The mineral zones of Cornwall

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THE

MINERAL

7

ZONES

OF

CORNWALL.

By HENRY DEWEY.

l'residcntial Address, Read Feb, 6th, '925.

INTRODUCTION. I NCE the early part of last century the occurrence of sequences of mineral deposits in the mines of Cornwall and Devon has been familiar to all. The papers in which these occurrences are described are far too numerous to mention, but the work of the following observers should be consulted by those who are interested in this subject :-Pryce, De la Beche, Carne, W. Smyth, le Neve Foster, B. Symons, Hunt, and J. H. Collins. Perhaps the chief honour, however, belongs to W. J ory Henwood, whose classic work on " The Metalliferous Deposits of Cornwall and Devon" (1)* is still an indispensable book of reference. Among later observers the following must be mentioned:-Hill,MacAlister, Scrivenor, Flett, Davison, and Cronshaw. Although the most superficial study of the lodes shows that the minerals do not all occur together in the same part of a mine, there are so many exceptions to the commonly accepted order of deposition, tin, copper, zinc, lead and iron, that it seemed desirable to examine afresh the evidences for the occurrence of these zones, and in pursuing this enquiry to refer rather to the existing exposures in the mines than to depend entirely upon published accounts, and the results of such an enquiry form the subject of my address. Reference to Plate 12 shows that the principal mining districts occur in the neighbourhood of the granite masses. The mining district of St. Just and St. Ives lies on the northern part of the Land's End Mass: the principal mines lie around the western part of the Carn Menellis and the adjacent Carn Brea and Carn Marth masses : south-east of the St. Austell granite an important group of mines was formerly worked and another group at the south-eastern margin of the Bodmin Moor granite. Between this granite and Dartmoor the two small bosses of Kit Hill and Hingston Down were the centre of an important mining district. In Devon, mines were worked on the west of Dartmoor near Calstock and Tavistock, and on the east in the Christow valley. In all these districts tin and copper ores were mined. Between the granite masses the country was barren of mineral matter except for restricted areas where lead, zinc and iron-ores were raised. The geology of Cornwall is so intimately associated with the distribution of the ores that a short account of the rocks and of their structure is necessary to enable one to follow the history of the mineral genesis.

S

'For list of references see p. '34, also' Special Repts. Min. Resources' (Mems. Geol, Surv.).

PROC. GEOL. Assoc., VOL. XXXVI., Part

2,

1925.

8

:roS

H. DEW EY,

Within the relatively small area of Cornwall th ere is a remarkable number of t ypes of rocks belongin g t o a long succession of outburs ts of ign eous act ivity. The great Lizard complex represents the hist ory of magmat ic differentiation in pre-Cambrian times, and includes rocks ranging from ultrab asic to acid in composition, the ultrab asic form ing by far the predominant m ass. Some metallic copper is associate d with this period of igneous activity, and, except for the spora dic occurrence of chromit e in th e serpent ine, this was th e only manifest ation of the deposition of minerals of economic import ance. The age of th e deposits of west ern Corn wall is not kn own ; th ey are probably pre-Silurian, as shear lenticles of Ordovician age occur in th em near Veryan Bay and elsewhere. Igneous activity was renewed , an d th e magma being spilitic in cha ra cte r was, afte r consolidation, attacked by emanations, whi ch conv ert ed the felspars into albite. No min erals of economic valu e were developed during this epoch, nor in Lower Devonian times, but sediments were quietly laid down during a prolonged period. Towards the end of the Middle Devonian a great outburst of volcanic activity affect ed th e whole region, and outpourings of lava spread far and wide. while intrusive dyk es and sills forced th eir way into the deep-seat ed sediments, ba king and met amorphosing them a t th eir junctions into hard cher ty rock. Th e land slowly and con tin uously sank and marine deposits covered th e lava-flows, until further and more in tense volcanic action recommenced in th e Upper Devon ian period. These eru ptions were probably th e grandes t that ever happened in Cornwall and th eir sub-marine flows now cove r wide tracts of North Corn wall and West Devon between the coas t north of P ad st ow and the English Cha nnel east of Dartmoor. Th ese flows were also of a sp ilitic character, while their hyp ab yssal representatives were closely related. No ore-bodies were form ed, but not able instances of mineral genesis are seen in t he relation of albit e to magnetite an d ilmenite. Th e fine-grained sills of 'diab ase' locally show in th eir central parts coarse-grained masses of a pegmatitic habit in which the augite and albite crystals are intergrown, with large skeletal crystals of ilmenite partly enclosing th em. Near Trebarwith and St. Clether masses of rock composed of an intergr owth of m agnetite and calcite form part of the spilitic lava : while veins of albite felspar and magnetite have broken through the sediments. The relati ons of albite to magnetite and tungst en h av e been considered by Lenarcic (2) and by Day and Allen (3). None of these bodies could be considered as of economic value. A long succession of outburs ts of spilite continued through the Upper Devonian and the Lower Carboniferous periods, and the lavas now form the domina ting topographical features of North and Ea st Corn wall.

THE MINERAL ZONES OF CORNWAL L.

109

Th ere is some evidence th at th e older Palseozoic sediments had been folded and perhaps cleaved before th e basal Lower Devonian conglomera te was laid down: for th e direction of the axes of the folds trends north-eastwards. At the close of the Carb oniferous peri od the great series of earth-movements, known as the Armorican , affect ed th e whole country and superimpressed upo n th e earlier fold-lines a new series of anticlines, synclines and oft en of isoclines closely pa cked together. The arenaceous rocks resist ed th ese st resses and becam e pleated and conto rted ofte n on such a small scale th at many compl ete zig-zags can be counted in an inch section. Th e argillaceous sediments gave way freely and th eir original consti tuents became dr awn and squeezed out a t right angles to th e dir ection of pr essure. The earlier folds of large amplitude were th emselves squashed together and packed into closer zig-zag folds along their limbs , whil e lat er the country gene rally app ears to have been thrown in t o a series of wide anti clines and syn clines. The best preser ved of thes e is th e anticline th at flanks the nor th and west of th e Bodmi n Moor gra nite. Its axis pitches north-westwards and near th e granite m any hundreds of feet of th e upper beds, including masses of pillow lava, ha ve been denuded, but at the coast another series of sharp folds crosses th e main axis and finally gives place to sever al grea t overthrus ts from th e northwest . A parallel and simila r series of overthrust s has carried forw ard th e Upper Devonian over th e Carboniferous rocks on th e south-east of Dartmoor and both probably belong to th e sam e period of folding, namely , th e Armorican. The Lower Devonian rock s betw een Watergate Bay and th e E nglish Channel form a second anticline, in to the dome of whi ch the granite mass of St . Austell ha s forced its way . The third fold, formed of (?) Ord ovician sediments, sur round s the Carn Menellis granite mass, and has dist ur bed the earli er Caledonian fold-lines of th e killas, while remains of a fourth anticline are seen around th e Land 's End ma ss. These four anticlines, with th eir at tendant gra nite masses, ar e arra nged along a zone trendin g north-eastwards, i.e., along the Caledonian axis, and it would appear that th e E .-W. folds of Armorican age ar e resting upon thi s earlier axi s; th at is, along a line of crust weakness, through which th e granite magma work ed its way. It is also probable that th e continual sinking of the land area was affect ed by thi s line of weakness, and that the imposing series of volcanic outpourings from the pre-Cambrian to the post- Carboniferous peri ods arose alon g this line as the resul t of readjustments between th e various masses of earthcrust on th e oceanic margin. As will presently be seen, th e period of ore-emanation also probably resulted from th ese adjustmen ts. Although th e magm as which had supplied th e lava during this

110

H. DEWEY,

succession of flows had been of a highly vapourous nature the deep-seated ore-reservoir had never been touched, and the lavas appear to have welled out without explosive violence. But the post-Carboniferous magma gave rise to a series of differentiation products, including granite of medium grain, porphyritic granite, pegmatite, and its end product, the quartz tourmaline dykes, aplites, quartz porphyry dykes and the mineral veins or lodes. Pegmatite veins locally pass, by the gradual dying out of the felspar constituent, into veins consisting of quartz and tourmaline. In one instance, north of Tintagel, such a dyke has broken through metamorphosed sediments many miles away from the nearest granite mass, and at Kit Hill a similar vein bears molybdenite. The successive emissions of these rock-types show a continuous process of magmatic differentiation, accompanied by an increase in the escape of the heavier constituents at a late stage of consolidation of the granite. THE MINERAL ZONES. From very early days of mining the banded nature of many of the ore-bodies had been noticed, especially from the walls inwards to the centre of the lodes, and the comby nature of the crystals showed that they had been free to develop their crystalline form before another series had grown upon them i rom fresh material. Records of the output had been saved, and the mine plans showed whence the ore had come. There are several such plans of old workings extant, but none shows the distribution of the ore-bodies better than the familiar section of the main lode of the Dolcoath Mine. This mine commenced work several centuries ago, and after having been stoped to a depth of 550 fathoms was abandoned, because the tin-zone was held to be bottomed. The upper levels were all driven in ores of copper, with only negligible quantities of tin-oxide. This predominantly copper-bearing section continued to a depth of 170 i athoms, when a thickness of some 20 fathoms of impoverished lode, containing ores of both tin and copper, was touched. Below that section the rich tin-ground was proved to extend to a depth of over 300 fathoms. The gangue minerals showed also a change, quartz, chlorite and tourmaline accompanying the tin-ore, and quartz with sulphides, the copper zone. In the higher parts of some of the workings important quantities of zinc blende were found. There is thus a clearly defined succession of ore-bodies which cut through the junction of the granite and killas. Most of the copper-ore came from the killas, but by no means all of it, as the granite in the eastern workings contained copper-ores of great value for a total depth of 60 fathoms, while tin-ore occurred in slate in the west of the mines. The occurrence of zones did not therefore depend upon the nature of the country-rock, but

THE MINERAL ZONES OF CORNWALL.

III

rather upon th e temperature and pressure obtaining at th e time of ore-deposition. In Dolcoath the seq uence was clearly one of tin, copper and zin c in upward success ion. Simila r depth zones have been encountered during th e exploitation of many other lodes, and th e following, worked during recent years, show such changes. At Tresavean, copper-ore did not app ear in the killas, but became rich in granite, and continued down to the 286 fa thoms level ; next came ores of ti n and copper mixed, while rich ti n-ores were found at a depth of 345 fathoms. The lode underlies sout h, and the ga ngue principall y consists of qu art z and" peach." Arsenical py rites comes in low down in t he copper zone, but never in association with fluorsp ar. Another lode carries large quantities of arsenical pyrites,and in th e upper levels zinc blende is abunda nt. H ardly any wolfra m has been found . Th ere is, th erefore, a su ccession of minerals similar to th at at Dolcoath , but the rel ati ve depths are different. At South Crofty th e copper-ores occur at present mainl y in the north-dipping Middle lode, which con t ains also mu ch fluorspar and iron-pyrites. Most of th e rich copper-ores occurred in th e killas above II O fat homs, when tinare came in abunda nt ly. Th e ga ngue consists of blu e " peach," " capel," qu artz, fluorspar and chlorite . In th e neighb ouring Tincroft Mine all the lodes form erly carried cha lcopyrite and chalcocite, especially in th e killas, but in th e North Tin croft lode copper-ores were abun da nt in th e gra nite to a depth of 4 0 fathoms. It was observed in th is mine th at, where fluorspar comes in, th e values of copper-ore decrease, and th at both fluorspar an d wolfram die out in depth. The Ea st Po ol and Agar Mines have yielded in th e past enormous quantities of copper-ores down to about 140 fathoms below adit level. Killas forms th e country-rock down t o 135 fathoms level, where th e granite is first encountered . Th e lodes underlie either northwards or southwards, the former being richer in copper and arsenic ores, and conta ining less " peach " th an th ose underlying sout h. According to Dr. Malco.m Maclaren (4) the zonal seq uence at E ast Pool is very clea rly developed , ores of copper extending t o a dep th of 140 fathoms, wolfram to 200 fathoms, and below that tin-oxide. Th e group of mines lying between the granite of Cam Brea a nd th at of Cam Menellis yielded an abundance of copper-ores down to about 100 fathoms below the surface ; most of the lodes underlie northwards, exce pt the Great Flat Lode, which inclin es to the south and carr ies mainly tin- ore. Th e largest producers of copper-ore were th e mines work ed near Gwennap and St . Day, from which upwards of a million t ons of copper-ores have been raised from an area of abo ut two sq ua re miles. Sla ty killas forms most of th e country-ro ck. Taylor 's Lode and the Grea t Lode were the principal lodes worked

II 2

H . DE WEY ,

a nd both dip north. Besides thes e two ~here ar e many others, all rich in copper-ores, which also underlie north . Some of the lod es have been work ed to a depth of over 290 fathoms, mainly for copper-ore, and the tin-zone has scarcely been touched. The gangue is quartzose, and at Wh eal Fortune blende occurred in considerable quantities. Further east , near the S1. Aust ell granite, copp er-ores were worked to a depth of 300 fathoms in th e F owey Consols Mine in a series of north-dipping lodes with quartzose gangue materials. Carbonat e of iron occur red in the upper work ings. Similar chan ges from carb onate of iron t o sulphide of iron and copper were observed at th e E ast Crinnis and P ar Mines, Some tin-ore was raised . The Caradon district is tr aversed by a grea t cross-course fr om nor th to south , and is divided by a mass of ba rre n granite into two rich areas. To the north the lodes were rich in tin-ore, and dip south, whereas on the south th e lodes dip north and were rich in copper, but poor in tin-ores. The lodes consist chiefly of quartz, chlorite and felspar, with, at intervals, small proportions of mica and t ourm aline. Th eir upper parts carried masses of brown iron-o re mixed wit h granular q uartz, with black and red oxid es of copper, th e carbonates of copper, vitreous copper and other rare minerals below. In th e deeper par ts of the mines fluorspar is more ab undant , while copper-pyrit es greatly increases in qu antity, but little or no tin-ore was found. The Ph oenix Mines (nort h of th e Caradon Granit e) have afforded in addition to t he copper-ores, large qu an ti ties of tin-ore, bu t contain no fluor. South Cara don mine was worked t o a depth of over 2 5 0 fathoms in a series of north-dipping lodes. Th e higher parts carried the oxides an d the carbonat es of copper , native copp er, oxides of iron, qu artz and kaolin . At depth these were succeeded by chalcopyrite, bornite and chalcocite, with mispickel and considerable quantities of chlorit e an d fluorspar , but tinoxide was not found . In neighbouring min es tin-ore occurred, especially th ose lying east of the cross-course t hat dro ppe d th e lodes 1 0 0 fel t east . The Cara don Mines were worked to a depth of 2 5 0 fat homs. The group of min es sit uated between Kelly Bra y and Dartmoor has produced enormous quantities of the ores of tin and copper, and also of arsenic and wolfram. Th e zonal sequence was not able at Gunn islake Clitters, where copper-ore decreased at about 2 7 5 fathoms, and also at Hingston Down, where th e lod es also underlie north and bear tin at depth. At Devon Gr eat Consols, four lodes underlying south wer e worked to a tot al depth of 2 2 0 fathoms below adit , and were everyw here rich in ores of copper. Cassit erit e, however , ha s not been met with. The Main Lode cont ains copp er-ores in the central part , with mispickel on bot h walls an d a gangue of peach and fluorspar.

TH E MI NERAL

ZO ~ E S

OF CORNW ALL.

Over 7 40,000 tons of copper-ores have been raised from this mine al one. In th e Bedford United Mines copper-pyrites and mispickel affect th e centre of th e lode, while tin and wolfram-ores lie next t o the walls, with a gan gue of quartz with some fluor and cap els of tourmaline and tinst one. Finally, on the west ern borders of Dartmoor, at Marytavy, th e Wheal Friend ship lodes show ed a similar zonal sequence. The lode contents near th e surface consisted largely of earthy iron-ore and qu ar tz, below this there was mu ch chalcopyrit e, iron-pyrites and arsenical p yrites with some chlorite, and at t he r r z-fath orn lev el cassiteri te m ad e it s appearance. Th e mine has been work ed t o a depth of 240 fathoms. North of the Dartmoor granite th e ores of copper form an interesting occurrenc e at St icklepath ; they are scattered throu ghout a mass of allochroite garn et rock , and are associa ted with axinite, actinolit e, ars enical pyrites and qu artz. This rock resem bles the "spelter " of Levant. From these descriptions one feels justified in inferring that the zonal distribution from above downwards of ores of iron, zinc, copper and tin is a fact , and that th e appar en t exceptions, as at East Pool and South Crofty, must have an ex plana tion. The distribution of the ores of wolfram , arsenic lead, silver and antimony will now be considered , an d th eir place in th e zonal sequ ence ascert ained. WOLFRA~1.

The modes of occurrence of wolfram in Cornw all present perh aps th e most instru ct ive instan ces of an y among th e orebodies of the relationship of th ose bodies to the gra nite magmas. For wolfram occurs as an origin al constit uent of th e pegmatite vei ns, possibly , th ough doubtfully , of th e granite it self, and commonly in th e greisened gra nite . Most habitually , however, it forms a constituent of t he sulph-arsenide gro up , and less frequently of some of t he tin veins. It may occur either in lodes in t he gra nite or in th e killas, and also forms a cons tituent of the sto ckwork s in killas at Mulberr y and at Great Beam in kaolinised granite . At Hemerdon Tor it is conspicuous in greisen veins that lie in kaolinised granite, and there the rela tive dates of format ion of th e severa l min eral bodies may be studied . Th e emanations that first mo dified the granite were those holding the wolfram, tin and t ourmaline and grea t qu an t ities of silica. After the original granite had been convert ed in to a network of vein s of greisen, th e un altered part s between th e meshes were attacked by other vapo urs that converted th em int o kaolin. At Great Beam th e granite has a similar histor y , but th e extent and dep th of the kaolinised gran it e rat her obscur es the greisen veining. The relat ive temperat ur es of th e formati on of wolfra m an d

H. DEWEY,

tin ores appear generally to have been that cassiterite crystallised out at higher temperatures than wolfram, for the zone of wolfram terminates at many fathoms above the base of the tin zone. But elsewhere crystals of cassiterite are observed to have grown on the large platy masses of wolfram and are (for these instances) of later formation. * Further, at Frementor, Bedford Consols, the wolfram formed a lining to fissures on which cassiterite afterwards grew. If one regards the emanation as a solution in which both cassiterite and wolfram are solutes, it is obvious that at certain temperatures there would be contemporaneity of crystallisation and even overlap, especially as cassiterite has a wider range of crystallisation temperature than wolfram. That some lodes were infilled with emanations that deposited only wolfram is seen at Castle-an-Dinas, and that others deposited cassiterite but no wolfram is also familiar to miners-the instance of the Main Lode at Do1coath may be cited. Another explanation is that the emanation belonged to successive periods of infilling, as seen in most lodes: such emanations representing first increasing and later steadily diminishing temperatures. Some of the wolfram pegmatites pass into quartz veins. At Kit Hill such a transition is seen, where the wolfram and felspar gradually disappear and leave only quartz, with sporadic occurrences of molybdenite. In the active mines near Pool wolfram is a usual constituent of the north-dipping lodes, but is found less frequently in those dipping south. It is associated with mispickel and cassiterite and locally with fluorspar and copper pyrites. Fluorspar in these mines, however, rarely occurs with tinstone. At Tincroft the rich wolfram zone extended down from the 80 to the 170 fathom level. In the neighbouring South Crofty Mine its downward extension was much greater, but it could not be depended upon below 245 fathoms, whereas at New Cook's Kitchen it formed rich ground between the 260 and the 290 fathom levels. The south-dipping Pryce's or Engine Lode carried copper-ore to a depth of 100 fathoms and cassiterite below that, but no wolfram. Thus, at the same level, one lode bears wolfram and copper and the other only cassiterite. In East Pool much of the Great Lode consisted of wolfram pegmatite, and yielded the ore between the 140 and the 300 fathom levels. The Middle Lode at 240 fathoms consisted of large microcline felspar crystals, quartz, tourmaline and wolfram. The frequency of the occurrence of copper,wolfram and arsenic in lodes dipping north, and of cassiterite in those with a southward dip, is continued in Wheal Busy, Poldice and Killifreth, and in the mines near Tavistock, but in the Dimson Mine wolfram was found in a north and south cross-course. ·Unless they were carried 'up as idiomorphic crystals from a lower layer..

THE MINERAL ZONES OF CORNWALL.

IIS

Perhaps the finest instances of wolfram pegmatite veins are those near Buttern Hill at the northern end of the Bodmin Moor granite mass. Large microcline felspar crystals up to 3 in. in length are inextricably intergrown with massive wolfram and quartz, while tourmaline is frequent and apatite also occurs. There are numerous veins of this composition over the north part of the granite which, by weathering, have yielded the alluvial wolfram worked profitably in the wide tracts of marshland of that area. Wolfram is also associated with mispickel and copper-ores in quartz veins at a few mines of this district. At Castle-an-Dinas a remarkable lode, consisting entirely of quartz and wolfram, traverses a hill of tourmalinised killas : at the base of this hill pegmatitic granite veins break across the cleaved slate and are kaolinised. In the neighbouring mines cassiterite occurs without wolfram. All are genetically related to the St. Austell granite mass, which is the most completely kaolinised granite in Cornwall. Further north the killas is mineralised throughout a wide tract with networks of tinstone and, rarely, of wolfram, while the kaolinised granite at Great Beam shows similar stockworks of the two minerals. Wofram is intimately associated with arsenical pyrites; it is a miner's axiom that" where wolfram is found there will be arsenic also," and also that" arsenic and (can) fluorspar cannot live together." MISPICKEL. Mispickel, the principal arsenic-bearing mineral throws similar light upon the zonal problem. Mispickel is by no means present in all lodes, but shows a marked preference for some minerals. and an antipathy to others, and, as already stated, it is a miner's maxim that fluor and arsenic cannot live together. It is only rarely that they have been found, and in such cases they were probably deposited from emanations of two different periods. traversing the same fissure. The range of temperature of formation as represented by depth zones is well marked at South Crofty and Tincroft. Results of ore-dressing in these mines bring out most clearly the quantitative development of mispickel, as the following figures show : From the New Cook's Kitchin Lode the following assayvalues for refined arsenic were obtained: From the surface down to the 80 fathom level 10 to 16 per cent. ; between the 80 fathom and the 100 fathom level not in payable quantities; between the 100 fathom and the 136 fathom level 4 per cent., and below the 136 fathom level hardly any arsenic. At Tineroft, arsenic is found more plentifully in the killas than in the granite, and follows copper and wolfram rather than tin-ores, but it tends to die out at depth gradually. The mispickel in the higher parts of the mine

n6

H. DEWEY,

is more coarsely crystalline than that in the lower. The assayvalues of the concentrates show that down to the 140 fathom level there is 14 per cent. of white arsenic present; from the 120 fathom to the 170 fathom about 6 per cent., but below that level only traces of arsenic are found. Fluorspar is seldom found with arsenical ores in these mines. ZINC AND LEAD ORES. The evidence of zoning derived from the study of the lodes formerly worked for the ores of zinc and of silver-lead can be summarized as follows : There are three principal areas of lead-bearing lodes, which are situated respectively north of Truro, between the St. Austell and the Bodmin granite masses and south of Calstock. Fields of minor importance are the St. Kew and Endellion and the Christow groups of mines. It has been found that the lead lodes never occur in the granite, and seldom in the metamorphosed rocks adjacent to it or in the basic sills and dykes, but in relatively unaltered sediments and in lava or ash. The relation of the country-rock to lead-ore has been exhibited in many of the mines. There is a significant association of lead-ore with argillaceous rocks; lodes coursing through interfolded beds of siliceous and argillaceous composition are rich in the latter and barren in the former. This phenomenon was well illustrated at the Chiverton Mines, where rich ore-ground was confined to soft shales; the lode, on coursing through sandstone, became barren, only the gangue minerals occurring, but where the lode entered another group of shales it at once became rich again. Similar conditions obtained at East Wheal Rose. At Beer Alston in Devon, the lodes while in shale were rich in lead-ore, but became barren near the granite. The lead-ore was found to deteriorate at depth and finally to die out. In some lodes it was gradually replaced by ores of copper, and where this occurred the miners regarded the advent of the copper as an extremely bad omen, and experience proved that their fears were justified. At points of intersection of east and west lodes by others coursing northward there was a notable enrichment of. the lead-ore, while ores of copper became argentiferous, The quantity of silver contained in such copper-ores has been estimated by J. H. Collins at 281 million ounces, and the proportion of silver at 21 ounces to the ton (5). The depth to which the ores of lead have been worked in the several districts of Cornwall and Devon does not greatly vary, and averaged about LtO fathoms; it was apparently not appreciably affected by proximity to granite. Near the Godolphin granite the mines are less than 100 fathoms deep. Between Truro and Newquay they lie about midway between the granite masses of Carn Mcnellis and St. Stephen's, but do not attain a

THE MINERAL ZONES OF CORNWALL.

II7

greater depth than 140 fathoms. At Wheal Mary Ann, four miles from the Bodmin granite, lead-ore was worked to a depth of 300 fathoms, the maximum depth for the West of England, which probably represents almost the total thickness of the lead zone. North-east of the same granite the greatest depth is lIO fathoms, while near the small bosses at Kit Hill the lodes were worked at Holmbush to a depth of 170 fathoms, at Beer Alston to 150 fathoms, and at Wheal Betsy to 140 fathoms. East of Dartmoor the Christow lodes became too poor to work at 145 fathoms. Throughout Cornwall the galena was highly argentiferous, and although no mines were worked for silver alone, ores of silver and the native metal have been found in many of the lead and copper lodes. The principal minerals are argentite, pyrargyrite or ruby-silver, cerargyrite or horn silver and the native metal. Silver-ores have been found down to 120 fathoms, mainly at about 20 fathoms, and frequently in the cross-courses. They seldom form long or deep shoots, and tend to be replaced by galena at depth. They are richest where two lodes intersect, and rich shoots were found in "slides" and "flucans." The amount of silver in the galena is variable, and the association of the two metals appears to be a mechanical mixture, and probably due to secondary enrichment. Where rich silver-ore was met in a lode bearing galena, the latter was always poor in silver. Variation with depth does not appear to have been constant, for in some lodes the galena near the surface was richer in silver than that at greater depths, while in other lodes the reverse condition obtained. The richest ores occurred in the Chiverton Mines, where they carried from 40 to 50 ozs. of silver to the ton of lead, and in the Herodsfoot-Ludcott groups, where the proportion rose to 55 ozs. The ores poorest in silver were those of the Cargoll and East Wheal Rose Mines, in which the proportion fel; to as little as 9 ozs. to the ton. The average for the whole of Cornwall between 1851 and 1882 amounted to 42 ozs. per ton. Collins estimated that 15 million ozs. of silver have been recovered from Cornish ores (6). There are very marked differences in the character and development of the gangue minerals in the lead lodes. At the Chiverton Mines quartz was the principal mineral, no fluorspar or barytes having been observed. The lodes at the Herodsfoot group contained some beautiful gossan minerals, including tetrahedrite, but fluorspar was rare, whereas in the neighbouring mines of Wheal Mary Ann, Trelawney and others there was an abundance of fluorspar, as well as barytes and chalcedonic silica. Barytes had a limited distribution and, except for a few pseudomorphs, died out at a depth of 40 fathoms. Some of the crystals of fluorite and barytes from this mine measure upwards of a third of a cubic foot. At the Great Retallack

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Mine, actinolite formed part of the gangue material, in which galena was developed in large crystals. In this lode the upper parts consisted of carbonates of iron with hsematits passing downwards into blende, and below the 6o-fathom level copper-ore came in. A similar change from blen de to copper-ore was observed at Wheal Alfred. In the Herodsfoot lode a remarkable intergrowth of crystalline and chalcedonic silica with chalybite and barytes was seen; tetrahedrite occurred here, but no fluorspar was found. But in the lodes at Mary Ann, Trelawney and adjacent mines fluorspar was abundant, and usually of whitish-brown colour. Barytes had often crystallized upon the fluorspar cubes and many of the galena crystals are studded with brilliant little doubly terminated pyramids of quartz. At Wheal Trelawney the downward distribution of the minerals was well marked. In the gossan there were masses of cerussite and lead phosphate; immediately below it quartz and fluorspar composed the gangue in which galena occurred. At still greater depths large quantities of iron pyrites and smaller amounts of blen de accompanied galena, and at depth copper pyrites with silver. Foster was fortunate enough to be able to examine a lode just before this mine closed, where the sequence of mineral deposition was clearly exhibited in the following order :-Quartz, galena, chalybite, and calcite. There were two periods of deposition of fluorspar, the galena crystallizing between the two and the above noted succession after the last growth of fluorspar. Evidences of secondary enrichment in the s i I v e r - I e a dan d z inc I 0 des. In the dump material at the West Chiverton Mine there are many examples of partly dissolved crystals of galena and blen de and also of complete negative crystals of both minerals. The former percolation of the ground-water must have led to secondary enrichment at depth; the water draining from the dumps shows on analysis appreciable quantities of lead and zinc. At Wheal Trelawney, extensive dumps, consisting largely of fluorspar and barytes with sporadic crystals of galena, are locally cemented into an intensely hard mass by a secondary growth of lead carbonate. The surfaces of the fluorspar debris are thus completely coated with a matt of cerussite crystals by which they are attached to other fragments. The gossan at Herodsfoot contained some beautiful crystals of the carbonate and phosphate of lead. ANTIMONY. The principal district where antimony ores are found lies on the north coast in the parishes of St. Minver, St. Endellion, St. Kew and St. Teath. The ore occurs there in slate and pillow

TH E MIN ERAL ZONE S OF CORNW ALL.

lava. Mixtures of jam esonite and stibnite are associa ted with ar gentiferous galena, and pa ss at depth into pure galena. The g angue consists of qu artz. At Trevinnick th e antimony-lode carries jamesonite, stibnite, antimony-ochre and also argentiferou s and auriferous st ibnit e. In th e gossan th ere are mas ses of pyromorphite and cerussite lining cavities form erly occupied by galena . Th e lodes course both from north t o south and fr om east to w est . Th e former carry stibnite and th e latter ja mesonite. Mispickel occurs in qu antity at Treore with native gold amounting t o from I to 3 dwt . to th e t on. Th e lodes are far removed from the granit e and the areas in whi ch th ey occur form out er frin ges of th e lead-silver zone. IRON-ORES. Th e principal sour ce of iron-ore was the Great P erran Lod e, which extends from th e coast north of Perranporth inland for a di stance of four miles. Nearly all th e other lodes lie in the country between S1. Aust ell and Wadebridge, i .e., in th e part of Corn wall tha t is farthest removed from th e gra nit e masses both horizontally and vertically . Both the anhy drous oxides of iron , magnetite and hsematite occur, and also th e carbonate or spat hic are. Magnetite form s a constituent of basic igneous rocks where it has become segregated into len ticular masses, as at Bot allack, Trebarw ith and Tintagel. It is associated at Treluswell with cassiterite and a t South Terras with ur anium ore. Massive hsematit e occur s in lodes traversing th e granite at Knighton, and at Ruby near St. Aust ell, and also in the great Restorm el Lode. Spathic iron is th e principal are of the Great P erra n Lode and also of the Pawt on Lode, and is found at many other localiti es. Near the surface it has been converted into limon ite. Copper pyrites, fluorspar, galena and blende are frequ ently found with it and the assemblage points to their derivation fr om graniti c ema nations. Th e country-roc k is usually the slaty killas. Most of t he lodes t rend in th e directi on of the principal cross-courses, nam ely, from north to sou th , but several trend in other direct ions-the princip al iron-lode, the Great P err an Lode, cour sing east and west. In this resp ect th e iron-lodes resemble th ose a t the silverlead ores. For the most part they were infilled at a later date than the tin-copper lodes, but ar e related to the same great period of earth movement. Nearly all of the lodes belong t o the upper zones of th e ore-bodies, and many may cont ain only such minerals as were dr iven off at temperatures to o low to carry tin and copper, while the zone of tin and copper lies too far below them to have

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repaid mining cost s a t th e pric es th en ruling. The lodes belong mainly t o th e radi al group. At the H erodsfoot Mine an interest ing intergrowth of qu artz and chaly bite is seen.

SECONDARY ENRICHMENT. T he dis ti nguishing charac ters of zonal development hav e been masked by: t hree gro ups of ph enomen.a: (1) Secondary enrichment of pn mary ores; (2) renewed fillmgs by later emanations ; and (3) th e effect of th e coun t ry -rock up on the ore deposit ed. Secondar y enr ichmen t is a natural proc ess that has been obser ved at nearl y all workabl e deposit s of cop per-ores throughout the world. The primary ores, usually coppe r pyrites, are leached by circulating atmosph eric water that becomes concentrated with copper salts, which on descent int o favourable places ar e deposit ed. The copper sulphide is dissolved in regions where sulphat es or chlorides occur and is precipitated from such acid waters by reaction upon the underlying sulphides where oxygen is excluded. Thi s process lead s to a vertical zonal distribution, the oxides and carbonates lying in th e leached zone and the ores of commercial import ance, cha lcocite, bornite, and chalcopyri te, in the zone beneath, whil e a third zone consists of unaltered sulphide. Such enrichment can oft en be obs erved in polish ed sp ecimens of ore, wh ere chalcopyrite can be seen to have been converted into bornite, bornite into covellite, and covellite into chalcocite. Chemically the change represents an increasing proportion of copper to sulphur, CuFeSz being converted into CU3Fe S3, that into CuS, an d finally by further abstraction of sulphur into CuzS. In th ese reactions the percentage of copp er in th e ore is increased from 34.5 per cent. to 79.8 per cen t (7). In the Bull er, Fran ces and Basset group of mine s th e zonal enrichment was well exhibite d. From adi t lev el t o 2 0 fathoms there was mostly oxidised ore ; from 20 to 50 fa th oms redruthite, vitreous ore and red ox ide occurred in ab un da nce; from 5 ~ to 70 fathoms only copper pyrites ; below the 70-fathom level th e lodes were too poor to work for copper. Such phenomena were noted by many observers during th e time that th e Caradon group of mine s, near Liskeard, were bein g worked. At South Caradon near th e surface the lod es contained much limonite with melanconite, cuprite, malachite, chessylite and native copper, while at lower levels chalcopyrite, bornite and chalcocit e appeared . Similar changes have been observed 'at many other min es. It is a change of paramount importance to the miner as the promise of a continued increase of richness at depth may deceive him into supposing the reserves to be very

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much greater than they are. Instances of the application of this knowledge to the economic working of a copper mine have recently been described in the United States (8). The solubility of copper sulphide has been put to use in many old adits and mine dumps. At the Devon Great Consols Mines there are extensive dumps, which were recently estimated to contain three-quarters of a million tons of material and assayed on an average one half per cent. of metallic copper. In the old mines there are 40 miles of levels. The drainage waters from above adit level and from the dumps are carried through launders, in which scrap-iron is placed. The copper is in the form of sulphate which, on reaching the iron, is converted into iron-sulphate and carried away in solution, while copper is precipitated. The mine waters on analysis show a maximum of 30 grains of copper per gallon, but the proportion varies with season and rainfall. The recovery of metallic copper has averaged for many years 45 tons per annum. Similar processes are adopted at the Caradon mines. On the beach at Perranporth water from the adit of the old St. George mine carries sulphate of copper in solution. It has formed a cement of green carbonate around the grains of sand at the old tailing dumps, which, described as " greens," were formerly worked. The famous copper mine of Parys Mountain in Anglesey now derives its principal output from water draining the old dumps of waste. Measurements of the daily flow are taken which show an average of Ii million gallons per day of 24 hours. The copper content equals from 3 to 4 ozs. to 2,240 lbs. of water. It has been found that the quantity of copper dissolved in the mine waters is greatest during the winter months on account of greater rainfall, but floods decrease the yield of copper. Lead and zinc occur in appreciable quantities in the water draining from the dumps at the Chiverton Mines. The effect of such solutions is beautifully shown at the Cathedral Stope at Aberllyn Mine near Llanrwst. A huge excavation was left where the lode had been stoped away about 40 years ago. Since the mine was abandoned a growth of hydrozincite measuring up to i-inch in thickness has covered the walls and the old mine timbers, while long corrugated stalactites hang from the roof. For the most part this deposit is glistening white, but locally is stained yellow, brown and red by oxides of iron and forms a beautiful sight when strong lights are thrown upon it. The influence of the country-rock upon the kind of are deposited has often been described. It is an obscure subject, and many of the effects ascribed to it are probably due to other causes. W. Jory Henwood devoted much attention to the subject, and published many details in his classic work (9). All observers agree that where a lode crosses rocks of different com-

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position its mineral contents change, as where a lode bears tinstone in granite and copper-ore in slate or greenstone. It is not an invariable rule as the reverse order obtained at Wheal Vor, De la Beche states that the slates most favourable for copperore are those of fine grain and light blue colour; Carne agrees with him, while Hunt remarks that clay-slate of a pale greyishyellow colour and in a state of decomposition accompanies the richer copper lodes. It appears that where lodes lie in igneous rock the principal copper-ore is the vitreous variety or chalcocite. It was observed by Henwood to occur in the greenstone at Levant, Botallack, Wheal Trenwith and the Providence Mines; also at Great Work, Wheal Vor, and Wheal Trannackin granite. At Tresavean the copper-ore is confined to the granite. At Wheal Jewel and East Wheal Damsel and also at Gunnislake, the vitreous ore was found only where the lodes were in the granite. Henwood states that the general law seems to be that a change of rock is characterised by an instant and corresponding alteration in both the metalliferous and the earthy minerals, of the lodes. He instances the occurrence at numerous mines, of copper ores in the sediments and greenstones, and of tin-ore in the granite, but points out that at Wheal Vor the lode was productive of tin-ore while in slate, but became worthless in the granite. At Botallack one of the lodes passes no fewer than three times from granite into slate, and at every change in the granite it yielded tm-ores only, but wherever it traverses the slate its metallic product was wholly vitreous copper-ore. From the number of exceptions to the rule of copper-ore in slate and tin-ore in granite, another determining factor has to be sought for; this may be the condition of temperature and pressure. Heat affects argillaceous rocks differently from those of siliceous composition; the former shrink on being heated, while the latter expand, and the fissures are accordingly wider in the slate. Instances of such a change have already been described at the Chiverton Mines (see p. n6). THE DISTRIBUTION OF THE LODES IN CORNWALL. Plate 12 shows in a generalized way the direction of the principal lode-groups in Cornwall, and from it the most significant fact appears that the general direction follows that of the land mass itself, curving round from south-south-west to south-west, and finally to west. This general lode-trend is crossed by other series which intersect the main lodes nearly at right angles, and over the whole arc are radially arranged. These radial lodes appear to have been formed after those of the main group and to have shifted the main lodes where they intersect.

THE MINERAL ZONES OF CORNWALL.

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Detailed study of a larger map reveals an even more important fact, and that is, that among the lo.ies of the main group itself some hade in directions opposite to those assumed by others, and these lodes are frequently adjacent to one another. This phenomenon is especially well seen in the neighbourhood of the active mines at Illogan, where many lodes underlie northwards, while an important number underlie southwards. It has long been known that the lodes hading to the north have been faulted by those hading in the opposite direction, and are therefore of an earlier date of formation. We have it on the authority of Dr. M. Maclaren that the mineral contents of the two series are different, especially as regards the relative proportions of their tin, wolfram, and arsenic ores. This is a fact of great moment and will be discussed later; meanwhile, from still further study of the lodes, the conclusion is drawn that they have not all had a similar history. S r J. Flett describes a series of lode-fillings from the Illogan district in his paper mentioned below (10). As a result of his petrographical investigations he arrived at several important conclusions with regard to the history of the lodes, which he summarises as follows : 1. Opening of the fissures. z. Infiltration of quartz, tinstone, and tourmaline. 3. Brecciation of the veinstuff by movement. 4. Formation of the peach by infiltration of much fine blue tourmaline, with tinstone and quartz. 5. The peach was shattered by renewed earth-movement. 6. The broken mass was healed up by the deposition of fine quartz with tourmaline and tinstone. 7. Renewed formation of fissures, this time on a smaller scale and not attended by severe detrition of the materials filling the veins. 8. The fissures were filled with fine quartz almost free from tourmaline. Copper pyrites occurs in these last veins, and may have been introduced along with the quartz. There is no evidence that tinstone was deposited at this period. The order in which the minerals crystallised was apparently tourmaline, tinstone and fluorspar, and as they are always surrounded by quartz it may be inferred that quartz was the last mineral deposited. The specimens examined came from Dolcoath at 375 fathoms level; from the Flat Lode in Wheal Basset at the 240 fathoms level, and from Carn Brea. It will be seen that they were taken from the tin-zone of this district, and the occurrence of the coppergroup would therefore not be expected. Mr. Davison, in his paper referred to below (11), found, as a result of detailed microscopic examination of veinstones, that the PROC. GEOL. Assoc., VOL. XXXVI., Part 2, 1925.

9

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H. DEWEY,

order of arrival of th e minerals is not always th e sa me, but the following is th e general successio n ;1. Quartz, mi ca, tourmalin e, topaz, cassiterite, wolfram, stannine and molybdenit e. 2 . Qua rtz, chlorite, fluor, chalcopy rite, mispickel, pyrite, and blende. 3. Lead and silver ores. 4. Secondary minerals. Now Dr. M. Macla ren pointed to the difference in the mineral cont ents of the lodes th at dip northwards from those th at fault th em and dip in th e opposite direction . The north -dippin g lodes are cha rac te rised by th e sulphide, arsenide and t ungstate asse mblage, whereas the latter at t he sam e level a re infilled with the tourrnaline-tinst one gro up. From the fact that th ese are later than the north-dipping lodes, and that the tin stone-bearing solutions were crystallising at th e same level as the other group 1HI.d crystallised, it can be inferr ed that the second period emanati ons were hotter, and th at th e country-rock was at a higher t emperature than it was at th e time when the north-dipping lodes were infilled. This secon d infilling led to the mixing of the two zones. In many lodes there is evidence of rep eated movement s, brecciation and subsequent infillings with changes of compos ition of both th e gangue a nd t he met alliferous minerals. From the throw of t hese faults, which at East Pool amou nt to as much as 1 2 0 fa th oms, the ex tent of th e earth-mov ement may be sur mised . The directions of lod es and cross-courses were grouped by W . J ory Henwood as follows, and are indicated in degrees on P late 1 2 : St. Just .. 35 S. of E . St. Ives . . 8 S. of E. Mar azion . . I N. of E . Gwinea r .. 2 S. of E . Camborne 20 N . of E. 22 N. of E . R edruth .. St . Agnes .. 22 N . of E . St. Austell 13 N. of E . 18 N . of E. Cara don .. Tavistock 9 N . of E. The inclination or dip was also calculated by Henwood, and the results are given in th e following table, the inclinati on being measured from the horizont al : Cross-courses 80°; Copper veins 70° ; Tin veins 60°. The lat er-formed veins th erefore are near er to the vertical than th e others . The north-dip ping lodes for the most part dip awa y from the granit e, whil e the oppos ite direction is assum ed by the south-dippers. Eman ati ons had , therefore, a longer course in the north-dipping lodes before th e killas was ente red, an d the resulting zones were at differen t levels

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from those developed in the south-dipping series which arise in the granite near the killas, and were therefore at a higher temperature. From this evidence the inference may be drawn that the several minerals have been developed at depths depending upon their temperatures and pressures of crystallisation, and probably upon other causes only remotely indicated, such as the physicochemical interaction of gas and magma. The emanations, consisting of the constituents of water, fluorine, boron, silica, and perhaps chlorine, attacked previously formed mineral masses and converted them into new substances as in the wellknown instances of tourmalinisation, greisening and kaolinisation. With this assemblage tin associates itself. Tourmaline (both brown and blue) is frequently its partner, but that boron is not necessary for tin-development is proved by the absence of tourmaline in the later tin deposits of Nigeria (12) and at Bundi, F.M.S. (13). Mixed with these fluorine emanations were the sulph-arsenides and the tungstates which could not crystallise at the high temperatures and pressures of the fluorine group, but which, on reaching the zone of suitable conditions, at once re-arranged themselves to form first wolfram and then, in order, the mixed sulphides of copper, iron, zinc, silver and lead. Carbon dioxide had its activity restrained until it had reached still cooler regions, when it formed mixed carbonates; in Cornwall, principally that of iron. But it also helped in the assault on the potash felspar of the granite and carried away the potash as carbonate. There is, however, evidence that the emanations of different periods differed in composition from each other, and that temperature and pressure in the fissures were also different at each period of infilling. This led to mineral changes, as, for instance, the earlier-formed tourmaline crystals were squat and brown, while those of later date are blue and hairlike; the earlier quartz contains numerous gas and liquid inclusions, while-the later quartz is free from such. * It has long been known that the gases mentioned, among others, normally e .cape from volcanoes, fumaroles and solfataras, and many observers have ascertained their temperature of emission. Fluorine appears among those that arise from the hottest parts, while sulphur and arsenic come next, and :finally carbon dioxide, the temperatures being about 575°C., and from 550°e. to 500°C. respectively. What then was the temperature of the country-rocks at that time? Wright and Larsen found that quartz alters its form above 800°e., that pegmatites crystallise out at 575°e., and that vein-quartzes crystallise below 575°e. (14). Here then we have glimpses of the temperatures at which • See Flett, Op, cit. and Crenshaw, Bull. I net, Mining and Metallurgy, 1"0.20.,1921.

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these arc-bodies formed. and such temp eratures ma y be accepted tcntativoly. * Where the two gro up s of halogen and sulph -arsenides lie side by side in adjacent lodes th e probable explanat ion is th at th e tin group lies above its normal position, and is due to another phase of emis sion when th e temperature had risen in th e country-rock.

METHOD OF FORMATION OF THE LODE-MINERALS. Th e method by which the fissures became infilled with th e lode-minerals has long form ed a subject of controversy, and several hypotheses have been advanced to explain it. Elie de Beaumont and Daubree early in the last century ascribed the origin of the tin-lodes to the action of gases arising from a cooling granite magma, and demon strated that the suggested reactions are chemically possible. Thi s proc ess has been studied by oth er observers, and especially by Prof. J. H. L. Vogt (15), who has term ed it pneumato-hydatogenesis, The common occurrence of ft.uorine-bearing minerals, such as tourmaline, ft.uorspar, topaz a nd apati te, in both the lodes and th e country-rock, pointed to ft.uorine as a " carrier" of the heavy metals in a state of vapour. The temperature of such vapours is unknown, but, from what has been already stated, is probably not lower than 800°C., which would be above the critical point, unless the emanations consisted of unknown chemical compounds with critic al points above 800°e. Th e country-rock frequently presents th e appear anc e of havin g been soak ed through with "emanations "t which att acked th e original min eral s and converted th em into other subst ances. The alterati on increases in compl et eness as th e fissures are approached, but where these are crowded together, as in many greisens, the whole original granite-substance has been metamorphosed. Where th e fissures are wider, veins or lodes form and th e country-rock is less alt ered. But it is difficult to believe th at any fissure could remain "open " for th e length of a mile upw ard s from the granite, and, judging from th e manner in which the mineralising material occurs , it appears to have been forced, under great pressure, into every crevice and cra ck in the wallrock. Thus the walls in th e tin -zone consist of schorl rock, composed chiefly of tourmaline and quartz. Such material has been computed by J. H . Collins to contain 2,578,000 ton s of fluorine, and 2,500,000 tons of boron, while cassiterite did not exceed 21,25° tons (16) per each yard in depth in the 2 00
THE :\U XE RAL ZONE S OF COR NWALL.

I 27

. Such tourmalinised rock has been found in both gra nite and kill as and Its for mation was certainly controlled more by th e t emperature than by th e nature of th e country- rock. The t em perature and pr essur e ca nnot be determined, but observations u pon th e temp erature of lav as and th eir gaseous emissions have been ma de by man y geologists . Suess (17) found th at the dr y v apours contained tin-ore. and boron, fluorine. tungst en , and uran ium-compounds. while at a lat er period strongly alkaline m agmat ic wate rs were given off, an d to these are attributable th e suiph-ars enide group of coppe r, lead and zinc. Finally th e carbo nic wat ers deposit ed carbona tes . The absence of tourmalin e from some of the tin-lodes of N igeria and th e F ederated Malay States ha s alread y been noti ced. In recent years Mr. Morrow Campbell was confronted with the fact th at the "pneum atolyti c min erals" are ab sent in T av oy, a nd thi s led him to reconsid er th e pneumatolytic th eory and to r eject it . He found th at t ungsten is soluble in the form of silicotungsti c acid and its alk alin e salts and that a soluble form of stannic acid is also known . From th ese facts he was led t o infer th at highly siliceou s water und er pr essure and at a modera te t em perature (say 300°C.) can carry both tungst en and tin in solution , and th at atmospheric wa te rs gained access to subterran ean granite, and th ere com bined with silica and the met als, a nd aft erwa rds welled up a nd deposit ed wolfram and tin-oxide. B y the study of polished specimens he found th e order in which the several ores were genera ted was :- molybdenite, wolfram, tin , bismuth, chalcopy rite, ars enopyri te , pyrrhot ite, galena , blend e. Not all of th ese were seen in an yone specimen, but by th e examination of man y sa mples th e successio n was built up. The whole sequence was probably not form ed by th e same infilling, but by seve ra l solutions at falling temperatures. . Th ere is no need to invoke th e aid of atmospheric waters as Gaut ier's expe riments have demonstrat ed th at a cubic kilomet er of gr an it e can yield 2 6 ,4 0 0 ,0 0 0 metric tons of water . and th a t h ydrogen on combus tion would yield another 4 ,26 6 .0 0 0 tons (18). Spurr has recently argued th at th e emana tions must have p ossessed a specific gravi ty higher th an th e country -rock, becau se .. horses " of all sizes ar e found in num erous lodes completely su rr ounded and deeply corroded by th e vein-filling. Th is is an experience of all wh o hav e examined lodes. There is pr obably much truth in all th ese hypotheses, and the emana tions may have consisted of various substances which changed state as temp erature and pressure fell. THICKNESS OF THE MINERAL ZONES. In Cornwall th ere is some evidence of the original thi ckne ss of the several min eral zones. Exact measurements cannot , of course, be mad e, and the mineral s. cha racte ristic by th eir abun-

128

H. DEvYEY,

dance in a particular zone, can, and often do, extend upward beyond the limits assigned to it into ground occupied by the minerals of the higher zone. Thus cassiterite occurs more or less in small quantities through the zone of copper, but except where the sulphides have been dissolved away leaving a spongy gossan specked with tin-ore, it would not pay to separate the mineral. At Dolcoath the rich cassiterite zone was entered at a depth of 190 fathoms below adit-level at Williams's Shaft, and was bottomed at 550 fathoms, thus prov.ng a thickness of 360 fathoms for this zone. This may be tentatively accepted as the general thickness. The zone of copper-ores can also be approximately measured. At the Tresavean Mine it was absent in the kill as, but commenced at a depth of 27 fathoms below the adit and was passed through at 345 fathoms, thus indicating a total thickness of 318 fathoms. Between the zone of tin and that of copper upwards of 20 fathoms. of poor ground containing the mixed ores occurred at both Dolcoath and Tresavean. The thickness of the zone of silver-lead ore can be closely calculated at Wheal Mary Ann, near Liskeard. Many feet of the upper part of the lodes were barren, but below this poor ground the ore came in and persisted to a depth of 300 fathoms, where it was so rare and so much mixed with copper as to be valueless. This figure, 300 fathoms, may therefore be accepted as the approximate thickness of the lead-zone. By combining the three zones the total thickness of more than 1,000 fathoms, or 6,000 ft., is obtained; this is, however, under the probable original thickness. The zone of antimony attains a thickness of perhaps 200 ft., while iron approaches 400 ft. In the following diagram an attempt has been made to represent the possible thicknesses of the zones as they were originally deposited. Knowledge of the temperatures ·of formation is so meagre that it is unsafe to use it except with great caution, and the figures are given as the merest approximations. But in spite of its obvious imperfections the diagram helps one to visualise the conditions under which the several zones and their gangue minerals may have formed. The distribution of tungstate and arsenide minerals is only approximate, and is based mainly upon the knowledge gained in the mines around Illogan, which, however, are typical Cornish mines. The sulphides are more definite, but even in this group iron-pyrites extends downwards to great depth and upwards to the surface. Fluorspar avoids the arsenide and tourmaline zones, and calcite as a veinstone is rare in Cornwall. Wolfram appears in two zones; the lower in the pegmatites, and the upper in the sulphide zone. The total thickness of the original mineralised zones could have been hardly less than It miles. This figure closely agrees with the thickness of sundry occurrences noted by Spurr (19).

TH E MINERAL ZONES OF CORNWALL.

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Thicknesaes

M ineral Zones

or gan~ue minera e,

In

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etc)

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Apprc»cimate

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400

I!SO

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200

Zone rtF 8ulP.hideo of lead and /Stille r; givinA pisco at depth to zinc.

' 8 00

400

Zone of' 8ulphldea of copper in ter mixed with wolfrem and tin for 700 feet.

2500

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9

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F IG. 7 . -DI AGRAM SHOWING THE VERTICAL DIST RUIU TION O F MINERAL ZONES IN CeRNW ALL.

I3 0

H. DEWEY, DISTRIBUTIO)/ OF THE LODES.

Thelimited distribution of the ore-material in lodes is indicated in any assay plan, and the comparison of records of output shows the geographical distribution of are to vary within narrow limits. Fig. 8 is drawn to illustrate the output of tin, copper and lead in Cornwall. From this map it will be noticed that a mineralised tract extends from the Land's End to eastern Cornwall; it is not uniformly rich, however, but consists of groups of enriched areas separated by barren stretches. The map also shows that by far the greater part of the ores raised in Cornwall have been obtained westwards of the St. Austell granite mass. This fact is further illustrated by the following figures of output grouped from west to east under the granite-districts in which the mines occur. The figures are in thousands of tons and are taken from MacAlister (20) and from Collins (21). Land's IGOdotPh' .1 Cam Brea. End. In etc.

Copper Tin

593

St. Agnes.

St. Austell.

Caradon. Tavistock

112

372

650

223

26

32

3°

18

8

117

85

37

3°0

Silver-lead

The totals are as follows :Western Cornwall. Tons. % of total. Copper Tin " Silver-lead

6,132,000 49 2,000 117. 0 00

88 95 49

Eastern Cornwall. Tons. % of total. 873.000 26,000 12'2,000

12 5 51

It will thus be seen that Western Cornwall has produced the bulk of the tin-ore; by far the larger part of the copper-ore, but not quite half of the silver-lead. Iron-ores are not included. but most have been contributed by Eastern Cornwall; of wolfram and arsenical pyrites the west has yielded the major part. The division between the two areas lies at the tract where the county rapidly widens out, which corresponds with the outcrop of the higher beds of the Devonian rocks. This area is where the effects of the Armorican movement are still best preserved, and also where the general level of the country suddenly rises. The maximum heights serve to indicate the general and relative elevations. Yes Tor and its neighbours rise to well over 2,000 ft. above Ordnance Datum; Brown Willy and Rough Tor arc each

THE MINERAL ZONES OF CORNWALL.

131

ever 1,200 ft., and much of the Bodmin Moor over 1,000 ft. South-westwards of this moor the land for the most part lies below the level of the 430 ft. platform, although isolated tors and earns rise well above that level. Eastwards the land mass averages approximately 600 ft. of greater elevation than that of the west. Has this land mass consisting mainly of Upper Devonian and Carboniferous rocks and granite, had any effect upon the mineral output? The diagram on p. 129 indicates that the mineral-zones above that of tin attain a thickness of over 4,000 ft. In 'Western Cornwall most of the land above the copper-zone has been removed by denudation, and no rocks of Upper Devonian or Carboniferous age remain. The lead and iron zones may have been about 2,000 ft. thick, and it is mainly this zone that has been worn away. The two figures approach one another, and the great removal of land in the west appear~ to have r, ndered more accessible to mining operations the zones of copper and tin, and hence led to the greatly increased outputs. Folds and faults have, however, often brought down the higher zones to lower levels, as, for instance, at the Great Perran Iron lode. From the foregoing evidence the vertical zones are apparent, and from fig. 8 the horizontal distribution can be traced. Tin and copper mines cluster around the granite masses of Western Cornwall, but eastwards of Scorrier and St. Day rapidly decrease in number, while a curved tract running parallel with their eastern boundary, and extending to the coast south of Newquay, has yielded zinc and lead ores in abundance. Flanking this zone lies another in which the principal iron mines were worked. The iron-lodes consist of oxides near the surface, but pass downwards into carbonate often mixed with manganese carbonate, and constitute a zone about 400 ft. deep, below which lead and zinc ores were met with. Another group of iron-lodes coursing northwards lies between the granite of St. Austell and that of Bodmin Moor; it is flanked on the north by a zone of antimony and silver-lead. The best defined zone of silver-lead ore is that lying between the copper and tin zones of St. Austell and Caradon respectively, forming the area near Liskeard, and bending eastwards to unite with the other lead-zone, south of Calstock. On fig. 8 the output of the mines is represented according to the parish to which they belong, and as has already been pointed out, the mineral ground lies in or adjacent to the granite masses. But perhaps the most notable fact in connection with the distribution of the lodes is the linear form of the mineralised tract. It commences in the Land's End granite and extends thence north-eastwards for a distance of more than 100 miles to the eastern side of Dartmoor. It is a narrow tract, being seldom more than 10 miles wide, and it cuts indifferently through

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THE MINERAL ZONES OF CORNWALL.

133

rocks of the most various ages and composition, from pre-Cambrian to Upper Carboniferous, and of both igneous and sedimentary origin. Neither structure nor topography affects its course, while within then arrow belt the lodes trend in all directions, and were formed at different periods. A rough parallelism from the centre to the margins of the tract is observed, the tin and copper ores lying centrally, while those of zinc, silver-lead, antimony and iron successively approach the margins. The mJ.p represents this tract as a straight line; in reality it forms a gentle curving arc, though slightly flexured near the subsidiary granite-bosses, generally following the trend of the land-mass, and therefore the direction of the Caledonian axis of fclding. Western Cornwall reveals the effect of the Caledonian folding, but east of St. Austell the Armorican fold lines cross it nearly at right angles without, however, causing any deviation of the main trend of the mineral belt. This direction appears to mark the position of a subterranean line of weakness, which has persisted throughout geological time, along which periodic movements have affected the crust, and up which successive emissions of magma have risen. The last migration of magma carried with it, from the deeper metalliferous zone, the emanations bearing the tin, copper and other compounds. This tract or belt, although of small dimensions, is perhaps the most clearly exposed instance in Europe of a world-wide phenomenon. Spurr (22) draws attention to the great silver-belt of North and South America, which extends for a distance of 6,000 miles due north and south, and seems to be unaffected by the upper layers of the earth's crust. He also remarks upon the similar zone of copper-ores and of the Missouri lead belts. His work exemplifies the remarkably local distribution of the ore-bodies and his wide experience of mines has led him to recognize the genetic relationship of members of families of are minerals to the different group~ of igneous rocks. It, moreover, appears that such a relationship is independent of time, for rocks of a given composition give rise to a characteristic group of ore-bodies, whether they belong to the pre-Cambrian, the Tertiary, or any other period. The minerals that belong most exclusively to the acid rocks appear to be those of tin, tungsten and molybdenum, where as platinum, nickel and cobalt are associated with the magnesian or basic magmas. Ores of copper, silver-lead and zinc are more catholic in their choice, and arise from both acid and basic rocks, while the intermediate rocks appear to be more closely connected with gold. But not every magma carries with it such metallic associates and their occurrence appears to suggest the rupture of a zone lying below the magmas.

1:34

H. DEWEY,

CONCLUSION. If this study of the mineral zones stopped here it would remain merely of academic interest, but fortunately, I think, practical results may proceed from its application in the pursuit of new ore-grounds. If the inferences drawn in this paper be correct they should act as a guide in the search for unworked tin-zones in Cornwall for future exploitation. The economic working of any mine depends upon the price ruling for the commodity raised and on the cost of raising it. Working costs increase as the mine is deepened and it therefore appears that the areas of Cornwall where tin-ore may still be found, but at depth, are not at present likely to be worked. Several areas formerly worked for copper-ore have not yielded tin-ore at the lowest levels, but should the zonal sequence be of general application such tin-zones should occur at depth. The richest copper-area was that around 51. Day and Gwennap. The copper-zone appears to have been bottomed, but the tin-zone was not proved. In an adjacent area, but genetically dependent upon another granite mass, the courage of the mine owners has led to success. I speak of the Tresavean Mine. The copper-zone was followed downwards into its impoverished region where cassiterite made its appearance; this poor zone was sunk through and at 395 fathoms the zone of rich tin-ore was entered. It is a good indication that the zon..l development still holds and leads one to hope that the great area where the United and Amalgamated Mines formerly raised copper may be underlain by tin at depth. Other areas are immediately suggested by the study of former output, but each one requires special consideration, for faults may bring together zones belonging to different depths, and thus the copper-zone may have been brought side by side with that of tin. Finally, all who have the interest of Cornwall at heart must have learned with satisfaction of the success attending the venture of the East Pool miners, who, on the strength of geological reasoning, boldly abandoned their old workings and sunk shafts in virgin ground, to find their anticipations were justified by the discovery of the rich lodes they had expected to reach.

1. 2.

3. 4-

5.

REFERENCES. Trans. Rov, Geol, Soc., Cornwall, vol. v ., 1843. Centrall . Min. Ceol. U. Pal., 1903, p. 705. The Isomorphism and Thermal Properties of the Feldspars. Carnegie Institute. 1905. p. 45. Mining Mag., vol. xvi., 1917, p. 248. [ourn, Roy. Lnst, Cornwall, vol. xvi., 1904, pp. I03-II9.

THE PERMIAN FORMATION IN DURHAM.

135

6. 7.

Trans. Roy. Ceol. Soc. Cornwall, vol. xii., 1903, pp. 715-717. See W. H. EMMONS, " The Enrichment of Ore Deposits," Bull. 625, United States Geol, Suru., 1917. 8. "The Copper Deposits of Ray and Miami, Arizona," Prof. Paper I IS, U.S.A. Geol, Surv., 1919. g. Trans. Roy. Geol, Soc., Cornwall, vol. V., 1843. 10. "Summary of Progress for 1902," Mem. Geol . Suru., 1903. Appendix II. II. Geoi. Mag., 1921, p. 511. 12. J. D. FALCONER, Abs. Proc, Ceol. Soc., No. II26, p. 18, 1924. 13. J. B. SCRIVENOR, "The Origin of Tin Deposits," Perak Chamber of Mines. 1909. 14. SPURR," Ore Magmas," vol. i., 1923, pp. 79-80. IS. Trans. American I nst, Mining Engineers, vol. xxxi., 1902, p. 125.. 16. "The West of England Mining Region," p. 332. 17. Geog; [ourn., vol. xx., 1902, p. 517. 18. "Data of Geochemistry," jrd edit., 1916, p. 214; see also CRONSHAW, o». cit. 19. "Ore Magmas," pp. 420-422. 20. "Summary of Progress for 1906," Mem. Geol. Suru., 1907. Ap. II. 21. "West of England Mining Region," 1912. Trans. Roy. Geot: Soc. Although they do not represent the entire Cornwall, vol. xiv. output between 1815 and 1905 they serve to illustrate the point under discussion, and, if complete. would still further emphasise the difference between the two areas. 22. "Ore Magmas," vol. i., chap. ix., 1923.

THE PERMIAN FORMATION IN DURHAM. c.

THE County of

T. TRECHMANN, D.Se., F.G.S. RUH~. Mard. !'i,Jr., 19'5.

Durham, together with the extreme southeast corner of Northumberland, may be regarded as the classical district in England for the study of these rocks. It comprises the area where the calcareous dolomitic facies of the Permian is most extensive and thickest; and the fossils which formed the subject of the early memoirs and most important investigations on the fauna were nearly all obtained from this district. Hitherto the production of a map of the whole Durham area with the Permian divided into its separate beds, has not been attempted. Dr. D. Woolacott* published a map showing his interpretation of the various beds in the northern area from Hawthorn dene northwards to South Shields. I am indebted to this m lp for many of the boundaries in the northern area of Durham, but I have made some alterations to them. For the main boundaries of the Marl Slate and Yellow Sands and the outliers of the Permian I am indebted to the r-inch drift Geolosical Survey maps of the district. The map herewith presented is a drift map and shows only tue outcrops of the beds, but the boundaries, as they are surmised to pass beneath the drift or elsewhere where uncertain, arc indicated by interrupted lines. • Proceediwgs UHiversity Durln,m Pbitos. SJciety, vol, iv., part S, 1912, p. 285. fig. S.