Mesothermal

Abstract: During Eocene time, 37 to 49 m.y. ago, a series of large hydrothermal systems was developed around a group of epizonal granite plutons in the Idaho batholith. These systems involved deep and extensive circulation of fluids derived from low-δ 18 O (∼−16) and low-δD (∼−120) meteoric waters. Water-rock interactions occurred at temperatures of 150 to 400 °C, lowering the 18 O/ 16 O and D/H ratios in the surrounding Mesozoic rocks (tonalite, granodiorite, and granite), such that the feldspar δ 18 O and biotite δD values became as low as −8.2 and −176, respectively. These values contrast markedly with the primary isotopic compositions of +9.3 ± 1.5 and −70 ± 5, respectively. Widespread propylitization of the Mesozoic plutonic rocks accompanied these isotopic exchange effects. Systematic mapping shows that anomalous δD and δ 18 O values occur over more than 15,000 km 2 , indicating the extensive lateral dimensions of the ancient circulating systems. The former zones of intense hydrothermal activity are marked by low- 18 O zones, which were mapped in the vicinity of the margins of several Eocene plutons (for example, at Rocky Bar) and also within a giant (5- to 20-km wide, 60- to 40-km diam) ring zone that surrounds the Sawtooth Mountains. The latter anomaly is coincident with the high-permeability ring fracture zone of an Eocene caldera system. Most of the ore deposits in the southern half of the Idaho batholith are epithermal and mesothermal Au-Ag veins that are located near the periphery of the low- 18 O zones (that is, near the outermost δ 18 O = 8 isopleth). This association links these deposits with the Tertiary hydrothermal activity and has great potential as an exploration tool in the heavily forested region. Evidence is presented that the Eocene ground-water circulation pattern was affected over large lateral distances (25 to 50 km) and great depths (5 to 7 km). These conclusions, together with the indications that large amounts of water (> 7,000 km 3 ) were involved in some systems and that the circulation patterns probably are related to caldera ring structures, may be of particular importance in geothermal exploration and exploitation of analogous modern systems. For example, the “fossil” hydrothermal activity mapped in the Idaho batholith may be characteristic of deep-level fluid circulation in geothermal systems such as Yellowstone National Park, Wyoming. In such regions, the major zones of hydrothermal activity seem to be principally associated with either (1) the caldera ring zones or (2) the central plutons (resurgent domes).

Abstract: Mesothermal gold provinces of Phanerozoic age are characteristically associated with regional structures along which allochthonous terranes have been accreted onto continental margins or arcs. A recurring sequence of transpressive deformation, uplift, late kinematic mineralization, and shoshonitic magmatism is consistent with thermal reequilibration of tectonically thickened crust. Mesothermal gold camps in the Superior province are spatially associated with large-scale structures that have been interpreted as zones of transpressive accretion of individual subprovinces or allochthonous terranes: these boundary structures are characterized by the sequence of significant horizontal shortening, uplift, late-kinematic mineralization, and shoshonitic lamprophyres and therefore may have the same geodynamic significance as Phanerozoic counterparts. In this model, thermal re-equilibration of underplated and subducted oceanic lithosphere and sediments in a transpressive regime, over time scales of 10 to 40 m.y., is a necessary precursor to gold mineralization. Hydrothermal fluids are released along boundary faults and their splays during uplift: the uniform temperature, low salinity and mole% CO2 signify uniform source conditions, whereas the variable O, C, Sr, and Pb isotopic compositions of fluids reflect lithological complexity of the source regions and conduits. Ou the basis of this model it is suggested that mesothermal lode gold deposits are the product of subduction-related crustal underplating and deep, late metamorphism, rather than magmatic or metamorphic events in the supracrustal rocks. Secular variations in the generation of Archean, Proterozoic, and Phanerozoic mesothermal Au provinces reflect the timing of collisional orogenies within terranes of these eras.