Summary:
Greisen formation is associated with the cooling of a highly fractionated H2O-rich granitic intrusion and the enrichment of incompatible volatile elements in the upper part of the intrusion such as F, Cl, B and Li during fractional crystallization.
The metasomatic greisen formation, called greisenization, is defined as a granite-related, post-magmatic process during which biotite and K- / Na-feldspars became unstable (ŠTEMPROK, 1987 [153]). Subsequently to Na-feldspathization it is commonly controlled by the further decrease of the alkali / H+ ratios (PIRAJNO, 2009 [141]). Granite minerals and textures are replaced by complex aggregates of micas, quartz, topaz, and fluorite with a considerable addition of some elements such as Sn, W, Li, Mo, Be and others. Highly aggressive, Fbearing solutions induce the formation of fluoride minerals, which are compared to other metasomatic rocks very common in greisen (ROMER et al., 2010 [146]). Greisenization can affect different wall rocks. Its intensity depends basically on the texture of the protolith.
A broad range of formation temperatures between 250°C – 500 °C and pressures of 0.3 kbar – 0.8 kbar is suggested by POLLARD, 1983 [142] for the formation of greisen minerals. Latest fluid inclusion studies indicate that all elements required for the formation of the mineralization at Zinnwald were contained in a single magmatic hydrothermal fluid that underwent two main processes, fluid rock interaction and depressuration (KORGES et al, 2017 [139]). The authors recorded homogenization temperatures of various generations of fluid inclusions ranging from 490°C to about 300 °C. These numbers are in good agreement with older data that have indicated an average homogenization temperature of 389°C ± 28 °C gained from two-phase fluid inclusions in quartz, Limica, cassiterite and fluorite of albite granite, stockscheider and veins from Zinnwald (UHLIG, 1992 [126]).
Styles of Mineralization
Greisen type mineralization at the Zinnwald / Cínovec deposit is related to flat dipping, sheet-like greisen ore bodies and veins in the apical part of a geochemically highly evolved granitic intrusion. Lithium, tin, and tungsten mineralization is potentially economic and occurs mainly as quartz-mica greisen.
Exploration at Zinnwald has defined a Li-Sn-W greisen deposit in several stacked continuous bodies with a dimension of 1.6 x 1.5 km on the German territory (corner points according to Gauss-Krueger coordinate system: 5,412,400; 5,622,650 – 5,414,000; 5,624,150). The deposit reaches from 200 m a.s.l. up to 850 m a.s.l..
Individual greisen beds show a vertical thickness between less than 1 m and more than 40 m.
No other areas of significant mineralization are known at present at the Zinnwald property, but surface exposures and drillings indicate various preliminary investigated or untested anomalies in the vicinity. Li-Sn-W- (Mo) mineralization is also known to exist to the north at the Altenberg “Zwitterstock” deposit. Furthermore, a Sn-W-Nb-Ta mineralization was intersected by drilling in the south-eastern portion of the deposit (NEßLER, 2017 [115], NEßLER et al., 2018 [116]).
1. The Zinnwald / Cínovec greisen deposit and subordinately the TR can be characterized by a number of different mineralization styles. The most important include: Independent or vein adjoining greisen bodies
II. Flat dipping veins ( “Flöze”)
III. Subvertical dipping veins (“Morgengänge”) IV. Metaalbite granite Sn-W-(Nb-Ta) mineralization
The vast majority of lithium and portions of the tin and tungsten mineralization within the Zinnwald / Cínovec granite stock can be found in the metasomatic greisen ore bodies (style I). The position of greisen mineralization is a result of late- to post-magmatic fluids, infiltrating the uppermost part of the granite stock. They were distributed in dependence on the granite’s joint system along cracks and intergranular pathways. Consequently, faults and joints played an important role in the dispersal of mineralizing fluids throughout the cupola. According to investigations of BOLDUAN & LÄCHELT, 1960 [104] and BESSER & KÜHNE, 1989 [110] greisenization as well as the development of the “Flöze” is closely linked to the flat dipping L-joints, representing cracks and joints resulting from the volume loss of the granite during cooling and crystallization. Areas of cross-cutting L-joints and sub-vertical faults / joints are considered to be favourable for the development of particular thick bodies of greisen mineralization.
Mineralization styles II and III are of subordinate importance for lithium but are well mineralized with cassiterite, wolframite and minor scheelite and played therefore an important role during historic mining. The predominant part of this resource was exploited in the past. Subordinate amounts of zinnwaldite can be found in the flat dipping veins (style II) along the endo- and exocontact of the deposit, where it forms selvages of very coarse grained zinnwaldite (up to 50 mm). Detailed information on veining in the deposit will be presented in Item 7.3.3.
Mineralization style IV represents an unusual type of ore mineralization in the Zinnwald deposit and will be discussed in the following chapter based on geological, mineralogical and geochemical findings.
Independent or vein adjoining greisen bodies
The lithium ore mineralization of the Zinnwald property is closely linked to the existence of metasomatic greisen ore bodies that are located at the endo-contact of the uppermost parts of the ZG stock (style I). They form curved, stacked and lensoidal compact greisen bodies that can be highly irregular in shape but commonly exhibit a larger horizontal and limited vertical extend. The presence of stock-like greisen, reported in literature (e.g., BOLDUAN & LÄCHELT, 1960 [104]), remains disputable owing to the lack of prove by drilling intersections. However, maximum intersected greisen thickness was about 44 m (ZGLi 06A/2013). This style of greisen mineralization occurs in the central uppermost part and along the flanks of the ZG and follows with subparallel dip the morphology of the granite’s surface. Frequency and thickness generally decrease with depth. True thickness of greisen bodies is consequently consistent with the vertical depth for the central parts where the dip angle is less than 10°. Towards the gently inclined (10° - 30°) flanks of the N, E and S and a steeply inclined (40° - 70°) W-flank the true vertical thickness needs to be recalculated, respectively. On average, thickness of potentially mineable greisen bodies in the property area is between 2 m and 15 m.
Other greisenized lithologies
Zinnwaldite is not restricted solely to greisen ore bodies. Subsequent greisenization affected various rock types of the ZG cupola and adjacent wall rocks to a different degree. Therefore, the term “greisenized” is used for rocks that are not completely transformed into a greisen, meaning that they exhibit remnants of feldspar. In terms of volume the ZAG is by far the most influenced lithology. Progressive greisenization produced an enormous amount of greisenized ZAG that exhibits typical features, e.g., beginning replacement of feldspar by the growth of metablastic quartz and zinnwaldite as well as advanced argillic, sericitic and haematitic alteration. A continuous succession of rocks that underwent a progressive metasomatic overprint can be described as follows:
Unmodified ZAG à slightly greisenized ZAG à intensely greisenized ZAG à greisen
Metaalbite granite Sn-W(-Nb-Ta) mineralization
Moderate to intermediate greisenization of albite granite associated with significant mineralization of Sn-, Wand Nb-Ta-oxides (style IV) represents an unusual mineralization style of the Zinnwald deposit. Spatially independent from major greisen ore bodies this style is characterised by greisenized albite granite of common appearance but with a disseminated ore mineraliza