Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 102

evidence that both the metals and the sulfide now found in the ore bodies were
derived from the detrital sedimentary rocks of the sedimentary basin. There is
equally strong evidence that the hydrothermal fluid was connate water; i.e. the
fluid, initially seawater, that filled the pore-space between the detrital grains of
the poorly consolidated sedimentary rocks. Even the mechanism that triggers the
precipitation of ore sulfides is well understood. Basinal fluids are relatively
oxidized and they most probably transported the metals as chloride or sulfate
complexes. The geochemical environment of the carbonates that now host the
ores was very different - it was reduced and contained abundant reductants in the
form of hydrocarbons (oil or gas) and other organic material. The redox reaction
destabilized and reduced the chloride or sulfate complexes, causing the precipita-
tion of Pb and Zn sulfides. Platform carbonates are often highly porous, due to the
presence of the breccias and cavities that develop during dolomitization, and the
ores were precipitated in these cavities or in zones of reaction between the fluid and
the carbonate rocks.
What is unclear is the process that sets the fluid in motion - stagnant connate
water cannot form a large ore body; to do so, the fluid must migrate to the margins
of the basin in order that it can interact with the reductants in the carbonates.
Various processes are debated in the literature. Dewatering of the basin during
compaction under the load of overlying sediment is commonly advocated, but this
process is probably too slow to explain the fluid fluxes inferred for many deposits.
A remarkable result of geochronological studies of the ore bodies and their broad-
scale geological setting has shown that the timing of ore formation in the
Mississippi Valley in central USA coincides with major deformation events in the
Appalachian mountain range at the Atlantic margin of the continent. This associa-
tion led to the idea that thrusting at the eastern margin of the sedimentary basin
drove the ore-forming fluids for over 1,000 km until they reached and reacted with
the marginal reefs at the other side of the basin (Fig. 4.11 ).
Detailed recent studies have shown that the ores formed when a pulse of warmer-
than-normal fluid was injected into the carbonate platforms. The duration of the
pulse was too short to have been formed by slow-acting processes like compaction
or orogenic movements and this has led to imaginative alternatives such as the
flooding of the basin by warm seawater following changes in the movement of
major ocean currents or transgression following melting of continental ice caps.
Analysis: MVT Deposits
Source of metals - detrital sedimentary rocks of the basin;
Source of S - biogenic H 2 S or sedimentary sulfide
Source of fluid - connate (interpore) water
Cause of fluid circulation - compaction (?), tectonic deformation (?), sea
level increase (?)
Precipitation process - redox reactions as oxidized basin water meets
organic material in carbonate facies