Help mindat.org|Log In|Register|
Home PageMindat NewsThe Mindat ManualHistory of MindatCopyright StatusManagement TeamContact UsAdvertise on Mindat
Donate to MindatSponsor a PageSponsored PagesTop Available PagesMindat AdvertisersAdvertise on MindatThe Mindat Store
Minerals by PropertiesMinerals by ChemistryRandom MineralSearch by minIDLocalities Near MeSearch GlossaryMore Search Options
Search For:
Mineral Name:
Locality Name:
Keyword(s):
 
The Mindat ManualAdd a New PhotoRate PhotosLocality Edit ReportCoordinate Completion ReportAdd Glossary Item
StatisticsThe ElementsMember ListBooks & MagazinesMineral Shows & EventsThe Mindat DirectoryHow to Link to MindatDevice Settings
Photo SearchPhoto GalleriesNew Photos TodayNew Photos YesterdayMembers' Photo GalleriesPast Photo of the Day Gallery

Little Duke Mine

Last Updated: 22nd Aug 2016

By Chris Popham

The mineralisation in relation to nearby mines.


Little Duke Mine was a small venture on the west bank of the River Tavy, high on the hillside and just down from the Orestocks Junction on the Bere Alston to Gulworthy Road. The mine was worked initially in the 1840s and 50s as a copper prospect in the higher levels, the majority of the production coming from around Whim Shaft. Tin production was from the western section of the mine around Gill's Shaft (now under disused bed of the Bere Alston to Tavistock railway) and West Shaft. The mine was reworked for arsenic and tin during the early 1900s though only in the upper levels where the highest yields of arsenic came from around Whim Shaft. There is no calciner, arsenic labyrinth or flue on the site, the ore was taken up to the Orestocks road junction and then to Gawton for refining. A brief walk in the woodland at the Orestocks road junction revealed numerous pieces of quartz having a white, massive form but none with any evidence of mineralisation.

In the present day the site is littered with part filled shafts and pits which form a rough line east-west from the crest of the hillside above the River Tavy to beyond the bed of the disused railway.

Access to the mine is via an open gunnis close to Whim shaft, a small entrance in level ground close to a ruined building. Whim shaft was originally sunk to the 30 fathom level with the workings finally reaching a depth of 50 fathoms. Off Whim Shaft levels at 10 and 20 fathoms opened onto the hillside. Deep adit was a further 20 fathoms below the 30 fathom level and was driven south-eastward opening onto the bank of the River Tavy.

Little Duke is a little to the south of nearby Virtuous Lady Mine on the opposite bank of the River Tavy. The lodes of both mines trend roughly east-west but whereas those of Little Duke are just sub-vertical: 80°S, those of the latter are flat lodes, highly unusual for the Tavistock area. As a consequence of this and the mineralisation in the lodes it has been put forward that the lodes worked in Little Duke were extensions of those worked in Bedford Consols approximately a mile away a little to the south and on the far western side of the hill in Maddacleve Woods above Gawton.

If the arsenopyrite bearing lodes at Little Duke are related to those at Bedford Consols and not to those at Virtuous Lady this may simply be due to the level of emplacement relative to the ground surface. Virtuous Lady was worked primarily at or below river level: 25m ordnance datum, whereas the productive ground in Little Duke was in the upper levels: approx 120m o.d. and Bedford Consols similarly 130m o.d. There is no major faulting of the ground between Little Duke and Bedford Consols nor along the course of the Tavy Valley between Little Duke and Virtuous Lady inviting the conclusion that the relative positions of the lodes observed in the present day are the same as at the time of emplacement. However, given the sub-vertical deposition of the lodes in Little Duke and the flat lode deposits in Virtuous Lady it may be that the lodes were deposited in distinct mineralisation phases .

The lode at Bedford Consols apparently also has quite a steep dip but in the absence of any large, accessible, stopeing it is impossible to estimate the actual dip. The Bedford Consols lode also yields small quantities of anatase, a mineral comparatively rare in the wider Tamar area but this mineral is also listed in some records as having been found at Little Duke.

Entering Little Duke the gunnis is about 8 to 10 meters deep above a modest stope. The lode is apparent in the roof of the stope as a thin 2 to 6 inch wide, slightly brecciated fissure with a centre of pale arsenopyrite occasionally with small stalactites of iron (limonite) and flecks of pyrite and chalcopyrite. However, at the foot of the first pitch there is a large area of copper secondary minerals on the footwall of a small offshoot stope indicating a more substantial copper deposit.

A low narrow passage heads off along the trend of the lode towards the river from close to the bottom of the pitch but ends after a short distance. The main stope heads off in the opposite direction (westwards back towards the railway track) over a low pile of debris under some well preserved timbering to the top of the second pitch. Below the pile of debris a small hole extends downwards onto a lower floor in the stope where there is a perfectly preserved wheel barrow. This has the remains of an iron band around the wheel and where water has dripped onto it flakes of native copper have been formed by electrolysis as well as green copper secondary deposits.

Small pieces of killas rock containing arsenopyrite were collected from the floor of the stope. The arsenopyrite has a good crystal structure, is bright and silvery when freshly broken but exposed crystal faces have taken on a golden surface lustre. The arsenopyrite, for the most part, appears to have crystallised directly into the country rock as disseminated strings without only traces of gangue minerals such as quartz and pale green fluorite. The killas has been much altered by the mineralisation and has softened in relation to the distance from the lode forming with dark grey-green chlorite as an alteration product and a grey-green clay where it is in direct contact. This appearance strongly supports the notion that the lodes here are extensions of those at Bedford Consols as that lode where it cuts across the end of the adit exhibits these same physical qualities. This physical appearance is uncommon for the Tamar area as a whole where arsenopyrite mostly occurs in shiny, massive form associated with quartz stringers: Gunnislake Bridge Adit and South Crebor, or a dull gritty form deposited on but not as stringers in the killas as in Devon Great Consols and Devon United.

Pyrite is also common in the loose material on the floor of the upper stope. It occurs mostly as massive lumps containing small 6 faced cubic crystals of 1 to 2 millimetres poorly bonded together in a matrix of gritty irregular pyrite. Some samples also contain chalcopyrite in pockets of very bright material.

The arsenopyrite in the dumps at Virtuous Lady is also found deposited in a softened grey shale and has a profound yellowish lustre almost to the degree that it could be confused with pyrite and an unusually high sulphur content. The sulphur content can be demonstrated by simply heating powdered mineral in a closed glass tube whereupon the sulphur will be driven off and will condense on the glass further up the tube, initially as a yellow deposit but burning to a black deposit with further heating. The purpose of prolonging the heating is that pure arsenic will also be driven off and will form a silver 'mirror' on the glass below the sulphur. The residue in the tube is magnetic being the reduced iron from the arsenopyrite. When heated in this way the arsenopyrite from Virtuous Lady evolves so much sulphur that it will form liquid droplets on the glass. The dumps at Virtuous Lady also contain well formed pyrite crystals up to 2cm along with abundant siderite and quartz.

The second pitch in Little Duke is down a narrow shaft that descends about 15m with a long ladder apparently glued to the side wall as it is certainly no longer supported from below. Several stope platforms are accessible from the shaft but none is extensive. The bottom of the shaft is quite confined with a small dry area and a flooded portion over a continuation of the shaft. This has been capped with discarded planks and ladders and submerged in about 2 feet of water. The timbers forming the platform-roof above this lowest level carry massive heaps of deads. Across the flooded shaft is a very well preserved level with large rounded uprights and roof bearers and well fitted planks, though it is quite deeply flooded. In an alcove off this passage are a number of relics including some sort of metal, paraffin or oil burning, lamp which stills shows a short length of wick. There are also bolts fixed together by candle wax that has run over them and a tin of approximately 1 pint capacity with pitch or wax around the inside making it quite weighty. There are also several candle stubs on the woodwork.

The water in the passage is about 2 feet deep but floor was apparently sound, initially littered with timber and ladders but then with rails. The timbering extends for about 6 to 8m before giving way to a low rock ceiling with only 12 inches clearance above the water, beyond which was a small stope apparently extending back over the timbered passage. There is some evidence of the lode in this stope though it is intermittent and not as clear as in the upper stope. Again there is abundant killas containing arsenopyrite to varying degrees amongst the loose material on the floor. From the stope the passage is tall enough to allow a person to walk upright and about 3 feet wide however it soon turns sharply left (south) and ends after about 20m at the foot of another shaft or winze which is completely choked with fallen debris.

Back over the flooded winze on the 'dry' side a narrow entrance gives access to a low passage under a massive pile of deads held back by only by very dubious looking timbers comprising some rough hewn planks but for the most part only 3 inch diameter interlaced pine poles resting against thicker round props. This passage winds for several hundred meters in the direction of the riverbank. However the timbering in some places has given way and it is very unsound throughout though even where it has fallen in it is impossible to determine the extent of the stope overhead. There are a number of artefacts including a galvanised bucket.

Four samples from Little Duke; the first and second from the upper stope the third & fourth from the lower level, were heated in this way. The first and second samples gave the tubes a coating of sulphur and a bright arsenic mirror. The third sample was obviously predominantly pyrite and though a dusting of sulphur coated the tube there was insufficient arsenic evolved to form a mirror. The fourth sample was dull silver-grey and visually gave the appearance of arsenopyrite. When heated it gave copious sulphur fumes no arsenic was evolved.

A sample of pyrite from the upper stope at Little Duke was also heated, sulphur was evolved and a magnetic residue remained but no mirror formed.

A reference sample from Bedford Consols was similarly treated and that gave a full coat of sulphur but perhaps a more substantial mirror than either of the Little Duke samples.

Despite close microscopic inspection anatase could not be identified in any of the samples collected at little Duke, though minute isolated cassiterite crystals were observed.

Based on the physical appearance of the arsenopyrite, the steep dip of the lode, the dissemination of the arsenopyrite as thin strings in the softened killas with little gangue mineralisation and the modest sulphur content it does seem fair to suggest that the arsenic bearing veins in the upper sections of Little Duke Mine are more akin to those of Bedford Consols rather than those of the much closer Virtuous Lady Mine. To the contrary however the abundance of pyrite and chalcopyrite but absence of anatase at Little Duke would be evidence for a link to the Virtuous Lady lodes. Consequently, it may be the case that the presently accessible lodes at Little Duke are upper vertical extensions of lodes that extend beneath the River Tavy at depth and that at depth they would be more characteristic of the lodes at Virtuous Lady.




Article has been viewed at least 1059 times.

Comments

In order to leave comments to this article, you must be registered
Mineral and/or Locality  
Mindat.org is an outreach project of the Hudson Institute of Mineralogy, a 501(c)(3) not-for-profit organization.
Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2017, except where stated. Mindat.org relies on the contributions of thousands of members and supporters.
Privacy Policy - Terms & Conditions - Contact Us Current server date and time: January 21, 2017 06:27:47
Go to top of page