Bruce E. Nesbitt

Dual origins of lode gold deposits in the Canadian Cordillera

From Late Jurassic to late Tertiary time, two geologically, geochemically, and genetically distinct gold mineralization processes were active in the Canadian Cordillera. One group of deposits can be characterized as epithermal because of its association with intermediate to felsic volcanics, regional caldera structures, low pH alteration zones, low Au/Ag values, and quartz-chalcedony-barite-fluorite gangue. The second group of deposits is mesothermal in character and has strong similarities to the Mother Lode deposits of California, being associated with transcurrent faults, intermediate pH alteration zones, and quartz ± carbonate, albite, mariposite, pyrite, arsenopyrite, scheelite gangue. Compared to epithermal deposits, mesothermal deposits have higher As, W, and Au/Ag values, higher CO2 content in fluid inclusions, and δ18O values of ore-forming fluids of +3‰ to +10‰ vs. −14‰ to −7‰ for epithermal deposits. Like the gold deposits in Nevada and Colorado, epithermal mineralization in the Canadian Cordillera formed from the shallow circulation of meteoric water in subaerial, intermediate to felsic volcanic complexes. In contrast, mesothermal gold deposits throughout the North American Cordillera are shown to be the product of deep circulation and evolution of meteoric water in structures associated with major, transcurrent fault zones. Similarities between Archean lode gold deposits and mesothermal deposits of the Cordillera suggest that Archean lode deposits may have been produced by processes similar to those involved in the formation of Cordilleran mesothermal deposits.

Origins and Movement of Fluids During Deformation and Metamorphism in the Canadian Cordillera

Stable isotope data from quartz veins in the Canadian Cordillera indicate that crustal fluids were heterogeneous in terms of sources and flow paths during Mesozoic-Cenozoic metamorphism and deformation. In regions of strike-slip and extensional faulting, the fluid regime to depths of at least 15 kilometers was dominated by convected, chemically evolved meteoric water. In contrast, in thrust faulted regions, the fluid regime was dominated by fluids derived from metamorphic devolatilization reactions. Deep convection of meteoric water implies that fluid pressures are hydrostatic in such systems not lithostatic, as had been commonly assumed. The occurrence of significantly lower fluid pressures would necessitate reevaluation of the manner in which metamorphic phase equilibria and stress relations in the crust are modeled. In addition, this study indicates that mesothermal gold deposits in the Canadian Cordillera are a product of the meteoric water convection process.

Chemical composition of biotite from the Casino porphyry Cu–Au–Mo mineralization, Yukon, Canada: evaluation of magmatic and hydrothermal fluid chemistry

Abstract The chemical composition of biotite has been determined for the Dawson Range batholith and the Casino, porphyry-style Cu–Au–Mo occurrence, Yukon Territory, Canada. Biotite from the propylitic, phyllic and potassic alteration zones of the Casino occurrence possesses higher X Mg , Al 2 O 3 , SiO 2 , F and Cl, and lower TiO 2 , BaO and MnO concentrations than biotite outside these alteration zones. This phenomenon is similar to that recorded for biotite of other porphyry Cu deposits. However, the absolute concentrations of the oxides and halogens vary significantly among deposits. Calculated log ( f H 2 O)/( f HF), ( f H 2 O)/( f HCl), ( f HCl)/( f HF) ratios, determined from biotite microprobe data, indicate that hydrothermal fluids associated with potassic alteration at Casino were similar to those associated with phyllic alteration. In contrast, fluids responsible for propylitic alteration had higher log ( f H 2 O)/( f HF) and less negative log ( f HF)/( f HCl) values. The log ( f H 2 O)/( f HCl) and ( f HF)/( f HCl) ratios established for Casino are similar to those for other porphyry Cu deposits. However, log ( f H 2 O)/( f HF) values vary greatly among porphyry deposits. This variation in log ( f H 2 O)/( f HF) values may be related to the source of the magmas and/or magmatic processes (assimilation/fractional crystallization) associated with melt on its ascent through the crust prior to the exsolution of magmatic fluids that are responsible for porphyry mineralization. The biotite compositions from the Dawson Range batholith are either not consistent with the Fe–F and Mg–Cl avoidance principles or record changes in the F/Cl ratio of the fluid during crystallization of the granitic body.

Paleohydrogeology of the Canadian Rockies and origins of brines, Pb-Zn deposits and dolomitization in the Western Canada Sedimentary Basin

Results of stable isotopic and fluid-inclusion studies of Cambrian carbonate units in the southern Canadian Rockies indicate that, during the Late Devonian or Early Mississippian, a warm brine migrated from miogeoclinal shales in the western part of the Western Canada Sedimentary Basin into the eastern carbonate sequences. This fluid had a temperature of 150 ±25 °C, salinity values of 20 to 25 equivalent wt% NaCl, and isotopic values of δ 18 O = -7‰ to 0‰,δ 13 C = -1‰ ±2‰, δD = -70‰ ±19‰, and 87 Sr/ 86 Sr >0.710. The fluids formed large bodies of coarse, sparry dolomite, as well as Pb-Zn, magnesite and talc mineralizations in the Cambrian units. It is probable that this event was also responsible for the development of Pb-Zn mineralization (specifically Pine Point) in the Devonian units, as well as a component of the present-day basinal brines in the basin.

Gold deposit continuum: A genetic model for lode Au mineralization in the continental crust

A model is presented dealing with the relations between convection in the continental crust and the formation of lode Au deposits. Sufficient permeability exists in the brittle rheologic regime in large sections of the continental crust to permit convection of meteoric water on both local and regional scales. The solubility of Au in this fluid is high enough to permit the mobilization of quantities of Au which are sufficient to form ore-grade deposits. This general process is shown to be responsible for a continuous series of deposit types ranging rom 1-2-km-deep epithermal deposits, to intermediate-depth Carlin-type deposits, to 10-12-km-deep mesothermal deposits. Major geologic and geochemical differences among the deposit types are products of fluid convection at variable depths and the resulting effects on water/rock ratios and gas solubilities.

Geochemical studies of the origins and effects of synorogenic crustal fluids in the southern Omineca Belt of British Columbia, Canada

Fluid inclusion and stable isotope investigations of 400+ samples of quartz ± carbonate veins and their zeolite to amphibolite grade host rocks from the southern Omineca Belt of the Canadian Cordillera have been used to determine origins, evolution, and effects of crustal fluids during and after orogenic activity. Correlations between fluid inclusion characteristics and tectonic and lithologic features indicate that salinities and gas compositions (CO 2 /CH 4 ratios) are controlled by host rock lithology. High total gas contents are linked to high inclusion homogenization temperatures, which parallel increasing metamorphic grades. The δD values of the vast majority of vein-forming fluids range from −100% to −150% (Standard Mean Ocean Water [SMOW]), indicating meteoric water as the source of the fluids. The δ 18 O Water values of the fluids were homogeneous over large sections of the crust, with a typical range of δ 18 O values of 6% to 11% (SMOW). Homogenization of δ 18 O values of vein-forming fluids is believed to reflect interaction of the meteoric water with a variety of rock types at temperatures in excess of 350 °C. This process resulted in resetting of δ 18 O values of both fluids and rock units. The absence of vein formation from low δ 18 O water requires that mixing of ascending and descending fluids does not occur. Carbon and strontium isotope analyses indicate a strong degree of host rock control on these isotopic ratios. Synthesis of the results yields a model for the hydrogeology of the brittle crust consisting of moderately high permeabilities in fractured brittle rocks with deep convection of surface, meteoric water. The maximum depth of penetration of the fluids is limited by the rheological brittle/ductile transition at temperatures of 350 to 450 °C and depths of 10 km or greater. The vast majority of veins found in greenschist and lower grades of metamorphic rocks are formed on the upflow limbs of the meteoric water convection cells.

Stable isotopic and fluid inclusion indications of large-scale hydrothermal paleoflow, boiling, and fluid mixing in the Keno Hill Ag-Pb-Zn district, Yukon Territory, Canada

The Keno Hill vein system of the central Yukon is restricted predominantly to the highly fractured, graphitic Keno Hill Quartzite unit of Mississippian age. Hydrothermal mineral zoning is related spatially to a Cretaceous granitic pluton which intrudes the quartzite. During mineralization, the quartzite acted as a district-scale aquifer. Subsequent erosion has exposed a 40-km long vein system, from its plutonic roots, outward to polymetallic Ag-Pb-Zn veins, and further to assemblages of epithermal character. The δ18Oquartz values from veins near the pluton increase outwards from +10.6 to +20.1%. as a result of cooling of the hydrothermal fluids and exchange with the quartzite. Contours of isotope values outline broad paths of fluid movement within the quartzite. Proceeding further from the pluton, δ18Oquartz values decrease to +10.5%. at the outer edge of the system. The presence of meteoric water is indicated here, where late stage quartz hasδ18OSMOW values as low as −7.l%. The outward decreasing trend appears to have been established by mixing of isotopically light meteoric water with exchanged fluids that were in isotopic equilibrium with the quartzite. Fluid inclusions from quartz in the orebodies demonstrate an evolving H2O-CO2-NaCl-CH4 system. Loss of CO2 and CH4 during water vaporization coincides with increasing salinity and decreasing temperature resulting from high enthalpy steam loss. Depressurization during active faulting is the principal mechanism. Late stage fluids are represented by dilute aqueous inclusions with lower homogenization temperatures. Quartz from silver-rich veins has been shifted to higher δ18O values, by up to 4%. relative to adjacent silver-poor veins, the result of a minimum 10–25% adiabatic boiling and fractionation dominated by water vaporization and associated cooling. Graphite initially buffered the hydrothermal fluids to a high CO2 content, with variable CH4. Involvement of organic carbon from the host rocks is indicated by the negative δ13CPDB values for the carbonates, from −4.0 to −12.9%. Variations in the carbon isotopes result from fluctuating CO2CH4 ratios, reflecting the contrasting volatility of the gas pair. Siderite formed as a late-stage product of the boiling event, and its formation coincides with a decreasing 18O trend in the water created by the equilibration of graphite and water in replacing exsolved CO2. The formation of CH4 during this stage had a reducing effect on the fluid, resulting in an increase in δ13Csiderite values in association with the decreasing δ18Osiderite values. A closed-system boiling model, together with calculations of water consumption during post-boiling CO2 and CH4 formation, indicates that greater than 50% of the original water in the ore fluid was removed. Relatively saline mineralizing fluids resulted.

Stable isotopic constraints on the nature of the syntectonic fluid regime of the Canadian Cordillera

Oxygen, hydrogen and carbon isotopic analyses of rocks from the Canadian Cordillera document regional variations in the origins and evolution of crustal fluids. High temperature systems in unmetamorphosed, felsic volcanic rocks are characterized by low {delta}{sup 18}O values. Veins in regional metamorphic rocks are characterized by {delta}{sup 18}O values that reflect approximate oxygen isotopic equilibration of the fluids with regional rock units. The {delta} D studies indicate that both systems were dominated by meteoric water. The {delta}{sup 13}C values for vein carbonate indicate a regional lithologic control on {delta}{sup 13}C values. The data indicate that convection of surface fluids to depth of 10 or more kilometers during orogenesis was common. The existence of deep convection cells involving meteoric water further indicates that fluid pressures to these depths must have been approximately hydrostatic. Regional structural style appears to exert an important control on the depth of penetration of surface fluids.

Surface fluid convection during Cordilleran extension and the generation of metamorphic CO2 contributions to Cenozoic atmospheres

Modeling of hydrogeological regimes associated with crustal extension indicate that flux values for deeply convected meteoric water are on the order of 10 −3 m 3 ċ m −2 ċ yr −1 . Calculated CO 2 fluxes produced by infiltration-driven, metamorphic decarbonation reactions along the circulation path are 2.4 × 10 −4 m 3 ċ m −2 ċ yr −1 . Appication of the model to Cenozoic extension in the North American Cordillera demonstrates that CO 2 generated in this manner may have been a major contributor to elevated CO 2 contents of Cenozoic atmospheres and the resulting global warming due to CO 2 greenhouse effect.

METEORIC WATER COMPONENT IN MAGMATIC FLUIDS FROM PORPHYRY COPPER MINERALIZATION, BABINE LAKE AREA, BRITISH COLUMBIA

Coexisting vapor-rich and high-salinity “magmatic” fluid inclusions from mineralized quartz ± carbonate veins in porphyry Cu systems of the Babine Lake area are depleted in deuterium (D) relative to accepted magmatic isotopic values. The δD compositions of high-salinity fluids from early mineralized veins range from −100‰ to −135‰ (standard mean ocean water), whereas fluid inclusions in late veins and breccias have δD values between −138‰ to −153‰, and unmineralized, regional veins have δD values from −94‰ to −150‰. Multiple δD analyses of fluids from inclusions are within ± 5‰ for 14 of 17 duplicate analyses, which suggests that mixing of fluid-inclusion populations did not contribute significantly to the D-depleted values. A correlation between δ 13 C values of vein carbonate and δD values of inclusion fluids indicates that postentrapment reequilibration did not significantly alter isotopic compositions of fluid inclusions. Incorporation of externally derived low-D material into the mineralizing intrusions, either from the influx into the melt of deeply convected, evolved meteoric fluids or by crustal assimilation of D-depleted country rock, is required to explain the observed D-depleted values of the magmatic fluids.