Sedimentary exhalative deposits

The sedimentary appearance of the thin laminations led to early interpretations that the deposits formed exclusively or mainly by exhalative processes onto the seafloor, hence the term SEDEX.

[7][9] Main ore minerals in SEDEX deposits are fine-grained sphalerite and galena, chalcopyrite is significant in some deposits; silver-bearing sulfosalts are frequent minor constituents; pyrite is always present and can be a minor component or the dominant sulfide, as it is the case in massive sulfide bodies; barite content is common to absent, locally economic.

The feeders themselves do not need to be mineralized[8][7] Near the seafloor, beneath or onto it, the ascending metal-bearing fluids eventually cool down and may mix with cold slightly alkaline, less saline seawater triggering precipitation of metal sulfides.

In an ideal exhalative model, hot dense brines flow to depressed areas of the ocean topography where they mix with cooler, less dense, sea water, causing the dissolved metal and sulfur in the brine to precipitate from solution as a solid metal sulfide ore, deposited as layers of sulfide sediment.

An example are arkosic strata adjacent to faults which feed heavy brines into the porous, permeable sediment, filling the matrix with sulfides.

[citation needed] Within disturbed and tectonized sequences, SEDEX mineralization behaves similarly to other massive sulfide deposits, being a low-competence low shear strength layer within more rigid silicate sedimentary rocks.

[11] This class includes also: As discussed above, one of the major problems in classifying SEDEX deposits has been in identifying whether or not the ore was definitively exhaled into the ocean and whether the source was formational brines from sedimentary basins.