CHARCAS INTRUSIVES COMPLEX: AGE AND GEOCHEMISTRY, CHARCAS MINING DISTRICT, MEXICO

Detailed knowledge of the magmatic history of the Charcas Mining District (ChMD) allows us to recognize the different magmatic events and their relationship to the skarn and vein mineralization.

The ChMD is situated in the northern part of the Taxco-San Miguel de Allende fault system and to the east of the Guerrero-Sierra Madre terrains contact. The Charcas intrusives complex (CIC) is constituted by intrusive rocks varying from quartz monzodiorite to monzogranite. The CIC mineralogical assemblage shows variable quantities of plagioclase + alkaline feldspar + quartz ± amphibole + biotite ± orthopyroxene + clinopyroxene + Fe-Ti oxides. The CIC was emplaced in Triassic to upper Cretaceous sedimentary rocks, but, only few dikes from the CIC developed metamorphism halo and related zinc mineralization.

13 intrusive samples were dated by LA-ICPMS on mono zircon crystals. 11 of them gave Ypresian-Lutetian age (mainly Lutetian), one Oxfordian age and one Rupelian age. Ypresian-Lutetian intrusives are emplaced over 5 Ma, from ca. 50 Ma to ca. 45 Ma with an age maximum distribution at ca. 47 Ma. Apparently only the intrusives at ca. 47 Ma induced metamorphism and mineralization. Inherited zircons crystal dated from all the Lutetian intrusives show the existence of at least three previous magmatic events in the region: Sinemurian, Cambrian / Ordovician and Stenian.

Lutetian intrusives show chondrite-normalized REE patterns enriched in light elements [(La/Yb)N = 7-17] with low Eu anomalies. Their trace-element geochemistry is comparable to high-Ba-Sr granitoids: (a) high Ba concentration (= 760-1437 ppm); (b) low concentrations of Y (= 18-23 ppm) and Nb (= 11-13 ppm); and (c) relative high values for Sr/Y (= 18-20).

The CIC is formed by multiphase magmatic events emplaced and controlled by a same regional structure. The Lutetian magmatic event is characterized by a 5 Ma successive dikes emplacement, inducing a long and continue period of hydrothermalism. The origin of CIC has been related to partial melting of an enriched lithospheric mantle, in a post-orogenic setting, followed by fractional crystallization coupled to crustal assimilation.