SEDIMENT-HOSTED LEAD-ZINC DEPOSITS: 20 YEARS OF PROGRESS

The MAC Short Course in Sediment-hosted stratiform lead-zinc deposits (Sangster, 1983) has proved to be a bench-mark in the development of concepts on the geology and genesis of this class of mineralisation. Detailed factual descriptions on a number of these deposits were published and models developed.

Classification and nomenclature remains problematic, in particular the use of genetic descriptors (e.g. SEDEX – sedimentary exhalative), but is now crystallising towards five distinct types of sediment-hosted lead-zinc deposits:

1. sediment-hosted massive sulphide (SMS, e.g. Isa, Sullivan, Rammelsberg, Red Dog – often known as SEDEX)

2. Irish-type (e.g. Navan, Lisheen, Reocin)

3. Broken-Hill-type (BHT)

4. Mississippi Valley Type (e.g. Pine Point, Upper Silesia)

5. sandstone-hosted lead (e.g. Laisvall)

These classes come close to empirically defining distinctive styles of mineralisation. Genetic concepts suggest a close relationship between the first 3 classes. Although these concepts continue to be modified, the empirical models that describe the architecture of the mineralisation, as well as its mineralogy and geochemistry, have proved to be robust.

The significant advances in understanding the tectonic setting of the SMS mineralisation will be illustrated by reference to Phanerozoic examples. Most of the Phanerozoic examples tend to cluster in age between Middle Devonian and Early Carboniferous, and occur on the margins of cratonic blocks within the Laurussian supercontinent. A later, but less productive event, occurred in the Jurassic – Cretaceous on the margins of Tethys. These SMS deposits occur at a specific stage of basin development related to tensional events that can be recognised in the stratigraphy.

In all SMS and Irish-type deposits, the resultant hydrothermal event imposed itself on the prevailing environment at the point of mixing with seawater on and beneath the sea floor. Sulphide mineralisation is preserved in a variety of permissive host rocks (shales, siltstones, carbonates), with the style of mineralisation (replacement, stockwork, stratiform laminae) reflecting both the sedimentary environment and host rocks at the site of mineralisation.