The
Coeur d'Alene mining district in northern
Idaho, USA is the second largest producer of
silver in the world. Mesothermal silver-lead-zinc veins occur in the
metasedimentary rocks of the Belt Supergroup with five main types of associated
hydrothermal alteration: 1) feldspar destruction, 2) hydrothermal bleaching, 3)
carbonatisation, 4) sulfidation, and 5) silicification. Hydrothermal bleaching
is characterised by hematite destruction with associated siderite and quartz
veins and veinlets, coarse euhedral pyrite and magnetite. Bleached zones envelop
both mineralised and barren veins and extend for metres to tens of metres into
the wall rocks. Carbonatisation consists of addition of siderite, which occurs
as veins, veinlets, stockwork, irregular blebs or spots, and coarsely
disseminated grains, depending on the relative distance from the main vein
structure. Siderite envelops nearly all mineralised veins, relates to all other
alteration types and extends from several metres to tens of metres into the wall
rocks. Pyrite and galena alteration dominate sulfidation and are associated with
some mineralised veins. Pyrite alteration strongly correlates with siderite
alteration and exists in three main forms: a) very fine-grained pyrite flooding,
which can extend for metres to tens of metres into wall rocks; b)
coarse-grained, euhedral pyrite, which is sporadic and associated with the redox
boundary in hydrothermally bleached zones; and c) small isolated zones (6 - 15
cm) of medium-grained, subhedral to euhedral pyrite associated with pervasive
siderite alteration. Where present, disseminated galena extends for metres to
tens of metres into the wall rocks. Silicification is characterised by veins,
veinlets, stringers, irregular patches and variable hardness in siltites and
commonly appears in association with hydrothermal bleaching and siderite
alteration. Although related to mineralisation, silicification is sporadic and
in most places a clear envelope around mineralised veins cannot be
identified.