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.

Fig. 2. (a) Immiscible-liquid fluid inclusions. (b) Multiphase-solid-bearing fluid inclusions. (c) Cluster of vapor-rich fluid inclusions. (d) Cluster of liquid-rich fluid inclusions.

Composition of fluid inclusions

The different types of FI show a wide salinity range. IL type vary from 1 to 3 wt% NaCl eq.; MS type vary from 30 to 38 wt% NaCl eq.; VR type vary from 2 to 3 wt% NaCl eq.; and LR type vary from 3 to 15 wt% NaCl eq. Raman analysis of the gas phase of the IL type shows presence of CO₂, CH₄ and H₂S, with average molar percentages of 63, 28 and 9%, respectively. LA-ICPMS analyses on the FI show that LR assemblages contain (in order of decreasing mean concentration) Cl, Na, K, Fe, Ca and S, each one reaching up to 10,000 ppm levels, followed by moderately-abundant B, Mn, Mg, Zn (each one up to 2,000 ppm), and by less abundant Cu, As, Sb, Pb, Cs, Sr (each one between 100s and 10s ppm), and, finally, Ag (< 5 ppm) and Au (≤0.2 ppm). The MS assemblages contain Cl, Fe, Na, K, and Mn always showing concentrations >1 wt%, followed by Zn (up to 0.8 wt%), Cu and Ca (both up to 0.7 wt%), and S (up to 0.6 wt%). Other minor, though omnipresent elements are Sb, As and Pb (each one up to 1,000 ppm levels), as well as Rb, Br, B, Bi, Sr, Cs, Ba, Ag (each one up to100s ppm) and Mo (<1 ppm). Gold was not detected in any of the inclusions.

Origin and evolution of the ore fluid

The structural and sedimentary architecture of the MFTB has controlled the emplacement of the intrusive bodies and dictated the pathways of the hydrothermal fluids and their interaction with the host, during and after thrust fault propagation. In the Miocene (period of Au-Ag-Cu deposition; Noble and Mckee, 1999), the Algamarca anticline was buried and formed a structural trap large enough to focus an important fluid flow, sufficient to produce the Algamarca deposit. However, this deposit is currently at the surface, as a result of modern Andean tectonic uplift and unroofing.

The IL type FI are strongly enriched in CO₂, CH₄ and H₂S, which is quite unusual for an epithermal or, for that matter, porphyry-style mineralization. This finding provides direct evidence suggesting that the hydrothermal fluid interacted with organic carbon, most likely in the sedimentary rocks. This led to massive production of CO₂ and CH₄(and H₂S), according to carbon dissolution reactions such as: 2 C + 2 H₂O = CO₂ + CH₄. 

The mineralogy and FI data for the pre-gold stage (Qz-I) point to a porphyry-style mineralization, as suggested by high salt (NaCl, KCl) and ore metal (Fe, Cu, Zn) contents of the MS type FI. Such high contents are diagnostic of magmatic-derived brines in porphyry systems (e.g., Kouzmanov and Pokrovski, 2012). Conversely, the homogenization temperature and salinity of the LR type FI (320 °C and 7 wt% NaCl eq., on average) in the main gold stage (Qz-II) are more consistent with epithermal conditions. 

It thus appears that the Algamarca veins recorded an epithermal-type fluid flow, resulting in mineralization, superimposed onto previous porphyry-type fluid circulation. Similar Zn/Pb ratios measured in the MS type FI (Zn/Pb = 5±1) and LR type FI (Zn/Pb = 4±1; Fig. 3) attest for a common magmatic origin of the fluids evolving from porphyry-stage to epithermal-stage. Such Zn/Pb ratios are also consistent with those reported in giant porphyry deposits (from 1 to 6; Kouzmanov and Pokrovski, 2012). It follows that our investigation on the origin of mineralizing fluids at Algamarca has revealed the presence of a porphyry system at depth, providing a potential targeting tool for exploration.

Fig. 3. Pb vs. Zn concentration analysed in the indicated types of fluid inclusion assemblages (FIAs). Each point represents an average concentration value, the error bars (shown when bigger than the symbol size) correspond to the minimun and maximun value within each FIA. MS = Multiphase-solid-bearing fluid inclusions, LR = liquid-rich fluid inclusions.

Acknowledgements

This work was funded by the Institut Carnot ISIFoR (Grants OrPet and AsCOCrit), Prociencia (project 425-2019) and French-Peruvian cooperation program ECOS-Nord (grants ECOS N°P21U01 and 020-2021-FONDECYT). We thank A. Marquet and P. de Parseval for help with the LA-ICP-MS and EPMA analysis. We are grateful to H. Valdez and W. Cotrina of Amasba association and HNS Consorcio SRL for assistance in the field.

References

Barker, S.L., Hickey, K.A., Cline, J.S., Dipple, G.M., Kilburn, M.R., Vaughan, J.R., Longo, A.A., 2009. Uncloaking invisible gold: Use of nanoSIMS to evaluate gold, trace elements, and sulfur isotopes in pyrite from Carlin-type gold deposits. Economic Geology, v. 104, p. 897–904.

Galdos R., Vallance J., Baby P., Pokrovski G.S., 2021. A common hydrothermal magmatic system generates different styles of gold mineralization at Algamarca and Shahuindo, Northern Peru. ProEXPLO 2021, Lima, Peru. Extended Abstracts, p. 100–104.

Kouzmanov, K., Pokrovski, G.S., 2012. Hydrothermal controls on metal distribution in porphyry Cu (-Mo-Au) systems. SEG Special Publication, v. 16, p. 573–618.

Mégard, F., 1984. The Andean orogenic period and its major structures in central and northern Perú. Journal of the Geological Society, v. 141, p. 893–900.

Noble, D.C., McKee, E.H., Mégard, F., 1979. Early Tertiary “Incaic” tectonism, uplift, and volcanic activity, Andes of central Peru. Geological Society of America Bulletin, v. 90, p. 903–907.

Noble, D.C., McKee, E.H., 1999. The Miocene Metallogenic Belt of Central and Northern Peru. In: Skinner, B.J. (Ed.), Geology and ore deposits of the Central Andes. SEG Special Publication, v. 7, p.155–193. 

Scherrenberg, A.F., Kohn, B.P., Holcombe, R.J., Rosenbaum, G., 2016. Thermotectonic history of the Marañón Fold-Thrust Belt, Peru: Insights into mineralisation in an evolving orogen. Tectonophysics, v. 667, p. 16–36.

Vallance, J., Galdos, R., Balboa, M., Berna B., Cabrera O., Huisa F., Baya C., Van De Vyver C., Viveen W., Béziat D., Salvi, S., Brusset, S., Baby, P., Pokrovski G.S., 2023. Combined effect of organic carbon and arsenic on the formation of sediment-hosted gold deposits: a case study of the Shahuindo epithermal deposit, Peru. Economic Geology, accepted.