The root zones of the Seridó W-skarn system, northeastern Brazil: Constraints on the metallogenesis of a large Ediacaran tungsten Province

Abstract

In the literature, skarn deposits have their genesis commonly explained by the interaction between a magmatic-hydrothermal fluid and carbonate rocks. There are, however, a large number of exceptions to this rule. One of the best examples may be the Seridó Mineral Province, where more than 700 scheelite deposits and occurrences have produced around 60,000 t of WO3 over the last hundred years. Important deposits, such as Bonfim, Brejuí, Bodó and Barra Verde, develop classic skarn alteration in the contact with marbles, forming diopside- and/or garnet-dominant parageneses. Other types of occurrences account for amphibole- and/or biotite-dominant assemblages interpreted respectively as Ca(Ba) and K hydrothermal alteration rocks that occur along granitoid basement rocks. Apart from pegmatitic dykes, the Ca(Ba) and K metasomatic rocks do not show any clear association with carbonate rocks and/or plutonic bodies. Instead, most of these W-bearing rocks are strongly associated with subvertical N20°E-trending shear zones, being more structurally controlled than lithologically dependent. Some of the most representative skarn, Ca(Ba) and K alteration deposits and occurrences of the area, which occur in different geological units, were studied in this work.Whole rock geochemistry analyses of the metasomatic rocks of the Ca(Ba) alteration point to Ca, Mg, Fe, Ba, Na and REE metasomatism, while rocks of the K alteration are characterized by Na, K, Si, Mo and W metasomatism.The scheelite-bearing metasomatic Ca(Ba) and K rocks are hosted in different basement granitoid wallrocks. In the Bonfim deposit, relict Paleoarchean zircon grains yielded an age of 3324 ± 33 Ma; Neoarchean crystals yielded an age of 2699 ± 11 Ma in Pedro Tomás; relict Paleoproterozoic grains from Salgadinho held ages of 2217 ± 6 Ma and 2177 ± 16 Ma; and a Paleoproterozoic age of 2194 ± 31 Ma was verified in the Caçador occurrence.W ± Mo mineralization is correlated with the Neoproterozoic-Ediacaran Brasiliano Cycle. In the Salgadinho occurrence, the core of a single grain along with the borders of some crystals provided concordant ages of 575 ± 3 Ma, 558 ± 4 Ma and 536 ± 3 Ma, which may be correlated to peak orogenic and metasomatic events. Results of similar age were identified by 40Ar/39Ar analyses in amphibole, with the ages 553 ± 3 Ma and 548 ± 2 Ma for Bonfim and 557 ± 7 Ma for Salgadinho; while analyses in biotite produced the ages 498 ± 2 Ma and 501 ± 2 Ma for Caçador.Following a new model for the area, the rocks studied in this work are interpreted as the root zones of the W ± Mo-skarns, presenting spatial and temporal associations with them. The Ca(Ba) and K alteration rocks along with skarns are part of the same system, but formed under different pressure, temperature, chemical, pH and REDOX conditions, as well as distinct fluid/rock ratios and hydrothermal solution and wallrock compositions. The regional-scale zoning between W ± Mo and relatively coeval Au deposits in the Seridó Mineral Province also suggests that they are cogenetic, being part of an Intrusion-Related system.