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.
(PLATE 6. 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.
(PLATE 7. 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.
(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.
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.