Geochemical footprints of IOA and IOCG deposits in Northern Norrbotten, Sweden, and Cloncurry District, Australia

Abstract

This paper addresses the complex hydrothermal evolution of Metasomatic Iron and Alkali-Calcic (MIAC) mineral systems based on a review of the lithogeochemical footprints of IOA and IOCG deposits in the northern Norrbotten province (Sweden) and the Cloncurry district (Australia). The use of Na-Ca-Fe-K-Mg molar barcodes on a lithogeochemical diagram tailored for these mineral systems allows to depict the evolution of MIAC systems along diagnostic metasomatic paths from high (HT) to low temperature (LT) alteration facies as follows: 1) HT or LT Na alteration (300–1000 °C); 2) HT CaFe alteration (400–1000 °C); 3) HT KFe alteration (350–450 °C); 4) HT K and HT K-Ca-Mg alteration; 5) LT KFe, Na-Ca-Mg-Fe, and/or Na-Ca-Mg alteration (≤ ~350 °C); and 6) epithermal alteration (≤ 150 °C) and later stage hydrothermal veining. A distinct range of whole rock compositions and metal associations characterizes each alteration facies and can be captured by diagnostic molar barcodes and alteration indices. In northern Norrbotten, the IOA deposits are hosted in HT CaFe alteration facies but regionally intensely albitized regions are overprinted by KFe alteration. The IOCG deposits are hosted in MIAC systems with zones of early Na (Ca) alteration related to the regionally extensive albitite or scapolite alteration (Facies 1) and localized skarns. These are overprinted by HT CaFe alteration (Facies 2) and HT to LT KFe alteration (Facies 3 and 5). The CuAu mineralization is not systematically associated with the iron oxide-rich breccias and the intense K-feldspar- or sericite-rich KFe alteration typical of many IOCG deposits worldwide. Instead, the lesser intensity of alteration and the abundance of mafic and ultramafic rocks in the environment lead to pattern enriched in Mg with relic of amphibole-rich alteration remaining in the assemblage as demonstrated for the Nautanen North IOCG deposit (Sweden). Consequently, the geochemical footprints of the Norrbotten CuAu deposits are distinct from magnetite-group (e.g., Great Bear magmatic zone, Canada) and hematite-group (e.g., Olympic Dam, Australia) IOCG deposits even if they have all the known alteration facies of MIAC systems. Conversely, IOCG deposits in northern Norrbotten show similarities to certain deposits in the Cloncurry district of Australia. In both regions, the IOCG deposits are associated with HT CaFe and KFe alteration facies that commonly overprint early Na and/or NaCa alteration. In northern Norrbotten, IOA deposits are characterized by early Na alteration evolving towards NaCa alteration, then Fe-rich CaFe alteration. These hydrothermal alteration types are subsequently superimposed by later KFe alteration. We conclude that the use of Na-Ca-Fe-K-Mg molar barcodes provides new insights to understand the evolution of MIAC systems and is a powerful approach for unraveling superimposed alteration trends, which can serve as an exploration targeting tool from the district- to the deposit-scale in complex metasomatized areas.