The Naturalist in Nicaragua

by Thomas Belt

(PLATE 8. 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,* (* "Quarterly Journal of the Geological Society" volume 17.) 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.* (* "Geological Magazine" September 1866.) 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.* (* "Quarterly Journal of the Geological Society" volume 28 page 308.) 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.* (* See "Geological Survey of Canada" pages 141 and 173.) 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,* (* "Mineral Veins" page 16.) 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.* (* Mr. John Phillips in "Memoirs, Geological Survey of Great Britain" volume 2 page 45.) Granitic, porphyritic, and trappean dykes* also sometimes contain gold and other metals; (* 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.) 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.* (* H.C. Sorby "Journal of the Geological Society" volume 14.) 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.

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,* (* "Mineral Veins" by Thomas Belt. John Weale 1861. ) I shall only now give a brief resume of the conclusions I have arrived at respecting the origin of mineral veins.

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.