THERMOBAROGEOCHEMICAL FUNDAMENTALS AND APPLIED POSSIBILITIES OF LOCAL FORECAST, RESEARCH AND ASSESSMENT OF POST-MAGMATIC MINERALIZATION

Abstract

Important issues of local forecasting, search and assessment of post-magmatic mineralization during various stages of geological exploration are analysed on a thermobarogeochemical (TBGC) basis. The theoretical basis of TBGC-forecasting, exploration and assessment of mineralization is the stability of the regime of physical-chemical conditions for the formation of productive mineral paragenesis. They are formed in a fairly narrow range of TBGC-parameters changes of the specific to chemical composition and aggregate-density state of ore-forming medium at certain (optimal) temperature and pressure values. This is revealed in the phase typomorphism of the respective families of fluid inclusions practically regardless of geotectonic conditions and metallogenic specialization of the ore regions. For example, the molybdenum-tungsten formation of the greisen type is characterized by the following: alkaline-halogen (fluoride-chloride-potassium-sodium) composition of inclusions with a high salt concentration (65-35 wt.% NaCl), and the spread of syngenetic families of inclusions with minerals-"prisoners" (which testify to the boiling processes of solutions) and solutions of inclusions of critical density. On the other hand, in gold ore formations, the productive gold-bearing stages in the corresponding paragenesises contain water-carbon dioxide inclusions, which homogenize at 290-180°C under conditions of intensive heterogenization and degassing (СО2) of solutions with extremely different phase ratios (G-LСО2-Н2О; LСО2-LН2О; G-LСО2; LСО2-L; LСО2-GСО2). Fluid inclusions of two- and one-phase CO2 with wide variations in density (from 1.02 to 0.4-0.1 g/cm3 for various depth formations) and simultaneous homogenization into liquid and gas phases (boiling) are common. So, we determine the distribution of molybdenum-tungsten or gold mineralization by the distribution of the corresponding families of syngenetic inclusions. An equally important prerequisite for the implementation of applied TBGC problems is the possibility of diagnosis and spatial extrapolation of these parameters (TBGC-zonation) with the determination of the spatial position of zones that are physically and chemically favourable for the development of post-magmatic mineralization. Such tools can be the vertical paleotemperature gradient ∆Т/100 m and its inverse functional relationship with the vertical spread of mineralization, the ratio of the amount of reduced gases to the amount of oxidized gases, etc. Specific examples of mineralization forecasting, search, and assessment using the genetic features of the deposit and the complex application of TBGC-criteria for spatial-temporal modelling of mineralization distribution are given.