Geology, Geochemistry and Age of Intrusion-Related Mineralisation in Eastern Mexico

Abstract

Examination of igneous intrusions and related mineral deposits in eastern Mexico (including the determination of 26 new zircon U-Pb ages, 6 new mineral Ar-Ar ages and 26 new sulphide and feldspar Pb isotope and 61 new whole-rock geochemical data) revealed a spectrum of Eocene to Pleistocene mineral deposits and important aspects of the metallogenic history of Mexico. The synthesis of these data with recent tectonic reconstructions suggest the character of igneous rocks and mineral deposits was controlled by two periods of ocean ridge subduction and contemporaneous slab steepening and flattening. Mineralisation occurred in four epochs: (1) Eocene ~ 43 to 38 Ma Cu (Au) skarn and porphyry deposits in northeastern Mexico; (2) Miocene, ~18 to 9 Ma Cu (Au) porphyry, skarn and high-sulphidation epithermal deposits in the Trans Mexican Volcanic Belt (TMVB) and Oaxaca; (3) Pliocene ~ 4.5 Ma low-sulphidation epithermal deposits in the TMVB; and (4) Pliocene to Pleistocene ~ 2.5-2.0 Ma Cu (Au) skarn and porphyry deposits in Chiapas. The intrusions of the Eocene and Miocene epochs are high-K, have adakite-like geochemistry including Sr/Y up to ~108 and ~85 respectively and La/Yb ratios up to ~38 and ~27 respectively, and were followed by alkaline magmatism. All episodes of mineralisation are interpreted to have occurred above areas of slab steepening following periods of flat slab subduction. Shallow emplacement of Miocene ~ 9.0-8.0 Ma diorite stocks in the TMVB caused flashing of hydrothermal fluids, banded quartz vein formation and Au deposition which overprint potassic alteration and Cu-Au mineralisation. Barren quartz-alunite zones formed contemporaneously with potassic alteration at higher elevations. Later fluids deposited Au in some quartz-alunite zones. There is little difference in Pb isotope compositions of feldspar and sulphides or variation over time, their compositions ranging from 18.3208 to 19.1637 for 206Pb/204Pb, 15.4541 to 15.6675 for 207Pb/204Pb and 38.3096 to 39.2193 for 208Pb/204Pb. This may reflect similar tectonic environments and magma sources for Pb.