For: Renzo Galdos^1,2, Jean Vallance^1,3, Patrice Baby^1,2, Stefano Salvi², Michael Schirra⁴, German Velasquez^2,5 and Gleb S. Pokrovski²

¹ Grupo de investigación en Geología de Yacimientos, Especialidad de Ingeniería Geológica, Pontificia Universidad Católica del Perú.

² Géosciences Environnement Toulouse, Université Toulouse III - Paul Sabatier.

³ Thin Section Lab.

⁴ Department of Earth Sciences, University of Geneva.

⁵ Instituto de Geología Económica Aplicada, Universidad de Concepción. 

Trabajo ganador del I Concurso Internacional SEG – SGA - IIM

Abstract

The nature of the gold-bearing fluid and the source of gold in sedimentary-hosted gold deposits of Carlin, orogenic or epithermal types is still matter of debate. Most authors point to magmatic/ metamorphic origin for the fluids, however, the properties of such fluids and their interactions with organic carbon – a common component in this type of deposits – are not sufficiently well known. Here we present a large set of novel geochemical, mineralogical and fluid-inclusion data obtained from the Algamarca Au-Ag-Cu epithermal veins, corresponding to lateral manifestation of the neighbouring sedimentary-hosted epithermal-gold deposit at Shahuindo. Our data reveal that strong interactions of a metal-bearing magmatic fluid with the organic carbon in the sedimentary basin exerted a primary control on gold transport, precipitation and distribution.

Introduction

Recent models inferred a magmatic source for gold-bearing fluids in sedimentary-hosted gold deposits. However, most of these inferences come from indirect data of sulfur isotopes or trace elements in arsenian pyrite (e.g., Barker et al., 2009; Vallance et al., 2023) - the main gold ore in this kind of deposits. Recent thermodynamic modeling suggests that interactions between hydrothermal fluids and organic carbon may favor gold transport rather than its precipitation (Vallance et al., 2023), as commonly assumed. However, the source of gold-bearing fluids is poorly understood and the consequences of their interaction with the organic carbon remain elusive.

The aim of this work is to assess the origin, properties and evolution of the gold-bearing fluids responsible for Au-Ag-Cu mineralization in the Algamarca deposit, which is hosted in sedimentary sequences rich in carbonaceous material. We studied a large set of samples of ore veins from the Algamarca deposit. The samples were analyzed by a series of analytical techniques: etching, scanning electron microscopy (SEM), X-ray powder diffraction (XRD), Raman spectroscopy (RS), electron microprobe analyses (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Fluid inclusions trapped in vein quartz were also investigated.

Geological setting

The Algamaca Au-Ag-Cu deposit is located 22 km west of the town of Cajabamba (Cajamarca region) of northern Peru, within the Marañon fold and thrust belt (MFTB). The MFTB hosts a suite of epithermal, porphyry and skarn deposits associated with intrusions from the Oligo-Miocene subduction-related magmatic arc (Noble and McKee, 1999; Scherrenberg et al., 2016). The Algamarca deposit consists of veins trending perpendicularly or obliquely to the Algamarca Anticline axis. This anticline corresponds to a partially eroded structure of the MFTB, which is an east-verging thin-skinned thrust system developed during late Cretaceous and the middle Eocene (early period of the Andean orogeny; Noble et al., 1979; Mégard, 1984). The close proximity (<1 km) between Algamarca and Shahuindo, and their similar age and ore mineralogy, suggest that both deposits correspond to the same magmatic/hydrothermal system (Galdos et al., 2021). The Mezosoic sediments that form the MFTB and host the Algamarca and Shahuindo deposits are enriched in carbon (mostly graphite, up to 2 wt%) that likely represents an overmature petroleum system.

Results

Sulfide ore minerals

Paragenetic sequence

Three mineralization stages have been identified: i) a pre-gold stage (stage A), characterized by quartz (Qz-I), pyrite (Py-I) and chalcopyrite; ii) a main gold stage (stage B), which presents an invisible-gold substage characterized by quartz (Qz-II) and arsenian pyrite hosting invisible gold (Py-II), and a visible-gold substage characterized by tetrahedrite-tennantite and chalcopyrite, where native gold occurs within microfractures in tetrahedrite-tennantite or along the contacts between the latter and chalcopyrite grains; iii) a barren post-gold stage (stage C), characterized by alunite, pyrophyllite, covellite and chalcocite. The paragenetic sequence for the Algamarca deposits is shown in Figure 1.

Fig. 1. Paragenetic sequence of the Algamarca veins. The line thickness is roughly proportional to the mineral abundance.

Sulfide mineral composition

Pyrite, chalcopyrite, tetrahedrite and tennantite, the most abundat ore minerals, and native gold from the veins were analyzed by EPMA and LA-ICPMS. Pyrite is the most important host of invisible gold (up to 160 ppm Au), whereas the other sulfides show much lower Au contents (<1 ppm Au). The pre-gold-stage pyrite (Py-I) presents lower invisible gold contents than the main-gold-stage pyrite (Py-II). The invisible gold contents in the latter significantly correlate with As contents and both show smooth patterns in the LA-ICPMS ablation signals, thus suggesting the absence of micro-particles of Au-bearing minerals. The main Ag host is tetrahedrite, with up to 1.6 wt% Ag, whereas tennantite and pyrite present much lower concentrations (<600 and <50 ppm, respectively). Native gold grains contain 14 wt% Ag, on average. Copper is mainly concentrated in chalcopyrite (in the pre-gold and main gold stages), tetrahedrite and tennantite (in the main gold stage). In the post-gold stage copper contents are lower, limited to scarse covellite and chalcocite.

Fluid inclusions

Fluid inclusions petrography

Four different types of fluid inclusions (FI), recognizable by their different phases present at room temperature, were identified in quartz from the Algamarca veins (Fig. 2). These types of inclusions are distributed in two quartz generations, Qz-I (from the pre-gold stage) and Qz-II (from the invisible-gold substage). In chronological order these are: i) Immiscible-liquid FI (IL; Fig. 2a), containing a CO₂-H₂S-CH₄ gas bubble, a CO₂-H₂S-CH₄-H₂O liquid, an aqueous liquid, plus an opaque daughter mineral. ii) Multiphase-solid-bearing FI (MS), containing a brine, a vapour bubble, a halite crystal plus two additional solid phases consisting of an opaque mineral and a rounded transparent mineral (Fig. 2b). The number and proportion of phases is consistent in each FI assemblage; however, either of the latter two solids may be absent from a MS-type FI population. iii) Vapor-rich FI (VR) containing a dominant vapor phase and a liquid phase (Fig. 2c); these FI are found to coexist with the MS FI type. iv) Liquid-rich FI (LR) contain a dominat liquid phase and a vapor phase (Fig. 2d). The IL type occurs in Qz-I as clusters, pseudosecondary trails, or isolated FI. The MS and VR types occur in Qz-I as pseudosecondary and secondary trails, also crosscutting the IL type, and in clusters. The MS and VR types commonly form the same FI assemblage, suggesting boiling of the fluid. The LR type is hosted by Qz-II, which is clearly discernable by a darker cathodoluminescence than Qz-I. The LR type comprises primary FI parallel to growth bands, pseudosecondary and secondary FI trails, and FI in clusters and isolated individuals.