Graphite morphologies from the Borrowdale deposit (NW England, UK): Raman and SIMS data

Abstract

Graphite in the Borrowdale (Cumbria. UK) deposit occurs as large masses within mineralized pipe-like bodies, in late graphite-chlorite veins, and disseminated through the volcanic host rocks. This occurrence shows the greatest variety of crystalline graphite morphologies rec ognized to date from a single deposit. These morphologies described herein include flakes, cryptocrystalline and spherulitic aggregates. and dish-like forms. Colloform textures. displayed by many of the cryptocrystalline aggregates, are reported here for the first time from any graphite deposit worldwide. Textural relationships indicate that spherulitic aggregates and colloform graphite formed earlier than flaky crystals. This sequence of crystallization is in agreement with the precipitation of graphite from fluids with progressively decreasing supersaturation. The structural characterization carried out by means of Raman spectroscopy shows that, with the exception of colloforrn graphite around silicate grains and pyrite within the host rocks. all graphite morphologies display very high crys tallinity. The microscale SIMS study reveals light stable carbon isotope ratios for graphite (b 13 C = -34.5 to 1. F. Barrenechea . F. 1. Luque (c:;]) .

Carbon isotopes of graphite: Implications on fluid history

“…In recent studies of the Borrowdale (Cumbria, UK) graphite deposit the relationships between fluid evolution and graphite precipitation in volcanic environments have been established (Barrenechea et al, 2009;Luque et al, 2009a, b;Ortega et al, 2010). The deposit is hosted by andesite lavas and sills belonging to the Upper Ordovician (Katian) Borrowdale Volcanic Group, and by a probably contemporaneous hypabyssal dioritic intrusion.…”

“…This situation appears to be unlikely in most cases since graphite precipitation from fluids proceeds through nucleation and crystal growth and because the sluggish diffusion kinetics of carbon in graphite once graphite is formed. The mechanism involved in the growth of graphite flakes is that of spiral growth (Santosh et al, 2003;Barrenechea et al, 2009;Satish-Kumar et al, 2011b). Thus, the Rayleigh precipitation process, in which every carbon atom incorporated to the graphite structure is isolated from the fluid, appears to be more appropriate.…”

“…Thus, graphite precipitation in open systems frequently results in different generations which can be recognized on the basis of textural features (e.g., changes in graphite morphology; Barrenechea et al, 2009;Ortega et al, 2010;Touzain et al, 2010) or from the association of graphite with different rock types linked to different stages of fluid evolution . It is important to note that in many studies the origin and evolution of the fluids are inferred from the isotopic variations in graphite, but studies in which fluid evolution is determined by independent and direct methods (i.e., fluid inclusion analysis) are very scarce.…”

“…For instance, metamorphic graphite usually occurs as flaky crystals, whereas a large number of crystalline habits (e.g. flaky, spherulitic, colloform, cryptocrystalline, rings, cones, tubes) have been reported for fluid-deposited graphite (Jaszczak et al, 2003(Jaszczak et al, , 2007Doroshkevich et al, 2007;Barrenechea et al, 2009). In addition, metamorphic graphite shows a large range of crystallinity (i.e.…”

Vein graphite deposits: geological settings, origin, and economic significance

“…8) and thermal data have been reported for igneous-hosted graphite veins both in the Serranía de Ronda, Beni Bousera, and Borrowdale deposits (Luque et al 1992;Crespo et al 2006a;Luque et al 2009;Barrenechea et al 2009;Ortega et al 2010). The different graphite morphologies found in the Borrowdale deposit are structurally highly crystalline ).…”