Hydrothermal Processes

Abstract

This Geochemical Perspectives follows along my path through the evolution and development of hydrothermal concepts as they became established during the latter half of the twentieth century, from conjecture through established precept. A powerful stimulus for the developments was a keen scientific interest in the genesis of hydrothermal ore deposits shared commonly by my colleagues, both geochemists and economic geologists. Underlying that interest was our optimistic belief that a fundamental understanding of ore-forming processes should have practical applications for mineral exploration.
Our early investigations of ore genesis were impeded, however, by a lack of adequate data on the chemistry of aqueous solutions at the high temperatures and pressures of hydrothermal processes. Even the prime parameters used typically by physical chemists to quantify acidity and oxidation state of solutions at elevated conditions were awkward. Instead, we soon adopted both the ionisation constants of nature’s common brine solutes in order to evaluate acidity, as well as oxygen fugacity index the oxidation state of hydrothermal fluids. Furthermore, we found that information on the high P-T volumetric and thermodynamic behaviour of volatiles like CO₂, CH₄, H₂S, SO₂, and especially H₂O were incomplete for applications which ideally extend to about 1,000 ºC and 10,000 bars. This resulted in extensive experimental investigations internationally by geochemical laboratories that painstakingly provided much of the fundamental new data still currently used.
That crucial, advanced chemical information could be applied effectively only if the physical conditions (P, T) of the environments where ores had formed were more accurately identified. To better resolve these temperatures and pressures, a series of geothermometers and geobarometers were invented and tested on various settings. Two of these, the quartz geothermometer and the sphalerite (ZnS) geobarometer, are special in that they are flexibly adaptable to a variety of mineral deposits. Their attributes favour relatively easy application so I have selected them for a more thorough examination. Another family of highly useful geothermometers are dependent upon the unique characteristics of the ubiquitous iron sulphides. There are more than seven of these sulphides and most ore deposits contain more than one of them. These sulphides are especially important as precise indicators of the environmental conditions where they crystallised, not only due to their occurrence within a deposit but also by differences in their solid solution composition and in morphology. We found that variations among the characteristics of the iron sulphides were largest precisely within the range of conditions of chemistry, temperature, and pressure where ores crystallise, attributes which made these indicators extremely valuable.
The nature of hydrothermal processes found from the rock record through mineralogical evidence on ore deposits was naturally supplemented by data from investigations of modern and active geothermal systems. Our initial consideration was to ascertain whether geothermal processes and hydrothermal ore genesis were related phenomena. In fact, it turned out that their ranges of temperatures and pressures and their deduced lifetimes were identical. Therefore, the measured solution flow rates and compositions of modern geothermal systems provided excellent baselines for modelling what must have transpired in ore-forming environments, which are simply ancient geothermal sites.

Having established the physical conditions and processes that characterise hydrothermal environments, our next problem was to determine the means of aqueous transport and how fluids could have caused ore-grade mineral deposition. It was clear that both transport and precipitation depended on the solubilities of ores in reducing fluids at near neutral acidity. In addition, the concentrations of the metals are controlled in such solutions by complexing (bonding) mostly with halide or sulphur-rich ligands. Yet, the stoichiometries (composition ratios) of each of the dissolved metals with these (and other) common ligands had to first be measured. Thus was done by generations of projects around the world and the research, has evolved as experimental and theoretical techniques have continued to provide increased resolution. These techniques and new data sets led to more accurate and precise thermodynamic descriptions of the complexes, which have been used to calculate, and then plot, variations in solubility as a function of oxidation state (log a_O2) and of pH at pertinent temperatures and pressures. Potential causes of ore deposition are implicit in such diagrams.

The maturity of our understanding of hydrothermal processes could be tested most readily against criteria from the lowest temperature and pressure ore deposits, the Mississippi Valley type zinc-lead ores. The extensive database for those in Illinois, Iowa and Wisconsin made that district ideal as a model. There, the ore-depositing solutions arrived about 270 million years ago from their source in the contemporaneously uplifted southern Appalachian Mountains. They flowed northward through the Illinois Basin becoming heated, highly saline, with a reduced oxidation state, slightly alkaline to neutral acidity, and with solute concentrations apparently especially rich in iron, barium, zinc, and lead. It was effectively a hot groundwater that flowed at about 11 metres per year from its source for 1100 km to the district to precipitate sulphides within an horizontal 35 metre depth range about 1 km deep. The mineral paragenetic sequence revealed that slight oxidation was sufficient to cause sulphide deposition dominantly at 125 ± 25 ºC and for sphalerite (ZnS) at a yearly rate of coating of about 0.2 μm. Four independent methods agreed that the process continued for about 0.25 million years. However, the aqueous complexes have not yet been identified for certain. Those metal-carrying aqueous species must have provided sufficient solubility to form deposits over an area of 10,000 square kilometres. Because the very regular banding in sphalerite could be correlated over very large area implies that there must have been continuing, recycling climate control of that ore-forming hydrothermal system. This observation is intriguing and still unresolved and calls for additional studies and a mechanistic explanation.

The development of this field is naturally as you will see closely intertwined with me and my colleagues’ research over the last 7 decades and what a journey it was. I hope this Geochemical Perspectives gives you, the reader, an insight into one such journey. Mine. Enjoy!