THE PROBLEM OF GRANITE-FLUID PORPHYRY-MINERALIZATION GENETIC ASSOCIATION

Tsarev D.I., Batueva A.A.

Geological Institute SB RAS, Ulan-Ude, Russia, dmtsarev@mail.ru

The subvolcanic bodies of porphyries are fixed in the ore deposits of various types in different regions and continents in close association with the mineralization. 70 % of various type ore deposits are spatially associated with the porphyry subvolcanic bodies in the North American continent.

In petrographical view, the subvolcanic porphyries were first studied in more detail by E.B.Yakovleva in Kazakhstan. She called them automagmatic breccias (AMB).

The significant autometamorphic changes are peculiar for them. The body forms are bedded, dyke-like, irregular that are correlated to thin apophyses into the host rocks.

G.F.Yakovlev and E.B.Yakovleva (1973) separated these formations into the fluid porphyry complex, where AMB, in their opinion, are derivates of the fluid-saturated magma. The higher contents of Al₂O₃, Na₂О, K₂О (with К₂О dominance over Nа₂О), volatiles (Н₂О, СО₂, SO₂, F, C1, В, etc.), higher contents of Pb, Zn, Sn, Mo, etc. compared to the Clark ones are noted in magmatic rocks associated with this magma. These rocks are very often closely associated with the intrusions of main composition (dykes and subvolcanic bodies of basalts-diabases). A great number of xenoliths with diabase structure, sharp inconformity of porphyry impregnations with the main mass by composition not rarely observed in them (Koptev-Dvornikov, Yakovleva, 1971). Thus, for instance, the porphyry P₁ extracts up to N92), have been described. If these are the first extracts of impregnations, then average composition of plagioclase should be N73 that does not correspond to the diparite- dacite composition of rock. Thus, this plagioclase evidently crystallized in magma of main composition.

A spatial association of fluid porphyries with pyrites-polymetallic, copper-pyrites and other ore deposits is widely spread in the Ore Altai, Salair, Urals, Caucasus, Transbaikalia and other regions of our country. P.F.Ivankin (1963) in the Ore Altai, G.L.Pospelov and A.S.Lapukhov (1971) in Salair, I.S.Vakhrameev (1969) and A.S.Bobokhov (1976) in the Urals; L.P.Khryanina (1967), D.I.Tsarev, A.P.Firsov (1988) in Transbaikalia and many other researchers paid much attention the essence of this association. Each researcher presented the observed facts and arrived to this or that conclusion. The conclusion that the association is not accidental results from all the observations in this direction. The difficulty of its deciphering is in the fact that the mineralization formed not only later than crystallization of fluid porphyries, but after their tectonic dislocation as well: cataclase and schist formation. Fluid porphyries themselves appear no rarely mineralized. These facts support the ideas by G.L.Pospolov and A.S.Lapukhov (1971) that the porphyry bodies were ways of filtrating the solutions from depth, containing the higher amounts of ore components. An association of pyrites-polymetallic mineralization with porphyry intrusions was well illustrated in Atlas of ore field morphostructures developed by the research group of SNIIGGIMS, edited by P.F.Ivankin (1973).

The fluid porphyries of the Ozeornoe deposit are ubiquitously altered by magmatogenic postmagmatic processes, and deciphering the initial appearance and composition is significantly difficult. The impression originates that initial, purely magmatic appearance of these rocks did not exist, as origin and alteration of melt and then recrystallized bodies as well under effect of transmagmatic solutions occurred simultaneously. It seems to us that fluid porphyries are the most typical representatives of magma formations under influence of transmagmatic solutions in the subvolcanic facies of depth. They support the idea of D.S.Korzhinsky (1972) about transmagmatic solutions and magma replacement.

The process of phases transformations in ore inclusions is as followed: the beginning of melting – 700-750°С, temperature of silicate phase melting – 850-975^oC, magnetite – 1000-1050°С, complete homogenization (dissolution of gas bubble) – 1050-1100°С. The inclusion volume increases after the 2-3 times heating, in addition, the main increase of the volume occurs during heating to 900°С (to 900°C, ore inclusion (OI) volume increases about 1,7 times). The water content is averagely 2,6% in the fluid phase of OI, 2,2 % - in silicate phase (part of muscovite is 0,5), 4,8 % - totally in OI; 2,1% - in the initial melt at the account of increase in vacuole volume to temperature 1100°C) (2,3 times); 2,8%.- to temperature 900°С (1,7 times). However, this water content can likely be considered minimal, as the fluid has more complex composition in OI.

The high initial water content, and particularly, the presence of the second, except water hardly soluble fluid should result in early distillation of melt. The melt boiling is the process that is followed by volume increase. Therefore, fluid pressure sharply increases during the retrograde boiling (Bernam, 1983). A discharge, i.e. melt introduction in overlapping rocks occurs at the moment when the internal pressure exceeds the limit of roof strength.

If magnetite grains of melt inclusions are considered as xenogenic hard phase trapped from undermelted rocks of dykes with main composition, then the temperature of melting the silicate phase of melt inclusions will be 850-875º С.

In geological literature, the information is available that fluid porphyries trace back to granitoid intrusions. And they rise into the upper crustal levels often along dykes of basites, replacing them and leaving traces as xenoliths. Rather large gabbroid xenoliths sometimes occur.

The subvolcanic intrusive bodies of intensely altered fluid porphyries are traces of transmagmatic solution outflows into the upper levels at magmatic replacement of gabbroids in formation of large granitoid plutons.

References

Bernam K.Y. (1983) The significance of volatile components. Evolution of igneous rocks. M. Mir. P.425-467.

Ivankin P.F. (1963) About morphological types of small intrusion clusters and hydrothermal streams // Doklady of the USSR Acad. of sci.Докл. v.149. N4 P.925-627.

Korzhinsky D.S. (1972) Flows of transmagmatic solutions and processes of granitization // Magmatism, formations of crystalline rocks and deapths of the Earth. М. Nauka. P.144-153.

Pospelov G.L. Lapukhov A.S. (1971) Structure and development of ore-forming fluid dynamic systems with polymorphic zonation (on example of the Salair ore field) // Physical and chemical proc. in dynamic ore-forming systems. Novosibirsk. Nauka.

Tsarev D.I., Firsov A.P. (1988) The problem of pyrites deposit formation. M. Nauka.

Vakhromeev I.S. (1969) About secondary volcanic type stratas of pyrites mineralization regions of development in the South Urals // Doklady of the USSR Acad. of sci. V.187. N3. P.625-628.

Yakovlev G.F., Yakovleva E.B. (1973 The ore-bearing fluid porphyry complexes of South-Western Altai // Vestnik of MSU. Series 4. Geology. N2. P.72-86.