Genesis of porphyry and plutonic mineralisation systems in metaluminous granitoids of the Grampian Terrane, Scotland
Authors: Lowry David , Boyce Adrian J. , Fallick Anthony E. , Stephens W. Edryd 
Published: November 2011

Mineralisation associated with Late Caledonian metaluminous granitoids in the Grampian Terrane has been investigated using stable isotope, fluid inclusion and mineralogical techniques.

A porphyry-stock-related style of mineralisation in the Grampian Terrane is characterised by a stockwork of veinlets and disseminations in dacite prophyries, consisting of quartz, dolomite, sulphides and late calcite, and well-developed wallrock alteration dominated by zones of phyllic, sericitic and propylitic alteration. On the basis of δ34S (+0·4±l·0‰), δ13C (−5·7‰ to + l·4‰) and δ18O (+10·8‰ to +19·9‰) it is likely that initial mineralising components were orthomagmatic with an input of external fluids during the later parageneses. Fluids were saline, boiling (up to 560°C), deficient in CO2, and ore deposition took place at depths of less than 3 km.

Plutonic-hosted mineralisation in appinites, diorites, tonalites and monzogranites is commonly represented by sporadic disseminations and occasional veins consisting of quartz, calcite and sulphides. Wallrock alteration is generally propylitic with phyllic vein selvages. Deposition from a cooling magma sourced fluid is indicated by δ34S (+2·6±l·5‰), δ13C (−7·2‰ to −4·5‰) and δ18O (+9·5‰ to + ll·8‰) data. Fluids were CO2-rich and of low salinity; inclusions were trapped below ≈460°C, and formed at estimated depths of 3–5 km.

Differences between these styles of mineralisation may due to multiple factors, the most important being the nature of the fluid: porphyry systems are dominated by greater volumes and much higher temperatures of hydrothermal fluids. Other controlling factors are likely to be the compositional characteristics of the melt source region, the mechanism of magma ascent, the level of emplacement, and the nature of the host metasediments. Variations in δ34S between the two groups are related, for the most part, to redox processes during magma and fluid genesis and not by crustal contamination.

Nolarge porphyry-related mineral deposits have been found in the Grampian Terrane, unlike those in Mesozoic and Tertiary continental margin environments. This is largely due to a combination of detrimental factors which massively reducesthe probability of economic mineralisation. These include the already metamorphosed nature of the host Dalradian, the absence of seawater (which entered many subduction-related magmatic systems), a poorly-developed system of deep faults (most deposits too deep to be influenced by surface-derived fluids), and the absence of supergene enrichment. The main processes which aid the concentration of mineralisation involve encroachment of external fluids (formation, meteoric and seawaters) into the magmatic system, but these fluids were largely absent from the Grampian host block at the time of granitoid intrusion.

The results of this study can be used in characterising the sources of fluids in sedimentary-hosted ore veins known (or considered) to be underlain by metaluminous granitoid batholiths, particularly in estimating the degree of magmatic fluid inputs into the vein systems: an example where this interaction has occurred (the Tyndrum Fault Zone) is discussed.