Howard Street Robinson Lecture Tour

The Howard Street Robinson Medal recognizes a respected and well-spoken geoscientist who will further the scientific study of Precambrian Geology and/or Metal Mining through a presentation of a distinguished lecture across Canada. The medal is named in honour of Howard Street Robinson, a founding member of the GAC whose bequest to GAC in 1977 of approximately $100,000 makes the lecture tour possible. The bequest was ”for the furtherance of scientific study of Precambrian Geology and Metal Mining”. Thus the GAC’s Mineral Deposits Division awards the medal in odd years, and the Precambrian Division awards it in even years. The lecturer is nominated and chosen by the Howard Street Robinson Medal Committee.

The 2025-2026 In-Person Lecture Tours are underway with Dr.Anthony (Willy) Williams-Jones !

Topic 1: ‘The Origin of Carbonatite Complexes and their Critical metals’

Abstract: Features of carbonatite complexes that need explaining are why they commonly host ultramafic rocks, including dunites, pyroxenites and glimmerites, why these rocks are intruded by ijolites and, in turn, by nepheline syenites, and why the carbonatites show a close spatial association with phoscorites. We also need to explain why these complexes are unusually enriched in the critical commodities, niobium, the REE and phosphate. The widely accepted explanation for the different rock-types is that they are the result of the fractionation of carbonated olivine nephelinite magmas that crystallise cumulates of olivine and clinopyroxene to form dunites and clinopyroxenites, and evolve through ijolitic and nepheline syenitic compositions to produce carbonatitic magmas. Experimental data, however, show that such fractionation to carbonate liquids is only possible at pressures > 3 GPa, i.e., that carbonatites cannot form by fractional crystallisation in the crust. We propose a unifying theory that we believe explains the various rock types present in carbonatite complexes and also the economic concentrations of niobium, the REE and phosphate in these complexes. According to this theory, the ultramafic rocks are the products of metasomatic reactions between the carbonatitic magmas and the feldspathic gneisses that host the complexes. These reactions consume magma, leaving behind a residue enriched in incompatible components, which crystallises to form banded carbonatite (alternating carbonate and silicate-apatite-magnetite-pyrochlore layers) and/or phoscorite, concentrating niobium to economic levels. These processes are accompanied and succeeded by the release of aqueous-carbonic fluids that fenitise the gneisses. The REE partition preferentially into these fluids to form economic concentrations of REE in the carbonatites and fenites. In addition to carbonatite-induced metasomatism, interaction of the carbonatitic magmas with the host rocks causes them to melt. The melting is restricted initially to the proximal, fenitised gneisses, generating alkaline silicate magmas that intrude the ultramafic rocks, forming ijolites, but with time the melting extends into the unaltered gneisses, to produce magmas of nepheline syenite composition that intrude the earlier-formed ultramafic rocks and ijolites. This unified theory satisfactorily explains the origin of the various rock types that make up carbonatite complexes and the processes that concentrate their critical commodities to economic levels.

Topic 2: ‘Metals, Vapours and Volcanoes’

Abstract: Until recently, conventional wisdom has held that the main agent of metal transport in hydrothermal systems is an aqueous liquid. However, there is increasing evidence from volcanic vapours, geothermal systems (continental and submarine), vapour-rich fluid inclusions, and experimental studies that the vapour may be an important and even dominant ore fluid in some hydrothermal systems, notably those that form porphyry and epithermal deposits. This is not a new idea. Indeed, as early as 1556, Georgius Agricola postulated that metal-bearing fumes were drawn up from the depths of the Earth to form ore-bearing veins. Support for the idea was provided by Sir Humphrey Davy, a British chemist and inventor who collected sublimates from Vesuvius during its 1820 eruption and showed them to contain cobalt chloride and copper sulphate. Further support came from the 1840 experiments of Auguste Daubrée, a French geologist and geochemist, who synthesised cassiterite, the main tin ore mineral, by reacting gaseous tin chloride (SnCl₄) with steam. The tide changed when the father of modern economic geology and Director of the US Geological Survey, Waldemar Lindgren, declared “In ore deposits, there are vast quantities of non-volatile material, particularly silica, which can hardly be transported by vapour”. The idea that metals could be transported by aqueous vapour was dead! Here we present evidence for the transport of metals by vapor (an aqueous fluid of any composition with a density lower than its critical density), clarify some of the thermodynamic controls that may make such transport possible, and propose a model for the formation of porphyry and epithermal deposits that involves precipitation of the ores from vapour. Analyses of vapour (generally >90% water) released from volcanic fumaroles at temperatures from 500° to over 900°C and near-atmospheric pressure typically yield concentrations of ore metals in the parts per billion to parts per million range. These vapors also commonly deposit appreciable quantities of ore minerals as sublimates. Much higher metal concentrations (ppm to wt.%) are observed in vapour inclusions trapped at pressures of 200 to 1,000 bars in deeper veins at lower temperature (400°–650°C). Experiments designed to determine the concentration of Cu, Mo, Ag, and Au in HCl-bearing water vapor at variable although relatively low pressures (up to 180 bars) help explain this difference. These experiments show that metal solubility is orders of magnitude higher than predicted by volatility data for water-free systems. They also show that, in the case of Cu and Au, the solubility increases sharply with increasing water fugacity and fugacity of HCl. Thermodynamic analysis demonstrates that this high metal solubility is due to the reaction of the metal with HCl (Cu, Ag and Au) and hydration of the resulting chloride complex, leading to the formation of species such as MeClm.nH₂O, or in the case of Mo, species such as MoO₃.nH₂O. Indeed, the solubility of Cu, Mo and Au, in vapour at 400 to 600°C and pressures corresponding to depths of 1km, can reach concentrations of several thousands of ppm, tens of ppm and a few ppm, respectively. In shallowly emplaced porphyry-epithermal systems, fluid inclusions provide evidence of the exsolution from the magma of a supercritical fluid of vapour-like density. On cooling and decompression, this fluid condenses a small fraction of brine by intersecting the two-phase surface on the vapour side of the critical curve, without significantly changing the composition of the expanding vapor. Vapour and brine reach Cu-Fe sulfide or MoS₂ saturation as both phases cool below 425°C. Vapour, which is the dominant fluid in terms of the total mass of H₂O, is interpreted to be the main agent of metal transport. Evolution of the system to lower temperature and pressure leads to a vapour that is enriched in H₂S, SO₂, Au, and variably enriched in Cu and As, which produces high sulphidation epithermal gold mineralisation or condenses in ground water to produce low sulphidation epithermal deposits.