GENESIS OF PORPHYRY AND PLUTONIC MINERALIZATION SYSTEMS IN METALUMINOUS GRANITOIDS OF THE GRAMPIAN TERRANE, SCOTLAND

Mineralisation associated with Late Caledonian metaluminous granitoids in the Grampian Terrane has been investigated using stable isotope, f luid 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, consist ing of quartz, dolomite, sulphides and late calcite, and well-develope d wallrock alteration dominated by zones of phyllic, sericitic and pro pylitic alteration. On the basis of delta(34)S (+0.4+/-1.0 parts per t housand), delta(13)C (-5.7 parts per thousand to +1.4 parts per thousa nd) and delta(18)O (+10.8 parts per thousand to +19.9 parts per thousa nd) it is likely that initial mineralising components were orthomagmat ic with an input of external fluids during the later parageneses. Flui ds were saline, boiling (up to 560 degrees C), deficient in CO2, and o re 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 c onsisting of quartz, calcite and sulphides. Wallrock alteration is gen erally propylitic with phyllic vein selvages. Deposition from a coolin g magma sourced fluid is indicated by delta(34)S (+2.6+/-1.5 parts per thousand), delta(13)C (-7.2 parts per thousand to -4.5 parts per thou sand) and delta(18)O (+9.5 parts per thousand to +11.8 parts per thous and) data. Fluids were CO2-rich and of low salinity; inclusions were t rapped below approximate to 460 degrees C, and formed at estimated dep ths of 3-5 km. Differences between these styles of mineralisation may due to multiple factors, the most important being the nature of the fl uid: porphyry systems are dominated by greater volumes and much higher temperatures of hydrothermal fluids. Other controlling factors are li kely to be the compositional characteristics of the melt source region , the mechanism of magma ascent, the level of emplacement, and the nat ure of the host metasediments. Variations in delta(34)S between the tw o groups are related, for the most part, to redox processes during mag ma and fluid genesis and not by crustal contamination. No large porphy ry-related mineral deposits have been found in the Grampian Terrane, u nlike those in Mesozoic and Tertiary continental margin environments. This is largely due to a combination of detrimental factors which mass ively reduces the probability of economic mineralisation. These includ e 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 e nrichment. The main processes which aid the concentration of mineralis ation involve encroachment of external fluids (formation, meteoric and seawaters) into the magmatic system, but these fluids were largely ab sent 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 un derlain by metaluminous granitoid batholiths, particularly in estimati ng the degree of magmatic fluid inputs into the vein systems: an examp le where this interaction has occurred (the Tyndrum Fault Zone) is dis cussed.