III. Metasomatism and modelling of the ore formation
(Leader Prof. G.P.Zaraisky)

GeoForschungsZentrum Potsdam Telegrafenberg, Potsdam, Germany (seltm@gfz-potsdam.de)

Geochemical features of the Orlovka and Etyka tantalum deposits, Eastern Transbaikalia, have been studied. The analyses were conducted predominantly by XRF, ICP-MS, ICP-AES, INAA, and ISE to develop a database for chemical and petrogenetic modeling.

According to the geological, petrological, and geochemical data of the district, the following rock groups can be distinguished:

1) barren bt and bt-ms granite massifs - Khangilay pluton (for Orlovka) and Oldanda pluton (for Etyka);

2) high-evolved ms-ab and amz granite stocks;

3) layered pegmatites-aplites, locally developed as line rocks, within the mineralized apical parts.

The low evolved Khangilay and Oldanda plutons are characterized by high values of Zr/Hf (25-32 and Nb/Ta (2.5-6.1), whereas the low values are common for more evolved granites and line rocks of the deposits (ca. 0.5-2.2 and 0.1-0.4, respectively).

Greisens situated within the layered complex are enriched in elements, such as Li (up to 8000 ppm), F (1.5-6.2 wt.%), and Ta (up to 700 ppm). The less-evolved granites of the Orlovka and Etyka deposits show intermediate values for these elements fitting the obvious evolution trend from the parental granites to the layered complex.

The evolution of the most typical granite groups of the Orlovka and Etyka regions, considered in the surrounding of the deposits the barren granites of Khangilay and Oldanda, respectively, is shown in Fig. 1.

The fractionation trend for the Orlovka region begins with bt-ms and subsequent bt granites of the Khangilay massif. More fractionated bt-ms-lith granites develop into ms-ab high Li-F granites, related line rocks and greisens. In the case of Etyka, the Oldanda bt-ms granites are followed by highly fractionated amz granites, amz-ab granites, and finally by the layered complex, whereas greisens are untypically and only rarely developed.

The REE plots for Orlovka (Fig. 2) show similar patterns for Khangilay granites and bt-ms-Li-mica granites at the base of the Orlovka granite cupola. The highly evolved granites at the Orlovka deposit and the layered rocks of the cupola show close relationships. Towards the layered rocks the Eu anomaly and tetrad effects are stronger developed. The latter is interpreted as reaction product caused by separation of exsolved from the melt highly reactive aqueous fluids . Similar fractionation trends are for Etyka.

The behaviour of W, Sn, and U in the system WO₃-SnO₂-UO₂-NaCl-H₂O was studied at 400-500^oC, 200-1000 bar using a Fe₂O₃-Fe₃O₄ buffer. It has been found that the acidic-basic interaction between WO₃ and NaCl occurs in the system at 500^oC resulting in the formation of (Na, Sn,U)_xWO₃ bronzes (x=0.1-0.4) whereas at 400^oC WO₃ undergoes no noticeable changes. At T=400^oC and P=200-250 bar tungsten preferentially redistributes to the L-phase, like the largest number of ore elements. The solubility of WO₃ in the L-phase is 0.02+0.01 mol/kg H₂O.

At T=500^oC and P=400-500 bar tungsten was found to behave in an unusual manner in the NaCl-HCl fluid two-phase region. At P=400 bar tungsten is almost completely in the V-phase (m^V_W=0.09+0.01 mol/kg H₂O) whereas at P=500 bar approximately equal concentrations (m^L_W=m^V_W = 0.07+0.03 mol/kg H₂O) are present in the V- and L-phases.

In the NaCl-HCl fluid homogeneous region at 500^oC, 1000 bar the solubility of Na_xWO₃ bronzes (x=0.1-0.2) grows from approx. 0.03 in pure water to 0.06 mol/kg H₂O in 400 wt% NaCl solution.

SnO₂ and VO₂ has a lower solubility than WO₃ or Na_0.1-0.2WO₃, and ore elements accumulate preferentially in the L-phase. At 400^oC and P=250 kbar m^L_Sn and m^L_Uamount to (2.0+0.8)^.10^-4 and (1,4+0,5)^.10^-5 mol/kg H₂O, at P=200 bar, respectively, (2,3+0,2)^.10^-4 and (2,4+0,2)^.10^-5 mol/kg H₂O. The partitioning coefficient of U between the L- and V-phases is 5<Kp=m^L_U/m^V_U<10. At 500^oC and P=400-500 bar the solubility of SnO₂ and UO₂ grows by 1-2 orders of magnitude due to the acidic-basic interaction of components in a solution.

It has been shown that in the range of 400-500^oC the conditions are realized wherein W,Sn, and U behave differently in the two phases of the hydrothermal NaCl-H₂O fluid. This fact explains the componental partitioning in a general hydrothermal process.

Ezhov S.V. Experimental study of the skarn-ore process.

Moscow State Geological Exploration Academy

The works were performed in the following directions:

1. Experimental study of transformation processes of ore aggregates under high - temperature conditions. In the laboratory of models of ore deposits (Institute of Experimental Mineralogy, RAS), we performed a series of experiments to consider in detail the previously advanced concept on active transformations of the ore substance under conditions when ore deposits are arranged between rocks different in compositions. We studied the behavior of associates involving pyrite, pyrrhotine, chalcopyrite, and magnetite. Granodiorite-porphyry, quartz porphyry, limestone, and dolomite limestone were used as side rocks. Experimental parameters: P = 1 kbar, T = 350-600^oC, 1 M solutions of NaCl and KCl. It is established that under these conditions, magnetite is the most stable in the whole temperature range. At T = 500-600^oC, sulfides undergo very strong changes: The structure of the aggregates and their mineral composition are intensely transformed (chalcopyrite is replaced by bornite, cubanite, and covellite), and redeposition of minerals with development of zoning occurs (Fig.1). The specific features of the zonal distribution of sulfides are determined by the composition of wall rocks (1).

Fig.1. Change in composition and development of zoning of ore aggregates in experiments at T = 600^oC (a) and T = 550^oC (b). Composition of the starting ore mixture: magnetite/pyrite/chalcopyrite = 60/20/20%, P = 1 kbar, solution 1 M NaCl. 1-4, side rocks: 1, granodiorite-porphyry; 2 - quartz porphyry; 3 - limestone; 4 - dolomite; 5-9 - zone compositions: 5 - magnetite; 6 - magnetite+garnet+chalcopyrite; 7 - magnetite+bornite; 8 - magnetite+cubanite; 9 - magnetite+chalcopyrite.

2. Experimental study of ore formation processes involving sulfur and iron. We checked an assumption that the

conjugated transition of sulfate sulfur to sulfide one and ferrous iron to ferric iron (8Fe²⁺ + S⁶⁺ = 8Fe³⁺ + S^2-) at high temperatures can be one of the main factors of the intense development of skarn, magnetite, and sulfide mineralization (2). The ratio of the atoms participating in the reaction determines the drastic predominance of minerals binding trivalent iron (andradite, epidote, and magnetite) over sulfides, which usually form a depleted uniformly distributed phenocryst. The results of the experiments at T = 600^oC and P = 1 kbar, when ferrous oxide or iron carbonates, some compounds of nonferrous metals, calcium sulfate, limestone, and granodiorite-porphyry were used as the starting substances, and magnetite, garnet, epidote, and various sulfides (pyrite, pyrrhotine, chalcopyrite, cubanite, galenite, and sphalerite) were found among the products, can be considered as the confirmation of the advanced supposition.

3. Study of natural forms of conjugated skarn and ore formation and transformation processes of skarn ores. The Peschanskoe skarn-magnetite deposit in the Northern Ural was considered as an example. Three types of magnetite ores were distinguished: 1) "medium-sulfide" type characterized by the proportionality between the content of sulfide and iron phenocrysts in the ore with the mean ratio S/Fe = 0.044 (this ratio shows that pyrrhotine was formed as sulfide simultaneously with magnetite); 2) "low-sulfur" type with the minimum content of sulfur (lower than 1%) at any variations of the iron content and without an interrelation between these elements; and 3) "high-sulfur" type with a high content of sulfur (units-tens percentage) also without an interrelation to the iron content. The influence of breakage tectonics is expressed only in the distribution of the third type of ores ("high-sulfur"); the spatial development of the two former types is independent of breakage distortions. The main ore bed of the Novopeschanskii region that lies as a slope lens at the boundary of limestones and overlapping rocks of the volcanogenic thickness is stratified when its upper part is formed by ores of the second (low-sulfur) type and the bottom part is formed by ores of the first (medium-sulfur) type (Fig. 2). This can be reasoned by the process of ore transformation accompanied by the sulfur redistribution in contours of the deposit: it is moved from the upper horizontals to develop low-sulfur ores and removed to the near-bottom part. Here sulfur is mainly fixed in the form of pyrite, which actively replaces primary pyrrhotine.

The results of the studies confirm the main statements of the convective model of formation of skarn deposits (3) developed by the author.

1. Ezhov S.V. (1997) Transformation of Ore Aggregates in Experiments and its Possible Geological Significance. // Dokl. Russ. Akad. Nauk, V.356, N.4, pp. 516-520 (in Russian).
2. Ezhov S.V. and Akinfiev N.N. (1997) Study of Mineral Formation Process in Contact of Silicate and Carbonate Rocks Using the BALANCE Program. // Geokhimiya, N.10, pp. 1058-1065 (in Russian).
3. Ezhov S.V. (1994) Convective Model of Formation of Skarn-Polymetallic Deposits of Altyn-Topkanskii Region as the Basis for Geological Prognostication of Mineralization. // Rudy i Metally, N.3, pp. 19-29 (in Russian).

This report is a follow up to our theoretical and experimental study of the effect of vibration on the filtration properties of rocks and of the effect of shock decompression on a change in the permeability of the crack neighbourhood upon the vein formation. The runs were performed in the pressure range 25-300 MPa and temperatures 25-500^oC [1]. Original data were obtained showing that with certain duration, frequency, and amplitude of cyclic actions the rock permeability can either increase (runs N 5 and 8) or decrease (run N 12) [2].

In the aspect of high temperature high pressure geochemistry the obtained data suggest the following conclusions:

1) as compression decompression waves corresponding to the P-wave at earthquakes pass in the crust, the migration ability of chemical elements can grow by a factor of 1.2 (run N 8) for M3 at a distance to 0.67-1.75 km, for M5 to 32-100 km, and for M7 to 1000-300 km away form the seismic focus [2].

2) long (several days) exposure of a rock to infrasound waves, corresponding to compression decompression waves at stormic microseisms, can decrease the fluid permeability of oceanic bottom rocks by ≈3.7 times (run N 12[2]).

^# The work has been performed in collaboration with A.V. Zharikov (IGEM RAS), RFBR grant N 95-05-15354

1	Sample	Change in the confining pressure; max / min (bar)	Fluid and effective pressure; (bar)	Minimal effective pressure (bar)	Period and frequency of vibration(s, Hz)	Vibration duration (h)	Permeability before and after vibration(m2)
1	Basalt DSDP -1	245+ 90 335/155	85 160	+70	16 0.065	18	5.97.10-19 6.00.10-19
2	Basalt DSDP-2	235+ 5 240/230	200 35	+30	30.33	1/4	3.73.10-18 3.63.10-18
3	Basalt DSDP-5	235+ 5 240/230	180 55	+50	3 0.33	1/4	3.60.10-17 3.61.10-17
4	Limestone 83086	245+ 90 335/155	45 200	+110	16 0.065	2	1.73.10-17 1.79.10-17
5	Limestone 83086	246+ 80 326/166	180 66	-14	16 0.065	2	2.08.10-17 2.19.10-17
6	Limestone K-809	239+ 5 244/234	84 155	+150	3 0.33	1/2	4.79.10-17 4.74.10-17
7	Limestone 83086	245+ 90 335/155	42 203	+113	16 0.065	22	1.73.10-17 1.70.10-17
8	Limestone 83086	245+ 90 335/155	200 45	-45	16 0.065	¼	2.21.10-17 2.63.10-17
9	Limestone 83086	245+ 5 250/240	100 145	+235	16 0.065	14 days 11 ∙ 5 h	8.59.10-17 8.26.10-17
10	Limestone K-788	245+ 10 255/235	100 145	+225	16 0.065	10 days 8 ∙ 6 h	6.16.10-16 6.22.10-16
11	Limestone 83086	245+ 25 270/220	100 145	+185	16 0.065	10 days 6∙6 h	1.15.10-17 1.12.10-17
12	Limestone 83086	245+ 50 295/195	100 145	+145	16 0.065	10 days 5 ∙ 8 h	2.08.10-17 5.64.10-18

Fig.1. Example of the experimental cycle for one of the samples: simultaneous recording of the confining pressure and pore pressure. Interval 1 at the initial parameters of the run. Interval 2 cyclic pressure change. Interval 3 return to the initial parameters of the run. Interval 4 decrease of the pore pressure due to the commenced spontaneous fluid filtration through the sample.

3) in the process of formation in a porous massif of a system of regular noncommunicating veins a shock decompression leads to an increase of fluid permeability of rocks at a distance no longer than 2 meters from the crack [3], the total massif of permeability growth not exceeding decimal order [4].

In all the cases along with vibration frequency, decompression rate and strength, the relationship between lithostatic and fluid pressures is of paramount importance for a change in the filtration properties of rocks.

1. Shmonov V.M., Vitovtova V.M., Zharikov A.V. // Experiment in Geosciences, 1995, v.4, No.4, p.5-7.
2. Shmonov V.M., Vitovtova V.M., Zharikov A.V. (1997) // Physicochemical and petrophysical studies in the Earth's sciences., Nauka (Science), pp.59-60.
3. Shmonov V.M., Lakshtanov D.L., Borisov M.V. // Experiment in Geosciences, 1996, v.5, No2, p.18-20.
4. Shmonov V.M., Zaraiskii G.P., Vitovtova V.M. et al (1997) // Experimental and theoretical modeling of mineral formation processes., M., Nauka (Sciences)

Previous Contents Next