Fluid and Mineral Inclusions and Inclusion Zones of Cave Calcite from Korsnäs Mine, Western Finland

FLUID AND MINERAL INCLUSIONS AND INCLUSION ZONES OF CAVE CALCITE FROM KORSNÄS MINE, WESTERN FINLAND

PENTTI REHTIJARVI and KARI A. KINNUNEN

REHTI JÄRVI, PENTTI and KINNUNEN, KARI A. 1979: Fluid and mineral inclusions and inclusion zones of cave calcite from Korsnäs Mine, western Finland. Bull. Geol. Soc. Finland 51, 75—79. Inclusions and crystal forms of calcite crystals from a cave in the Korsnäs lead-lanthanide mine in western Finland have been studied. The primary fluid inclusions in the scalenohedral yellowish calcite show homogenization temperatures of 79° to 103°C and salinities of 3.6 to 4.9 eq. wt. °/o NaCl. The prevailing crystal form is scaleno- hedron. In some specimens it reveals a rhombohedral phantom. Four mineralization stages are proposed on the basis of correlation between inclusion zones in multiple-zoned calcite crystals. Pentti Rehtijärvi and Kari A. Kinnunen, Geological Survey of Fin- land, SF-02150 Espoo 15, Finland.

Introduction K/Ar-method (Dr. Eric Welin, written com- munication in 1975). The isotopic composit- During mining operations in the Korsnäs ion of the galena indicates an age of 1770 lead-lanthanide mine (closed 1972), situated Ma for the sulphide ore (Isokangas 1975). about 30 km south of the town of Vaasa in The cave mineralization was studied by western Finland, a cave was found 190 m collecting information from the fluid and below the earth's surface (Tuominen 1961). The cave (0.5 m X 1.5 m X 30 m) is part of mineral inclusions of different zones in indi- a vertical fracture zone. Its walls were vidual calcite crystals. On the basis of the partly covered by euhedral crystals of calcite, mode of occurrence of the inclusions an in- harmotome, apophyllite, pyrite and apatite; clusion »stratigraphy» was established that the main mineral was calcite. A small amount is comparable to the sphalerite »stratigraphy» of euhedral quartz and ancylite was also described by Roedder (1977) as a result of a found. Apophyllite and harmotome have study of sphalerite zoning and paragenesis been described by Sahama (1965) and Sahama at Creede, Colorado. The calcite »strati- and Lehtinen (1967). An age of 88 Ma was graphy» established gives an overall view of determined for yellow apophyllite by the the cave mineralization in the Korsnäs mine. 76 Pentti Rehtijärvi and Kari A. Kinnunen

PREVAILING INCLUSION

CRYSTAL FORMS ZONES SOM E

OF IN O F MINERAL S STAG E CALCITE CALCITE INERALIZATIO N M ORDE R CAV E CRYSTALLIZATIO N

PROPOSED INDICES FOR THE CRYSTAL FACES OF CALCITE :

a a (121) (302) b V \ i 9 // b (511) h (100) — D c (101) i (010) d (883) j (041 ) e (001) k (561) HARMOTOM E f (102) 1 (011 ) 0 o

? 3.

iiiniiiiiii — 1 1 1 EXPLANATION FOR THE 1 1 1 INCLUSION ZONES IN CALCITE:

9 APOPHYLLIT E lTr( \ J 2 O PRIMARY FLUID INCUSIONS III PSEUDOSECONDARY FLUID INCUSIONS h k \ C ll / \ I PYRITE

A APOPHYLLITE HARMOTOM E

* REDEPOSITED CLAY

n—i—n f^Vj MONTMORILLONITE - CHLORITE ^^ CLAY LAYER K^ C \ • • B d \

Fig. 1. The proposed mineralization stages, prevailing crystal forms of calcite, inclusion zones in calcite and the suggested crystallization order of the main cave minerals.

Materials and methods logy. Some of these yellowish translucent scalenohedrons contain a white pyrite-cov- Thirty euhedral calcite crystals, varying ered rhombohedral phantom marking the in length from 2 to 7 cm, were examined. earlier growth stage of the crystal. Indi- Calcite shows parallel growth, and most of vidual rhombohedral calcite crystals were the specimens exhibit scalenohedral morpho- also found. A few of the calcite crystals are Fluid and mineral inclusions ... 77 yellow and either transparent or translucent. individual calcite crystals. In mineralization The transparent crystals are characterized stage A the pyrite inclusions simulate a cube; by a basal plane. In some crystals the pyra- in stage B they are elongated cubes. The midal faces are curved. The three prevail- calcite generation of mineralization stage C ing crystal forms are depicted in Fig. 1. contains pyrite inclusions showing parallel The homogenization temperatures of the growth. The oldest pyrite generation in this fluid inclusions were determined with a mineralization stage is a cube whose growth Leitz 350 heating stage and a Chaixmeca mi- started with the formation of the (112) crystal crothermometry apparatus. The Chaixmeca face. In the following zones of stage C, pyrite exhibits parallel growth along the stage was also used for the freezing point triad axis. The habit of pyrite in the fifth determinations. The temperature readings and sixth inclusion zones is similar to that were calibrated with organic melting point of the first. The youngest pyrite of stage C standards. The accuracy of the temperature shows parallel growth along the triad axis. measurements was ± 2°C, at 50° to 150°C, The pyrite in the youngest calcite from stage and ± 0.1°C at —5° to 0°C. D is combination of a cube and a pentagonal dodecahedron.

Mineralization stages Apophyllite and clay are found in three different inclusion zones (Fig. 1). Primary The cave mineralization was divided into fluid inclusions were usually formed in these zones when pyrite, apophyllite or clayey ma- four stages on the basis of the mineral para- terial disturbed the growth of the crystal genesis and the prevailing crystal forms of faces of calcite. Calcite crystals of similar calcite (see Fig. 1). Mineralization stage A habit have similar outermost inclusion zones, is characterized by the crystallization of which confirms their contemporaneous cry- montmorillonite-chlorite clay, stage B by the stallization. crystallization of rhombohedral calcite and Pseudosecondary fluid inclusions charac- pyrite, stage C by the crystallization of scale- terize two of the inclusion layers situated be- nohedral calcite, harmotome and apophyllite tween the mineralization stages. At the end and stage D by the crystallization of yellow- of stage B white calcite with a rough surface ish scalenohedral calcite. The crystallization had crystallized over the rhombohedron in order of the cave minerals was inferred from a layer about 0.5 mm thick that was found the mineral inclusion zones in calcite crys- to contain appreciable pseudosecondary fluid tals, the mineral crystallizing in the cave inclusions. Before the youngest calcite gen- often having been trapped inside growing eration crystallized, a fracture layer about calcite. 0.2 mm thick developed on some of the faces of the calcite crystals. Pseudosecondary fluid inclusions are abundant in these fractures Inclusion zones as well.

The crystal habit of pyrite is distinct and hence characteristic in the different inclusion Fluid inclusions zones of the proposed mineralization stages (Fig. 1). This observation was utilized in The primary fluid inclusions in calcite were correlating the fluid inclusion zones found in found to be concentrated in specific inclusion 78 Pentti Rehtijärvi and Kari A. Kinnunen

inclusions was —1.9° to -—2.7°C, correspond- ing to a salinity of 3.6 to 4.9 wt.%> NaCl equivalent. During cooling many of the in- I clusions decrepitated when ice was crystallized at about — 50°C. The homogenization temperature of about 600 secondary fluid inclusions in the calcite of mineralization stages B and C ranges from 50°C to about 80°C. In addition, micro- cracks have been found to radiate from some of the primary fluid inclusion cavities of mineralizing stage D. These cavities contain secondary fluid filling with a freezing temperature of about 0.0°C.

Fig. 2. Photomicrograph of typical primary fluid inclusions with liquid (L) and vapour (V) in yellowish scalenohedral calcite; homogenization Conclusions temperatures 92°C, 93°C and 88°C; freezing temperatures —2.4°C, —2.4°C and —2.3°C correspond- ing to salinities of 4.5, 4.5 and 4.3 eq. wt. °/o NaCl. The study of fluid and mineral inclusions Transmitted light, one nicol, bar length 10 /

References

Holland, H. D. (1967) Gangue minerals in hydro- Roedder, E. (1971) Metastability in fluid inclusions. thermal deposits. Pp. 382—436 in Geochem- Soc. Min. Geol. Japan, Spec. Issue 3, 327—334. istry of hydrothermal ore deposits, ed. by — (1977) Fluid inclusions as tools in mineral Barnes, H. L.; Rinehart and Winston Inc., exploration. Econ. Geol. 72, 503—525. New York. Sahama, Th. G. (1965) Yellow apophyllite from Isokangas, P. (1975) The mineral deposits of Fin- Korsnäs, Finland. Mineral. Mag. 34, 406— land. Phil.Lie-thesis, Dept. of Geology, Uni- 415. versity of Helsinki, 128 p. — and Lehtinen M. (1967) Harmotome from Kors- Potter, R. W., II. (1977) Pressure corrections for näs, Finland. Mineral. Mag. 36, 444—448. fluid-inclusion homogenization temperatures Tuominen, H. (1961) Outokumpu Oy, Korsnäsin based on the volumetric properties of the kaivos. English summary. Vuoriteollisuus 19 system NaCl-H=0. U.S. Geol. Survey Jour. (2), 23—33. Research 5, 603—607. Manuscript received, April 11, 1979