Cathodoluminescent quartz textures in transitional magmatic-hydrothermal systems

This contribution deals with cathodoluminescent textures in quartz from transitional magmatic-hydrothermal systems, such as those related to rare-metal granites and Cu-Mo porphyries, in order to better understand quartz crystallisation conditions and the evolution of silicate liquids and associated hydrothermal fluids. The early magmatic stage of these high level magmatic systems is recorded in quartz phenocrysts as growth textures with dominant blue cathodoluminescence (CL). These textures are due to variations in Ti content in quartz (40-250 µgg^-1) which reflect fluctuations in temperature due to stepwise magma ascent and temporary storage, and/or mixing and mingling of magmas. The CL contrast of growth textures decreases in groundmass quartz reflecting the late magmatic stage and ultimately disappears if the granitic magmas become saturated in water and the crystallisation temperature decreases. CL colour changes to red brown and Al increases from 100-200 µgg^-1 in early magmatic phenocrysts to 200-400 µgg^-1 in groundmass quartz.
The quartz textures of rare metal granites and porphyry systems advance in two different ways. Batches of rare metal granite magmas become highly fractionated and enriched in volatile/fluxing agents (H₂O, Cl, F, B). These interact with the solidifying magma producing snowball quartz textures and line rocks with comb quartz showing planar oscillatory growth zoning, red to orange CL and Al >>400 µgg^-1. In a few cases sector zoning is developed suggesting crystallisation from aqueous fluids. However, these quartz types contain inclusions of silicate glass (as well as vapour and liquid phases) indicating that silicate melt was present.
In porphyry systems most of the magmatic vapours and fluids escaped from the magmatic source forming stockworks of polygeneration quartz veins. During the initial hot stage (>600°C) high-quartz precipitates from magmatic, hypersaline liquids coexisting with a low-density vapour reacting with the country rock causing potassic alteration. This early barren stockwork quartz combines signatures of magmatic and hydrothermal quartz: comb crystals with oscillatory zoning, bluish CL, high Ti (80-250 µgg^-1) and moderate Al (100-400 µgg^-1). Polyphase silicate glass inclusions are common. Quartz overgrowths with red-brown CL and related to the sulphide mineralisation corroded the hot-stage quartz. This dull luminescent quartz has high and variable Al (>1000 µgg^-1), exhibits sector zoning and crystallised at <500°C from hydrothermal fluids.
The study reveals difficulties in ascribing a magmatic or hydrothermal origin for snowball and comb quartz in rare metal granites and hot-stage stockwork quartz in porphyry systems. This is due, in part, to the problem of defining the state of systems during the transitional stages of magmatic-hydrothermal evolution. Late-stage magmatic systems contain a variety of immiscible phases, silicate melts, hydrous saline melts, aqueous fluids, and CO₂-rich vapours.