Lead sources in Mesozoic and Cenozoic Andean ore deposits, north-central Chile (30–34°S)

Abstract

Mesozoic and Cenozoic ore deposits in the Chilean Andes between La Serena (~30°S) and Santiago (~34°S) include polymetallic vein, low- and high-sulfidation epithermal vein, skarn, porphyry copper-molybdenum and porphyry copper-gold. These deposits are associated with volcanic and plutonic complexes emplaced in eastward-migrating longitudinal arcs which formed during subduction along the continental margin of South America since the Middle Jurassic. Stratabound, but epigenetic, volcanic rock- and sedimentary rock-hosted manto deposits contain additional copper resources. Lead isotopic compositions in ore minerals from 29 deposits vary with age and geographic location, and hence with basement and host rocks. Lead in most ore deposits is derived from temporally related igneous rocks, except for the manto deposits whose lead is derived from host volcanic and sedimentary rock sequences. Lead in the ore deposits is dominated by two crustal sources. Low 207Pb/204Pb characterizes one source whereas high 207Pb/204Pb characterizes the second source. Lead isotopic compositions of Jurassic and Miocene ore minerals (206Pb/204Pb>18.50; 207Pb/204Pb>15.61) lie along the average crustal growth curve. By contrast, most Cretaceous deposits have ore minerals with lower 206Pb/204Pb (<18.39) and 207Pb/204Pb (<15.58) than Jurassic ore minerals. The shift in lead isotopic composition to lower lead isotopic values precludes derivation of lead from a source of similar composition to those in the Jurassic or Tertiary deposits. For Cretaceous deposits, polymetallic and low-sulfidation epithermal veins and a skarn have lower 206Pb/204Pb than a porphyry copper-gold system and peripheral gold veins at Andacollo (18.43–18.50). Late Cretaceous veins from the Bellavista deposit have the lowest 206Pb/204Pb (18.33) of all deposits. Ore minerals in Miocene and Pliocene porphyry copper-molybdenum deposits have higher 206Pb/204Pb (18.58–18.67) than Cretaceous deposits, consistent with their age being younger. The Miocene and Pliocene ore minerals also have higher 207Pb/204Pb (15.58–15.66) than Cretaceous ore minerals, thereby requiring an additional input from the high-207Pb/204Pb source into the younger deposits. Miocene auriferous deposits in the north have similar 206Pb/204Pb values as the Miocene and Pliocene porphyry copper-molybdenum deposits in the south, but they are distinguished by higher and variable 207Pb/204Pb (15.61–15.66) and 208Pb/204Pb (38.54–39.01), which are arrayed along steep mixing trends. These ore minerals have the largest input of high-207Pb/204Pb material in the deposits studied. By contrast, lead in the epigenetic manto deposits appears to be derived from the host volcanic or sedimentary rock-dominated sequences, and locally exhibits large-scale isotopic heterogeneity within a deposit. Overall, the lead isotopic compositions of ore minerals mimic the values and variations established in age-equivalent rock sequences. The low-207Pb/204Pb material in the deposits is derived from Cretaceous igneous rocks or their sources as they evolved with time; low 207Pb/204Pb characterizes these rocks. By contrast, high-207Pb/204Pb material is likely derived from Carboniferous to Triassic igneous rocks or their sources, as this lead isotopic characteristic dominates these rocks.