Herrera 1968

conomi Geology Vol. 63, 1968, pp. 13-29

Geochemical Evolution of Zoned Pegmatites of Argentina AMILCAR O. HERRERA

Abstract

The pegmatites in the Sierras Pampeanas of Argentina are classified into four main types on the basis of their internal structure, composition, and mineral paragenesis. The first type is distinguished by oligoclase-bearing wall zones and the second type by intermediate zones of quartz, plagioclase and microcline, and by higher overall percentage of potassium feldspar. The third type is characterized by very simple zonal structure, by a greater diversity and abundance of accessory minerals and by the presence of sodium-potassium replacement units. In the fourth type spodumene is present, the percentage of plagioclase (albite) is greater and that of microcline is lesser than in the other types.

The four types of pegmatites constitute an essentially continuous series, their characteristics reflecting the course of a single broad genetic process. Their formation is described in terms of the evolution of paragenetic associations that retain their identity throughout all or a great part of this process. These associations, listed in the inferred order of formation are: 1) border association; 2) plagioclase association; 3) microcline association; 4) albite association; 5) spodumene association; 6) core margin association; 7) Na, K and Li replacement; 8) quartz core.

A brief analysis of some of the more complete descriptions of pegmatite districts published over recent years, shows that the adopted scheme of classification and genesis is applicable to pegmatites of other regions of the world.

The generalized sequence of mineral assemblages that characterizes the evolution of the Sierras Pampeanas pegmatites is established and correlated with the paragenetic associations. The sequence is as follows: 1) plagioclase-quartz, 2) microcline-quartz- plagioclase, 3) microcline-quartz, 4) albite, quartz, 5) albite-microcline-quartz, 6) microcline-albite-quartz-spodumene, 7) microcline-quartz-spodumene. The comparison of this sequence with that listed by Cameron et al. for the principal districts of U.S.A. shows ' that both groups of pegmatites followed a similar genetic course.

The evolution of pegmatite development can be characterized according to the role of the cations Na, K and Li in the different phases of the process. The average content of CaO, Na20, K20, and Li2 in the four types of pegmatites are calculated. The variations in composition throughout the genetic series show a close similarity with those of other groups of pegmatites from different parts of the world.

Accepting the hypothesis that pegmatites result from the crystallization of residual magmatic fluids, the observed series would represent the general trend of evolution of the fluids. The process itself, though essentially continuous, does not develop uniformly but is instead characterized by a succession of compositional steps.

The occurrence of bodies of different bulk composition does not necessarily imply the existence of different magma types. The zonal distribution indicates that the type of pegmatite would depend largely on the depth of consolidation of the intrusive body. Furthermore, the chemical evolution of the fluids must be supplemented by the adequate tectonic Conditions to enable the separation and injection of the derived fluids.

In conclusion, the following genetic scheme for the pegmatites of the Sierras Pam- peanas is proposed: a) the pegmatites originated from residual liquids of changing composition successively emitted from cooling magma chambers, b) the process in all the emission centers was essentially similar, c) the present distribution of the pegmatites was determined by the depth of the magmatic loci and by the subsequent erosion.

Introduction

IN recent decades, and mostly since Fersman's mas- terly works, pegmatites have been intensively studied all over the world, with the result that great advances have been made in the understanding of the processes

by which they were formed. Nevertheless, no genetic theory yet advanced seems totally in accord with all the observed facts. The main difficulty in this field is the lack of experimental data applicable to the complex physico-chemical processes that take place

13

14 AMILCAR O. HERRERA

in a residual aqueous-magmatic fraction. Progress is further hindered by the diversity of field criteria applied in the determination of paragenetic groups and sequences and from the scarcity of reliable data on the composition of pegmatites.

During the last years, the author (Herrera, 1958, 1961, 1963, 1964) has made studies in almost all the pegrnatite districts of Argentina, giving special atten- tion to the internal structures of the pegrnatites and related mineral distribution and to the composition and abundance of the replacement and fracture filling units. The information gathered and herein discussed seems sufficient to justify a tentative interpre- tation of the geochemical evolution of pegrnatites, which the author feels could also be applied to other regions of the world. Nevertheless, it is evident that this first assay can constitute only a very general approach, because the information about some as- pects of the problem--particularly detailed mineralogy and distribution of trace elements---is too poor to establish a more elaborated genetic theory. In spite of these deficiencies, the ordering of known facts within a coherent scheme should be useful as a working hypothesis for other investigators inter- ested in this subject.

The Genetic Classifications o Pegmatites The view that differences between various types of

pegrnatites can be attributed to their having developed during different stages of a fundamentally continuous genetic process has been expressed by many authors, especially Fersman (1930), Cameron et al. (1949), Guinsbourg (1960) and Brotzen (1959).

Fersman (1930, p. 228) distinguishes on the basis of their mineralogy ten types of granitic pegrnatites in correspondence with seven genetic phases: epimagmatic (700/800 ), pegrnatitic (700/600 ), pegmatoid (600/500), hypercritic (500/400), high hydrothermal (400/300), medium hydrothermal (300/200 ) and low hydrothermal (200/100 ).

Fersman's classification has strongly influenced many later ones, but is now rarely used in its original form. One of its main disadvantages lies in placing certain minerals in too definite temperature ranges and phases of formation that makes it somewhat un- realistic and difficult to apply. Furthermore, Fers- man's classification is based mostly on the presence of accessory minerals--tourmaline, mica, beryl, topaz, etc., which fails to reflect the evolution of the genetic process characterized, as demonstrated by other investigators (Guinsbourg, 1960; Solodor, 1959) by the differentiation of the alkalies.

Variations in quantity and distribution of the accessory minerals are too great for them to constitute the basis of a useful classification. This may easily be shown by reference to several Argentine examples.

In Fersman's scheme, muscovite appears in the beginning of the pneumatolytic stage and therefore it follows potassium feldspar, which corresponds to the epimagmatic stage (Ferman 1930, fig. 65-66, p. 464). Nevertheless, in most of the Argentine mica-bearing pegmatites, commercial muscovite appears in quartz- plagioclase wall zones and is consequently earlier than the potassium feldspar (Herrera, 1958, 1961, 1964). The same happens with the phosphates, which in Fersman's scheme (1930, p. 303) characterize the medium hydrothermal stage and appear as a subordinate phase in high hydrothermal stage. In - the pegrnatites of San Luis and Catamarca (Herrera , 1963, 1964), the phosphate phase is fully developed in the beryl-bearing pegrnatites, which correspond to Fersman's hypercritic stage. Similar discrepancies in other parts of the world have also been pointed out by other authors (Shand, 1942; Brotzen, 1959).

Cameron et al. (1949), while not dividing pegmatites in expressly differentiated groups, establish a sequence of mineralogical assemblages that reflects, in a general way, the entire course of the genetic process. This classification has been very useful in describing the zoning and paragenesis of individual pegmatites or groups of bodies belonging to the same genetic type, but it presents limitations that spring mainly from its structural character. Thus, when comparing pegmatites belonging to different genetic types, the zones are grouped according with their mineralogy and structural position, which gives sequences that do not represent accurately the evolution of the total genetic process. In the tables giving the sequences of mineral assemblages of the Black Hills pegmatites, for instance, the plagioclase-quartz assemblages of the wall zones of the spodumene-bearing pegmatites and that of the non-lithium-bearing bodies appear as a single unit due to their equivalent structural position in the bodies. In my opinion, however, these two assemblages, as will be seen later, represent two different stages of genetic development. Another drawback is that the sequence does not show clearly the place and importance of alkali- replacement stages because they do not occur in definite spatial zones.

Solodov's (1959) classification refers to rare metal- bearing granitic pegrnatites and establishes four types based on the relative abundance of microcline, albite, and spodumene. This classification is fundamentally descriptive and does not consider the characteristics and relative importance of the various phases of the genetic process (primary crystallization, replacement, etc.).

Guinsbourg (1960) groups pegrnatites into three types that differ in mineralogy, structure, age and specific geochemical characteristics such as trace ele- ment content and the nature of replacement. This

GEOCHEMICAL EVOLUTION OF ZONED PEGMATITES 15

classification, besides being essentially descriptive, considers factors such as depth, age, type and composition of the associated intrusives, which not only are difficult to determine, but also vary considerably in different parts of the world as judged from the existing literature. Furthermore, in considering the composition of the deepest group the author assigns more importance to the role of country rock assimila- tion than most current investigators are ready to accept.

Brotzen (1959) distinguishes four main stages within an essentially continuous genetic process, each represented by a pegrnatite type of characteristic mineralization, which is more or less regularly distributed in the zonal sequence. The types and series thus resulting are similar to those proposed by Solo- dor and Guinsbourg. This classification is based on the evolution of paragenetic groups or units that maintain their identity and succession throughout the whole process. In contrast previous classificationsin especially those of Fersman and Cameron et al.-- reflect principally the course of the process in individual pegrnatites. Thus, Brotzen's classification allows an adequate grouping of bodies which are closely related from a genetic point of view, even when they have considerable mineralogical differences, especially as regards accessory minerals. Fur- ther, it does not establish rigid limits between the types, these being based on the division of a continuous genetic process, hence it is sufficiently flexible to include transitional or atypical forms. However, there are some limitations in this approach, particularly regarding the criteria used to characterize the paragenetic units.

Brotzen's paragenetic units are: 1--granitoid zone (border); 2--graphic granite zone; 3--pegmatoid zone; 4 core margin; 5--alkali replacements; 6 core margin mica; and 7--alteration products.

From a genetic point of view, units 2 and 3 are the most important ones, but in my opinion they have been differentiated following questionable criteria. The distinction of a graphic granite and a pegrnatoid zone is evidently based on Fersman's scheme that distinguishes--following an epimagrnatic phase--a pegrnatitic phase during which graphic granite forms followed by a pegmatoid phase characterized by a coarse-grained, irregular texture. The pegrnatitic phase corresponds to the end of the epimagmatic stage, and the pegrnatoid to the pneumatolytic stage (Fersman 1930, Vol. I, p. 53-57).

The difficulty in assigning a precise genetic mean- ing to the units considered, can be clearly exemplified

x Both these authors consider graphic granite to be a quartz-microcline or quartz-plagioclase intergrowth, indis- tinctly. Therefore, in all their descriptions, the quartz- plagioclase wall zones characteristic of many of our type 1 pegmatites, are included under graphic granite.

by the following facts. The graphic granite zone, as Brotzen himself admits (1959, p. 42) is not constant, even within a given type of pegrnatite. It is completely absent from some bodies and in others it is only represented by a few dispersed crystals. In Argentine districts it is common to find bodies without graphic granite but having all the other charac- ters such as zonality, mineralogy, etc. found in bodies with well developed graphic granite.

With regard to the pegmatoid zone, Fersman uses the term "pegrnatoid" in two different senses: as a textural term--following Evans (Fersman 1931, Vol. 1, p. 139)--to name "coarse-grained facies in igneous rocks of a pegrnatitic habit, but which differ from real pegrnatites due to the absence of graphic granite" and as a genetic term, as discussed before. Brotzen uses it mainly in the latter sense, the pegmatoid zone being the part of a pegrnatite comprised between two limits which, in his opinion, constitute discontinuities in the genetic process: the point at which graphic granite ceases to form and the transition zone, "core margin," between the core and the pegmatoid zone. Defined thus, the pegmatoid zone comprises units of markedly different composition and texture that range from granitoid zones, almost exclusively composed of plagioclase and quartz, to giant porphyritic zones, consisting principally of microdine or spodumene. These variations in composition evidently correspond to different stages of the genetic process, as they are always accompanied by marked changes in zonality, in the type and relative abundance of the essential minerals, and in the composition and quantitative importance of replacement units.

These problems stem from the difficulty in estab- lishing accurately the physical-chemical characteristics of the pegmatogenic process. This is one of the reasons why most recent investigators have aban- doned Fersman's system and have favored classifications and genetic schemes based on the changes in composition of the pegrnatitic system, as reflected principally in the sequence of essential mineral associations. This last approach underlies the work of the modern American school and of most contem- porary Russian authors.

The Pegmatites of the Sierras Pampeanas The pegrnatite bodies of the Sierras Pampeanas 2

(Fig. 1) can be grouped into four fundamental types, according to their internal structure, composition and mineralogical associations. In the descriptions

2 The Sierras Pampeanas is a morpho-structural province of the Argentine territory consisting of block-mountains of crystalline rocks surrounded by flat, wide tectonic depres- sions filled up mostly by Tertiary and Quaternary sediments. This province comprises a considerable part of central and northwestern Argentina (see GonZalez Bonorino, 1950).


LA RIOJA

66

SANTIA_G_O DEL ESTERO:_

--

__

___-_?;?*--- _ _

HETAHORPHIC RO. CKS ( Precombrian ? ) 6RANITIC ROCKS

SEDIHENTARY COVER /"'"'"":/" ( Pa[eozoic to Tertiary) [1 QUATERNARY

VALLE FERTI O

--

0 ALTA

PEGHATITES:

TYPES 1 AND 2

TYPE 3

TYPE 4

0 S0 100 Km : ...... LUIS i

O RIO CUARTO

FxG. 1. Map showing the distribution of pegmatites in the Sierras Pampeanas region. Each symbol represents from one to several pegmatites. (Geology based on Mapa Geo16- gico de la Repfiblica Argentina. Scale X1-2.500.000 Direcci6n Nacional de Geologla y Minefla. Repfiblica Argentina, 1964.)


that follow the zonal terminology established by Cameron et al. (1949) will be used.

Type /.--These pegmatites show generally thin tabular or lenticular bodies. The thickness to length ratio can reach 1:40, and rarely exceeds 1:15.

This type of pegmatite is found in Valle Frtil, Alta Gracia, and in the western group of pegmatites of Catamarca. Its mineralogy and internal structure are simple and characterized by a plagioclase-quartz wall zone. Plagioclase (An 15-25) constitutes about 60 percent of this rock. A quartz-microcline core or a quartz-microcline intermediate zone and quartz core typically lie inside the wall zone. The quartz and microcline of the intermediate zone occur either in approximately equivalent amounts or the feldspar predominates. They appear in more or less equal amounts in bodies without quartz core. A microcline-quartz-plagioclase zone, the most characteristic unit of type 2, is represented in many bodies of type I as a poorly developed, discontinuous intermediate zone. The plagioclase is more sodic than that of the wall zone. A pronounced feature of this group of pegmatites is the presence of clearcut zone boundaries.

These pegmatites, together with those of type 2, constitute practically the only source of commercial muscovite in this region. This mineral is found mostly in the plagioclase-quartz wall zone, either dispersed throughout it or as a discontinuous band on its inner or outer boundary. The most common accessory minerals are biotite, apatite, garnet and monazite. Topaz occurs in the border zone and tourmaline (schorlite) is also common, though far' less so than in the other types of pegmatite bodies.

Typical replacement units are not found in this group, though an incipient replacement of the fracture walls can be observed in certain tabular fracture filling units composed bf muscovite and quartz.

Type 2.--Lenticular or tabular forms predominate. Like those of type 1 they are relatively thin, but their length-thickness ratio seldom reaches the extreme values frequent in the first group; this ratio ranges from 1: 25 to 1: 10. These pegmatites occur north of the city of Catamarca, especially in the Sierras de Farifiango and Humaya, and make up the greater part of those of Valle Fdrtil, San Juan, and Sierra de Comechingones.

This group is characterized by a microcline-quartz- plagioclase wall zone. In this zone potassium feldspar generally predominates averaging about 50 percent of the zone; plagioclase (An 5-20) does not amount to more than 30 percent. There is also an intermediate zone of quartz and microcline--microcline constitutes 60 to 80 percent of the rock---and a quartz core is present in most of the bodies. In some bodies one of these units is absent, so that the other

constitutes its central part. Graphic granite is commonly found in the quartz and microcline intermediate zones.

Some pegmatites in this group lack a quartz- microcline intermediate zone; apart from the border zone and core, they are composed essentially of potassium feldspar, with some quartz and very little plagioclase. These are considered transitional with type 3 from which they differ by having more calcic plagioclase (An 5-15%) and more muscovite and in accessories. Biotite commonly is present, but beryl iron, manganese and lithium phosphates are rare or absent. On the other hand, some pegmatites of this group are similar to those of type 1 in having a narrow quartz-plagioclase wall zone.

The main economic product of these bodies is muscovite, which appears principally in the wall zone. Tourmaline is more abundant than in type 1 pegmatites and beryl is scanty. Like those of type 1, these pegmatites do not contain typical replacement units.

Type 3.--Tabular bodies also predominat. e, though irregular forms are more common. The thickness to length ratio is difficult to determine due to bad exposures, but is estimated to range from 1:10 to 1:15.

The pegmatites of Sierra de Velazco, La Rioja, although belonging to this group, are a notable exception with regard to shape. Nearly all of them are irregular, and globular forms predominate. This probably can be attributed to their having consoli- dated within an incompletely crystallized granite. The pegmatites of other districts, in contrast, were injected into fractures in schists.

These pegmatites are the most widely distributed in Argentina. They are the predominant type in San Luis and in the Sierras de Ancasti, Comechin- gones and Velazco.

Bodies of this group are characterized by simple zonal structure, predominance of potassium feldspar, greater abundance and wider diversity of accessory minerals as compared with the two previous types, and by the occurrence of sodium-potassium replacement units. Apart from the border zone and quartz core, practically all these pegmatites contain only a thick quartz-microcline zone, which constitutes 60 to 70 percent of the total volume. The composition and texture of these zones typically change from the periphery to the center of a body. The percentage of quartz is generally larger at the external border, where the grain size is smaller than in the rest of the unit. Then follows a band of graphic granite with quartz inclusions that are larger towards the center. This layer occurs only near the margin of the zone. The percentage of quartz diminishes inwards, and near the core the unit is composed almost exclusively of large microcline masses. The average composition


of these zones is approximately microcline 70-90 percent, quartz 30-10 percent.

A narrow and generally discontinuous plagioclase- quartz zone of fine to medium grain size appears in some bodies. It occurs between the border zone and quartz-microcline zone and seems to represent a par- tial development of the wall zone characteristic of the preceding groups, bfit with a more sodic plagioclase (An 4-12).

The accessory minerals are markedly different from those of the two previous groups. Muscovite is relatively scarce, is fine to medium grained and is uniformly distributed throughout, except in the core. Biotite is almost completely absent, and garnet and monazite are rare. Topaz, which in the previous types occurs almost exclusively near the outer margin of the border zone, occurs in places as large masses in internal zones, especially near the border of the core. Beryl, the main economic product of these bodies, is found in every zone except the center of the quartz core, but the main concentrations, comprising idiomorphic crystals of 4 or more meters long, occur at the contacts between the core and microcline-quartz zone. Among the other minerals present in bodies of this group the most characteristic are the phosphates of iron, manganese, and lithium: triplite, triphyllite, and lithiophyllite. They generally occur in the inner parts of the quartz-microcline zone, especially near the core and commonly in association with albitic replacement bodies. Bismuth minerals, cassiterite and columbite-tantalite also are common.

Unlike the previous types, these pegrnatites include abundant sodium-potassium replacement units, which almost always are found in the inner parts of the quartz microcline zones. They are composed of cleavelandite, cleavelandite and muscovite, cleavelandite and quartz, and muscovite. The characteristics of these bodies have been described elsewhere (Herrera, 1963, pp. 56-58; 1964, p. 49-50). The relative volume of the replacement units, compared with the total volume of the pegmatites, does not exceed 5 percent.

Type 4.----As in the three previous types, elongated forms predominate, but ovoid or globular bodies are much more common. The thickness to length ratio is greater than in the other groups, and ranges between 1/5 and 1/15, exceptionally reaching 1/20.

The pegrnatites known to belong to this group are found in San Luis, C6rdoba and in the Sierra de Ancasti, Catamarca.

The main characteristics that distinguish this group of pegrnatites from the previous ones are the presence of spodumene-bearing zones, a greater abundance of plagioclase, and a considerable lesser abundance of microcline.

The zonal structure is rather complex, and zone boundaries are difficult to establish with precision. The wall zone is composed of quartz and albite or of quartz microcline, and albite. A quartz-albite wall zone commonly is accompanied by an intermediate zone of quartz-microcline and albite. Then follow the spodumene-bearing zones. This mineral is associated with quartz, albite and microcline in varying amounts, but quartz and albite generally are predominant. Spodumene occurs in idiomorphic and generally large crystals, included in a matrix formed by the other minerals. In some bodies, a quartz core is absent, and in these the central part has a composition similar to that of the intermediate zones described.

The plagioclase of these pegmatites is almost wholly cleavelandite, Ab 98-100 percent in composition. Microcline averages approximately 10 percent albite in perthitic intergrowth, which contrasts with that of the other groups, where it averages 20 percent.

This type of pegrnatite differs notably in bulk composition from the previous groups. Microcline rarely exceeds 10 or' 15 percent, and the average abundance is considerably less (see chapter on composition); in other groups it is invariably greater than 30 percent, and can surpass 50 percent. Plagio- clase, in contrast, is more abundant in this type amounting to almost 35 percent (including that con- tained in perthite), whereas in the other types it averages 25 percent. Spodumene averages 10 to 25 percent.

Some pegrnatite bodies that contain no spodumene and very little lithium also belong to this group as shown by their other characteristics. A typical example is the San Elias pegrnatite, in San Luis (Her- rera 1963), which does not contain spodumene; lithium is found in a relatively small discontinuous unit composed of lepidolite, cleavelandite, and amblygonite. The total lithium content is very small, but the presence of a typical quartz albite wall-zone and the composition of the intermediate zone--where quartz and albite predominate over microcline--are both characteristic of this group.

The accessory minerals include those in the other pegrnatite groups, together with several others such as amblygonite and lepidolite. Muscovite is scarce and fine grained. Beryl is consistently present, but is far less abundant than in type 3. Differences in composition are observed in some of the minerals common to both groups. Thus, in this group, the Ta2Os:Nb20 ratio in the columbite-tantalite minerals ranges between 1.3 and 4, whereas in type 3, it is generally less than 1 (Herrera 1963). Tourma- line, rarer than in the previous group, is generally an alkaline variety. Also, cassiterite is far more


abundant than in type 3, and is generally associated with albite masses.

Replacement bodies are predominantly albitic. It is sometimes difficult to estimate the amount of replacement as these pegrnatites also are rich in primary albite that is not easily distinguished from the replacement variety. In some of the bodies practically all the albite appears to be primary, whereas in .others, widespread replacement of microcline and spodumene by masses of albite is observed. Ambly- gonite, triplite, triphyllite, lithiophillite, apatite, cassiterite, and tantalite-columbite are generally associated with these replacement bodies. Replacement of microcline and spodumene by fine-grained green- sh muscovite also is recognizable in all these bodies.

Genesis o1 the Pe#matites.--The pegmatites of the Sierras Pampeanas were formed by crystallization, from the wall inwards, of a magmatic liquid force- fully injected into fractures. The main evidences that favor this hypothesis are the following:

1. Most of the pegmatites are discordant, tabular bodies, parallel to regional fracture systems commonly occupied by dikes of varied composition. In many instances the schists in contact with the pegrnatites show small folds that were probably produced by differential movements along the fault planes prior to the intrusion. In the wall zone of the pegmatite La Totora, in San Luis, there are included randomly oriented slabs of a granitic dike, up to 40 cm wide, which constitutes one of the walls of the body. The dike was probably fractured by movements that pre- ceded or accompanied the forceful injection of the pegmatitic fluids, which engulfed the fragments in their ascension.

2. In some pegmatites there are fracture-filling units composed mostly of quartz or microcline and quartz, which starting from inner zones of the same composition transect the outer zones. These units, and the replacement bodies concentrated close to the core, are convincing evidences of the existence of residual fluids in the central parts of the pegmatites after the outer zones have crystallized.

3. Many of the features of the pegmatites show a systematic change from the walls inwards. The most important are the increasing grain size, the distribution. of minerals in concentric zones, and the changes in mineral composition such as the increase of anor- thite content of plagioclase and the change from black tourmaline in the outer zones of type 4 pegmatites to alkaline varieties in the inner parts.

4. In the lithium-bearing zones of some pegmatites the crystals of spodumene are tapered and normal to the contact, with the broad end pointing inwards (Herrera, 1963, Fig. 4). This feature, as has been pointed out by Jahns (1953) can be best explained

by assuming that the crystallization of the pegmatites proceeded from the walls inward.

5. The contacts are sharp, with the only exception of some of the pegmatites of La Rioja, that are emplaced in a granitic country rock.

Discussion of Paragenesis The four types of pegmatites described above seem

to reflect the course of a single genetic process. Fol- lowing Brotzen's basic genetic scheme, I consider their genesis as the result of the evolution of paragenetic units or groups that appear within each peg- 'matite in a certain order. Unlike Brotzen, however, and for the reasons given before, mineralogical criteria will be used to differentiate the associations representing the stages between crystallization of the border zone and that of the core margin.

The following paragenetic associations, placed in the order of formation within individual bodies, de- scribe the process: t--Border association, 2--plagioclase association, 3--microcline association, 4 albite association, 5--spodumene association, 6--core margin association, 7--Na, K and Li replacements, 8-- quartz core.

Associations 2, 3, 4 and 5 are named after the minerals that characterize them although they may not always be the most abundant constituents. All the associations, excepting 7, correspond to zones or groups of zones and are thus of limited distribution within a body. Replacement units, on the other hand, though more abundant near the core, can occur anywhere. The paragenetic units follow each other regularly throughout the pegmatogenic process. In general terms, the evolution of each pegmatitic body, as evidenced by its internal structure, shows the general trend of the process originati.ng the different groups of pegmatites.

Each paragenetic group can be characterized briefly, as follows:

Border tssociation.--This association was amply described in previous papers and only its variations in the different types of pegmatites will be mentioned. It is characterized by its small width (1 or 2 mm up to a few centimeters), by its granitic texture, and by the crystals tending to be perpendicular to the contacts. The composition is relatively constant in terms of the essential minerals--plagioclase, quartz and muscovite--but muscovite is less abundant in type 3 and scarce in type 4 wherein a little microcline appears. Greater variation is found in the accessory minerals. In types 1 and 2 they commonly are topaz, biotite, vermiculite, garnet, apatite and tourmaline, the last two being much more abundant in type 2. In type 3, biotite is absent, tourmaline and apatite increase and beryl, cassiterite and tantalite-


columbite appear. In type 4 the same accessory minerals are found, but part of the tourmaline commonly belongs to the alkaline varieties, and there is a greater amount of cassiterite and tantalite-columbite.

Plagioclase .4ssociation.--This paragenetic unit comprises all zones containing An 5-25 plagioclase. It is characteristic of types 1 and 2 and can be divided into 2 sub-associations. The first sub-association is composed of a fine to coarse-grained quartz and plagioclase intergrowth and is characteristic of wall- zones of type 1. The mineral content is constant: 60-70% plagioclase; 30-40% quartz. The main accessory is muscovite, which can constitute up to 5 percent of the zone and tends to concentrate in dense bands at the contacts. Biotite, garnet and monazite appear in lesser quantities.

The second sub-association is genetically later and occurs in type 1 pegmatites as an intermediate zone, and in type 2 as a characteristic wall zone. It is composed of microcline, quartz and plagioclase, the first two generally predominating. The texture varies from granitoid to porphyritic, microcline crystals being the most developed ones. In contrast to the first subtype, its average composition varies considerably, not only in the same body, but also in different pegrnatites of the same group. Within the same body the sodium content in plagioclase increases towards the unit's interior and, within the paragenetic series, from type 1 to 2. Graphic granite is very common and occurs in the zone's outer margin, its texture becoming coarser and more irregular inwards. Muscovite is again the most important accessory mineral. Biotite, garnet and monazite also occur. Apatite and tourmaline, rare in the quartz- plagioclase sub-association, are characteristic com- ponents.

Microcline .4ssociation.--This is the most important paragenetic association in volume as well as genetically. It appears in the four types and is composed of coarse to very coarse-grained perthitic microcline and quartz; microcline as subhedral to anhedral crystals and quartz filling the interstices. In type 1, potassium feldspar constitutes 50--60 percent of the total volume of this association. The proportion increases in the remaining types up to 80-90 percent in type 3, where graphic granite shows its greatest development. The plagioclase content in perthires is about 20 percent. Potassium feldspar is usually gray near the center and pink towards the periphery.

A notable feature in this association is the scarcity of accessory minerals. Muscovite is scanty and tourmaline is found in type 2 and especially in type 3.

.41bite .4ssociation.--This association is found in some pegrnatites of type 3, and in most of type 4. In type 3 pegrnatites it occurs as a thick zone of

saccharoidal albite, in contact with the core. The most characteristic accessory minerals are beryl, cassiterite, tantalite-columbite and manganese, iron, and lithium phosphates. Type 3 pegrnatites bearing this association are poorer in potassium than the rest. According with their bulk composition, they constitute a group transitional between types 3 and 4. In Argentina they are found in San Luis, especially in the Independencia Argentina group. The albite zone in type 4 pegmatites differs from the above in containing abundant quartz (60 to 70%) and laminar cleavelandite. The exact position of this zone within the genetic sequence is somewhat difficult to establish. In Argentine pegmatites with spodumene it is found as a wall zone surrounding the spodumene zones (La Rosada, Maria del Huerto and Cema pegmatites; Angelelli and Rinaldi, 1963). This has also been observed in some of the U. S. A. peg- matires (Edison Spodumene, Bob Ingersoll No. 1, Hardesty Homestead and Buckhorn pegrnatites), described by Cameron et al. In all these bodies, as well as in those of group 3 already mentioned, this zone appears before the unit with lithium silicates and after the microcline zones.

In other bodies, however, this association occurs in a somewhat different position. In San Elias pegrnatite (Herrera, 1963, p. 48), a quartz-albite zone surrounds a microcline, quartz and albite unit, and similar situations in other pegmatites have been described by Cameron et al. (1949), by Jahns (1953) and by Solodor (1960). In these bodies, the albite zone is external to the microcline units. However, these microcline units are different from those in paragenetic association 3. In the former, microcline (80--90%) and quartz are the essential com- ponents, whereas in the latter units the proportion of potassium feldspar is much smaller, and albite is always present, sometimes averaging up to 70 percent. The proportion of albite in perthitic intergrowth--approximately 10%---is lower than that in type 3. In addition, the accessory minerals--lithiophillite, amblygonite, lepidolite, cassiterite--are typical of lithium-bearing pegrnatites and are rarely found in p/tragenetic association 3. There is also an important group of lithium-bearing pegrnatites--as Cabeza de Novillo, La Totora and La Viquita (Her- rera, 1965, p. 67)---containing wall zones of microcline quartz and albite, but lacking in quartz-aibite zones. All this suggests that both units are really varieties of a single paragenetic association, that appears in the final stages of the potassium phase or in the initial stages of the lithium phase.

oCpodumene .4ssociation.--This association is found in type 4 and its spodumene content ranges from 10 to 30 percent. In most occurrences large spodumene crystals are surrounded by a much finer grained


quartz-albite matrix. Microcline also occurs, though less commonly. Spodumene crystals may be normal to the contacts. The most characteristic accessory minerals are amblygonite, apatite, triplite, tantalite- columbite, manganotantalite, and beryl.

Core-margin lssociation.--Many authors have acknowledged the existence of one or several mineralogical associations typical of the core margin, especially in relation to the distribution of beryl. Brotzen distinguishes in his evolutive scheme the core margin as a paragenetic unit "marked by the appearance of individual crystals which project into the quartz core from the pegmatoid zone, or are entirely surrounded by quartz" (Brotzen 1959, p. 45). He also

'considers convenient, for chemical reasons, that it be restricted to comprise only those minerals that formed after microcline had ceased to develop. It is, therefore, a paragenetic group originated by primary crystallization'at the end of the formation of the peripheral zones and prior to the beginning of the crystallization of the quartz core.

In Argentine type 3 pegmatites and in some of type 4, there are units composed of beryl and smoky quartz, commonly containing triplite, apatite and tantalite-columbite, that may belong to this paragenetic group. However, these units, which have been described by the author elsewhere (Herrera, 1963, p. 63; 1964, p. 47-48) are commonly associated with typical replacement bodies; hence its place within the core margin paragenetic group is some- wht uncertain. In pegrnatites of types 1 and 2, fine-grained smoky quartz and biotite intergrowths, as well as small concentrations of sulfides, probably belong to this unit.

Na, IC and Li Replacements.--This association occurs only in type 3 and 4. Replacement units do not have restrictions in their spatial distribution but are much more common and better developed in the inner parts of the microcline and quartz zones. The predominant replacement mineral in type 3 is albite, mostly laminar cleavelandite. The cleavelandite masses commonly contain quartz or colorless or greenish muscovite. The most characteristic accessory mineral is cassiterite; garnet, tantalite-columbite, apatite, triplite and lithiophillite occur less commonly.

Many of the cleavelandite and cleavelandite-quartz replacement units are clearly controlled by fractures. In many pegrnatites where these units cut through microcline they are flanked on each side by a band of fine-grained muscovite. These bands were probably produced by the same fluids that originated the replacements units and which, after losing their load of sodium and becoming enriched in potassium, pene- trated the microcline (Herrera, 1963).

In type 3 pegmatites microcline is commonly pseudomorthically replaced by muscovite or by

medium- to fine-grained aggregates of albite and muscovite. In some pegmatites of San Luis the albite-muscovite intergrowth is very fine grain.ed and has replaced selectively the potassium feldspar, preserving the veinlets of pertitic albite that are included in the aggregate (Herrera, 1963, Lam. I).

In pegrnatites of type 4 the albitic replacement is similar to that of type 3. Most of the spodumene crystals have been totally or partially replaced by sericite, muscovite and clay mineral aggregates.

In the San Elias pegrnatite, San Luis, there are two elongated bodies, about 10 m long and 0.5 to 1.5 m wide, composed of a fine-grained intergrowth of albite lepidolite and quartz, included in a quartz- microcline-plagioclase intermediate zone. These units, which transect the minerals and textures of the zone, have irregular and gradational contacts, and include islands of surrounding rock, are supposed to have been formed by replacement (Herrera, 1963). Bodies of similar composition have been attributed to replacement by other investigators (Jahns, 1953; Hutchinson, 1959; Norton et al. 1962; Sheridan et al., 1957).

Quartg Cores.--This paragenetic association has similar characteristics throughout the whole series. The accessory minerals, which occur in small amounts, and commonly in the outer border are, for each type of pegmatite, the same described for the inner zones.

The position of this association in the pegmatogenic process is somewhat uncertain. It occurs in the central part of the bodies of the four types and its formation seems to be related more to the conditions prevalent in the final stages of the individual pegrnatite evolution, than to any definite stage on the genetic process.

Paragenetic and Classification Diagram Figure 2, following Brotzen's method of repre-

sentation, shows schematically the inferred sequence of paragenetic associations and the position of the four types of pegrnatites previously described. Ac- cording to the hypothesis that pegrnatites originate from the crystallization of a residual magmatic liquid, the paragenetic sequence depicts the system's evolution as the temperature decreases, both by the type succession and by the order of crystallization within each type. The genetic position of each of the pegmatite groups studied by the author, is also shown. The minerals are shown in the associations where they are most common and abundant. Quartz, being common to all the associations, was not included in order to simplify the diagram.

The position of the minerals in the diagram is almost exclusively based on the author's personal observations, because in most of the reports pub-

22 .4MILC.4R O. HERRERA

CORE N=, Li end K BORDER SPODUNENE NARSIN REPLACEHENT TYPE OF ;OClATION ASS __ION ASSOCIATION QUARTZ

al. bit . *' cr be mu Ab be . ..

-- c t mc x Ni H; cb m

mi c - % cb op o' cb - Ab tr v

P[ Ab ' be Mu mu '% mu am be -- Li to _ i A be

Mi mu

Mu -'

-- Ni mu Mu bt Mi P to tm

X PL X . ,.

I X X -- association Nu

Nu x bt N Mi

P[ mu P essocietion Xx Nu P

to go

-Ve mo Mu

SAN LUIS

ANCASTI

ANCASTI SAN LUIS VELASCO ( Lo Riojo)

VALLE FERTIL

CATAMARCA

SIERRA BRAVA ( Lo Riojo )

ALTA $RAClA

CATAMARCA VALLE F'ERTIL ALTA $RACIA

Fro. 2. Paragenetic scheme of zoned pegmatites in the main districts of Argentina. The 21 minerals included are those for which the position in the sequence has been best established by the author. Basic diagrama from Brotzen (1959)..

Ab--Albite P1--Plagioclase Mi--Microcline Mu--Muscovite Bt--Biotite Be--Beryl Tm--Tourmaline (Schorlite)

Tc--Colored tourmaline Ap--Apatite Sp--Spodumene Am--Amblygonite Le--Lepidolite Ga--Garnet Mo--Monazite

.4 b--Predominant Ab--Important ab--Common

Bi--Bismuth Cb--Columbite-tantalite Li--Lithiophillite Tr--Triplite Ca--Cassiterite To--Topaz Ve4Vermiculite


lished in Argentina, quantitative estimates and exact position of the minerals within the pegmatite bodies are not usually given. Furthermore, even when the latter information is available, it usually proves in- sufficient to determine which association a mineral belongs to.

Most of the districts appear in two positions in the .diagram. The interval between these two positions represents the part of the genetic sequence appearing -in the district. The position of each district in the diagram represents the stage of evolution of the group of pegmatites observed, and not of any pegmatite in particular. In some bodies, parts of the sequence may be lacking. A typical example occurs among pegmatites of type 4 in San Luis; as previously noted, some of them lack the quartz-albite asso- .ciation, the sequence starting with the albite, quartz, and microcline association.

This paragenetic and classification scheme though based on the characteristics of Argentine pegmatites, can be applied, however, to other pegmatite districts of the world. Without attempting an exhaustive review of existing descriptions, a few can be taken as examples. The pegmatites in Southeastern U.S. A. (Cameron et al., 1949, p. 63) mostly belong to type 1, showing transitions to type 2 as is evidenced by the presence, in most of them, of plagioclase in the quartz-microcline zones and the absence, :in some, of quartz-plagioclase zones without microcline. Most of the New England pegrnatites belong to type 1 excepting Anderson No. 1, which belongs to -type 4. Muscovite is the only important commercial product--besides microcline and quartz--in both districts, the same as in similar Argentine districts. The pegrnatites in the Black Hills, South Dakota, comprise a wider composition range. The largest group is that of the lithium-bearing pegrnatites, which in the diagram corresponds to the base of type 4, as indicated by the presence in most of the pegmatites, of a quartz-perthite zone enveloping the lithium silicate units (Cameron et al., 1949, Table 5, -p. 68). Another group, comprising the first four bodies in the table, belongs to type 2. New York, Earl Lode, Punch and Old Mike pegrnatites are ap- parently transition forms between types 3 and 4.

Some of the lithium-bearing pegrnatites of this .district (Bob Ingersoll Dike No. 2, Beecher Lode, Giant Volmey and Huge) have quartz plagioclase units that are correlated with the quartz-plagioclase association of the non lithium bearing pegmatites. As it has been pointed out before, this correlation is only structural, as both assemblages appear in wall zones. In the genetic sequence, however, they correspond, in my opinion, to different stages of evolution. The plagioclase-quartz assemblage of the lithium-bearing pegmatites corresponds to the albite

association of my scheme, and the assemblages of the other pegrnatites correspond to the plagioclase association. This assumption, based fundamentally on the position of the assemblages in the genetic series as discussed previously, finds support also in the brief description of the zones accompanying Table 5: "By far the largest number of the pegmatites contain albite (An 4-10) in the outer zones, although many of the lithium-bearing pegmatites contain more sodic albite in the outer zones. This change in anor- thite content is accompanied by a change in physical shape of the plagioclase' crystals. Oligoclase and calcic albite have a more or less equidimensional habit and the more sodic albite has a platy form (cleavelandite)" (Cameron et al., 1949, p. 67).

Taken into account the previous considerations it is easy to verify that the genetic evolution (reflected in the sequence of mineralogical associations) of the above-mentioned U.S. pegrnatite districts corresponds to that depicted in my paragenetic diagram. The correspondence is perfect for Petaca, New Eng- land and the Southeastern States district. The lithium-bearing pegmatites in Black Hills are the only ones that show an appreciable variation due to the presence, in two of the bodies (Giant-Volney and Hugo), of microcline-quartz zones after the lithium-bearing units.

In his scheme, Brotzen (1959, p. 56) includes a Swedish group of pegmatites with a composition range covering my four types. Although this author uses different criteria in distinguishing paragenetic units, his scheme shows clearly that the evolution of such bodies corresponds to that of the Argentine pegmatites.

Guinsbourg's .classification includes 3 types, to which he attributes different depths of formation. Although this author does not give information as to zones, from his geochemical, mineralogical and structural descriptions (Guinsbourg, 1960, p. 113), it follows that the first group (deep-seated pegrnatites) corresponds to types 1 and 2; the second group (pegmatites from average depths) to type 3 and the third (shallow-seated pegrnatites) to type 4.

Sequence of Primary' Mineralogical 4ssemblages As stated before, the main limitation of the se-

quences of mineral assemblages based on the zonal analysis, is that they do not represent accurately the evolution of the total genetic process. However, this limitation can be reduced by correlating the mineral assemblages and the paragenetic associations in con- structing the sequences. The mineral assemblages, as described by Cameron et al. (1949), are associations of essential minerals that appear in individual bodies in definite sequences from the walls inward and are closely related to the zonal structure. The

24 .JIMILC/IR O. HERRERA

Tab !.- Generalized secuence of the miner assemblages of the main nematite districts 0 Argentina.

_ _

Paraenetic Mineral Alta Valle Sierra La San Cata- Ancasti associations assemblages Gracia Frtil Brava Rioj a Luis marca

Plagioclase plagioclase quartz with or without X X X muscovite

(2) ' microcline

association quartz X X X X X X plagioclase (3)

Microcline microcline X X X X X X X association quartz

(4) Albite albite X

' quartz '(5) ,

association ' albit e microcline X quart z

,

(6) microcline

Spodnene albit e' quart z X X spodumene (7) ' ' ' microcline

association quartz X X spodumene

paragenetic associations, as here defined, are units that appear in regular succession and maintain their identity throughout the whole genetic evolution. In individual bodies each paragenetic association includes one or more mineral assemblages.

Table I shows, in terms of essential minerals, the generalized sequence of mineral assemblages, ex- cluding the replacement associations, that characterizes the evolution of Argentine pegmatites. It is based on the concepts discussed above, and on the study of the internal structure of the pegmatites of the main districts of the Sierras Pampeanas (Her- rera, 1958, 1961, 1963, 1964).

To obtain a clearer picture of the essential characteristics of the genetic evolution, the assemblages characteristic of the border zone and quartz core have been omitted. With regard to the border zone, its composition is probably influenced in most cases by an interaction between the magmatic liquid and wall rock. This is principally shown by its relatively uniform composition throughout the entire series; by

the common transitional nature of its contact with the wall rock, especially when emplaced in granite, and by the commonly abrupt increase of the calcium content in the plagioclase in comparison with that of adjacent zones. Furthermore, the formation of certain minerals in the wall rock--particularly muscovite, tourmaline, and plagioclase--also indicates an active exchange near the contact. In some occurrences, as indicated by Cameron et al. (1949, p. 48), the composition of the border zone is similar to the bulk composition of the pegrnatite itself, and can be interpreted as a chilled margin. Consequently this unit's composition is not only the result of the "normal" course of crystallization of a pegrnatitic liquid, but is also dependent on the physical and chemical conditions prevalent wi,thin the area of contact with the wall rock. The quartz core has not been included in the sequence, for the reasons already stated.

Uncommon assemblages have not been included in the sequence, as they are considered variations of some fundamental one. For example, in some peg-


-matites of type 4, an albite, quartz and spodumene zone occurs that may be considered a variation of assemblage 6, for which the microcline content varies widely. In types 1 and 2 pegrnatites, subzones rich in muscovite accompanied by quartz, with or without feldspar, are sometimes found. These units have been included in assemblages 1 and 2.

It is interesting to compare the sequence in Table 2 with that established by Cameron et al. (1949, p. 61) for the principal pegrnatitic districts of the United States. The first four assemblages in the latter coincide with the three first in Table 1, the only difference being that I omitted the border zone assemblage. Assemblages 5 to 7--spodumene-bearing units--also correspond in composition to our sequence. Assemblage 8, composed of lepidolite, plagiaclase and quartz, appears with certainty in only one pegmatite and is probably similar to the author's replacement units. Assemblages 9 and 10quartz, microcline, and microcline, plagioclase, lithium micas, quartz--have not been found by the author in Argen- tine pegmatites. This, however, does not appreciably alter the character of the sequence. Further- more, the first assemblage was found in only one peg- matire and the second, in two (Cameron et al., 1949,

p. 68). The main difference between the two sequences is the separation in my scheme of the quartz- plagioclase assemblage in types 1 and 2 pegmatites from that in type 4. The reason for this was explained in the previous section. Consequently, the comparison of the two sequences of mineralogical assemblages shows also that the course of the genetic process was very similar in both cases. This is sig- nificant, as in both studies the sequence is based on groups of pegrnatite districts that cover large areas, and belong to widely different geological environ- ments.

The Chemical Composition of Pegrnatites The sequence of paragenetic associations shows

clearly that the general evolution of the pegmatogenic process is mainly expressed by the role of Na, K and Li in its different stages. Each of the pegmatite types is characterized by a given alkali content.

The bulk composition of zoned pegmatites is diffi-, cult to ascertain, as the texture is very coarse and the composition varies in the different zones. This is further complicated by the irregular distribution of the accessory minerals. In Argentina no data on the bulk composition of pegmatites have been published.

Table 2.- Content of alkalis in Argentine pematttes belonging to the four t7pes of the proposed classification. Values obtained b Solodov (199) for $roups of pematites corresponding to types 3 and 4 sme also in.cuded.-

microclime albite type 1 type 2 type 3 pegmatites type 4 spodumene Pegmatite (Solocloy) pegmatites San Luis II

SoZodov) from

to 4.6 - 5.5 5.5 - 7 4.5 - 10.7 5.5 - 10.1 0.7 - 2.4 - , 0.9

aver ge 4.9 6 7.3 7.5 1.3 2

from 2.4 3.8 2.3 - 2.6 1.4 - 2.4 2.7 - 3.5 1.8 - 3.8 - to aO 3.5

aver 2,8 2.4 1.8 3 3 4.5 ge frm to - - - 0.01 - 0.05 1.5 - 2.4 1.1 - 1.5

Li20 2.4 avera

ge .... 2 1.3

fram 1.4 - 1.8 2.1 - 2;9 3.1 - 4.5 2 - 3.3 0.4 1.3 0 to - _ ' 0.25 Na20 avera 1.75 2.5 4 2.5 0.43 0.5 ge

,

26

?

6

$

.4MILC..4R O. HERRERA

1 2 3

KzO KO _.... ---- No:, O

No O

CoO

g

?

6

4

3

2

1

Micr ocLine Atbite - Spodumene pegmatites peg matires (SoLodor) ( Sotodov )

KO

KO

No:,O No20 x.. xx LiaO /

Fro. 3. Variation in the alkalis content of Argentine pegmatites of the four types of the proposed classification. Values obtained by Solodor (1959) for groups of pegmatites corresponding to types 3 and 4 are also included.

Kz O Nolo

KO

However, these difficulties are reduced if, instead of determining bulk compositions, the variations in the relative amounts of alkalis are estimated. For this, only the amounts of plagioclase, microcline, spodumene and muscovite need be determined. In general, each of these minerals constitutes more than 5% of the bodies and are more or less regularly distributed in zones.

The author has tentatively calculated the bulk composition of Argentine pegrnatites on the following assumptions: a) the specified elements--No, K and Li--occur only in the essential zone-forming minerals plagioclase, microcline and spodumene. In pegmatites of types 1 and 2 muscovite have been also considered. These minerals, together with quartz, constitute practically the entire volume of the bodies. Albite occurring in replacement units was also included; b) the pegrnatites considered are all tabular or long elongated bodies, and the volumes of the zones are proportional to their mean widths. All zones have the same specific gravity; c) the average composition of a body does not vary appreciably with depth. This hypothesis--though undoubtedly imply- ing a certain degree of risk is based on the observa-

tion of the zonal structure of many Argentine pegmatites in different stages of exploration?

On the basis of these assumptions, the relative volume of the zones, the mode of each zone and the mode of each pegmatite, were calculated. To cal- culate the contents of alkalis the theoretical composition of the minerals concerned was used (Dana,. 1949). To determine the average albite content in microcline, 30 thin sections of perthites were mea- sured micrometrically. In non-lithium pegmatites albite averages approximately 20 percent, and around 10 percent in lithium bearing ones. The mode of a zone was mainly calculated on the basis of visual estimates complemented by some measurements in order to verify the results. The results thus obtained are consistent and also agree with data found by other authors, as will be seen later. However, the final

8According to several authors (Norton et al., 1962; Johns, personal communication, 1967) many pegmatites that have been extensively exposed by mining show a decreasing KO/NaO ratio with depth. In my calculations this possi- ble source of error is important only in types 2 and 3, because some of the bodies of types 1 and 4 examined are. sufficiently exposed as to preclude the possibility of serious miscalculations due to that cause.


figures obtained have to be considered only as rough approximations.

Figure 3 shows the variations in content of the principal alkalis for the four types of pegmatites. 4 CaO is also included, but refers only to that con- tained in plagioclase. The amount of K20 increases regularly from types 1 to 3 and decreases abruptly in type 4. The Na20 contents varies only slightly, though the change of free plagioclase content is large. This is because in potassium feldspar rich pegmatites most of the plagioclase occurs as perthite.

The K20/NaO ratio (Fig. 3) is a distinguishing feature. In bodies intermediate between types 3 and 4, the K20/N%O ratio can be similar to that in type 1, but the distribution and composition of plagioclase are different. In type 1, plagioclase is limited to peripheral zones, and its composition ranges from medium albite to calcic oligoclase; in type 4, it is sodic albite--generally cleavelandite--and occurs in all the units.

The diagram also shows the alkali content variation in the four types of rare metal-bearing granitic pegmatites distinguished by Solodov (1959, p. 785). This sequence corresponds to the interval between my types 3 and 4. Despite small variations due to the intermediate forms considered by Solodor the graphs show the remarkable similarity of compositions.

Table 2 shows the extreme and average alkali contents of groups of pegmatites belonging to each type. The two extremes of Solodov's series, equivalent to my types 3 and 4, are also included. The last column gives the composition of a sample from the San Luis II pegmatite, which is similar to the average for type 4 pegmatites. This body was described briefly in a previous report (Herrera, 1963, p. 51); it is composed of crystals of spodumene in a finer intergrowth of quartz, albite and microcline. It is fine grained--the spodumene crystals measure from 2 to 5 cm--and of uniform composition throughout the area examined. This makes an estimate of its composition from sample analysis unusually accurate.

The author believes that this body was formed from a fluid equivalent in composition to that which produced other "normal" lithium-bearing pegrnatites of the district; the lack of both, zoning and giant texture, may be due to the loss of volatiles due to fracturing of the wall-rock. The similarity between

4 The composition of the following pegmatites was calculated: Type 1: Maria Inks, Santi Spiritu, Santa Maria, La Chiquita and La Codicia I, from Alta Gracia, C6rdoba. Type 2: Payahuaico and Martita from Catamarca, and Manto Rosado from Valle Frtil, San Juan. Type 3: Majada Oeste from Ancasti, Catamarca, San Jos and Paso Grande from San Luis, and E1 Resuello from Sierra de Comechingones. Type 4: La Viquita and San Luis II from San Luis, and Reflejo del Mar from Ancasti, Catamarca.

TABLE 4

San Luis IP

North Lacorne Pegmatite D. Derry (1950)

SiO2 73.8 73.05 A1203 17.2 17.21 Na20 3.5 5.02 K.oO 0.9 0.79 Li. 2.4 1.64 CaO 0.2 1.29 K. 0.25 0.29 Na.

Analyzed by B. L. de Lafaille, Laboratorio de An&lisis de Rocas CNICT-FCEN.

the alkali content of this body and that of group 4 pegmatites tends to corroborate this hypothesis.

There is a striking similarity between the San Luis II pegmatite and the North Lacorne pegmatite, in Quebec, Canada, described by D. Derry (1950, p. 101, Table II). The chemical composition of the two bodies is practically the same, as shown in Table 2. Like in the San Luis pegmatite, the rest of the bodies in the Canadian district are "normal" lithium-bearing pegrnatites.

It would appear, considering the above data on types 3 and 4, that the bulk composition of pegmatites belonging to a given type is similar in different parts of the world, in spite of differences in shape, zonality, grain size, accessory minerals, etc.

Genetic Considerations

If it is assumed, as I believe it is the case, that pegmatites originate through the crystallization of residual magmatic fluids, the sequence described above would represent the general trend of evolution of the fluids. This process, although essentially continuous, does not proceed uniformly but rather by a succession of compositional steps. This is suggested by the scarcity of pegmatites intermediate in composition between those of the four types described. 5

The zonal distribution of the different types of pegmatites in certain areas also points to successive emissions from a differentiating fluid. In most districts where several types of pegrnatite are presuma- bly associated with an intrusive body, a distinct zonaI distribution related to the igneous body is observed. Sodium or lithium rich bodies are nearly always peripheral, potassium bodies are intermediate, and microcline-oligoclase bodies are closest to the intrusive. In districts where different types occur together their transgressire relations prove that the

5 Solodor (1959, p. 790) attributes the unequal concentrations of rare elements in pegmatites of our types 3 and 4 to a "spasmodic" introduction of the pegmatogeni fluids.

28 ,4MILC,4R O. HERRERA

sodium-lithium bodies are later than the potassium ones, and that the latter are prior to the oligoclase- microcline bodies. The following districts are good examples of those relationships: Eight Mile Park, Colorado (Heinrich, 1948); Namaqualand (Gevers, 1936); Turquestan Range (Strelkin, 1938); Yellow- knife-Beaulieu Area, Northwest Territory (Rowe, 1954); West Hawk Lake (Stockwell, 1933), and Preissac La Corne, Quebec (Rowe, 1954).

Although the composition of a pegmatite depends on that of the initial magma, different bulk compositions do not necessarily imply differences in the original magmas. The zonal distribution indicates that the type of pegrnatite would depend largely on the depth of consolidation of the intrusive body and the degree of subsequent erosion.

Furthermore, the chemical evolution of the fluids must be supplemented by the adequate tectonic conditions to enable the separation and injection of the derived fluids. If these conditions fail to exist during part of the process, some types may be absent or, more probably, mixed types may appear by telescop- ing of two or more of the "normal" types (e.g., pegmatites of Sierra de Velazco, Fig. 2). In Brotzen's paragenetic scheme (1959, p. 56) groups of mixed- composition bodies also appear.

The predominance of more or less "pure" types in most pegrnatite districts of the world, however, indicates that there is a parallelism, or an intimate rela- tionship, between chemical differentiation and tectonic conditions. In my opinion, all or most granite magrnas are capable to produce, during their final phase of crystallization, the series of pegmatite types. Whether part or all of the members of the series are formed would depend more on tectonic conditions acting during the pegrnatogenic period, than on initial composition.

The. compositional differences found in pegmatites of the same type are generally related to variations in the content of rare elements that, in turn, determines the type and quantity of their accessory minerals. These variations are always regional and probably reflect differences in the parent-magrna's genetic history.

In the main districts of the Sierras Pampeanas of Argentina the pegmatites of the same type have very similar bulk composition and accessory minerals associations, vhich suggest.a remarkable similarity in the composition of the parent magmas. The areal distribution of the bodies does not show any obvious spatial relation to the granitic intrusions, but there is a distinct grouping of the pegrnatite bodies according to their genetic types (Fig. 1). On the other hand, the considerable distances separating such groups of pegmatites makes it highly improbable that

they are zonally arranged in relation with common centers of emission.

Considering the observed facts and the conclusions of the previous discussion, the genesis of the pegmatites of the Sierras Pampeanas can be best explained, in my opinion, as follows: a) the pegmatites were produced by successive emissions of residual liquids that changed in composition as the magmatic chamber cooled; b) the process was essentially similar in all emission centers; c) the present distribution of the pegmatites was determined by the original depth of the magmatic loci and by the subsequent erosion.

Acknowledgments The author is grateful to Dr. R. H. Jahns, who

critically reviewed the manuscript and made sugges- tions for its improvement. The writer is also in- debted to Dr. F. Gonz/tlez Bonorino for his helpful advice during the preparation of the manuscript.

The field and laboratory work and the first draft of this paper were completed at the Department of Geology of the University of Buenos Aires. The author left the University of Buenos Aires in July, 1966.

UAIIVERSIDAD DE CHILE, FACULTAD DE CIENCIAS FISICAS Y MATEMATICAS,

DEPARTAMENTO DE GEOLOGIA, 5'ept. 23,1967

REFERENCES ,

Angelelli, V., and Rinaldi, C. A., 1963, Yacimientos de mi- nerales de litio de las prov{ncias de San Luis y C6rdoba: Comisi6n Nacional de Energia At6mica de la Rca. Argen- tina, Inf. No. 91, Buenos Aires, p. 1-99.

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Documents

Herrera 1968