The Cristal Zinc prospect (Amazonas region, northern Peru). Part I: New insights on the sulfide mineralization in the Bongará province

Abstract The Cristal Zn prospect consists of a mixed sulfide and nonsulfide mineralization located in the Bongara province (Amazonas region, northern Peru). The mineralization is hosted by carbonate rocks of the Pucara Group, deposited in a Mesozoic extensional basin on the western margin of the Brazilian-Guyana shield. Zinc sulfides at Cristal occur in the roots of the nonsulfide concentrations, and are locally present also nearer to surface. The sulfide mineralization postdates two hydrothermal dolomitization phases (Dol1 and Dol2) and the sulfides occur mainly in veins, cavity fillings or as disseminated mineralization, within sparry to saddle dolomite. They mostly consist of dark-brown sphalerite, intergrown with smaller amounts of pyrite. Sphalerite shows a distinct Fe-zonation, with average ca. 7 wt% Fe (max. ca. 12 wt% Fe), and is Ge-rich (mean concentration of 142 ppm). Galena is rare. The Cristal sphalerite has sulfur isotopic compositions of δ 34 S = 14 to 15 ‰ VCDT. Oxygen isotopic compositions of dolomites are: δ 18 O = 24.4 to 24.7 ‰ VSMOW for Dol1 and 18.4 to 22 ‰ for Dol2. δ 34 S and δ 18 O values of Cristal sulfides and dolomites are similar to those observed in two Mississippi Valley-type (MVT) deposits located ca. 20–30 km south of the Cristal prospect, namely the Florida Canyon and Florcita deposits. This could be consistent with one or more events within a same MVT mineralizing system, acting at the district scale. The Pb isotope compositions of sphalerite from two different areas of the prospect (named Cristal s.s . and Charlita North) define two distinct data-point clusters (centered around averages of 206 Pb/ 204 Pb = 18.850 ± 0.002, 207 Pb/ 204 Pb = 15.685 ± 0.002, 208 Pb/ 204 Pb = 38.752 ± 0.004, and 206 Pb/ 204 Pb = 19.042 ± 0.002, 207 Pb/ 204 Pb = 15.712 ± 0.002, 208 Pb/ 204 Pb = 39.080 ± 0.004, respectively). This difference requires distinct metal-bearing hydrothermal pulses in the mineralized area and/or distinct Pb sources. The Pb isotopic compositions of the Cristal s.s . and Charlita North sulfides are intermediate between the compositions of galena from the San Vicente and Shalipayco MVT deposits, and record a contribution from an old crustal component. The Paleozoic basement, which has Pb isotopic ratios roughly matching those of dolomites and sulfides from Cristal and Charlita North areas, represents the most reliable candidate to be an end-member source of the metals of the Cristal sulfide mineralization. The second end-member could be an igneous source, isotopically identical to the Late Paleozoic to Early Mesozoic intrusives of the Peruvian Eastern Cordillera, or the Triassic volcanic rocks occurring within the Mitu Group.