Constraints on the genesis of the mineralization of the Alston Block, Northern Pennine Orefield, northern England

Abstract

SUMMARY New evidence from fluid inclusion compositions and simple physical modelling provide important constraints on the genesis of the ores of the Alston Block in northern England. Evidence from fluid inclusions shows that probably at least four, hydrothermal fluids were involved, two of which are formed by evaporation of seawater, in some cases well beyond halite saturation. Some of the high-temperature fluid dissolved evaporites during its evolution. Physical modelling shows that neither compactional dewatering of the adjacent Carboniferous sedimentary basins nor compressional tilting and throughflow within the Northumberland Trough could have contributed significantly to the hydrothermal fluids of the orefield. Most of the high temperature hydrothermal flow must have been generated by penetrative circulation of surface brines to 8 to 10 km depth in the basement, with the upflow focused and accelerated by the high heat production of the Weardale Granite that unconformably underlies the Carboniferous rocks of the Alston Block. The orefield is considered to have formed during the Upper Permian by the following sequence of events. Initially, fluid movement in the basement was prevented by sealing of pre-existing cracks, and temperatures rose at depth. An episode of extension allowed saline water, probably from the Upper Permian Zechstein Sea, to penetrate deep into the basement, reacting with the rocks there, and becoming heated to as much as 200°C. Penetration to 10 km took a few tens of thousands of years. The granite, at 300°C at that depth, up to 100°C hotter than the surrounding basement, acted to focus the flow, and to generate concentric zones of increasing temperature fluid towards the centre. Once the basement had become cooled, the flow slowed, cracks became sealed, and the episode of hydrothermal circulation and ore formation was complete.