Fluid inclusions in sphalerite as negative crystals: a

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The secondary fluid inclusions, healing fractures in sphalerite crystals mainly at 160-185 °C, are strongly controlled by the thickness .... vertical defect lines. h) Negative crystals, an enlarged detail of f). .... Clark & Kelly (1973), sphalerite is readily deformed plasti- .... even distribution and the same crystallographic faceting by.
Eur. J. Mineral. 2002, 14, 607–620

Fluid inclusions in sphalerite as negative crystals: a case study IVAN K. BONEV1 and KALIN KOUZMANOV2 1

Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria e-mail: [email protected] 2 Sofia University, Faculty of Geology and Geography, 1000 Sofia, Bulgaria

Abstract: The crystal morphology of vacuoles of fluid inclusions in some transparent, low-iron and relatively low-temperature sphalerites from the vein and replacement Pb-Zn ore deposits of the Madan ore district, Bulgaria, was studied both by optical microscopy in oriented thin sections and by SEM on open cleavage surfaces. The vacuoles represent negative crystals bounded by faces of d{110}, o{111} and -o{111} forms together with small faces of a{100}, e{210}, n{211}, p{221}, -n{211}and -p{221} forms. Their corners and edges are rounded. The primary inclusions, formed mainly at 200-220 °C, have an isometric, elongated, tubular, spindle-like or acute-angled habit depending on their position and the influence of the growth defects (growth zones, subgrain and sector boundaries, twin planes, etc.). The secondary fluid inclusions, healing fractures in sphalerite crystals mainly at 160-185 °C, are strongly controlled by the thickness and local configuration of the crack surfaces. They are flat, irregular or isometric in shape. It is suggested that the fluid inclusions in the low-T sphalerite usually preserve their initial high-energy and non-equilibrium crystal morphology formed during periods of rapid growth. Key-words: sphalerite, fluid inclusions, negative crystals, crystal morphology, Madan ore district.

Introduction Fluid inclusions, common in transparent gangue minerals of hydrothermal ores, can also be found in some opaque sulphide minerals such as galena (Bonev, 1977), stibnite and bournonite (Lüders, 1996), pyrite (Lüders & Ziemann, 1999; Kouzmanov et al., 1999), enargite (Mancano & Campbel, 1995; Bonev et al., 2000), etc. IR microscopy, SEM, and other methods have to be applied for their study. Light-coloured transparent sphalerite is a rare case of sulphides, which can be studied by using the standard optical microscopy along with the SEM methods. Fluid inclusions in sphalerite have been known and examined at a number of Pb-Zn-Ag-deposits such as Providencia (Mexico), Casapalca (Peru), Creede (USA) or Cerro del Toro (Spain) (Sawkins, 1964; Rye & Sawkins, 1974; Roedder, 1977, 1984; Morales-Ruano et al., 1996). In general, these studies concern the homogenisation temperature (Th), salinity, and partly, the chemistry and stable isotopes. Only fragmentary data are available about the morphology of inclusion cavities, and the manner of their formation has not been fully accounted for. MincÏ eva-Stefanova & Veselinov (1981) gave the first detailed morphological description of such fluid inclusions seen as densely located open cavities on rough surfaces of crystal fragments of high-temperature sphalerite. These inclusions are considered to be of primary origin. The faceted vacuoles of fluid inclusions are usually reDOI: 10.1127/0935-1221/2002/0014-0607

ferred to as negative crystals (Laemmlein, 1973; Kern, 1987; Sunagawa, 1987). It is assumed that after originally taking the primary (more or less irregular) shape, the fluid inclusions usually transform it through dissolution and recrystallisation into the equilibrium crystal shape of minimum surface free energy (under definite P-T conditions). This is why Roedder (1968, 1971, 1984) considered the shape of most fluid inclusions to be a “transient feature of little diagnostic value”. However, Roedder (1984) mentioned that this statement is not universal. It can be expected that at low T, fast cooling, and for highly insoluble phases, as a result of the slow recrystallisation rate, the inclusions and especially the larger ones, can preserve their original metastable shape. Sunagawa (1987, p. 567) emphasised that the morphologies of negative crystals had not been studied systematically, particularly in relation to equilibrium morphology, so that further efforts were required for their understanding. Our aim was to perform a case study on the crystallography and mechanisms of formation of vacuoles of fluid inclusions in transparent, relatively low-temperature undeformed sphalerite crystals having high symmetry and well preserved growth banding. The abundance of primary and secondary fluid inclusions in these crystals enables a comparative characterisation of their morphology and origin. Some preliminary results were presented at the ECROFI XIV Symposium in Nancy, France (Bonev & Kouzmanov, 1997). 0935-1221/02/0014-0607 $ 6.30 2002 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

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Sphalerite samples Sphalerite from the Tertiary (30-35 Ma), economically important vein and replacement lead-zinc deposits of the Madan ore district in the Central Rhodope Mts., Bulgaria, were sampled where well-developed sphalerite crystals can often be found ( MincÏ eva-Stefanova, 1973; Bonev, 1982; Kolkovski et al., 1996). Most samples come from the Mogilata, Ossikovo, Shoumachevski Dol, Strashimir, and Erma mines. The crystals of the most widespread, non-transparent, brown, iron-bearing early sphalerite generation, have a tetrahedral or pseudo-octahedral habit. They were formed at a rather high temperature (Th 350-280 °C, determined by homogenisation of fluid inclusions in contemporaneous quartz), and at low hydrostatic pressure (of the order of 100 bar) in open vein cavities. The late, lower-temperature (< 220 °C) sphalerite generation, which is the object of this study, is represented by clear and banded, light brown, yellow or green sphalerite of cleiophane type. The crystals, reaching 2-3 cm or more (up to 10 cm), have a dodecahedral habit bounded by the rhombododecahedron d{110}, both tetrahedra, the positive

o{111} and the negative -o{111}, and some rarer forms like a{100}, e{210}, n{211}, -n{211}, p{221} and -p{221}. Their Fe content is low, e.g., 0.3-2.0 wt.% for the yellow sphalerite, and about 0.1% or lower for the green sphalerite whose colour is related to some Co content ( MincÏ eva-Stefanova et al., 1983). The sphalerite usually contains abundant primary and secondary fluid inclusions of various faceting and habit, size, and position.

Methods of study Doubly polished oriented sections cut parallel to (111), (110), or (100), and coming across the centre or parallel to some crystal face, were used for optical microscopic studies. Mostly, sphalerite exhibits distinct optical anisotropy, thus helping to visualise fine internal defects such as growth banding, lamellar twinning and grid-iron textures similar to those described by More et al. (1991). Due to the high refraction index and total reflection, the fluid inclusions have thick, dark contours so that details of their morphology are not visible under an optical microscope. Microthermometric measurements were performed on the same samples. An USGS-type microthermometric stage mounted on an Olympus microscope was used. The reproducibility of the ice melting and bulk homogenisation temperature was 0.1 °C and 1 °C, respectively. The surface morphology of the fluid-inclusion cavities was examined by SEM on (110) cleavage planes after mechanical opening and carbon coating (Fig. 1). It was interpreted by means of oriented crystal projections constructed by using Dowty’s SHAPE programme (1989). Additional gold coating was applied for more detailed observation of the surface structure.

General characteristics of the fluid inclusions Inclusion cavities

Fig. 1. a) The flat (110) face of a tabular secondary inclusion opened by decrepitation, with small isometric and dendritic salt crystals of halite and sylvine epitaxically deposited from the evaporated fluid. Transmitted light. b) Enlarged view of the dendritic salt deposits. SEM.

The inclusion cavities represent negative crystals (Fig. 1a and 2) generally bounded by the most important sphalerite forms: the positive o{111} and the smaller or equal negative -o{111} tetrahedra, together with the dodecahedron d{110} and the cube a{100}. Additional small faces of n{211}, -n{211}, and e{210} are usually developed next to the cube faces, and p{221} and -p{221} at the transition between o and d faces. Due to the hemihedry of sphalerite and the polarity of its [111] axes, the two tetrahedra have different surface structures (Komatsu & Sunagawa, 1965; Kern, 1987). The outward surface of the positive tetrahedron o{111}, terminating with Zn atoms, shows triangular stepped faces, whereas the negative -o{111} tetrahedron, terminating with S atoms, is smooth. Reciprocity exists between the tetrahedral faces of the negative-crystal inclusions and the embracing crystal: the negative -o faces of the inclusions (Fig. 2) correspond to the positive o faces bounding the host crystal over them. These observations are in agreement with the data of MincÏ eva-Stefanova & Veselinov (1981). However, many rare

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Fig. 2. Idealized morphology of negative crystals: a) with complex morphology and o > -o; b) and c) with simple morphology and o > -o (b) or o -o (c). Crystal forms: d{110}, o{111}, -o{111}, a{100}, e{210}, n{211}, -n{211}, p{122}, -p{122}, etc. The edges and corners are usually slightly rounded.

Fig. 3. Microthermometry of primary and secondary fluid inclusions in sphalerite. a) Temperature of homogenisation (Th). b) Temperature of melting of ice (Tm ice). c) Th versus Tm ice diagram.

forms with high indices mentioned by these authors, e.g., of the {hk0}, {hkk} or {hhl} type, were not identified in our SEM studies. The negative-crystal inclusions have rounded edges and corners. Surrounded by such surfaces, the flat faces of the inclusions also have curved angles (Fig. 1a). Nature of the fluid After splitting mechanically a sphalerite crystal, or as a result of its decrepitation, the solution liberated from the opened fluid inclusions evaporates rapidly, epitaxically precipitating small cubic and branched dendritic salt crystals on the freshly cleaved (110) surface (Fig. 1a, b). They were

identified by EDS analyses as halite, NaCl (strongly prevailing), and sylvite, KCl (minor). Two epitaxic orientations were clearly recognised: (110)[110] halite and sylvite // (110)[110] sphalerite, (001)[110] halite and sylvite // (110)[110] sphalerite. Slightly increased Ca content was also registered around the precipitates, though no specific Ca phases were observed. Due to the complicated morphology and high refraction of sphalerite, microthermometric measurements were possible only in relatively flat fluid inclusions, transparent in transmitted light. Both types of inclusions, the primary and secondary ones, are aqueous, two-phase, with a vapour-toliquid ratio not higher than 0.30, without daughter minerals.

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Fig. 4. Primary fluid inclusions in the growth pyramid. Microphotographs in transmitted light. a) A twinned striated natural (110) face intersected by a transverse (111) twin plane (horizontal, in fact, a narrow band of three parallel twin planes). b) Orientation of the negative crystals-inclusions in two adjacent twin areas (tw – twin plane as traced on a): view in (110) – 1 and 3; view in (111) – 2 and 4. Note that twinning in sphalerite requires 180° rotation around the polar three-fold [111] axis, and not reflection. Forms developed are: d{110}, o{111}, -o{111}, a{100}. c) Inclusions in a deeper (110) growth zone just below the surface in a). d) and e) Enlarged details of c). f) A transverse section of the same crystal // to the (111) twin plane. g) The same but focused on the twin plane, in which the inclusions are elongated following vertical defect lines. h) Negative crystals, an enlarged detail of f).

All inclusions homogenise into a liquid phase during heating. No evidence for boiling was found. The microthermometric determination of the homogenisation temperature (Th) for primary inclusions is within the range of 200-220 °C, rarely to 230 °C, as shown in Fig. 3a. A slight decrease in Th is observed from the core to the rim in some zoned crystals (e.g., from 215 °C to 206 °C in the

case shown in Fig. 5a). The melting temperature of ice (Tm ice) for primary inclusions is between -6.1 °C and -3.4 °C and mostly within the -3.8 °C to -3.4 °C range (Fig. 3b). Such values characterise fluids of relatively low salinity, about 5-6 wt.% NaCl equiv. The first melting temperatures for primary inclusions range from -24.6 °C to -24.2 °C. These values are to some extent lower than the eutectic point

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Fig. 5. a) Zoned distribution of primary fluid inclusions (zones 1-4) in the growth pyramid of a large crystal. A (110) section with the [110] growth direction upward. Partly crossed polars. b) Pseudo-octahedral habit of an inclusion, as seen along the [100] direction. c) Elongated inclusion in the (110) plane, a detail of zone 2 on a). d) An acute-angled fluid two-phase inclusion trapped between the subparallel blocks of a mosaic crystal. e) Large fluid inclusions (black) elongated along (111) twin planes (vertical). Partly crossed polars. f) Surface morphology of an opened primary inclusion from these in zone 1 on a). SEM.

(Te) of the NaCl–KCl–H2O system (-22.9 °C, Sterner et al., 1988), thus implying the presence of other salts in the fluid as well. Most probably, these are minor amounts of CaCl2, as was mentioned earlier. Piperov et al. (1977), analysing the liquid phase of primary fluid inclusions in galena from the same deposits by means of atomic-absorption spectrophotometry, determined its low salinity (up to 5 wt.% NaCl equiv.) and Na-K-Ca composition at a molar ratio of approx. 10:2:1. The presence of CO2 in the gas bubble was registered by Raman microspectroscopy. The low Fermi resonance splitting of 103 cm-1 between the bands at 1388 cm-1 and 1285 cm-1, registered at room temperature conditions, determine the low density of CO2 (~0.18 g/cm3) in the gas phase (Rosso & Bodnar, 1995). The measured Th for secondary inclusions varies mainly between 160 °C and 185 °C (Fig. 3a). The Tm ice for these inclusions ranges from -5.4 °C to -0.7 °C, thus determining a wider interval of salinities, i.e., from 1.2 up to 8.4 wt.% NaCl equiv. On a Tm/Th plot (Fig. 3c) the primary and secondary inclusions form clearly different areas. Dobrovols-

kaya et al. (1979) and Krusteva & Gadzeva (1986) determined similar and lower Th in sphalerite from the Shoumachevski Dol and Erma mines, though without distinction between primary and secondary inclusions.

1. Primary fluid inclusions Morphology and location Various primary inclusions have been distinguished according to their morphology and position inside sphalerite. Isometric inclusions arranged along growth zones are most abundant. They are bounded by d, o, -o, and small a faces, and exhibit mostly a pseudo-octahedral-rhombodo decahedral habit with o -o. In some cases the positive tetrahedral faces are predominant. The fluid inclusions are up to 100 µm or more in size. A typical case of zoned distribution of isometric inclusions in a growth pyramid of yellow sphalerite (up to 1-2 wt.% Fe and 0.5 % Cd) is presented in Fig. 4a-h, in two perpendicular sections, parallel

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Fig. 6. Primary fluid inclusions of the growth pyramid and in a (111) plane below the crystal face. a) Giant, channel-like inclusions elongated in [112], nearly perpendicular to the [110] growth striation. b) and c) Enlarged striated bottom faces of the macroinclusions in a). A trapped tiny bilateral quartz crystal (Q) is also seen. d) A group of [112] tube-like and isometric inclusions near a large fluid inclusion. Bottom faces of the inclusions display unchanged fragments of the conserved growth striation (horizontal steps), when the top walls are flat. e) and f) Rounded surfaces of an opened large inclusion with some small arched flat (110) terraces. The solid crystals trapped in the lower part of the inclusion are identified as galena (GA) and tennantite (TN). a)-d): transmitted light; e) and f): SEM. [Inset: The position of fluid inclusions elongated in the [112] direction of advancing [110] surface growth macrosteps].

and perpendicular to (110). It can clearly be seen that the twinning is due to a 180° rotation around the polar [111] axis, and not to the reflection in a mirror plane. The (111) inclusion side faces have their own triangular striation due to layer growth. Single, randomly distributed, primary large inclusions also occur. Spindle-like or partly flattened are some inclusions from the same zone (Fig. 4g), they adhere to the perpendicularly

located (111) twin plane, or follow cross and oblique (at 60°) defect lines – narrow bands of intersection with {111} equivalent planes. Elongated along the vertical twin planes and partly irregular are the inclusions shown in Fig. 5e. Spindle-like inclusions occur sometimes along the boundaries between neighbouring (111) and (110) growth pyramids. Thus, the planar or linear defects controlling location of some primary inclusions also modify their shape.

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Depressions on crystal faces

Fig. 7. The striated tetrahedral face of a brown sphalerite crystal with two oval depressions (arrows) indicating the locations of fluid inclusions below the surface.

Isometric fluid inclusions in four growth zones have been observed in another crystal (Fig. 5a) of green, fine-banded sphalerite with 0.12 wt.% Fe. The size of the inclusions is uniform in each zone, but decreases from 200-500 µm in zone 1, to 10-15 µm in the last peripheral zone 4. The negative crystals in zones 1 and 4 exhibit a nearly perfect octahedral-dodecahedral habit (Fig. 5b). The inclusions in zones 2 and 3 are uniformly elongated in an oblique direction (Fig. 5c) due to unequal development of the two tetrahedral forms. Large rounded areas occur between the flat faces (Fig. 5f). These four successive crystal zones have been formed under slightly decreasing temperature (probably isobaric) conditions, the measured Th (from 1 to 4) being 215-210-208-20 6 °C. For the different localities and druse cavities in the area, the crystal size and zoning are not identical due to some local changes in the environment. Enormous channel-like elongated fluid inclusions occur in the (111) growth pyramids of some large yellow crystals. They are elongated in the [112] direction, nearly perpendicular to the [110] growth striation (Fig. 6a). Their basal o planes inherit the macrostepped structure of the face, slightly rising to the face centre, and the side faces are rounded or rough (Fig. 6b, c, e, f). The largest of these inclusions are up to several mm long (maximum 7 mm!). Smaller, flat-elongated inclusions in (111) planes are associated with them but on a different level. They also follow the [112] direction (Fig. 6d), and their basal o planes are also striated. Solid inclusions, up to 100-120 µm in size, were found in some large fluid inclusions. The solid phases identified by SEM and EDS are: bilateral prismatic quartz (Fig. 6b), cubo-octahedral galena and tetrahedral zincian tennantite with 8.8 % Zn (Fig. 6f). Evidently, these crystals were trapped accidentally and are not true daughter minerals. Some small irregular fluid inclusions are related to the frontal parts of tiny solid chalcopyrite inclusions deposited on (111) sphalerite growth zones, as described by Sawkins (1964) for the Providencia mine. Acute-angled irregular inclusions occur trapped between the blocks of some mosaic crystals and bounded by their flat tetrahedral faces (Fig. 5d).

A remarkable phenomenon are the face depressions, up to 1-2 mm in diameter, related to large isometric primary inclusions just below the sphalerite crystal surface (Fig. 7). They were found only on some brown, tetrahedral crystals of the early higher-temperature sphalerite from the Mogilata deposit, formed at Th 270-310 °C, as measured in the paragenetic quartz. The depressions have been formed by plastic deformation of the covering surface layer due to internal pressure reduction of the trapped fluid during isobaric cooling. Despite this irreversible deformation, the inclusions have not imploded and have preserved their fluid filling. The inclusions located more deeply inside the crystal have not been distorted. As was established experimentally by Clark & Kelly (1973), sphalerite is readily deformed plastically at a relatively higher T, with ductile-brittle transition and strength reversal at 250-300 °C. This explains why such depressions were not observed in the lower-temperature light sphalerite described above. It should be noted that similar surface depressions related to fluid inclusions are characteristic of the highly ductile galena crystals of the same deposits. As it was shown (Bonev, 1977), in such an aqueous inclusion with concentation of 5 wt.% NaCl, trapped at 310 °C, by isobaric cooling to 200 °C, due to difference in the liquid/solid volume reduction, the internal pressure can drop to 25 bar. Thus, the resultant outer pressure of about 75 bar acting inwards, is enough to cause deformation of the still ductile covering layer of the vacuole.

Crystallisation on the walls and “ghost”-inclusions Some large fluid-inclusion cavities, as shown in Fig. 8a and b, are surrounded by an outer, closely disposed, dark zone of chalcopyrite dust from which tiny, µm-sized chalcopyrite needles and “bead chains” developed in the enclosing sphalerite. They are arranged along parallel (111) twin planes, and along oblique [110] dislocation lines, clearly decorating them, being absent far from this zone. The solid inclusions are a typical manifestation of the “chalcopyrite disease”, generally interpreted to be the result of diffusion and solidstate reaction within the sphalerite matrix (Barton & Bethke, 1987; Barton, 1991; Bonev & Radulova, 1994). Their formation is obviously controlled by the structural defects in sphalerite. The long-lived open channel-like cavities have been in contact with the solution reservoir during the short time of chalcopyrite deposition. The sphalerite crystallisation on the cavity walls continued for some more time until the closure of the inclusion without any disturbance of the primary habit. An incomplete thin chalcopyrite aureole can be seen in the small fluid inclusion in Fig. 8c, due to its opening for only a short time. Around some other adjacent fluid inclusions, closed at that time, as around the secondary inclusions, chalcopyrite formations are absent. Another specific case of crystallisation on the walls is the full healing of an open inclusion cavity before its closing (Fig. 8d). Fine chalcopyrite precipitates on the walls mark

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Fig. 8. Microphotographs of primary fluid and solid inclusions in sphalerite. a) and b) Vacuoles of large fluid inclusions (black) in a (110) section outlined by an outer dark zone of piled chalcopyrite dust and needles (“chalcopyrite disease”) controlled by (111) twins (horizontal lines) and [110] dislocations (oblique). c) Similar inclusion in a (111) section. d) Inclusion-“ghost” infilled by sphalerite, with boundaries marked by black chalcopyrite inclusions (focused on two levels). A diagonal cleavage plane intersects both the matrix and the inclusion.

the original outlines of such a ghost-inclusion again. An oblique cross-cutting cleavage plane indicates its crystalline state.

2. Secondary fluid inclusions Structure of the fractures in sphalerite The secondary inclusions are always attached to healed fractures in sphalerite. More detailed information about the geometry of such fractures can be obtained by SEM observation on freshly broken crystal surfaces. Such studies showed that the fracture surfaces consist of flat and rounded portions of variable structure depending on their particular crystallographic orientation. Several characteristic areas occurred: – Flat areas of rhombododecahedral perfect (110) cleavage planes with only rare steps. – Stepped or striated areas formed by two systems of intersecting {110} cleavage planes at an angle of 120° and with step direction [111] = (110):(101). – Kinked areas formed by the intersection of three different

{110} cleavage planes and consisting of uniform small triangular pyramids. When the three pyramidal faces are equal, the whole surface approximates a rough tetrahedral (111) face. However, smooth {111} areas cannot be found. – Rounded areas with smooth shell-like oval surfaces and highly variable spatial position sometimes complicated by concentric arched, flat (110) terraces. Since the characteristic {111} rotation twinning in sphalerite maintains the parallel position of the (110) planes, a (110) cleavage surface usually extends as a continuous plane intersecting the adjacent parts of the twinned crystal. However, the twin domains can be recognised on this surface as bands with different orientation of striations on both sides of the twin suture (at an angle of 54½° = [110]:[100]). The large sphalerite crystals contain healed internal fractures of different orientations. Morphology of the secondary inclusions The secondary fluid inclusions are formed by the same crystallographic forms as the primary ones (Fig. 2): o{111}, -o{111}, d{110}, a{100}, n{211}, -n{211}, e{210},

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Fig. 9. SEM images of secondary fluid inclusions revealed on a (110) cleavage crystal surface. a) Isometric and slightly elongated in [111] direction inclusions. b) Irregular and [111] elongated tabular inclusions. c) Idealized morphology of the negative crystal. d-h) Diversely distorted inclusions bounded by the same flat surfaces and rounded edges. The morphology is influenced also by (111) twinning (horizontal planes in d).

Fig. 10. Secondary fluid inclusions healing a thin cleavage crack in sphalerite. Microphotographs in transmitted light. a) Irregular, elongated and platy inclusions along the (110) cleavage plane. b) Tubular inclusions elongated in [110] direction. c) Dot-like inclusions on a wavy cleavage surface.

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Fig. 11. a) SEM micrograph of densely spaced secondary fluid inclusions as seen on a healed and freshly re-opened cleavage surface in sphalerite. Continuation of the surface can be seen optically below the undisturbed transparent upper covering crystal part, preserved and displayed on the right side of the picture (black and smooth area). All unopened inclusions there have two-phase liquidgas filling. The preserved vacuole of a considerably larger inclusion of primary origin (PI), cut by the same fracture (and secondary refilled), can also be seen. b) and c) Details of a) showing the uniform crystallographic faceting of the secondary inclusions as dependent on their local position on the uneven fracture surface.

p{221} and -p{221}. An idealised projection of a negative crystal is shown also in Fig. 9c. However, the real habit of inclusions frequently deviates from the ideal, structurally determined isometric one due to the influence of some morphological characteristics of the healed fractures (Fig. 911). Several morphological types of secondary fluid inclusions can be distinguished: The large flat tabular inclusions (Fig. 1a and 9a, b, g) are most typical. They have two large, parallel smooth d faces developed in the cleavage plane. The side walls are smaller transverse and oblique d faces together with o and -o faces. Small cubic faces a, surrounded by n, -n and e, form the crystal surface around the four-fold axes, and p and -p, the transition between tetrahedral and rhombododecahedral faces. Typical are the rounded edges and corners of the inclusion cavities, and as a result the flat single faces have curved angles as well (Fig. 1a). The flat inclusions have isometric outlines or are slightly elongated, e.g., in a [111] (Fig. 9e, f) or [110] (Fig. 9g) direction. Frequently, they are much larger and irregular (Fig. 9b) or are straight linear

along [110] or [111]. The size of the flat inclusions ranges from 20-50 µm, sometimes to 100 µm and more. Twinning planes intersecting them clearly influence their morphology with the development of jagged o:o surfaces (Fig. 9d). The thin tube-like inclusions arranged in the (110) cleavage plane are rarer, usually forming groups with uniform [110] orientation due to the intersection of the tetrahedral faces (Fig. 10a and b). These inclusions are up to 100-200 µm long and a few µm wide. Abundant are also very small, dot-like isometric inclusions (Fig. 10c and 9h). A typical example of a complex healed fracture (fluid-inclusion plane) in a large green sphalerite crystal from the Shoumachevski Dol deposit, as seen under the SEM after its partial mechanical re-opening, is shown in Fig. 11. This plane, cross-cutting the growth zones, is undoubtedly of epigenetic origin. It contains numerous small uniform pits of even distribution and the same crystallographic faceting by d, o and -o forms. However, the shape of each single fluid inclusion depends on its local position over the uneven fracture surface. Inclusions around the convex edges are more

Fluid inclusions in sphalerite as negative crystals

irregular in shape (Fig. 11c – centre). The size of the isometric fluid inclusions is within the range of 15-40 µm, sometimes reaching 50-60 µm (Fig. 11b – upper left). The distance between them is about 2-3 times their diameter. Some tiny inclusions, one order smaller (some µm in diameter), occur beside the larger ones. The quite similar numerous small cavities on such a rugged sphalerite surface, that were described by MincÏ eva-Stefanova & Veselinov (1981) as primary fluid inclusions, are most probably of secondary origin as in the case discussed above. Over the last years fluid-inclusion planes (FIP) have been extensively studied by microscopic methods in rock-forming quartz and the statistical measurement of their spatial orientation has been developed as an effective method for determining the stress field causing rock deformations (Cathelineau et al., 1994). In a quasi-isotropic granite these planes crosscut numerous randomly oriented grains not essentially influenced by the crystallographic anisotropy of the single quartz grains with their indistinct cleavage. On the contrary, the fractures in single sphalerite crystals are strongly controlled crystallographically and physically owing to the perfect {110} cleavage.

Comparison between primary and secondary inclusions A comparison between the primary and secondary fluid inclusions in sphalerite (Table 1) shows that along with the similarities, some clear differences exist in their morphology, dimensions, distribution, etc. All these characteristics, coinciding well with Roedder’s criteria (1984), are useful for distinguishing between the primary and secondary nature of fluid inclusions.

Discussion – Mechanism of formation Modern crystal-growth theories (Chernov, 1984; Sunagawa, 1987; Bennema, 1993) agree that the surface of a growing polyhedral crystal is bounded by interfaces of varied atomic roughness, which determines the different growth mechanisms and growth rates thus controlling the crystal morphology. Conversely, from the macroscopic growth forms it is possible to make conclusions about their atomic growth mechanisms (Chernov, 1984). The habits determining smooth faces are formed by the layer-growth mechanism at low supersaturation, whereas the rough surfaces (e.g., stepped, kinked or rounded), occurred at increased supersaturation (“kinetic roughening”), and grow by a continuous mechanism with a high normal growth rate. The negative crystals, representing cavities of fluid inclusions and also bounded by crystal faces, show a clear reciprocity with respect to the ordinary polyhedral crystals since the growth of the faces leads to a decrease in the volume of the negative crystal. As was shown above, the sphalerite crystals and the fluid inclusions in them are formed from relatively low T and low-salinity aqueous solutions in a regime of slightly decreasing T, i.e., 220-200 °C for the primary, and 185-160 °C

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for the secondary ones. The available data do not establish considerable differences in the fluids in different morphological types of inclusions, which often occur together. Therefore, it is likely that the morphological differences are related mostly to the specific growth mechanisms. Primary fluid inclusions The fine growth banding in the large crystals is characteristic of layer growth at low supersaturation with small alternating changes in crystallisation conditions, whereas the attachment of primary inclusions to some distinct zones (e.g., Fig. 4, 5a) points to short episodes of some disturbance in the morphological stability (Chernov, 1984; Sunagawa, 1987), and transient skeletal growth of the flat faces at increased supersaturation and hampered diffusion. Some rough superficial growth defects like pits, grooves and striations arising in such episodes are transformed into fluid inclusions by their subsequent covering during the following episode of calm layer growth (Roedder, 1984). The straight linear banding suggests that this surface relief is usually a growth, not a dissolution feature, although formation of etch pits and grooves in some cases cannot be excluded. Similar crystallographically specific surface growth defects were observed uncovered on some natural sphalerite crystal faces as well, e.g.: – On o{111} faces: triangle pyramidal pits; layer growth macrosteps and striation; [112] grooves; oriented re-entrant contact ridges of oblique twin planes; subgrain boundaries; the -o faces are usually smooth. – On d{110} faces: pyramidal pits and grooves bounded by four d, and two tetrahedral -o and o faces, with or without a flat bottom d face; face striation. – On a{100} faces: pyramidal pits of four oblique d faces and two o faces. Incomplete covering of some channel-like grooves by overhanging rather thick layers (up to 0.3 µm height) was also observed on natural sphalerite o faces. Chernov (1984) discusses the role of overhanging macrosteps in the fluid-inclusion formation. From a genetic point of view it is important to evaluate the possible morphological changes in the inclusions after their trapping. The following observations are relevant: 1) The bottom faces of adjacent fluid inclusions in a {111} growth sector (Fig. 6d, and inset) show uniform parallel [110] oscillatory striation with a lower symmetry than their own face symmetry (3m). In fact, these are small “fossilised” fragments of the striated (111) face with its characteristic triangular pattern (Komatsu & Sunagawa, 1965) composed of growth macrosteps elevated towards the face centre. The covering upper faces of these negative crystals are flat and smooth -o planes. Similar relationships were also found in other low-temperature sphalerite, e.g., from Creede, USA (see Pl. 8, Fig. 1 in Roedder, 1972) and Cerro del Toro, Spain (Fig. 2B in Morales-Ruano et al., 1996). No visible signs of dissolution or morphological changes on the straight linear steps can be observed directly after heating of inclusions, as documented by Roedder (1972) on the cited figure as well.

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Table 1. Primary and secondary fluid inclusions in the low-iron sphalerite – a comparison. Main characteris- Primary fluid inclutics sions

Secondary fluid inclusions

Morphology Main crystal forms Additional forms

negative crystals o{111}, -o{111}, d{110} a{100}, n{211}, -n{211}, e{210}, p{221}, -p{221} rounded

negative crystals o{111}, -o{111}, d{110} a{100}, n{211}, -n{211}, e{210}, p{221}, -p{221} rounded

curved angles

curved angles

Edges and corners Outlines of the flat faces Habit

isometric, spindle- and channellike, flat (// to o or d faces), irregular, acuteangled Size large: 100-200 µm and more Occurrence groups in growth zones; single – random, or along: twin planes, sector boundaries, subgrain boundaries, dislocations, beyond solid inclusions Distance between 3-5 diameters or much inclusions larger Fluids aqueous, 2-phase (L+V) Daughter miner- none als Accidental solid quartz, galena, tennaninclusions tite Th, °C 199-230, mainly 200220, mean 208.2 ± 4.7 (n = 85) Tm ice, °C -3.8 to -3.4, mean -3.7 ± 0.7 Salinity (wt.% 5-6 NaCl eq.) CO2 ~ 0.18 kbar Pressure ca. 100 bar Peculiar cases - chalcopyrite overgrowths, - cavity infilling -“ghosts” Growth mecha- - transient periods of nisms and stages skeletal growth, arising of formation of pits, - closing by overlying layers Subsequent insignificant changes

flat (// to (110) cleavages), tubular (along [110]), isometric

relatively small: 5-1050 µm groups in healed crosscutting fractures composed of different areas: flat (cleavages), stepped, kinked, rounded areas 1-3 diameters aqueous, 2-phase (L+V) none no found 146-184, mainly 160185, mean 176.3 ± 5.1 (n = 53) -5.4 to -0.7, mean -4.1 ±1.2 1.2-8.4 no precise data ca. 100 bar re-filled primary inclusions

- rapid normal growth of stepped and rounded areas, - slow layer growth of the flat cleavage areas insignificant

2) The specific zoning around some fluid inclusions marked by an earlier zone of fine solid chalcopyrite inclusions (Fig. 8a-c) also proves that no morphological changes have been performed after inclusion trapping. 3) Many inclusions, especially the largest ones, have large rounded non-singular surfaces with sometimes small arch-like flat terraces (Fig. 6e) or concave areas. There are no signs of changes in these highly non-equilibrium surfaces of high surface energy. 4) The linear position of the zoned primary fluid inclusions does not suggest any subsequent changes. It can be concluded that after being trapped in this lowtemperature low-iron sphalerite, the fluid inclusions maintained their own original morphology and position. Most probably, the rounding of edges and corners of a negative crystal was completed just before its closing, when the reentrant corners acted as intensive layer sources. In some cases the whole inclusion cavities were infilled by sphalerite (the “ghost”-inclusion in Fig. 8d). Secondary fluid inclusions The healing of cross-cutting open fractures with the formation of secondary fluid inclusions, i.e., negative crystals, is an important case of regenerative crystal growth. This process is realised from low-salinity (a few wt.%) fluids at a relatively low temperature (mainly 185-160 °C) and most probably under conditions of low supersaturation. The fractures inside the cooled brittle sphalerite crystals are usually formed as a result of internal or external stresses, e.g., by interaction with adjacent crystals of different thermal expansion coefficients, etc., as generally discussed by Völkl (1994). The geometry and width of a fracture is determined by the real crystal morphology of the two opposite complementary cleavage surfaces and by their mutual displacement. The specific regenerative crystal growth differs considerably from the usual layer growth of the flat-faced polyhedral crystals (see e.g., Chernov, 1984; Sunagawa, 1987) due to the enormous number of steps generated by the fracturing. They operate as active layer sources and growing sites as long as the link with the solution reservoir is not interrupted. The various surface areas of the fracture walls grow by different mechanisms and with different rates: – The curved (atomically rough) areas grow by a continuous growth mechanism advancing in a normal direction at the highest rate. In the course of time they can be faceted by low-index flat planes of lower surface energy. – The kinked and stepped areas, having a large number of reentrant angles as active growing sites, also grow at a rather high rate. – The flat (110) cleavage areas with smooth surfaces grow by a layer mechanism with tangential spreading of the layers originating from the available rare single steps. Thus, the flat character of these slowly growing areas is preserved. Due to such differentiation, the fast-growing rounded and kinked areas of the opposite fracture walls are closed first, entrapping many, usually small and more isometric fluid inclusions (Fig. 12). In the slowly growing flat sections of

Fluid inclusions in sphalerite as negative crystals

619

n{211}, p{221}, etc.; and b) rounded non-singular surfaces binding the flat faces instead of sharp edges and corners. 2) The location, morphology, and size of the vacuoles is controlled: a) for primary inclusions, by the roughness and defects on the growing face (steps, elongated channels, twinning and sector sutures), and by the crystallographic specificity of the different growth pyramids; b) for the secondary inclusions, by the crystallographic orientation, morphology and width of the healed epigenetic fractures inside the host sphalerite. 3) Fluid inclusions are formed in a regime of decreasing temperatures: 220-200 °C for the primary, and 185-160 °C for the secondary inclusions. 4) The position and metastable non-equilibrium morphology of the vacuoles are preserved largely unchanged after their trapping, in the same way as the unaltered finecompositional metastable growth zoning of the embedding sphalerite crystals.

Fig. 12. Successive stages (a-c) of regenerative crystallisation and formation of secondary fluid inclusions by healing of a cleavage fracture in sphalerite. A scheme.

fractures, larger and mostly flat inclusions arise. The tubelike inclusions are formed in the fine-stepped areas. The vacuoles of secondary fluid inclusions thus formed are negative crystals with unequal development of crystallographically equivalent faces, elongated, flat and highly irregular shapes, and large rounded surfaces. These features characterise a metastable, rather non-equilibrium surface configuration of large surface energy and suggest preservation, to a large degree, of the original inclusion shape owing to kinetic restrictions. Processes of “necking-down”, known for high-temperatures and high-soluble crystals (Laemmlein, 1973; Roedder, 1984) have not been of any importance at lower and gradually declining temperatures, especially for such a highly refractory sulphide as sphalerite (Barton, 1991). This was proved by the uniform microthermometric determinations of neighbouring and synchronous inclusions.

Conclusions The systematic studies of fluid inclusions in transparent low-T sphalerite show that: 1) The vacuoles of fluid inclusions represent well-formed negative crystals bounded by: a) flat, low-index, slowly growing faces, including both tetrahedra o{111} and -o{111}, d{110}, and additional small faces of a{100},

Acknowledgements: Authors are indebted to J. Dubessy, E.A.J. Burke, C. Marignac, A.M. Van den Kerkhof, N.B. Piperov and I. Veselinov for the very useful comments and suggestions. A. Dikov and K. Peryov are thanked for some donated samples. This work was supported by the Bulgarian National Science Foundation, by a grant to K. Kouzmanov from the University of Geneva (Institutional exchange programme between the Universities of Sofia and Geneva), and a grant for visit from the GEODE programme.

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