W–Au skarns in the Neo-Proterozoic Seridó Mobile Belt, Borborema Province in northeastern Brazil: an overview with emphasis on the Bonfim deposit

Abstract

The Serido Mobile Belt (SMB) is located in the Borborema Province in northeastern Brazil and consists of a gneiss basement (Archean to Paleo-Proterozoic), a metasedimentary sequence (marble, quartzites, and schists), and the Brasiliano igneous suite (both of Neo-Proterozoic age). In this region, skarns occur within marble and at the marble–schist contact in the metasedimentary sequence. Most of the skarn deposits have been discovered in the early 1940s, and since then, they have been exploited for tungsten and locally gold. Recently, the discovery of gold in the Bonfim tungsten skarn has resulted in a better understanding of the skarn mineralization in this region. The main characteristics of the SMB skarns are that they are dominantly oxidized tungsten skarns, with the exception of the Itajubatiba and Bonfim gold-bearing skarns, which are reduced based on pyrrhotite as the dominant sulfide, garnet with high almandine and spessartine component, and elevated gold contents. In the Bonfim deposit, pressure estimates indicate that the skarns formed at 10- to 15-km depth. The mineralized skarns present the prograde stage with almandine, diopside, anorthite, and actinolite-magnesio-hornblende, and titanite, apatite, allanite, zircon, and monazite as accessory minerals. The retrograde stage is characterized by alkali feldspar, clinozoisite–zoisite–sericite, calcite, and quartz. Scheelite occurs in four ore-shoots distributed within the marble and at the marble–schist contact. The main ore body is 5–120 cm wide and contains an average of 4.8-wt.% WO3, which occurs in the basal marble–schist contact. Fold hinges appear to control the location of high-grade scheelite. The late-stage gold mineralization contains bismite (Bi2O3), fluorine-bearing bismite, native bismuth, bismuthinite (Bi2S3), and joseite [Bi4(Te,S)3], and also chlorite, epidote, prehnite, chalcopyrite, and sphalerite. This gold–bismuth–tellurium mineralization exhibits a typical late character and occurs as a black fine-grained mineral assemblage controlled by conjugate brittle-ductile faults (and extensional fractures) that crosscut not only the banding in prograde skarn but also the retrograde alkali feldspar and clinozoisite–zoisite–sericite assemblage. The Au–Bi–Te-bearing minerals are intergrown with retrograde epidote, prehnite, chlorite, chalcopyrite, and sphalerite, indicating that gold mineralization at Bonfim is linked to a late-stage skarn event. The polymetallic nature of the Bonfim deposit can be used as an important guide for the exploration of this type of skarn deposit in the Borborema Province, which potentially contains significant new, undiscovered gold and polymetallic deposits.