MINERÍA la mejor puerta de acceso al sector minero EDICIÓN 572 / MAYO 2025 50 Norte HS deposit in northern Peru (~14 Moz Au) is mainly hosted in quartz sandstones (~80%; with the remaining ~20% in volcanic rocks; Cerpa et al., 2013), and La Arena (porphyry and HS, >2 Moz Au; Santos & Li, 2020; HS mineralization is mainly hosted in sandstone), which demonstrates that HS deposits may form in siliciclastic rocks, and opens extensive geologic terranes composed of siliciclastic host rocks for HS deposits exploration. This study is focused on La Virgen, another HS district mainly hosted in sandstones, to help reveal how siliciclastic rocks react with hydrothermal fluids, especially with acid fluids, how HS mineralization occurs in sandstones, and identify features, patterns and vectors that can be used to explore in siliciclastic terrains. Additionally, it reveals new ages and mineralization styles in the district. La Virgen District (LVD) Geology Lithology The sedimentary rocks of LVD are Lower Cretaceous siliciclastic rocks with minor limestones that are strongly folded and faulted with evident displacement from various events. The faults had multiple movements, generating tectonic breccias that were later altered and cemented by hydrothermal fluids. The sedimentary rocks were intruded by andesite porphyry, feldspar porphyry, and phreatomagmatic breccias of ~22- 21 Ma, and later dacite porphyries and their related phreatomagmatic breccias of ~17 Ma (Figure 1). Structural The sedimentary rocks in LVD were affected by intense Eocene folding and thrust faulting associated with the Inca II orogeny (Mégard, 1984). Mineralization in LVD was localized by both Inca folds and thrust-related structures and by later N-S, NW, and E-W trending faults. The major faults (Alumbre, Suro, Tres Ríos, Escalerilla, and Cuypampa) and the minor faults (Cerro Blanco, F1-F4, E1, E2, and C1) are highly fractured zones and local faults. These major and minor faults control the distribution of alteration and mineralization (Figure 1). Alteration The alteration occurred in two events and two domains: There are five types advanced argillic alteration (AA1: vuggy quartz + pyrite ± alunite; AA2: quartz + alunite + pyrite; AA3: dickite + pyrite ± alunite; AA4: kaolinite + pyrite ± illite; and AA5: quartz + pyrite + pyrophyllite), intermediate argillic alterations (IA: illite and/or smectite + pyrite, and chlorite + smectite ± pyrite) in the HS deposits, and K-feldspar alteration and slightly later phyllic alteration (quartz-sericite- pyrite) associated with the porphyry- style mineralization. The alteration is structurally and lithology-controlled. The AA1 to IA alterations are typically zoned in magmatic rocks such as at Cerro Alumbre, Alumbre Norte and Piedras Gordas but in siliciclastic rocks occurs AA2 to AA4, the AA5 group occurs mostly in clay-rich siltstone rocks. The highly fractured and brecciated sandstone of the Chimu Formation are altered by AA alterations in which alunite, dickite, pyrophyllite, and quartz fill the interstitial open spaces between the resistant quartz grains, fill microfractures, and replace minor feldspar grains. The alteration is cryptic, with the altered sandstone appearing fresh. The alteration of minerals also cements and alter clasts of tectonic- hydrothermal breccias (Figure1). Mineralization The mineralization occurred in two events and two domains ~22-21 Ma porphyry-HS mineralization in the Escalerilla-Cuypampa Domain in the east, with the source being the feldspar porphyry in its Cuypampa block, and the ~18-17 Ma HS mineralization in the La Virgen Domain, with its magmatic center possibly below Cerro Alumbre. The Cu- Au HS mineralization occurs filling breccia open spaces, filling quartz grains interstitial and mainly fractures around the breccia bodies, the mineralization consists of pyrite, enargite, and minor famatinite, luzonite, covellite, digenite, sphalerite, galena, tennantite, tetrahedrite, and visible gold. Au also occurs in pyrite2, pyrite3, and enargite, as revealed by LA-ICP-MS mapping. HS mineralization is accompanied by silicification that occurs partly
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