm x 15 cm wide and ~1 m manually deepened channel. The channel, in cross section, is composed of inner part built by hydrothermally altered grey-green rhyolite cut with secondary mineralization veinlets, and outer part, composed of red clay (Fig. 3, 4).
Fig. 3. A photo of a horizontal channel filled with red clay and altered grey-green rhyolites containing veins of secondary Cu mineralization, taken while deepening and collecting samples, ~50 cm depth.

Fig. 4. A photo of a horizontal channel containing veins of secondary Cu mineralization, at about 80 cm depth.

Mineralization in this channel appeared at various stages while deepening. The border between altered rhyolites and surrounding non-altered rocks is sharp. Due to high fragility of the veins and limited space in the channel, extraction of larger samples was difficult and the size of most does not exceed a few centimeters (Fig. 5).

Fig. 5. Fresh samples of green Cu secondary minerals, hammer as scale.

4.1. Chrysocolla (Cu_2-xAl_x)H_2-xSi₂O₅(OH)₄·nH₂O _(x<1)

It is one of the most common secondary minerals forming in copper ore weathering zones. In Okrzeszyn, the chrysocolla occur in different forms:
a) blue-green veins with botryoidal accumulations in voids, up to 3 cm thickness, locally filling horizontal and vertical fractures (Figs. 6.1-6.2)
b) blue-green encrustations up to 0.5 cm thickness, being surfaces of veinlets cutting rhyolites (Fig. 6.3)
c) green nodular accumulations up to 4 cm length, embedded in highly weathered rhyolites (Fig. 6.4)

Fig. 6.1. Porous and partially botryoidal fragments of vein chrysocolla, specimen size: A: 8.5 x 5 cm, B: 6 x 5 cm.

Fig. 6.2. Botryoidal chrysocolla samples. Specimens size: A: 4.5 x 3 cm, B: 6 x 5 cm.

Fig. 6.3. Surfaces of thin chrysocolla veinlets cutting rhyolites. Specimens size: A: 8,5 x 5,5 cm, B: 7 x 6 cm, found among loose material.

Fig. 6.4. Sample of nodular chrysocolla collected directly from weathered rhyolites, size: 4 x 3 cm.

Moreover, fresh chrysocolla samples, collected in the quarry, behave like opal - they are sensitive to low humidity and dehydrate quickly, what results in cracking.
Chrysocolla was initially identified on the basis of EDS micro-area analysis of two samples. Both samples show similar element composition which is dominated by Cu, Si and Al with small admixtures of Ca, Mg and Fe (Figs. 7.2, 7.2). In one of the samples admixtures of K were also detected. Presence of small amounts of calcium, magnesium, potassium and iron may indicate the presence of carbonate microinclusions and/or clay minerals.

Fig. 7.1. BSE images of sample no.1 with marked analysed micro-areas, with corresponding EDS patterns.

Fig. 7.2. BSE images of sample no.1 with marked analysed micro-areas, with corresponding EDS patterns.

Diffraction analysis of chrysocolla indicate, the mineral has a partially amorphous structure. The studied chrysocolla, based on the PXRD pattern, partially lacks crystalline structure, however, it is not fully amorphous. Several broad and low intensity reflections at d = 4.411, 2.871, 2,618, 2.513, 1.611, 1.485, 1.319 Å are observed (Fig. 8). The d-spacing of these peaks closely matches the references of chrysocolla JCPDS 11-0322, ICDD 27-188, and the chrysocolla data from Stary Lesieniec (Tab. 1).

Fig. 8. PXRD pattern of the studied chrysocolla with marked reflections, and their d-spacing [Å] numbers.

Tab. 1. Comparison of d-spacing of the studied chrysocolla with ICDD and JCPDS reference data and chrysocolla data from Stary Lesieniec (Mederski et al. 2021).

5. Discussion

Chrysocolla as a mineral approved by IMA, has a structure which differs from typical crystalline structures. According to Farges et al. (2006) chrysocolla should be redefined to a phase composed of spertiniite, water and amorphous silica, however, Frost and Xi (2013) proof chrysocolla is rather an ‘amorphous hydrated copper silicate’ not related to spertiniite. In terms of crystallinity, chrysocolla from Okrzeszyn has more crystalline structure than chrysocolla from DR Congo and Arizona, what is noticeable when comparing the diffraction pattern with these published by Frost et al. (2012).
Chrysocolla from the Okrzeszyn also differs from that occurring at Stary Lesieniec. Although diffraction data of the mineral from both localities is similar, chemical compostion is slightly different. The chrysocolla from Stary Lesieniec is additionally enriched in Ti, Cl, S and P, what may be caused by using more precise analytical method. Recently forming chrysocolla in volcanites of Radzimowice (Siuda, Kruszewski 2013) also has a different chemical composition. The main reason is lack of Al in all the samples analysed by these authors and small admixtures of Zn. However, to compare elements quantitatively, chemical EPMA analysis should be performed.
Taking into account the presence of chalcocite, bornite and chalcopyrite as primary minerals and malachite in the Okrzeszyn area (Lis, Sylwestrzak, 1986), a preliminary model for the crystallization of chrysocolla can be applied: (1) migrating hot solutions within rock fractures began oxidation of sulphides, which caused pH decreasing and further interaction of solutions with rock-forming minerals, being source of silicon and aluminum. (2) Si, Al and Cu remained in solutions until pH increased. (3) Interaction of migrating acidic solutions with locally inflowing meteoric waters caused lowering their acidity until pH>6, which started the precipitation of chrysocolla. According to Nicol and Akilan (2018), such pH conditions favour chrysocolla stability. Moreover, the temperature during crystallization must have been much lower than 125°C, because at this point, according to Frost et al. (2012), chrysocolla start to decompose rapidly. In the final stage, weathering processes must have led to carbonate anion local enrichment in migrating solutions, as indicated by presence of malachite.

6. Conclusions

1. Chrysocolla veins, encrustations and nodules were found in the old rhyolite quarry in Okrzeszyn.
2. Chrysocolla as the only analytically recognised phase, may contain small amounts of micro-inclusions of other minerals
3. Based on literature data, crystallization of chrysocolla could take place under close to neutral pH conditions and temperature below 125°C.
4. Taking into account the size, purity, appearance of gathered chrysocolla samples, and their occurrence 'in situ', the discovery, among other occurrences of chrysocolla in Europe is exceptional and is a good educational example of hydrothermal activity in the Intra-Sudetic Basin.
5. Chemical EPMA analysis should be performed, in order to determine an accurate chemical formula of the chrysocolla.