The origins of late-stage rocks in the St Austell granite—a re-interpretation

Abstract

The origin of the granite varieties present in the St Austell granite body (biotite granite, megacrystic and non-megacrystic lithium-mica granite, fluorite granite) is discussed in the light of new petrographic, chemical and experimental evidence. There were essentially two stages of magmatic activity—the first represented by the intrusion of several phases of biotite granite, the second by a later intrusion of non-megacrystic lithium-mica granite which crystallized from a volatile-rich residual differentiate of the biotite granite magma. Both intrusions suffered hydrothermal alteration which primarily involved at least two stages of exchange of components via a single aqueous fluid phase. Within the non-megacrystic lithium-mica granite, topaz and lithium-mica were among the first phases to break down, releasing F and Li to this aqueous phase. F was locally fixed as fluorite, removing Ca (derived from the host biotite granite) from the fluid and giving rise to pockets of fluorite granite. Apart from losing Ca from plagioclase, the biotite granite gained Li, expressed as lithium-rich mica, resulting in an aureole of megacrystic lithium-mica granite surrounding the second intrusion. At least one pervasive hydrothermal system, using joints and grain boundaries as fluid pathways, may have been in existence. Water-rock interaction during fluid circulation is believed to have modified considerably the composition of predominantly meteoric water to give rise to the hydrothermal fluid responsible for greisening and mineralization within its sphere of influence. The alteration phenomena initiated by the proposed hydrothermal system may have culminated in the formation of the major kaolinite deposits currently worked in the area.