Bulletin of the Geological Society of Malaysia, Volume 17, Dec.1984, pp. 323 – 356
1Billiton (U.K.) Ltd., Exploration Division, Wheal Andrew Farmhouse, Chacewater, Truro, Cornwall, England.
2Camborne School of Mines, Pool, Redruth, Cornwall, England.
‘And still they gaz‘d, and still the wonder grew,”
Goldsmith, “The deserted village”
SUMMARY: The widespread and numerous stanniferous placers of Cornwall have been exploited intermittently from the Bronze Age to the present.
Although a variety of placers are known, the fluviatile ones, some of which extend under the sea, are, by far, the most important. Superimposed, in part, over the wholly natural placers are others which have developed by the natural beneficiation of the mill tailings from hard-rock tin mines.
Generally the natural fluviatile placers consist of a basal layer of stanniferous sand and gravel, overlain, in the upper reaches, by peat and fluviatile sediments. In the lower reaches of the valleys the cover consists of peat and estuarine and marine sediments.
The stanniferous province of Cornwall is made up essentially of Devonian and Lower Carboniferous non-calcareous metasediments and intercalated metabasites into which was emplaced, during the Permo-Carboniferous, a polyphase granitoid batholith whose surface was ornamented by ridges with undulating crestlines. Many granitoid dykes were then emplaced and these tend to strike about parallel to the granitoid ridges. The vein swarms and hydrothermal breccias were generated in and about the high spots (cusps) on the ridges, and lodes, often zoned, and containing tin, copper, etc., were established with strikes about parallel to neighbouring ridges and dykes. Mesothermal lodes with such species as galena, siderite and sphalerite were formed later in fault structures striking approximately at right angles to the tin lodes.
Little is known about the palaeogeography of the region before the Pliocene, but there is reason to believe that parts of the granite batholith were unroofed in the Permian, and subsequently from that time there was progressive destruction of the primary tin deposits with accompanying release of cassiterite which may have been concentrated in placers, in part only to be reworked on a number of later occasions.
The destruction of the primary tin deposits occurred under a number of very different climatic conditions. From the Permian to the end of the Tertiary, hot and arid conditions prevailed. From the Palaeocene until the end of the Oligocene the climate was becoming more temperate but during the time of the Alpine Storm all but the highest parts of the region were submerged. Subsequently, in the Pliocene, or possibly a little earlier, when a temperate climate prevailed, there was emergence in stages which led to the development of a number of platforms of which on those at 229 metres and 131 metres respectively, there are the so-called mid- to high-level tin gravels. Of all the platforms developed during and after the Pliocene the 131 m one is by far the most extensive. Elevation continued in stages throughout the Pleistocene when permafrost conditions prevailed and ‘Head‘, a periglacial solifluction and mass wasting product, some of which was tin-bearing, migrated during the warmer interglacial and interstadial times on to lower ground and into the valleys which, when occupied by water, were deepened rapidly due to the rejuvenation, the drainage systems were undergoing. At the end of the Pleistocene and during the early part of the Recent (Flandrian) the climate ameliorated and the ‘Head‘ and other debris in the valleys were subjected to marked natural beneficiation which resulted in the final stages of development of most of the placers known today. During the later phases of placer generation, cassiterite from marine sediments on the Pliocene platform were washed into the drainage systems, and, in addition, as the valleys were being deepened cassiterite was liberated from lodes transected by the streams and rivers. Subsequent changes in base sea-level resulted in the partial drowning of the valleys with the formation of rias and the forest-covered adjacent lowlands thus creating the present situation in which some of the placers lie beneath the sea.
Finally, it is abundantly clear that when endeavouring to understand the disposition, nature and genesis of the placers under review it is necessary to take into account the nature and geographic disposition of hard-rock sources of the cassiterite, the characteristic of the host-rocks, the varying climatic environments to which the region has been exposed since the primary deposits were uncovered, the variations in land and sea relationships, the history of drainage development and Man‘s activities. All these factors have been dealt with, at some length, in this paper which concludes with pictorial summaries of the disposition of the placers, largely with respect to the marine platforms, and of the authors‘ views of the genesis.