Abstract

Aqueous inclusions in oil-bearing vug-filling barite which postdates the karstic and fissural ore stages at Les Malines, one of the largest Mississippi Valley-type Zn-Pb deposits in Europe, have been studied by microthermometry and Raman spectrometry. The correlative variations of the melting points of ice (T (sub m ice ) and of the homogenization temperatures (T h define three main stages: (1) stage I, gas-rich inclusions characterized by T (sub m ice ) = -7 degrees + or - 1 degrees C and T (sub h (sub L-V(V)) ) = 180 degrees to 380 degrees C, (2) stage II, liquid-rich and gas-rich inclusions homogenized in the temperature range of 160[degree to 175 degrees C, with T (sub m ice ) = -8.2 degrees to -4.1 degrees C--stage I and stage II inclusions contain CO 2 and no CH 4 , N 2 , or H 2 S detectable by Raman spectrometry, and (3) stage III, liquid-rich inclusions, characterized by T (sub m ice ) which increased from -8 degrees to -1.7 degrees C as their T (sub h (sub L-V(L)) ) decreased from 160 degrees to 130 degrees C. They are essentially aqueous.The H 2 O-CO 2 -NaCl vapor-dominant stage I inclusions in barite cannot have leaked because their gas content is distinct from air. They are interpreted to have trapped a hot (T approximately 300 degrees C), CO 2 -bearing vapor which was cooling. Oil was thermally degraded during trapping in barite. The source of heat and CO 2 is considered to be volcanism contemporaneous with the major uplift of the horst during the Upper Jurassic, Dissolution of the carbonated wall rock is related to stage I. Stage II inclusions are interpreted to represent the end-product of the mixing of pore fluid with the hot vapor. The temperature of the pore fluid exceeded 150 degrees prior to the addition of magma-heated fluids. Such anomalous pore fluid temperatures are tentatively related to the steady dewatering of the Terres Noires shales from below, with the fluids moving up along the east-west fault of Les Malines near adiabatic conditions. The formation waters were probably responsible for the migration of the oil and for the recrystallization of the basement. Gas-rich and liquid-rich stage II inclusions are compatible with minor unmixing of the H 2 O-CO 2 -NaCl liquid with precipitation of calcite. The V-X properties calculated for a stage II liquid-rich inclusion (96.1 mole percent H 2 O, 1.85 mole percent NaCl, and 2 + or - 0.2 mole percent CO 2 ; V = 21.5 + or - 1.3 cm 3 mole (super -1) fix the pressure of unmixing at 175 degrees C in the range 285 + or - 45 bars. The T h -V-X properties of secondary CO 2 -bearing inclusions in barite from the earlier fissural stage (Charefand Sheppard, 1988) point to higher fluid pressures in the horst, in the range of 365 + or - 45 bars minimum. CO 2 -rich inclusions in the two barite generations imply that the metamorphosed Cambrian basement at Les Malines has been transiently geopressured below the Triassic shales. The addition of a small amount of magma-derived CO 2 to the pore fluid probably accounts for the rapid increase of pressure. The fluid pressure in the horst reached near-lithostatic values repetitively, first at the end of the Middle Jurassic subsidence, when the burial depth was around 1,600 + or - 200 m, then at a later stage, probably during the Upper Jurassic, when the sedimentary cover was partly eroded to around 1,200 + or - 200 m. The overpressuring, with a possible structural deformation resulting from the rising magma, probably caused the opening of the vertical, late barite-filled veins by hydraulic fracturing. Stage III inclusions are interpreted to indicate the influx of colder diluted fluid to the horst under hydrostatic pressure conditions, i.e., after decompression.

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