Abstract
The Rockliden Zn–Cu volcanic-hosted massive sulphide deposit is located approximately 150 km south of the Skellefte ore district, north-central Sweden. Most of the mineralisation is found at the altered stratigraphic top of the felsic volcanic rocks, which are intercalated in the metamorphosed siliciclastic sedimentary rocks of the Bothnian Basin. Mafic dykes cross-cut all lithological units, including the massive sulphides, at the Rockliden deposit. The relatively high Sb grade of some parts of the mineralisation results in challenges in handling of the Cu–Pb concentrate in the smelting process. The aim of this study is to characterise different host rock units and ore types by their main mineralogy, as well as by their trace mineralogy with focus on the Sb-bearing minerals. Ore types are distinguished largely on the basis of their main base-metal bearing sulphide minerals, which are chalcopyrite and sphalerite. Several Sb-bearing minerals are documented and differences in the trace mineralogy between rock and ore types are highlighted. Based on the qualitative ore characterisation, rock- and ore-intrinsic parameters, such as the pyrite, pyrrhotite and magnetite content of the massive sulphides, the trace mineralogy and its association with base-metal sulphide minerals, are outlined and discussed in terms of relevance to the ore processing.
Keywords
Introduction
The Rockliden volcanic-hosted massive sulphide (VHMS) deposit is located in the Västernorrlands Län, north-central Sweden, ca. 150 km SW of the Skellefte ore district and the nearby existing processing facilities (e.g. the smelter at Rönnskär, Fig. 1). The inferred mineral resource is 3·53 Mt with 4·2 wt-%Zn, 1·9 wt-%Cu, 0·7 wt-%Pb and 71 g t–1 Ag (New Boliden, 2013). The Rockliden deposit is relatively Cu-rich compared to other Swedish VHMS deposits, e.g. Kristineberg, Renström, Maurliden and Rakkejaur (Allen et al., 1996; New Boliden, 2013). Similar to other VHMS deposits of the Skellefte ore district, the Rockliden mineralisation shows high concentrations of penalty elements such as As, Sb and Hg (Allen et al., 1996; Raat and Årebäck, 2009).
a location of the Rockliden VHMS deposit in the Bothnian Basin limited by the Bergslagen and Skellefte ore districts (Kumpulainen, 2009), south of the Luleå-Jokkmokk line (Mellqvist et al., 1999; Weihed, 2004) and b road map locating the exploration site (coordinate system: Swedish grid RT90 2·5 gon west)
Shortly after the discovery of the Rockliden deposit in the 1980s, the project was put on hold when metallurgical tests indicated a high Sb content in the Cu–Pb concentrate, thus lowering the quality of this product (Raat and Årebäck, 2009). In 2007, exploration drilling in the Rockliden area restarted, indicating a decrease in the average Sb content at depth and some variation along strike, and new flotation tests were performed with new samples (Raat and Årebäck, 2009; Bolin, 2010). There is now an alternative treatment considered to remove Sb from the Cu–Pb concentrate by alkaline sulphide leaching (Awe, 2013; Bolin, 2010). However, in order to determine when it is appropriate to use this optional treatment, it is important to know what parts of the ore are likely to generate high Sb contents in the Cu–Pb concentrate (Minz, 2013). Studies of the Sb mineralogy of Rockliden have pointed towards several parameters controlling the distribution of Sb-bearing minerals during flotation and hence the Sb content of the Cu–Pb concentrate (Minz et al., 2013). One potential process-relevant parameter is the interlocking of penalty element-bearing minerals with base-metal and Fe sulphides in mineralogically complex particles (Minz et al., 2013).
Details of the Sb mineralogy of Rockliden are published by Minz et al. (2013). A detailed petrographic and mineralogical description of the different rock and mineralisation types at the Rockliden exploration project, i.e. the framework to the Sb-bearing minerals, is presented in this paper. The main purpose of this study is to characterise and classify the Rockliden mineralisation qualitatively based on the occurrence and distribution of the major sulphide minerals, i.e. pyrite, sphalerite, chalcopyrite and pyrrhotite. The parameters used in this petrographic study to characterise the massive sulphides include not only the mineralogical composition but also textural information such as grain size, oriented intergrowths and association with penalty and bonus element-bearing minerals (cf. Walters and Kojovic, 2006). Herein, the current knowledge of the local geology and mineralogy of Rockliden is compiled, and is discussed in terms of petrographic parameters that are expected to influence the ore processing. This qualitative evaluation gives guidance for quantifying potential process-relevant parameters which might form the background of a process-adopted geological model for the Rockliden deposit (Lamberg, 2011; Minz, 2013).
Geological setting
Regional geology
The Rockliden massive sulphide deposit is located within the Bothnian Basin (Welin, 1987; Lundqvist et al., 1998; Kousa and Lundqvist, 2000), in the Svecofennian rocks south of the Luleå-Jokkmokk line (Weihed, 2004). The basin is located south of the Skellefte ore district and north of the Bergslagen ore district (Kumpulainen, 2009; Fig. 1). The supracrustal rocks of the Svecofennian Domain are dominated by turbiditic metasedimentary rocks of the Härnö group which is estimated to be at least 10 km thick (Lundqvist, 1987; Lundqvist et al., 1990; Kousa and Lundqvist, 2000; Kumpulainen, 2009). Volcanic rocks in the Bothnian Basin are divided into basalt-andesites and fractionated basalts to rhyolites (Bergström, 2001). In the Rockliden area, Mattsson and Heeroma (1985) described mafic rock types comprising gabbros and basaltic rocks, which are found in outcrops within a distance of 5 to 10 km from the Rockliden deposit. Acid volcanic rocks occur locally. Metarhyolitic to metadacitic volcanic rocks, located just 1·5 km west of Rockliden, were dated 1875 Ma (Welin, 1987). As with the volcanic rocks of the Skellefte group (Allen et al., 1996; Rutland et al., 2001), a number of different tectonic settings for the Rockliden volcanic rocks have been suggested. Depauw (2009) proposed an active continental margin setting based on the geochemical classification of Schandl and Gorton (2002), or an extensional setting within the Bothnian Basin rimmed to the south by the Bergslagen volcanic arc and to the north by the Skellefte volcanic arc.
Based on geochronological age data the following post-depositional history was proposed. Peak metamorphism was reached in the Bothnian Basin during the Svecokarelian orogeny (1·9 to 1·8 Ga) and large parts of the basin were metamorphosed to upper greenschist to upper amphibolite facies, and partly migmatised (Lundqvist et al., 1990; Weihed, 2004). Late-orogenic S-type granitoids (1·82 to 1·80 Ga) and post-orogenic A- to I-type Revsund granitoids (1·80 to 1·77 Ga) intruded into the basin (Claesson and Lundqvist, 1995; Weihed, 2004; Fig. 2d). Between 1·27 to 1·24 Ga the basin was intruded by mafic sills and dykes of the Central Scandinavian Dolerite Group (Söderlund et al., 2006).
Geological maps and section of the Rockliden area and mineralisation. Abbreviation: CSDG – Central Scandinavian Dolerite Group a surface map of the geology of the Rockliden area (based on geochemical classification of percussion drill core samples, coordinate system: Swedish grid RT90 2·5 gon west); b detailed surface map showing the extent and shape of the massive sulphides (enlarged map from picture a); c cross-section (see graphic b for location) showing the vertical distribution of alteration zones in the felsic volcanic host rock, and mineralisation (from Depauw (2009)), drill core profiles are found in Fig. 7; d stratigraphic column (modified from Depauw (2009))
Local geology
The Rockliden massive sulphide deposit is located primarily at the contact between the Härnö sedimentary rocks and the Rockliden volcanic rocks (Depauw, 2009). On the basis of the geochronological age data of Welin (1987), the Rockliden felsic volcanic rocks are located in the stratigraphic upper part of the sedimentary rocks (Depauw, 2009). They are classified as calc-alkaline rhyolites to rhyodacites and dacites (Mattsson and Heeroma, 1985) and are interpreted as quartz–feldspar phyric coherent domes, tuffitic rocks, lavas and hyaloclastites, and volcanoclastic mass flows, autoclastic breccias, and agglomerates. The stratigraphic sequence of these rocks is not well understood. Metamorphosed turbiditic shales and siltstones stratigraphically overlie the mineralisation and generally show a sharp contact against the massive sulphides (Mattsson and Heeroma, 1985; Raat and Årebäck, 2009; Depauw, 2009). To the north and west, the Rockliden area is bordered by granitic rocks, whereas to the south and east metagreywackes of the Härnö group are found as gneisses and migmatites (Mattsson and Heeroma, 1985; Depauw, 2009). Various mafic intrusions occur in the Rockliden area including decimetre- to metre-wide sills and dykes. They mostly occur along structural zones of weakness, such as faults (Raat and Årebäck, 2009). Dolerite dykes with chilled margins crosscut all rocks of the Rockliden area in a WSW–ENE to SW–NE direction (Fig. 2; Mattsson and Heeroma, 1985; Raat and Årebäck, 2009). Mafic sills and dykes, lacking distinct chilled margins, are found close to, or within, the mineralisation (Depauw, 2009). At deposit-scale, these mafic dykes are steeply dipping, mostly lying parallel to bedding or along the foliation of the host rocks, but they do not appear to be continuous over vertical distances larger than 50 m and often appear to coincide with possible fault structures (Raat and Årebäck, 2009).
While the metamorphic grade in large parts of the Bothnian Basin reached upper amphibolite facies conditions during the Svecokarelian orogeny (e.g. Lundqvist et al., 1998), the host rocks to the Rockliden mineralisation have reached only a maximum metamorphic grade of upper greenschist facies (Depauw, 2009). According to Raat and Årebäck (2009) and Depauw (2009), the volcanic rocks show, locally, impregnations of carbonate minerals and silicification in irregular zones in distal parts of the massive sulphide mineralisation. Sericitic alteration is the most prominent type in the Rockliden volcanic footwall rocks, extending several tens to hundreds of metres into these rocks and transgressing into a strong to intense sericite-quartz dominated alteration within tens of metres to a few metres of the massive sulphides. A zone of chlorite alteration of several metres to only a few decimetres wide occurs in close proximity to the massive sulphides and locally contains neoblasts of andalusite, biotite, magnetite and red garnet (Depauw, 2009).
The contact between the altered volcanic rocks and the metasedimentary rocks of Rockliden is steeply dipping (Fig. 2c) and the sedimentary rocks are generally younging to the north (Depauw, 2009; Raat and Årebäck, 2009). The geometry of the massive sulphide bodies is complex due to deformation (Fig. 2b and c). Structural features generally trend E–W (Evins, 2011). Two deformation events were proposed by Mattsson and Heeroma (1985): a first folding event with N–S trending fold axis and a second event with E–W trending fold axis. However, the E–W trending fold axes are prominent in the massive sulphides and the folds are tight, nearly concentric at the outcrop of the massive sulphide mineralisation (Raat and Årebäck, 2009; Evins, 2011). These folds are occasionally accompanied by axis-parallel shear zones with an apparent sinistral offset of up to 30 m (Evins, 2011). Additionally, NW–SE and NE–SW striking and steeply dipping faults transect the Rockliden area (Mattsson and Heeroma, 1985). Dextral offset was observed on a late shear zone cutting the massive sulphide mineralisation (Evins, 2011).
Mattsson and Heeroma (1985) sketched an idealised mineralogical zoning for the undeformed massive sulphide mineralisation at Rockliden. Modifications of the Rockliden massive sulphides and deviations from the idealised zoning pattern are thought to be due to deformation and metamorphism (Depauw, 2009; Raat and Årebäck, 2009; Minz, 2013; Fig. 2c). This includes structural repetition of massive sulphide bodies and remobilisation of ore shoots along fault planes, as well as enrichment of galena and chalcopyrite in fold hinges and along faults and transformation of pyrite to pyrrhotite (Depauw, 2009; Raat and Årebäck, 2009).
Methods
In preparation of drill core sampling for this ore characterisation study, a simplified mineralogy (pyrite, chalcopyrite, sphalerite, galena, arsenopyrite and bournonite) was calculated from chemical assays (S, Cu, Zn, Pb, As and Sb) for drill core intervals using the Geo module of the HSC Chemistry software (Roine, 2009). The calculated total for modal pyrite includes other sulphide minerals which are not present in the list for the element-to-mineral conversion yet might be present in the ore, for example pyrrhotite. Bournonite was chosen to represent the Sb-bearing minerals in these calculations. Based on the sphalerite and chalcopyrite content of the plain sulphide fraction (i.e. sulphides calculated to 100 wt-%) three preliminary classes were defined: a sphalerite-, chalcopyrite- and pyrite-rich class. These classes were further subdivided by their Ag and Sb content representing critical bonus and deleterious elements.
Boliden's database of chemical assays from the Rockliden exploration project were imported and interpolated using a spheroidal model for kriging with Leapfrog 3D modelling software (Zaparo Ltd, 2009) and qualitatively evaluated together with the preliminary classification. Based on this evaluation, six drill cores were selected and 59 samples collected from the preliminary defined classes throughout the spatial extent of the deposit for the study of variability in mineralogy. All 59 samples were assayed by the ALS Minerals Division (Piteå, Sweden) for 64 major and minor elements. The accuracy and precision of the data is stated with ±5% for elements at high level concentration and ±10% for elements on trace level concentration. The ranges of the analytical precision affect the recalculated modal mineralogy in the same order of magnitude. The geochemical data were studied against reference data from the Boliden database for Rockliden host rock material.
Polished thin sections were prepared by Vancouver Petrographics Ltd (Canada) and were studied under an optical microscope (Nikon ECLIPSE E600 POL, Luleå University of Technology, Sweden) to refine the HSC Chemistry based preliminary classification and to provide additional textural information. Further, mineralogical studies were continued with energy-dispersive X-ray spectroscopy (EDS) in the scanning electron microscope (SEM) and the chemical composition of minerals was measured by wavelength-dispersive X-ray spectroscopy (WDS) at the electron probe micro-analyser. EDS and WDS analysis were conducted using the following instrumentation with the following settings: Merlin SEM (Zeiss Gemini, FESEM, Luleå University of Technology, Sweden) with acceleration voltage of 20 keV and emission current of 0·9 to 1·1 nA and JEOL JXA-8200 microanalyser (Center of Microscopy and Nanotechnology, Oulu University, Finland; Table 1) with acceleration voltage of 15 keV, emission current of 15 to 30 nA and a beam diameter of 5 μm.
Standards used for EPMA/WDS analysis.
Results
Based on the total calculated sulphide content the following broad distinction was made for the drill core samples: massive sulphides (samples with >50 wt-% total sulphide content) and disseminated to semi-massive sulphides (<50 wt-% total sulphide content). The latter group is dominated by silicon (Table 2) and hence by silicate minerals (Table 3). In the following these are referred to as host rocks and geochemical tools are employed for their classification.
Average concentrations of chalcophile and lithophile elements in disseminated and massive sulphide samples
Abbreviations: Ccp – chalcopyrite, Mgt – magnetite, Po – pyrrhotite, Py – pyrite, Sp – sphalerite, felsic (altered) – felsic (altered) volcanic rocks, mafic – mafic dykes, sed – sedimentary rocks.
Number of samples.
Relative abundance of minerals in the sulphide fraction and non-sulphide fraction (xxx – major, xx – common, x – minor, (x) – trace).
Abbreviation: felsic (altered) – felsic (altered) volcanic rocks, mafic – mafic dykes, sed – sedimentary rocks, Amp – amphiboles, And – andalusite, Anh – anhydrite, Ap – apatite, Apy – arsenopyrite, Bt – biotite, Boul – boulangerite, Bour – bournonite, Cal – calcite, Carb – carbonate minerals, Ccp – chalcopyrite, Zn-Chr – Zn-rich chromite, ClayMin – phyllosilicates, Epi – epidote group minerals, Fl – fluorite, Fsp – feldspar minerals
plagioclase,
alkali-feldspar), Gd – gudmundite, Il – ilmenite, Mene – meneghinite, Mgt – magnetite, Po – pyrrhotite, Py – pyrite, Qtz – quartz, Ser – sericite, Sp – sphalerite, Ttn – titanite, Ttr – tetrahedrite.
As shown in Fig. 2, felsic volcanic and meta-sedimentary rocks form the main host to the Rockliden massive sulphide mineralisation. The felsic volcanic rocks constitute the stratigraphic footwall and will be described in more detail below. Several mafic rock types are also present, locally in close relationship and partly interfingering with the massive sulphides. Mafic rocks showing an elevated Sb content are included in this study.
Petrography and geochemistry of host rocks
From drill core logging, three main rock types are distinguished: felsic volcanic, mafic and sedimentary host rocks to the massive sulphides. Regarding the base-metal sulphide minerals, mafic rocks tend to contain more chalcopyrite than the felsic volcanic and sedimentary rocks, both of which have higher sphalerite content (Fig. 3a, cf. Table 2). The three host rock types are described in more detail below.
Mineralogical and chemical classification of Rockliden host rocks (based on 20 samples collected for this study and ca. 380 lithological reference samples from the host rock database (Exploration Department, Boliden Mines). Mineral abbreviations: Ab – albite, Adu – ‘adularia’, Ill – illite, Chl – chlorite, Py – pyrite, Ank – ankerite, Dol – dolomite, Cal – calcite, Ep – epidote a chalcopyrite(Ccp)–sphalerite(Sp)–pyrite(Py) ternary plot based on mineralogical composition of the sulphide fraction calculated with HSC Chemistry (Roine, 2009); b chemical classification based on Le Maitre et al. (1989) with violet dots representing least altered felsic volcanic rocks (based on classification by alteration box plot); c chemical classification based on Winchester and Floyed (1977); d chemical classification based on alteration box plot after Large et al. (2001); e discrimination of sedimentary rocks based on Yb–La diagram; f discrimination of mafic rocks in Cr–V diagram
Felsic volcanic rocks
The volcanic rocks cover an area of 25 km2 (Fig. 3a). The unaltered felsic volcanic rocks are characterised by porphyritic textures with variable abundance of feldspar and quartz phenocrysts. Feldspar phenocrysts range in length from 1 to 5 mm and, within feldspar-rich units, they form up to 10 area-% of the rock (Raat et al., 2011). Both plagioclase and alkali feldspar phenocrysts are identified. Quartz phenocrysts constitute generally less than 5 area-%, with a maximum size of ca. 5 mm (Raat et al., 2011). Foliation in the volcanic rocks is defined by alignment of phyllosilicates (Fig. 4b). In most cases the groundmass of these rocks is too fine to be resolved by optical microscopy (Fig. 4a and b).
Volcanic host rock and alteration types (optical microscopy, plane and cross polarised light) a volcanic host rock with feldspar (Fsp) and quartz (Qtz) phenocrysts; b slightly sericite-altered rock with quartz (Qtz) veins (minor alteration minerals are andalusite (And) and calcite (Cal)); c intensively sericite-altered rock (Ser – sericite); d alteration assemblage comprising quartz (Qtz), chlorite group minerals (Chl) and biotite (Bt); e amphibole crystals (Amp) and epidote group minerals (Ep); f quartz (Qtz), Zn-spinel (Zn-Spl, Fe-rich gahnite) and biotite (Bt)
The volcanic rocks plot mainly in the field of rhyolites and dacites in the TAS diagram (Le Maitre et al., 1989; Fig. 3b) and are mostly found in the range of rhyolites and dacites in alteration box-plots, i.e. they plot within the lower part of the box which composes analysis of the least altered rocks (cf. Large et al., 2001; Fig. 3d). The geochemical classification of the volcanic rocks is supported by the immobile trace element plot by Winchester and Floyd (1977), which shows two clusters within the rhyolitic–dacite field (Fig. 3c). However, systematic differences in the mineralogical composition, i.e. potential correlation with the geochemical classification and clustering, have not been observed during drill core logging.
All studied samples have been taken close to the massive sulphide mineralisation and the felsic rocks described below are all affected by alteration. Also, those plotting in the box of least altered rocks show features of alteration such as silicification (Fig. 4b), but remnants of alkali feldspar crystals are locally preserved. The volcanic rocks plotting outside the box of least altered rocks, i.e. AI >80 (cf. Large et al., 2001), comprise quartz and sericite (Fig. 4c) with varying amounts of phyllosilicates, partly identified as chlorite group minerals (Fig. 4d). In reference to optical microscopy, they appear to become more chlorite-rich with increasing CCPI index (Fig. 3d). Gahnite, with 7 to 8 wt-%Fe, is found locally in intensely altered samples (Fig. 4f). Amphibole minerals were documented in one sample of host rock classified as a felsic volcanic rock (Fig. 4e). Locally, the altered volcanic rocks show elevated values of Sb and are transected by quartz veins or calcite veinlets (Fig. 4b). Besides gudmundite, various Sb-bearing sulphosalts were documented with EDS analysis.
Turbiditic sediments and greywackes
The siliciclastic sedimentary rocks comprise alternations of shale and fine-grained sandstone (Fig. 5a). In contrast to the volcanic host rock, alteration is less pronounced in the sedimentary rocks. Drill cores display bleached zones (decimetre to several metres in width) of silicification in contact with the massive sulphides. However, primary sedimentary textures such as graded bedding, indications of soft-sediment deformation (Evins, 2011) and fracturing in the form of calcite and quartz veinlets were documented (Fig. 5a). In contact with the massive sulphides, the fracturing is locally accompanied by elevated concentration of Sb in drill core assays. The sedimentary rocks are mostly distinct from the (felsic) volcanic rocks in Yb–La and Cr–V trace element plots (Fig. 3e and f) and partly distinct in e.g. Al2O3/TiO2–Zr/Al2O3 plots.
Textures in sedimentary and mafic host rocks (transmitted cross polarised light and reflected light microscopy) a variation in the grain size of quartz in the sedimentary rocks. Bedding is transected by calcite veinlets (Cal); b mafic dyke with clusters of amphibole (Amp) crystals in an amphibole-biotite (Amp-Bt) dominated groundmass; c amphibole (Amp), biotite (Bt) and titanite (Ttn) crystals in a mafic dyke sample; d Zn-rich chromite (Chr) enclosed by magnetite (Mgt) and associated with amphibole (Amp) and arsenopyrite (Apy) in a mafic dyke sample
Mafic rocks
Dolerite dykes are a few decimetres to metres wide and are largely aphyric (Raat and Årebäck, 2009). They have chilled margins and 1 to 5 mm long plagioclase crystals commonly with ophitic texture, and carbonate–quartz-filled amygdules in the central parts (Raat and Årebäck, 2009). Complementary mafic rock types, termed mafic sills and dykes by Raat and Årebäck (2009), have a similar thickness as the dolerite dykes in drill core intersections, but show no distinct chilled margins. To simplify, they will be referred to as mafic dykes in this paper. Generally, the mafic dykes are composed of amphibole minerals, phyllosilicates such as biotite, and plagioclase forming a fine-grained groundmass (Fig. 5b). The grain size of amphibole varies from 50 to 500 μm (Fig. 5b and c) and chemically it is hornblende, with relatively large variation in Fe, Mg, Si and Al content (Table 4). The main carbonate mineral is calcite infilling fractures together with quartz and chlorite group minerals. Most of the mafic dyke samples contain traces of rounded Zn-rich chromite grains, with ca. 9 wt-%Zn, surrounded by Cr–V-rich magnetite rims or ilmenite rims (Fig. 5d, Table 4). In a Cr–V diagram the mafic dykes form a distinct cluster at Cr>300 ppm and V>150 ppm (Fig. 3f, Table 2). The studied mafic dyke samples show elevated concentrations of Sb (Table 2), and bournonite, meneghinite and tetrahedrite are the main Sb-bearing minerals (Table 3). More detailed research on all mafic rock units, occurring within and around the Rockliden deposit, is needed in order to give a full petrological description and to confine the timing of formation of different mafic rocks relative to the ore formation.
Selected EPMA/WDS analysis showing the compositional variation of some silicate and oxide minerals
Abbreviations: Plg – plagioclase, Amp – amphibole, Bt – biotite, Ep – epidote group, Grt–garnet group, Mgt (mafic) – magnetite in mafic dykes, Chr – chromite, felsic – altered felsic volcanic rocks, mafic – mafic dykes, MS – massive sulphide samples.
LLD – lower limit of detection.
Massive sulphide mineralisation types
The variation in chemical composition of the massive sulphide mineralisation is shown in Fig. 6a and the metal proportion of the inferred mineral resource at Rockliden (New Boliden, 2013) plot in the Zn–Pb–Cu field in the Cu–Pb–Zn ternary plot of Franklin et al. (1981).
a Zn–Cu–Pb ternary plot showing the variation of chemical composition of massive sulphide samples (424 assays with total sulphide content.50 wt-%) and the grade of the inferred mineral resource at Rockliden (New Boliden, 2013). Classification lines for massive sulphide deposits associated with volcanic rocks are taken from Franklin et al. (1981) and b calculated pyrite, chalcopyrite and sphalerite content for the mineralogical groups defined for the studied massive sulphide (33 samples)
The massive sulphides show variable textures in the drill core. Banding is due to the variation in grain size of pyrite or in the abundance of sphalerite in the groundmass surrounding the pyrite grains (Fig. 7). Locally, irregular chalcopyrite patches (referred to as chalcopyrite mesh texture, Fig. 7) or abundant host rock clasts 1 are observed in the massive sulphides.
Massive sulphide intersections in drill core (for location see Fig. 2c) Mineral abbreviation: Ccp — chalcopyrite a undisrupted; b disrupted (massive sulphide package enclosed by sedimentary rocks); c disrupted (massive sulphide intersections in contact with brecciated altered volcanic host rock at ca. 172 m and sedimentary rocks below 245 m). Mineral abbreviation: Ccp – chalcopyrite
In Fig. 7a, a continuous massive sulphide intersection is shown with a gradual change in Cu/(Cu+Zn) ratio from the sericite and chlorite altered volcanic host rock into the massive sulphides. Arsenic is found in relatively high concentrations in the semi-massive sulphides (Fig. 7a). The stratigraphic upper contact of the massive sulphide intersection with the sedimentary host rock is sharp (Fig. 7a). At this contact, the sedimentary host rock is quartz-veined and characterised by elevated Sb grade.
Most other massive sulphide intersections are disrupted by mafic dykes but also intervals of altered felsic volcanic rocks. Two of the disrupted massive sulphide profiles are shown in Fig. 7b and c. In neither profile are systematic changes in the Cu/(Cu+Zn) ratio of the massive sulphides observed. Massive sulphides showing chalcopyrite mesh textures are more common in profiles with high Cu content (compare Fig. 7a and b with Fig. 7c). Some of the interrupted massive sulphide intersections show high Ag (>100 ppm) and Sb (>1000 ppm) contents over parts or the entire massive sulphide intersections. Relatively narrow (max. 2 m wide) massive sulphide intervals with abundant host rock clasts are occasionally found (Fig. 7b) and these intervals are typically dominated by pyrrhotite.
On the basis of optical microscopy, the massive sulphides are grouped according to the dominant sulphide (and oxide) minerals, i.e. pyrite (Py), pyrrhotite (Po), magnetite (Mgt), chalcopyrite (Ccp) and sphalerite (Sp) (Fig. 6b). Some textures of the massive sulphide samples are shown in Fig. 8. It must be noted that transitions between the various groups are common; there are no well-defined boundaries between these mineralisation types.
Massive sulphides (reflected light microscopy). Mineral abbreviation: Ant – native antimony, Apy – arsenopyrite, Ccp – chalcopyrite, Gd – gudmundite, Gn – galena, Mgt – magnetite, Po – pyrrhotite, Py – pyrite, Sp – sphalerite a large fractured pyrite grain in a pyrrhotite–chalcopyrite–sphalerite groundmass and intergrowths of galena and bournonite; b augen texture of silicate minerals hosting gudmundite; c dense packing of pyrite grains; d pyrite–magnetite dominated sample with chalcopyrite meshes; e orientated texture of pyrrhotite and partly also chalcopyrite; f close association of chalcopyrite, pyrrhotite and arsenopyrite (details given at higher magnification in the insert); g pyrite showing atoll texture (white frame) and sphalerite–pyrrhotite diablastic intergrowth; h cataclastic pyrite associated with pyrrhotite, galena and native antimony
In the pyrrhotite–sphalerite and pyrrhotite–chalcopyrite groups (Po–Sp, Po–Ccp), pyrrhotite is the main sulphide mineral. Locally, isolated grains of pyrite (up to 500 μm diameter) are preserved (Fig. 8a). Distinctively high grades of more than 1000 ppm Sn are measured in clast-rich, isolated (max. 2 m width), massive sulphide intersections (Fig. 7b). The Sn-bearing oxide and sulphide minerals include cassiterite and stannite, respectively. The silicate clasts form so-called augen textures (Fig. 8b; cf. Petruk, 2000). The clasts comprise mainly quartz, biotite, chlorite and other phyllosilicates, minor epidote group minerals and feldspars. Beside silicates, carbonates are the main non-sulphide phases in samples of this group (Table 3). Zoning of dolomite group minerals is observed (back-scattered electron imaging at the SEM and EDS analysis), comprising Fe-rich cores surrounded by Fe-poor margins, which in turn, might be rimmed by calcite. Magnetite is rarely found in samples of this group.
Massive sulphide samples of the pyrite (Py) group show banding related to the clustering and grain size variation of pyrite (50–500 μm). The packing of pyrite grains locally reaches high density (Fig. 8c). The combined Cu and Zn content of this group is generally below 6 wt-% (Table 2, cf. Fig. 6b). Samples with Cu values >2 wt-% are characterised by irregular-shaped chalcopyrite±pyrrhotite meshes, imposing a deformation texture on these samples (Fig. 8d). Magnetite grains are mostly rounded to subangular and range from 100 to 200 μm in diameter (Fig. 8d). Occasionally, they contain inclusions of sulphides, mainly pyrite.
The Py–Sp–Ccp and Py–Ccp–Sp groups can be subdivided by their Zn content (Py–Sp–Ccp with Zn>5 wt-%, Py–Ccp–Sp with Zn<5 wt-%, cf. Fig. 6b). The Cu content is generally above 2 wt-% (Table 2, cf. Fig. 6b). Samples with high chalcopyrite content tend to also have relatively high pyrrhotite content. They often lack distinct banding, but deformation textures such as chalcopyrite meshes are documented (Fig. 8d–f, Fig. 7). A close association of arsenopyrite with chalcopyrite and pyrrhotite is locally observed in samples of this group (Fig. 8f). Some samples of the Py–Ccp–Sp group contain both abundant magnetite and pyrrhotite. Thus, they are allocated to a separate group, termed Py–Po–Mgt–Ccp–Sp group (Fig. 6b). However, they do not form a separate cluster in the Py–Ccp–Sp ternary plot (Fig. 6b).
The pyrite–sphalerite (Py–Sp) group has relatively low Cu content (generally below 2 wt-%), while the Zn content is relatively high (ranging from 7 to 15 wt-%, Table 2, cf. Fig. 6b). Pyrite grains often show embayments of sphalerite (atoll texture, Fig. 8g). Grain size of pyrite ranges from 200 to 500 μm (up to 1000 μm diameter) and it can host inclusions of sphalerite and occasionally galena or tetrahedrite (Fig. 8h). Despite sphalerite forming the dominant mineral embedding pyrite grains, pyrrhotite is locally present and in this case imposes a banded texture on the ore. Locally, pyrrhotite is found in diablastic intergrowth in the sphalerite groundmass or as fractures within pyrite grains (Fig. 8e and h). Magnetite is commonly found in this group.
So far, no systematic differences are noticed in the Sb mineralogy of different base-metal groups of the massive sulphides (Table 3). Based on optical microscopy it is observed that tetrahedrite, bournonite, meneghinite and boulangerite are associated with sphalerite and galena, as well as pyrrhotite and arsenopyrite, whereas gudmundite is often found associated with chalcopyrite, pyrrhotite, silicate minerals and calcite (Minz et al., 2013).
Discussion
Based on the sulphide content, massive (>50 wt-%), semi-massive (15 to 50 wt-%) and disseminated (<15 wt-%) mineralisation types are distinguished in the Rockliden deposit. The latter is dominated by silicate minerals and forms the host rock to the mineralisation. Several lithologies can be distinguished in the geochemical classification diagrams: felsic volcanic rocks, sedimentary and mafic rocks. In the following the characteristics of the studied lithologies are summarised and the distinguished host rocks are discussed, as is the mineralogy of the massive sulphides and the implications on mineral processing.
Lithological constraints on the classification of the Rockliden deposit
Several mafic rock types have been described from the Rockliden area, including gabbro intrusions, dolerite dykes and basalt (Mattsson and Heeroma, 1985; Raat and Årebäck, 2009; Depauw, 2009). Mafic rocks such as gabbro intrusions and dolerite dykes have been suggested to post-date the formation of the massive sulphides at Rockliden (Söderlund et al., 2006; Depauw, 2009). So far only one mafic rock type, referred to as mafic dykes, is studied in detail. These mafic dykes are dominated by amphiboles, and contain traces of Cr–V-rich magnetite, and can be distinguished in Cr–V diagrams (Fig. 3f). They are found in relatively narrow drill core intersections, partly located within the massive sulphides. Although they represent a volumetrically minor host rock, they are expected to influence the content of critical metals (e.g. Sb and Ag) in the deposit and in the respective flotation products. Thus, their petrographic and geochemical differentiation from other mafic rock units occurring in the Rockliden area and their relative timing in relation to the massive sulphide mineralisation, i.e. the potential overprinting effect on the mineralisation, should be studied in more detail. However, this is beyond the scope of this paper.
It is shown that the sedimentary and felsic volcanic rocks can be largely distinguished by their Yb and La contents (Fig. 3e). Given the combination of felsic volcanic and sedimentary host rocks, and the base-metal content of Rockliden (Fig. 6a; cf. Barrie et al., 1999), the mineralisation is suggested to belong to the bimodal felsic VHMS group (cf. Fig. 3c), in accordance with earlier studies by Depauw (2009) and Raat and Årebäck (2009). It is also consistent with Mattsson and Heeroma's (1985) schematic for pre-deformational zoning within the Rockliden deposit, which is typical of massive sulphide deposits associated with felsic volcanic rocks (Galley et al., 2007). The felsic volcanic rocks show spatially extensive alteration zones and are dominated by sericite and quartz with minor chlorite group minerals. The sequence of alteration zones resembles the zoning outlined by Galley et al. (2007) for the bimodal felsic VHMS group (cf. Raat and Årebäck, 2009).
Mineralogical indications of deformation and metamorphism of the massive sulphides
The metamorphic grade of the host rocks varies from lower greenschist facies, up to amphibole facies (Depauw, 2009; Raat and Årebäck, 2009). However, the geometry of the ore body is complex due to deformation and most massive sulphide intersections are disrupted by thin, partly brecciated host rock intervals. Within the massive sulphides, textural features related to metamorphism and deformation are:
banding may have been enhanced during deformation since galena, sphalerite, pyrrhotite and chalcopyrite are suggested to be relatively ductile compared to pyrite at the maximum metamorphic conditions for Rockliden (Clark and Kelly, 1973; Raat and Årebäck, 2009). However, the extent to which metamorphism is responsible for this texture is unknown, as banding might also be a primary feature or a recrystallisation event in unmetamorphosed ‘prototype’ VHMS deposits (Eldridge et al., 1983; Mattsson and Heeroma, 1985).
corrosion of pyrite grains (by sphalerite, chalcopyrite and pyrrhotite) and recrystallisation of pyrite and related changes in grain sizes has been suggested to occur during metamorphism (Petruk, 2000). However, rounding of pyrite grains and the formation of atoll textures have also been ascribed to modification by ore forming solutions (Eldridge et al., 1983). Similarly to banding the impact of metamorphism on this texture is unknown for the Rockliden massive sulphides
locally, pyrrhotite is the predominate Fe-bearing sulphide mineral in thin, massive sulphide intersections; it is found marginal to massive sulphides or enclosed by sedimentary rocks. Also, pyrrhotite is found to fill fractures in large pyrite grains (Fig. 8h). These observations are in accordance with the interpretation of pyrrhotite formation during metamorphism and deformation (Mattsson and Heeroma, 1985; Depauw, 2009). Furthermore, pyrrhotite is documented in diablastic intergrowth with sphalerite, and pyrrhotite and chalcopyrite often show foliated, elongated and schistose textures. These features were ascribed to metamorphism of massive sulphides (Petruk, 2000)
Mattsson and Heeroma (1985) suggested that the formation of magnetite is related to secondary (ore-modifying) events as they observed magnetite bands cross-cutting the banding in the massive sulphides. Similar observations were made in this study. Moreover, magnetite locally contains abundant inclusions of pyrite and pyrrhotite (Fig. 3.5e of Minz (2013)), resembling magnetite porphyroblasts, which were interpreted as metamorphic features in VHMS deposits studied by Petruk (2000).
It is suggested that metal zoning occurs on the deposit scale, i.e. Cu dominated drill core intersections become more abundant towards depth, i.e. ca. 400 m below surface (Raat and Årebäck, 2009). By correlation this would mean that textures such as the chalcopyrite meshes are also expected to be more abundant at depth, which is apparent from drill core logging. However, no clear metal zoning pattern was found and the absence of such pattern was related to structural modification and deformation at Rockliden (Minz, 2013). Moreover, the occurrence of pyrrhotite-dominated sulphide pods within largely unaltered sedimentary rocks was interpreted as remobilisation along faults cross-cutting the mineralisation, stressing the importance of deformation on the shape and mineralogy of the Rockliden massive sulphide deposit (Depauw, 2009; Fig. 2c). However, the above listed metamorphic features are observed on a microscopic scale and vary at a centimetre to metre scale. Thus, their implementation into a process-adapted geological model is challenging (cf. Minz, 2013) and evaluation in terms of exploitation has not been performed at the current stage.
Regarding the trace mineralogy, the association of Sb-bearing minerals is rather complex at Rockliden (Minz et al., 2013). Such complex associations are known also for other polymetallic sulphide ore deposits. For example, bournonite is commonly observed adjacent to galena and tetrahedrite or meneghinite (Anderson, 1940; Wen et al., 1991; Wagner and Cook, 1997). Additionally, Sb-bearing minerals are found intergrown with pyrrhotite at Rockliden (Fig. 8h). It is expected from aS2-temperature diagrams that sulphosalts such as meneghinite and bournonite are stable in the pyrrhotite field at higher temperatures, i.e. > ca. 200°C (Mookherjee and Mishra, 1984). Given these considerations on the ore formation, a hydrothermal ore modification event cannot be excluded at Rockliden. It was suggested in earlier studies that remobilisation of Sb occurred during the intrusion of dolerite dykes and a related increase in temperature (Depauw, 2009).
Mineralogical parameters in context of mineral processing
Based on the qualitative mineralogical results of this study, it is proposed that the content of chalcopyrite and sphalerite can be calculated reliably from Cu and Zn assays as they are the main carriers of the base metals in the massive sulphides. Exceptions are only Fe-rich gahnite and Zn-rich chromite for Zn and tetrahedrite, bournonite and meneghinite for Cu. In most cases, these minerals are not expected to be a significant carrier of either Cu or Zn as they are found as minor or trace minerals in the massive sulphides and host rocks. This means that the Cu and Zn content of the massive sulphides (cf. Fig. 6a), will be reflected in the variation of the chalcopyrite and sphalerite content. However, the modal mineralogy does not directly reveal textural features. Some textural features potentially affect the processing of the massive sulphides. For example pyrite–sphalerite banding will cause anisotropic mineralogical hardness contrast, and microtextures will control grain and related liberation size (cf. Butcher, 2010).
From the overlap of Py–Po–Mgt–Ccp–Sp and Py–Ccp–Sp massive sulphide groups in the ternary Py–Ccp–Sp plot (Fig. 6b); it is clear that the entire modal mineralogy cannot be calculated from chemical assays based on the limited number of elements commonly assayed in drill core. Furthermore, the direct calculation of pyrite from simple Fe assays is not possible since other Fe-bearing minerals such as magnetite occur with pyrite and pyrrhotite in the massive sulphides. The importance of quantitative information on Fe-bearing minerals in massive sulphides was illustrated for the magnetite content in ore from the Black Mountain polymetallic base metal mine, South Africa (Williams and Holtzhausen, 2001). The magnetite content was shown to influence the comminution performance by increasing the hardness of the ore, and the flotation performance by lowering the grade of valuable minerals (Williams and Holtzhausen, 2001). In this study, no systematic variations in the magnetite content have been documented at Rockliden. In a study by Mattsson and Heeroma (1985), it was found that magnetite is more abundant in massive sulphides from the northwestern part of the deposit. The overall magnetite content in the Rockliden ore is expected to be low compared to the Black Mountain ore and might not affect the processing of the Rockliden massive sulphides significantly (Bolin, pers. comm., 2013). However, the pyrite content and the silicate mineralogy might have a significant impact in ore processing, e.g. due to hardness and grindability characteristics of the ore and therefore controlling the plant throughput.
A relationship between the Sb mineralogy and the rock types of Rockliden has been proposed whereby the mafic dykes contain mostly bournonite, meneghinite or tetrahedrite; the altered felsic volcanic rocks contain locally gudmundite and various Sb-bearing sulphosalts; and the massive sulphides contain tetrahedrite, bournonite or gudmundite as the main Sb-bearing trace minerals (Minz et al., 2013). There are also indications that the Sb mineralogy of the massive sulphides might change with depth, i.e. below ca. 400 m below surface (cf. Minz et al., 2013), and towards the northwest of the Rockliden deposit (Mattsson and Heeroma, 1985). Generally, the Sb-bearing minerals are associated with base-metal minerals and this association will have an influence in the comminution characteristics of the ore and their distribution during flotation (Minz et al., 2013).
Conclusions
Based on the total sulphide content of whole rock samples, it is possible to distinguish between disseminated to semi-massive and massive sulphides at the Rockliden deposit. The former comprise the lithological units which are hosting the massive sulphides, including felsic volcanic and sedimentary rocks and mafic dykes. Felsic volcanic and sedimentary rocks form the main host rocks. The altered felsic volcanic rocks show locally elevated concentrations in Sb, often connected to quartz veining in the host rock. Among the mafic rock types, one unit, referred to as mafic dykes, shows elevated concentrations in Sb and, although it is found in relatively narrow intervals, it is concluded to be an important rock type when the Sb distribution of the Rockliden ore is considered.
Massive sulphide samples are classified based on their chalcopyrite (Ccp), sphalerite (Sp), pyrite (Py), pyrrhotite (Po) and magnetite (Mgt) content. The following groups were distinguished: Po–Sp, Po–Ccp, Py, Py–Po–Mgt–Ccp–Sp, Py–Ccp–Sp, Py–Sp–Ccp and Py–Sp. The content of pyrite, pyrrhotite and magnetite is expected to be important to the ore processing. However, this cannot be directly calculated from drill core assays. The chalcopyrite and sphalerite content is expected to be approximated from chemical assays. However, ore textures cannot be subtracted from chemical assays. Textures which are suggested to be related to deformation and metamorphism include banding and variation of grain size and shape of pyrite on mm to μm-scale, pyrrhotite-filled fractures in pyrite, diablastic intergrowth of pyrrhotite and sphalerite, schistose textures and foliation of pyrrhotite and chalcopyrite, and magnetite with abundant sulphide inclusions. In order to evaluate the impact of these textures on the ore processing they would need to be quantified using (automated) microscopic tools.
Systematic differences in the trace mineralogy, especially the Sb-bearing fraction of the massive sulphides, could not be determined by qualitative ore characterisation and the usage of automated SEM-based mineralogical tools is suggested for the quantification of the Sb mineralogy within different rock and ore types. Further, the content of the Sb-bearing minerals as well as their association is expected to control their distribution during flotation and by extent the Sb grade of the Cu–Pb concentrate (cf. Minz et al., 2013). The Sb grade of this flotation product will decide whether it has to be further treated by hydrometallurgical methods, such as alkaline sulphide leaching (cf. Awe, 2013), or during pyrometallurgical processing. In either case the production planning would benefit from the forecast of the Sb grade of the Cu–Pb concentrate. Thus, determining and measuring the rock- and ore-intrinsic parameters, which control the distribution of the Sb-bearing minerals, is needed to develop a model predicting the Sb grade of the Cu–Pb concentrate (Minz et al., 2013). SEM-based methods (automated mineralogy) should lead to the quantification of these trace minerals. Furthermore, quantifying and understanding the variation in the Sb mineralogy in relation to lithological units and structural features (potentially favouring hydrothermal modification) will help in predicting the Sb-mineralogy of the ore and plant feed based on a geological model.
Footnotes
Acknowledgements
This study is part of a PhD project financed by CAMM (Centre of Advanced Mining and Metallurgy). New Boliden AB is acknowledged for financing the analytical work in this study, and for permission to publish this paper. ALS Minerals Division (Piteå, Sweden) is thanked for financial support for preparation of chemical assays and analytical work. Hein Raat (Boliden Mines, Exploration Department) is thanked for advice about the geology of Rockliden and the personnel in the Boliden drill core archive for their help with finding and mounting drill cores which made it possible to obtain samples in an efficient way.
The authors also acknowledge the comments and corrections of three anonymous reviewers and our colleague Riia Chmielowski.
1
Rounded up to 5 cm wide non-sulphide spots within the massive sulphides. Please note that the term is not used in a strict sedimentary sense. As it is used here it has no implication on rock formation.