The Bongarà-Mina Grande (Amazonas, Peru) Zn-nonsulphide deposit

The Mina Grande deposit (Amazonas, Peru) consists of several mineralised areas, where the ore occurs as Zn-nonsulphides in karst cavities (Workman & Breede 2016). Lead is scarce or absent. The primary deposit is considered to have been a MVT (Basuki & Spooner 2009), located in carbonate rocks of the Condorsinga Fm. (Pliensbachian–Toarcian) of the Pucarà Group. The nonsulphide mineral assemblage, mostly consisting of hydrozincite, smithsonite and hemimorphite, is associated with concretionary calcites and, locally, with a gossanous cap. The measured resources were considered ∼315 000 Tons @ 24% Zn in a 2008 evaluation, but at Mina Grande there is still considerable economic potential both for nonsulphides in the surficial karst network, and for primary sulphides, possibly occurring along several tectonic structures in the whole area. Several stages of alteration and mineral formation, mirrored by the mineralogical paragenesis of the deposit, have been confirmed by stable isotopic analyses of the newly formed carbonate minerals. Isotopically different smithsonite and hydrozincite generations have been detected. Early smithsonite was the first product of sulphide oxidation and secondary precipitation, and was partially replaced by early generations of hemimorphite and hydrozincite. Late smithsonite, hemimorphite and hydrozincite generations can be considered the products of a second oxidation phase. Early and late smithsonites have different δ18O ratios (from 26.9 to 27.2‰ and from 26.0 to 26.3‰ VSMOW, respectively). The average O-isotope ratios of spongy and needle-shaped hydrozincite (24.6 and 23.7‰ VSMOW, respectively) are relatively similar to those of concretionary calcite in the deposit (24.45‰ VSMOW). The different oxidation stages might be related to several periods of uplift that occurred in the Bongará district at least since ∼10 Ma, when the switch from the Pebas to Acre systems affected the Amazonas foreland basin in Miocene. The karstic activity, to which the supergene mineralisation is related, can be restricted to a period spanning between Late Miocene and Early Pliocene. Climatic models of the ecosystems persisting in the region from Late Tertiary to Recent also suggest that the Mina Grande supergene mineralisation was related to several weathering episodes that occurred under a climate not very different from today.