Iain M. Samson's research while affiliated with University of Windsor and other places

The Wawa Gold Corridor, a series of Archean orogenic Au deposits in the Michipicoten greenstone belt, Canada, comprises two styles of Au mineralization: (1) syn-deformation gold associated with pyrite and arsenopyrite; and (2) late- to post-deformation gold associated with chalcopyrite and Bi-Te(-S) phases. Through petrographic and mineral–chemical analysis, it was determined that gold in the latter assemblages precipitated from Bi-rich polymetallic melts during hydrothermal overprinting of the earlier Au-As-S mineralization; this event was likely driven by the emplacement of Archean lamprophyres. The formation and evolution of these melts was governed by fluid–pyrite reaction interfaces, where the bulk composition of the melts was broadly controlled by the trace-element chemistry of the sulphide minerals in the local host rocks. This suggests that the melt-formation event involved mobilization of existing metal endowments related to early Au events, rather than addition of new Au, Bi, and Te. Thus, the deposition of high-grade Au by Bi-rich melts was dependent on pre-existing sulphide mineralization, both as a source of metals and as micro-environments that stabilized the melts. The paragenesis documented in the Wawa Gold Corridor (i.e., early hydrothermal Au-As-S mineralization and late melt-related Au-Bi-Te mineralization) has been previously recognized in numerous other orogenic and non-orogenic Au deposits. Herein, it is suggested that this apparent consistency in the timing of melt events across multiple systems probably reflects the physicochemical conditions (i.e., fO2-aH2S) of orogenic fluids being incompatible with molten Bi. Bi-rich polymetallic melts are hence unlikely to form primary Au mineralization in orogenic systems but can, however, have a significant impact on the ultimate deposit-scale distribution of Au via secondary mobilization and enrichment.

The Wawa gold corridor, located in the Michipicoten greenstone belt of the Superior province, Canada, comprises Au-bearing shear zones that crosscut the 2745 Ma Jubilee stock and that evolved during protracted deformation (D1-D3). Numerous generations of sulfide minerals crystallized before, during, and after these deformation events, and gold is associated with D1 arsenopyrite, D2 pyrite, and Bi-Te phases and chalcopyrite in assemblages that crosscut D3 veins. Observations of porosity and inclusions in D1 arsenopyrite and D2 pyrite suggest these sulfides underwent coupled dissolution-reprecipitation reactions. By coupling these textural observations with trace element analysis by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), it is evident that such replacement reactions generated gold from Au previously in solid solution. Conversely, textural relationships among paragenetically late gold and Bi-Te minerals are consistent with the precipitation of these phases from Bi-rich polymetallic melts. Mass balance calculations involving comparisons of the mass of Au in sulfides and the total mass of Au in their associated host rocks indicate that only D1 arsenopyrite contained enough Au to account for whole-rock Au content and overall deposit grade. Although D1 arsenopyrite is less volumetrically abundant than the various pyrite generations present in the deposit, it is often replaced by the later pyrite types, which is compatible with higher initial volumes of arsenopyrite than what is presently observed. It is concluded that the D1 Au + arsenopyrite event was the principal Au-mineralizing event in the Wawa gold corridor and that the other gold-bearing assemblages (i.e., gold + D2 pyrite, gold + Bi-Te phases + chalcopyrite) largely represent secondary mobilization of this primary enrichment. Given that LA-ICP-MS sulfide chemistry is regularly used in orogenic Au research, the approach outlined herein to assess the relative impact of distinct Au- and sulfide-mineralizing events could easily be applied to the study of other Au deposits in which complex hydrothermal parageneses are recognized.

Deep burial of sedimentary basins results in the development of complex diagenetic environments influenced by pressure, temperature, and metasomatic chemical processes. Fracture systems resulting from deep tectonic-related burial can provide archives of physio-chemical characteristics during burial helping unravel diagenetic events such as hydrocarbon migration and paleobarometry. The Early Triassic Montney Formation in the Western Canadian Sedimentary Basin is a highly productive unconventional hydrocarbon reservoir that has undergone multiple phases of tectonic-related burial and uplift resulting in the formation of a series of calcite-filled fracture systems. These fracture systems occur as vertical to sub-vertical fractures, brecciated zones, and horizontal bedding-plane parallel fractures that are rich in co-occurring, but not co-genetic aqueous and petroleum fluid inclusion assemblages. Fluid inclusion microthermometry, Raman spectroscopy, and stable isotope analysis of these fracture systems and host rock reveals paleobarometric and temperature conditions during fracture formation. Vertical fractures formed at temperatures exceeding 142°C during peak burial associated with the Laramide orogeny ∼50 Ma. Similarities in modeled oxygen isotope values of calcite parent fluids and pore water implicate locally sourced carbonate in fracture calcite. Therefore, low permeability and closed system-like conditions were prevalent throughout initial fracture formation and cementation. Petrographic analysis of brecciated and horizontal fractures show evidence of hydrocarbon generation and migration into fracture-filling calcite. Modeling of petroleum inclusion paleobarometry indicates entrapment pressures approaching or even exceeding lithostatic pressure consistent with the development of overpressure associated with the thermal maturation of organic matter following peak burial. Combined use of aqueous and petroleum fluid inclusions in this deeply buried sedimentary system offers a powerful tool for better understanding diagenetic fluid flow, the timing of hydrocarbon migration/maturation, and helps constrain the pressure-temperature history important for characterizing economically important geologic formations.

Magmatic and metasomatic zircon occurs in many alkaline igneous rocks and both are potential economic reservoirs of Zr, and in some places, rare-earth elements. The Baerzhe deposit in China is an example of a system where both types of zircon occur. Previous studies recognized deuteric and variably altered magmatic zircon in a transsolvus miaskitic granite, as well as four types of metasomatic zircon in a transsolvus agpaitic granite. In this study, the relationships among, and origins of, zircon and how these relate to models for rare-metal mineralization are assessed. In-situ back-scattered electron (BSE) and cathodoluminescence (CL) imaging, Raman spectroscopy (including mapping), and chemistry of zircon from the agpaitic granite were conducted, combined with evaluation of published data on zircon from Baerzhe. Their textural, spectroscopic, and chemical characteristics suggest that the four types of metasomatic zircon in the agpaitic granite were not subjected to metamictization or intense alteration, with trace-element accommodation largely following a xenotime substitution mechanism. The most abundant type of metasomatic zircon in the agpaitic granite occurs in zircon-quartz pseudomorphs and exhibits comparable CL, Raman spectral, and chemical features to rare zircon that has partially replaced elpidite. This confirms that the pseudomorphs formed by complete replacement of elpidite. The pseudomorph zircon occurs in association with snowball quartz that contains inclusions of zircon, aegirine, and albite, and with secondary quartz containing aegirine. This is consistent with their coeval formation during Na metasomatism. The restriction of Na metasomatism to the agpaitic granite indicates that this event and the associated zircon formation resulted from early autometasomatism of the agpaitic phase. REE- and Be-rich zircon that replaced magmatic amphibole crystallized as a result of reaction with a REE- and Be-rich fluid that most likely was responsible for the later REE-Nb-Be mineralization that affected both the miaskitic and agpaitic granites. The miaskitic granite contains deuteric and altered magmatic zircon with different chemical characteristics to the four types of metasomatic zircon in the agpaitic granite. This suggests that secondary Zr mineralization in the miaskitic granite formed from different fluids to those that metasomatized the agpaitic granite and may also have resulted from autometasomatism. This study reveals a complex picture for the formation of zircon at Baerzhe, the character of which can vary significantly, both temporally and spatially. Such variable chemistry of the various types of zircon resulted not only from their different origins (magmatic vs. metasomatic), but also from localized water-rock interaction that involved multiple stages of fluids. Zircon in both the miaskitic and agpaitic phases was mainly the product of autometasomatism that was constrained to their parental granites.

The Geordie Lake Cu-Pd deposit is associated with troctolite at the base of the Geordie Lake intrusion, located near the center of the Coldwell Complex (1106.5 + 1.2 Ma). It is the only platinum group element deposit in the Midcontinent Rift associated with alkaline rocks. This study focuses on the long-standing questions regarding genetic relationships among the Geordie Lake gabbros, the Wolfcamp basalt, and the various syenites that make up the east-central portion of the Coldwell Complex. Primitive mantle-normalized trace-element patterns for the Geordie Lake intrusion are nearly flat from Th to Ce and show negative Sr, Eu, and Zr anomalies. Characteristic ratios for the Geordie Lake gabbro and troctolite include Th/Nb (0.12), La/Nb (1.1), La/Lu (150), La/Sm (6.9), Zr/Sm (18), and Gd/Yb (2.8). Trace-element patterns that are useful for determining petrogenesis for gabbros are similar to the Wolfcamp basalt and augite syenite with some key exceptions, notably the middle rare earth element and Zr abundances. Affects due to metasomatism or crustal contamination in Wolfcamp basalt and Geordie Lake gabbros and syenites are negligible. Results of Rayleigh fractionation modeling show (1) the Geordie Lake intrusion and Wolfcamp basalt are very similar but not directly related by crystallization, (2) the gabbros and basalt are not related to the syenites, (3) the lower augite syenite can be related to the upper augite syenite and amphibole quartz syenite by fractionation of a hypothetical crystal cumulate composed of orthoclase (78%), clinopyroxene (15%), olivine (1%), and titanomagnetite (6%). We conclude that the Geordie Lake intrusion, Wolfcamp basalt, and saturated syenites in the Coldwell were derived by separate partial melting events in a common mantle source. The origin of the sulfide mineralization is enigmatic because it exhibits characteristics of both magmatic and hydrothermal processes. The sulfide assemblage changes from disseminated bornite and chalcopyrite in the basal zone to pyrrhotite plus chalcopyrite in the upper zones. Sulfides occur as coarse blebs interstitial to fresh or partly altered silicates, or as very fine grains intergrown with clusters of biotite and actinolite. Primitive mantle-normalized platinum group element patterns exhibit a W-shape for Pd-Pt-Rh-Ir-Ni, indicating a relative depletion of Pt and Ir. The Cu/Pd ratios in the mineralized zones are within the range of mantle values (1000–10,000), Pd/Pt is 14–19, Pd/Rh is 91 + 37, and Pd/Ir >16,000. The Pd/Pt, Pd/Rh, and Pd/Ir are considerably higher than in the Wolfcamp basalt (<1, 17, and 75, respectively). If the sulfides are magmatic in origin, then either the Geordie Lake magma was, unlike the Wolfcamp basalt magma, depleted in Pt, Rh, and Ir, or these elements were selectively removed from the sulfide assemblage. Alternatively, Pd was enriched by late-stage hydrothermal processes. Additional work is recommended to constrain petrogenesis of the sulfides by detailed base-metal and TABS (Te, As, Bi, Sb, and Sn) element analysis.

The South Mountain Batholith (SMB; Nova Scotia, Canada) is the largest composite batholith exposed in the Appalachians and lies entirely within the most outboard Meguma terrane. In-situ and CA-TIMS U-Pb dating and in-situ isotopes (Lu-Hf, O) and geochemistry for zircon from all phases of the SMB constrain its source as well as its evolution. CA-ID-TIMS for zircon yield emplacement (autocryst) ages indicating a transition from granodiorite (378.7 ± 1.2 to 375.4 ± 0.8 Ma) to leucogranite (375.4 to 371.8 ± 0.8 Ma) over several million years. Furthermore, in situ SHRIMP, LA-MC-ICP-MS, and SIMS analyses of distinct zircon domains reveal: 1) abundant ancient xenocrysts (~420 Ma to 2.2 Ga); 2) antecryst ages ca. 3–15 million years older than SMB emplacement; 3) autocryst δ18O values between +7.3 and +9.1‰ (V-SMOW); 4) similar isotopes, REE signatures, and derived fO2 values among antecrysts and autocrysts; and 5) εHf values from the 371.8 ± 0.8 Ma Davis Lake Pluton (DLP) autocrysts that are higher (+1.74 to +4.38) than the rest of the SMB (-2.99 to +1.68). Collectively, these data suggest a protracted magmatic evolution for the SMB with melt generation and assembly from ~390 to 370 Ma via melting of a metasomatized mantle source followed by contamination, first from the structurally underlying Avalonian terrane and later by metasedimentary wall rocks of the Meguma terrane. The most southwesterly part of the SMB (i.e., DLP) represents a petrogenetically distinct magmatic phase that underwent less overall contamination than the rest of the SMB.

The Porto Nacional Gold District, in the northern Tocantins Province, Brazil, is one of the few crustal-scale shear systems that contain both hypozonal and mesozonal orogenic gold deposits. It is situated within a Paleoproterozoic terrane, hosted by the crustal-scale Neoproterozoic Transbrasiliano-Kandi Shear System. The Cachimbo Shear Zone (CSZ) hypozonal deposits are hosted by phyllonites characterized by staurolite, garnet, and median to high-crystallinity graphite, consistent with amphibolite facies conditions. The mesozonal deposits, hosted in the Mutum Shear Zone (MSZ) and the Conceição Shear Zone (CoSZ), show a paragenesis that reflects greenschist-facies conditions. All zones host quartz fault-fill veins and adjacent extensional quartz-carbonate veinlet swarms. Two mineralizing stages are recognized. Stage 1 occurs in old quartz porphyroclasts in the fault-fill veins of the CSZ and MSZ deposits, and comprises pyrite, Type-2 graphite, gold, and other sulphides. In the CSZ stage 1, pressure-temperature data derived from mineral thermometry, quartz microstructure, and fluid inclusion isochores are compatible with a ductile deformation regime (433° to 580°C and 3.5 to 6.3 kb), equivalent to 16 to 24 km depths under lithostatic conditions. Conditions for MSZ stage 1 are 350° to 415°C and 2.0 to 3.1 kb, where the calculated depth (9 to 12 km) suggests the MSZ developed at shallower levels in the crust than the CSZ. In both shear zones the ore fluids are represented by H2O-CO2 fluid inclusions that are CH4-bearing and low salinity (0.3 to 6 wt. % NaCl), and have low ƒO2. Reactivation along the shear zones caused folding and boudinaging of the extensional veinlets and recrystallization by subgrain rotation in the vein quartz, suggesting temperatures of ∼ 400°C. Ore stage 2 (gold-galena) occurs in vug-fill quartz in the veins and in extensional veinlets of the MSZ and CoSZ. It is estimated to have formed at lower temperatures (340° and 390℃) and pressures (1.8 to 4.5 kb) than ore stage 1, at depths of 6 to 18 km. The fluids were H2O-CO2 rich, with salinities of 3 to 8 wt. % NaCl. The high and fluctuating pressures, above lithostatic conditions, suggest supralithostatic conditions. The wide fluid pressure variation is consistent with the fault-valve model for both ore stages. Later, higher salinity aqueous fluids (up to 15 wt. % NaCl) with no CO2 or CH4 circulated at 300℃ and at 1.0 to 1.7 kb, which is more compatible with hydrostatic conditions at depths of 4 to 7 km. The stable isotopic composition of the fluids (δ¹⁸OH2O = 2 to 12 ‰, δ¹³CCO2 = -4 to -8 ‰) and the local geological constraints indicate that the mineralizing fluids were metamorphic. The P-T characteristics of the system, which indicate a decrease in T and P from the CSZ to the MSZ and CoSZ suggests that the PNSZ developed concurrently in the greenschist-amphibolite window. The further development of footprints for the hypozonal-mesozonal orogenic gold deposits of the PNGD could help identify new target areas for exploration in Brazil and elsewhere in this Neoproterozoic crustal-scale shear system.

The Eastern Gabbro of the alkaline Coldwell Complex, Canada, represents a Ni-poor conduit-type system that comprises two rock series, the Layered Series and Marathon Series, which intruded into a metabasalt package. Based on distinct variations in magnetite compatible (e.g., Ni, Cr) and incompatible (e.g., Sn, Nb) elements in Fe–Ti oxide intergrowths, the metabasalts, Layered Series, and Marathon Series must have crystallized from magmas that originated from compositionally distinct sources. Of these rock units, the metabasalts crystallized from a more primitive melt than the Layered Series as Fe–Ti oxides in the former have higher concentrations of magnetite-compatible elements. Unlike the metabasalts and Layered Series, the Marathon Series crystallized from multiple, compositionally distinct magmas as Fe–Ti oxides in this series exhibit large variations in both magnetite compatible and incompatible elements. Accordingly, the various rock types of the Marathon Series cannot be related by fractional crystallization of a single batch of magma. Rather, the magmas from which the rock types crystallized had to have interacted to variable degrees with a late input of more primitive melt. The degree of this magma interaction was likely controlled by the geometry of the conduit and the location of emplacement given that Fe–Ti oxides in the oxide-rich rocks occur in pod-like bodies and exhibit no compositional evidence for magma mixing. Mirrored variations in magnetite compatible and incompatible elements in Fe–Ti oxides in the Footwall Zone, Main Zone, and W Horizon of the Marathon Cu–PGE deposit indicate that these zones could not have formed from a single, evolving magma, but rather multiple batches of compositionally distinct magmas. Fe–Ti oxides exhibit no compositional difference between those hosted by barren and mineralized rock. This is likely because sulfide liquated at depth in all of the magmas from which the Marathon Series crystallized. The composition of Fe–Ti oxides in the Eastern Gabbro fall outside of the compositional fields for Ni–Cu mineralization defined by Dupuis and Beaudoin (Mineral Deposita 46:319–335, 2011) and Ward et al. (J Geochem Explor 188:172–184, 2018) demonstrating that their discrimination diagrams can distinguish between Ni-rich and Ni-poor systems that contain disseminated and massive sulfides.

Magnetite (mag)–ilmenite (ilm) intergrowths are more common than mag–ulvöspinel (usp) intergrowths in mafic–ultramafic Ni–Cu–PGE systems, yet the former has no known solid solution. The most accepted model for the formation of mag–ilm intergrowths in terrestrial environments is fluid-induced oxidation of mag–usp assemblages by oxygen in water. In this study, we re-examine this model in light of the fact that crustal fluids have very low pO2 and that mag–ilm intergrowths commonly occur in rocks that show little or no evidence of hydrothermal alteration. We also characterize the chemical changes that occurred during the formation of mag–ilm intergrowths and how they affect the use of Fe–Ti oxide chemistry for petrogenesis and mineral exploration. In the Eastern Gabbro, Coldwell Complex, a continuum of Fe–Ti oxide intergrowths occur ranging from cloth (mag–usp) to trellis (mag–ilm) types. Trellis-textured intergrowths have higher bulk Fe3+:Fe2+ ratios and are predominantly enriched not only in some multivalent (Ge, Mo, W, Sn) elements, but also in Cu and Ga, consistent with their formation via oxidation by a metal-rich fluid. These compositional changes are significant relative to typical elemental abundances in Fe–Ti oxides and could potentially lead to erroneous interpretations regarding primary magmatic processes if they are not taken into consideration. The irregular distribution of the intergrowths throughout the Eastern Gabbro suggests that different rock series and mineralized zones experienced variable degrees of fluid-induced oxidation. It is proposed that C in CO2 rather than O2 in water could potentially be an important oxidizing agent in mafic systems: 9Fe2+2TiO4+0.75CO2+1.5H2O⇋9Fe2+TiO3+3Fe3+2Fe2+O4+0.75CH4 The applicability of this model is supported by the common occurrence of CO2 and CH4 in fluid inclusions in mafic rocks.