Carbonate-hosted lead-zinc deposits

Sphalerite, galena and pyrite. Navan, Ireland

A euhedral crystal of galena (white, centre) has cleavage pits and lies within a botryoidal sphalerite (light grey) overgrowth, which itself occurs within less well polished sphalerite that contains abundant fine-grained pyrite (yellow-white, right centre). Black areas are polishing pits.

Polished block, plane polarized light, x40, air

Sphalerite, galena and pyrite. Navan, Ireland

Galena (blue-white, left) is overgrown by botryoidal sphalerite (light grey), which in turn has a botryoidal pyrite rim (yellow-white, centre). Coarse-grained carbonate (dark grey, centre) is the main gangue. Black areas are polishing pits.

Polished block, plane polarized light, x80, air

Galena, sphalerite and pyrite. Navan, Ireland

Galena (white, top left) has triangular pits (black, centre top) due to plucking along the perfect (100) cleavage and is intergrown with sphalerite (light grey). Euhedral quartz (dark grey), showing a prismatic section (centre) and basal section (centre), occurs within sphalerite and has a lower reflectance than dolomite (small rhomb, centre right). Sphalerite inclusions occur in both gangue phases.

Polished block, plane polarized light, x80, air

Pyrite, marcasite, sphalerite and galena. Navan, Ireland

Sphalerite (dark grey, bottom left) is overgrown by banded pyrite (yellow-white, centre) and marcasite (blue-white), which shows reflection pleochroism (blue-white to white-blue, centre top) and bireflectance. The outer growth zone of marcasite is overgrown by a thin band of low reflectance pyrite (brown, centre). Minor amounts of galena (blue-grey, lower reflectance than pyrite, centre bottom) occur within sphalerite.

Polished block, plane polarized light. x 160, oil

Sphalerite, bournonite and galena. Navan, Ireland

Sphalerite crystals display abundant light yellow to brown internal reflections and are cemented by bournonite, which shows its characteristic polysynthetic twinning on (110) 'parquet-twinning' (centre). Minor amounts of galena (lilac-white, bottom centre,) are intergrown with bournonite. The lilac-white colour of galena is due to the partial crossing of the polars.

Polished block. partially crossed polars, x 160, oil

Galena and bournonite. Navan, Ireland

Galena (blue-white) has a characteristic triangular pit (black, top left) due to the plucking along its perfect (100) cleavage. It carries abundant sulphosalt inclusions (blue-green, centre) which have the optical properties of bournonite. The inclusions range in size from submicroscopic to approximately 10 lam in diameter and many lie along grain boundaries of the galena. The presence of small sulphosalt inclusions is very common in galena. They are largely responsible for the trace and minor element chemistry often mistakenly reported to be in solid solution. Their similarity in optical properties, and small grain size (which makes chemical analysis difficult to impossible), means that they are often unreported or unidentified.

Polished block. plane polarized light, x 160, oil