Depósitos de escarnitos mineralizados em ferro e cobre do arco magmático de Santa Quitéria, Ceará, Provincia Borborema do nordeste do Brasil

Abstract

W-Mo skarns deposits in the Borborema Province have been known since the 40’s, however, until this study there were no records of Fe-Cu skarns. In Ceará State, Brazil, the first occurrences of Fe-Cu skarns were identified recently in the Magmatic Arc of Santa Quiteria, product of superposition of several magmatic arcs, ranging from juvenile arc (870 – 800 Ma), up to continental collision (625 – 600 Ma). The skarns are associated with calc-silicate rocks and dolomitic and/or calcitic marbles, cut by Neoproterozoic, early to tardi-collisional quartz monzonite. In the area several skarn system types are found such as endoskarns, hornfels, skarnoids and exoskarns. The exoskarns show two mineral assemblages, reflecting different carbonate protolith: the first consists of clinopyroxene-garnet (prograde stage) and magnetite-pyrrhotite-chalcopyrite-pyrite-biotite (retrograde stage), characteristic of calcitic protolith, and the second, consists of olivine-spinel-diopside in prograde phase and serpentine in retrograde phase, representative of dolomitic protolith. Garnets partly zoned exhibit composition dominated by andradite (82 – 52 mol%) with subordinate grossularite (16 – 45 mol%) and minor pyrope (2 –3 mol%). The pyroxene of the marble calcitic protolith is typical of the diopside-hedenbergite series, with average composition Diopside-46Hedenbergite52Johannsenite2. Three types of iron-dominated skarn related ores are distinguished: i) banded magnetite; ii) disseminated magnetite; iii) vein massive magnetite. Takes place also a sulphide copper ore with and without associated magnetite and gossans blocks. The geological and metallogenic data obtained indicate that the genesis of these skarns are associated with the influence of monzonitic Brasilian granites and although there are two types of skarn, there is a predominance of Fe-Cu Skarn.