The Physical Hydrogeology of Ore Deposits

Abstract

Hydrothermal ore deposits represent a convergence of fluid flow, thermal energy, and solute flux that is hydrogeologically unusual. From the hydrogeologic perspective, hydrothermal ore deposition represents a complex coupled-flow problem—sufficiently complex that physically rigorous description of the coupled thermal (T), hydraulic (H), mechanical (M), and chemical (C) processes (THMC modeling) continues to challenge our computational ability. Though research into these coupled behaviors has found only a limited subset to be quantitatively tractable, it has yielded valuable insights into the workings of hydrothermal systems in a wide range of geologic environments including sedimentary, metamorphic, and magmatic. Examples of these insights include the quantification of likely driving mechanisms, rates and paths of fluid flow, ore-mineral precipitation mechanisms, longevity of hydrothermal systems, mechanisms by which hydrothermal fluids acquire their temperature and composition, and the controlling influence of permeability and other rock properties on hydrothermal fluid behavior. In this communication we review some of the fundamental theory needed to characterize the physical hydrogeology of hydrothermal systems and discuss how this theory has been applied in studies of Mississippi Valley-type, tabular uranium, porphyry, epithermal, and mid-ocean ridge ore-forming systems. A key limitation in the computational state-of-the-art is the inability to describe fluid flow and transport fully in the many ore systems that show evidence of repeated shear or tensional failure with associated dynamic variations in permeability. However, we discuss global-scale compilations that suggest some numerical constraints on both mean and dynamically enhanced crustal permeability. Principles of physical hydrogeology can be powerful tools for investigating hydrothermal ore formation and are becoming increasingly accessible with ongoing advances in modeling software. * ### Notation a : total fracture aperture after dilation a : initial aperture A : cross sectional area [L2] b : thickness [L] c : specific heat capacity (usually isobaric heat capacity) [E M−1 T−1] c b : bulk compressibility of porous medium at constant fluid pressure [L t2 M−1] c s : bulk compressibility of rock matrix [L t2 M−1] c u : uniaxial compressibility of the porous medium [L t2 M−1] C : aqueous concentration [M L−3] D : hydrodynamic dispersion [L2 t−1] D w : diffusion coefficient in open water [L2 t−1] E : energy [E] F : fluxibility [M L−3 t−1] g : gravitational acceleration [L t−2] G : shear modulus, [M L−1 t−2]. H : specific enthalpy [E M−1] k : intrinsic permeability [L2] k : reference intrinsic permeability [L2] k r : relative permeability [dimensionless] K : thermal conductivity [E t−1 L−1 T−1] L : characteristic length or distance [L] M : mass [M] P : pressure [M L−1 t−2] P c : capillary pressure [M L−1 t−2] q : volumetric flow rate per unit area (volume flux, specific discharge or Darcy velocity) [L t−1] R : general source/sink term for mass, heat, or chemical reactions [variable] s s : specific storage [L−1] S : volumetric saturation [L3 L−3, dimensionless] t : time [t] T : temperature [T] u : displacement vector [L] U s : shear displacement [L] v : average linear velocity (seepage velocity) [L t−1] X : mass fraction H2O, NaCl, or CO2 in an H2O-NaCl-CO2 mixture [dimensionless] z : elevation above a datum, vertical Cartesian coordinate, or depth [L] z g : elevation parallel to the direction of gravity [L] α : dispersivity [L] α e : effective stress coefficient, [dimensionless] α T : porous medium linear thermal expansivity [T−1] β : bulk fluid compressibility [L t2 M−1] θ : potential per unit mass [E M−1] λ : coefficient of friction [dimensionless] μ : dynamic viscosity [M L−1 t−1] v : Poisson’s ratio [dimensionless] ρ : density [M L−3] σ : stress [M L−1T−2] σ eff : effective normal stress σ nref : effective normal stress to a fracture τ : tortuosity [dimensionless] φ : porosity [dimensionless] φ e : effective porosity [dimensionless] Φdil : shear dilation angle ( ∧ ) : indicates increase or decrease in a quantity (-) : indicates a nondimensionalized quantity * ### Subscripts f : refers to the fluid mixture in place (either a single phase or a two-phase mixture) l : refers to liquid m : refers to the porous medium r : refers to the rock v : refers to vapor (steam) : refers to an initial state