Regional fluid and metal mobility in the Dalradian metamorphic belt, Southern Grampian Highlands, Scotland

Abstract

Abstract.A prominent set of veins was formed during post-metamorphic deformation of the Caledonian Dalradian metamorphic belt. These veins are concentrated in dilational zones in fold hinges, but apophyses follow schistosity and fold axial surface fractures. The veins are most common in the cores of regional structures, especially the Dalradian Downbend and consist of quartz, calcite, chlorite and metallic sulphides and oxides. Metals, including gold, have been concentrated in the veins. The fluid which formed the veins was low salinity (1 -5 wt% NaC1 and KCI) COg-bearing (3-16 wt% CO2) water of metamorphic origin. The fluid varies slightly in composition within and between samples, but is essentially uniform in composition over several hundred km 2. Vein formation occurred at about 350+50~ and 200-300 MPa pressure. Further quartz mineralization occurred in some dilational zones at lower temperatures (160-180 ~ This later mineralization was accompanied by CO2 immiscibility. Dilution and oxidation of the metamorphic fluid occurred due to mixing with meteoric water as the rocks passed through the brittle-ductile transition. A similar metamorphic fluid is thought to have been responsible for gold mineralization in the nearby Tyndrum Fault at a later stage in the Dalradian uplift.Late Caledonian, post-metamorphic, metalliferous veins commonly occur in the Caledonian Dalradian metamorphic belt. Many of these veins are clearly associated with magmatic processes (Plant et al. 1983;Alderton 1985), but for others, the origin of the mineralizing fluid is not so clear (Pattrick et al. 1988). It is the purpose of this paper to show that metamorphic fluids formed an important part of the hydrothermal fluid flux through the Dalradian rocks in late Caledonian times, that these fluids carried metals, and that at least some of the post-meta-* Present address: Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand morphic metalliferous deposits, particularly Au-bearing ones, may have been formed from such fluids.Fluid movement during uplift of a metamorphic belt is a well-established phenomenon. Fluids have been investigated in active mountain belts (Pecher 1979;Craw 1987;Holm et al. 1989), and in more deeply eroded belts (Hollister and Burruss 1976;Crawford et al. 1979), principally by fluid inclusion studies of veins. In most such studies, vein formation occurred throughout the uplift history of the metamorphic belt, representing long-continued fluid mobility. Thus numerous veins and generations of veins cross-cut earlier veins in a complex pattern of fluid pathways. Metallic mineralization commonly accompanies one or more of these vein generations (Kerrich and Fyfe 1981;McKeag and Craw 1989).In contrast to the above situation, portions of the Dalradian metamorphic belt of Scotland show only minor vein formation in the early and late stages of uplift. Instead, vein formation is concentrated into a relatively narrow part of the uplift history. This vein generation coincides with the formation of a set of struc...

The origin and significance of non-aqueous CO2 fluid inclusions in the auriferous veins of Bin Yauri, northwestern Nigeria

“…Although an H20 phase is not visible there is a general agreement that such inclusions can contain up to 15 vol.% H20 which can remain unobservable, because the H20 tends to adhere to the inclusion walls as a thin film (Kreulen 1980, Oslen 1988, Hollister 1988, Craw 1990). Since there is a concensus of opinion that fluids responsible for gold mineralization are low-salinity H20+CO2 (Phillips and Groves 1983, Cameron 1988, Goldfarb et al 1988, Kerrich 1989, Naden and Shepherd 1989, Bowers 1991, it would be reasonable therefore, to assume that the original fluids from which the Bin Yauri gold mineralization precipitated were CO2-H20 fluids.…”

“…Metamorphic devolatilization and dehydration of subducted crustal materials along collisional suture zones have been proposed as an important control on the origin of mesothermal gold deposits irrespective of their age (Goldfarb et al 1988, Kerrich 1989. In some instances widespread oxidation of metamorphic fluids during gold-vein formation have been interpreted to be due to mixing by downward-percolating meteoric water (Nesbitt et al 1986, Craw 1990, Johnstone et al 1990), but Goldfarb et al (1988) have argued against the significant role of meteoric water in the genesis of mesothermal gold deposits in North American Cordillera as earlier suggested by Nesbitt et al (1986). As already pointed out, meteoric water has probably contributed to the formation of late barren quartz veins in Bin Yauri (characterized by low TnHzO fluid inclusions).…”

Meteoric incursion and oxygen fronts in the Dalradian metamorphic belt, southwest Scotland: a new hypothesis for regional gold mobility

“…The rocks contain greenschist facies assemblages dominated by quartz, albite, muscovite and chlorite, with minor biotite in some rocks. Accessory minerals are pyrrhotite or pyrite, graphite, and ilmenite, An earlier study (Craw 1990) examined fluid inclusions in postmetamorphic veins in these host rocks to document regional fluid composition during the early stages of Dalradian uplift. The present study extends this work to later stages of Dalradian uplift.…”

“…Inset (lower right) shows the location of the study area (open square) in Scotland folds ( Fig. 1) were accompanied by formation of veins in fold hinges on the scale of decimetres of 10"s of metres (Craw 1990). The veins are irregular in shape, have locally diffuse margins, and lie both parallel and across the foliation (Fig.…”

“…1) and gold genesis is still controversial (Pattrick et al 1988;Craw 1990;Curtis et al 1993). Late state meteoric incursion into the Dalradian belt has been documented previously from studies of post-metamorphic mineralogy, stable isotopes, and fluid inclusions (Craw 1990;Jenkin et al 1992;Curtis et al 1993). The present paper describes the regional extent of meteoric involvement in late stage vein formation, and examines possible relevance of this meteoric incursion to gold mineralization.…”