Location:

Los Bronces, Chile

In many porphyry deposits the internal vapour pressure builds up until it exceeds the tensile strength of the solidified carapace and the resulting hydrofracturing produces crackle breccias. Where this takes place explosively, breccia pipes and pebble dykes are formed. At Los Bronces a granodiorite with porphyry-style mineralization has intruded Tertiary andesites. Locally, the granodiorite is cut by hydrothermal breccia pipes that carry a quartz-tourmaline-sulphide-oxide assemblage. Later mineralized veins carry a more diverse assemblage which includes enargite group minerals, tennantite, galena and sphalerite in a quartz, tourmaline, baryte and siderite gangue.

Major Minerals:

Chalcopyrite, pyrite, haematite, molybdenite, chalcocite

Minor Minerals:

Magnetite, bornite, secondary delafossite, covelline, chalcanthite, malachite, ferrimolybdite, native copper

Trace Minerals:

Include luzonite, enargite, tennantite, valleriite

Textures:

Chalcopyrite is massive, intergrown with quartz and tourmaline, and cements and replaces pyrite along its cleavage and fractures. Haematite forms acicular crystals and locally is replaced by magnetite. Chalcopyrite is replaced by bornite and chalcocite, enargite replaces haematite.

References:

Holmgren and Marti, 1984; Warnaars et al., 1985; Stoiser, 1986