Abstract -- This report considers the principal features metasomatic processes in granites and specifies the leading role of fluid transport in metasomatic evolution.. The results obtained substantiate preferential localization of the most pervasive metasomatic alteration zones, along with ore mineralization, in the periphery of high level granite intrusions, whereas their core parts are of low ore potential. Using experimental data, quantitative characteristics are first obtained for the border formation conditions (T, P, chemical composition and concentration of dissolved components in the solutions, pH, etc.) for all major types of the near-ore granite metasomatites: greisens, quartz-feldspar metasomatites, secondary quartzites (selexites), propylites, alkaline albitites, aegirine-riebeckite apogranites, ultra-alkaline nepheline, sodalite, and cancrinite metasomatites. Experimental zonal column (the regular sequence of metasomatic zones) is obtained for each type. Using the behavior of rare metals (i.e., that of W, Mo and Sn) as a paradigm, a possibility of the ore deposition during the course of greisenization and alkaline metasomatism of granites has been proved. Determination of physicochemical parameters for major types of near-ore metasomatism may serve as a clue to origin of various granite-related ore deposits.
1. Metasomatism in granites.
Types of metasomatic processes. After Zharikov and Omel’yanenko (1978), the following principal types of metasomatic assemblages related to the granite magmatism are as follows (in approximate time sequence and in decreasing order of temperature):
1. Early K-feldspathization;
2. Early albitization;
3. Alkaline Na (less frequently, K) metasomatism;
4. Magnesian metasomatism (amphibolization and biotitization);
5. Quartz-feldspathic metasomatism;
6. Greisenization;
7. Formation of secondary quartzites (a somewhat loose term for an assemblage of quartz and high-alumina minerals taken in Russian literature);
8. Quartz-tourmaline metasomatism;
9. Propylitization;
10. Quartz-sericite metasomatism;
11. Gumbeitization (quartz-orthoclase-phlogopite-dolomitic alteration, the name given after the Gumbeika river, the South Urals, Russia);
12 Beresitization (quartz-sericite- ankerite-pyrite alteration, after the Berezovskoe gold deposit, the Middle Urals);
13 Argillization;
14. Quartz-adularia-sericite metasomatism;
15. Carbonate-orthoclase-metasomatism;
16. Aceitization (quartz-albite-chlorite-carbonate-apatite alteration, the name derived from the Ace U deposit, the Lake Beaverlodge area, Canada, v. Omel’yanenko et al., 1974)
17. Zeolitization.
The metasomatic assemblages (the metasomatic formations, after Zharikov and Omel’yanenko, 1978) are subdivided into the metasomatic facies. The above sequence portrays the most common evolutional features of metasomatism inherent in the granite fluid-magmatic systems. At first, SiO2 content of altered rocks increases from earlier to later assemblages reaching its maximum during the course of formation of greisens and secondary quartzites; further, the acidity of hydrothermal fluids attenuates. Korzhinskii (1953) has outlined an early alkaline stage of metasomatism, with, a subsequent stage of acidic leaching and, finally, the late alkalic stage. Members 15-17 of the above list align to the latter.
Naturally, nobody ever observed this complete 17-member metasomatic sequence in any real granite massif. More, over most of these assemblages are inherent in different genetical and petrochemical varieties of granites. Thus, alkaline Na metasomatites are inherent in ancient Precambrian granites, whereas in Phanerozoic granites such metasomatites occur only in the high-alkali varieties. Quartz-tourmaline metasomatites, propylites and beresites ocur in more basitic varieties of granites. On the contrary, greisens ocur exclusively in extremely acidic leucogranites and alaskites, whereas the rare metal albitites that bear Li micas are predominately related to the latest phases of Li-F granites.
With respect to the dissolved component transport, the diffusion and infiltration types of metasomatism are specified. (Fig. 1). Mineral compositions of metasomatites and speciation of zones within the metasomatic column are of little relation to the mode of transport, because they depend mainly on the chemical composition of rock and solution, temperature, pressure, and other fundamental characteristics. Presumably, under the natural conditions (as in granites, as well as in other rocks), the diffusion and infiltration metasomatic alterations are combined.
Metasomatism and ore deposition. Orebodies are practically rarely by unaltered rocks: they are enveloped by halos of metasomatically altered rocks. Ore deposition and wall rock alterations result from a single process of the rock-fluid interaction. Hence, disclosure of the physicochemical conditions causing the metasomatic wall rock alterations is a clue to understanding the ore deposition environments with respect of temperature, pressure, chemical composition of solutions, and duration of rock-fluid interaction, thus obtaining a basis for revealing causes and mechanisms of ore deposition.
Fluid transport. The character of metasomatic alterations depends greatly on structural factors. Dissipated percolation of solutions through granites results in weak low-contrast alterations that are frequently missed by the field observations. In case the hydrothermal solutions move along macro-fissures, the flow becomes focused. Thus, the metasomatic alterations here are getting maximally localized, and the products are of maximal different from the host rocks with regard to their mineralogy and geochemistry.
Thermoelastic stress. Thermoelastic stress may be one of the most common factors responsible for fracturing of granites. As demonstrated for with high-level (hypabyssal) granite intrusions, the contraction fissures, along with the thermal gradient fracturing related to cooling of a pluton, can occur only within the peripheral zone of the latter. Thickness of the zone is about 0.2-0.3 of an intrusion radius; at greater depth the thermoelastic stress becomes lower than the rock strength (Khudyaev et al., 1989). As calculated, within intrusives crystallized at depth of about 3 km, shear fractures occur in an outer zone of about a quarter of the estimated radius of the bodies, whereas tensile fractures penetrate to depth of approximately 1/20 of their radii (Zharikov and Zaraisky, 1995). It is the absence of fractures focusing the fluid flow that makes the core parts of the granite massifs barren of notable metasomatic alterations and ore mineralization.
Metasomatic zoning Immediately near a fracture, the metasomatic alterations are maximally controlled by composition of the fluid, so the rock-solution interaction takes place under the “fluid-dominated” regime. According to Korzhinskii (1970), these are the maximally favorable hydrodynamic characteristics for development of the metasomatic zoning in the wall rock. Here the differences in mobility of the components (at a local chemical equilibrium) control the zoning pattern as a sequence of sections contrasting in their mineral and chemical composition. The outer zones of the metasomatic column are most similar in their composition to the wall rock (the “rock-controlled” regime). Most of the rock components remain inert here.
Korzhinskii’s theoretical concept of a correspondence between a metasomatic process and its metasomatic column that should include the whole zonal pattern, from the parent rock to final products of its metasomatic alteration (at equilibrium to the reacting solution) proved to be exclusively useful in a methodical aspect. Not the individual alterations, but instead the entire zonal sequence may give a maximally complete characteristic of a given metasomatic process and present the needed data on the physicochemical features of its evolution.
2. Results and discussion of experimental study of metasomatism.
During the last decade, major types of near-ore metasomatic rocks were reproduced experimentally, and their boundary conditions were determined, i.e., T, P, PCO2, chemical composition and concentration of solutions, pH, fO2, etc. (Zaraisky, 1989, 1993a,b; Zharikov and Zaraisky, 1991). In this paper results of these studies are used for interpretation of the physicochemical evolution of metasomatism in granite systems.
2.1. Methods and techniques.
Experiments were conducted to study diffusion metasomatic zoning. An essential feature of the method was experimental reproduction of an open system with perfectly mobile (in Korzhinskii’s terms, 1953, 1970) components. Activity of the components outside the rock was kept constant by use of a large buffer volume of the solution where a small gold capsule was placed, open at one side (di=5 mm, l=50 mm), filled with a compacted rock powder (the particle diameter below 0.1 mm). Leucocratic granite, biotite granite and granodiorite were used as the starting rocks (Table 1). The rock-filled capsule was vertically placed into an air-tight autoclave insert (150 cm3) made of a non-corrosive Ti alloy (Fig. 2). The vessel was filled with water and, if needed, the buffers (the oxygen buffer, etc), solid CO2, powdered quartz in order to achieve saturation of the solution with SiO2, and the ore components, depending on the individual experimental task. The water-rock proportion was ca 100:1. The leaching agents used were dilute HCl and HF, alkali (NaOH and KOH), and solutions of salts of major chemical elements: Na, K, Ca, Mg, and Fe chlorides and fluorides, along with Na and K carbonates and metasilicates in various concentrations, depending on the problems to be studied by a given experiment. In many cases, NaCl or KCl are major components of the solutions, and some solutions were acidified with HCl or NaOH-alkalinized in order to obtain the needed pH. Concentration of solutions varied widely, from 1?10-5 to 5 mol per kg H2O, and most frequently ranged from 1?10-3 to 1.0 mol per kg.
During the experiments the solution remained immobile, having reacted with the rock through an open end of the capsule by diffusion. The diffusion counter-flow transported the rock components from the capsule into the solution. Because of relatively large volume of the solution, no significant changes in its chemical composition apparently occurred during the course of experiments. Granite came to equilibrium with the solution only near the open end of the capsule. Deeper into the capsule the rock remained disequilibrated relative to the outer solution, being locally close to equilibrium relative to that portion of the intergranular pore solution. The front zone (the farthest one from the open end of the capsule) often remained similar in composition to the source rock. Activity of the components just as temperature and pressure here were controlled by an outer medium. In the experiments with F solutions, smaller gold and platinum inserts instead of titanium ones were used; accordingly, the capsules were of smaller size. The experiments were performed at T=250-600śC and P=1 kbar; duration of the experiments ranged from 7 to 28 days.
Upon completion of an experiment, a zoned metasomatic column was formed, instead of a homogeneous rock, frequently with sharp borders separating the zones (Fig. 3). The most pronounced alteration took place in the rear zone, at the open end of the capsule, where the rock was in immediate contact with the reacting solution. Further-more, the experimental columns were organic glue-cemented, then cut lengthwise into sections with a diamond saw and studied using an electron microprobe the ordinary points mode and using a narrow beam in the scanning regime along transverse profiles in order to determine the bulk rock composition over the zones.
In this paper we discuss the experimental results only for those major types of metasomatic rocks that immediately replace granites. We avoid an important group of the metasomatites that are genetically related to granites but normally occur in the granite-hosting rocks (e.g., magnesian metasomatites of the exocontact zones, propylites, beresites, quartz-sericite and quartz-adularia-sericite metasomatites, etc). Since metasomatism and ore deposition are closely related, we conducted experimental study of deposition of the rare metal ore deposition simultaneous to the processes of acidic and alkaline metasomatism in granites.
2.2. Reproduction of acidic metasomatism in granites.
The term acidic metasomatism (acidic leaching, after Korzhinskii, 1953, hydrogen metasomatism, after J. Hemley and W. Jones, 1964) is a process that generates large group of metasomatic assemblages characterized by a general tendency of extraction of major components (Fe, Mg and Ca) and alkalis (K and Na), along with increasing of total acidity of a rock. Acidic metasomatism results in formation of assemblages like that found in the rear zones composed exclusively of silica and alumina minerals (sometimes, solely of silica).
2.2.1. Metasomatism of granites caused by acid chloride solutions.
2.2.1.1. Secondary quartzites (silexites). Metasomatic columns were obtained as a result of interaction between leucogranite or granodiorite and aqueous quartz-saturated solutions of low mKCl/mHCl, mNaCl/mHCl, and mCaCl2/mHCl ratios.The metasomatic columns contain rear zones constituted by quartz in combination with one of the aluminosilicate minerals: kaolinite (at temperature below 300?C) pyrophyllite (300-420?C) or andalusite (above 420?C), depending on temperature (Zaraisky et al., 1981). The assemblages of the minerals of these synthetic metasomatites are comparable to kaolinite, pyrophyllite and andalusite facies of natural secondary quartzites. This metasomatite could be considered an extreme result of leaching of granite, and the acidic reacting solution containing very low contents of components other than silica.
The structure of the metasomatic column originating from treatment of a biotite granite sample (Table 1) with 0.1M HCl solution at 400?C examplifies the above statement (Fig. 4). Normative mineral compositions of minerals (mol.%) constituting the zones of the experimental column were calculated from the microprobe scanning data (Table 2). To avoid excessive details in the figure, the porosity variations (28-41% ) are omitted; the total of mol% is reduced to 100%. Note that in Table 2 the influence of porosity is responsible for low total content of the components, as microprobe scanning techniques estimate content of a component per unit of the porous medium volume. The sum could be 100% only when there no pores. Table 2 demonstrates that during the course of metasomatism, FeO, MgO, CaO, Na2O, and K2O were intensely removed, whereas contents of SiO2, Al2O3, and TiO2 remained approximately constant although some removal of silica was observable in the rear zones (i.e., zones 1 and 2).
A decrease in temperature down to 300?C and below resulted in formation of kaolinite instead of pyrophyllite in the rear part of the column, whereas at 500?C andalusite formed instead of pyrophyllite. In all cases, the second phase constituting the rear parts of the experimental columns was quartz. In the middle zones muscovite formed at any temperature within the range studied. The following sequence of replacement, feldspar ® muscovite ® pyrophyllite ® (kaolinite, andalusite), illustrates increasing acidity of the solution from the front parts of the columns backwards. Replacement of kaolinite by pyrophyllite and andalusite with increasing temperature corresponds to the natural sequence of the temperature facies of secondary quartzites (Creasey, 1966; Beane, 1974)
2.2.1.2. Formation of quartz-muscovite greisens and quartz-K-feldspathic
metasomatites. With increasing mKCl/mHCl ratio in the reacting
solution, intensity of the acid leaching of biotite granite decreased,
and micas, feldspars and other aluminosilicates arose in the rear zones.
Thus, forming the experimental columns are comparable to quartz-muscovite
greisens and quartz-feldspar metasomatites. Reaction of biotite granite
with solution (1M KCl + 0.05M HCl + Qtz, pHini 1.4 at T=400?C,
P=1 kbar, and t=500 h) produced a column similar to greisens of the quartz-muscovite
facies (the numerals below the following schemes denote the distance from
the open end of the capsule):
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0 2.8 6.1 33.7 43.7 48.7 mm
In experiments involving quartz-undersaturated solutions, the rear zone consisted solely of highly porous muscovite. Such columns are similar to greisens of the muscovite facies that are less common under natural conditions than quartz-muscovite varieties.
Interaction between biotite granite and highly acidic highly potassic
solution (3m KCl + 0.1m HCl + Qtz, pHini=0.8) at T=500?C,
P=1 kbar and t=240 h resulted in a column of the quartz-K-feldspathic metasomatism
where an assemblage of quartz and K-feldspar replaced the starting granite
to a relatively large depth:
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0 35.0 50.0 mm
Here the quartz-undersaturated solution produced a monomineral K-feldspathic rear zone.
As acidity increased (i.e., at decreased mKCl/mHCl ratio), at temperature above 450śC quartz-muscovite greisens graded into quartz-andalusite facies of secondary quartzites or greisens, whereas at temperature below 300śC -- into quartz-kaolinite facies.
With respect to temperature and the equilibrium mKCl/mHCl ratio, the border formation conditions of secondary quartzites, quartz-muscovite greisens and quartz-K-feldspathic metasomatites, could be specified using an experimental phase diagram of the granite-solution system, Al2O3-SiO2-H2O-KCl-HCl (Fig. 5). The latter is similar in its composition to the most altered rear zones of the acidic metasomatic columns with major chemical elements removed, except Si, Al and K. Hence, the phase composition of the fields on the chart resembles the mineral composition of the rear zones inherent in corresponding natural metasomatic assemblages. The upper temperature limit of quartz-muscovite greisens is specified as a wedging out of the stability field of muscovite associated with quartz at 555śC. The limit will shift with the variation of PH2O (up to 590śC at 2 kbar and down to 500śC at 0.5 kbar). In the high-K solutions, quartz-K-feldspathic metasomatites may exist within a very wide range of temperature. Here, with decreasing temperature at a stable HCl concentration, progressively higher concentrations of KCl are required to enable replacement of the quartz-muscovite assemblage by the quartz-K-feldspathic one.
2.2.1.3. Investigation of quartz-albite metasomatism and propylitization
of granitic rocks. In Na dominated solutions, albitization became
a leading process of metasomatic alteration of granites. However, strongly
acidic Na solutions (1m NaCl+0.1m HCl+[Qtz] ) reacted with a leucogranite
were almost as effective as purely HCl solutions, i.e., they produced the
Qtz+Kln,
Qtz+Prl or Qtz+And assemblages in the rear zones of the columns.
Such columns correspond to a variety of the secondary quartzite facies.
High acidity is the major control here, not Na content. In case the chloride
solutions were moderately acidic to neutral at various concentrations of
Na (0.01m to 5m NaCl), all the minerals of the starting leucocratic granite
(Table 1), quartz excluded, were becoming albitized. The resulting metasomatic
column presents quartz albitites (T=400śC, P=1 kbar, t=336 h, 1m HCl +[Qtz],
pHini=5.8):
| Ab+Qtz | Ab+Qtz+Bt | Ab+Qtz+Kfs+Bt |
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0 6.0 15.0 50.0 mm
Albite first replaced K-feldspar, then biotite. Relicts of the primary plagioclase were occasionally preserved. Albite and quartz constituted the rear zone. Increasing acidity resulted in formation of muscovite in association with biotite, albite, and quartz in the front of the column.
Interaction of the near-neutral and weakly-acidic sodic solutions with granodiorite (Table 1) at T=300-500śC produced columns of albite propylites with rear zones consisting of Ab+Chl+Am+Zo (the solution was undersaturated with SiO2). At T=500śC, P=1 kbar, t=336 h, 1m NaCl+0.0003m HCl and pHini=3.7, the resulting column was as follows:
| Ab+Am+Chl+Zo | Qtz+Ab+Am+Chl+Zo | Qtz+Pl+Am+Bt+Chl+Zo | Granodiorite |
| 0 | 2.0 | 4.5 |
0 2.0 4.5 21.0 50.0mm
Along with increasing acidity, Am, Zo and Ab successively dropped out from the four-component mineral assemblage of the rear zone. Chlorite in assotiation with paragonite remained as the most stable phase when exposed to moderately acidic Na-bearing solutions. At high acidity, chlorite disappeared, and Kln, Prl or And constituted the rear zones of the columns. In SiO2-saturated solutions, Qtz joined the assemblage.
2.2.2. Acidic fluoride metasomatism.
2.2.2.1. Study of greisenization of granite.
Acidic fluoride solutions reacted with leucogranite are most characteristic of processes of greisenization, especially in formation of quartz and quartz -topaz greisens with fluorite and F-bearing micas. (Shcherba, 1970, Burt, 1981). Shapovalov (1988) was first to obtain experimental data on the mineral equilibria in the system Al2O3-SiO2-H2O-KF-HF (Fig. 6). As established, formation of topaz required rather high HF concentration and low KF in equilibrated solution (HF>0.025m and KF<0.006m, at T=400śC and P=1 kbar). This explains relatively low abundance of the topaz greisens in nature. Depending upon temperature, the lower limit of stability of topaz was controlled by equilibria with andalusite, pyrophyllite or kaolinite. The upper limit, within the studied range of temperature (300-600śC), was controlled by equilibria with fluellite (AlF3*H2O).
Greisens of quartz-topaz facies. Silica and alumina are highly soluble in the acidic fluorine solutions (Zaraisky, 1994). Thus, it was necessary to add powdered quartz and corundum to the solution in order to prevent dissolution of granite. The following column was obtained under these conditions (T=500śC, P=1 kbar, t=336 h; 1m HF + [Qtz]+[Al2O3]):
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0 3.0 4.1 7.2 12.1 25.0 mm
Quartz prevailed over topaz in the rear zone. In deeper zones muscovite substituted for quartz; K-feldspar was more stable, as well as albite in the starting leucogranite. Fluorite was found in all zones (1-3%).
Greisens of quartz-muscovite facies. A slight increase in K content in solution (KF>0.006m) reacted with leucogranite resulted in replacement of topaz in the rear zone for muscovite and increasing K-feldspar content in other zones:
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Greisens of the quartz facies. Alumina may be totally removed as a result of interaction between granite and quartz-saturated HF solutions. The following column with its rear zone composed almost exclusively of quartz was obtained (T=500śC, P=1 kbar, t=336 h; 1m HF+ [Qtz] in excess):
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| 0 | 3.0 | 5.0 | 7.5 | 25.0 |
0 3.0 5.0 7.5 25.0 mm
To obtain the most strongly developed zonal column with the quartz-topaz zone between the quartz and quartz-muscovite ones, it was necessary to increase the activity of aluminum in solution (T=500śC, P=1 kbar, t=336 h; 1m HF+0.17 m AlF3+[Qtz]):
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0 1.0 4.5 5.5 7.0 25.0 mm
This zoning is inherent in a symmetrical pattern of steep-lying greisen veins of Akchatau, a W-Mo ore deposit in Central Kazakhstan (Zaraisky, 1995).
Results of experimental modeling confirm that acidic fluoride solutions are essential in formation of greisens. Really, as a result of the fluoride attack all major types of greisens were reproduced (i.e., quartz-muscovite, muscovite, quartz-topaz, and quartz varieties), whereas he acidic chloride attack produced only quartz-muscovite and muscovite facies. In chloride solutions we failed to obtain a column corresponding to the monoquartz greisens because of stability of alumina in the rear zone. Greisens exemplify a subsidiary role of temperature relative to the chemical composition of the solutions. All types of the greisen columns here were obtained within a wide temperature interval, from 300 to 600śC (Soboleva et al., 1988). Composition of the experimental columns here practically did not correlate with the temperature variations, being essentially dependent on chemical composition of solutions.
2.3. Study of alkaline metasomatism of granites
2.3.1. Metasomatism caused by solutions of Na and K chlorides, carbonates and silicates
2.3.1.1. Aegirine albitites and K-feldspathites: Methods of formation. During the course of alkaline Na metasomatism of biotite granite with quartz in excess, K2O remained the only component being actively removed from granites, whereas Na2O accumulated significantly in the rock (Zaraisky, 1989). Albite and aegirine replaced, accordingly, feldspars and biotite. These alterations resulted in formation of the rear column zones composed of quartz, albite, and aegirine (T=500śC, P=1 kbar, t=336 h; 1m HCl+0.1m NaOH+[Qtz], pHini=12.8):
1 2 3 0
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0 8.2 27.7 44.5 mm
Table 3 presents the chemical composition of the above zones. It seems of interest to compare their composition to those produced by acidic leaching (Table 2). During the course of acidic leaching of the biotite granite, including that with acidic Na solutions, all the components, alumina (and, to a certain extent, Na) excluded, were actively removed from the rock. In case of alkalic Na metasomatism, only K2O and, partially, MgO were intensely removed, whereas Na2O was notably accumulated. Alumina from granite concentrated in albite, and aegirine formed from the granitic components (Fe, Mg, Ca, and Ti).
Treatment of the rock with sodic alkalic SiO2-undersaturated solutions resulted in pervasive desilicification of the granite. Instead of albite, feldspathoids were formed in the rear zone (nepheline, sodalite or cancrinite, depending on the anion composition of the solution -- OH-, Cl-, or CO32-), in association with aegirine. Evidently, this high alkalinity is not inherent in granite massifs because of high concentration of silica in granite.
Potassic alkaline solutions cause K-metasomatism in granites, as expressed by pervasive K-feldspathization. The rear zone in this case consisted of the following mineral assemblage: Qtz+Kfs+Bt. In case the solutions were undersaturated in SiO2 , K-feldspar was the only mineral component of the rear zone. At a very high alkalinity (1m KOH) kalsilite in association with tetraferribiotite was formed instead of Kfs. In the intermediate and front zones of these columns Kfs was formed instead of kalsilite, whereas aegirine and riebeckite replaced biotite.
2.3.2. Metasomatism caused by alkalic fluoride solutions
2.3.2.1. Alkaline albitites, riebeckite and aegirine apogranites: Methods of formation. The presence of fluorite, cryolite, F-bearing amphiboles and micas is a common feature of many alkaline metasomatites developed after normal and alkaline granites. One of interesting problems here is the metasomatic formation of riebeckite and aegirine granites (Zaraisky, 1989).
In alkaline fluoride solutions the most intensive transformations of biotite granite occurred when the solutions were undersaturated in SiO2 (T=500śC, P=1 kbar, t=95 h; 1.0m NaF, pHini=7.2):
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0 1.0 12.0 40.0 mm
Riebeckite albitite formed in the rear zone of the experimental column, whereas quartz was removed, K-feldspar and plagioclase were completely albitized, and alkaline amphibole replaced biotite. The front zone of the column consisted of a typical riebeckite granite. As temperature increases to 550-600śC, aegirine developed instead of riebeckite.
In case of interaction with SiO2-saturated solutions, the intensity of alterations in granite was lower. In the rear zones quartz remained intact, and the stability of K-feldspar increased. As activity of SiO2 grew, aegirine was formed instead of alkaline amphibole. The front zone of the column corresponded in composition to riebeckite granite. The same assemblage of typical riebeckite granites, i.e., Qtz+Ab+Kfs+Rbk, was formed in the front zone even if biotite granite interacted with alkalic potassic fluoride solutions instead of sodic ones (T=500śC, P=1 kbar, t=96 h; 1.0m KF, pHini=7.3):
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0 1.5 14.0 35.0 40.0 mm
This observation is attributed to the buffering effect of rock that supplied Na to the pore solution while K-feldspar replaced albite. However, depending on the cation composition of the source solution, the rear zone of the column consisted either of albite and riebeckite (NaF solution) or K-feldspar and tetraferribiotite (KF solution). It should be noted that in the intermediate zone of the column narsarsukite (Na2(Ti, Fe)Si4O10(O,OH, F)) was formed of interaction with KF.
2.3.3. Facies of alkaline Na metasomatites versus temperature and log(mNaCl/mNaOH) of the reacting solution.
Granodiorite (Table 1) served as the source rock in this experimental series, and 1.0M NaCl was a major dissolved component of the reacting solutions. Alkalinity was controlled by addition of NaOH (from 0.0001 to 0.1m). Initial pH varied from 7.0 to 12.8. The solutions were SiO2-undersaturated (Zaraisky,, 1989).
As a result of increasing alkalinity, the type of resulting metasomatites varied from albite propylites (near-neutral medium) to alkaline amphibole-aegirine albitites and, further, to ultra-alkaline aegirine-riebeckite-sodalite metasomatites (Fig. 7). With temperature decreasing from 400 to 350-300śC, analcime was formed instead of sodalite and albite. An important boundary separating areas of acidic and alkalic metasomatism was indicated by formation of alcalic amphibole in albitite instead of the actinolite amphibole in propylites. Within the P-T interval studied, the boundary runs parallel to a line of neutral pH(T,P), being slightly shifted to the alkaline field. Chemical analysis of the solutions after experiments demonstrated a sharp decrease (by a factor of two decimal orders) in content of dissolved Fe, Mg and Ca in the alkaline solutions relative to acidic ones.
2.4. Study of the rare metal (W, Mo, Sn) ore deposition during the course of greisenization and alkaline metasomatism of leucogranite.
In this series of experiments powdered wolframite, molybdenite and cassiterite served as sources of the ore elements. The minerals were placed into the solutions in separate platinum cups (Zaraisky, Stoyanovskaya, 1995). Powdered leucogranite was placed in the gold capsule, like in other experiments. The rock-solution interaction resulted in formation of metasomatic columns bearing trace amounts of W, Mo and Sn in their rear zones. Acidic chloride and fluoride solutions were used as the “greisen-forming” agents (1m KCl+0.05m HCl and 1m KF+0.05m HF). To compare the effects, we studied the results of interaction with a near-neutral 1m NaCl solution that caused albitization of the leucogranite, along with two alkalic solutions (0.1m Na2SiO3 and 0.1m Na2SiF6). In the latter case alkaline albitites were formed after the leucogranite. The experimental columns were cut lengthwise, with subsequent electron microprobe analysis for the petrogenic elements and atomic emission quantitative spectral analysis for the ore elements.
As established, W, Mo and Sn could have been transported in significant quantities by the solutions of various composition that caused greisenization, albitization and alkaline metasomatism of the granite. These chemical elements could have been deposited simultaneously with formation of the metasomatites, mainly in the rear zones of the metasomatic columns (Fig. 8). Along with it, the mobility of these chemical elements was significantly different: this fact explains zonal patterns observed at the mineral deposits.
Amounts of transported and deposited rare metals depend on the chemical composition of solutions including their pH (Fig. 9). Thus, W, Mo and Sn were actively transported by alkaline solutions. On the other hand, acidic chloride solutions are favorable transporting media for W, but not for Mo. The largest travel distances were observed for Sn in Na2SiF6 and NaCl solutions. Moderately acidic fluoride solutions were the worst transporting media for W but better ones for Sn and Mo. Tungsten was best transported by strongly alkaline solutions and to a much lesser extent by the near-neutral ones. Molybdenum was deposited in the columns to a much lesser extent relatively to W and Sn. Maximal accumulation of Mo was observed in the alkaline media. The averaged sequence for the three ore elements, in decreasing order of accumulation in the metasomatic zones and the migration distance along the column, was W>Sn>Mo. Tin was the most indifferent of the three in respect of chemical composition of the solution.
Conclusions
The type of metasomatic process is controlled by a combination of numerous physicochemical parameters and is specified, most distinctly, by a zonal metasomatic column, a series of zones beginning from “fresh” granite to the final product of its alteration. In our experiments we obtained various types of metasomatic columns reproducing almost all varieties of natural metasomatites (metasomatic assemblages) developed after granites.
Major factors controlling the type of resulting metasomatism are composition of rock, solution, including the pH of the latter. Thus, in order to form topaz greisen, granite is attacked by acidic fluoride solution (>10-3m HF at 400śC and >10-2m HF at 500śC) at high content of alumina (>5?10-4m Altot) and low activity of K (<10-2m KF). Albite propylites are formed in near-neutral media and indicate the boundary area between acidic and alkalic metasomatism. The temperature interval of 350-450śC in combination with essentially sodic solution at pHT,P >6.6-8.5 is favorable for formation of alkaline aegirine albitites.
Metasomatic columns corresponding to secondary quartzites result from interaction of granite and highly acidic chloride solutions with low mKCl/mHCl ratio. Sequential increase in the mKCl/mHCl ratio results first in formation of columns of quartz-muscovite greisens or quartz-sericite metasomatites, and then -- those of quartz-K-feldspar metasomatites (Fig. 6).
Many minerals of the granite metasomatites (quartz, feldspar, muscovite, albite, topaz, tourmaline, epidote, riebeckite, aegirine, etc.) are formed within a wide P-T interval and are not indicative of pressure and temperature, although they are informative on composition of the solutions. Temperature is a major control in case when quartz-kaolinite facies of secondary quartzites grades into quartz-pyrophyllite and quartz-andalusite facies. The presence of kaolinite in metasomatites specifies that the temperature of the process did not exceed 270-300śC. Andalusite or corundum indicate that the temperature was above 400śC, whereas pyrophylite is stable at 300-400śC. Corundum and diaspore are incompatible with quartz, testifying undersaturation of the solutions in SiO2.
The experiments demonstrated a possibility of ore metal deposition immediately during the course of acidic and alkalic metasomatism of granites and showed differences in the migration properties of W, Mo and Sn distinctly controlled by composition of the leaching hydrothermal solution.
Acknowledgments. The work was financially supported by RFFR (Project 9605-64709, INTAS (Project 93-1783) and the DFG-RFFR Russian-German Fund (Project 96-05-00020).
Abbreviations
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References
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Burt D.M. (1981) Acidity-salinity diagrams - application to greisen and porphyry deposits Econ.. Geol., 76, pp. 832-843.
Creasey S.C. (1966) Hydrothermal alteration. In: Geology of the Porphyry Coper Deposits. S.R. Tutley and C.L. Hicks (eds.), Tucson, Univ. of Arizona Press, pp. 51-74.
Hemley, J.J. Jones, W.R. (1964) Chemical aspects of hydrothermal alteration, with emphasis on hydrogen metasomatism. Econ.. Geol., 59: 238-369
Khudyaev, V.S., Balashov, V.N., Zaraisky, G.P. (1989) On the model of formation of the greisen ore deposits: Estimation of the thermoelastic stress for a simple spherical model of cooling and crystallizing intrusion (in Russian), Geol. Rudn. Mest., no. 4, pp. 3-11
Korzhinskii, D.S.(1953) An outline of metasomatic processes, in: Fundamental problems in studies of magmatogenic ore deposits (in Russian), Izd. Akad. Nauk SSSR, Moscow, pp. 332-452.
Korzhinskii, D.S. (1970) Theory of metasomatic zoning, Clarendon: Oxford, 162 pp.
Omel’yanenko B.I, Lisitsyna, G.A., Naumov, G.B. (1974) Aceites (the low-temperature Na metasomatites): Determination of formational conditions (in Russian), in: Metasomatism and Ore Deposition, Nauka: Moscow, pp. 160-171);
Shapovalov, Yu.B. (1988) Mineral equilibria in the system K2O-Al2O3-SiO2-H2O-HF at 300-600śC and P=1000 bar (in Russian). Ocherki fiziko-khimicheskoi petrologii, issue 15, Nauka: Moscow, pp. 160-167.
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Soboleva, Yu.V., Zaraisky, G.P., Shapovalov, Yu. V. (1984) Experimental modeling of the diffusion zoning in topaz greisens (in Russian). Ocherki fiziko-khimicheskoi petrologii, issue 15, Nauka: Moscow, pp. 148-160.
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List of figures
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(*) Note:
1-leucogranite, Akchatau, Central Kazakhstan: Qtz 35.0, Pl 34.0, Kfs 29.0, Bt 1.0; others 1.0.
2-biotite granite, Altai, Kalba, Belaya Gora: Qtz 25.0, Pl 45.5, Kfs 19.0, Bt 9.5; others 1.0.
3-granodiorite, Tajikistan, Maikhura: Qtz 23.5, Pl 45.5, Kfs 14.5, Bt 10.0, Am 5.5; others 1.0
Table 2. Pyrophyllite facies of experimentally reproduced secondary quartzites: chemical composition (wt%)of the metasomatic zones (Fig. 4).
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(*) starting biotite granite, porosity 30%
(**) Total Fe as FeO
Table 3. Experimentally reproduced column of aegirine albitites (alkaline Na metasomatites): Chemical composition of the zones (wt%)
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Notes:
(*) Total Fe as FeO
(**) starting biotite granite, porosity 30%