The Touro volcanogenic massive sulfide (VMS) deposit is located in the NW of the Iberian Variscan massif in the Galicia-Trás-os-Montes Zone, an amalgamation of several allochthonous terrains. The Órdenes complex is the most extensive of the allochthone complexes, and amphibolites and paragneisses host the deposit, characterized as being massive or semimassive (stringers) sulfides, mostly made up of pyrrhotite and chalcopyrite. The total resources are 103 Mt, containing 0.41% copper. A 3D model of the different orebodies and host rocks was generated using data from 1090 drill core logs. The model revealed that the structure of the area is a N–S-trending antiform. The orebodies crop out in the limbs and in the hinge zone. The mineralized structures are mostly tabular, up to 100 m in thickness and subhorizontal. Based on the petrography, geochemistry and the 3D model, the Touro deposit is classified as a VMS of the mafic-siliciclastic type formed in an Ordovician back-arc setting, which was buried and metamorphosed in Middle Devonian.

Electromagnetic (EM) methods belong to the main geophysical techniques used in the mineral exploration of massive sulphides. For selecting EM anomalies as possible massive sulphide targets, it is important to combine the geophysical results with other geological and/or geochemical techniques. In 2015, Atalaya Mining started a new mineral exploration project in the Touro Cu deposit, combining geological, geochemical (ore over 0.2% Cu), and geophysical techniques. The geophysical survey consisted of helicopter-borne EM using the versatile time-domain electromagnetic (VTEM™) max system operated by Geotech Ltd. with full-waveform processing. In total, 509 line-km of geophysical data were acquired during the survey that was completed in 2018. The results showed the massive sulphide Touro ore to be typically of the order of 0.25 ohm·m (4S/m conductivity) and host rock in the range of 1000–30,000 ohm·m, measured directly on the drill core. This modelling agreed well with the sub-horizontal dips observed for the known Touro ore bodies. The conductance modelled by the plate estimation of the VTEM data were also in good agreement with those provided by Geotech Ltd. and the resistivity/conductivity measurements we made on the massive sulphide samples from several Touro ore bodies. The combination of flat dips, good conductance, shallow depth, and, lastly, lack of conductive overburden or noneconomic conductive stratigraphy, i.e., graphitic shales and sulphide iron formation make the Touro project an ideal target for airborne electromagnetic prospecting. This paper presents the excellent correlation observed between the EM airborne anomalies and the massive sulphide blocks of the Touro copper deposit. Favourable factors contributing to the success of the survey were the high contrast in resistivity/conductivity between the massive sulphide Touro ore and the amphibolite host rock and minimal interference from “nuisance” conductors, such as graphitic shales.

Characterization of metamorphic rocks to evaluate waste material acid rock drainage potential is particularly challenging as commonly used laboratory methods can result in its significant underprediction. Static tests were conducted for over 300 samples from the Touro copper project and indicate that carbon-based methods frequently overestimate acid neutralization potential due to the presence of both graphite and manganese–iron carbonates. The Modified Sobek method more accurately accounts for the buffering capacity of carbonates and does not account for graphite, although aluminosilicate dissolution kinetics need to be evaluated in the context of sulfide oxidation rates. Historic sulfur assays for the project relied on methods insufficient to fully digest metamorphosed sulfides and required correction. The more aggressive Leco method provides accurate sulfur estimates and has now been adopted for the project. Static test metrics such as the net neutralization potential or neutralization potential ratio, therefore, can give misleading results when incorrect characterization methods are employed. Such metrics should be considered as screening level, used with caution, and complemented with careful field and laboratory kinetic tests. Preliminary humidity cell testing of five Touro samples suggests that terminal pH values for cells that have become acidic closely match predicted net acid generation (NAG) pH values. The NAG pH test avoids some of the challenges associated with sulfur and carbon predictions in metamorphic rocks as it directly buffers sulfide oxidation acidity with available material neutralization potential. As such, NAG pH has been adopted as the accepted project metric for segregating acid-generating from non-acid-generating waste. Supplementary material: All Touro project static and kinetic test data are available at https://doi.org/10.6084/m9.figshare.c.5389948 Thematic collection: This article is part of the Hydrochemistry related to exploration and environmental issues collection available at: https://www.lyellcollection.org/cc/hydrochemistry-related-to-exploration-and-environmental-issues

Title: Breathing new life into assay data using machine learning methodsAuthors: T. Meuzelaar, M. Warren, A. Alex and P. Núñez Fernández

A geochemical study was conducted on the Touro deposit, which is situated within the Iberian Variscan Massif on the allochthonous terrain of the Galicia–Tras-os-Montes Zone. This study encompassed both mineralogical and geochemical analyses of the host rocks, with a particular focus on the high-grade Fornás metamorphic unit of the Órdenes Complex. The deposit is composed of massive and semi-massive sulfides, the host rocks are amphibolites and paragneisses, and the ore is hosted in garnet amphibolites and mineralized paragneisses. A microscopic study of thin sections and over 300 electron probe microanalyses on various minerals were conducted with the objective of geochemical characterization. Furthermore, a study of approximately 6000 samples processed by mining companies for multielement analyses of over 1350 drill cores was conducted to geochemically characterize the host and mineralized rocks for use as exploration guides. Additionally, five samples underwent Sm-Nd isotope analysis. The data from the Touro Cu (Zn-Co) deposit are consistent with its classification as a mafic–siliciclastic (Besshi-type) VMS deposit constructed in a back-arc environment during the Ordovician period. Following burial and high-grade metamorphism during the Middle Devonian period, these rocks were subsequently exposed during the later Variscan deformation phases, resulting in the formation of the Arinteiro antiform.