Key factors controlling massive graphite deposition in volcanic settings: an example of a self-organized critical system

Abstract

Massive graphite deposition resulting in volumetrically large occurrences in volcanic environ ments is usually hindered by the low carbon contents of magmas and by the degassillg processes occurring during and after magma emplacement. In spite of this, two graphite deposits are known worldwide associated with volcanic settings, at Borrowdale, UK, and Huelma, Spain. As inferred from the Borrowdale deposit, graphite mineralization resulted from the complex interaction of several factors, so it can be considered as an example of self-organized critical systems. These factors, in turn, could be used as potential guides for exploration. The key factors influencing graphite mineralization in volcanic settings are as follows: (1) an unusually high carbon content of the magmas, as a result of the assimilation of carbonaceous metasedimen tary rocks; (2) the absence of significant degassing, related to the presence of sub-volcanic rocks or hypabys sal intrusions, acting as barriers to flow; (3) the exsolution of a carbon-bearing aqueous fluid phase; (4) the local structural heterogeneity (represented at Borrowdale by the deep-seated Burtness Comb Fault); (5) the structural control on the deposits, implying an overpressured, fluid-rich regime favouring a focused fluid flow; (6) the temperature changes associated with fluid flow and hydration reactions, resulting in carbon supersaturation in the fluid, and leading to disequilibrium in the system. This disequilibrium is regarded as the driving force for massive graphite precipitation through irreversible mass-transfer reactions. Therefore, the formation of volcanic-hosted graphite deposits can be explained in terms of a self-organized critical system.

Magnetotelluric evidence for massive sulphide mineralization in intruded sediments of the outer Vøring Basin, mid-Norway

“…Various studies have connected the occurrence of massive magnetite and metallic sulphides with thermal aureoles (Gillett, 2003;Katz et al, 1998). Massive graphite can also precipitate in volcanic intrusions and host-rocks in the presence of carbon-bearing aqueous fluid phases (Luque et al, 1998(Luque et al, , 2012Ortega et al, 2010). A clear horizontal seismic reflector is interpreted as layer-parallel igneous intrusions located in the middle of conductor C2 at 9 km depth (Figure 7a), which reinforces the mineralization hypothesis.…”

“…Fluid-deposited graphite in intruded carbonaceous host-rocks can also explain the low magnetic response in the north Gjallar Ridge. However, this type of massive volcanogenic graphite is often precipitated in thin veins and steep pipe-like bodies (Luque et al, 1998(Luque et al, , 2012Ortega et al, 2010). As opposed to Noril'sk stratified iron-sulphides, this type of graphite deposits (veins, vertical pipe-like body) is unlikely to provide a well-connected mineral network required to increase electrical conduction across regional distances.…”

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

“…The volcanic host rocks of the Borrowdale Volcanic Group show field, mineralogical, and geochemical evidence of assimilation of metapelitic material from the underlying Skiddaw Group (Ortega et al 2010). The process of assimilation of carbonaceous materials increases the carbon content of the magma and appears to play a key role in the formation of many graphite deposits hosted by igneous rocks as recently reported by Luque et al (2012a) for graphite mineralization in volcanic settings and by Chukhrov et al (1984) for vein graphite in nepheline syenites from the Botogol massif.…”

“…The yellow-brown matrix within the mineralized pipe-like bodies comprises intensely altered wall-rock and brecciated quartz. Also, both the andesite and dioritic host rocks veins have been intensely hydrothermally altered to a propylitic assemblage containing quartz, chlorite, epidote, sericite, and albite, along with some disseminated small aggregates of graphite and late calcite veinlets (Ortega et al 2010;Luque et al 2012a).…”