Formation of Gold Alloys during Crustal Differentiation of Convergent Zone Magmas: Constraints from an AU-Rich Websterite in the Stanovoy Suture Zone (Russian Far East)

Nikolai Berdnikov,Natalia Konovalova,Nikita Kepezhinskas,Pavel Kepezhinskas

doi:10.3390/geosciences12030126

Nikolai Berdnikov, Natalia Konovalova + Show 2 more

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https://doi.org/10.3390/geosciences12030126

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Abstract

Gold is typically transported by mafic and evolved magmas into the upper crust to be deposited in shallow oxidized porphyry and epithermal environments. However, the magmatic behavior of gold is still poorly understood and warrants further attention. Additional insights into the magmatic evolution of gold and other noble metals can be provided by investigations of primitive convergent zone magmas and products of their differentiation that contain primary-textured Au-alloys. One of the best examples of such Au-rich ultramafic cumulates is the Triassic (232–233 Ma) Ildeus intrusion, which was emplaced within the Mesozoic Stanovoy subduction zone in the Russian Far East. Some websterites from the Ildeus intrusion, representing cumulates crystallized from a primitive convergent zone magma, are enriched in Au (up to 596 ppm) and contain abundant Cu-Ag-Au micro-particles. Most of these Au-alloy micro-particles display compositions similar to those previously found in explosive pyroclastic rocks in the Lesser Khingan iron district, mantle wedge peridotites in Kamchatka and Cretaceous adakites in the Stanovoy suture zone. Textural and compositional characteristics suggest that Cu-Ag-Au alloys precipitated from a primitive calc-alkaline melt during its crustal differentiation in a Mesozoic paleo-subduction zone. Some large Cu-Ag-Au grains display an internal honeycomb-like structure with alternating Cu-rich and Cu-poor zones. Heating experiments under atmospheric conditions recorded a substantial loss of Cu from primary magmatic Cu-Ag-Au alloys, which appears to be a process characteristic of oxidized hydrothermal ore systems. We suggest that the later-stage hydrothermal alteration of differentiated igneous conduits containing magmatic gold alloys results in the formation of Cu-free gold mineralization comparable to the upper crustal porphyry and epithermal environments.

Highlights

IntroductionGold is an important commercial metal in various types (porphyry, epithermal, volcanogenic massive sulfide, Carlin-type, intrusion-related, etc.) of ore deposits at convergent plate margins [1–12]
We present in this paper new results of detailed electron microscope investigation of Au-bearing alloys in ultramafic rocks from the Ildeus intrusion in the Stanovoy Suture Zone of Far East Russia
Rocks of the Triassic (232–233 Ma) Ildeus intrusion in the Stanovoy suture zone represent ultramafic cumulates from the primitive convergent zone magma. lldeus ultramafic rocks are enriched in Au and contain micro-particles of Cu-Ag-Au alloys

Summary

Introduction

Gold is an important commercial metal in various types (porphyry, epithermal, volcanogenic massive sulfide, Carlin-type, intrusion-related, etc.) of ore deposits at convergent plate margins [1–12]. Because of their high solubility in a wide range of silicate melts and hydrothermal fluids, gold and associated base and precious metals (Ag, Cu, Mo, Pt, Pd) are transported into the upper crust by metal-rich, basaltic to dacitic magmas and deposited in epithermal and porphyry environments in subduction and collision zones [3–6,11,13–20]. Well-characterized mineralized plutonic complexes in ancient subduction settings, representing cumulates of primary arc magmas [30–32]) are capable of providing critical insights into the magmatic behavior of gold in crustal environments. The main goals of this paper are to (1) report new type of gold-rich mineralization associated with arc-related ultramafic-mafic intrusive complexes;

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