Hydrothermal orefields and ore fluids

Abstract

AbstractThe development of Hydrothermal Theories of Ore Genesis during the past 150 years owes much to the pioneering work of two eminent British geologists, H. C. Sorby and K. C. Dunham, who published benchmark papers in the Quarterly Journal of the Geological Society on the classic mining districts of Cornwall and the English Pennines. Sorby’s paper of 1858 laid the foundation for fluid inclusion studies. Despite considerable scepticism or indifference that has lasted well into the twentieth century, fluid inclusions are now widely regarded as the best way of establishing the nature and composition of ancient mineral-forming fluids. Dunham’s paper of 1934 provides a classic account of mineral zonation away from a focus of mineralization in the North Pennine orefield. By analogy with the Cornish deposits this focus was considered to be the result of ascending mineralizing fluids from a hidden granite at depth. Although subsequent drilling confirmed the existence of granite (the Weardale granite) directly beneath the North Pennine Orefield it was older than the Carboniferous Limestone which hosts the mineralization and therefore cannot be its direct source.With the ‘coming of age’ of fluid inclusion techniques in the 1960s, widespread and systematic studies were carried out on samples from the Cornubian and Pennine orefields and other classic mining districts of the world over the succeeding three decades. These, together with stable isotope studies, established that hydrothermal, ore-forming fluids cover a wide temperature range (50° to >500°C) and compositional range (0 to >60 wt % dissolved salts), and are of diverse origin. The composite and protracted nature of hydrothermal events in Cornwall, and indeed in areas of the world where mineralization is spatially associated with granites, is now evident. Most authors agree that, whilst components of the early mineralization may be due to metalliferous fluids directly evolved from cooling granite bodies, much of the later base-metal mineralization is due to thermal convection and pulsation of meteoric fluids and basinal brines within the intrusion and surrounding country rocks.The basinal brine expulsion theory is generally favoured for carbonate-hosted, epigenetic, base metal deposits of the Mississippi valley type. The nature, geological setting and fluid inclusion characteristics place the Pennine ores clearly in this broad class. However, most recent work suggests that the mineral zonation patterns in the northern part and temperature differences between the northern and southern part are still best explained by the hydrogeological and geochemical influence of granite at depth.