ORE METAL SOURCES IN TERTIARY MINERAL DEPOSITS OF THE GUERRERO AND SIERRA MADRE TERRANES, MEXICO, AS INFERRED FROM LEAD ISOTOPE STUDIES

New TIMS (Thermal Ionozation Mass Spectrometry) Pb isotope analyses indicate that Triassic schist samples from the Arteaga Complex and Triassic-Jurassic phyllite and slate samples of the Tejupilco metamorphic suite (Guerrero terrane) contain radiogenic Pb relative to bulk earth models, plotting in a quite scattered area to the right of the average Pb crust evolution curve of Stacey and Kramers (1975). Lead isotope ratios of Cretaceous sedimentary rocks of the Zihuatanejo Sequence are distinctly more radiogenic than samples from the Huetamo Sequence (Guerrero terrane). Overall, Pb in the sedimentary rocks is less radiogenic than that in the metamorphic basement; the sedimentary cover was therefore not simply derived from the basement and other rocks were involved in their provenance. Lead isotope ratios of Tertiary intrusive rocks from La Verde, Inguaran, and La Esmeralda (Guerrero terrane) resemble the orogene reservoir in the plumbotectonics model of Zartman and Doe (1981), while Tertiary intrusive rocks from Zimapan (Sierra Madre terrane) show values that are slightly more radiogenic compared to the ones from the Guerrero terrane.

Ores from La Verde and La Esmeralda porphyry copper deposits (Guerrero terrane) yield isotopic ratios between 18.677 and 18.723 for ²⁰⁶Pb/²⁰⁴Pb, 15.568 and 15.617 for ²⁰⁷Pb/²⁰⁴Pb, and 38.419 and 38.598 for ²⁰⁸Pb/²⁰⁴Pb, plotting within the sedimentary rocks field of the Huetamo Sequence and defining a narrow field just below the average Pb crust evolution curve of Stacey and Kramers. Minerals from Zimapan skarn mine (Sierra Madre terrane) yield values varying from 18.774 to 18.974 for ²⁰⁶Pb/²⁰⁴Pb, 15.591 and 15.842 for ²⁰⁷Pb/²⁰⁴Pb, and 38.549 and 39.400 for ²⁰⁸Pb/²⁰⁴, plotting generally above the Stacey-Kramers reference line and showing much higher radiogeneity compared to the ones from the Guerrero terrane. Ores from La Negra skarn deposit (Sierra Madre terrane) have similar Pb isotopic signatures to ores from Zimapan mine, implying similar ore metal sources. The distribution of data from these ores on co-variation diagrams appears to define a steep array, reflecting variable degrees of mixing between a mantle-derived Pb and a more radiogenic component, represented either by the basement or by the sedimentary rocks.