- © 2009 Canadian Institute of Mining, Metallurgy and Petroleum
The Bongara area, northern Peru, contains Zn-Pb mineralization with characteristics typical of Mississippi Valley-type (MVT) systems. The northwest–southeast-trending late Triassic–Jurassic carbonate host rocks of the Pucará Group were locally replaced by early (D1) and late (D2) dolomite. Fluid inclusion microthermometric data from dolomite and sphalerite are typical of MVT ore fluids (78° to 187°C; ~15–23 wt.% CaCl2 equivalent), and do not show apparent covariation between temperature and salinity.
Homogenization temperature data from D1 and D2 dolomite show a decrease from northern (mean Th = 138°C) to central (mean Th = 126°C) to southern areas (mean Th = 102°C), suggesting fluid flow from north to south. Assuming a geothermal gradient of ~30°C/km and an average surface temperature of ~20°C, the lower temperature limits might approximate the ambient host-rock temperatures, whereas the upper temperature limits may represent minimum temperatures of a deeper aquifer, probably at a maximum depth of ~6000 m. The lower temperature limits are also similar to the estimated temperatures if dolomitization and mineralization took place in the Late Cretaceous at ~2000–3000 m depth, probably during the formation of the Peruvian fold and thrust belt near the present day Peru–Ecuador border.
Fluid inclusion data show that the fluids in each studied location had relatively uniform temperatures and salinities during dolomitization and mineralization. A significant salinity drop and a slight temperature decrease are observed during late calcite precipitation, suggesting mixing of brines with dilute, cooler fluids.