Structural features and age of gold mineralization in the Troia Massif, Borborema Province, NE Brazil: A Paleoproterozoic (~ 2029 Ma) hypozonal orogenic gold deposit overprinted by the late Neoproterozoic Brasiliano/Pan-African orogeny

摘要

Hypozonal orogenic gold mineralization has been recently recognized in the Troia Massif, an Archean/Paleoproterozoic basement inlier within the Neoproterozoic Borborema Province, NE Brazil. The mineralization occurs in association with quartz veins hosted by the amphibolite facies Paleoproterozoic Serra das Pipocas greenstone belt. The main mineralized areas are located near parallel to lithological contacts, siting on shear zones established at the metavolcanic and metasedimentary boundary. Four deformation events are recognized: (1) Dn was responsible for the early Sn foliation, parallel to the bedding (So) of the greenstone units. (2) Dn + 1 is characterized by a pervasive, southeasterly-dipping Sn + 1 foliation that is axial-planar to several asymmetric, tight to isoclinal and recumbent folds. (3) Dn + 2 represents a strike-slip NE-trending shear zones, and (4) the late Dn + 3 event is characterized by ductile-brittle deformation. Evolution from Dn + 1 to Dn + 3 is interpreted as a progressive deformation, with the maximum compressive stress in the WNW-ESE direction. The main stage of gold mineralization is associated with the Dn event, and comprised by deformed quartz veins and associated high-temperature calc-silicate alteration (diopside, hornblende, titanite) and albitization of the host rocks. Lower temperature gold (+/- Te, Ag) occurs along late stage brittle structures (Dn + 3). The U-Pb titanite age of 2029 +/- 28 Ma for the calc-silicate alteration of the early gold mineralization is similar to the age of many Transamazonian/Ebumean gold mineralizations elsewhere. However, the strong Pb loss of titanite grains defines a 574 +/- 7 Ma lower intercept age, indicating that early gold mineralization was affected (remobilized) by Neoproterozoic deformational events and metamorphism (Brasiliano/Pan-African orogeny). This is in agreement with the U-Pb zircon age of 575 +/- 3 Ma obtained for the syn-tectonic Guaribas dikes, which bracketed the age of Dn + 3. Therefore, as Dn + 1, Dn + 2 and Dn + 3 evolved by progressive deformation; they are all of Neoproterozoic age. This two-stage gold mineralization is here interpreted as product of crustal exhumation, with (1) early hypozonal orogenic gold mineralization occurring shortly after the high-grade Paleoproterozoic metamorphism and first exhumation processes of the greenstone pile; and (2) late gold mineralization occurring at shallow levels (second exhumation process), associated with the Neoproterozoic Brasiliano/Pan-African orogeny.