Ordovician Mineralizations in Argentina

Isidoro B. SCHALAMUK1-2 and de R.E. de BARRIO1

Abstract: ORDOVICIAN MINERALIZATIONS IN ARGENTINA. The regional geological knowledge of the Lower Paleozoic of South America has developed especially in the last years important advances in the characterization of the great geotectonic lineaments. The development of extensive sedimentary basins affected by important tectomagmatic events has favored and controlled the generation of extensive mineralized belts in terranes of Lower paleozoic age.The Ordovician sequences that extend from the south center of Peru, Bolivia up to the north of Argentina, are the most important from the metallogenic point of view. They register mineralizations that cover a wide areal distribution and in some cases they constitute very important ore deposits. In Argentina, the Ordovician sedimentites are emplaced in different regions, comprising especially in some cases, areas with exceptional mining potential in Sedex type deposits (Aguilar-Esperanza) and volcanogenic potential in Kuroko type deposits, associated with an Ordovician volcanic arch. In the northwest region they comprise the provinces of Jujuy, Salta, Catamarca and La Rioja, and in the last one, it is found the Famatina basin which also shows favorable conditions to host sedex and VMS type deposits. On the other hand, in the Precordillera region of La Rioja, San Juan and Mendoza provinces, deposits associated with the Ordovician are found. They have an enormous development of limestones, dolomites and pelitic rocks with stratabound barite deposits and possible sedex, polymetallic and massive sulphides deposits. Its potential is not very well-known yet. Finally, in Buenos Aires provinces, and associated with the Ordovician sequence of the Tandilia System, clay deposits have been exploited for several decades.

Resumen: MINERALIZACIONES ORDOVÍCICAS EN ARGENTINA. El conocimiento geológico regional del Paleozoico Inferior de América del Sur ha operado en los últimos años importantes avances especialmente en la caracterización de los grandes lineamientos geotectónicos. El desarrollo de extensas cuencas sedimentarias afectadas por importantes eventos tectomagmáticos ha favorecido y controlado la generación de extensas fajas mineralizadas en terrenos de edad paleozoica inferior. Las secuencias ordovícicas, que se extienden desde el centro sur de Perú, Bolivia hasta el norte de Argentina, son las que alcanzan mayor importancia metalogénica. Registran mineralizaciones que cubren una amplia distribución areal y en algunos casos constituyen yacimientos de gran significación económica. En Argentina, las sedimentitas ordovícicas se emplazan en diferentes regiones, conformando en algunos casos áreas con excepcional potencial minero en especial en depósitos tipo SEDEX (Aguilar-Esperanza) y potenciales depósitos volcanogénicos tipo Kuroko, vinculados a un arco volcánico ordovícico. En el noroeste comprenden las provincias de Jujuy, Salta, Catamarca y La Rioja, encontrándose en esta última la denominada cuenca del Famatina, que también muestra condiciones favorables para albergar yacimientos tipo sedex y VMS. Por su parte, en la región precordillerana de La Rioja, San Juan y Mendoza, es de destacar la presencia de depósitos ligados al Ordovícico, con un enorme desarrollo de calizas, dolomías y rocas pelíticas con depósitos estratiformes de baritina y posibles sedex, polimetálicos y sulfuros masivos cuyo potencial aún no es bien conocido. Finalmente, en ámbito de la provincia de Buenos Aires y vinculados a la secuencia ordovícica del Sistema de Tandilia, existen depósitos de arcillas que se explotan desde hace varias décadas.

Key words: Mineralizations. Ordovician. Argentina

Palabras clave: Mineralizaciones. Ordovícico. Argentina

Introduction

The regional geological outline of the Lower Paleozoic of South America has widely developed with regard to the knowledge of its large geotectonic features, in the last years. The high degree of regional metamorphism of some paleozoic units located in the tectonically active areas, especially in the western sectors, has hindered its differentiation regarding the lands of upper Precambrian that crop out rarely and fragmentally along the whole Andean belt. On the other hand, in the most stable ones, the paleozoic rocks show a weak metamorphism that has practically not obliterated the original sedimentary and paleontological features and this has allowed a better characterization of the stratigraphy and of the paleoenvironmental conditions as well.

From the evolutionary point of view, in South America the tectomagmatic events and the properly sedimentary ones favored and controlled the mineralization settings with the formation of big mineralised belts in units of the Lower Paleozoic (Fig. 1). The ordovician sequences are those that reach greater metalogenic importance in the central Andes and the Precordillera area, since they register mineralizations that cover a wide areal distribution and, in some cases, they constitute very important ore deposits from the economic point of view. The Ordovician sedimentites that occur from the south center of Peru and Bolivia up to the north of Argentina, contain an exhalative submarine volcanism that constitutes the main source of Pb-Zn-Cu-Ag and Ba concentrations of South America. In our country they are emplaced in two well differentiated basins, one in the

northwestern area (Fig. 2) and another one defined as the Famatina basin. A third area, with deposits associated with the Ordovician, is emplaced in the precordillera area of La Rioja, San Juan and Mendoza provinces.

The great extension of the Ordovician basin in the northwest of Argentina, with well defined stratabound and world class deposits (El Aguilar mine) and numerous Au and sulphides occurrences (Fig. 3), associated with the sedimentary sequence, forms an area with an exceptional mining potential especially in SEDEX and VMS type deposits. Metallogenic aspects in the region were considered, among others, by Coira et al., (1982), Angelelli (1984), Zappettini (1990), Schalamuk et al. (1992); Pérez et al. (1998), Sureda (1999), Méndez et al. (2001), Peralta et al., (2001) and Lencina and Peralta (2002) who emphasize the potential of mining resources that characterizes the area.

The submarine volcanism, simultaneous with the ordovician sedimentites, favored the process of concentration of large accumulations of Pb-Zn-Cu and Ba of the South America Paleozoic that extends like a belt, from the northwest of Argentina (with the SEDEX type Aguilar-Esperanza, La Colorada-Limeca deposits) up to Cerro Pasco latitude, in Peru.

Northwestern region

El Aguilar-Esperanza District:

Without any doubt this Pb-Zn an Ag district is the most important in its type in Argentina as well as in all South America. It is located in Jujuy province, in a transition environment between the Puna and the Cordillera Oriental. The district comprises two important mines, El Aguilar and La Esperanza, being the first one the most significant producing center, because of the volume yielded since 1936, registering during decades a sustained daily production of 2.000 to 2.200 t. of ore with grades of 8,4% in Zn; 5,55 in Pb and 90 to 120 g/t Ag. Even nowadays it is in full operation and with a good reserve level.

El Aguilar ore deposit shows a syngenetic mineralization with the Ordovician sedimentation. (Padrioc and Lampazar Formations), Fig. 4. It is a stratified deposit associated with orthoquartzites. Essentially the ore minerals are: iron bearing sphalerite; galena, pyrite and as minor components, pyrrhotite, tetrahedrite-tennantite, pyrargirite, arsenopyrite and molybdenite (Sureda, 1999). The

great volumes of existent ore are due to geothermal sources, partly exhalative, that caused the flow of thick metal-bearing brines in the sea floor of the lower Tremadocian. (Sureda 1999).

Esperanza mine (Pb-Zn-Ag) is emplaced in oceanic siliciclastic sediments (Lampazar Formation) deposited in the Upper Cambrian-Tremadocian. The mineralogy consists of pyrite, galena, iron bearing sphalerite and pyrrhotite, and in lesser proportion silver sulphosalts, marcasite and baryte gangue.

La Colorada-Limeca District.

It is partially explored and it also shows characteristics similar to the sedimentary-exhalative pattern. It is considered that the mineralization took place from hydrothermals fluids exhaled in the sea floor and then channeled by pipes controlled by faulting (Lurgo Mayón et al., 1999). The district is located in Sierra de los Cobres, the Puna Oriental first step, in Salta province.

La Colorada mine shows stratified bodies and levels of massive or semimassive sulphides with thickness of 2,5 to 10,8 m and grades of up to 10% of Zn, 7% of Pb, 2,11% of Cu and 13 g/t of Ag and some levels with values of up to 0,7g/t of Au. (Lurgo Mayón et al., 1999). The wall rocks of the ore deposit comprise meta-pelites, with metaquartzite intercalations and metadolomites that are assigned to the Chiqueros Formation (Lower Ordovician). The group is affected by metasomatism due to the intrusion of the Granodiorite Cobres that produced the development of skarn rocks.

The mineralization is emplaced in the eastern slope of a great antiform and the granodiorite Cobres is located in its nucleus. The main mineralization consists of sphalerite, pyrrhotite, arsenopyrite, chalcopyrite, and minor lollingite, magnetite and cassiterite.

Limeca is a very interesting prospect; its sulphide outcrops occur interbedded with the ordovician

sedimentites, affected by a low metamorphism. The recognized mineralogy belongs to pyrite, esfalerita, graphite sheets and chalcopyrite (Méndez and Méndez, 2001; Méndez et al., 2001).

In the central area of the Calalaste range, Southern Puna, in Catamarca province, a mineralization of Fe, Zn and Cu rich sulphides is found in a complex environment of pelites, quartzites, wackes, lavas, tuffs and ignimbrites, of an Ordovician age. The group shows a structural configuration that belongs to a folded and thrusted belt. The sulphide bodies are synsedimentary associated with the dacitic volcanism and with pelites deposits in restricted environments (Méndez et al., 2001).

In the same central area of the range, basic and ultrabasic rocks are found, consisting of cumular gabbros, gabbros and basalts that constitute a volcaniclastic intrarc sequence. Hongn et al. (1999), recognize strong metallic anomalies and they state a model of sinextensional basin and the occurrence of a potential massive deposit of submarine volcanogenic type or Kuroko type.

Pumahuasi Polymetallic District

The district is located in the Cordillera Oriental environment, Yavi department, in Jujuy province.

They occur in Pb, Zn, and Ag bearing vein structures hosted in shales and interbedded sandstone of the Acoite Formation, affected by low metamorphism. The district that was exploited in the first decades of the last century, is included as part of a belt that extends up to about 500 kms to the north, penetrating in Bolivia.

In a zone of 15 km strike north-south and approximately 3 km wide, three systems of veins are recognized, with thickness that oscillate between 20 and 40 cm and lengths of some hundreds meters. The main system shows a strike N55º-80ºW; a second one strike N40º-65ºE and a third one formed by a single vein strike N 5º E. They occur as fault fillings, showing a strong vertical zoning, with high Pb concentrations in its upper portion and Zn in the lower levels.

The mineralization consists of galena, sphalerite, chalcopyrite, pyrite, arsenopyrite, silver sulphosalts and supergenic copper sulphides. The gangue minerals consist mainly of quartz, barite and siderite. The mineralogy and its structural and isotopic characteristics and the setting environment, allow to classify them as veins in sedimentary clastic terranes according to Beaudoin and Sangster,

1992 (in Segal et al., 1999), where the cortical faults and deformational and metamorphic events constitute an important role in their genesis.

La Rinconada District

This important district of auriferous veins is emplaced in the area called Sierra La Rinconada, in the Puna environment and it extends from the limit of Bolivia up to the 23º00’ parallel of south latitude, in Argentina. The numerous mineralized structures were focus of sporadic exploitation and they constitute Azules, El Torno, Santo Domingo, Farillón, Pabellón, El Carmen mines, among others. In general, lengths of few meters up to some hundreds of meters and thickness from centimeters to several decimeters, are observed. The gold grades are intermediate, although contents of up to 120 g/t and even higher are shown.

They are generally located, in a sedimentary turbiditic sequence made up of ordovician marine leptometamorphic deposits belonging to the Acoite Formation (Zappettini and Segal, 1999), locally covered by continental sediments and volcanic and porphyric rocks of tertiary age. The district is characterized by a group of anticlines and synclines with inclinations to the north, originated by ocloyic movements. The sequence was affected by a faulting associated with the Andean tectonic that gave origin to the range. There are several authors who refer to the different primary gold mineralized structures that were synthesized by Angelelli (1984) and more recently, among others, Sureda et al., 1986; Zappettini and Segal 1999 and Rodríguez et al., 2001. In general, the mineralized veins are located in the anticline axis, in both flanks and parallel to the sedimentary sequences constituting saddle reef type structures (Fig. 5). There are also discordant veins next to lens parallel to the ordovician deposits. The mineralization consists of gold and electrum, abundant arsenopyrite and pyrite, and in lesser proportion sphalerite, galena, chalcopyrite, among other subordinated minerals. It is defined an initial paragenetic sequence with accumulation of gold, arsenopyrite, pyrite and quartz gangue, ankerite, calcite and graphite, a second stage of gold, sulphides and sulphosalts and a final stage with native gold and quartz gangue and carbonates.

The origin of these primary auriferous deposits is presently associated with lateral exudation or secretion process in an advanced diagenetic stage during the Ordovician (Sureda et al., 1986). As regard the primary gold source, it is usually attributed to the exhalative volcanic activity (Coira and Koukharsky, 1994) that occurs, at regional scale, as domes and lavic intercalations, in the west of La Rinconada area.

Incahuasi District

In the Incahuasi Sierra, Southern Puna, in the province of Catamarca, it is located an auriferous ore deposit with veins that was exploited by the indian natives (Incas) and Jesuits and, in an active form, between 1936 and 1954. It is considered a total production lower than 2.000 kg of gold, starting from high grade auriferous ore, higher than 20 g/t.

The ore deposit is of the banded type, with quartz veins and veinlets with gold and minor pyrite, arsenopyrite and chalcopyrite. The mineralized structures are emplaced in a pelitic and grauwackic sequence of Falda Ciénaga Formation. The sequence suffered alterations in its composition and texture, due to metamorphic diagenetic and hydrothermal processes. The area is affected by faults and fractures in two preferential systems: a north-south one and another one NE-SW (González, 1999). The deposit is associated with the N-S faulting that would have affected the Ordovician sequence during the Ocloyic Phase (Navarro Garcia and Rosello, 1989). The gold bearing quartz veins show lengths of some hundreds meters and thickness of few centimeters up to three meters. In general plunging to the east, with values higher than 45º.

It is considered that the gold source is related to the ordovician magmatism (González, 1999). It is considered that the regional metamorphism and the tectonic deformation facilitated the generation and migration of hydrothermal fluids bearing gold that gave as a result the formation of the deposits during the Upper Ordovician.

Barite Districts of Santa Victoria and Zenta ranges

In the Cordillera Oriental, barite deposits are located in Santa Victoria and Zenta ranges, in the Salta province. These deposits present bedded or in vein formations hosted in the ordovician sedimentary sequence. They consist of barite of high purity concentrations or they accompany lead, zinc, copper sulphides and occasionally native gold and silver.

The Santa Victoria District that comprises the eastern sector of the homonymous hill, presents barite concentrations as bodies with veins, in shales and shaly sandstones from the Ordovician.

There are several registered deposits, however the most important, for the exploited volume and for the existing reserves, belongs to the mine known as Mono Abra. The area shows intense deformation with overturned, fractured and thrust folds. The mineralization fills faults of the sedimentary sequence. Several veins, of different thickness (0,45 to 10 m) and up to 800 m of length are found, with strikes E-W and NE-SW (Schalamuk et al., 1983).

La Ciénaga mine, according to Méndez et al., (2001) consists of semiconcordant lenses or beds rich in sulphides. The mentioned authors consider that the sulphides horizons were deposited in the sedimentary sequence and they attribute to synsedimentary faults used as feeding channels favoring the solution circulation and then galena, sphalerite, pyrite precipitation, with moderate barite occurrence. This characterization constitutes a good indication for the exploration of new bedded bearing sulphide deposits in Santa Victoria district.

In the Sierra of Zenta, in the south of Santa Victoria, another barite district is located. There are several deposits that mainly register textures with veins, they are emplaced in Tremadocian shales and sandstones of the Santa Rosita Formation (Castillo et al., 1996). They show variable lengths of up to some hundreds of meters and from few decimeters to three meters of thickness.

Aluminous belt

An extensive metamorphic aluminous belt extends from the Salar de Pastos Grandes, Salta, going toward the south of Catamarca province. The outcrops are not continuous, but they show a good degree of association with the crystallophilic complex of the Oire Formation (Schalamuk et al., 1983).

The aluminous deposits consist of fibrous-prismatic aggregates of andalusite, sillimanite and cyanite associated with mica schists and concordant quartz veins. The origin of these deposits is attributed to a conjunction of processes among which regional metamorphism of pelitic sediments, with B hydrothermalism and biotite, feldspar and muscovite transformations into aluminium silicates and quartz are included.

The age of this singular aluminous mineralization is simultaneous with the metamorphic processes that have been dated and assigned to the Medium Ordovician (472 Ma). According to Sureda et al. (1986) that age would coincide with the metamorphic climax event of the Northern Sierras Pampeanas crystalline basement during the orogenic Famatinian cycle.

Precordillera of La Rioja, San Juan an Mendoza provinces

It is characterized by the great development of deposits from the Lower and Mid Paleozoic and for the lack or rare occurrence of evident magmatism. The limestones and dolomites constitute the most important item in the mining of the application rocks in the Precordillera environment. Their distribution coincides with the development of the Cambrian and Ordovician. Contents exceptionally high in calcium carbonate locally occur (Jachal; Villicum and Ullum) and they are widely exploited.

While the calcite, in irregular bodies and vein forms, preferably appears in the ordovician limestones.

Canota barite Belt

In an attractive belt of about 10 kilometers of length, with dominant northeast strike, a group of barite deposits is emplaced in shales of the Villavicencio Formation that constitutes the barite bearing rock. Moreover, grauwackes, quartzites and phyllites of Canota Formation (Silurian- Devonian) are found in the area (Brodtkorb et al., 1988).

The stratabound deposits, belong to the sedimentary Ordovician sequence where the barite occurs as nodules, lenticular bodies and stray beds hosted conformable in the planes of fissility of the black shales from the Caradoc. Disseminations of barite crystals are also observed in shales.

The barite nodules shows ellipsoid to subspheroidal forms, with diameters of 15 to 20 cm and 4-8 cm of thickness. The stray beds extend up to 20 m of length and 1 meter of thickness. They show dark gray and black coloration. The density is about 4. Some mines (Pirucha, Don Manuel, Victoria, among others) were object of intermittent exploitation from the decade of the 50s until the end of the 80s. The district offers good reserve perspectives, taking into account the outcrop distribution along several kilometers in marked a continuity with the stratigraphy.

Its genesis is associated with the sedimentary–diagenetic processes in a not very deep basin with intense organic activity, where the nodular occurrence of barite took place in supersaturated muds of barium sulphate. The typology is common to different places of the world and they constitute deposits that make a very important contribution to the international supply.

Calingasta and Rodeo Al-Mg Sulphate Deposits

Among the non metal-bearing resources of San Juan province, it is important to point out the wide deposits of aluminium and magnesium sulphates that have been exploited intermittently since 1939. The deposits are located in the western watershed of the Precordillera, assembled in the sectors km 114 - Calingasta (northwestern of Sierra del Tontal) and Rodeo (Cuesta del Viento).

The Alcaparrosa Formation of the Caradoc comprises the sulphate deposits that occur as impregnations, crack fills and big pockets of very different sizes. The defined mineral species are: alunogen, pickeringite, halotrichine and epsomite (Angelelli et al., 1980). The ordovician formation registers an estimated thickness of 1300 m. It consists of fine, argillo-slimy sediment: shales and black and gray limolites, in continuous alternation (rhythmites) that show frequent effusion of volcanic products as pillow lavas. The black shales register metallic sulphurated elements (Fe, Zn, Cu), besides vanadium and high contents of carbon. It is frequent the presence of framboidals pyrites (Schalamuk et al., 1990) which may be associated with small sulphide contents and in some cases gold traces.

The sulphate formation is considered as the product of the iron sulphide oxidation in the shales, the liberation of sulphuric acid and its etched to the components of the rock itself. The sulphates originated in this way, all them very soluble, were carried by the waters toward lower levels, crystallizing as different mineral species in the open spaces of the shales. The formation process, originated in recent times, becomes effective in greater or lessor degree in the whole shaly fraction of the Alcaparrosa Fm, and as a product of this, the sulphate accumulation is very extensive and of big reserves. The areas more tectonically disturbed are the richest in sulphates. In general the grades vary between 2 and 6% of aluminium sulphate.

La Helvecia mine

La Helvecia mine represents a Pb-Zn-Ba mineralization located in the northwestern sector of La Rioja province. It is associated with the ordovician sedimentites of San Juan (limestones), Gualcamayo (shales) and Conglomerado Las Vacas Formations. It mainly occurs as concordant levels and irregular bodies with galena, sphalerite and lesser amount of pyrite, chalcopyrite, tetrahedrite and barite.

The ore deposit, considered as a stratabound deposit, by Brodtkorb and Brodtkorb (1999), could belong, according to its mineralogy, to a SEDEX type deposit with later removilization processes.

The presence of other mineral prospects of similar characteristics in the Urcushúm range, enlarges significantly the mineralized area that shows an interesting prospecting potential.

Mineralizations related to ophiolitic rocks

Santa Elena Deposit

It is a polymetallic deposit located in the western flank of the Precordillera, in Calingasta area, province of San Juan. The mineralization is interpreted as a deposit of volcanogenic massive sulphide (VMS) with hydrothermals facies associated with a basic magmatism, represented by dikes, sills and effusion of submarine lavas with pillow structure, of basaltic alkaline tholeiitic composition.

Above the lavas the sedimentites of the Alcaparrosa Formation are found.

Talc Deposits

Basic rocks and serpentine bodies partially altered in talc and other elements, are located in different metamorphic units, essentially of low grade. The bodies extend in a belt of ultramafic rocks that begins in San Juan, go throw the Sierra Cortadera and Uspallata up to the Cordillera Frontal. This belt is integrated by peridotites, gabbros, serpentines and deposits of talc of 20 to 40 meters long, 1 to 6 meters of thickness and depths of up to 70 meters. Most of the deposits are located in the Bonilla Formation, Uspallata range, Mendoza province.

Famatina System

This large morphostructural unit registers important sedimentary and volcanic-sedimentary sequences of the Ordovician, with lavic intercalations. There is no information that allows to associate definitely the contribution from the ordovician magmatism with the formation of mineral deposits.

The occurrence of spilitic breccias with disseminated sulphides is mentioned. At the same time some polymetallic concentrations are associated with the granitic plutons of the Famatinian cycle.

Summarizing, although there is not much information about the metalogenic frame in the Famatina basin, the great extension and the regional continuity of the ordovician sequences, justify the development of exploration programs.

Clayds deposits associated to the Ordovician sedimentites of the Tandilia System

In the province of Buenos Aires, in the Tandilia System environment, there is an important package of clays associated with the Ordovician sedimentites of the Balcarce Formation that have been exploited during decades.

The argillaceous horizons in general are interbedded among quartz rocks levels, mainly outcropping in Balcarce and Claraz areas. In general they are refractory and in lesser proportion plastic clays.

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Recibido: 23 de Julio de 2002

Aceptado: 16 de Septiembre de 2002