Carbonate-hosted lead-zinc ore deposits are important and highly valuable concentrations of lead and zinc sulfide ores hosted within carbonate (limestone, marl, dolomite) formations and which share a common genetic origin.
These ore bodies range from 0.5 million tonnes of contained ore, to 20 million tonnes or more, and have a grade of between 4% combined lead and zinc to over 14% combined lead and zinc. These ore bodies tend to be compact, fairly uniform plug-like or pipe-like replacements of their host carbonate sequences and as such can be extremely profitable mines.
This classification of ore deposits is also known as Mississippi Valley Type or MVT ore deposits, after a number of such deposits along the Mississippi River in the United States, where such ores were first recognised; these include the famed Southeast Missouri Lead District of southeastern Missouri, and deposits in northeast Iowa, southwest Wisconsin, and northwest Illinois.
Similarly Irish-type carbonate lead-zinc ores, exemplified by Lisheen Mine in County Tipperary, are formed in similar ways.
Sources
The ultimate source of the mineralizing fluid(s) in MVT deposits is unknown. The ore fluids of MVT deposits are typically low temperature (100 °C-150 °C) and have the composition of basinal brines (10-30 wt.% NaCl equivalent) with pH's of 4.5-5 (buffered by host carbonates). This hydrothermal fluid may or may not carry the required sulfur to form sulfide minerals. Mobile hydrocarbons may have played a role in delivering reduced sulfur to certain MVT systems, while methane and other organic matter can potentially reduce sulfate carried by an acidic fluid. The ore fluid is suspected to be derived from clastic red bed sequences (potential metal source) that contain evaporites (potential sulfur source).
Transport
Two potential transportation mechanisms for the metal-bearing ore fluid have been proposed. The first involves compaction of sediments in basins with rapid sedimentation. Mineralizing fluids within the basin become trapped within discrete, over-pressured aquifers and escape episodically and rapidly. The second fluid transportation mechanism is topographically-driven gravitational fluid flow. This occurs during uplift that is commonly associated with an orogenic event. One edge of a basin is uplifted during the formation of a foreland fold and thrust belt, and basinal fluids migrate laterally away from the deformation front as the basin is uplifted. Migration of the fluids through deep portions of the basin may result in the acquisition of metals and sulfur contained within the basin.
Trap
The trap for carbonate-hosted lead-zinc sulfides is a chemical reaction which occurs as a consequence of concentration of sulfur, often hydrocarbons, and zinc and lead which are absorbed by the hydrocarbons. The hydrocarbons can either leak out of the fault zone or fold hinge, leaving a stockwork of weakly mineralized carbonate-sulfide veins, or can degrade via pyrolysis in place to form bitumens.
Once hydrocarbons are converted to bitumen, their ability to chelate metal ions and sulfur is reduced and results in these elements being expelled into the fluid, which becomes saturated in zinc, lead, iron and sulfur. Sulfide minerals such as galena, sphalerite, marcasite and pyrite thus form.
Commonly MVT deposits form by the combination of hydrocarbon pyrolysis liberating zinc-lead ions and sulfur to form an acidic solution which dissolves the host carbonate formation and replaces it with massive sulfide accumulations. This may also take the morphology of fault-hosted stockworks, massive tabular replacements and so forth.
Porous limestones may form disseminated ores, however most MVT deposits are massive sulfides with knife-like margins between carbonate and sulfide mineralogies.
Mineralogy and alteration
Ore minerals in carbonate replacement deposits are typically lead sulfide, galena, and zinc sulfide sphalerite. Weathered equivalents form anglesite, cerussite, smithsonite, hydrozincite and secondary galena and sphalerite within the supergene zone.
MVT and Irish type deposits are commonly associated with a 'dolomite front' alteration, which manifests as a yellow-cream wash of dolomite (calcium-magnesium carbonate) within calcite-aragonite assemblages of unaltered carbonate formations.