|Trenwith Mine - St. Ives|
It is known as Trenwith Car Park. The north side of the parking area was the best place to look for samples.
(Lower Car Park Lot)
What a View!!
|Trenwith.In the western parts of St. Ives. 1-in. geol. 351, 358; 6-in. Corn. 61 S.E.; |
A.M. R 15. Country: metamorphosed greenstone and killas overlying granite
Main Lode of this mine, the north-eastward extension of Standard Lode of St. Ives Consols, crops out on the southern slopes of the Stennack Valley, just south of the St. Ives-Zennor road, coursing E. 25° N. and underlying up to 20° N. About 20 fms. S. of and parallel to Main Lode is South Lode but this has not been extensively developed. The country rock at surface is metamorphosed killas and greenstone and the granite contact, which crops out some 500 yds. W., is encountered underground, with a steep northerly slope. At the 60 fm. Level at Victory Shaft, near the centre of the workings, granite lies a few feet south of the lode and between the 70-fm and 80-fm. levels the lode follows the contact, passing into granite country at about 5 fms. above the 90-fm. Level. Westwards of Victory Shaft the trace of the penetration of the lode into granite rises gently, but eastwards crosscutting south from the lode proves only metamorphic rocks. From 2 to 16 ft. in width and with well-defined walls, Main Lode consists generally of brecciated country rock cemented by quartz end chlorite with cassiterite, chalcopyrite, chalcocite and bornite. Pitchblende with bismuth, molybdenum, galena and zinc blende are also present but these are late arrivals which occur in the lode, between about the 30-fm. and 70-fm. levels as veins and lenses, generally at the footwall but occasionally at the hanging wall. These late ores also occur in joints and fissures in the country rock and have been noted at 40 ft. from the lode on the south side; they are here associated with siderite and bornite, with which the pitchblende is sometimes intergrown (see Cann 1917, pp. 16, 17; Dines 1930, p. 215). A specimen of the mine ore (11977), containing arsenopyrite and pitchblende, is reported by Dr. J. Phemister to show, in thin section, granular vein quartz full of minute inclusions of flaky chlorite and irregular, spongy masses of pyrite and chalcopyrite with which zinc blende is associated.
Main Lode is opened up by Wills Shaft, 120 yds. E. by N. of Nanjivey cross-road, to the 20-fm. Level; Victory Shaft, 160 yds. E. by N. of Wills, on the underlie to the [lO-fm. Level; Berriman's Shaft, 130 yds. E. by N. of Victory, on the underlie to the 90-fm. Level, and Old Sump Shaft, 100 yds. E.N.E. of Berriman's, to the 80-fm. Level. Adit Level enters the mine from St. Ives Consols, to the west, but its course between the mines is not shown on the plan. It extends through the sett to its portal near Western Hotel in St. Ives, connecting with Wills Shaft at a depth of 28 fms., Victory Shaft at 33 fms., Berriman's Shaft at 30 fms., and Old Sump Shaft at 22 fms.; water issuing from the edit is now used as part of the water supply of St. Ives. The 10-fm., 20-fm, and 30-fm levels block out the lode from about 50 fms E. of Old Sump Shaft to Wills Shaft, a distance of 230 fms., although the latter shaft does not reach below the 30-fm. Level. The 40-fm. and SO-fm. levels extend from SO fms. E. of Old Sump Shaft to 35 fms. W. of Victory Shaft. The 60-fm. Level is driven from 85 fms. E. of Old Sump Shaft to 35 fms. W. of Victory Shaft; the 70-fm. and 80-fm. levels from 38 fms. E. Of Old Sump Shaft to 80 fms. W. of Victory Shaft; the 90 fm. Level connects Berriman's and Victory Shafts and continues 50 fms. W. of the latter; the 100 fm. Level extends 40 fms. E. and 46 fms. W. of Victory Shaft and the 110-fm. Level is short. Stoping is rather patchy but spread over most of the blocked out area, only the 60 fm. Level east and the 70-fm. and 80 fm. levels west extending some 30 and 40 fms. beyond stoped ground; in all about 20 per cent of the developed ground has been removed.
South Lode, according to the plan, seems to have been developed only at Adit Level from a crosscut IS fms. S. from Victory Shaft and from another 22 fms. E. of Berriman's Shaft; the latter crosscut continues 60 fms. S. of South Lode but there are no other drives from it.
Beyond the fact that both copper and tin ores were produced at Trenwith (13,080 tons of 11 per cent copper ore and 20 tons of black tin were raised between 1825 and 1856) little is known of its early history, but owing to the occurrence of pitchblende its name has become well known in later years. In 1843, Henwood (p. 19) stated that difficulties in smelting the copper ores produced here were found to be due to the presence of pitchblende which the miners took for black copper oxide. On the discovery of the mistake, the minerals were thrown on the dumps. This caused Henwood to write: "Was there ever an instance in which an acquaintance with Mineralogy and Chemistry would have been more useful" . After lying idle for more than 50 years the dumps were being worked over for uranium ores in 1907, and in 1908 the property, together with the two adjacent mines to the west (St. Ives Consols and Rosewall Hill and Ransom United) and Giew Mine, 2 miles to the south, was taken over by a company known as St. Ives Consolidated Mines Ltd., which continued to work the levels of Trenwith above thc 60 fm. selectively and pick over the dumps for uranium ores. Between 1911 and 1917, 694 tons of uranium ore were produced, mainly from dumps; the small amount that came from underground occurred in scattered patches as thin films in Joints in the gangue or country. In 1917 the group of mines was taken over by the Thermo Electric Corporation which, however, abandoned the northern mines and concentrated on Giew Mine.