Heat and Mass Transport in Hydrothermal Mineralizations

P. Möller GeoForschungsZentrum Potsdam PB 4.3 Telegrafenberg 14473 Potsdam, Germany


R. Ondrak GeoForschungsZentrum Potsdam PB 3.4 Telegrafenberg 14473 Potsdam, Germany

Calcite and quartz are frequent minerals in hydrothermal vein mineralizations. Above 150°C, both minerals become more soluble with increasing temperature and pressure. Quartz precipitation is nearly independent of solution composition and pH under natural conditions, whereas calcite deposition depends on ligand contents and pH of solution. Furthermore, effervescence of CO2 favors calcite precipitation. However, calcite can be deposited from CaCl2 fluids if small amounts of HCO3- are present.

In extensional regimes dilatational jogs open and hot fluids can enter from below. These hot fluids become oversaturated if cooled. The amount of precipitate is controlled by temperature decrease of the fluid during its ascend in the jogs. On their way back the fluids are heated up again but still remain cooler than the source fluids because of heat loss to the host rocks.

Using known solubility data as a function of temperature and pressure in combination with information on fluid flow, the time needed for deposition of a 10 cm thick vein mineralization covering the wall rock uniformly is calculated. Because the opening of veins is tectonically controlled, the vein will not be clogged completely by precipitates, its width is considered to extend during mineral deposition.

Two procedures are applied to solve the problem:

(i) Using a geometrical approach for heat dissipation and fluid flow in veins and assuming a dependence on flow velocity and open vein width, cooling of fluids is calculated. The derived decrease of temperature and pressure is applied to quantify the amount of mineral precipitated in one cycle.

The time to precipitate a given quantity of precipitate is inversely proportional to flow velocity and nearly independent of the vein width.

From the expected total amount, the number of cycles and, thereby. the total time needed for precipitation is derived. This is in the range of 103 to 105 years for calcite depending on pH. Because of assumed fluid circulation the total amount of fluids is 10-3 to 10-5 of the quantity needed when the cooled fluid would leave the system.

(ii) As a second approach a schematic numerical model simulating coupled heat and mass transport is used to model the temporal and spatial evolution of hydrothermal vein deposits. The computer model allows to calculate the temperature evolution in the fracture zone and its surroundings resulting from convective and conductive heat transport. The coupled mass and temperature calculations are used to model the temperature-dependent precipitation of vein minerals like quartz or calcite.

Present calculations show that high vertical flow rates (v>10 m/a) are necessary to obtain the measured high temperatures in the mineralizations. The upward directed flow must be very localized to avoid an undue heating of the host rock. Every flow system consists of two branches therefore a downward direct flow is assumed in wide zones of lower porosity and permeability. This flow branch will cool its surroundings with time. Eventually, it will lead to a short cut of the thermal system, therefore, bringing mineral precipitation to a halt. Beside the large circulation system, local convection cells in the fracture system may develop which recycle the fluid within the fractures. First modeling results show that a combination of local convection cells and large scale circulation can explain the temperature distribution and amount of mineral formation. To precipitate up to 10 cm in a fracture system of 50 cm total width a time span of 2*105 years is anticipated according to previous simulations.

Coupling of heat and mass transport in the numerical model may help to gain a better understanding of the mechanisms controlling the formation of vein deposits. Especially the combination of temperature distribution and mass balance calculations should open new possibilities to constrain time span and mass transport necessary to form vein deposit