Multi-isotopic chronology of the polygenetic San Pedro epithermal vein system, Andacollo Mining District, Argentina

The Ag-Au (Cu,Pb,Zn) San Pedro epithermal system at Andacollo, Argentina, has been dated using ³⁹Ar-⁴⁰Ar together with Sr and Pb stepwise leaching. A precondition for multi-isotope analysis was a very detailed petrographic study of 12 hydrothermal pulses. Element maps were obtained by SEM-EDS on three thin sections from two drillcores, DHA-30 and -35. Twelve samples for isotope analyses, weighing ca. 2-10 mg, were cut with a steel blade from the analyzed thin sections, intentionally collecting "fresh" paragenetic adularia mantled by secondary alteration products to different degrees, in order to constrain the alteration trends in the Ar, Sr and Pb isotopic systems. Nine were analyzed by ³⁹Ar-⁴⁰Ar step-heating and three were step-leached [1] measuring both Sr and Pb isotopic compositions.

The Ar results demonstrate a plurality of alteration phases. A broad correlation trend between Cl/K (which should be zero in fresh feldspars) and step ages confirms the variable mass balance between primary adularia and secondary sericite, smectite, chlorite, kaolinite, and apatite. The end-members of the alteration trends observed in the scattered Ca/K-age correlation are different from those of the Cl/K-age correlation, requiring ≥ 4 compositionally different alteration phases. A regression to zero Cl/K in the three DHA-30 aliquots indicates that the primary adularia is 77±6 Ma old (2 sigma), consistent with cutting relations with dacitic dykes. The saline fluid inclusions, observed in different thick sections of the same cores, were decrepitated at low furnace temperature giving a very high ³⁸Ar/³⁹Ar ratio and an ⁴⁰Ar*/Cl ratio of 7.7×10^-5, the same order of magnitude as the ⁴⁰Ar*/Cl ratio measured by Turner in fluid inclusions from Cornwall [2], whereby primary and secondary fluid inclusion in Andacollo have different ⁴⁰Ar*/Cl ratios.

The six DHA-35 aliquots also show a broad Cl/K-age correlation; Cl/K ratios are higher, indicating a higher overall contamination. The extrapolated adularia age, 85±13 Ma, is statistically indistinguishabe from DHA-30.

The three Sr-Pb leach fractions gave alteration signatures very distinct from the adularia proper, both in Sr and Pb. The signature of the DHA-30 alteration is different from DHA-35. The Sr, and especially Pb, signature of both alteration fluids is very radiogenic, i.e. compatible with circulation through old country rocks. The host rocks are Carboniferous; however, significant inherited and detrital zircon contributions of Eo- to Mesoproterozoic age have been reported in both volcaniclastic and sedimentary rocks respectively [3,4]. This suggests the reworking of a Proterozoic basement, and hence a contribution to the isotopic signature of the analysed hydrothermal minerals.

The compositional and isotopic differences between the two drillcores are compatible with, but not proof of, a long-lasting alteration history as documented in other large ore deposits [5,6].

 

 

[1] Villa & Hanchar (2013) Geochim. Cosmochim. Acta 101, 24-33

[2] Turner (1988) Geochim. Cosmochim. Acta 52, 1443-1448

[3] Dicaro et al (2024) J. South American Earth Sci 148, 105158

[4] Pons et al, in preparation

[5] Kang et al (2020) Ore Geol. Rev. 122, 103527, 1-23

[6] Virmond et al (2024) Contrib. Min. Petrol. 179, 88