Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 77

1000

Arkansas
Mississippi

Louisiane

100

10

1

0,1

sea water

0,01
10 2 10 3 10 4 10 5 10 6

Cl - (mg /l)

Fig. 4.2 Solubility of zinc as a function of chlorine of the hydrothermal fluid. The solubility
increases dramatically when the Cl content is higher than 105 mg L 1 through the formation of
chlorine complexes. If the Cl content decreases, for example when the fluid is diluted, Zn
precipitates and can form deposits (Modified after Cathles and Adams 2005 )

4.2.3 The Trigger of Fluid Circulation

In past decades a process called lateral secretion was discussed as a possible ore-
forming process. The idea was that diffusion of metals and other elements along a
thermal or chemical gradient could lead to the precipitation of metals in a restricted
location and thus to the formation of an ore body. This idea has since fallen out of
favour because it has been recognised that only under exceptional circumstances
can elements diffuse up a chemical gradient, as is needed if elements present in low
concentrations in a solution can precipitate as a high-concentration ore body. It is
now recognised that most ore deposits form as a result of the circulation of
hydrothermal fluids, and indeed that high fluid fluxes are required to form large
ore bodies. An important question therefore is the nature of the process or motor
that causes the fluid to circulate.
As for the other parameters discussed above there are several possibilities.
For deposits related to magmatic activity release of fluids from magmas is the
main driving force. The exsolution of fluid entails a large increase in volume which
is capable of fracturing the rocks overlying the magma chamber and the low-density
fluids thus released ascend through the fractures. Heating of groundwater surrounding
the intrusion causes it to convect, enhancing the primary circulation.
Convection is the main cause of circulation in deposits that form at the ocean
floor. Seawater penetrates into the crust where it acquires heat from still-hot lavas or
from high-level intrusions. The less-dense warm fluids then ascend to the surface
along fractures. In the case of deposits in sedimentary basins the driving mechanism