Economic Geology Vol. 59, 1964, pp. 428-442

STRATIGRAPHY AND MINERALOGY OF THE MANGANESE

SEDIMENTARY DEPOSITS OF COQUIMBO PROVINCE, CHILE •

LUIS AGUIRRE AND SONIA MEHECH

CONTENTS

PAGE

Abstract .............................................................. 428 Introduction .......................................................... 428

Stratigraphy .......................................................... 429 Arqueros formation ................................................. 431

Distribution, stratigraphic relations and lithology .................... 431 Age and alepositional environment ................................. 433

Quebrada Marquesa formation ....................................... 433 Distribution, stratigraphic relations and lithology .................... 433 Age and depositional environment ................................. 434

Depositional cycles of the manganese .................................... 435 Lower Cycle deposits ............................................... 435 Middle Cycle deposits .............................................. 437 Upper Cycle deposits ............................................... 439

Depositional environment and processes in the accumulation of manganese .... 440 Conclusions ........................................................... 441 References ............................................................ 442

ABSTRACT

The stratigraphy of two formations of Early Cretaceous age exposed in the Coquimbo province has been studied. The older Arqueros forma- tion is marine; the younger, Quebrada Marquesa, is continental, and over- lies conformably the Arqueros formation; both units contain manganese layers.

The stratigraphic sequence of these units shows that the manganese deposits originated during three sedimentary cycles. From their lithologic and paleontologic characteristics it is inferred that the manganese was introduced during submarine volcanism and that the concentration and later deposition of the manganese took place under near shore or lagoonal conditions.

The manganese minerals, in order of abundance, are braunire, pyrolu- site, manganite, psilomelane and hausmanite. Piedmontite is important as a gangue constituent. The comparative study of the mineral samples from the different cycles reveals that psilomelane is more abundant in the Lower Cycle and that hausmanite is found only in the Middle Cycle.

INTRODUCTION

As part of a program of quadrangle mapping on the scale of 1'50,000 and of detailed studies of the manganese mining districts being conducted by the

x Published with permission of the Director Instituto de Investigaciones Geo16gicas (IIG), Chile.

428

M/tNG•INESE SEDIMENTARY DEPOSITS OF CHILE 429

Instituto de Investigaciones Geo16gicas (IIG) in the Coquimbo province, the authors have obtained detailed information on the stratigraphy and mineralogy of the two manganese-bearing units exposed in that region. These are the Arqueros formation, of marine origin, and the conformably overlying Que- brada Marquesa formation, which is continental. These units constitute a north-south trending belt, approximately 25 km wide, in the west-central part of the province.

This region has been intensively studied for economic purposes, because it contains numerous copper, silver, gold, and manganese ore deposits.

The mineralogy of the manganese ore occurring at least at three different horizons in the Arqueros and Quebrada Marquesa formations was determined through the microscopic study of 55 polished sections of representative sam- ples; besides, 25 thin sections of samples from the unmineralized beds that occur above and below the manganese-bearing layers were also studied. Some of the samples were microchemically tested and others were studied by the X-ray diffraction method by Mauricio Tabak of the IIG. Eight of the mineral samples considered in this report were sent to Professor Paul Ram- dohr of Heidelberg University in West Germany, for study. Credit is given for the data reported by Prof. :Ramdohr.

The authors are indebted to Messrs. Carlos Ruiz, Director of the Instituto de Investigaciones Geo16gicas, and Dr. Jos• Corval/m, geologist of the same Instituto, who critically revie•ved the manuscript and made many helpful sug- gestions. Also acknowledged is the collaboration of Gast6n Bustamante, engi- neer of the Empresa Nacional de Mineria and of Lino Tapia, engineer of the Compafiia Manganesos Chile, who gave information and mineral samples of some of the ore deposits. Dr. Jos• Corvalfin and Mr. Kenneth Segerstrom of the U. S. Geological Survey kindly revised the English translation.

STRATIGRAPHY

The manganese-bearing Arqueros and Quebrada Marquesa formations consist of sedimentary and volcanic rocks, both marine and continental; they are of Early Cretaceous age. The lower unit, named Arqueros formation by Aguirre and Egert (1, p. 26), is marine and contains manganese beds near the top. The upper unit, named Quebrada Marquesa formation by the same authors (1, p. 28), is continental and contains manganese layers in its middle and upper parts. The contact between these two units is conformable and, in places, the top of the older interfingers with the base of the younger. In the index map (Fig. 1), they are undifferentiated and bear the symbol K1.

Although units of similar lithology and of the same age are of wide distri- bution along the Chilean territory, the occurrence of manganese is restricted almost completely to the Coquimbo province, where economically important districts are located (Fig. 3). These yield 87 percent of the total production of manganese in the country with approximately 17,000 tons per year (3, p.

430 L. AGUIRRE AND S. MEHECH

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MANG•tNESE SEDIMENTsfRY DEPOSITS OF CHILE 4,31

ztrqueros Formation

Distribution, Strati•7raphic Relations and Litholo#y.--The name Arqueros formation was proposed by Aguirre and Egert (1, p. 26) to designate a sequence of porphyritic andesites and marine limestones containing manganese beds in its upper part. The unit is exposed throughout the length of the

Thickness

(in meters)

EXPLANATION

d 14o

30

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35

• Shole

,1• Limestone

. '.':• Colcoreous sondstone

".'•.• Sondstone

; oF;;•J:• Conglomerote

ß •:•1 Breccio

•1 Tuff

,,,,•:• Flow-brecclo

• ,•;:• Andesire

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•' Mongonese-beoring bed

• Gypsum Ioyer

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Fro. 2. Colunmar section of the Arqueros and Quebrada Marquesa formations.

432 œ. .4GUIRRE .4ND S. MEHECH

Fro. 3.

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LAMBERT • QU['•"'• RQU E SA ..v,//Ar royones

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Index map showing the location of the manganese districts in Coquimbo province.

Coquimbo province in its west-central part. The type locality is Llano de Arqueros, at approximately 35 km northeast of La Serena.

The Arqueros formation rests unconformably on the volcanic La Negra formation of Jurassic age, and is overlain conformably by the Quebrada Mar- quesa formation. The following stratigraphic section (from top to bottom) represents the complete sequence of the unit and is based on the measurement of numerous sections at different localities (Fig. 2).

Thickness

a) Grayish-red-purple, porphyritic andesite with aphanitic groundmass and abundant yellowish-white plagioclase pheno- crysts 2 to 3 mm long. 30 m

b) Grayish and dusky-red, fine to medium-grained volcanic sand- stone. Manganese beds, ferruginous chert, and copper-bearing beds in some places. 15 m

c) Grayish-red (due to weathering), porphyritic andesite with grayish-green groundmass in unaltered samples. Tabular yellowish-white plagioclase phenocrysts, approximately 2 cm long. 35 m

M•lN'G./t2?ESE SEDIMEN7.4RY DEPOSITS OF CHILE 433

d) Medium gray, aphanitic limestone, containing numerous tubu- lar rudistic shells (.4#ria blumenbachi Studer), Nerinea, and corals; alternated limestone and medium to coarse-grained calcareous sandstone 5 to 10 cm thick; light gray, aphanitic limestone. Also calcareous breccia with a dusky-red matrix and angular fragments of limestone and andesite 4 to 5 cm in diameter. 140 m

e) Grayish-red, porphyritic andesite with grayish-green ground- mass in unaltered samples. 115 m

f) Medium to dark gray aphanitic limestone, with yellow to orange- yellow weathering surface; it contains abundant Ostrea and a few Pecten (?); pale-yellow chert; grayish purple, medium to coarse-grained, calcareous sandstone with abundant Ostrea. 100 m

g) Grayish-red, porphyritic andesite with numerous vesicles filled with zeolites, epidote and chlorite. 150 m to

200 m

Total thickness: about 585-635 m at the type locality.

•4ge and Depositional Environrnent.--The best paleontologic evidence available to assign an age to the Arqueros formation, is the presence of •/gr/a blurnenbachi Studer (2, p. 321-324), which indicates that the unit is of Barremian (Early Cretaceous) age.

The fossil content and petrographic characteristics of the calcareous mem- bers of the Arqueros formation indicate that the unit was deposited in a littoral environment. Due to a later accumulation of lavas this environment

became shallower until finally grading into lagoonal. This latter condition appears to have been well defined at the beginning of the depositional cycle of Quebrada Marquesa formation.

Quebrada Marquesa Formation

Distribution, Stratigraphic Relations and Lithology.--The name (•uebrada Marquesa formation was proposed by Aguirre and Egert (1, p. 28) for a sequence of continental sedimentary rocks and andesitic lava, that contains manganese-bearing beds at the base of the middle part. In some places, layers of marine calcareous sandstone with fragments of Ostrea and Trigonia sp. aft. Trigonia coihuicoensis Weaver are found in the basal part of the unit. The base of the formation interfingers with the top of the Arqueros formation. The most typical exposures are found in (•uebrada Marquesa. 40 km east of La Serena; several other sections are exposed throughout the province.

The (•uebrada Marquesa formation overlies, conformably, the Arqueros formation, and is overlain unconformably by a continental volcanic unit of possible Late Cretaceous age.

At the type locality, the lithology of the (•uebrada Marquesa formation, from top to bottom, is as follows: (Fig. 2).

Thickness

a) Medium gray porphyritic andesite with aphanitic ground- mass and few tabular plagioclase phenocrysts. The pheno- crysts are up to 2 mm long; they are partially altered to hematite and chlorite. Vesicles filled with epidote and chlorite are abundant. At the base of the middle part of this section, there are fine-grained sandstones xvith intercalations of pale grayish red shale. 150-200 m

434 L. .4GUIRRE .4ND S. MEHECH

b) Red clastic unit with the following sub units (from top to bottom):

1. Coarse conglomerate, matrix formed by grayish purple red, medium-grained sandstone; predominant clasts are rounded cobbles of pale grayish purple porphyritic ande- sire, up to 20 cm in diameter.

2. Grayish red, medium-grained, well stratified sandstone alternating with shale of the same color; mudcracks and rainprints occur in the shale.

3. Medium gray lacustrine limestone; in some places it con- tains manganese layers that are concordant with the strati- fication; in others, it bears intercalations of well laminated chert.

4. Manganese layer, very discontinuous in its lateral extent. 5. Grayish red, medium-grained sandstone.

150-180 m

c) Yellow sandstone unit with the following sub units (from top to bottom):

1. Yellowish-olive-gray, calcareous sandstone, medium to fine-grained; it contains fragments of petrified wood and spherical seeds of 1.5 cm in diameter

2. Olive gray, finely laminated, medium-grained calcilutite. 3. Porphyritic andesite with aphanitic groundmass and

abundant phenocrysts of plagioclase and tabular augite up to 3 mm long.

4. Gypsum lens. 5. Yellow, fine-grained, calcareous sandstone. In many

places the continuity of the yellow sandstone unit is inter- rupted by channel structures filled with sedimentary breccia.

d) Grayish red, fine to medium-grained sandstone containing manganese (main manganese-bearing bed of Quebrada Mar- quesa formation); orange claystone, gypsum and red sand- stone

e) Pale greenish gray, porphyritic andesitc with aphanitic groundmass, white plagioclase phenocrysts up to 2.5 mm long, and abundant amygdules of zeolite, epidote, chalcedony, chlorite, glauconite and prehnite.

f) Coarse conglomerate; the matrix is medium to coarse- grained sandstone and contains gray, well-rounded cobbles of porphyritic andesite up to 30 cm in diameter. Sedimen- tary breccia (mud flows ?), andesitic flow breccia, and inter- calations of red porphyritic lava. Reddish gray sandstone with Ostrea, Exo#yra and Crinoids. Grayish dusky-red sandstone, tuff and breccia.

Total thickness of the Quebrada Marquesa formation at its

40 to 60 m

10 to 15 m

12 to 15 m

Total thickness

approx. 800 m

1150 to 1300m

Age and Depositional Environment.--Due to the scarcity of stratigraphi- cally significant fossils in the Quebrada Marquesa formation, its age cannot be accurately determined. However, on the basis of the occurrence of Tri-

M•tNG•tNESE SEDIMENTdRY DEPOSITS OF CHILE 435

#onia sp. aft. Tri#onia coihuicoensis Weaver, the unit can be assigned an Early Cretaceous age.

The conditions under which the Quebrada Marquesa formation appears to have been deposited, are those of a shallow-water marine environment rapidly filled with coarse clastic material and lava flows. This filling would have conditioned the development of marginal lagoons in which the manga- nese deposition and the formation of gypsum layers would have taken place. A gradual change to continental conditions is indicated by the occurrence of plant remains in the sedimentary unit overlying the manganese beds. These conditions appear to have persisted throughout the deposition of the upper part of the formation as indicated by the occurrence of red sediments with mudcracks and rainprints.

DEPOSITIONAL CYCLES OF TYIE MANGANESE

The occurrence of manganese-bearing beds at three different stratigraphic horizons in the Arqueros and Quebrada Marquesa formations and the lith- ology of their underlying and overlying strata give enough basis to infer that there was accumulation of manganese during three heterochronous episodes of sedimentation, here referred to as depositional cycles. The economically more important deposits are those of the older Lower and Middle Cycles which are widespread and continuous; the youngest (Upper Cycle) deposits contain only one thin and discontinuous manganese layer.

Lower Cycle Deposits

Exposures of the Lower Cycle deposits are found in the Llano de Ar- queros (Fig. 3) (Palmira, Arrayanes and Leoncito mines), in the upper course of the Array•n de Tunilla valley, and in the central part of Lambert quadrangle. In this last area these deposits are mihed at Escondida, San Luis, Alto San Luis, Jamelas and other mines. In the west-central and southern parts of Quebrada Pangue quadrangle, they are mined at Arrayan, Liga de Algodones and Porvenir mines.

The manganese in the Lower Cycle deposits occurs in well defined beds, varying in number from one to three, and having an average thickness of one meter. The highest grade of the handpicked ore is 51 percent and the lowest 31 percent. The manganese beds are intercalated in a sequence of grayish red to dusky red, fine to medium-grained volcanic sandstone, 12 to 15 m thick, of the Arqueros formation (Fig. 2). The microscopic study of representative samples of this sandstone unit shows that the cement, generally calcitic, is scarce: the predominant clasts are of andesitic and traquitic lavas and of plagioclase; the average grain size is 0.2 to 0.3 mm. Piedmontite occurs dis- seminated in the cement of the sandstone beds closest to the manganese layers.

In some places, as in Jamelas and Alto San Luis mines, the manganese is associated to a copper bed (chrysocolla, malachite), which either overlies or underlies the manganese layer.

Several representative samples of the manganese layers occurring in the

436 L. AGUIRRE AND S. MEHECH

d

FIG. 4. (a) Pyrolusite crystals radially arranged. Arrayfin mine. X 50. FIG. 4. (b) Psilomelane showing banded colloform texture. Arrayfin mine.

x 50.

Fro. 4. (½) Alternating bands of braunite and gangue. Corral Quemado dis- trict. X 15.

Fro. 4. (d) Euhedral crystals of braunite disseminated in the gangue. X 400 (Oil immersion ).

MANGANESE SEDIMENTARY DEPOSITS OF CHILE 437

Lower Cycle were taken at San Luis, Escondida, Arrayira and Porvenir mines, and studied under the microscope. The minerals present in these mines are braunite, pyrolusite, manganite and psilomelane in order of abundance.

Braunite appears finely disseminated in the gangue, and in places, at Porvenir mine (according to Ramdohr), showing an octahedric habit or constituting aggregates of octahedric crystals.

Pyrolusite occurs as an alteration product of braunite, as euhedral crystals with clearly defined cleavage; in parts these crystals are radially arranged (Fig 4a). This type of occurrence is characteristic in Arrayan mine. In Porvenir and Array/m mines pyrolusite shows a botryoidal colloform texture; in some specimens pyrolusite is finer in the center, coarser at the edges; others contain alternating bands of pyrolusite and psilomelane. According to Ram- dohr, in Porvenir mine, pyrolusite in part replaces calcite.

Manganite is only present in San Luis and Escondida mines and it occurs mainly as numerous needle-shaped crystals distributed around the braunite crystals in the form of a thick border zone; in some specimens manganite occurs as inclusions in braunite. Rarely, manganite occurs filling veinlets.

Psilomelane is present only at Porvenir and Arrayin mines. In most of the samples where psilomelane is present, the texture is banded colloform (Fig. 4b). In one of the samples where this texture was observed, the central part of the bands is formed by a very fine aggregate of pyrolusite and limonite and the edges by psilomelane. In some specimens the psilomelane occupies the center of the structure whereas pyrolusite constitutes the edges; in others, alternating bands of both minerals are observed.

The gangue is constituted chiefly by calcite; in some specimens, piedmontite accompanies the carbonate. In Porvenir mine the manganese-bearing layers have a relict clastic texture corresponding to sandstone and fine conglomerate. The matrix appears partially cemented by the manganese minerals; rare fragments of quartz, feldspar and volcanic rocks are preserved.

Middle Cycle Deposits

The main exposures of the Middle Cycle deposits are found in the area between Quebrada Marquesa to the north and Hurtado river to the south. Several mines of the E1 Romero, Corral Quemado and Fragua mining dis- tricts are located in that area. There, the manganese occurs in well defined layers, generally two, with a thickness varying from 0.25 m to 3 m; the average grade of the handpicked ore is 45 percent. These layers are inter- calated in a grayish red, fine to medium-grained volcanic sandstone that also contains orange claystone beds and gypsum lenses (unit labeled "d" in Quebrada Marquesa formation, Fig. 2).

The petrographic characteristics of the red sandstone that accompanies the manganese are similar to those of the Lower Cycle deposits. Presence of abundant piedmontite xvas also noticed. The thickness of this sequence varies from 10 to 15 m; it is overlain by a very distinctive unit of pale-yellow color, represented by calcareous sandstone, laminated calcilutites, porphyritic ande-

438 L. ,'tGUIRRE AND S. MEHECH

sites and gypsum lenses. The Middle Cycle deposits overlie a sequence of pale greenish-gray, porphyritic andesires containing abundant amigdules filled with chlorite, chalcedony, zeolite, prehnite and glauconite.

Representative samples of the manganese beds of this cycle were taken from several mines of the E1 Romero, Corral Quemado and Fragua mining districts and were studied under the microscope.

El Romero District.--Samples were studied from Liga de Talcuna, Alta del Romero, San Carlos and E1 Romero mines. The minerals present are braunite, pyrolusite, manganite and psilomelane. Braunite is predominant.

Braunite generally occurs finely disseminated in the gangue. In some specimens from San Carlos mine, braunite appears filling small fractures and also (according to Ramdohr) as euhedral crystals cemented by a fine-grained matrix of pyrolusite. At E1 Romero mine braunite is present as euhedral crystals isolated in the gangue. In one of the samples Ramdohr established the presence of manganite as the cement of braunite or as xenomorphic aggre- gates, while in other specimen the same author found that braunite is ce- mented by psilomelane including remnants of manganite crystals. Based on these observations, Ramdohr suggests that psilomelane was originated directly from manganite without passing through the intermediate phase of pyrolusite, this last being the most general transition observed.

Pyrolusite occurs as a coarse impregnation in the gangue, as very small crystals mixed with calcite grains, and as scattered compact masses.

Manganite is very rare and occurs as inclusions in braunite or as veinlets cutting across braunite crystals.

The gangue is formed mainly by calcite and piedmontite. At San Carlos mine the mineralized rock corresponds to a very fine-grained

sandstone cemented by manganese minerals and calcite. The fragments are generally subangular and correspond chiefly to feldspar and quartz; scarce fragments of porphyritic lavas are also present.

Corral Quemado District.--Samples from Toda la Vida, Loma Negra and Macanuda mines were studied under the microscope. The minerals present are braunite, pyrolusite, manganite and psilomelane, the two first being pre- dominant.

Braunite appears intermixed with the gangue, and in some specimens it forms a banded texture consisting of alternating bands of braunite and gangue (Fig. 4c). In cases braunite also occurs as small patches associated to others of pyrolusite, both minerals being enclosed by manganite.

Pyrolusite is the only mineral present at Loma Negra mine. In some specimens this mineral is in the form of small needle-shaped crystals inter- mixed with the gangue; a slightly banded texture is determined by alternation of mineralized bands and gangue. In other samples pyrolusite constitutes lamellar aggregates or is finely disseminated in the gangue.

Manganite, when abundant, constitutes fine-grained aggregates. Psilomelane is very rare and only occurs in a small amount at Macanuda

mine intermixed with the gangue. The gangue is formed by piedmontite and calcite.

MANGANESE SEDIMENTARY DEPOSITS OF CHILE 439

Fra•7ua District.--Samples from Carolina, San Joaquin, Blanquita, Esme- ralda and Dura mines were studied under the microscope. The minerals present in the samples studied are braunire, manganite, pyrolusite, haus- manite, psilomelane. Braunite and manganite are predominant.

Braunire appears under different forms: a) intermixed with manganite and pyrolusite; b) crystals forming compact groups devoid of gangue; c) small euhedral crystals disseminated in the gangue, some of them with quad- rangular and pentagonal sections (Fig. 4d); d) filling of pores as concentric bands alternating with calcite; e) occupying spaces between pores, and f) as larger crystals, some of them having the edges replaced by pyrolusite and others containing needle-shaped inclusions of slightly oriented pyrolusite.

Manganite generally occurs as filling of fractures, in part as needle-shaped radial crystals. In some samples it is associated with braunite to which it seems to replace partially. Manganite also appears surrounding hausmanite crystals or forming, together with braunite, part of the concentric bands in the filling of pores. Also it appears finely disseminated in the gangue.

Pyrolusite is rare and appears intermixed with braunire; as inclusions in manganite and braunite and, in part, as needle-shaped crystals included in braunire; in some samples these crystals correspond to a replacement of braunite according to two preferential directions.

Hausmanite is only present in Carolina mine in the form of a coarse- grained aggregate having a mosaic texture; some of the crystals have twin lamellae.

Psilomelane is very scarce; in several samples the identification of this mineral is only tentative because the crystals are too small to be accurately determined.

The gangue is mainly calcite, quartz, and a very finely crystallized red mineral (?).

Upper Cycle Deposits

The deposits of this cycle are the more restricted in areal distribution as compared to those of the previous cycles. The only outcrops known in the region are present in the slopes of Shangay Mountain in E1 Romero mining district. There, the manganese occurs in a very lenticular layer 10 to 20 cm thick, located near the base of unit "b" of Quebrada Marquesa formation (Fig. 2). This unit overlies the yellow sedimentary sequence "c" which constitutes the top of the manganese-bearing beds of the Middle Cycle deposits.

Thin manganese layers occupying bedding planes of the limestones of the lower half of unit "b" of Quebrada Marquesa formation could also be attributed to the Upper Cycle deposits. Outcrops of these limestones are present in Algarrobilla valley, in the neighborhood of Balmaceda mine of E1 Romero district.

In the exposures at Shangay Mt., the manganese-bearing intercalated beds are in a sequence of red volcanic sandstones with similar characteristics to those of the older cycles.

440 L. .4GUIRRE .4ND o c. MEHECH

Herbert Thomas (oral communication) found manganese layers overlying a limestone and chert series in a locality approximately 5 km east-northeast of Fragua mine and 7 km south of Corral Quemado. These layers are younger than those of the principal manganese-bearing sequence of Corral Quemado mining district and would represent deposits of the Upper Cycle that constitute a lens included in a sequence of red and black jasper.

The microscopic study of representative samples from Shangay Mt. indicates that the predominant manganese mineral present is braunite; man- ganite occurs in very small amount. The samples consist mostly of braunite showing colloform texture; very seldom it occurs in the form of euhedral crystals. This mineral is generally altered to manganite. Manganite occurs in the form of very small needle-shaped crystals. Pyrolusite is very scarce and occurs as very small isolated crystals disseminated in braunite. These manganese minerals are partially interlnixed with the gangue.

DEPOSITIONAL ENVIRONMENT AND PROCESSES IN THE ACCUMULATION

OF MANGANESE

The fossil fauna consisting of pelecypods (Ostrea), gastropods (Nerinea), corals and rudistids, and the lithology of the older manganese-bearing se- quence, the Arqueros formation, indicate that this unit was deposited in a shallow-water marine environment. Sedimentation in this environment was

accompanied by a permanent volcanic activity, which contributed to the rapid filling of the basin, in places deterlnining the existence of transitional condi- tions of sedimentation. This is inferred from the presence of calcareous breccia containing abundant andesitic lava fragments in unit "d" of the Arqueros formation.

The volcanic activity recorded by the units "a" and "c" of the Arqueros formation was probably developed under submarine conditions in its begin- ning. This volcanic activity appears to have produced an important man- ganese mineralization, as can be inferred front the presence of numerous veins that cut across the andesitic lavas of units "a" and "c" throughout the Arqueros region.

The uninterrupted acculnulation of lava flows produced the filling of a great part of the shallow water depositional basin, leaving isolated troughs of very little depth filled with waters saturated with manganese. In these local, elongate troughs the precipitation of the manganese minerals that con- stitute the manganese-bearing beds of the Lower Cycle deposits took place. The presence of copper and ferruginous chert beds, generally associated to the manganese layers, also suggests the possibility that the same volcanic cycle would have contributed important amounts of Cu, SiO 2 and Fe. It is also possible titat desertic climatic conditions contributed to the depositional mechanism by the existence of an oxidizing environment, as suggested by the red sedimentary unit enclosing the manganese layers.

Deposition of the manganese layers included in the Lower Cycle deposits was followed by a new effusive activity characterized by andesitic lavas. This vulcanism took place just before the accumulation of a great thickness of

MANGANESE SEDIMENTARY DEPOSITS OF CHILE 441

continental clastic sedi•nents. The basal part of this continental section is characterized by grayish-red tuffaceous sandstones, sedimentary breccias, sandstones and lutites which are interpreted to have been deposited under arid conditions. The mudflows, flow breccias, sedimentary breccias and ande- sitic lavas overlying the red basal beds produced almost a complete elimination of the previously existing local troughs. A few isolated lagoons appear to have persisted in the southern part of the region described. In these, the water was probably saturated with manganese, originated from the vulcanism that followed the Lower Cycle deposits. This depositional environment pre- vailed during the sedimentation of the manganese layers included in the Middle Cycle deposits. This cycle contains gypsum lenses, accompanying the mineralized beds. Lenticular gypsum layers also occur in the overlying yellow sedimentary unit "c" of the (•uebrada Marquesa formation. Following the deposition of the yellow sedimentary unit "c," this was cut by small channels that were filled by sedimentary breccia; this breccia locally inter- rupts the lateral continuity of unit "c."

The manganese layer of the Upper Cycle deposits originated at the begin- ning of the deposition of the red sedimentary rocks of unit "b" of the (•uebrada Marquesa formation. Sedimentation took place in persisting lagoons that appear to have been located in the easternmost part of the region.

CONCLUSIONS

The detailed study of the stratigraphy of the manganese-bearing Arqueros and Quebrada Marquesa formations in the Coquimbo province affords enough basis to state that the manganese layers exploited in that region originated during three heterochronous cycles of sedimentation. The Lower and Middle Cycle deposits are separated by a thick clastic sequence and their stratigraphic setting is completely different. The manganese beds exploited in the mines of the Lambert-Arqueros region represent a lower stratigraphic horizon than those being worked at E1 Romero, Corral (•uemado and Fragua mining districts.

The deposition in local troughs characteristic of these three cycles and the fact that these basins became progressively separated and isolated, determined the great variability in thickness of the mineralized beds. In many places, the manganese-bearing beds are absent although the stratigraphic sequence that includes them in other regions is present.

This could be the result of non-deposition or partial elimination of the mineralized beds by channel scouring. At many places there are channel structures filled with sedimentary breccia cut through a large part of the yellow sedimentary rocks overlying the beds of the Middle Cycle deposits and in some places reaching as far as the top of them.

The manganese-bearing layers that occur in the different mining districts of the Coquimbo province contain braunite, pyrolusite, manganite, psilomelane and hausmanite. Piedmontire is, in general, an important constituent of the gangue. Braunite is by far the predominant mineral. Psilomelane is more abundant in the Lower Cycle deposits, occurring with a colloform botryoidal

442 L. .4GUIRRE .4ND 5'. MEHECH

texture. Hausmanite is formed only in the deposits of the Middle Cycle. A constant association, between the minerals braunite and piedmontite, has been observed; the latter ndneral occurs only associated with braunite and is absent in all samples where braunite is not present.

INSTITUTO DE INVESTIGACIONES GEOLOGICAS, SANTIAGO, CHILE,

Sept. 5, 1963

REFERENCES

1. Aguirre, L., and Egert, E., 1962, Las Formaciones Manganesiferas de la Regi6n de Ouebrada Marquesa, Provincia de Coquimbo: Revista del Instituto de Ingenieros de Minas de Chile, Santiago, Chile.

2. Fritzsche, C. H., 1923, Neue Kreide fanhen aus Siidamerika: Neues Jahrbuch fiir Mineralo- gle, p. 321-324, Stuttgart.

3. Servicio de Minas del Estado, 1961, Anuario de la Mineria de Chile, Santiago.