Geology of the Souter Head subvolcanic complex, Aberdeenshire, Scotland: an Ordovician granite-related Mo–(Bi–As–Au) system

ABSTRACTAn Ordovician subvolcanic intrusive complex hosted by Neoproterozoic metasediments crops out at Souter Head about 6km S of Aberdeen, Scotland. The complex is composed mainly of two-mica red granite and breccia with minor dykes of pegmatite, quartz porphyry, felsite and dolerite, and widespread quartz veining, hydrothermal alteration and minor molybdenite mineralisation. Anomalous levels of bismuth (Bi), arsenic (As) and gold (Au) occur in quartz–pyrite veins. The complex has been mapped and the major- and minor-element geochemistry, including rare-earth elements of intrusives and mineralisation, has been determined. These data reveal a complex tectonic, intrusive and hydrothermal history. The intrusives are peraluminous and magnetite-, muscovite- and garnet-bearing. The youngest member, a quartz porphyry, is highly fractionated. There are two stages of hydrothermal activity: the first is linked to the explosive release of volatiles from a granite cupola and breccia formation; and the second, widespread quartz veining. Mo is associated with both stages, and Bi–As–Au anomalies are found in late quartz–pyrite veins. The mineralisation is classified as a granite-related vein-type Mo system. The unique preservation, in the Grampian terrane, of an Ordovician subvolcanic complex may be attributed to pre-Devonian movements on the nearby Dee fault and possibly also the collapse of the magma chamber following the explosive release of volatiles. The combination of large size, poor exposure and abundant multi-stage hydrothermal activity suggests that there is potential for further Mo and possibly Au mineralisation in this complex. Further mineralisation of this style may be present in the NE Grampian terrane.

Auriferous Quartz Veining Due to CO2 Content Variations and Decompressional Cooling, Revealed by Quartz Solubility, SEM-CL and Fluid Inclusion Analyses (The Linglong Goldfield, Jiaodong)

Quartz is the most common gangue mineral in hydrothermal veins. Coupled with capacities of hosting fluid inclusions and recording varieties of microtextures, its solubility behavior may provide unparalleled insights into hydrothermal processes. In this study, the Linglong goldfield in Jiaodong is targeted to investigate gold-producing quartz veining process. Scanning electron microscope (SEM)-cathodoluminescence (CL) imaging uncovered three episodes of quartz deposition, intervened by an episode of quartz dissolution. Based on newly-developed quartz solubility diagrams and CL-aided fluid inclusion microthermometry, it is proposed that precipitation of the earliest quartz (Qz1) was controlled by CO2 content increase and subordinately affected by decompressional cooling, leading to the formation of the early thick gold-barren veins (V1); the second generation of quartz (Qz2a) was formed by the same fluids that may have been diluted and cooled by meteoric water, leading to a greatly reduced quantity of quartz and the deposition of pyrite and gold; and the third generation of quartz (Qz2b) was deposited along with polymetallic sulfides, due to fluid cooling following a quartz dissolution event likely induced by cooling in retrograde solubility region and/or CO2 content decrease. This research may elucidate gold formation processes in orogenic intrusion—related deposits, and points to imperative CL-based in situ analyses for future studies.

The Miocene igneous rocks in the Basal Unit of Lavrion (SE Attica, Greece): petrology and geodynamic implications

The Miocene igneous rocks in the Basal Unit of the Lavrion area form part of the granitoid province of the central Aegean. Undeformed, subvertical dykes of quartz-syenite to granodiorite and granite porphyries, and a little deformed but variably altered granodiorite stock intrude metamorphic rocks of the Basal Unit. A 9.4 ± 0.3 Ma K–Ar age on feldspar for a dyke rock provides a minimum age for the igneous activity in the Basal Unit. East–west orientation of porphyry dykes is indicative of a regional extensional stress field with roughly north–south direction. Substantial extension in the Basal Unit after granodiorite emplacement is evident from widespread quartz veining associated with hydrothermal alteration of the granodiorite and the occurrence of mineralized tension gashes cutting the hydrothermally altered hornfelses. Final emplacement of the Blueschist Unit over the Basal Unit by extensional detachment post-dates contact metamorphism of the rocks surrounding the granodiorite. Geochemical diagrams show a continuous range of compositions from the dykes to the granodiorite. Radiogenic isotope compositions are compatible with a common magmatic source for the two lithologies. Elemental variations, as well as the considerable geochemical similarity of the dyke rocks to the Hercynian paragneiss of the central Cyclades, indicate that crustal melts were significant components during the evolution of the igneous rocks with fractional crystallization as an important process during later stages of evolution. The granodiorite displays geochemical signatures indicative of a significant mafic mantle-derived magma component.

40Ar/39Ar dating of ribbon-textured veins and wall-rock material from Meguma lode gold deposits, Nova Scotia: implications for timing and duration of vein formation in slate-belt hosted vein gold deposits

Results of 15 40Ar/39Ar age spectra for whole-rock argillite samples collected from within and adjacent to veins from eight Meguma gold districts in the Meguma Terrane of southern Nova Scotia are presented. The samples give excellent plateau ages (i.e., concordance of plateaux, correlation, and integrated ages) that range from ca. 379 to ca. 403 Ma, and there is no apparent correlation of age with bulk rock composition (K2O, Al2O3) or sampling localities. In addition, apparent ages for samples from a district are similar within analytical error, except for one deposit, and there is no difference for samples from within ribbon-textured veins versus samples collected outside of the gold districts (i.e., in areas free of quartz veining). The results of this work compare well to previous whole-rock 40Ar/39Ar dating of Meguma Group samples and we concur with previous workers that the range in dates (i.e., 380-410 Ma) reflects diachronous cooling of the area through the intracrystalline retention temperature for argon in mica (i.e., approx. 300-350°C). However, with respect to the vein samples, there is a marked difference between the 40Ar/39Ar ages of vein-hosted whole-rock samples and hydrothermal minerals (amphibole, muscovite, biotite) from the same deposits previously dated, which indicates that whole-rock samples have retained their metamorphic ages and have not been reset by the later hydrothermal event responsible for vein formation, despite being incorporated within the high-temperature fluids (approx. 400-450°C). This discrepancy in ages indicates that the wall rocks and veins were in thermal disequilibrium, as the vein temperatures were well above that required to cause diffusion of argon out of mica phases within the whole-rock samples, and implies therefore that the fluids must have been derived from depth. The results have the following important implications for models of vein formation: (i) vein formation was rapid and is consistent with models of hydrofracturing due to fluid overpressure, and (ii) the vein-forming fluids were derived from depth and cannot have been produced by a lateral secretion processes whereby fluids and gold are derived from the Meguma Group.

Bismuth sulphosalts within quartz veining hosted by the Loch Shin monzogranite, Scotland

The Loch Shin monzogranite is host to quartz veins bearing the sulphosalts aikinite, hammarite, lindströmite, krupkaite, gladite and pekoite, which belong to the aikinite-bismuthinite series, and represents the first significant occurrence of this series in the United Kingdom. Inclusions of the sulphotelluride tetradymite occur in krupkaite-gladite. Berryite is present as inclusions in chalcopyrite. Electron microprobe analyses reveal a range of compositions in individual crystal masses from hammarite to krupkaite in one sample, and from krupkaite to gladite in a second. Compositions between friedrichite and hammarite and gladite and pekoite are notably absent.

Sinistral transpression and hydrothermal activity during emplacement of the Early Proterozoic Julianehåb batholith, Ketilidian orogenic belt, South Greenland

The first systematic investigations of the central part of the Early Proterozoic Ketilidian orogen in the vicinity of Søndre Sermilik in the early 1960s suggested that this part of the orogen comprised a mixture of the Julianehåb granite, altered supracrustal rocks and older orthogneisses. Recent field work has shown that the area consists only of a variably deformed suite of granitic to dioritic plutonic rocks and a range of hornblende-bearing dykes of the appinite suite which all belong to the Julianehåb batholith. Steep to vertical shear zones with widths from a few centimetres to more than one kilometre are a significant element of the structure. The principal shear zones trend north-east and they are parallel to the schistosity and subhorizontal linear structures in the granitoid rocks. Kinematic indicators in many of the shear zones indicate sinistral transcurrent displacements. The relationships between granite fabrics, shear zones and mafic dykes suggest that the Julianehåb batholith was emplaced during subduction from the south towards the Archaean craton in the north-west in a sinistral transpressional system. Effects of hydrothermal alteration, mainly in the form of quartz veining, silicification, chloritisation, epidotisation and pyritisation, are common within and adjacent to the largest shear zones. These effects are believed to be related to late stages of the evolution of the batholith. Gold anomalies appear to be closely tied to the hydrothermal phenomena.