The GBMZ has experienced a prolonged arc-related magmatism that intruded felsic-intermediate plutonic bodies into the ~1.885 Ga Echo Bay and Port Radium supracrustal marine sedimentary rocks and volcanic suits. This magmatism created a perfect geological environment for the formation of a variety of ore deposits and introduced a substantial amount of metals such as Fe, Cu, Pb, Zn, Ni, Co, U, Bi, and somewhat Au, Ag, and Th. The Richardson high heat production pluton located 9 km south of Port Radium (Fig. 1) is enriched in some of these metals (particularly U and Th) and may have partially been the source of some of these metals (Somarin and Mumin, 2012).
The main ore mineralization of the Port Radium deposit occurred during epithermal veining which followed an extensive and intensive IOCG-style hydrothermal alterations including albitic, MAA, potassic, phyllic and propylitic. It appears that occurrence of phyllic-propylitic alterations before epithermal veining is a common feature in IOCG deposits of Port Radium and other IOCG deposits of the GBMZ (Somarin and Mumin, 2014; Klapheke, 2018). Various types of veins and hydrothermal assemblages formed during fluid-rock interactions and evolution of the Port Radium hydrothermal system. These complex textural and compositional variations in the ore mineralogy reflect rapid changes in temperature and chemical activities in the system (Somarin and Mumin, 2014).
Although sulfide mineralization is younger than MAA alteration, drill core logging and drill assay data of IOCG-style deposits in GBMZ (Somarin and Mumin, 2008; Somarin and Mumin, 2014; Klapheke, 2018) indicate that MAA zones always have high base metal concentrations. This may suggest that either both MAA and sulfide mineralizations utilized the same channels for precipitation, or MAA prepared the ground for sulfide mineralization by alteration of the country rocks which were subsequently fractured easily forming channels for movement of the base metal-bearing hydrothermal solution. As the hydrothermal system evolved, magnetite content of the MAA alteration was reduced and sulfide mineralization progressed. This manifests as MAA transitioning into AA-pyrrhotite-pyrite-chalcopyrite reflecting changing in the fugacity of oxygen and sulfur in the hydrothermal system.
Sulfide and sulfarsenide mineralization in the strongly altered wall rocks occurred possibly due to reaction of the hydrothermal solution with the country rocks. Such reactions commonly formed fine-grained hydrothermal assemblages at Port Radium (Fig. 3e). In contrast, similar minerals precipitated as coarser grains in the veins due to boiling or sudden changes in the pressure and temperature of the hydrothermal fluid (Somarin and Mumin, 2014). Intense brecciation of the wall rocks of the veins suggests high fluid pressure during hydrothermal activity that caused hydrofracturing of the surrounding rocks. Propylitic alteration of the wall rock fragments shows that main ore mineralization of Port Radium occurred after propylitic alteration. The cross cutting relationship and textural features show that uranium mineralization occurred almost simultaneously with sulfides and sulfarsenide mineralization in veins and continued after them, probably suggesting change in the oxidation state of the system. This temporal and spatial relationship between U and sulfides-sulfarsenide mineralization can be used as an exploration tool for uranium mineralization.
As the Port Radium hydrothermal system was enriched in Ni-Co, not only the common sulfides (e.g. pyrite and pyrrhotite) but also others such as sphalerite, chalcopyrite, arsenopyrite, tetrahedrite, emplectite, and aikinite have traces of these metals (as substitutes for Fe and possibly Zn and Cu).
Safflorite, gersdorfite, skutterudite, cobaltite, niccolite, and rammelsbergite are common minerals of the Fe-Co-Ni-As-S system that are found at Port Radium. Three unknown minerals with (Co0.96Ni0.45Fe0.03)S0.27As4, (Ni2.13Co0.38)S0.97As4, and (Ni1.83Co0.83)S1.25As4 formulas are also found that show distinct compositional differences with known minerals of this system (Figs. 6, 7). The nomenclature guidelines of International Mineral Association (IMA) states that in a continuous solid solution, only end members are considered as species (Nickel and Grice, 1998). For example, if a mineral has at least 50 mole% of an end member, it is named after that end member. This rule is easily applicable to continuous solid solutions where the end members have similar structures; its application becomes complicated in minerals with complex structures and multiple substitutions (Nickel and Grice, 1998; Kiefer et al., 2017). Mineral unknown 1 (Co0.96Ni0.45Fe0.03)S0.27As4 is possibly a solid solution product of cobaltite and gersdorfite. The resulted mineral is chemically and optically different than both cobaltite and gersdorfite.
Mineral unknown 2 (Ni2.13Co0.38)S0.97As4 and unknown 3 (Ni1.83Co0.83)S1.25As4 are chemically comparable to the Port Radium rammelsbergite (Ni1.01Co0.04S0.09As2 or Ni2.02Co0.08S0.18As4) with substantial addition of S and Co. Similarly, addition of S to rammelsbergite by hydrothermal solution and formation of gersdorfite has already been documented at Port Radium (Fig. 3g). It is notable that the added S and Co did not substantially replace As and Ni, respectively, as apfu values of As and Ni are similar in these unknown minerals and rammelsbergite. Mineral unknown 2 and unknown 3 could also be a solid solution product of gersdorfite-cobaltite or safflorite-rammelsbergite (Hem, 2006). The addition or vast substitution of As-S and Ni-Co-Fe in the Fe-Co-Ni-As-S system changes unit cells and form different minerals (Hem, 2006). These changes may affect optical properties including anisotropism of the sulfarsenide (Kiefer et al., 2017).