Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 101

Mississippi Valley in the beginning of the eighteenth century. He found
plentiful shiny gray mineral (galena) at the surface and since this discovery,
Missouri has been the major source of lead of USA. The metal originally was
used as a roofing material.
The southeastern Missouri Mississippi Valley-type Mineral District
contains some of the highest concentration of lead on the world as well as
large quantities of zinc, copper and silver. The ore are primarily hosted by
bacterial stromatolite reefs and associated oolitic rocks of a Cambrian dolo-
mitic formation deposited in a shallow sea. They formed when warm metal
and organic-carbon bearing fluids migrated from adjacent sedimentary basins
through this formation.
For many years deposits Mississippi Valley deposits, located in relatively
young (Paleozoic) sedimentary basins, provided almost all the lead consumed
in the USA; Europe, in contrast, was supplied by large Australian deposits
such as Broken Hill and Mt Isa, which are located in rocks of Proterozoic age.
(The latter, both SEDEX deposits, were described in the previous section).
The difference in age is transmitted to the isotopic composition of the lead,
which is considerably more radiogenic in the case of the older Australian
deposits. Particularly during the period 1950-1990, before lead was with-
drawn from petrol, this isotopic difference was used as a tracer of pollution.
Dust and other pollutants blown eastward across the Atlantic could easily be
distinguished from material from local European industry by the isotopic
composition of its lead. And in archeological studies, lead from local sources
mined from Roman to modern times could readily be distinguished from
modern industrial sources.
Ore deposition temperatures determined from fluid inclusion studies are low
(50-200 C), but somewhat higher than those attributable to normal thermal gradients
within the sedimentary pile. Ore fluids were dense basinal brines, typically containing
10-30 wt.% dissolved salts. Lead and sulfur isotopic data indicate that the sources
for both metal and reduced sulfur were the sedimentary rocks of the basin.
Within each ore district, deposits display remarkably similar features, including
mineral assemblages, isotopic compositions, and textures. Ore controls typically
are district-specific; examples include shale edges (depositional margins of shale
units), limestone-dolostone transitions, reef complexes, solution collapse breccias,
faults, and basement topography. Most MVT ore districts are the product of
regional or sub-continental scale hydrological processes. Therefore, diversity
among MVT districts is expected because of wide ranging fluid compositions,
geological and geochemical conditions, fluid pathways, and precipitation mecha-
nisms possible at the scale of MVT fluid migration.
Origin : As with many other types of deposits, the broad outline of the ore-
forming process is well understood but the details, some of crucial importance,
remain obscure. As mentioned above, there is strong geological and geochemical