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Hydrothermal Zeolite Occurrence from the Smrekovec Mt. Area, Slovenia, Yugoslavia
Hydrothermal Zeolite Occurrence from the Smrekovec Mt. Area, Slovenia, Yugoslavia P. Kovic and N. Krosl-Kuscer Geological Survey of Ljubljana, Parmova 37, 61000 Ljubljana, Yugoslavia Hydrothermal waters emanating from an andesitic magma affected the pyroclastics seated on deeper levels below the sea floor. Consequently, albite, laumontite and subordinate prehnite were widely developed as the replacements of the primary constituents, as interstitial fillings and as vein minerals. Analcime occurs locally, predominantly at the expense of preexisting albite and laumontite. Its appearance can be ascribed to the effects.of heated marine water or its admixtures to hydrothermal fluids when the previously affected rocks were displaced near or at the sea floor. INTRODUCTION In the region of NE Slovenia, the Mesozoic Era closed and the Cenozoic Era opened with the Alpine orogeny which produced extensive deposits of terrigenous sediments and volcanic rocks and the principal structural features younger than Paleozoic. A first manifestation of the orogeny was uplift and it was closey followed by volcanism producing large masses of extrusive igneous material, accumulated in the bordering Panonian Sea which was a part of the Para tethys basin [10J. The center of volcanic activity was in the Smrekovec Mt. area (Fig. 1), where an over 600 m thick sequence [8] of explosive and non-explosive volcanic rocks occurs. The outcroping volcanics are encountered as andesites, andesitic pillow lavas, autobrecciated andesites, pyroclastic breccias and tuffaceous rocks [4].
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Fig. 1. - Map showing the location of Slovenia and the Smrekovec Mt.area.
87
88 (GM-S-3) Pyroclastics from the study area have undergone the changes in mineralogy, characterized by the development of zeolites and related silicate minerals predominantly upon hydrothermal conditions. Some of the investigated authigenic mineral assemblages are rather complex. Their relationship seems to offer the possibility of development by more than one contemporaneous and subsequent processes including: - the activity of hydrothermal fluids of occasionally modified ionic composition, - the activity of marine water when the pyroclastics were found at the sea floor, and - the variability of temperature and pressure related to the uplift as mentioned above. COMMON MINERAL ASSEMBLAGES Authigenic mineralization was initiated in the early stages of tectonic and volcanic activities when hydrothermal waters emanating from an andesitic magma affected the pyroclastics seated on deeper levels below the sea floor. Consequently, albite, laumontite and subordinate prehnite were widely developed as the replacements of the primary constituents, i.e. pyrogenetic plagioclases, volcanic lithic fragments and a fine-grained matrix, as interstitial fillings and as vein minerals. When replacing the primary constituents, this mineral assemblage rarely exceeds 20 wt.% of the bulk rock composition and it is mainly accompanied by quartz, smectite, chlorite and sphene. Plagioclases are largely albitized or altered to albite and prehnite while the replacements by laumontite are rare in occurrence. The intensity of zeolitization of the pyroclastics from the Smrekovec Mt. area may be profoundly different. It was influenced by the distance from the source of hydrothermal waters and by porosity and permeability of sediments (and rocks). The latter relationship is reflected in an enhanced hydrothermal alteration of the coarser pyroclastics compared to that in the finer ones where zeolites occur as vein and fissure fillings only. Furthermore, the distribution of authigenic minerals in the pyroclastic rocks is commonly unequal and for this reason the resulting mottled textures can frequently be observed. Chemical and physical conditions in hydrothermal fluids were an important factor controlling the processes of mineral modification. In the early stage of hydrothermal activity being discussed, minor replacements of prehnite by laumontite occured (Fig. 2). Surdam [11] has reported that the stabili~ies of prehnite and laumontite can be explained in terms of the activities of Ca +, Si0 and H+ ions in 2 the aqueous phase. Accordingly, the replaceme~ts as previously mentioned can also be related to the decreased activity ratio Ca +/H+ and/or the increased activity Moreover, the supposedly modified ionic composition of hydrothermal fluof Si0 2• ids was likely accompanied by lowered temperatures due to constant cooling of this geothermal system.
Fig. 2. - Prehnite (Pr) and laumontite (L) from a lapilli tuff. A, In plane-polarized light prehnite shows a dusty appearance and supposedly, it is partially replaced by laumontite. B, Under crossed polars the less common fibrous form of laumontite is revealed.
P. Kovic and N. Krosl-Kuscer
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In the northern margin of the Smrekovec Mt. area analcime occurs, predominantly at the expense of albite and laumontite being developed in the preceding hydrothermal episode as discussed above (Fig. 3,4,5). The local character of the analcime appearance is reflected by its restricted vertical and horizontal distribution range. The rocks intensively altered to analcime and pqyllosilicatc minerals (Fig.4 and 5) rapidly grade upward in the pyroclastics of an identical texture where albite and laumontite are completely preserved. The horizontal distibution of analcime is characterized by imperfections of· the mineral transformations (Fig.3). A relatively small extent of the described alteration processes can be attributed to a locally modified ionic composition of hydrothermal fluids and to fairly reduced porosity and permeability of the rocks which have undergone the preceding stage of hydrothermal mineralization.
Fig. 3. - Albite, laumontite and analcime from a lapilli tuff. A, In the photograph taken under crossed polars albite (Ab) replacing volcanic lithic ) can be seen. The central part of the field of view is by albite which is partially altered to analcime (An). B, The view under crossed polars reveals laumontite (L) being corroded by analcime (An). The reaction from analcime to albite in the aqueous medium was experimentally studied by Coombs et al (2), Campbell and Fyfe 11] , Thompson [13] and Liou (7) • The dissolved mineral substances reduce the chemical activity of water in fluids and thus lower the temperature of the equilibrium with respect to that in pure water at the same fluid pressure (Campbell and Fyfe (1) , Iijima (5) , Surdam and Boles [12) ). Furthermore, it was reported by Coombs et al.[2] and Wilkinson and Whetten (14) that analcimes found in sedimentary and
90 (GM-5-3) burial rocks are commonly rich in silicon and poorer in aluminum and sodium than the stoichiometric composition NaAlSi 206.H20.
Fig. 4. - Specimen from a lapilli tuff largely altered to analcime and pqy~llicate minerals. Preexisting albite occuring as the replacement of the volcanic lithic fragment (L ) has been inverted to analcime (An) and quartz (Q). Ab - theVremnants of albite. A, In plane-polarized light quartz stands out clearly in relief against the analcime with which it is intergrown. 5, The same as A, under crossed polars. The analcime can be seen to be slightly birefringent. The plagioclase grain (PI) which has been replaced by albite and prehnite (Pr) in the preceding hydrothermal episode is also mainly altered to analcime. Among tne complexly modified chemical conditions in hydrothermal fluids controlling the reactions from albite- and laumontite to analcime the increased activities of Na+, water and HCO- are the most obvious. The evaluation of microanalysis data is not easy since albite and laumontite are often incompletely altered to analcime, thus a wide range of partial mineral transformations occurs. Their detail study is
Kovie and
N.
Krosl-Kuscer
91
the sUbject of our future investigations. In general it can be suggested that the analcimes developed at the expense of albite sometimes are enriched with respect to silicon whereas the tendency of the excess silica to be stabilized as quartz is largely restricted to the strongly affected pyroclastic rocks (Fig. 4). In analcimes replacing minor interstitial substitutions of 2Na+ for are evident. The + ions could have reacted to form calcite (Fig. 5) or being removed from the system.
Fig. 5. - Specimen from a lapilli tuff predominantly replaced by analcime and pqy~ilicate minerals. Preexisting laumontite is inverted to analcime (An) and calcite (Ca). Au - relic augite, Pl - plagioclase grain altered to analcime, L - remnants of laumontite. A, The view in plane-polarized light. B, The same as previous, under crossed polars. Considering t2e reactions being discussed, a marine water seems to be an apparent source of Na + and HCO- ions. The Alpine orogeny reached a,structural climax in the early Neogene when ehe Smrekovec Mt. area was already uplifted on land. For this reason the analcime development can be ascribed to the effects of heated marine water or its admixtures to hydrothermal fluids when the previously affected pyroclastics were displaced near or at the sea floor. Clinoptilolite, occasionally accompanied by analcime exclusively replaces the overlain dacitic rocks which are localized in occurrence. The intermediate character of plagioclases is completely preserved and none of the high-temperature minerals including albite, laumontite and prehnite was developed. Evidence suggests the genesis of clinoptilolite either in the distal parts of this hydrothermal system during the early stage of authigenic mineralization or in the latter stage as discussed above. The pyroclastics displaced in a marine environment were likely subjected to alteration processes being reflected in an extensive further development of the smectite-like phyllosilicateminerals. The zeolites from the study area, occuring as vein and fracture fillings include laumontite, analcime, stilbite [91 and yugawaralite. Laumontite is the dominant vein mineral and it was obviously developed before and after the analcime appearance. We suppose that the Smrekovec Mt. area was already uplifted on land before the cessation of hydrothermal activity. The tectonic event as mentioned above, originated widespread fissure systems in which laumontite was precipitated in the latest stage of authigenic mineralization.
92 (GM-5-3) CONCLUSION The Tertiary volcanism in the Smrekovec Mt. area related to the Alpine orogeny produced large masses of explosive igneous material deposited in a shallow-water submarine environment of the bordering Panonian Sea. The most prominent tectonic event was uplift and it was accompanied by hydrothermal activity which took place over a relatively long period of time. This is 'evidenced by the presence of zeolites and related silicate minerals in the pyroclastic rocks from the study area. Authigenic mineralization was initiated in the early stages of tectonic and volcanic activities when hydrothermal waters emanating from an andesitic magma affected the pyroclastics seated on deeper levels below the sea floor. Consequently, albite,laumontite and subordinate prehnite were widely developed as a high- to moderate-temperature (6) mineral assemblage, replacing the primary constituents and filling interstices, veins and fissures. Analcime occurs locally, predominantly at the expense of preexisting albite and laumontite. Its appearance can be ascribed to the effects of heated marine water or its admixtures to hydrothermal waters when the pyroclastics, affected in the preceding hydrothermal episode, have been displaced near or at the sea floor. The Smrekovec Mt. area was already uplifted on land before the cessation of hydrothermal activity. In the latest stage of authigenic mineralization laumontite was precipitated in the widespread fissure systems originated by the tectonic movements as mentioned above. REFERENCE 1. A.S. Campbell and W.S. Fyfe, Am.J.Sci., 263:807-816 (1965). 2. D.S. Coombs, A.J. Ellis, W.S. Fyfe and A.M. Tylor, Geochim. Cosmochim. Acta, 17:53-107 (1959). 3. D.S. Coombs and J.T. Whetten, Geol.Soc.Am.Bull., 78:269-282 (1967). 4. A. Hinterlechner-Ravnik and M. Pleniear, Geologija, 10:219-236 (1967). 5. A. Iijima, Nat. Zeolites (F.A. Mumpton Ed), 175-189 (1976). 6. A. Iijima, Proc.Fifth Int.Conf.Zeol. (L.V. Rees Ed.), 103-118 (1980). 7. J.G. Liou, Lithos 4:385-402 (1971). 8. P. Mioe, OGK Slov. Gradec, L 33-35 (1978). 9. V. Osterc, Proe. Eight Yug.Geol.Conf., 199-207 (1974). 10. M. Pleniear, Geologija (A. RamovB Ed.), 175-188, 1978 11. R.C. Surdam, Geol.Soc.Am.Bull. 84:1911-1921 (1973). 12. R.C. Surdam and J.R. Boles, Asp.Diag. (P.A. Scholle and P.R. Schluger Ed.), Soc.Ec.Pal.Min.Spec.Pub.26, 227-242 (1979). 13. A.B. Thompson, Con.Min.Petr., 33:145-161 (1971). 14. J.F.T. Wilkinson and Whetten, J.Sed.Petr., 34:548-553 (1964).
YUGOSLAVIA o
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Fig. 1. - Map showing the location of Slovenia and the Smrekovec Mt.area.
87
88 (GM-S-3) Pyroclastics from the study area have undergone the changes in mineralogy, characterized by the development of zeolites and related silicate minerals predominantly upon hydrothermal conditions. Some of the investigated authigenic mineral assemblages are rather complex. Their relationship seems to offer the possibility of development by more than one contemporaneous and subsequent processes including: - the activity of hydrothermal fluids of occasionally modified ionic composition, - the activity of marine water when the pyroclastics were found at the sea floor, and - the variability of temperature and pressure related to the uplift as mentioned above. COMMON MINERAL ASSEMBLAGES Authigenic mineralization was initiated in the early stages of tectonic and volcanic activities when hydrothermal waters emanating from an andesitic magma affected the pyroclastics seated on deeper levels below the sea floor. Consequently, albite, laumontite and subordinate prehnite were widely developed as the replacements of the primary constituents, i.e. pyrogenetic plagioclases, volcanic lithic fragments and a fine-grained matrix, as interstitial fillings and as vein minerals. When replacing the primary constituents, this mineral assemblage rarely exceeds 20 wt.% of the bulk rock composition and it is mainly accompanied by quartz, smectite, chlorite and sphene. Plagioclases are largely albitized or altered to albite and prehnite while the replacements by laumontite are rare in occurrence. The intensity of zeolitization of the pyroclastics from the Smrekovec Mt. area may be profoundly different. It was influenced by the distance from the source of hydrothermal waters and by porosity and permeability of sediments (and rocks). The latter relationship is reflected in an enhanced hydrothermal alteration of the coarser pyroclastics compared to that in the finer ones where zeolites occur as vein and fissure fillings only. Furthermore, the distribution of authigenic minerals in the pyroclastic rocks is commonly unequal and for this reason the resulting mottled textures can frequently be observed. Chemical and physical conditions in hydrothermal fluids were an important factor controlling the processes of mineral modification. In the early stage of hydrothermal activity being discussed, minor replacements of prehnite by laumontite occured (Fig. 2). Surdam [11] has reported that the stabili~ies of prehnite and laumontite can be explained in terms of the activities of Ca +, Si0 and H+ ions in 2 the aqueous phase. Accordingly, the replaceme~ts as previously mentioned can also be related to the decreased activity ratio Ca +/H+ and/or the increased activity Moreover, the supposedly modified ionic composition of hydrothermal fluof Si0 2• ids was likely accompanied by lowered temperatures due to constant cooling of this geothermal system.
Fig. 2. - Prehnite (Pr) and laumontite (L) from a lapilli tuff. A, In plane-polarized light prehnite shows a dusty appearance and supposedly, it is partially replaced by laumontite. B, Under crossed polars the less common fibrous form of laumontite is revealed.
P. Kovic and N. Krosl-Kuscer
89
In the northern margin of the Smrekovec Mt. area analcime occurs, predominantly at the expense of albite and laumontite being developed in the preceding hydrothermal episode as discussed above (Fig. 3,4,5). The local character of the analcime appearance is reflected by its restricted vertical and horizontal distribution range. The rocks intensively altered to analcime and pqyllosilicatc minerals (Fig.4 and 5) rapidly grade upward in the pyroclastics of an identical texture where albite and laumontite are completely preserved. The horizontal distibution of analcime is characterized by imperfections of· the mineral transformations (Fig.3). A relatively small extent of the described alteration processes can be attributed to a locally modified ionic composition of hydrothermal fluids and to fairly reduced porosity and permeability of the rocks which have undergone the preceding stage of hydrothermal mineralization.
Fig. 3. - Albite, laumontite and analcime from a lapilli tuff. A, In the photograph taken under crossed polars albite (Ab) replacing volcanic lithic ) can be seen. The central part of the field of view is by albite which is partially altered to analcime (An). B, The view under crossed polars reveals laumontite (L) being corroded by analcime (An). The reaction from analcime to albite in the aqueous medium was experimentally studied by Coombs et al (2), Campbell and Fyfe 11] , Thompson [13] and Liou (7) • The dissolved mineral substances reduce the chemical activity of water in fluids and thus lower the temperature of the equilibrium with respect to that in pure water at the same fluid pressure (Campbell and Fyfe (1) , Iijima (5) , Surdam and Boles [12) ). Furthermore, it was reported by Coombs et al.[2] and Wilkinson and Whetten (14) that analcimes found in sedimentary and
90 (GM-5-3) burial rocks are commonly rich in silicon and poorer in aluminum and sodium than the stoichiometric composition NaAlSi 206.H20.
Fig. 4. - Specimen from a lapilli tuff largely altered to analcime and pqy~llicate minerals. Preexisting albite occuring as the replacement of the volcanic lithic fragment (L ) has been inverted to analcime (An) and quartz (Q). Ab - theVremnants of albite. A, In plane-polarized light quartz stands out clearly in relief against the analcime with which it is intergrown. 5, The same as A, under crossed polars. The analcime can be seen to be slightly birefringent. The plagioclase grain (PI) which has been replaced by albite and prehnite (Pr) in the preceding hydrothermal episode is also mainly altered to analcime. Among tne complexly modified chemical conditions in hydrothermal fluids controlling the reactions from albite- and laumontite to analcime the increased activities of Na+, water and HCO- are the most obvious. The evaluation of microanalysis data is not easy since albite and laumontite are often incompletely altered to analcime, thus a wide range of partial mineral transformations occurs. Their detail study is
Kovie and
N.
Krosl-Kuscer
91
the sUbject of our future investigations. In general it can be suggested that the analcimes developed at the expense of albite sometimes are enriched with respect to silicon whereas the tendency of the excess silica to be stabilized as quartz is largely restricted to the strongly affected pyroclastic rocks (Fig. 4). In analcimes replacing minor interstitial substitutions of 2Na+ for are evident. The + ions could have reacted to form calcite (Fig. 5) or being removed from the system.
Fig. 5. - Specimen from a lapilli tuff predominantly replaced by analcime and pqy~ilicate minerals. Preexisting laumontite is inverted to analcime (An) and calcite (Ca). Au - relic augite, Pl - plagioclase grain altered to analcime, L - remnants of laumontite. A, The view in plane-polarized light. B, The same as previous, under crossed polars. Considering t2e reactions being discussed, a marine water seems to be an apparent source of Na + and HCO- ions. The Alpine orogeny reached a,structural climax in the early Neogene when ehe Smrekovec Mt. area was already uplifted on land. For this reason the analcime development can be ascribed to the effects of heated marine water or its admixtures to hydrothermal fluids when the previously affected pyroclastics were displaced near or at the sea floor. Clinoptilolite, occasionally accompanied by analcime exclusively replaces the overlain dacitic rocks which are localized in occurrence. The intermediate character of plagioclases is completely preserved and none of the high-temperature minerals including albite, laumontite and prehnite was developed. Evidence suggests the genesis of clinoptilolite either in the distal parts of this hydrothermal system during the early stage of authigenic mineralization or in the latter stage as discussed above. The pyroclastics displaced in a marine environment were likely subjected to alteration processes being reflected in an extensive further development of the smectite-like phyllosilicateminerals. The zeolites from the study area, occuring as vein and fracture fillings include laumontite, analcime, stilbite [91 and yugawaralite. Laumontite is the dominant vein mineral and it was obviously developed before and after the analcime appearance. We suppose that the Smrekovec Mt. area was already uplifted on land before the cessation of hydrothermal activity. The tectonic event as mentioned above, originated widespread fissure systems in which laumontite was precipitated in the latest stage of authigenic mineralization.
92 (GM-5-3) CONCLUSION The Tertiary volcanism in the Smrekovec Mt. area related to the Alpine orogeny produced large masses of explosive igneous material deposited in a shallow-water submarine environment of the bordering Panonian Sea. The most prominent tectonic event was uplift and it was accompanied by hydrothermal activity which took place over a relatively long period of time. This is 'evidenced by the presence of zeolites and related silicate minerals in the pyroclastic rocks from the study area. Authigenic mineralization was initiated in the early stages of tectonic and volcanic activities when hydrothermal waters emanating from an andesitic magma affected the pyroclastics seated on deeper levels below the sea floor. Consequently, albite,laumontite and subordinate prehnite were widely developed as a high- to moderate-temperature (6) mineral assemblage, replacing the primary constituents and filling interstices, veins and fissures. Analcime occurs locally, predominantly at the expense of preexisting albite and laumontite. Its appearance can be ascribed to the effects of heated marine water or its admixtures to hydrothermal waters when the pyroclastics, affected in the preceding hydrothermal episode, have been displaced near or at the sea floor. The Smrekovec Mt. area was already uplifted on land before the cessation of hydrothermal activity. In the latest stage of authigenic mineralization laumontite was precipitated in the widespread fissure systems originated by the tectonic movements as mentioned above. REFERENCE 1. A.S. Campbell and W.S. Fyfe, Am.J.Sci., 263:807-816 (1965). 2. D.S. Coombs, A.J. Ellis, W.S. Fyfe and A.M. Tylor, Geochim. Cosmochim. Acta, 17:53-107 (1959). 3. D.S. Coombs and J.T. Whetten, Geol.Soc.Am.Bull., 78:269-282 (1967). 4. A. Hinterlechner-Ravnik and M. Pleniear, Geologija, 10:219-236 (1967). 5. A. Iijima, Nat. Zeolites (F.A. Mumpton Ed), 175-189 (1976). 6. A. Iijima, Proc.Fifth Int.Conf.Zeol. (L.V. Rees Ed.), 103-118 (1980). 7. J.G. Liou, Lithos 4:385-402 (1971). 8. P. Mioe, OGK Slov. Gradec, L 33-35 (1978). 9. V. Osterc, Proe. Eight Yug.Geol.Conf., 199-207 (1974). 10. M. Pleniear, Geologija (A. RamovB Ed.), 175-188, 1978 11. R.C. Surdam, Geol.Soc.Am.Bull. 84:1911-1921 (1973). 12. R.C. Surdam and J.R. Boles, Asp.Diag. (P.A. Scholle and P.R. Schluger Ed.), Soc.Ec.Pal.Min.Spec.Pub.26, 227-242 (1979). 13. A.B. Thompson, Con.Min.Petr., 33:145-161 (1971). 14. J.F.T. Wilkinson and Whetten, J.Sed.Petr., 34:548-553 (1964).