Porphyry copper and molybdenum ores




Haematite, chalcopyrite, pyrite and rutile. Los Bronces Mine. Chile


Click hereCoarse-grained haematite (light grey-blue, left), chalcopyrite (yellow, centre left) and pyrite (light yellow-white, high reflectance, centre left) are intergrown with radiating laths of tourmaline (light grey, left). An aggregate of rutile crystals (grey-white, centre right) is enclosed within a mosaic of coarse-grained quartz crystals (dark grey with faint internal reflections, right). Black areas are polishing pits.



Polished block, plane polarized light, x 80, air


Chalcopyrite, haematite and pyrite. Los Bronces Mine, Chile


Click hereTabular haematite (blue) and subhedral pyrite (pale yellow-white, centre bottom) are intergrown with chalcopyrite (yellow). The relief difference between the harder haematite and softer chalcopyrite can be seen clearly (centre right). Pyrite carries small chalcopyrite (centre bottom) and haematite inclusions (centre). Radiating prismatic crystals of tourmaline (dark grey, bottom left) form the main gangue. Black areas are polishing pits and the section shows relief.



Polished block, plane polarized light, x 80, air


Chalcopyrite, enargite group mineral, haematite and pyrite. Los Bronces Mine, Chile


Click hereChalcopyrite (yellow, bottom left) and an enargite group mineral (brown, top right) enclose relict crystals of haematite (blue). Subhedral pyrite (yellow-white, high reflectance, left) carries small chalcopyrite inclusions (top left). Chalcopyrite shows incipient alteration next to haematite (grey, centre top, centre bottom), which at higher magnification is seen to be bornite. Quartz (bottom left) and tourmaline (top left) are the main silicate phases. Abundant black areas are polishing pits. The relict haematite grains within enargite are in optical continuity showing that enargite has replaced haematite.



Molybdenite. Henderson Mine, Colorado, USA


Click hereA molybdenite veinlet cross-cuts quartz (dark grey, internal reflections, bottom). Individual molybdenite laths show their basal cleavage along (0001) and bireflectance and reflection pleochroism (light grey when east-west and dark grey-brown when north-south).



Polished block, plane polarized light, x80, air


Molybdenite. Henderson Mine, Colorado, USA


Click hereStubby crystals of molybdenite show cleavage along (0001) (centre) and marked bireflectance (grey to grey-brown). Lath-shaped crystals are well polished but basal sections are poorly polished (centre). The gangue is quartz showing diffuse internal reflections. Black areas are polishing pits.



Polished block. plane polarized light, x 80, air


Magnetite, molybdenite and pyrite. Henderson Mine, Colorado, USA


Click hereSubhedral magnetite crystals (brown-grey, centre right) enclose small rounded pyrite crystals (white, high reflectance) and a cube of pyrite (centre left). Curved laths of molybdenite show strong bireflectance from dark grey (when oriented north-south, bottom left) to light grey (oriented east-west, top left). Quartz shows strong internal reflections.



Polished block. plane polarized light, x 160, oil