Exceptional Concentrations of Gold Nanoparticles in 1,7 Ga Fluid Inclusions From the Kola Superdeep Borehole, Northwest Russia

V Lüders,D A Banks,K V Lobanov,V A Milichko,S L Selektor,V Yu Prokofiev,N N Akinfiev,M V Chicherov,A A Borovikov

doi:10.1038/s41598-020-58020-8

Abstract

In the drill core of the Kola super-deep borehole (SG-3, 12,262 m depth) gold-bearing rocks of Archaean age have been located at depths of 9,500 to 11,000 m. In veins, between 9,052 and 10,744 m, within this gold zone, quartz contains fluid inclusions with gold nanoparticles. There are 4 types of fluid inclusions (1) gas inclusions of dense CO2, (2) liquid-vapor two-phase aqueous inclusions, (3) three-phase inclusions with NaCl daughter crystals, and (4) CO2-aqueous inclusions. In all inclusion types, there are extremely high concentrations of gold. The highest gold concentrations were found in the type 3 and 4 fluid inclusions with an average concentration of c. 750 ppm and may be as high as 6,000 ppm. The presence of gold as nanoparticles in the solutions of these fluid inclusions was determined by optical and spectroscopic methods. We suggest that these fluids could be a precursor of “orogenic gold fluids” which, at the gold concentrations determined, would reduce the requirements for large volumes of metamorphic fluids to form orogenic ore deposits. Further, as nanoparticles, gold could be transported in larger amounts than in true solution.

Highlights

The aim of this work was to study the physicochemical parameters, chemical composition and gold content of the fluids that formed the deep gold mineralization, which was discovered during the drilling of the Kola superdeep well SD-3 at a depth of 9.5–11.0 km
The gold mineralization is localized in the metamorphic rocks of Archean age (2.6–2.8 Ga), that were overprinted by Proterozoic regional epidote-amphibolite and amphibolite facies metamorphism
Fluid inclusions in quartz from 9,052 to 10,744 m can be divided into four types (Fig. 1): type 1, or V – gas inclusions of dense CO2, type 2, or L-V – vapor-liquid two-phase aqueous inclusions, type 3, or L-V-S – three-phase inclusions containing NaCl daughter crystals, and type 4, or L-LCO2 – CO2-aqueous inclusions

Summary

Introduction

The aim of this work was to study the physicochemical parameters, chemical composition and gold content of the fluids that formed the deep gold mineralization, which was discovered during the drilling of the Kola superdeep well SD-3 at a depth of 9.5–11.0 km. Different types of fluid inclusion are present, liquid-vapor-halite, liquid-vapor, liquid-vapor-CO2 and pure CO2 in quartz veins, all containing exceptionally high concentrations of gold and silver. The Kola superdeep borehole (SG-3, 12,262 m) was drilled from 1,970 to 1,992, in the Pechenga ore district of the Russian Federation through approximately a third of the Baltic Shield continental crust It was the deepest man-made hole on Earth, and was drilled to provide information about the deep structure of the Baltic Shield[8] and to investigate seismic discontinuities. Taking into account the erosion, which has cut out about 7 km of the Baltic Shield in the Kola region, the actual depth of the gold mineralized zone was at 16,600–18,000 m during the Proterozoic. Raman spectroscopy indicates only low-density N2 was present in the gas phase of type 2 fluid inclusions

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