Abstract Tengiz is a giant oil field on the northeastern coast of the Caspian Sea in Kazakhstan that produces a high-gravity, hydrogen sulfate (H 2 S)-rich oil from a reservoir containing abundant solid bitumen. Several lines of geochemical and petrographic evidence suggest there were at least two stages of petroleum migration into the Tengiz reservoir, both generated off structure from a marine source rock. The initial charge gave rise to solid bitumen, perhaps by a de-asphaltening process. Bitumen formation was followed by a period of hydrothermal activity, which thermally matured the bitumen to an insoluble pyrobitumen, produced bitumen-freepores, precipitated calcite on the bitumen, and mineralized parts of the Tengiz flank. Finally, a second petroleum charge, most likely from the same source at higher maturity, accompanied by a significant in-flux of H 2 S arising from thermochemical sulfate reduction (TSR) deep in the basin, filled Tengiz with its present-day oil. The Tengiz reservoir consists of Carboniferous and Devonian limestones with mostly grainstone and packstone textures that define an isolated mound with a central platform and surrounding flank. Do-lomitization and silicification are sparse; cements are sparry calcite. The reservoir is divided into unit 1 (∼3950-4500 m; ∼12,959-14,763 ft), unit 2 (∼4500-5100 m; ∼14,763-16,732 ft), and unit 3 (∼5100 to >5600 m; ∼16,732 to >18,372 ft). Porosity average is 7% bivolume (BV) (range 0-20%). Where solid bitumen is abundant, it typically occupies 3% BV (range 0-15%), but bitumen occupies less than 1% BV in large parts of the reservoir. Bitumen is commonly encapsulated with calcite cement. Typically production preferentially enters boreholes from a few meter-thick intervals. Top reservoir temperature is 105°C, and initial pressure is 11,500 psi (79.2 MPa).

Abstract Gold mineralization at the Victory mine, Kambalda, is associated with discrete metasomatic alteration zones around quartz breccia zones, shear zones, and quartz vein arrays. The mineralogy, textures, and whole-rock chemistry of the wall-rock alteration zones are described for several different host rocks. Mineral assemblages at zone boundaries, calcite-dolomite geothermometry, and amphibole geobarometry have been used to estimate the temperature, pressure, and fluid composition associated with metasomatism. Fluid inclusion data have been used to estimate independently these conditions. Wall-rock alteration zones extend up to 3 m from veins and breccias at the Victory mine. Textures indicate that the zoned wall-rock alteration and associated gold mineralization postdated regional metamorphism and outlasted retrograde carbonation. Chemical variations across zoned alteration profiles indicate that alteration occurred at approximately constant volume. Outer alteration zones are characterized by the addition or loss of H 2 O, CO 2 , Na, and K whereas Al, Mg, Ca, Fe +2 , and Fe +a were mobile in the inner alteration zones. Chemical changes and mineralogy of the alteration envelopes depended critically on the initial composition of the host rock which affected the resultant mineral assemblages. Assuming that local equilibrium conditions existed at alteration zone boundaries, mineral compositions from microprobe data have been used to model equilibria in the system SiO 2 -Al 2 O 3 -MgO-CaO-K 2 O-H 2 O-CO 2 . The mineral equilibria together with calcite-dolomite geothermometry provide an estimate of 390° ± 40°C for metasomatism which is similar to a minimum temperature estimate of 370° + 30°C from fluid inclusion data. Mineral equilibria and fluid inclusion data suggest that pressure during metasomatism was approximately 1.7 to 2 kbars. Fluid inclusion data indicate that metasomatism was associated with a homogenous H 2 O-CO 2 -NaCl fluid containing 19 to 36 wt percent CO 2 CX CO2 = 0.1-0.2) and 8 to 9 equiv wt percent NaCl. The data presented in this study indicate that metasomatism occurred at considerably lower temperatures and pressures than those estimated for peak metamorphic conditions at Victory. Thus after peak metamorphism, substantial uplift occurred before the hydrothermal emplacement of gold.