The volcanic and volcaniclastic lithologies of the Unit result from submarine eruptions ranging from basalts to basaltic andesites, accompanied by lesser dacites and rhyolites, with tholeiitic to calc-alkaline affinity. The VHMS ore deposits are hosted at different levels of the Main Macuchi sequence, whereas no analogs are known in the Basal Macuchi. This result has a justification in the geotectonic model presented previously, according to which multiple subduction-related extractions from the same mantle reservoir might have occurred, thus yielding superimposed arc sequences derived from a progressively depleted source. Pelagic sediments, incorporated in the mantle magma source, are therefore the only ultimate possible source of radiogenic lead present in the Main Macuchi sequence. The ore minerals display layered textures.
Early pyrite is replaced by chalcopyrite.
Chalcopyrite is substituted by the bornite-pyrite association. Bornite is replaced by covellite-digenite and by idaite. Digenite may host rare enargite. Low-Fe sphalerite is coeval to or postdates bornite. In general the ore mineral assemblages indicate a progressive increase in sulfidation conditions with time.