Note: We are pleased to reprint this paper on the Colombian emerald mines.
The Emerald Deposits of Muzo, Colombia
By Joseph E. Pogue,* Ph. D., Evanston, Illinois
Transactions of the American Institute of Mining Engineers
Vol. LV, 1917 (Arizona Meeting, September, 1916)
*Assistant Curator, Division of Mineralogy and Petrology, U. S. National Museum.
Received June 28, 1916
The writer visited the Muzo emerald mines in July, 1915, and spent six days in their study. This paper embodies the results of his observations, plus information personally communicated by Robert Scheibe, Professor of Geology in the Mining Academy of Berlin, who at the time of the visit was completing a detailed field investigation of nearly a year’s duration of the emerald deposits of Colombia. An elaborate account of this valuable work may be expected at a future date from the pen of Professor Scheibe.
The Muso emerald deposits are situated in the western foothills of the eastern branch of the Colombian Andes and are distant about 96 km.1 (60 miles ±) in a direct northwesterly line from Bogotá, the capital of Colombia (Fig. 1). They lie about 8 km. by trail west of the small village of Muzo in the Department of Boyaca, and embrace about eight great open cuts, closely grouped, occupying a portion of a steepwalled valley, that of the Itoco, also called Quebrada del Desaguadero, an affluent of Rio Minero which empties into the northward-flowing Magdalena, the great artery of commerce for central Colombia. Though distant but 33 km. from La Dorada, the head of steam navigation on the lower Magdalena, they are inaccessible from that point, and may be reached practicably only from Bogotá via rail to Cipaquira or Nemocon and thence by mule for 2 1/2 days over an execrable trail, nearly impassable in the rainy season2 (Fig. 1).
The region about the deposits is intensely tropical, characterized by excessive heat and high humidity, with a rank jungle growth that quickly obscures abandoned workings and makes exploration peculiarly difficult and costly.3 The region round about is sparsely inhabited by Indians who live in squalor and poverty modified descendants of warlike aborigines, docile and peaceable, even servile, speaking a Spanish patois.
The region in general is unhealthful; the natives suffer from tropical anaemia, malaria, dysentery, and other complaints incidental to the latitude. Work in the mines, however, is reasonably safe owing to the excellent location of the workmen's quarters (Fig. 3) and the medical attention and sanitation enjoyed under recent management.
As shown in Fig. 1, the emerald occurs at other points in the mountainous region of Boyaca, but only the Muzo locality has been productive in modern times. The so-called Somondoco deposits (marked Chivor on the map) and those of Coscuez are important historically and enjoy the reputation of being very rich. The other localities indicated are prospects merely, though locally known as minas. The total number of emerald localities in Boyaca has been stated to be 157,4 but this figure is probably a rough approximation. Outside of the Department of Boyaca, the emerald is not definitely known to occur in South America. It has been reported, however, in Colombia near Bolivar, Province of Velez, Department of Santander,5 and tradition points to the Manta Valley near Puerto Viejo in Peru as a source,6 but it seems probable that all the Peruvian emeralds came from the Colombian deposits.
About the year 1594 the Spaniards succeeded in finding the Indian workings nearby at the site of the present-day Muzo mines,15 and then commenced active work and for some 15 years or more the output was considerable.16 The production, however, soon fell away, owing to labor difficulties,17 and toward the middle of the 17th Century the Spanish Crown, whose fifth portion18 had dwindled, reorganized the industry under the direction of the Royal Treasury. Its administration seems to have been singularly inefficient; excessive labor in the mines was imposed on neighboring tribes, a burden resulting in heavy mortality and serious depopulation of the region; dishonesty on the part of both workers and officials19 still further lessened the output; and the galleries that were earlier worked were abandoned for open-cut operations, a change not immediately productive of results. Ineffective mining continued to the middle of the 18th Century or thereabouts, when a disastrous fire terminated activities for a time. Work was later resumed but prosecuted only in a desultory fashion until the success of the War of Independence in 1819 transferred the holdings to the new-born Republic. The republican Government at the outset lacked the organization necessary for running the mines, but realizing their possibilities as a source of revenue, soon contracted for their private exploitation, the terms being 10 per cent. of the net profits.20 The mines were worked under lease from 1824 to 1848,21 when Congress in Bogotá decreed that all emerald deposits found in the country should be worked under the direction of the nation.22 This decree does not seem to have been strictly adhered to, for contracts with private parties were subsequently entered into by the Government, some being in the nature of partnerships, others being strict concessions.
It would be scarcely profitable, even if trustworthy data were available, to follow in detail the vicissitudes of the various arrangements made from 1848 to 1909.23 Suffice to say that the Muzo mines were worked almost continuously during that period, but their development suffered from a lack of any sustained policy of administration as well as from the want of engineering and geological advice. In 1909, the Government closed a partnership contract with an English company, The Colombian Emerald Mining Co., Ltd., controlled by South African diamond interests,24 and the deposits were actively exploited for a time; but after a few years the contract was rescinded25and the Government reassumed sole control of the mines. Operations have been totally suspended, however, since Jan. 1, 1913, owing to the fact that the appropriations for administration and exploitation, being entirely insufficient for reestablishing mining activity,26 were applied merely to the maintenance of the property. The European War, in its effect on the precious stone market, precludes profitable exploitation in the immediate future.
The geology of the Eastern Cordillera of the Colombian Andes is known only in the most general way.27 The principal geological formations exposed in the emerald-bearing region are shown in the following columnar statement:28
These rocks are compressed into great north-south folds and igneous phenomena are largely lacking.
General.The geological relations of the emerald deposits are well exposed in the great open cuts made in exploitation of the emerald veins, but the surrounding conditions are almost completely hidden by dense jungle growth (Fig. 3). The emeralds are found almost entirely in calcite veins that traverse a black, carbonaceous, rather intensely folded formation consisting of thin-bedded shale and limestone (Fig. 4).
This emerald formation29 lies discordantly30 upon steeply dipping strata, barren of emeralds, composed of heavier beds of carbonaceous limestone intercalated with black shale, and called the Cambiado from the Spanish word cambiar, to change. Between the emerald formation and the Cambiado and ever in close proximity to the plane of discordance are three rock types of great significance in furnishing direct evidence of the origin of the emeralds. These are (1) albite rock, (2) a light-gray rock composed of a soft granular aggregate of calcite, dolomite, quartz, pyrite, and other minerals, called by the miners Cenicero (Spanish cenicero = ash) in allusion to its ash-like appearance, and (3) aggregates of large, well-formed calcite rhombs in a fine-grained matrix, forming rock masses known locally as Cama, from the Spanish word cama, meaning bed.31 In addition, a few pegmatite veins have recently been discovered in the deposits.
The Cambiado. (See Figs. 8a and 8b). This formation consists of beds of black, crystalline limestone, averaging in thickness about 25 cm. and alternating with thin-bedded shale similar to that of the overlying emerald formation. The limestone shows itself under the microscope to be composed of ragged, granular masses of calcite, inclosed in black carbonaceous matter, and carrying a few to many fragmental crystals of albite. This rock in places grades upward into a phase in which albite predominates, the so-called albite rock described later; downward it grades presumably into albite-free limestone, but only the topmost few meters of the Cambiado are in any place exposed.
The Cama. This
formation is composed of conspicuous rhombs
and rhombic twins of calcite, most of them
from 5 to 10 cm. in diameter, set in calcareous
cement along with some quartz, the whole
forming a breccia-like mass (Fig. 10). The
habit of the calcite, which occurs as
unit rhombohedrons alone or modified by base,
and as twins of the first-named form with
(1010) as the twinning plane, probably reflects
the temperature range of development.34 This
formation rests directly upon the Cambiado, but
is not continuous, being found only here
and there, either alone or close to the somewhat
similarly occurring Cenicero (see
The Cama in places shows plainly a connection with calcite veins both in the emerald formation above and in the Cambiado below. Some calcite veins in the latter have the peculiar calcite crystallization of the Cama.
The Cenicero. This formation occurs as irregular lenses or beds up to a meter or so in thickness present in many places between the emerald formation and the Cambiado, either with or without the Cama (Fig. 10). It is connected below with the albite rock, into which it locally grades, but unlike the Cama, shows no connection with the calcite veins traversing the overlying and underlying formations. In a few places it was noted forming vein-like bodies in the emerald formation itself.
The ordinary Cenicero is a crumbly, light-gray aggregate of crystals, chiefly of calcite, dolomite, quartz, and pyrite. A typical specimen under the microscope shows the minerals noted as well-formed, fragmental, and rounded crystals, set in a fine-grained ground, difficultly resolvable, but probably mainly calcareous matter, stained with a little carbonaceous matter. Toward its top the Cenicero in many places carries abundant fragments of black shale a centimeter long and smaller, forming masses of breccia rarely seen over 2 m. in thickness.
There are three more or less strongly marked phases of the Cenicero dolomitic, pyritic, and bariticthe normal sequence upward being in that order (Fig. 9). In addition, the lowermost part in many places is albitized, while the topmost portion is nearly everywhere connected with the emerald formation by the breccia phase just noted. The baritic phase is locally seen as an almost pure layer of massive to nodular barite, with a maximum thickness of about 40 cm.
Pegmatites. Pegmatite dikes were discovered in 1915 by Robert Scheibe in the Cambiado near Banco Amarillo and in a ravine back of Banco Central. The last-named locality was visited35 and the pegmatite, here about 2 m. in width, found to consist of quartz, in part well crystallized (the low-temperature form), and decomposed feldspar, together with a few crystals of albite and apatite, much greenish to clear allophanite, many small form-rich crystals of pyrite, and a little hyalite.
The Muzo deposits present a notable assemblage of minerals, many of them well developed crystallographically and some of particular chemical interest. The present section assembles the geologically significant characteristics of these minerals, but attempts no detailed mineralogical description. A good crystallographic study was published in 1904 by H. Hubert36 and an accurate mineral list with brief characterizations in 1915 by Lleras Codazzi.37 The statements here given are the results of the writer's observations, except where otherwise noted.
found in pockets, or embedded, in calcite
veins traversing the emerald formation; rarely
embedded in that formation itself or in the Cenicero. Closely
associated minerals forming the emerald gangue
are: Calcite, dolomite, parisite, pyrite,
quartz, barite, fluorite, and apatite,
the last three very rare. The emerald occurs
as six-sided prisms with base, some with
rarer forms also. Few crystals are larger
than the thumb. Most crystals are clear when
first taken from the matrix, but later develop
cracks; some fall to pieces upon removal.38
Choice specimens show a rich green color surpassed by the product of no other locality. Some crystals display zones of color; a few are dark to black with inclusions of carbonaceous matter. In some specimens recently found, the carbonaceous matter is arranged in a six-rayed figure centering about a tapering hexagonal core. One such specimen was examined optically in basal section and proved to be of the same orientation throughout; it therefore does not represent a twinned crystal as suggested by Lleras Codazzi.39 Its re-entrant angles are presumably the effect of solution and the disposition of the carbonaceous inclusions, the expression of crystallizing forces, as shown also, for example, in chiastolite.
Calcite. Forms emerald-bearing veins in the emerald formation and barren veins in the Cambiado, and is well crystallized where occurring in vugs. Crystals are water-clear to opaque from disseminated carbon; they show a rich variety of forms, with two dominant habits, rhombohedral and prismatic. Closely associated with emerald, pyrite, and parisite. Occurs in the Cama as conspicuous unit rhombohedrons (some modified by base) and as rhombic twins, twinning-plane (1010). Is an important component of the Cenicero as small rhombs and grains.
Ankerite. Occurs as yellowish or brownish rhombs in the emerald gangue; one variety has been reported as carrying a little CeCO3.40
Pyrite. Occurs in well-formed crystals in the emerald veins; as seams, disseminated crystals, and concretions in the emerald formation; as crystals in the Cenicero; and as crystals and concretions in the Cambiado. The crystals, which range in diameter from a fraction of a millimeter to several centimeters, show a profusion of crystal forms and present three habits, cubic, octohedral, and pyritohedral.
Parisite. This rare mineral, of the composition (CaF)(CeF)Ce (CO3)3,41 was first discovered in the Muzo mines by J.J. Paris, a lessee of the deposits, and named in his honor by Bunsen,42 who investigated the mineral. It occurs as crystalline masses and crystals in immediate association with the emerald. The crystals are double hexagonal pyramids, with or without the base, and most are under 1 cm. in length.
Quartz. Occurs as well-formed colorless to greenish (rare) crystals in the emerald veins; inclusions of parisite, pyrite, and emerald have been noted.43 Less perfect crystals are found intergrown with calcite rhombs of the Cama. Crystals rich in forms occur in the pegmatite vein back of Banco Central; these are low-temperature quartz, formed below 575°.44
Fluorite.45 Occurs as small, colorless to greenish crystals in some of the emerald veins; very rare; forms are cubes or cubes modified by small octahedrons; inclusions of emerald noted.46 Has also been noted by Scheibe (oral communication) in the albite rock at one spot.
Gypsum. Present in the emerald formation as well-formed, clear, slender crystals. Presumably a weathering mineral, but Lleras Codazzi47 noted inclusions of parisite, and Olden48 mentioned green gypsum as an associate of the emerald.
Barite. Occurs in conspicuous layers, up to 40 cm. or so in thickness, forming in places the uppermost part of the Cenicero; this phase is nodular to massive.50 Found also as small, glassy, tabular crystals, 2 mm. across, associated with crystallized calcite in some veins in the emerald formation. A small crystal perched on an emerald crystal has been noted by Scheibe.
Anthracite. Small fragments of impure anthracitic to graphitic carbon are found in joints in the emerald formation.51
Marcasite. Noted by Lleras Codazzi52 in the form of nodules.
Chalcopyrite. A few imperfect crystals, stained with a little malachite and azurite, have been found in the workings. Also occurs sparingly in the Cenicero.
Native Sulphur. Found in conspicuous masses in parts of the Cenicero.
Allophanite. Found only locally developed in clay-like masses forming lenses in the emerald formation. Noted by Lleras Codazzis53 in blue masses in a vein in the Cambiado.
Fuchsite. Noted by Lleras Codazzis54 as green laminae adhering in places to the surface of the shale of the emerald formation.
The ages of the emerald formation and Cambiado are fixed as Cretaceous by the fossils, chiefly ammonites, that have been found rather abundantly in them. Miguel Gutiérrez55 places the Cambiado as lower Cretaceous56 and the emerald formation as middle Cretaceous.57 The present writer presents no fossil evidence but feels that further paleontological study is needed before a correlation closer than lower Cretaceous can be accepted for the rocks of the emerald deposits. An ammonite collected by the writer from the stream bed below the workings has been identified by Dr. T.W. Stanton as Pulchellia zaleatoides, Karsten, from the upper part of the lower Cretaceous.
The evidence bearing on the origin of the emerald has been presented in descriptive form. It may be summarized under four heads, as follows:
1. The association of such minerals as emerald, parisite, fluorite, apatite, albite, and barite in a sedimentary formation implies the introduction of material from an external source. This is so obvious from the composition of these minerals and their known occurrence elsewhere as to render further elaboration unnecessary.
2. The presence of pegmatites is significant, because the conditions under which pegmatites form are fairly definitely understood. The mineral content of the pegmatites is thought to correlate their formation with the general period of mineralization.
3. The presence of albite rock (highly albitized limestone) and its spatial relation to a zone occupied by the Cenicero and Cama indicate the passage of strongly effective mineralizing solutions. The albite rock itself is thought to represent a contact rock, not of the normal type (because of the absence of such characteristic minerals as garnet, epidote, pyroxene, amphibole, etc.) but of the type characterized by V.M. Goldschmidt58 as that due to pneumatolitic contact metamorphism, a type that develops later in the cooling of, and more distant from, the parent magma than the normal type.
4. Structural conditions indicate that the emerald formation was overthrust to its present position upon the Cambiado, and that this movement was followed by a period of mineralization which attained its most conspicuous results along the fault plane and its economic results above (and not below) that plane. That the emerald veins are the result of the same period of mineralization that produced the Cenicero, Cama, and albite rock, is thought to be clearly indicated by the mineral content and spatial connection that may be traced between the four. The barren calcite veins in the Cambiado are probably of the same period of mineralization also; for they are post-faulting (Figs. 8a and 8b) and in places are connected with the Cama.
The emerald is won exclusively by open-cut mining. The steep slopes of the emerald formation, stripped of their covering of jungle, are worked in great terraced banks (bancos), affording benches60 on which lines of peons stand and attack the bench below with long iron crowbars (Figs. 11 and 12). The comparatively soft limestone and shale are easily broken away in this way without recourse to blasting (which would shatter the fragile emerald crystals) and the emerald-bearing calcite veins are carefully removed by hand and taken to a sorting shed above. The debris falls down the step-like slope and the accumulation at intervals is swept down to the creek below by water led from reservoirs in the mountains above the workings (Fig. 13).
In the sorting shed, the calcite veins are carefully broken by hand and the emerald crystals picked out. The finer material, together with gem-bearing debris gathered from bed-rock and from the water channels below the banks, is washed on sloping tables and the emerald fragments withdrawn by boys (Fig. 14). The stones are separated into a number of grades according to color, size, transparency, and freedom from flaws. The product goes by mule to Bogotá from time to time, and there awaits transportation to London in larger consignments.
The labor is done by Indian peons drawn from the neighborhood. Mining officials and police are supplied from Bogotá or other towns. Great vigilance is exercised, when the mines are in operation, to reduce loss by theft. A body of military police is assigned to the mines; the exits are carefully guarded; watchmen are always on duty in small guardhouses on prominent points above the workings; overseers are in constant attendance during hours of work; and the workmen are impounded and not allowed to leave the mines until the culmination of a suitable period of search.
The mine buildings are commodious and comfortable, maintained in good condition (Fig. 3). The mining equipment is simple, but the fragility of the emerald precludes the use of most types of equipment that would increase the quantity of ground handled.
It is impossible to present an approximation of the total production of the Muzo mines. The pre-Spanish output, undoubtedly significant, is of course not open to any measure. In historic times, the exploitation was so irregular and the records so incomplete, that a fair basis for judgment is entirely lacking. Nevertheless, it is certain that the total output may be estimated in terms of tens of millions of dollars, and that in many single years the production has run in value from $1,000,000 to perhaps $2,000,000, or more.
The Coscuez and Somondoco emerald deposits have already been mentioned as the only other important known occurrences of this mineral in South America.
Coscuez Deposits. These lie about 12 km. in direct, line north-northwest of the Muzo mines (Fig. 2) but are exceedingly difficult of access. They were known before the Conquest and won a reputation for richness (see p. 914). No information is yet available concerning their geology, but the writer has been informed that a geological study of them was made late in 1915 by Robert Scheibe.61
Deposits. These lie
about 130 km. in direct line southwest
of the Muzo mines on the Orinoco watershed
(Fig. 1), and a careful
survey of available information suggests
that possibly they are richer than the
Muzo deposits. They have been visited
and described by W. Lidstone62 and
by E. B. Latham;63 and
in 1915 a detailed geological survey
of them was made by Robert Scheibe, but
the results are not yet published.
These deposits have a romantic history. They were richly productive before the Spanish Conquest, were seized and worked a while by the Spaniards, were subsequently abandoned and lost, and only rediscovered in 1896.64 They have not been productive in recent times, though foreign capital has interested itself in their exploitation.
According to Latham65 the emerald here occurs in veins of semi-decomposed quartz traversing folded beds of dark gray to black clay-slate and limestone, and is found either directly embedded, or in pockets, in these veins, very rarely in the rock itself. Lidstone66 describes the occurrence here in a similar manner, without, however, specifying the vein matter to be quartz.
For courtesies and valuable help, both during the writer's visit to the Muzo mines, and later during the preparation of this paper, the writer extends his appreciative acknowledgment to the following: Hon. Marco Fidel Suárez, Minister of Foreign Affairs, Bogotá; Hon. Daniel J. Reyes, Minister of Hacienda, Bogotà; Hon. Thaddeus A. Thompson, American Minister, Bogotá; Dr. Juan de Dios Vasquez, Director of the Muzo mines; Dr. Alfredo Angueyra, Acting-Director of the Muzo mines at the time of the writer's visit; Prof. Dr. Robert Scheibe, Professor of Geology, Royal Mining Academy, Berlin; Dr. Ricardo Lleras Codazzi, Professor of Mineralogy and Geology, National University, Bogotá; Dr. Lucas Caballero, Bogotá; Dr. Hermano Apolinar Maria, Bogotá; Phanor James Eder, New York; Douglas B. Sterrett, U. S. Geological Survey, Washington; Dr. George P. Merrill, and Dr. Edgar T. Wherry, U. S. National Museum, Washington; Dr. Ronald S. Crane, Evanston, and Mrs. Leonard G. Shepard, Evanston.
EDGAR T. WHERBY, * Washington, D. C. (communication to the Secretary) .Dr. Pogue’s presentation of the facts concerning the emerald deposits is very clear and convincing, and the only addition that I can suggest is a summary of previous theories of origin. He makes it evident that the pegmatite theory is the only one capable of explaining the existing relations, but upon certain details there may be some difference of opinion. If I understand the term pneumatolytic, it does not imply that all the elements concerned in a given deposit were transported as gases, but rather that the crystallization of these elements into the various minerals was favored by the presence of certain gaseous substances, notably H20, CO2 and HF. It is highly improbable that the oxides of glucinum, aluminum, chromium, and silicon, which enter into the composition of the mineral emerald could have been transported in the gaseous form. The same is true of the metallic constituents of the parisite and other associated minerals. The explanation suggested, that solutions separated into liquid and gaseous portions, the latter ascending and forming the emerald in the upper portions of the rock only, therefore, seems to me untenable.
When two formations exist side by side and one, A, is mineralized while the other, B, is barren, the possible explanations may be classed as (1) chemical, and (2) physical.
1. Some chemical feature of A not found in B might have caused crystallization of certain minerals in the former, which did not appear in the latter. In the present instance both rocks appear to be so similar chemically that no such effect can be looked for. Pogue mentions carbon as a possible precipitating agent, but describes both formations as carbonaceous, so that the difference in the minerals of the two can not be thus explained.
2. The physical condition of A might have permitted or encouraged the passage of the solutions, while that of B retarded or prevented it. In the present deposit some mineralization occurs in both formations, calcite veins, albite, and pyrite being found in both, whereas emerald, parisite, and a number of minerals of minor importance occur only in the upper, A. It seems to me that this difference may have been produced by a change in the composition of the solutions during the progress of mineralization; at first these brought in only the constituents of albite and pyrite and deposited them with calcite dissolved from the wall rock, in both formations; the openings in B became completely filled, while the more numerous or larger ones which would naturally have developed in A, since it was the uppermost formation, remained partially open. Then, when during later phases of the mineralizing activity the constituents of emerald and parisite appeared, they were deposited only in A because B had become impermeable.
* Associate Professor of Geology and Mineralogy, Northwestern University.