The Geology of the Poldark Mine and its surrounding area.
For Advanced Students
N. G. LeBOUTILLIER BSc., PhD., MCSM., EurGeol., CGeol., FGS.
THE GEOLOGY OF POLDARK MINE: PART TWO – THE CARNMENELLIS GRANITE.
5.6.1 Introduction: The geology of The Carnmenellis Granite.
The Carnmenellis Granite (and its satellite Carn Brea Granite and Carn Marth Granite intrusions) forms a broad sub-circular outcrop (see Figure 10), between Camborne [SW656400] and Constantine [SW733290], that stretches 11 km N-S and 14.5 km E-W. The granite is a composite intrusion with a central sub-circular medium-grained granite within an outer coarse-grained megacrystic granite, and with variably megacrystic coarse-grained granites disposed in incomplete ring-like outcrops on the east side of the pluton. Each of the granites is chemically distinct indicating a number of (evolving) source magmas. The Carnmenellis (Ga) Granite has been dated at 293 Ma, making it the oldest of the Cornubian granites.
Figure 10. A geological map of the Carnmenellis Granite. After BGS 1:50,000 sheet 352.
Leveridge et al. (1990) in mapping the Carnmenellis Granite and its satellites have classified the various granite types on textural grounds; their classification is a modification of that used by Ghosh (1934) to map the same pluton and divides the granite into seven varieties (Ga to Gh). The Ga granite, within which Poldark Mine lies, is a megacrystic (15-20 mm) granite with a coarse (2-4 mm) groundmass and forms an open circular outcrop making up the outer section of the pluton. This granite also makes up the Carn Brea and Carn Marth satellite bodies. The granite contains xenoliths close to the margin and often displays preferred linear and planar alignments of the feldspar megacrysts.
The Gb granite outcrops on the eastern margin of Carnmenellis and is characterised by a coarse groundmass, but a relative scarcity of feldspar megacrysts. Those that do occur show no preferred alignments, except in exceptional cases. Textural changes in the groundmass are also used to distinguish Ga from Gb.
The Gc granite outcrops in a 1-2 km wide arcuate strip between Ga and Gb (with which it has some observed faulted contacts relating to a NNE-SSW trending fault). It is distinguished from Ga (which it is broadly similar to) on minor textural grounds (groundmass feldspar crystal form and mode of biotite occurrence).
Gd outcrops as a roughly circular mass in the centre of the pluton. It is characterised by a scarcity of megacrysts (which are generally < 15 mm) and a medium grained (about 1 mm) groundmass. These properties are not, however, consistent and finer/coarser areas are known to occur.
The Ge and Gf granites outcrop as a series of minor stocks and dykes around the margins of the Carnmenellis pluton and occasionally within it. Both are medium grained with feldspar laths up to 2 mm long in a finer matrix of quartz, feldspar, mica and accessories with a grain size of 0.2-0.4 mm. Gf also contains subhedral megacrysts up to 10 mm in length.
The Gh granite occurs in two masses with subcircular outcrop patterns. One (2 km in diameter) is located, straddling the Ga/Gd boundary at Boswyn [SW664367]; the other (1 km in diameter) outcrops on the margin of Carnmenellis at Praze-An-Beeble [SW636357]. The groundmass of this type is fine grained (0.5 mm) with alkali feldspar laths up to 3 mm in length and occasional 10 mm length megacrysts.
The nature of the contacts between the various granite types on Carnmenellis is often obscure, hampered by poor exposure. The Gb/Gc contact is nowhere exposed, although much of its length is marked by a fault. The displacement on the pluton margin by this fault is small; therefore it has been postulated that its control of the Gb/Gc contact implies this to be gently inclined (a sheet-like feature?). The Ga/Gc contact, only exposed in quarries, is transitional, but no clue to its overall geometry can be ascertained. The Ga/Gd contact is well defined and suggests control by steep joints. The Gh granite occupies high ground and this, in conjunction with its outcrop pattern, may also indicate a sheet-like form, intrusive into the Ga granite.
The granite/killas contacts of the Carnmenellis Granite are nowhere exposed, although an exposure at Tremough [SW77503492] lies close to the eastern contact. The contact of the Carn Brea Granite was formerly visible at several points in the now-flooded workings of South Crofty Mine [SW66384090] and the contact of the Carn Marth Granite can only be seen in the underground workings (currently being re-excavated by mining enthusiasts) of Wheal Gorland [SW73904188] near St Day. Internal exposure on Carnmenellis and elsewhere is very poor and is limited to isolated tors, underground sites and quarrying operations.
The pattern of jointing across the Carnmenellis Granite (see Figure 11) reveals the presence of two major joint sets, trending NNW-SSE (320°-325°) and ENE-WSW (070°-080°), with the NNW set being dominant. The formation of this joint system, as already noted, was a product of the regional stress field in operation during the emplacement, and subsequent cooling, of the pluton – σ 1 vertical, σ 3 NNW-SSE. Current principal stress orientations are radically different to those in effect during emplacement/cooling of the Carnmenellis Granite, with σ 1 (horizontal) trending NW (~310°), σ 3 (horizontal) trending NE (~040°) and σ 2 vertical, a stress state that would facilitate further movements on the NNW-trending faults under the right conditions.
Figure 11. A contoured stereonet and rose diagram, of joints in the Carnmenellis Granite.
The strong NNW-trending joint set was almost certainly influenced by mineral alignments in the granite formed in response to movements on NNW-trending faults. Leveridge et al. (1990) record alignments in the Ga and Gc granites (defined by the feldspar megacrysts) trending 330°-350°, which is consistent with fabrics seen in the Land’s End Granite. Evidence of a pronounced zone of mineral alignment close to the granite contact was seen at Tremough, where there was evidence of contemporaneous movement on an ENE-WSW trending fault, also recorded as a mineral alignment in the granite. Evidence of near simultaneous movement on the two sets of faults already identified as being the sets of structures involved in the development of crustal pull-aparts lends further weight to the suggestion that these structures played a major part in creating the free space necessary for the emplacement of the major plutons of the Cornubian Batholith at shallow crustal levels.
The roughly circular outline of the Carnmenellis Granite and the parallel arrangement of the cleavage, which appears to be draped around the pluton, has been used, in the past, to suggest a diapiric origin for the pluton; however, the observed structures are consistent with those produced in a crustal pull-apart, where the original intrusion has been rotated between pairs of faults (with fresh pulses of magma injected into the centre of the existing intrusion, which becomes a marginal unit). This leads to the development of a composite pluton, with ‘draped’ structures in the country rocks. The potential development of ductile structures in response to granite emplacement, is inconsistent with the data collected for the Land’s End Granite and Tregonning-Godolphin Granite, but may be reconciled by the possibility that the Carnmenellis Granite is exposed at a deeper structural level (due to differential erosion of the batholith caused by the vertical ‘staggering’ of individual, pan-pluton’ fault blocks) than the other granites in the local area, where there is an overlap of brittle and ductile structures related to emplacement.
Recent work on the Carnmenellis Granite, using AMS (anisotropy of magnetic susceptibility) techniques detected two main lineations within the granite, trending NW to NNW and ENE-WSW. The ENE-WSW lineation was associated with higher concentrations of tourmaline and was interpreted as a pan-pluton tension gash which acted as a conduit for fresh batches of magma and the concentration of volatiles. This suggests that movement on extensional faults (which were also undergoing some degree of lateral shear movement to form the pull-aparts) during D3, between pairs of NNW-SSE faults controlled the flow of magma into the extending pull-aparts in SW Cornwall.