PACROFI VI - Electronic Program

Fluid Inclusions in the Study of Crustal Scale Paleo-Fluid Regimes

Bruce Nesbitt

Dept. of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3

Over the last several years, we have developed and pursued an extensive regional program on the study of origins, movement, chemical evolution, and effects of crustal fluids in relation to stratigraphic, structural, metamorphic, and igneous processes in the Canadian Cordillera. This program has covered in excess of 1,000,000km2, examined fluid regimes in a wide variety of lithologic and tectonic settings, and documented the linkages between crustal fluid processes and the generation of mineralization and hydrocarbons (Nesbitt and Muehlenbachs, 1995a).

The study of fluid inclusions has played a key role in this program providing regional information on the temperatures, salinities, gas contents and compositions, isotopic values, and ionic ratios of paleo-fluids. The unique aspects of our application of fluid inclusion studies to questions of geological significance deal with the scale of the area covered and the number of analyses conducted, totaling 1000's of heating and freezing determinations and over 1000 analyses of the deltaD values of inclusion fluids.

To illustrate the application of fluid inclusion analyses to the study of regional fluid regimes, three examples will be discussed showing how inclusion studies in combination with other mineralogical and geochemical work combine to produce comprehensive and innovative characterizations of crustal fluid regimes.

Extensional Regimes
Principal area of focus for studies of fluid regimes in extensional terranes has been the Eocene age, Omineca core complex of south-central British Columbia (Nesbitt and Muehlenbachs, 1995b). Regional fluid inclusion studies of metamorphic quartz +/- carbonate veins have documented a high degree of continuity of crustal regimes throughout the area. Th gradients are mapable and correspond to regional variations in metamorphic grade. A prominent correlation is present between high Th values and high concentrations of CO2 +/- CH4, indicating temperature as the principal control on total gas content. In contrast, gas compositions (CO2/CH4) vary with lithology with a distinct 5000 km2, high CH4 region corresponding in distribution with a particular lithologic package.

Fluid salinities are uniformly low (<5 eq. wt. % NaCl) as are deltaDFI(Fluid Inclusion) values (-100 to -150o/oo) indicating a meteoric origin for the vein-forming fluids. Synthesis of the fluid inclusion, isotope, and mineralogical studies indicates that the syn-extensional fluid regime was dominated by deeply convected meteoric water. The lower limit for penetration of the fluids appears to have been the brittle/ductile transition, with quartz veins, some with economic concentrations of Au, forming on the upflow limb of the convection cells.

Continental Margin Sedimentary Basins
From the Middle Proterozoic through the Paleozoic, the North American continental margin was an Atlantic style passive margin, with irregular rifting events. A coarse, hydrothermal, white, sparry, saddle dolomite is present in carbonate units of the margin, ranging in age from middle Cambrian in the southern Canadian Rockies to middle and upper Devonian in the northern Canadian Rockies. This dolomite is economically important, because of its association with major gas reserves, and Mississippi Valley type deposits, such as Pine Point and Kicking Horse.

Regional scale fluid inclusion studies of these dolomites (Qing and Mountjoy, 199x; Nesbitt and Muehlenbachs, 1994) have documented consistently high salinities (21 to 25 eq. wt. % NaCl) and low eutectic temperatures for the inclusion fluids. Homogenization temperatures range from 90 to 175oC and decrease from west to east along the Presqu'ile trend (Qing and Mountjoy, 1992). Studies of extracted inclusion fluids have documented consistently high deltaDFI values of -20 to -80 o/oo, throughout the north-south extent of the dolomite and high Br/Cl and I /Cl ratios (with Dr. W. Prochaska, Leoben).

Previously, the origin of this dolomite and associated MVT mineralization has been ascribed to topographically driven flow of meteoric fluids associated with the rise of the Rockies in the late Cretaceous to Tertiary (Garven, 1985). However the results from this study preclude this origin, since such fluids have deltaDFI values <-120o/oo, low salinities and low Br/Cl values. Instead, it is concluded that the dolomites and MVT mineralization is a product of pre-Laramide, west to east fluid flow, which probably occurred during the latest Devonian or early Mississippian. This interpretation is consistent with constraints on the age of dolomitization and MVT formation from Morrow and Aulstead (1995) and Nakai et al. (1993).

Fold and Thrust Belts
The third example to be examined is the evolution of fluid regimes during and following Late Cretaceous to Early Tertiary, development of the fold and thrust belt of the Canadian Rockies. At deep levels within the fold and thrust belt, deltaDFI values of quartz +/- carbonate veins record the presence of early devolatilizational fluids (deltaDFI >-90o/oo). This is overprinted by the widespread development of subvertical quartz +/- carbonate veins with deltaDFI values <-100o/oo, indicative of the influx of surface fluids at later stages of structural evolution. Fluid inclusion studies indicate that both early and late fluid regimes were characterized by low salinity fluids, with relatively low Th values.

The results from the Rockies document a clear contrast between syn-tectonic fluid regimes in extensional, such as the Omineca, versus compressional regimes. In compressional regimes, surface fluid penetration occurred relatively late in the tectonic sequence and was restricted to shallow depths. In addition, fluid temperatures relative to metamorphic grade of the host rocks are consistently lower in the fold and thrust belt in comparison to the extensional regimes. The differences in the nature of the fluid regimes is to a large extent a reflection of the difference in the nature of fracturing, since in extensional systems vertical fracturing provides ready access for surface fluids to penetrate to substantial depths (>10 km), whereas in compressional systems, sub-horizontal fracture systems are more restrictive to the deep penetration of surface fluids.

General Observations

  1. Using geological and geochemical techniques it is possible to identify and map distinct, crustal, paleo-hydrogeological systems.
  2. In extensional settings, surface fluids penetrate to great depths (>10 km) and precipitate most "metamorphic" quartz veins.
  3. In compresional settings, penetration of surface fluids is more restricted, with devolatilizational fluids dominating the fluid system at depth.
  4. The preservation of high deltaDFI values in quartz and/or carbonate veins from a number of different settings in the Canadian Cordillera indicates that contamination of deltaDFI values by unrelated secondary inclusions is not a common problem.
  5. In general, mineralization is restricted to those areas with regional indicators of high paleo-flux and flow rates.