D. H. M. Alderton

Rare earth element mobility during granite alteration: Evidence from southwest England

Geochemical analyses of granitic rocks from southwest England reveal that the rare earth elements (REE) were potentially mobile during hydrothermal and supergene alteration. In particular, trivalent REE were removed from the system during K-silicate alteration, Eu was lost during sericitic alteration; all REE were lost during tourmalinization, and light REE were lost during chloritization and argillic alteration. The fluids themselves had low concentrations of REE; in only one case (chloritization) were heavy REE introduced during alteration. Analysis of separated minerals indicated that the behaviour of the REE could be partly explained in terms of their different affinities for the primary and secondary assemblages. Thermodynamic calculations indicated that REE mobility is enhanced by the presence of fluorine in the alteration fluids partly because REE form more stable complexes with F than with Cl and partly because elements such as Ti, Zr and P that form REE-bearing minor phases are themselves potentially mobile.

The character and evolution of hydrothermal fluids associated with the kaolinized St. Austell granite, SW England

A complex assemblage of fluid inclusions is present in quartz from the St. Austell granite. Homogenization temperatures range from below 100°C to above 570°C, and salinities vary from <l to 42 wt % NaCl equivalents. On a regional scale the fluid inclusion populations show marked variations in their abundance and distribution of types. These variations can be correlated with the petrology of the host rock. Low-temperature, low-salinity fluids are most abundant in extensively-kaolinized granite. These represent the latest phase of fluid activity in the St. Austell granite and were probably responsible for the formation of the extensive ‘china clay’ deposits in the region, at temperatures ranging from about 150°C to below 70°C. The most saline fluids (with halite as a daughter mineral) are restricted to the lithionite and fluorite granites. They could either represent the earliest fluids associated with these granites or the concentrated residue formed by boiling of an earlier, more dilute fluid. In the biotite granite, the earliest fluids have salinities of about 10–20 wt % NaCl equivalents, and it can be demonstrated that these fluids boiled at temperatures around 520°C. Abundant gas-rich inclusions in all rock types show further that episodic boiling did occur during the evolution of the hydrothermal phase associated with all these granites.

P-T conditions in the South Wales Coalfield: evidence from coexisting hydrocarbon and aqueous fluid inclusions

Siderite nodules in the Carboniferous Coal Measures of South Wales contain cavities which are often infilled with quartz, carbonates, sulphides, and hydrocarbons. The quartz contains a mixture of hydrocarbon and aqueous fluid inclusions. The aqueous fluid inclusions consist of a dilute brine (3 wt % NaCl equivalent) and have homogenization temperatures in the range 97–212 ºC (mean 143 ºC). The hydrocarbon fluid inclusions are dominated by methane with a small component of higher order hydrocarbons; their homogenization temperatures are in the range 35–78 °C (mean 54 °C). It is assumed that the two fluids were trapped simultaneously during growth of the quartz and thus a P–T estimate of entrapment can be obtained by graphical intersection of the hydrocarbon isochores and the aqueous fluid bubble point (homogenization) temperatures assuming hydrocarbon saturation. This method gives temperatures between 130 and 160 ºC, and pressures between 40 and 55 MPa. The timing of mineralization is uncertain, but it is suggested that it took place during burial and low grade metamorphism of the subsiding sedimentary basin (i.e. in the Upper Carboniferous). The hydrothermal fluids were probably derived from evolved meteoric or connate waters expelled during subsidence and sediment compaction.

Fluid inclusion petrography of SW England granites and its potential in mineral exploration

Fluid inclusions in granite quartz from SW England provide a record of the complex and protracted hydrothermal history of this important metallogenic province. Regional variations in terms of the different types of inclusions can be correlated on an inter-pluton scale with both the texture of the host granite and the extent to which it is mineralized. On an intra-granite scale those areas where mineralization is particularly pronounced show a higher overall inclusion abundance than areas where little or no mineralization is known to occur. The types of fluid inclusion most commonly related to Sn-W-Cu mineralization are halite-free, moderate temperature inclusions. Inclusions containing visible CO2 at room temperature are restricted to two localities in SW England. Both of these contain stockwork/vein-swarm tungsten mineralization. These regional ‘fluid inclusion anomalies’ show that fluid inclusion petrography using a simple petrographic microscope has potential application in the field of mineral exploration.

Developments of the ICP-linked decrepitation technique for the analysis of fluid inclusions in quartz

Abstract Further developments in the analysis of fluid inclusion decrepitates by inductively-coupled plasma emission spectroscopy (ICP) are outlined. Decrepitates from samples of quartz have been analysed and give useful results. As several mechanisms of transfer of material into the plasma are feasible it is important that the temperature of heating should be restricted to the temperature range of decreptiation. However, contamination by silicate phases in the quartz appears to be of little significance. Reproducibility in our analyses is usually better than 30% (relative standard deviation, RSD) and most of this appears to be due to heterogeneity in the abundance of fluid inclusions. We have been able to estimate the ore metal content of some hydrothermal fluids from SW England and have obtained values in the range one hundred to a few thousand parts per million for some elements (Cu, Pb, Zn, Sn and B). Because of the sensitivity of the method we feel that its greatest potential is in the field of routine mineral exploration, and we have had considerable success in this application.

Fluid inclusion chemistry as a guide to tin mineralization in the Dartmoor granite, south-west England

Abstract Sixty-two quartz samples were separated from stream sediments draining the Dartmoor granite (southwest England) and chemically analyzed using the ICP-linked decrepitation technique. The samples were derived from three environments: an area of granite peripheral to the mineralized area, and a region of granite-devoid of mineralization. On a local scale, the fluid inclusion decrepitates in samples from the mineralized region show relatively higher concentrations of Rb, B, Sn, Cu, and S, and lower concentrations of Na, K, Ca, Fe, Li, Ba, Sr, and Zn. Principal component analysis of the whole multi-element data set confirms that the elements Rb, B, Sn, Cu, and S behave independently from the major constituents of the fluid inclusion decrepitates (Na, K, Ca, Fe, etc.), and possibly represent a “mineralization” factor. Discriminant function analysis, using the concentrations of Na, Ca, S, Ba, Sr, and Rb, is able to produce an efficient separation of the samples into the three groups. The regional variation in chemistry of the fluid inclusion decrepitates can be explained by invoking two main sources of quartz for the stream sediments: quartz veins associated with the mineralization, and quartz from the unaltered granite. The fluids in quartz from the unaltered granite also seem to show a regional variation, as elevated concentrations of Na, K, Ca, Fe, and Zn are found in decrepitates from samples adjacent to the region of mineralization. The results of this study indicate that the multi-element chemical analysis of fluid inclusion decrepitates may be a viable method of geochemical exploration for granite-hosted tin deposits.

A eukaryote assemblage intercalated with Marinoan glacial deposits in South Australia

Composite hematite–silica structures recovered from a siltstone bed in the Elatina Formation of South Australia include (1) sub-circular to whorl-shaped forms, (2) elongate to half-moon-shaped forms and (3) and lozenge-shaped forms locally linked into chains. They range from 200 to 500 µm in diameter and are interpreted as eukaryote tests. Evidence for internal etching of a calcite core of some tests indicates that at least some of the hematite–silica fabrics were acquired through replacement. Carbon isotope values of −20‰ δ13C are suggestive of precipitation by microbial activity, and imply a change in ambient fluid chemistry associated with a pH reduction. The tests occur within sandstone beds that were deposited on a tidally modulated braidplain during the Marinoan glaciation at the end of the Cryogenian. The quartz grains in the sandstone sample lack the typical textures (surface striae, internal fractures or irregular grain boundaries) expected for glacially transported material. Thus, on textural grounds we argue that the eukaryote tests represent a proglacial ecosystem during a late Cryogenian snowball Earth event. Supplementary material: Video files of digital X-ray tomographs (μCT) in the longitudinal and transverse planes are available at: https://doi.org/10.6084/m9.figshare.c.2209723.

Ore mineralogy of the mesothermal gold lodes of the Dolgellau gold belt, North Wales

Abstract The auriferous quartz-sulphide veins of the Dolgellau gold belt represent the finest example of a pre-Acadian, mesothermal gold-lode metallogenic province in the United Kingdom. The gold lodes are composite, multi-stage, ribbon-quartz bodies occupying shear zones in clastic marine sedimentary rocks, including black mudstones, of Middle to Upper Cambrian age and in associated altered, basic-intermediate intrusives. Petrological studies have revealed a complex history of multiphase mineralization and anomalously pervasive hydrothermal alteration. The mesothermal gold-lode mineralization has a four-stage paragenetic sequence comprising early Fe-Co-As followed by localized, bonanza-style Au-Ag-Bi-Te-Pb. Stage three is Cu-Fe-dominated, whereas the final stage is dominated by Pb-Zn. Quartz and carbonates accompany all stages, their relative abundances varying considerably from locality to locality. The textural relationships of the ore minerals have been partially modified by post-depositional processes related to regional, strain-related metamorphism and Acadian deformation, but the veins still reveal a consistent regional paragenesis, a detailed account of which is published here for the first time. A discussion of the redefined paragenetic sequence of the gold lodes is followed by a reinterpretation of the extensive existing pool of previously published analytical data.

Fluid inclusion and stable isotope studies at Gunung Limbung Cu/Pb/Zn deposit, west Java

Abstract Base metal mineralization at Gunung Limbung (west Java) occurs in several steepy-dipping quartz veins which are hosted by a Miocene monzodiorite stock. The mineralization is dominated by pyrite, chalcopyrite, sphalerite and galena. Wallrock alteration is intense and consists of an innermost zone of K-feldspar enrichment, surrounded by an outer zone of propylitic alteration; argillic alteration is sporadically developed close to some of the mineralized veins. Fluid inclusions in the quartz veins indicate that two distinct fluids were associated with the hydrothermal activity—one of low salinity (0–4 wt% NaCl equivalents) and relatively high Ca and Fe contents, the other of higher salinity (11–23 wt% NaCl equivalents) and relatively high Na contents. Both fluids probably had a similar temperature of around 350°C. Oxygen- and hydrogen-isotope analyses of vein quartz, altered rocks and fluid inclusions support this separation into two distinct fluid types: the dilute fluids had a composition in the range δ 18 O = 0 to + 2‰ and δ D = −80 to − 50‰ , whilst the more saline fluids fall in the range δ 18 O = + 3 to + 5‰ and δ D = −55 to −40‰ . The fluids responsible for the potassic and propylitic alteration had a similar isotopic composition to that of the higher salinity fluids. The fluid inclusion and stable isotope data support a model in which a metal-bearing magmatic fluid was exsolved from the crystalliziing monzodioritic melt and subsequently mixed with heated, exchanged, local groundwaters. Mineralization at Gunung Limbung could have been caused by the dramatic dilution of the ore fluid during this mixing.

Characterisation of granites from the Midyan terrain, NW Saudi Arabia

Five, Neoproterozoic, poorly studied felsic intrusives from NW Saudi Arabia have been subjected to a detailed geological, geochemical and mineralogical study to identify their characteristics and to understand their processes of formation. The results have indicated that the individual plutons in the region are each subtly different. This is mainly displayed by the existence of different types of granites, based on chemistry and petrography (peralkaline, alkaline and calc-alkaline) and mineralogy (particularly the presence of different types of amphibole, both sodic and calcic). The granites were most likely derived by low-pressure, partial melting of crustal rocks with possible crustal contamination, followed by fractional crystallization and later sub-solidus alteration by fluids. The geochemical differences between the granites could be related to their formation during different stages of the region’s evolution. The granites contain relatively high contents of rare earth elements and rare metals, mostly hosted by phosphates (monazite-Ce and xenotime), Nb oxide (fergusonite-Y) and possible rare earth carbonates (synchysite). Further detailed geochemical study would determine the economic significance of the studied granitoids and allow understanding how the plutons were fit into tectonic setting of the region.