Cligga Head

A small outcrop of the Permian-aged Cligga Granite extends across the cliff section and is connected at depth to the St Agnes Granite to the west. The granite intruded into the Devonian-aged Porthtowan Formation, which is made of biotite-andalusite hornfels (metasedimentary rocks). On the contact between the granite and the hornfels, there is a large fault. To the south, there is a chalcopyrite-rich elvan (quartz porphyry) which also intruded into the metasedimentary rocks. This elvan intersects another elvan at 90° in a large cavern locally known as the prism.

An extensive Permian-aged vein system extends across the cliff section. Near-vertical east-west orientated veins contain wolframite (tungsten ore) with cassiterite (tin ore). Old mine workings, dominantly from the 19^th Century relating to extraction of these ores, can be seen in the cliff section.  The veins have a greisen border. The wolframite-cassiterite veins are overprinted by later Permian-aged sulphide veins. These are all crosscut by north-south trending, steeply dipping Jurassic lead-zinc crosscourse mineral veins.

The Cligga Head Granite is medium-grained with quartz, plagioclase feldspar and alkali feldspar phenocrysts up to 25mm. Micas, the shiny platy minerals, silver-gold muscovite micas, with less common biotite micas that contain lithium. Due to the abundance of muscovite, the Cligga Granite is therefore termed a “muscovite granite”. Accessory minerals in the granite include tourmaline, topaz, ilmenite, fluorite and rutile.

In the greisen-bordered veins, muscovite, tourmaline, quartz and topaz are common. Feldspars are commonly altered to a mixture of clay minerals. Wolframite is the main mineral in the greisen-bordered veins with minor cassiterite. Quartz, cassiterite, stannite, arsenopyrite, bismuthite and sphalerite veins overprint the earlier wolframite veins.

The elvans are fine-grained, with quartz phenocrysts and occasional clay-replaced alkali feldspar phenocrysts. The east-west orientated elvan at the southern end of the site contains disseminated chalcopyrite mineralisation. Jurassic mineralisation crosscuts all earlier mineralisation and these mineral veins contain galena, sphalerite, iron oxides, quartz, chlorite and clays.

Cligga Granite: On the left is the plane polarised image, with the cross polarised image on the right. Mineral abbreviations: Fld – Feldspar; Li Mca – Lithium mica; Msc – Muscovite; Qtz – Quartz;  Tur – tourmaline.

As the granite cooled, internal stresses resulted in the formation of vertical joint sets (fractures in the rock). In some places, due to tectonic movement at the time, the joint sets have buckled. As the granite cooled, fluids escaping from the granite used these joint sets, resulting in the greisen-bordered veins. These fluids were 350-400°C. Lower temperature fluids that were 210-260°C used these same joint structures after the granite and greisen-bordered veins had formed.

The large fault between the granite and the metasedimentary rocks would have aided the granite intruding. A large fault system is a weakness that would be exploited by an intruding granite magma. A common interpretation is that granite melts act like lava lamps, but in southwest England they are commonly associated with faults.

The Porthtowan Formation was altered by the heat and fluids of the intruding granite, changing from a mudstone to a hornfels. Minerals recrystalised, forming biotite and needles of andalusite. The metamorphic aureole, or area of rock altered by the intruding granite, extends several kilometres beyond the granite.