For: Raul Montesinos, Zhaoshan Chang and Shiqiang Huang, Colorado School of Mines; Juan Carlos Castelli, Mappex Consulting; Willis Hames, Auburn University, and Robert A. Creaser, University of Alberta.

Trabajo ganador en la categoría posgrado del II Concurso Internacional de Estudiantes SEG-SGA-IIMP.

Abstract

This study seeks to enrich our understanding of high-sulfidation (HS) ore-forming processes in siliciclastic environments and guide exploration in siliciclastic terranes by studying the La Virgen district in northern Peru, 17 km east of the giant Lagunas Norte HS deposit, in which the HS mineralization is mostly hosted in the sandstone of the Chimu Formation and other siliciclastic rocks. In addition, this is the first detailed study of La Virgen District and has led to the discovery of porphyry style mineralization in the eastern part of the area, which, based on mapping, geochronology, SWIR (Short Wavelength Infra-Red) spectral analysis, petrography, LA-ICP-MS and whole rock geochemistry, is shown to have significantly older mineralization than the western side.

Introduction

Most of the major HS epithermal deposits around the world are hosted in magmatic rocks, including Yanacocha (Peru) with >70 million ounces (Moz) Au hosted in dacite porphyry domes and ignimbrites (Longo et al., 2010; Pilco and McCann, 2021); Pascua Lama (Chile- Argentina) with >21 Moz Au hosted in Permian-Triassic granite (Chouinard et al., 2005); Pueblo Viejo (Dominican Republic) with >41.7 Moz Au hosted in mudstone, microdioritic intrusion, and basaltic volcanic rocks (Kesler et al., 2005; Vaughan et al., 2021); Lepanto (Philippines), hosted in dacitic pyroclastic rocks (Chang et al., 2011; Calder et al., 2022); Mt Carlton (Australia), hosted in rhyodacite (Sahlstorm et al., 2020), and Zijinshan (China), hosted in granite (Chen et al., 2019). However, the giant Lagunas 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 as blackish silica with fine-grained pyrite and partly as creamy silica. The silicification also fills interstitial open spaces between grains of the sandstone, fractures in sandstones, and as cement of breccias. The porphyry style mineralization at Cuypampa displays minor quartz - pyrite veinlets (D vein) mostly cutting fine-grained sandstone of Carhuaz Formation and the main mineralization occurs inner feldspar porphyry developing quartz- moly-pyrite (B vein) and traces of remaining chalcopyrite in white mica + pyrite ± quartz stockwork zone.

Geochronology

This study collected 16 samples for dating: 7 alunite (Ar-Ar), 1 molybdenite (Re-Os) for alteration-mineralization and 8 Zircon (U-Pb) for magmatic rocks.

The dating shows that although both the La Virgen Domain and the Escalerilla block of the Escalerilla-Cuypampa Domain have HS mineralization and related AA alteration, they formed in two different events (ca. 18-17 Ma vs. ca. 22- 21 Ma) and are unrelated. The HS mineralization-alteration in Escalerilla block has similar ages as the porphyry-style mineralization in the Cuypampa block, therefore, it is inferred that the Escalerilla HS is related to the Cuypampa porphyry (ca. 22-21 Ma), while in La Virgen domain the HS mineralization-alteration is developed ca. 18-17 Ma (Figure 1).

Discussions and Conclusions

AAA in siliciclastic rocks

At LVD is largely cryptic to the naked eye. The AA1 alteration develops intensely in hydrothermal breccias, with silica (hydrothermal quartz) cementing open spaces and percolating tiny interstices between grains in quartz sandstones. Alunite forms platy crystals, and as we move away from the breccia zone, its occurrence increases, filling microfractures and interstices of quartz grains. Additionally, dickite appears, while silicification decreases. In more distal areas, dickite and kaolinite dominate, eventually, this transition leads to the IA zone, where fresh sandstones contain minor pyrite veins filling fractures as distal escape fluids. The major mineralization is associated with the AA1 assemblage. The rocks, especially sandstones, with AA alteration appear fresh, mainly due to the lack of vuggy texture, the low abundance of alunite, dickite and pyrophyllite, and the small grain sizes as such minerals could only grow in the tiny interstices between sandy grains.

Traditionally, pyrophyllite is used to indicate a higher temperature and deeper position than alunite (e.g., Chang et al., 2011). This study shows that this vector should be used more carefully in siliciclastic terranes, because pyrophyllite may preferably replaces clay-rich lithologies therefore there is an additional lithological control in addition to temperature/depth/acidity control. As a result, it would not indicate transition zones to high-temperature areas. This idea should be validated through further studies in other similar deposits.

Magmatic rocks show visible alteration through mineral changes, while quartz sandstones appear fresh but may have cryptic alteration, undetectable to the eye but present at a microscopic level. SWIR spectral analysis is highly effective for detecting alteration minerals. A dry, hand-sized sample is enough for quick reconnaissance, while a detailed sampling grid is ideal for district mapping.

Two Domains in the LVD: 

La Virgen and Escalerilla-Cuypampa

LVD has only been explored, recognized, and mined under the concept of a single HS mineralization since its discovery (Noble and Mckee, 1999; Ristorcelli and Prenn, 2001). This study found that it is actually composed of two domains, based on mapping and dating of alunite, molybdenite, and zircon. In the La Virgen Domain, the hypogene alunite Ar-Ar dates are 18.1 ± 0.4 to 17.1 ± 0.4 Ma, whereas in the Escalerilla-Cuypampa Domain, the hypogene alunite Ar-Ar dates are 22.1 ± 0.4 to 21.4 ± 0.4 Ma (related HS mineralization), and the molybdenite Re- Os date is 21.89 ± 0.09 Ma (Moly from B vein; porphyry mineralization style) and consistent with the alunite Ar-Ar dates. The alteration and mineralization in the two domains formed in two distinct events.

Exploration Implications of the Findings

The AA alteration in siliciclastic rocks is cryptic, filling microfractures and quartz grain interstices; therefore, SWIR instrument is essential, even if the rock appears fresh. The AA1 alteration developed more prominently in breccias and intensely fractured rocks, forming coarse alunite, the farther from the feeder zone, the alunite becomes increasingly cryptic, and dickite, kaolinite, illite, and smectite begin to appear.

Pyrophyllite develops in silt- and clay- rich rocks; therefore, in siliciclastic terrains, it should not be used as a vector toward the porphyry environment. The development of quartz veining (D veins) in siliciclastic rocks displays a very faint and thin halo of white mica and abundant FeOx box works.

According to our understanding, HS mineralization in siliciclastic rocks almost always require a magmatic component, such as subvolcanic intrusions, domes, and phreatomagmatic events, as these generate fracturing in siliciclastic rocks and enhance permeability for hydrothermal fluids.

In northern Peru, where there is over ~45,500 Km² Lower Cretaceous siliciclastic rock, the potential for discovering new deposits is quite high. Other siliciclastic districts worldwide should also be explored even if satellite images do not show color anomalies.

Acknowledgments

We appreciate Mochica Resources and the geological team, Katharina Pfaff, Moises Mera, and Kelsey Livingston for financial and technical support.

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