Exploration vectoring from the white mica spectral footprint in the atypical auriferous Lavra Velha deposit, San Francisco Craton, Brazil
Carlos Martín Medina, Diego Fernando Ducart, Josué Souza Passos, Leandro Rocha de Oliveira
Abstract
The Lavra Velha deposit is a poorly understood gold deposit hosted by the 2.1 Ga Ibitiara Granitoid, within the São Francisco Craton (Brazil). Although not consensually agreed, Lavra Velha has been inserted into the Iron Oxide-Copper-Gold (IOCG) class. The major alteration footprint of the deposit involves an upward mineralogical zoning from widespread chlorite-epidote-calcite (propylitic) to white mica-hematite-quartz association with an intervening magnetite-tourmaline-quartz-white mica realm. The ore occurs mostly as massive lenses composed of magnetite, hematite, pyrite, tennantite, bismuthinite, digenite and native gold. Gold is present as inclusion within hematite, pyrite, quartz and cassiterite, in association with white mica, but also with supergene jarosite-goethite. The proximal footwall and hanging walls of the mineralization are commonly anomalous in Cr-Ni-Co and, where weathered, a talc-nontronite association is documented. Such features are considered to represent ultramafic rocks closely associated with mineralization. Reflectance spectroscopy results show that white mica, hematite and Fe-tourmaline are closely and spatially associated with gold mineralization, thus representing a proximal alteration mineralogy. Notably, white mica becomes progressively more phengitic (>2210 nm) as higher Au grades are laterally approached, whereas it has muscovitic composition (<2198 nm) where gold grades are waning. This chemical gradient can be used as an exploratory guide to find zones of higher-grade mineralization. The crystallinity of white mica reveals a strong zonation from the top to the bottom of the deposit defined by the downward variation from muscovite to illite. The passage zone from muscovite to illite represents potential sites for gold deposition. In weathered profiles, the spectral mapping of talc-nontronite would be key in detecting gold-related ultramafic rocks. This work found that SWIR hyperspectral measurement of white mica features provides an efficient means for vectoring to gold mineralization, at least to district scale. The unusual mineralogical signature of Lavra Velha is not diagnostic of any specific style of known deposit and further in-depth studies are required to decipher what kind of deposit it belongs.