Iron-oxide and alkali-calcic alteration ore systems and their polymetallic IOA, IOCG, skarn, albitite-hosted U±Au±Co, and affiliated deposits: a short course series. Part 1: introduction

Abstract

Worldwide, iron oxide copper-gold (IOCG) deposits form world-class mining districts. In a single deposit, such as Olympic Dam, resources can reach 10 billion tons at 0.78% Cu, 0.25kg/t U3O8, 0.30g/t Au, 1.0g/t Ag. Rare-earth resources can also be significant. Systems that form IOCG deposits also host many other deposit types. This short course illustrates the metasomatic growth of polymetallic magmatic-hydrothermal iron-oxide and alkali±calcic alteration systems and the genetic linkages among their iron oxide-apatite (IOA), iron oxide copper-gold (IOCG) and affiliated deposit types, using the Great Bear magmatic zone (Northwest Territories, Canada) as a prime example. Complementary information are also sourced from the Central Mineral Belt of the Makkovik Province (Labrador), the Romanet Horst of the Labrador Trough (Quebec) and the Bondy gneiss complex of the Grenville province (Quebec) in Canada as well as from deposits worldwide. The Great Bear systems are differentially uplifted, tilted, transcurrent-faulted, and locally exhumed. They escaped orogenic metamorphism and pervasive deformation. Their former sedimentary covers are largely eroded and Quaternary glaciation left only a sporadic till cover. Consequently, outcrops are non-weathered, glacially polished and nearly continuous exposing in structural 3D the metasomatic growth of iron-oxide and alkali±calcic alteration ore systems from paleo-depth to paleo-surface. Within systems, metasomatism is pervasive and intense at regional to deposit scale. From paleo-depth to paleo-surface and away from heat sources (sub-volcanic intrusions), the diagnostic alteration facies prograde from: From depth to surface and away from heat sources, the diagnostic alteration facies prograde from: Facies 1 Na, transitional Na-Ca-Fe and skarn; Facies 2 high temperature Ca-Fe; Facies 3 high temperature K-Fe; Facies 4 transitional K and K-skarn; Facies 5 low temperature K-Fe (± low temperature Ca-Mg); and Facies 6 epithermal alteration. The prograde, retrograde, telescoped and cyclical metasomatic reaction paths lead to a regular series of deposit types with varied metal associations and mineralisation styles from paleo-depth to paleo-surface: Iron oxide-apatite (IOA) and their REE mineralised variants; Iron oxide copper-gold (IOCG) and low Cu, Co, Bi variants; Polymetallic potassic-skarns; Albitite-hosted U and Au-Co-U; Mo-Re and other affiliated deposits. Space-time relationships between metasomatism, magmatism, deformation and mineralisation constrain element addition and depletion in ore fluids from sources to deposits. They also record fluid pathways, sources of fluid rejuvenation, and heat sources across the ore-forming environments. The information is synthesised into an ore deposit model adaptable to the variety of fluid (and potential melt) sources, host rocks and compositions these ore systems have. New petrological mapping protocols, rock nomenclatures, chemical map methodologies, discriminant chemical diagrams, and geological vectors to mineralisation stem from these findings. They collectively unify the complex and disparate attributes of these ore systems into effective exploration concepts and provide a novel geoscience framework to explore Canada's prospective terrains for IOCG and affiliated deposits.