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© F.Coakley ,
2001
[From Lamplugh, Geology of IoM, 1903]
While the lodes which have been mined to commercial advantage in the Isle of Man are only two in number, viz. that of Foxdale and that of Laxey, it has been demonstrated by the numerous other workings that ores of lead, zinc, iron and copper are present in a minor in veins in almost every part of the Manx Slate Series. These veins have in all cases the character of infilled fissures which break across the stratification of the rocks. Relative displacement of the walls of these fissures has probably occurred in most cases, but usually only to a slight amount; and it is exceptional to find indications of considerable faulting along them. The principal vein is generally accompanied by rudely parallel veins and branches which are sometimes in themselves metalliferous, the whole forming a more or less complex group. Occasionally they are broken and displaced by later transverse faults, as will subsequently be described. Almost invariably they are inclined at angles of from 5° to 30° or more degrees from the vertical this "hade " varying in amount as the vein is followed downward, but usually constant in direction. In rare instances the " hade" is temporarily reversed, a change considered by the miners to be unfavourable to the productiveness of the lode.
The veins are subject to great and sudden variations in breadth and in mineral contents, the metalliferous deposits rarely form more than a small proportion of their infilling, the greater part of the fissure being occupied by crystalline quartz, calcite, dolomite, with sometimes a little barytes, flour-spar, etc., and by breccia and decomposed material derived from the walls. Gas- or water-filled cantles known as "lochs" or "vughs" are likewise frequent. The valuable ores sometimes occur in definite ribs in the vein-stuff, and sometimes in disseminated crystalline grains, or in both forms combined. It is clear that the crystalline constituents, both metalliferous and non-metalliferous, have been slowly deposited in open cavities. In some cases, mostly in the Foxdale Lode, there are indications of movement of the walls after partial infilling of the fissure, causing portions of the vein-stuff to be displaced and brecciated. The fissures have been found to extend vertically, with or without ore, as deeply as the deepest mining works have gone, viz., not far short of 2,000 feet below the present surface at Mondale and at Laxey; laterally, the Foxdale lode in one or another of its branches has been traced almost continuously for 2½ miles, and the Laxey Lode for over a mile; but in most cases the veins have been found to split up or otherwise become indistinguishable within much shorter distances.
The main lode-system at Foxdale has a nearly east-and-west course, and the smaller veins at the old Cornelly Mine a mile north of Foxdale, and at the Bishops Barony Mine three miles east of Foxdale, have also this direction; but at Laxey, Snaefell, Ballacorkish, Bradda, and in fact at almost all the other workings from which ore has been obtained the direction of the lode has been approximately north and south, or more strictly, a little (5°—30°) to the west of north and east of south. In only one or two unimportant instances has any ore been found in N.W. and W.N.W. veins; while apparently not a single case of a productive north-easterly vein is known, although this is the direction of strike of the rock-masses and of innumerable quartz-veins accompanying planes of cleavage, crushing and fracture (see pp. 86-7). It is true that the predilection of the miners for north-and-south veins has led to these being tested in far greater numbers than those in any other direction, and that this selection may in some degree have affected the result; but the extent to which veins of every kind have been cross-cut in underground workings on the productive lodes is sufficient to prove that the occurrence of the metalliferous ores in other than the recognised directions must be extremely rare.
The north-and-south veins are rather frequently dislocated by normal faults, known to the miners as ~slides,' which usually strike about E.20°N.—W.20°S., or approximately at right angles to the metalliferous vein. Several of these have been observed in the Laxey Mine (see p. 521) and others at Ballacorkish (p. 527), Snaefell (p. 535) and one or two other places (pp. 528, 532). The amount of vertical displacement which they represent is as a rule small; but if the disturbances by which the Laxey vein is lost southward, and by which the Ballacorkish vein is broken at the "Great Douk Lode," be due to faulting, these cases may be of great extent. It is not clear whether the north-and-south lodes received their metalliferous infilling before or after these transverse dislocations took place; with one supposed exception (p. 522) the 'slides' have never been found to contain ore, but some of the facts at Laxey suggest that the principal deposition of the metals took place there subsequent to the faulting of the lode (p. 522).
At Foxdale, while the chief productive lode strikes east and west, this intersects north-and-south metalliferous veins both in the central portion of its course, and farther west at Beckwith's Mine; and in the latter case the E.—W. lode is said to have thrown the N.—S. lode at the intersection (p. 505), thus playing the role of a 'slide'. In the Foxdale Mines slickensided surfaces are abundant, and the striations are generally nearly horizontal, showing that some degree of lateral movement had taken place along the line of the lode; moreover, the metalliferous vein-stuff is in places broken up, sometimes into partially rounded blocks, and recemented by undisturbed material subsequently introduced, showing that there has been movement along the fissure after an interval of quiescence.
It seems possible that the later movement may have taken place here at the time of the cross-faulting of the N.—S. lodes in other districts, and that the Foxdale vein was affected along, and not across its course because of its E.—W. direction
A point of importance in regard to the position of the lodes is, that all the larger and more productive, including Bradda, Ballacorkish, Foxdale, Laxey, and Maughold Head, occur on or in the vicinity of the structural axis of the Manx slates near where the dominant dips of the folded strata and of the cleavage form an anticline. As elsewhere shown (p. 118), this axis is probably the centre of a synclinorium of the slates as a whole; but it is remarkable that in many mining districts abroad, e.g., in Nova Scotia and Queensland l a close association of metalliferous deposits with anticlines of folded strata has been observed, though not in veins of the Manx character.
While from the limited range of Manx stratigraphy direct evidence as to the period at which the metalliferous fissures were formed is not forthcoming, we possess sufficient data to show that it must have been comparatively late in the geological history of the Island. It was certainly later than all the PreCarboniferous earth-movements and dyke-injections described in a previous chapter (p. 71-2), by which the Slate Series was packed into folds, brecciated, foreshortened by overthrusts, and interpenetrated by basic and afterwards by granitic intrusions.
The segregation-vein of quartz and other minerals which were formed so abundantly during the later stages of these movements contain no metals of economic value; and the metalliferous fissures have been cleanly gashed through rocks in which all the above indications of earth-movement are present, and are clearly subsequent. The presence of copper pyrites in a vein in the Carboniferous Basement Conglomerate at Langness (p. 538), and of galena in a similar vein in the Carboniferous Limestone at Castletown (p. 537), brings down the date of, at any rate, some of the metalliferous deposits into Post-Carboniferous times. The only 'solid' rocks newer than Carboniferous accessible to observation above sea-level in the Isle of Man are the intrusive dykes of olivine dolerite, which, on grounds already discussed (chap. VIII, p. 327), are believed to be of Tertiary age. The study of the relationship of these dykes to the lodes has led me to the unexpected conclusion that, although the fissures were in existence before the date of these intrusions, some part of their metalliferous filling was of later date. The grounds for this conclusion are fully stated in the descriptive details of the Bradda, Ballacorkish, Langness, East Foxdale and other mines and will here be only briefly recapitulated.
At North Bradda, Ballacorkish, and Langness there is evidence to show that dolerite dykes, following the usual north-westerly course, have been diverted northward for a short space on intersecting the fissures. In the first-mentioned place, where the lode is of great size and clearly revealed in the cliff, it is seen that the dyke is intrusive into the vein. But at Ballacorkish and East Foxdale, portions of the metalliferous lodes have been found in the underground workings to intersect the igneous rock (see Smyth's reports quoted on pp. 515 and 534), and the ores (sulphides) lie alongside the margin of the dyke-rock in positions which they could not have occupied before or during the injection of the molten matter; and similarly at Langness, where the ore slightly interpenetrates the dolerite. Besides the places above mentioned, smaller quantities of the metalliferous ores have been found in the vicinity of olivine-dolerite dykes at Kerroo-mooar (p. 546), Glen Auldyn (p. 545), Maughold Head (p. 541), and Castletown Harbour (p. 537); but in all except the last case no evidence is now available as to the exact relationship.
It is of course possible that scattered ores already in existence in the fissures may have been displaced and concentrated into larger bodies by the invasion of the dykes; but it seems more likely that the deep-seated channels which permitted the upward egress of the molten rock may also have served, at a somewhat later stage of the same period of thermal activity, as conduits for the vapours and waters which supplied the crystalline infilling to the reopened fissures.2
It does not follow, however, that the whole of the Manx metalliferous veins are of this age; for it is only in a few instances, and these not of the first importance, that the connection between the dolerite and the ores has been observed; and, moreover, the majority of the olivine-dolerite dykes are not known to be accompanied by ore-deposits. All that can be learned from the evidence is that where the dykes of this late age are in contact with the metalliferous accumulations, the latter are the newer. As no association of olivine-dolerite dykes with ore deposits appears to be known in Western Scotland and Northern Ireland where these intrusions are so numerous, there must be some additional factor in the local conditions of the Isle of Man which has favoured the production of the metals in the veins. What this may be has not yet been discovered.
The outcropping of the metalliferous veins and of the dolerite dykes at the present surface shows how extensive must have been the erosion in the area during later Tertiary times, as the infilling of the fissures in both cases can only have occurred at come considerable depth.
I was not aware until after the above passages were written that my colleague, Mr. J. G. Goodchild, had some time ago reached a similar conclusion in regard to the metalliferous veins of another district by a different chain of reasoning. The following quotation from his suggestive paper entitled "Some Observations upon the Mode of Occurrence and the Genesis of the Metalliferous Deposits,'3 will show that the Manx evidence is in close agreement with his results.
" As bearing upon the age of the North of England lead-veins one or two points remain to be considered. The faults wherein the veins occur probably date back in many cases to very remote geological periods. As zones of disturbance and of dislocation many of them certainly may be referred to periods long anterior to the date of the rocks they now affect at the surface. There is plenty of evidence to show that, as zones of weakness, they have acted as faults again and again at various periods since. The fault breccias are scrunched and slickensided in a manner that points to the repeated exertion of powerful mechanical forces in times past. In remarkable contrast to this evidence of powerful grinding and crushing is that afforded by the crystalline contents of the veins. There may be, in a few instances, some kind of evidence of these vein-minerals having been disturbed since they were formed; but as a rule the evidence tends to show that since the date of formation of these minerals there has been no vertical displacement of the opposite cheeks of the fault. Large masses of crystals occur without the slightest sign of any bream during their growth from first to last, and the crystalline faces of many of the minerals, such as Fluor, Baryte, Clalena and others are just as perfect as when the minerals were first formed. The significance of this very common feature of mineral veins does not seem to have been generally perceived."
" Another equally well-known feature in connection with mineral veins calls for remark here. This is the ' comby' structure of lodes, and the evidence of the deposition of vein minerals in successive layers. This clearly points to the fact that during the filling of the vein there were occasional interruptions of deposition, which were contemporaneous with more or less lateral disruption of the veins and their contents. After the formation of the earliest stages of the comby structure there has been no displacement along the plane of the fault."
" Such a condition of things, extending over a large area, can only have obtained under conditions of upheaval. This seems to indicate that the introduction of the lead ores of the North of England took place at, or about the close of, the last period of upheaval. This, by independent reasoning, I have inferred to be contemporaneous with the last manifestations of volcanic energy in Tertiary times; and all the evidence yet brought forward seems uniformly to support that conclusion" (pp. 56, 57).
In many other parts of the world it has been proved that certain metalliferous veins have received their infilling at comparatively late periods in geological history and in association with the intrusion of igneous rocks. If we take, for example, Part III. (Economic Geology) of the 18th Annual Report of the U.S. Geological Survey, which came to hand while this chapter was m preparation, we find an account (p. 69) of the celebrated Treadwell-Mexican Gold Mine on Douglas Island, Alaska,by G. F. Becker, in which it is stated that the mineralisation of the lode is probably connected with the intrusion into it of a vein of analcite-basalt, the author concluding that " the basalt was injected after the mineralisation began, but before it had ceased "; he adds, " so far as I know this is the first known case of an association of analcite-basalt with ore generation." The age of the intrusion is not definitely established, but is supposed to be either Late Mesozoic or Tertiary.
Moreover, in the same volume three out of the four other reports on mining districts, in widely separated parts of the Western States, embody the conclusion that the metalliferous infilling of the veins must be assigned to periods of eruptive activity in Tertiary (or in one case possibly Cretaceous) times: these are " On the Judith Mountains of Montana," by W. H. Weed and L. V. Pirsson t veins not older than early Tertiary, . 463); " On the mining districts of the Idaho Basin," by W. Lindgren (veins probably Cretaceous or early Tertiary, p. 631); and " On the Telluride Quadrangle, Colorado," by C. W. Purington (veins not older than Late Tertiary, p. 825).
The Manx metalliferous veins are developed principally in that portion of the Manx Slate Series which is intermediate in character between the grits on the one hand and the clay-slates on the other. Their usual matrix is the bluish-grey thin-bedded flags or flashy slates which consist of a more or less intimate admixture of argillaceous and fine-grained sandy or silty material. In no case has a productive lode been found among the quartz veined grits ("Agneash and other Grits," of published map); and where belts of this character have been encountered in underground workings, as at North Laxey and in the northern extremity of Great Laxey, the vein has been found to contract or become ' tight,' and to lose its value. On the other hand the homogeneous dark-blue " Barrule Slates " have been found equally barren; and where bands of this composition have been Sassed through, as at Snaefell and other places, the lode has become soft ('douky') and shattered, splitting up into small branches and losing its individuality. At Foxdale the main E. and W. lode, and also one of the N. and S. cross-courses (Magee's), has been followed downward from the slate into granite, and has been found equally as productive in the one rock as in the other, though the quality of the galena has shown a remarkable variation, that in the slate being richer in silver than that in the granite.
In this locality the opinion, alluded to by Cummings4, is commonly held that the metalliferous vein owes its riches to the granitic intrusion. But it is clear not only that the granite was consolidated, but also that its offshoots affected by post consolidation movements (p. 316) long before the lode was formed. Similarly at Laxey, the lode cuts across elvans given off from the Dhoon Granite, in which incipient cleavage had been produced by earth-movement subsequent to their consolidation(p. 52] ); and at the old Cornelly Mine, a mile N. of Foxdale, where massive granite was found beneath the slate (p. 517), the lode traversed both rocks. At Bradda, Ballacorkish, Snaefell, and North Laxey considerable ore bodies occur in the slate-rocks where there is no reason to suspect the proximity of granitic intrusions; and the highly productive western portion of the Foxdale main lode (Beckwith's Mine, p. 504), lies amid slates nearly 2 miles beyond the proved underground extension of the granite. The rich vein at Laxey, again, is about a mile distant from the nearest outcrop of the Dhoon Granite; while other veins in close proximity to that intrusion have proved barren or nearly so (p. 528). Hence the whole of the facts tell against the supposition that the metalliferous deposits were a direct result of the granitic eruptions.
In an indirect manner, however, it is possible that the Foxdale Granite, long after its consolidation, may have favoured the production of the lode, by affording a massive homogeneous rockbasement, in which a fissure produced by unequal earth-movement might descend to great depths and remain sufficiently open to permit the ascent along it of the vapours and solutions from which the empty spaces received their infilling. The broad shelving laccolitic outline of this granite (p. 166) would be more favourable to the production of such a fissure than would be the case in the Dhoon Granite, which appears to descend as an irregular pipe-take core with steep walls (p. 144).
The same explanation may be applied to the distribution of the veins in the sedimentary rocks. Where these were of a character to break cleanly to considerable depths, and to preserve an open space when fissured, as in the case of the firmly packed and welded mass of Loner flags enclosing the Laxey lode productive metalliferous veins have been formed: but where the rocks were, like the Agneash Grits, of such a character that regular fissures could not readily be produced in them; or, like the Barrule Slates, where if produced, such fissures would be filled with the broken debris of the walls, the conditions were unfavourable either for the percolation or deposition of the ore-bearing solutions, and the veins are consequently scanty and of no economic value. This explanation seems adequate for the known occurrences of ore in the Island; but fails to account for the fact that there are large tracts of the firm slate flags apparently equally favourable for the production of Ore-conditions, in which only inconsiderable quantities of ore have yet been discovered. Here again some additional factor in the deep seated structure, to which we have no clue, is probably involved.
On this point but little information has been obtained. The lead and zinc ores (sulphides) are usually found together, but the relative quantities are extremely variable; so that while at Laxey zinc-blende is by far the most abundant ore, at Foxdale the quantity present is too insignificant to be recognised commercially. Copper, in the form of pyrites, generally occurs, in very thin strings or finely disseminated around the outskirts of the other ore-bodies, as at Laxey, where its appearance in the southward drivings heralded the deterioration, of the lode (see p. 520); at Foxdale it is rarely found and in very small quantities (see Smyth's report in 1883 quoted on p. 511). Pyrites and chalybite are more or less present in all the lodes, the latter sometimes in large bodies at Mondale.
The hematite ore of the Maughold veins, as suggested on p. 291, may have been introduced into the lode when the slates were overlapped by Triassic strata.
Lists of the minerals of the individual lodes and particulars respecting the rarer varieties are given in the subsequent detailed descriptions of the mines.
1 Victoria Government Blue-book, Dept. of Mines, Reports on the Bendigo Gold Field, by E.J. Dunn, pp 9-13 (Melbourne 1896)
2 Sir W. W. Smyth's description of the quiet and undisturbed condition of the delicate fibres of Plumosite in the Foxdale lode, quoted on p. 503
3 Proc. Geologists' Assoc., vol. xi (1888-9), pp. 45-68.
4 " Isle of Man," p. 308.
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