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

 

Mondillo, N., Arfè, G., Boni, M., Balassone, G., Boyce, A.,

Joachimski, M., Kang, J.S., Villa, I.M.
 
ORE GEOLOGY REVIEWS, vol. 94, p. 261-276, (2018)
doi: 10.1016/j.oregeorev.2018.01.021
 
 
Abstract
The Cristal Zn prospect consists of a mixed sulfide and nonsulfide mineralization located in the Bongará province (Amazonas region, northern Peru). The mineralization is hosted by carbonate rocks of the Pucará 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 δ³⁴S = 14 to 15 ‰ VCDT. Oxygen isotopic compositions of dolomites are: δ¹⁸O = 24.4 to 24.7 ‰ VSMOW for Dol1 and 18.4 to 22 ‰ for Dol2. δ³⁴S and δ¹⁸O 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 ²⁰⁶Pb/²⁰⁴Pb = 18.850 ± 0.002, ²⁰⁷Pb/²⁰⁴Pb = 15.685 ± 0.002, ²⁰⁸Pb/²⁰⁴Pb = 38.752 ± 0.004, and ²⁰⁶Pb/²⁰⁴Pb = 19.042 ± 0.002, ²⁰⁷Pb/²⁰⁴Pb = 15.712 ± 0.002, ²⁰⁸Pb/²⁰⁴Pb = 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.
 
Net-intensity maps for Zn-L and Fe-L spectral lines (high resolution EDX system FEI Quanta), and qualitative EDX spectra of Fe-poor and Fe-rich sphalerite enclosed in the green squares. Note: in the EDX spectra, C is related to carbon coating. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)