Carbonate-hosted lead-zinc ore deposits

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.

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).

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.

Some MVT deposits can, however, be very iron-rich and some sulfide replacement and alteration zones are associated with no lead-zinc at all, resulting in massive accumulations of pyrite-marcasite, which are essentially worthless.

During the area selection phase, attention must be paid to the nature of the carbonate sequences, especially if there is a 'dolomite front' alteration identified within oil exploration wells, which is commonly associated with lead-zinc mineralisation.

Finally, once a basin model of the carbonate sequence is formulated, and the primary basin-margin faults are roughly identified, a gravity survey is often carried out, which is the only geophysical technique which can directly detect MVT deposits.

Gravity surveys aim to detect significant accumulations of lead and zinc due to their greater density relative to their surrounding host rocks.

Finally, the 'pointy end' of an exploration programme is to drill each and every one of the gravity targets in sequence, with no favour or prejudice given to the strength or amplitude of any anomaly.

Examples include the Dharwar Basin zinc-lead deposits, India where sulfides are hosted in shears within dolomite sequences.