Massive sulphides in ophiolites




Chalcopyrite and pyrite. Lasail, Oman


Click hereChalcopyrite (yellow, right) surrounds poorly crystalline, fine-grained pyrite (brown, bottom centre) which has well crystalline margins (pale yellow-white, centre). The fine-grained nature of pyrite lowers its reflectance and gives a characteristic brown surface colour. The fine pyrite could be primary or altered pyrrhotite, but here it is probably primary. The gangue phases (dark grey) are silicates.



Polished block, plane polarized light, x80, air


Pyrite, chalcopyrite and sphalerite. Bayda, Oman


Click hereEuhedral pyrite (light yellow-white) crystals are extensively replaced by chalcopyrite (yellow) along the (100) cleavage directions of pyrite. Although pyrite is fractured, much of the chalcopyrite is replacing, rather than cementing, pyrite. Sphalerite (light grey, bottom centre) overgrows pyrite as small subhedral crystals (right centre) and is inclusion-free. Quartz is the main gangue phase (dark grey, top centre).



Polished block, plane polarized light, x 80, air


Pyrite, magnetite, chalcopyrite and pyrrhotite. Lasail, Oman


Click hereCoarse pyrite crystals (light yellow-white, right) enclose pyrrhotite (brown, centre right), chalcopyrite (yellow, centre top) and cubes of magnetite (grey, centre right). Euhedral to subhedral magnetite (grey-brown, centre) and chalcopyrite (centre left) are intergrown with pyrite. Black areas are quartz and polishing pits.



Polished thin section, plane polarized light, x 160 oil


Sphalerite, pyrite and chalcopyrite. Bayda, Oman


Click hereEuhedral to poorly crystalline pyrite (light yellow-white, left) surrounds sphalerite (centre) which has suffered extensive replacement by chalcopyrite; this is an extreme example of chalcopyrite disease. The replacement of sphalerite by very fine-grained chalcopyrite is crystallographically controlled, following growth zones and poorly defined twinning. The differences in surface colour (orange to purple) are due to orientation effects of the fine chalcopyrite inclusions. Under crossed polars the anisotropy effects are enormous. The outer margin of the sphalerite (grey, top left) is chalcopyrite-free and belongs to a later generation of sphalerite. Quartz (dark grey with internal reflections) is the gangue.



Polished block, plane polarized light, x 160, oil


Sphalerite, pyrite and chalcopyrite. Bayda, Oman


Click herePyrite (light yellow-white) forms euhedral crystals (top centre) or poorly polished spongy aggregates (right centre). Sphalerite (left, centre) has suffered extensive chalcopyrite disease along twin planes. The variation in surface colour of the diseased sphalerite (yellow-brown to purple-brown) is controlled by the density and crystallographic orientation of the fine chalcopyrite inclusions. Other areas of the sphalerite are inclusion-free and show characteristic grey surface colours and low reflectance (left). The crystal on the right, which appears to be chalcopyrite, enclosing a low-reflectance phase; is an example of extreme chalcopyrite disease with little relict sphalerite. The degree of chalcopyrite replacement is clearly related to crystallographic domains in the original sphalerite.



Polished thin section, plane polarized light. x 160 oil


Pyrite and haematite. Lasail, Oman


Click hereEuhedral pyrite (light yellow, centre) is fractured and surrounded by aggregates of fine-grained, often botryoidal, haematite (light green-grey, centre left). The differences in reflectance and surface colour (bottom centre) between the core and margin of the haematite aggregates are due to variations in grain size of individual haematite crystals. The coarser crystals on the margin have a slightly higher reflectance. Quartz (dark grey) is the gangue.



Polished block, plane polarized light. x 160, air