From micro to mega: fluid inclusion data sets influence large-scale genetic models of orogenic gold deposits

More than three decades of fluid inclusion studies of orogenic deposits have provided geologists with the knowledge of the fundamental physical-chemical properties of Au-depositing hydrothermal fluids within the Earth’s crust. To date, it is common knowledge that these ore fluids are aquo-carbonic in nature (CO2 = 5-15 mol%), with salinities that typically do not exceed 10 wt% NaCleq and Thtot in the 300-400 °C range. It is also common knowledge that these properties are documented with surprising consistency in Archean terranes of Canada, Africa, Australia, and India, and are not much different from those of much younger orogenic belts (e.g., Lachlan fold belt of Central Victoria, Australia). This consistent occurrence of similar fluid properties in distinct orogenic belts has represented an important argument for a unique genetic process of orogenic deposits, valid across time and space. This model would consider a stage of fluid production at the root zones of orogenic belts (source region), and a stage of fluid focusing and ore genesis typically within quartz-filled, brittle-ductile faults (veins) at shallow structural levels. \nBeing conceptually simple and appealing, this model has generated a lot of consensus in the scientific community; however, it also generated a vigorous debate on the actual identity of the source fluid, as virtually all the imaginable genetic processes at the source region were proposed in the last years, i.e. mantle vs. magmatic vs. heating of a deeply-driven meteoric fluid. This spectrum of models is based on stable isotope data, petrologic and timing arguments, and field associations, and keeps the fluid inclusion data at the background of data interpretation and discussion. In this talk, I will show how the fluid inclusion data collected during the last decades of research have progressively lost their initial influence in the construction of genetic models of orogenic deposits. A detailed survey of these data shows that past studies (1) do not identify unequivocal petrographic relationships between the entrapment of ore fluid within vein minerals and Au precipitation, (2) are based on poor quantitative constraints of the phase proportions in the fluid (i.e., the liquid/vapour ratio), and (3) do provide a quantitative estimation of the concentration of Au and other major and trace fluid components (i.e., Na, K, Ca, B, W, As, Sb, and Ag) in the ore fluid. Thus, the previous database should be re-considered and amended where needed. \nThe data I will present show how the systematic application of an up-to-date set of fluid inclusion analytical techniques in a limited number of well-known Au deposits of different categories (i.e., not only orogenic in the classical sense) amends and completes the previous knowledge on Au-transporting hydrothermal fluids, providing a more effective background for comprehensive genetic models of Au deposits.

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