The Naturalist in Nicaragua, by Thomas Belt

Chapter 6.

Configuration of the ground at Santo Domingo. Excavation of valleys. Geology of the district. Decomposition of the rocks. Gold-mining. Auriferous quartz veins. Mode of occurrence of the gold. Lodes richer next the surface than at lower depths. Excavation and reduction of the ore. Extraction of the gold. “Mantos”. Origin of mineral veins: their connection with intrusions of Plutonic rocks.

THERE is scarcely any level land around Santo Domingo, but in every direction a succession of hills and valleys. The hills are not isolated; they run in irregular ranges, having mostly an east and west direction, but with many modifications in their trend. From the main valleys numerous auxiliary ones cut deeply into the ranges, and bifurcate again and again, like the branches of a tree, forming channels for carrying off the great quantity of water that falls in these rainy forests. The branching valleys, all leading into main ones, and these into the rivers, have been excavated by subaerial agency, and almost entirely by the action of running water. It is the system that best effects the drainage of the country, and has been caused by that drainage.

The wearing out of valleys near Santo Domingo proceeds more rapidly than in regions where less rain falls, and where the rocks are not so soft and decomposed. Even during the few years I was in Nicaragua there were some modifications of the surface effected; I saw the commencement of new valleys, and the widening and lengthening of others, caused not only by the gradual denudation of the surface, but by landslips, some of which occur every wet season.

The rocks of the district are dolerytes, with bands and protrusions of hard greenstones. The decomposition of the dolerytes is very great, and extends from the tops of the hills to a depth (as proved in the mines), of at least two hundred feet. Next the surface they are often as soft as alluvial clay, and may be cut with a spade. This decomposition of the rocks near the surface prevails in many parts of tropical America, and is principally, if not always, confined to the forest regions. It has been ascribed, and probably with reason, to the percolation through the rocks of rain-water charged with a little acid from the decomposing vegetation. If this be so, the great depth to which it has reached tells of the immense antiquity of the forests.

Gold-mining at Santo Domingo is confined almost entirely to auriferous quartz lodes, no alluvial deposits having been found that will pay for working. The lodes run east and west, and are nearly perpendicular, sometimes dipping a little to the north, sometimes a little to the south, and near the surface, generally turning over towards the face of the hill through which they cut. The trend of the main ranges, also nearly east and west, is probably due to the direction of the outcrops of the lodes which have resisted the action of the elements better than the soft dolerytes. The quartz veins now form the crests of many of the ranges, but are everywhere cut through by the lateral valleys. The beds of doleryte lie at low angles, through which the quartz veins cut nearly vertically. Excepting that they are very irregular in thickness, and often branch and send thin offshoots into the enclosing rocks, they resemble coal seams that have been turned up on edge, so as to be vertical instead of horizontal. They run for a great distance. Near Santo Domingo they had been traced for two miles in length, and probably they extend much further. They are what are called fissure-veins, owing their origin to cracks or fractures in the rocks that have been filled up with mineral substances through chemical, thermal, aqueous, or plutonic agencies. In depth, the bottom of fissure-veins has never been reached, and taking into consideration the deep-seated forces required to produce fissures of such great length and regularity, we may safely assume that they run for miles deep into the earth — that their extension vertically is as great as it is horizontally. The possibility that they extend to immense depths is increased when we reflect that mineral veins occur in parallel groups that run with great regularity for hundreds of miles; and further by the fact that, in all the changes of the earth’s surface, by which deep-seated rocks have been brought up and exposed by denudation, no instance is known of the bottom of a fissure-vein having been brought by such movements within the reach of man.

The gold-mines of Santo Domingo are in veins or loads of auriferous quartz that run parallel to each other, and are so numerous that across a band more than a mile in width one may be found every fifty yards. All that have been worked vary greatly in thickness; sometimes within a hundred yards a lode will thicken out from one to seventeen feet. Their auriferous contents vary still more than their width. The richest ore, worth from one to four ounces per ton, occurs in irregular patches and bands very small in comparison with the bulk of the ore stuff, which varies in value from two to seven pennyweights per ton. The average value of all the ore treated by the Chontales Mining Company, up to the end of 1871, has been about seven pennyweights per ton, and during that time small patches have been met with worth one hundred ounces of gold per ton. The gold does not occur pure, but is a natural alloy of gold and silver, containing about three parts of the former to one of the latter. Besides this metallic alloy (to which, for brevity, I shall, in the remarks I have to make, give its common designation of gold), the quartz lodes contain sulphide of silver, peroxide of manganese, peroxide of iron, sulphides of iron and copper, and occasionally ores of lead.

The quartz is generally very friable, full of drusy cavities, and broken up into innumerable small pieces that are often coloured black by the peroxide of manganese. The gold is in minute grains, and generally distributed loosely amongst the quartz. Pieces as large as a pin’s head are rare, and specimens of quartz showing the gold in it are seldom met with, even in the richest portions of a lode. The fine gold-dust can, however, easily be detected by washing portions of the lode-stuff in a horn. The quartz and clay is washed away, and the gold-dust sinks to the bottom, and is retained in the horn. This is the usual way in which a lode is tested by the mining agents, and long practice has made them very expert in valuing the ore by the wash in the “spoon.” Although most of the gold occurs loose, amongst the soft portions of the lode, the hard quartz also contains it disseminated in minute grains throughout. These can be obtained in the horn by pounding the quartz to powder and then washing it.

Machinery of Chontales Gold-Mining Company.

One feature in the distribution of gold in the quartz lodes of Santo Domingo led to a most exaggerated opinion of their value when they were first mined by English companies. On the hills, near the outcrops of the lodes, the ore was in some places exceedingly rich. One thousand ounces of gold were obtained from a small patch of ore near the surface of the Consuelo lode, and at Santo Domingo, San Benito, San Antonio, and Javali lodes, very rich ore was also discovered within a few fathoms of the surface. When, however, these deposits were followed downwards, they invariably got poorer, and at one hundred feet from the surface, no very rich ore had been met with. Below that, when the works are prosecuted still deeper, there does not appear to be any further progressive deterioration in the value of the ore, and it varies in yield from two to seven pennyweights of gold per ton, upon which yield further depth does not seem to have any effect. The cause of these rich deposits near the surface does not appear to me to be that the lodes originally, before they were exposed by denudation, contained more gold in their upper portions than below, but to be the effect of the decomposition and wearing down of the higher parts, and the concentration of the gold they contained in the lode below that worn away. We have seen that in the decomposed parts of the lode the gold exists in loose fine grains. During the wet season water percolates freely from the surface down through the lodes, and the gold set free by the decomposition of the ore at the surface must be carried down into it, so that in the course of ages, during the gradual degradation and wearing away of the surface, there has, I believe, been an accumulation of the loose gold in the upper parts of the lodes from parts that originally stood much higher, and have now been worn away by the action of the elements.

This accumulation of loose gold near the surface of auriferous veins, set at liberty from its matrix by the decomposition of the ore, and concentrated by degradation, is probably the reason of the great richness of many of what are called the “caps” of quartz veins; that is, the parts next the existing surface, and has also, perhaps, originated the belief that auriferous lodes deteriorate in value in depth. I at one time, after having studied the auriferous quartz veins of Australia, advocated this theory, which was first insisted upon by Sir R.I. Murchison, but further experience in North Wales, Nova Scotia, Brazil, and Central America has led me to doubt its correctness, excepting in cases such as we have been considering, where there has been an accumulation of gold in the superficial portions of lodes since their original formation. Gold is distributed in quartz veins in bands, and in patches of richer stone of more or less extent. These richer portions of the lodes, if sunk upon perpendicularly, will be passed through, but so also they would be if followed horizontally, their extent in one direction being as great as it is in the other. The chances of meeting with further patches of rich ore in depth, after one has been passed through, are about the same as they are in driving horizontally, and the frequency therefore with which the auriferous ores are met with along the surface will, as a rule, be an index of their occurrence in depth, if we be careful in distinguishing deposits belonging to the original condition of the lodes, and those due to subsequent concentration. To do this we must get below the immediate surface, and take as our guide the gold occurring in the solid undecomposed quartz, and not the loose grains contained in the fissures and cavities.

Section of Mine Showing Method of Extracting the Ore.

Section of Gold Mine.

Diagram showing method of excavating ore at Santo Domingo Mines. A, Levels. B, Rise, down which the ore is thrown. D, Stopes. C, Stopes refilled with clay and barren rock. Lowest level, Tramway to Stamps.

The lodes of Santo Domingo are worked by means of levels driven from near the bottoms of the valleys that intersect them. When these levels have entered sufficiently far into the hills, shafts are driven upwards from them to the surface, and other levels driven sixty feet higher than the first. This process is continued until the lode lying above the lowest level has been divided off into horizontal bands, each about sixty feet in depth. The quartz is then excavated above the topmost level, and thrown down the shafts to the lowest, where it is received into waggons and conveyed to the reduction works. As both the ore and the enclosing rocks are greatly decomposed and very soft, the whole of the ground has to be securely timbered as the work proceeds. The levels are timbered with “nispera,” a wood of great durability and strength, but the excavated portions between them are only temporarily secured with common soft wood, and at the end of every fortnight filled up with clay and barren rock. The mining is entirely executed by native workmen, principally Mestizos from the border lands of Honduras and Nicaragua, where they have been engaged in silver-mining. They are paid according to the amount of ground excavated, and are very industrious when poor; but when they accumulate a little money, they take fits of idleness and dissipation until it is spent.

The ore is taken down to the reduction works in waggons that run down by gravitation, and are drawn up by mules. It is then stamped to powder by iron beaters, each of which is lifted by cams, and let fall seventy times per minute. The stamped ore, in the form of fine sand, is carried by a stream of water over inclined copper plates covered with mercury, with which is mixed a little metallic sodium. Nearly the whole of the free gold is caught by the mercury, for which it has a great affinity, and accumulates as amalgam on the copper plates, from which it is cleaned off every twelve hours. The sand and water then pass over inclined tables covered with blankets, the fibres of which intercept particles of gold and mercury that have escaped from the first process, and afterwards into a concentrating box, where the coarsest grains of sand and the sulphurets of iron, copper, and silver are caught, and with the sand from the blankets re-treated in arrastres. These arrastres are round troughs, twelve feet in diameter, paved with stones. Four large stones of quartz are dragged round and round in this trough, and grind the coarse sand to fine powder. The gold liberated sinks into the crevices in the stone pavement, a little mercury being put into the trough to form it into amalgam. The arrastres and all the amalgamating apparatus is cleaned up once a month. The amalgam obtained is squeezed through thin dressed skins, and is then of the consistence of stiff putty, and of a silver colour. These balls of amalgam are placed in iron retorts, and the mercury driven off by heat and condensed again in water. The balls of gold so obtained are then melted into bars weighing about one hundred ounces each, and in that state sent to England. At Santo Domingo about two thousand tons of ore are treated monthly, and the whole cost of treatment, including all charges for mining, carriage, reduction, amalgamation, and management, is only about eight shillings per ton. The loss of mercury is about twenty pounds for every thousand tons of ore treated; the smallness of the loss in comparison with that of many other gold-extracting establishments being greatly due to the employment of sodium in the amalgamating process. The loss of mercury usually occurring in amalgamation work is principally caused by its mineralisation, and sodium has such an intense affinity for oxygen and sulphur, that it reduces the mercury to its metallic form again, and prevents its being carried off in light mineralised flakes and powder.

Section across San Antonio Lode.

Section across San Antonio Lode.

A, Lode. B, Decomposed doleryte. C, Surface soil. D. Quartz rocks in surface soil.)

The band of auriferous quartz veins worked at Santo Domingo continues westward for eight miles, as far as the savannahs near Libertad, and has been largely mined in the neighbourhood of that town, and between that point and Santo Domingo. Besides the working of the mines proper, some surface deposits, called by the Spaniards “Mantos,” are also worked for gold, especially in the neighbourhood of Libertad. The “Mantos” consist of broken quartz, covering the faces of the hills in the neighbourhood of some of the lodes. In some places they form a broken but regular stratum over the whole side of a hill, and I was much puzzled at first to account for their origin.

I have already mentioned that the lodes near their summit incline over towards the face of the hill through which they cut. In some cases, as in the San Antonio mine, the lode is in parts bent completely round, as shown in the section in Plate 8. This bending over of the lodes is always towards the face of the hill, and is, I think, produced by successive small landslips. It is evident that if carried still further than in the case shown in the diagram, the lode would be brought down over the face of the hill, and the result has, I think, been achieved in some places, and a regular “Manto” produced. I have already stated that small landslips are of frequent occurrence on the sides of the hills. We had several times the entrance to our mines temporarily closed by them in the wet season.

Mr. David Forbes,1 in his account of the geology of Peru and Bolivia, has advanced the opinion that auriferous quartz veins belong to two different systems, one occurring in connection with Granitic, the other with Diorytic intrusive rocks. In later papers he has shown that this occurrence of gold is not confined to South America, but appears to prevail in all parts of the world.2 One of the latest writers on the subject, Mr. R. Daintree, in his “Notes on the Geology of Queensland”, has shown that the auriferous veinstones in that colony occur in connection with, or in the near vicinity of certain intrusive trap-rocks, and that even some of the trappean dykes themselves are auriferous.3 Several years ago, I endeavoured to show that mineral veins in granitic districts occurred in regular sequences, with certain intrusive rocks, as follows:— first, Intrusion of main mass of granite; second, Granitic veins; third, Elvan dykes; and, lastly, Mineral veins, cutting through all the other intrusive rocks.4 Later observations have led me to conclude that a similar sequence of events characterised the occurrence of auriferous quartz veins in connection with the intrusive rocks, commonly designated Greenstones, in some districts consisting of diabase, as in North Wales, near Dolgelly; in others of dioryte, as in Santo Domingo; and in many parts of South America and Australia. In North Wales we have, firstly, an intrusion of diabase, occurring in great mountain masses; secondly, Irregular tortuous dykes of diabase; thirdly, Elvan dykes; and, lastly, auriferous quartz veins. In every region of intrusive plutonic rocks that has been thoroughly explored, a similar succession of events, culminating in the production of mineral veins, has been proved to have taken place,5 and it appears that the origin of such veins is the natural result of the plutonic intrusion. There is, also, sometimes a complete gradation from veins of perfectly crystallised granite, through others abounding in quartz at the expense of the other constituents, up to veins filled with pure quartz, as at Porth Just, near Cape Cornwall; and, again, the same vein will in some parts be filled with felspar; in others, contain irregular masses of quartz, apparently the excess of silica beyond what has been absorbed in the trisilicate compound of felspar.6 Granitic, porphyritic, and trappean dykes7 also sometimes contain gold and other metals; and I think the probability is great that quartz veins have been filled in the same manner — that if dykes and veins of granite have been an igneous injection, so have those of quartz. By an igneous injection, I do not mean that the fused rock owed its fluidity to dry heat. The celebrated researches of Sorby on the microscopical fluid cavities in the quartz of granite and quartz veins, have shown beyond a doubt that the vapour of water was present in comparatively large quantities when the quartz was solidifying. All strata below the surface contain water, and if melted up would still hold it as super-heated steam; and M. Angelot has suggested that fused rock under great pressure may dissolve large quantities of the vapour of water, just as liquids dissolve gases. The presence of the vapour of water would cause the liquefaction of quartz at a much lower temperature than would be possible by heat alone, unaided by water.8 I know that this opinion is contrary to that usually held by geologists, the theory generally accepted being that mineral veins have been produced by deposits from hot springs; but during twenty years I have been engaged in auriferous quartz-mining in various parts of the world, and nowhere have I met with lodes, the phenomena of which could be explained on this hypothesis. The veinstone is pure quartz containing water in microscopical cavities, as in the quartz crystals of granite, but not combined as in the hydrous siliceous sinter deposited from hot springs. The lodes are not ribboned, but consist of quartz, jointed across from side to side, exactly like trappean dykes. There is often a banded arrangement produced by the repeated re-opening and filling of the same fissure; but never, in quartz veins, a regular filling up from the sides towards the centre, as in veins produced by deposits from springs. Quartz veins extend sometimes for miles, and it is necessary to suppose on the hydro-thermal theory that the fissures remained open sufficiently long for the gradual deposition of the veinstones, without the soft and shattered rocks at their sides falling in, nor yet fragments from above; although there are many lodes, fully twenty feet in width, filled entirely with quartz and mineral ores, without any included fragments of fallen rocks, and nowhere showing any trace of regular deposition on the sides. The gold also found in auriferous lodes is never pure, but forms varies alloys of gold, silver, copper, lead, iron, and bismuth; and no way is known of producing these alloys except by fusion.

1 “Quarterly Journal of the Geological Society” volume 17.)

2 “Geological Magazine” September 1866.)

3 “Quarterly Journal of the Geological Society” volume 28 page 308.)

4 See “Geological Survey of Canada” pages 141 and 173.)

5 “Mineral Veins” page 16.)

6 Mr. John Phillips in “Memoirs, Geological Survey of Great Britain” volume 2 page 45.)

7 Sir R.I. Murchison “Siluria” pages 479, 481, 488 and 500; and R. Daintree “Quarterly Journal of the Geological Society” volume 28 pages 308, 310.)

8 H.C. Sorby “Journal of the Geological Society” volume 14.)

It is true that mineral veins contain many minerals that could not exist together undecomposed with even a moderate degree of heat; but it is only here contended that the original filling of the lodes was an igneous injection, not that the present arrangement and composition of all the minerals is due to the same action. Since the lodes were first filled they have been subjected to every variety of hydro-thermal and aqueous influence; for the cooling of the heated rocks must have been a slow process, and undoubtedly the veins have often been the channels both for the passage of hot water and steam from the interior, and of cold water charged with carbonic acid and carbonate of lime from the surface, and many changes must have taken place. Auriferous quartz veins have resisted these influences better than others, because neither the veinstone nor the metal is easily altered, and such veins therefore form better guides for the study of the origin of mineral lodes than fissures filled with calc spar and ores of the baser metals, all readily dissolved and re-formed by hydro-thermal agencies. Our mineralogical museums are filled with beautiful specimens of crystals of quartz, fluor spar, and various ores deposited one on the other; and the student who confines his attention to these is naturally led to believe that he sees before him the process by which mineral veins have been filled. But the miner, working far underground, knows that such crystals are only found in cavities and fissures, and that the normal arrangement of the minerals is very different. The deposition of various spars one on the other in cavities is a secondary operation even now going on, and has nothing necessarily to do with the original filling of the lodes; indeed, their arrangement is so different that it helps to prove they have been differently formed.

It would take a volume to discuss this question in all its bearings, and as I have already entered more fully into it in another place,1 I shall only now give a brief resume of the conclusions I have arrived at respecting the origin of mineral veins.

1 “Mineral Veins” by Thomas Belt. John Weale 1861.

1. Sedimentary strata have been carried down, by movements of the earth’s crust, far below the surface, covered by other deposits, and subjected to great heat, which, aided by the water contained in the rocks and various chemical reactions, has effected a re-arrangement of the mineral contents of the strata, so that by molecular movements, the metamorphic crystalline rocks, including interstratified granites and greenstones, have been formed.

2. Carried to greater depths and subjected to more intense heat, the strata have been completely fused, and the liquid or pasty mass, invading the contorted strata above it, has formed perfectly crystalline intrusive granites and greenstones.

3. As the heated rocks cooled from their highest parts downwards, cracks or fissures have been formed in them by contraction, and these have been filled from the still-fluid mass below. At the beginning these injections have been the same as the first massive intrusive rocks, either granite or greenstone; but as the rocks gradually cooled, the fissures reached greater and greater depths; and the lighter constituents having been drawn off and exhausted, only the heavier molten silica, mingled with metallic and aqueous vapours, has been left, and with these the last-formed and deepest fissures have been filled. These injections never reached to the surface — probably never beyond the area of heated rocks; so that there have been no overflows from them, and they have only been exposed by subsequent great upheaval and denudation.

4. Probably the molten matter was injected into the fissures of rocks already greatly heated, and the cooling of these rocks has been prolonged over thousands of years, during which the lodes have been exposed to every degree of heat, from that of fusion to their present normal temperature. During the slow upheaval and denudation of the lodes, they have been subjected to various chemical, hydro-thermal, and aqueous agencies, by which many of their contents have been re-arranged and re-formed, new minerals have been brought in by percolation of water from the surrounding rocks, and possibly some of the original contents have been carried out by mineral springs rising through the lines of fissures which are not completely sealed by the igneous injection, as the contraction of the molten matter in cooling has left cracks and crevices through which water readily passes.

5. Some of the fissures may have been re-opened since they were raised beyond the reach of molten matter, and the new rent may have been filled by hydro-thermal or aqueous agencies, and may contain, along with veinstones of calcite derived from neighbouring beds of limestone, some minerals due to a previous igneous injection. Crevices and cavities, called “vughs” by the miners, have been filled more or less completely with crystals of fluor spar, quartz, and various ores of metals from true aqueous solutions, or by the action of super-heated steam.

6. By these means the signs of the original filling of many mineral lodes, especially those of the baser metals, have been obscured or obliterated; but in auriferous quartz lodes both the metal and the veinstone have generally resisted all these secondary agencies, and are presented to us much the same as they were first deposited, excepting that the associated minerals have been altered, and in some cases new ones introduced, by the passage of hot springs from below or percolation of water from the surface.

http://ebooks.adelaide.edu.au/b/belt/thomas/naturalist_in_nicaragua/chapter6.html

Last updated Wednesday, March 12, 2014 at 13:32