Origin of gold mineralization in the Borden Mine and a new tectonic model for the Kapuskasing Structural Zone, Ontario, Canada

Abstract

The Kapuskasing Structural Zone (KSZ) containing high–pressure granulites is one of the best exposed mid–to–lower crustal sections in the world. However, the timing and mechanism for its formation/exhumation remain highly controversial. Previous studies of the KSZ and its neighboring Wawa Gneiss Domain (WGD) focused predominantly on Proterozoic brittle deformation in the Ivanhoe Lake Deformation Zone (ILDZ), with tectonic models restricted to political boundaries. In this study, the KSZ and WGD are proposed as a continental metamorphic core complex (MCC) with the high–grade footwall of the former exhumed in comparison to the southern Superior Province, and the latter as the hanging–wall, high–strain listric regional structure. The KSZ exposes granulite–facies ductile sinistral strike–slip shear zones and amphibolite–facies brittle–ductile sinistral and dextral shear zones that predate the ILDZ. Neoarchean oblique collision between the Minnesota River Valley terrane and southern Superior Province formed the Pontiac subprovince accretionary prism and equivalents in the “Quiet Zone” by ca. ≥2667 Ma. The formation and exhumation of the WGD and KSZ as a MCC were synchronous with retrograde amphibolite–facies metamorphism by at least ca. ≥2630 Ma. Plutonism and continued transtension, rotation, and related folding continued to ca. 2600 Ma with magmatism ceasing by ca. 2580 Ma. The enigmatic Borden Mine (4.17 Mt at an average grade of 6.38 g/t Au) in the Chapleau Belt within the southern KSZ is hosted in sillimanite–garnet–biotite gneisses or schists within metamorphosed mafic and felsic volcanic rocks (with mid–ocean ridge basalt and arc affinities), and banded iron formations. Laser ablation inductively coupled plasma mass spectrometry (LA–ICPMS) analysis of zircon in sillimanite–garnet–biotite gneiss from the Discovery Trench of the Borden Mine yielded three age populations: 2703.9±7.3, 2667.1±7.2 and 2590±17 Ma. The oldest population represents magmatic crystallization that formed the volcanic protolith of the sillimanite–garnet–biotite gneiss, while the latter two record prolonged high–grade metamorphism. Gold mineralization in the Borden Mine most likely formed initially as part of syn–volcanic seafloor hydrothermal alteration and was remobilized throughout prograde upper amphibolite– to– granulite–facies metamorphism during orogenesis and retrograde amphibolite–facies metamorphism during exhumation.