Goudafzettingen in Suriname (Gold deposits in Surinam)
Leidse Geologische Mededelingen , Volume 21 - Issue 1 p. 1- 246
THE GOLD DEPOSITS IN SURINAM AND THE DISTRIBUTION OF CONCESSIONS THROUGH THE COUNTRY The fieldwork on the occurrence of primary and secondary gold deposits in Surinam on which this thesis is based was carried out by order of the Welfare Fund Surinam (Welvaarts Fonds Suriname) during the periods December 1950—September 1952 and September 1953—January 1954. The regions investigated form part of Surinam (formerly Dutch Guiana), South America. These regions are indicated by heavy lines on the enclosed geological survey map of Surinam, after Schols and Cohen (1953) and IJzerman (1931). As participant of a medical scientific expedition to the southern borderlands of Surinam, I made investigations on the occurrence of gold in this part of Surinam. The trail of the expedition is indicated on the survey map of Surinam (enclosed map). As was stated by statistical (figure 4) and by field evidence, the goldfields of Surinam occur in the northern and eastern part of the country, south of the coastal plain. The goldfields are geo-topografically connected with an area of metamorphic sedimentary schists (Orapu-Formation and Bonnidoroseries) and, mainly epimetamorphic, basic to intermediate volcanic rocks (Balling-Formation without Bonnidoro-series). Two belts of relative concentration of goldconcessions can be constructed from the distribution of the goldconcessions through the area where the goldfields are found. Most of the workable deposits are alluvial or colluvial placer deposits, but in some places in Surinam, primary deposits of hypogene origin are worked. Here the gold is found in gold-bearing quartz veins, -veinlets and -lenses, and in gold-bearing pyrite, which is limonitised as a result of weathering, and which occur in the metasomatically altered wall rock along, as well as in, some of the goldand pyrite-bearing quartz veins and -lenses. These primary deposits, being the result of hypogene processes which are genetically related to intrusive quartzdiorites and granites, occur in the invaded bedrock of the Balling-Formation (Headley-Reef, Km 124,3 L. S., figures 21 and 22) and the Orapu-Formation (Mindrinetti goldfields, Km 93—106 L. S., figures 22 and 25). The placer gold in the alluvial and colluvial deposits of the mentioned areas must be considered to be the concentrated debris of the gold-bearing quartz veins and -lenses, as a result of mechanical transport from chemically weathered, enclosing bedrock. This is in contradistinction with earlier theories of Middelberg (1908), who thinks the placer deposits a result of mainly chemical transport by the help of supergene solutions. Transport of gold by supergene solutions as a result of weathering under most favourable conditions (Bateman, 1949) in the presence of limonitising pyrite, has occurred in some cases over short distances only, from the pyrite to the surrounding matrix. Mostly however the limonitised gold-bearing pyrites are still gold-bearing (table 9). As a process of concentration of placer gold those supergeneous processes are very unimportant. As was stated earlier by Gruttebink and de Haan (1940, 1950, 1952) and as could be reascertained during our fieldwork in the concessions of the Sarakreek Goudvelden N.V. on the upper course of the Lawa river, the placer gold is also the concentrated debris of quartz veins and -veinlets. The same holds good for most of the other investigated areas, i. c. the concession Ettenberg opposite Stoelmanseiland (survey map) and probably the concession Doorson on the southern slope of the Nassau mountains (figure 12). On the other hand in the investigated area of the Kabalebo river near Stone creek (figure 35) the conglomerates of the Orapu-Formation must be regarded as the possible sourcerock of the scarcely scattered gold in the gravels, mostly formed by quartz pebbles from the conglomerate outcrop, at a distance of some tens of metres upstream the creek. This gold shows signs of long-distance transport, because of its roundness (figure 67 e). Workable deposits of this type are not known in Surinam. The gold from the workable placer deposits was not transported over long distances from its source deposit. Therefore the geology of the underlying bedrock of the placer deposits is an important source of information with respect to the genetic relationship between the occurrence of gold and the processes of primary mineralisation, even if the primary gold deposits are not known or hidden by the result of chemical decay of the deposits and their enclosing bedrock. The genetic relation between gold in placer deposits and gold bearing quartz veins is proved by the occurrence of gold-bearing quartz pebbles in the gravels, whereas the geological processes that caused the mineralisation arc detected by the coincidence of the distribution of the placer deposits and contactzones between the Balling-Orapu-Formations and younger intrusive granites and quartzdiorites. The primary deposits have already been found or must be sought for in the invaded bedrock of the Balling- and Orapu-Formation, in the direct contact aureoles of the quartzdiorites and granites (figure 12) or at a certain distance from the outcrops of the contacts, but seldom further than 2000 m (figure 16). In the case of the Mindrinetti-goldfields (figure 25), the placer and primary deposits are concentrated on both sides of a savannah area. This savannah area consists of the weathered products of contact metasomatieally altered schists and conglomerates of the Orapu-Pormation (Grosgroup). The contact metasomatieally altered schists and -conglomerates are here considered as the contact aureole of a hidden granitic intrusion, comparable to the granite of Km 109 L. S. (figure 26). The gold-bearing hypogene deposits don't occur in the direct contactzone, but at a short distance from there, not exceeding 2000 m, along shear planes and minor faults. Study was made of the type of granites and quartzdiorites that caused mineralisation, because igneous rocks of the same mineralogical composition are found in the southern and western part of Surinam, where as far as known, workable gold deposits are not found. The granites and quartzdiorites in this part of Surinam are formed as large migmatite-granite complexes of an inhomogeneous mineralogical composition over short distances (de Munck 1953, IJzerman 1931). Because most of the Surinam granites belong to this type, the chemical analyses of the granites represent mostly granites of this migmatite-granite type. In the QLM-diagram (P. Niggli 1945) it can be seen that they belong to the pacific province (calc-alkali magma), but they are very irregularly distributed in this diagram (figure 44). There is a shifting of the plotted analyses to the side of sedimentary rocks, probably meaning that the analyses represent igneous rocks which assimilated a lot of material from the invaded rocks of a different chemical composition. These granites are interpreted as being formed by granitisation. The granitic and quartzdioritic magmas which caused gold mineralisation are characterised by their homogenity, chemically as well as mineralogically, over short and long distances. Their contacts are well marked, either by xenolith's (figure 12) or by zones of marked contact-metasomatism (Grosgroup, figures 22 and 26). Four chemical analyses of quartzdiorites which are genetically related to gold mineralisation were made. The result of the analyses are published in table 2. The analyses are plotted against the analyses of other quartzdiorites (figure 45). The same has been done for the granites. The granite of Km 109 L. S. represents an extremely acid granite (granitaplite). These granites and quartzdiorites are interpreted as real intrusive granites. As the quartzdiorites from the concessions on the upper course of the Lawa river are porfiritic, they have a double sequence of crystallisation. The gold deposits of the SE—NW goldbelt of Surinam (figure 4) and those of the concessions Ettenberg (survey map) and Doorson (figure 12) are genetically related to the intrusive quartzdiorites. The same quartzdioritic complex that causes mineralisation on the concession Doorson has been found discordantly covered by the Orapu-Formation (Schols and Cohen 1953). The conglomerates and schists of the Orapu-Formation were never seen to have been intruded by quartzdiorites. The granites of Km 109 L. S. are intrusive into the Orapu-Formation. This proves that granites and quartzdiorites of different age do occur and makes it probable, that both can have caused gold mineralisation. The quartzdiorites (granites no. 1) are supposed to be older than the granite of Km 109 L. S. (granites no. 2). The SE—NW goldbelt of Surinam is genetically related to the granite no. 1 (quartzdiorites); the B—W goldbelt of Surinam to the granite no. 2 (granite of Km 109 L.S.). For both goldbelts we can give the same sequence of events causing workable gold deposits. Only in the Mindrinetti-goldfields and near Km 124,3 L. S. (Headley-Reef) workable primary deposits, as a result of hypogene processes, are known and here have been studied especially, whereas the placer deposits on the upper course of the Lawa river and in the alluvial deposits of this river and the Marowyne have been studied especially there. As a result of the fieldwork and laboratory investigations we can subdivide the different types of gold-bearing deposits into the following groups. A. Secondary deposits 1. Deposits formed as a result of mainly mechanical transport a. Alluvial deposits. b. Colluvial deposits. 2. Deposits formed as a result of mainly chemical transport a. Residual deposits. b. Gold-bearing, deeply weathered bedrock. B. Primary deposits as a result of hypogene processes 1. Gold-bearing quartz veins, -veinlets and -lenses. a. Gold- and pyrite-bearing quartz veins. b. Gold-bearing blue quartz veins. c. genetically related, not gold-bearing turmaline-quartz veins. d. Gold- and ferberite-bearing quartz veins. e. genetically related, not gold-bearing white quartz veins. 2. Gold-bearing bedrock. a. Gold-bearing pyrite in metasomatically altered wall rock of the gold- and pyrite-bearing quartz veins. b. Gold-bearing bedrock. a. dolerites. p. granites. y. conglomerates. The workable gold deposits in Surinam must be considered to be the result of a genetic sequence of events from intrusive granites or quartzdiorites which intruded older rocks of the Balling- or Orapu-Pormation, causing goldbearing quartz veins and gold-hearing pyrites in the wall rock, which has often been altered contact-metasomatically. The placer deposits are formed from the debris of these primary deposits, mainly as a result of mechanical transport. A. SECONDARY DEPOSITS 1. Deposits formed as a result of mainly mechanical transport Most of the workable gold deposits in Surinam are of this type. The gold occurs in these placer deposits as flour gold (up to 0,1 mm), dust gold (0,1—0,5 mm) and coarse gold (more than 0,5 mm). Nuggets too occur and are called “pepieten”. The gold is marked by irregular forms, flakes and fine specks. Grutterink (1940) described some large nuggets from the concession area on the upper course of the Lawa river and states a hypogene origin of these nuggets, because chalcopyrite was found as inclusions in the gold. Morphologically the gold from the placer deposits is sharp-angular to subrounded (figure 67). Gold-bearing quartz pebbles often occur in this type of deposits. These pebbles are also sharp-angular to subrounded. More seldom fully rounded quartz pebbles occur. This proves that the transport of the gold and the gold-bearing quartz did not occur over long distances. a. Alluvial deposits The alluvial deposits can be subdivided into a. recent alluvial deposits; ß. terrace deposits. a. Recent alluvial deposits are the most common type of workable gold deposits in Surinam. The gold occurs in one or more layers of the deposits, mostly concentrated in the gravel-bottomlayer, on a kaolinitic-weathered bedrock. Examples of this type of deposits are illustrated by the figures 15, 17, 18, 23. Figure 15 represents an alluvial deposit of the Lawa river, opposite Stoelmanseiland. The deposit consists of gold-bearing sands and gravel on a kaolinitic-weathered quartzdioritic bedrock. Fine and coarse gold occurs in the gravel layer. The gold has irregular subrounded forms. Nuggets do not occur in this place. Gold-bearing quartz pebbles are found in the gravel layer. Some 1500 m upstream the river, the contact between quartzdiorite and the Balling-Formation is hidden by swampy lands and the Lawa river. In the Balling-Formation many quartz veins occur. According to the concessional, nuggets (0,5—10 grams) have been found in the area of the Balling-Formation, which is marked by more accidentated lands and ferritic weathering. So the primary deposits must be sought in this area of the Balling-Formation. Figures 17 and 18 illustrate the morphology of the gravel-covered bedrock and the distribution of goldconcentrations in the Bas Rufin (figure 16). Here the gold occurs in a gravel-layer of an average of 70 cm, resting on a kaolinitic-weathered bedrock and covered by layers of sands and clays of 2—3 m. The workable deposits are found in the bottom layer of the gravel and the first 2 or 3 cm of the weathered bedrock. Here the gold is found as fine and coarse gold, together with gold-bearing quartz pebbles and irregularly formed nuggets with rounded and subrounded forms and platy sharpangular nuggets weighing up to several tens of grams and not showing any results of transportation. In the weathered bedrock under these gravel layers, gold-bearing quartz veins occur. This proves that the gold from these deposits has partly been transported over short distances, whereas soms of the gold has to be considered as eluvial. Figure 23 shows a section, situated on the borderline of the savannah area and the hills of the gold bearing zone of De Jong Zuid and Gros Placer (figure 25). Here the gold-bearing deposits consist mostly of material of the gold-bearing zone, whereas the colluvial deposits of the savannah area prove to be barren of gold. The morphology of the gold, found in these deposits, is subrounded. ß. Terrace deposits, in Surinam prospection, is the name given to all gravel deposits above creek- or river level. Deposits of this type are illustrated by figures 5, 6, 7, 17, 18. Figures 5, 6, 7 reproduce a type of terrace deposit found in the Marowijne river. At main waterlevel, in the river and along the banks of the river we find cemented riverconglomerates, which rise up to 2 meters above waterlevel. The quartz pebbles of the conglomerates (2—20 cm) are perfectly rounded. Along the banks they are covered by sands and clays. Some of the islands (tabbetjes) in the river are the remains of older riverbank terraces (Nason). The conglomerates proved to bear a small amount of gold. The fine gold has irregular rounded forms. Deposits of this type are not worked in Surinam. Another type of terraces is found in the valleys along the creeks on the upper course of the Lawa river. This type is illustrated by figures 17 and 18. The gold in these terraces has the same morphology as the gold in the recent alluvial deposits. The terraces represent an earlier stage of the creek, which carved a valley in preexisting rocks. The terrace deposits are characterised by their topographical height above creek level and by their more ferritic appearance as a result of weathering processes above water level. Sometimes the gold in this type of deposits occurs together with limonite. b. Colluvial deposits The colluvial deposits link together the alluvial deposits of the type A. 1. a and the primary deposits of the type B. They are characterised by yellow and redbrown sandy granular clays with angular gold and quartz fragments and often ferritic iron stones. The deposits represent the slightly transported debris which resulted after the lateritic weathering of the underlying bedrock. They are transported as a result of creep and of the removal of large volumes of peliticweathered bedrock by erosion. The occurrence of quartz fragments and free gold with angular morphology in this type of deposits proves that in most instances the chemical weathering did not affect the gold to an important degree. In some cases gold-bearing limonitised pyrite is found in this type of deposits. Sometimes the colluvial deposits form a foothill plain. The colluvial deposits are affected by chemical weathering, resulting in the formation of so called “kraskouw” layers. This kraskouw, being a stage of ferritic weathering of transported material above water level, occurs in the clays of the terrace deposits too. Kraskouw is characterised by its irregular reticulated ferritic enrichment and pore-space filling by clay matter. Figures 19 a. b. c. illustrate some types of colluvial deposits. Figure 19 a represents the most common type of colluvial deposit. The clay matrix of the upper layer is washed out by erosion, by which the layer has undergone enrichment in quartz fragments and iron stones. The lower layer has more pelitic contents but also belongs to the colluvial deposit transported by creep, because quartz-fragments and gold occur, whereas the underlying bedrock does not show quartz veins. Figure 19 b illustrates colluvial deposits which cover an older terrace deposit. Kraskouw has been formed in the colluvial layers. Figure 19 c represents a deposit that has been considered as a result of filling of a creek valley with residual quartz boulders, by mainly colluvial deposits. The quartz boulders, measuring up to several cubic feet, are subrounded to angular. In this type of deposit, nuggets have been found with sharp-angular platy forms, which prove that the nuggets were formed as a result of fissure filling from gold-bearing solutions of hypogene origin (Grutterink 1940). 2. Deposits formed as a result of mainly chemical supergene processes Deposits of this type have not yet been carefully studied in Surinam. Some random observations prove the existence of this type of deposits but nowadays no workable deposits of this type are known in Surinam. These deposits seem to be a consequence of the weathering processes in primary and secondary gold deposits. They result into: a. Residual deposits. b. Gold-bearing, deeply weathered bedrock. a. Residual deposits Residual concentration of gold in the ferritic final members of the lateritic weathering of gold-bearing deposits in the form of gold-bearing quartz fragments and free gold, belongs to the type already discussed as colluvial deposits. Beside this type of residual concentration there is also another type which probably is a result of the above mentioned processes. Here the ferrites themselves are gold-bearing as was proved by van Kooten (1953). Assays of ferrites above the gold-bearing deposit of Pakira Hill (De Jong Zuid) showed a grade of 2,5 g Au/ton. b. Gold-bearing, deeply weathered bedrock Probably a piece of gold-bearing clay, which was found in the kaoliniticweathered bedrock under the gravel layers of the Rufin (figure 82), belongs to this type. The morphology of the gold and the distribution through the clay prove it perfectly impassible that this gold is placer gold, which penetrated mechanically into the clay. There is always a possibility that the gold was already formed in the unweathered bedrock by hypogene processes but this doesn't explain the clayey-weathered bedrock that is enclosed in the gold. To a certain degree some results of the limonitic weathering of gold-bearing pyrites in the quartz veins and in the contact-metasomatically altered wall rock along these veins also belong to this type, which will be treated together with B. 1. a., because the deposits formed in this way are mainly formed by hypogene processes. B. PRIMARY DEPOSITS, AS A RESULT OF HYPOGENE PROCESSES Primary deposits are only worked in the environment of the country railroad between Km 93 and Km 133. These deposits were studied especially, although here too, placer deposits of the same types which have already been discussed, are more important as contributors to the gold production of Surinam than the primary deposits. Two types of primary deposits can be distinguished. The first type is that of the Mindrinetti-goldfields and can be found in several zones on both sides of the savannah area of figure 25, between Km 99,5 and Km 105,5 L. S. (country railroad). The second type occurs near the contact of the Kabeltonalite (figure 22). The primary deposits of the Mindrinetti-goldfields lie in deeply-weathered, contact-metasomatically altered wall rock which consists of (sandy-) kaolinitesericite-quartz-bearing clays (figure 31). Often limonitised pyrite occurs in the contact zones along gold-bearing quartz veins. The kaolinitic-weathered contact-metasomatically altered wall rock is distinguished from the normal regional-epimetamorphic schists and conglomerates of the Orapu-Formation, by its colour, which is often white or a pastel shade of rose, violet, yellowbrown and redbrown; by its often high kaolinite percentage (table 5, column VI); by its unconsolidated pelitic character (table 6 and figure 50), conglomeratic relicstructure and its topographic position along goldbearing quartz veins. The normal epimetamorphic conglomerates and schists of the Orapu-Formation are weathered to a smaller degree and are still consolidated rocks. Lateritic weathering of these rocks results in the formation of ferrites which occur in the colluvial deposits on the tops and the slopes of the bills. The weathering products of the Grosgroup (figure 22) are characterised by the same weathering features as the contact-metasomatically altered rocks along the gold- and pyrite-bearing quartz veins, but here limonitised pyrite, nor gold occur. The unconsolidated sandy kaolinite-sericite-quartz clays change into the consolidated conglomerates and schists of the Orapu-Formation, loosing their pelitic character and later on their non-ferritic weathering. As was proved by two drillings into the unweathered parts of this contact-metasomatically altered bedrock, which were located by reason of the results of the laboratory investigations on the weathering products, the contact-metasomatism resulted in important changes of the mineralogy and chemistry with respect to the epimetamorphic schists and conglomerates of the Orapu-Formation. Mineralogically the contact-metasomatically altered rocks of the Grosgroup (figure 25, WFI) consist of chlorite- and calcite-bearing epidote-sericite-albitequartz schists, whereas the regional-metamorphic schists of the Orapu-Formation consist only of (chlorite-, chloritoïd- and plagioclase-bearing) sericite-quartz schists and -conglomerates. The contact-metasomatically altered wallrock of the gold-bearing zone of Pakira Hill (De Jong Zuid, LB 65) consists of a biotite-bearing quartz-sericiteepidote-albite rock. Table 4 shows the estimated mineralogical changes caused by the contact-metasomatism. The process of contact-metasomatism mineralogically results in the formation of albite and epidote mainly and also of calcite in the rocks of the Grosgroup and of biotite in the rock of the Pakira Hill. Chemically the contact-metasomatism is characterised by the supply of soda and lime into the schists and conglomerates of the Orapu-Formation, which form albite and epidote in a first stage, synchronous with desilicification of the pre-existing conglomerates and schists; whereas in a later stage carbonatisation and silicification occurred. This stage of silicification was found in the drilling WF I. Here small quartz and calcite veins cut through the newly formed albite (figure 46). In the drilling LB 65 only the first stage was found. Possibly the equivalent of the second stage of the contact-processes here can be found in the gold-bearing quartz veins. It seems very probable that the processes, which caused contact-metasomatism and those which caused gold mineralisation and quartz veins, are genetically related and that both result from the intrusion of a hidden granitic intrusion which exists under the savannah-area (figure 26). It is probable that the residual solutions which escaped from the intruding magma, reacted with the invaded bedrock. As a result of the reactions with the invaded bedrock an exchange of elements occurred, whereby the residual solution became more concentrated with Si02, whereas the invaded rock became enriched in sodium and calcium. The contactmetasomatism of the Grosgroup is here interpreted as a result of diffuse processes, whereas the gold-bearing quartz zones are the results of more concentrated solutions, which acted along shear planes and minor faults. The figures 51 and 52 illustrate the relative supply and removal of the elements, joined into groups si, al, fm, c and alk after P. Niggli (1945). In figure 51 the removal and relative concentration of elements from contactmetasomatically altered rocks to their weathering products is shown and in figure 52 the relative supply and removal from the epimetamorphic subgraywacke (WB III) to the contact-metasomatically altered rocks of the drillings WF I and LB 65 and then to the weathering product of LB 65 is shown. Figure 53 illustrates the same supply and removal of the elements on the supposition that Al2O3 was supplied nor carried off during the processes. The analyses are compared by the same calculated atom equivalent of 282 for ½(Al2O3). 1. Gold-bearing quartz veins, veinlets and lenses Figures 25 and 26 show the relations between granites, zones of contactmetasomatism and the gold-bearing and related quartz veins, as they were interpreted by field evidence and laboratory studies. Along the northern boundary of the savannah-area (De Jong Noord) five zones of gold-bearing and genetically related quartz veins are found. Each zone is characterised by its own paragenesis of the quartz veins which usually strike in a similar direction and occupy a certain area. The gold-bearing deposits occur as separated groups of quartz veins and -lenses, called reefs (figures 27, 28, 29 and 30). Thus a zone has several reefs, characterised by the same paragenesis. Every reef consists of several quartz veins and -lenses either with the same paragenesis (De Jong Noord) or of more types of quartz veins and -lenses each with their own paragenesis (De Jong Zuid). Five types of paragenesis are distinguished, being: a. Gold- and pyrite-bearing quartz veins. b. Gold-bearing blue quartz veins. e. genetically related, non-gold-bearing turmaline-quartz veins. d. Gold- and ferberite-bearing quartz veins. e. genetically related, non-gold-bearing white quartz veins. This subdivision is made of the quartz veins in the Mindrinetti-goldfields but holds good for the other concession areas too, i. c. the concessions on the upper course of the Lawa, the concession Ettenberg opposite Stoelmanseiland, Headley-Reef near Km 124,3 L. S. and Concession Doorson on the southern slope of the Nassau mountains. Some other types of paragenesis are found on these concessions but they are of secondary importance with respect to the gold-bearing primary deposits. Not all types occur on the same concession. Type d was found only at one place in Surinam (Pakira Hill, De Jong Zuid). a. Gold- an pyrite-bearing quartz veins The gold- and pyrite-bearing quartz veins are the most common and most easily discovered type of primary gold deposit of the Mindrinetti-goldfields. Here they are characterised by contact zones of metasomatically altered wall rock, as was described before. This wall rock has been altered by weathering into kaolinitic-clay and often shows schistose- or conglomeratic relicstructures. Gold-bearing, limonitised pyrite often occurs in this wall rock. Figures 27, 30 and 31 show sections through the gold-bearing primary deposits of this type, whereas figure 25 shows the distribution of the zones where reefs of this type occur (reefs marked P.). Assays of the quartz are given in table 8. Table 9 shows assays of goldbearing, limonitised pyrite from the wall rock of the quartz veins of this type. The gold occurs scattered through the quartz between the boundaries of grains and in cracks of the quartz. Gold also occurs in the surrounding matrix of the limonitised pyrite, which has been impregnated by iron oxides and hydroxides (figures 57 and 58). Here it appears as fine specks or sometimes as flaky gold in the crystal-negatives of completely-weathered pyrite. Figures 59 and 60 show the stages of weathering of the pyrite. The white parts of figure 60 are the non-weathered pyrite, surrounded by low reflecting material (limonite I). The dark grey parts further away from the pyrite also consist of limonite (limonite II). The light-grey parts represent the final stage of the weathering of the pyrite. They consist of hematite. Macroscopically visible gold is seldom encountered in these gold-bearing limonitised pyrites. One sample of the last mentioned type was presented to me by the Sarakreek Goudvelden N.V. for laboratory investigations. The figures 61 and 62 show the morphology of the gold (white) in the limonitised pyrite (grey). This morphology is considered to originate from the weathering processes of the limonite and as a result of transportation of the gold by supergene solutions. Beside gold-bearing, limonitised pyrites and gold-bearing quartzes which result from the weathering of the pyrites, primary gold of hypogene origin occurs in the gold-bearing quartz veins of this type of deposits. Figure 64 shows the outlines of primary gold from these quartz veins. As a result of limonitic-weathering of gold-bearing pyrites in the wall rock of these quartz veins, limonite and hematite walls occur which enclose the veins. Secondary enrichment has occurred on the boundary between the limonite-hematite and the quartz. Here the gold has been concentrated as a result of supergene processes. These gold-bearing limonite-hematite walls along the quartz veins are known as """"tjaps"""" in Surinam. The supergene processes however didn't contribute to the amount of gold of the gold-bearing deposits as a whole. Only slight transport and concentration over a distance measurable in cm's, resulted from these supergene processes. Of course it is possible that relative concentration occurred by carrying off of other formerly existing minerals. b. Gold-bearing blue quartz veins Gold-bearing greyblue, blue and blueblack quartz veins occur in the goldfields of Surinam. They were studied especially on De Jong Noord (figure 25), De Jong Zuid and on the concessions on the upper course of the Lawa river. One specimen of an extremely deep blueblack colour proved to enclose millions of semi-opaque inclusions. The walls of the inclusions are coated by particles of solid matter, whereas the content of the inclusions consists of liquid- and gaseous CO2. By concentrating the solid matter from the inclusions, through treatment of the quartz with HF, and by X-ray analyses of the thus obtained concentrate, the solid matter which causes the colour of the blue quartz was proved to be graphite. Primary gold of hypogene origin was studied in a specimen of bluegrey quartz from the concessions on the upper course of the Lawa river. The gold is irregularly distributed through the quartz (figure 65) and contains inclusions of three different minerals beside quartz, which is replaced by the gold (figure 66). One of the included minerals is probably chalcopyrite, which possibly encloses valleriite. The blue quartz veins on De Jong Noord are not characterised by contactzones of kaolinitic-weathered, metasomatically altered wall rock. On De Jong Zuid this type of quartz vein occurs together with the gold- and pyrite-bearing type of quartz veins in a zone of strongly contact-metasomatically altered and later weathered wall rock. Here they are probably younger than the gold- and pyrite-bearing quartz veins because they change into turmaline-bearing blue quartz veins as a transition between the gold-bearing blue quartz veins and the non-gold-bearing turmaline-bearing white quartz veins. e. Genetically related, turmaline-bearing white quartz veins On De Jong Zuid the quartz veins of this type are certainly younger than the gold- and pyrite-bearing quartz veins, because they cut through these veins. On De Jong Noord two zones of non-gold-bearing turmaline-quartz veins border the zones of gold-bearing quartz veins. d. Gold- and ferberite-bearing quartz veins Ferberite was found at only one place on De Jong Zuid. The deposit occurs on Pakira Hill in a zone of contact-metasomatically altered wall rock
|Leidse Geologische Mededelingen|
|Released under the CC-BY 4.0 ("Attribution") License|
|Organisation||Naturalis journals & series|
Brinck, J.W. (1956). Goudafzettingen in Suriname (Gold deposits in Surinam). Leidse Geologische Mededelingen, 21(1), 1–246.