The volcanogenic perspective on the world-class Chapada Cu-Au deposit in Central Brazil revisited

The Chapada deposit, located in the Neoproterozoic intraoceanic Mara Rosa Magmatic Arc, is a low-grade, world-class metallic concentration with a minimum endowment of 3.78 Mt Cu plus 7.76 Moz Au. Ore consists of pyrite-chalcopyrite disseminated mainly in early Neoproterozoic biotite-rich schist pertaining to a mafic-dominated volcanic-sedimentary sequence intruded by 884 to 865 Ma-old gneissic diorite-tonalite. The deposit was metamorphosed and isoclinally folded in a fold-and-thrust system under upper amphibolite facies conditions at 760-730 Ma, and underwent further metamorphism and deformation under greenschist facies conditions at 630-600 Ma, leading to difficulties to unravel its nature. Pre-metamorphic hydrothermal alteration includes an ore-bearing potassic biotitic core with haloes of phyllic (muscovite-pyrite-rich), argillic (quartz-muscovite-kyanite-bearing), advanced argillic (quartz-kyanite-rich), and propylitic (amphibole-epidote-bearing) alteration. Based on these features –– along with the identification of A- and D-type veinlets, and sulfur isotope results close to zero per mil - Chapada has been considered a porphyry-type deposit. This conception, from 1986, has gained general acceptance, whereas another early proposition, from 1989, of a volcanogenic nature for Chapada, has been practically dismissed. Nonetheless, the geologic setting of Chapada is permissive for volcanogenic deposits. Considering this, the volcanogenic perspective on Chapada is re-evaluated here.  The exercise has been carried out by scrutinizing essential attributes of volcanogenic deposits searching for features comparable to those of Chapada. The outcome is that the evidence leading to the acceptance of the porphyry model for Chapada applies equally to support a volcanogenic interpretation. This situation probably reflects a shallow-crustal, sub-volcanic environment, where both deposit types share attributes as, e.g., epithermal-style high-sulfidation alteration. The picture obtained allows for the working hypothesis that a volcanogenic relationship exists between epidote-rich amphibolite source rocks in the west, and quartzitic cap rocks in the east, of Chapada to produce disseminated ore, with accompanying K-Al-Si-propylitic alterations and a magnesian zone made up by early-recognized gedrite-staurolite rocks. The situation envisaged is that ore deposition occurred in sub-seafloor conditions from a poorly focused hydrothermal upflow discharge of fluids from a convective system with magmatic contribution. The volcanogenic model for Chapada is reinforced by the recent discovery of active submarine volcanogenic Cu-Au systems with high-sulfidation silica-aluminous alteration, by the incidence of equally high-aluminous alteration in the 40 km-distant VMS-affiliated Zacarias Au-Ag-Ba deposit, and by the localization of the deposit in a regional zone of alteration, with a suggested exhalative contribution, that seems to fit the concept of favorable, or equivalent, ore horizon.