[A~IERICA:-1 JouRNAL oF SciENCE, VoL. 261. MAY 1963, P. 4N.432)

AMBIENT PYIUTE GRAINS IN PRECAMBRIAN CHERTS STANLEY A. TYLER* and ELSO S. BARGHOOHN**

ABSTRACT. Pyrite grains, with appendages composed of either quartz or carbonate, oc· cur in the Lower Algal zone of the Gunflint formation, Ontario, and the Biwabik forma· tion of Minnesota. The pyrite grains with appendanges appear to have been propelled through solid rock by the force of crystallization of quartz or carbonate, leaving a record of their path in the form of a carbonate· or quartz.filled trail.

INTHODUCTlON

The black cherts of the Lower Algal zone of the Gunflint formation of Ontario and the Biwabik formation of Minnesota owe their color largely to the presence of pyrite and carbonaceous organic matter. The pyrite occurs pre· dominantly as fine grains disseminated throughout the chert but locally masses of pyrite attain a size of 5 em or more. Most of the individual pyrite grains fall iu the size range from less than a micron up to t('n microns. but occasional grains attain a size of 1 to 5 mm. The pyrite grains commonly exhibit the pyritohedral habit. hut the cubic habit is not rare. Pyrite grains in local ar<>as in the chert and occasional pyrite grains throughout the chert have pt>culiar appendages composed of eitht>r carbonate or quartz. These pyrite grains appear to havt' moved through the cht>rt matrix. lt>aving a record of tlwir path in the form of a carbonate· or quartz.filled trail.

CAHBONATE APPENDAGES

Pyrite grains exhibiting carbonate appendages (trails) range in size from less than a micron to more than l 0 microns. The diameter of the car· bonate appendage is always the same as the diamt>ter of the pyrite grain to which it is attached, whereas the length of the appendage is variable ranging up to an observed maximum of about .1 mm. The carbonate appendages on grains observed in the Gunflint cherts tend to be either straight (pl. 1, fig. 1) or curved within a single plane (pl. l. fig. 6). whereas the spatial form of the carbonate appendages on pyrite in the Biwabik cherts is more diversified. The forms observed include straight trails. plano curves. plano spirals (pl. 2, fig. 5), loose and tight sinistral and dextral spirals, and irregular forms (pl. 3. figs.

* Deceased. Formerly of Department of Geology, University of Wisconsin, Madison. Wis· consin.

** Department of Biology, Harvard University, Cambridge, Massachusetts.

PLATE 1 Ordinary Light

Fig. l--2197X. Straight carbonate appendage attached to pyrite crystal. Biwabik Iron Formation, Mesabi Range.

Fig. 2-616X. Pseudo branching effect of carbonate appendage produced by two pyrite grains. Biwabik Iron Formation, Mesabi Range.

Fig. 3-1612X. Two pyrite crystals. Fig. 4--ll25X. Pseudo branching effect of carbonate appendage produced by two

pyrite grains. Biwabik Iron Formation, Mesabi Range. Fig. 5--16l2X. Plano spiral carbonate appendage. Biwabik Iron Formation, Mesabi

Range. Fig. 6-1650X. Carbonate appendage curved within a single plane. Biwabik Iron

Formation, Mesabi Range. Fig. 7-14-0X. Chlorite appendage replacing carbonate, attached to a magnetite

grain. Ironwood Iron Formation, ;\Iichigan.

424

Stanley A. Tyler and Elso S. Barghoom 425

l and 2). The appendages in general tend to assume a smooth and regular ex­terior throughout their extent (pl. L figs. l, 5. 6). but some (pl. 3. fig. 2) exhihit irregular boundaries with the enclosing chert but of the same diameter as the associated grain. The border tends to become more regular as the pyrite grain at the end of the appendage- is approached. The profile of the pyrite

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PLATE 1

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426 Stanley A. Tyler and Elso S. Barghoorn

grain-appendage interface closely approximates the profile of the distal end of the appendage (pl. 1, fig. 1) in cases where the pyrite grain and appendage lie within the plane of the thin section. A pseudo branching effect is produced when the distal end of an appendage of one pyrite grain lies adjacent to or near the point of origin of another pyrite appendage (pl. 1, figs. 2, 4). All the appendages that lie completely within the thin section have a pyrite grain with the same diameter as the appendage at one and only one end. Appendages that intersect the surface of the thin section may or may not exhibit pyrite grains at their termini. depending upon which end of the appendage was ground away in the preparation of the thin section.

The specific type of carbonate in the appendages can not be determined due to the fact that the diameters of the appendages are less than the thickness of the rock slice. The lack of a granular or fibrous appearance in the carbonate indicates that the appendages are probably composed of a single crystallo­graphic unit. Siderite and ankerite grains and masses as well as siliceous oolites occur in proximity to pyrite grains with appendages. There are also numerous pyrite grains without apparent carbonate appendages.

The chert matrix consists of chalcedony and quartz. The chalcedony grains average .01 to .04 mm in diameter hut attain a maximum size of 0.5 mm or larger in local areas. The grain size is commonly highly variable, rang­ing from less than .01 mm to more than .05 mm in a distance of 0.1 mm or less. The chalcedony and quartz grains form a mosaic pattern with the grain boundaries ranging from roughly linear to highly sutured. The quartz grains range in size from 0.2 to 0.4 mm or larger.

When a relatively large field containing numerous pyrite grains with ap­pendages is viewed under the microscope, one is impressed by the worm-like shapes. the branching forms, and the intricate winding of the carbonate ap· pendages. A complete lack of any preferred orientation of the carbonate appendages in the chert is readily observed by raising and lowering the focus of the microscope.

QUARTZ APPE~DAGES

Pyrite grains with quartz appendages have been observed in the black cherts of the Lower Algal horizon of the Gunflint formation of Ontario and in jasper from the upper portion of the Plymouth member of the Ironwood iron

PLATE 2

Polarized Light Fig. l-l2X. Curved quartz appendage attached to pyrite crystal. Gunflint Iron

Formation, Ontario. Fig. 2-13X. Two pyrite crystals with quartz appendages extended in opposition.

The profile of distal end of appendages conformg to the profile of the pyrite crystal­appendage interface. Gunflint Iron Formation, Ontario.

Fig. 3-l2X. Curved quartz appendage attached to pyrite crystal. Gunflint Iron Formation, Ontario.

Fig. 4-llX. Pyrite crystal with quartz appendage. Gunflint Iron Formation, Ontario. Fig. 5-SX. Straight quartz appendage attached to pyrite crystal. Gunflint Iron

Formation, Ontario, Fig. 6-ISOX. Black area at top is a pyrite crystal with a border zone of elongated

quartz grains which are in optical continuity with the chert grains; a narrow dark line marks the contact of the chert with the border zone.

1.

3.

5.

Ambient Pyrite Grains in Precambrian Cherts 427

formation of Michigan. The pyrite grains range in size from .65 mm to 1.4 mm with the quartz appendages ranging in length up to 2.95 mm. As in the case of the carbonate appendages, the diameter of the quartz appendage is always the same as the diameter of the pyrite grain to which it is attached. The quartz

PLATE 2

2.

4.

428 Stanley A. Tyler and Elso S. Barghoorn

appendages observed are either straight (pl. 2, fig. 5) or curved within a plane (pl. 2, figs. 1, 3). The profile of the pyrite grain-app!:'ndage interface conforms to the profile of the distal end of the appendage, provided the pyrite grain and entire appendage lie nearly parallel or within the plane of the thin section (pl. 2, fig. 2). The relatively large size of the pyrite-quartz appendag!:'s makes it possible to study these forms readily and to note morphological dt'tails that are obscure in the case of the pyrite-carbonate appendages.

The appendages stand out distinctly in transmitted light due to the fact that the quartz of the appt'ndage is clear and colorless, whereas the chert matrix is stained brown or red by particles of organic matter or hematite less than a micron in diameter. The quartz of the appendages is not a single crystallo­graphic unit but consists of numerous small and fewer larger units without common optic orientation. Small quartz grains occur invariably along the entire border of the appendage, with the grain size increasingly rapidly inward away from the border. The largest quartz units lie in the center of the appendage extending to the interface of the pyrite grain. When the border zone of an ap· pendage is examined with polarized light, chert grains with random optic orientation are observed to pass through the border of the appendage (the border is usually marked by a dark line) and continue in optical continuity within the appendage as crystalline units elongated perpendicular to the border (pl. 2, fig. 6). The quartz grains within the appendage also exhibit random optic orientation (pl. 3, figs. 4 and 5). An almost equal scatter in each quad­rant of a Schmidt net point diagram of the projections of the optic axes of the grains indicates random orientation of the latter. The size of the elongated quartz grains increases rapidly toward the pyrite-quartz interface.

The Gunflint cherts contain aggregates of fine-grained pyrite as well as individual grains. Some of the aggregates exhibit a border zone or rim of quartz grains elongated perpendicular to the pyrite-quartz interface. The quartz border zones are developed on only one side of the pyrite aggregates (pl. 3, fig. 3). The physical relationship of the quartz to the pyrite aggregate is very similar to the relationship of the quartz appendages to single pyrite grains. Small pyrite grains with carbonate appendages have been observed penetrating quartz appendages attached to pyrite aggregates.

CHLOHITE APPENDAGES ATTACHED TO MAGNETITE

Chlorite appendages have been observed on two magnetite grains in a single slide of granular ferruginous chert from the Ironwood iron formation of Michigan (pl. 1, fig. 7). The rock is composed of chert, siderite, magnetite, hematite, and chlorite. The chert, which is the dominant component consists of a mosaic of quartz grains ranging in size from 2 to 30 microns. Numerous siderite rhombs averaging about .1 mm in diameter and magnetite grains up to 50 microns in diameter are scattered throughout the chert. Hematite occurs as very fine particles usually less than a micron in diameter, which outline chert granulce. Chlorite occurs in subordinate amounts, usually confined to the carbonate grains.

The chlorite appendage:-; have the same (liameter as the magnetite grains and closely resemble the carbonate and the quartz appendages previously

Ambient Pyrite Grains in Precambrian Cherts 429

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s PLATE 3

Fig. l-260X. Several pyrite grains with carbonate appendages (ordinary light). Fig. 2-658X. Carbonate appendage with irregular boundaries which becomes more

regular as the pyrite grain at the end of the appendage is approached. Biwabik Iron Forma­tion, Mesabi Range (ordinary light).

Fig. 3-8X. Pyrite aggregate with coarse quartz on one side. Fracture in pyrite aggregate is offset from fracture in enclosing chert by the growth of coarse quartz. The two round areas within the pyrite are holes in the slide (polarized light).

Fig. 4. Sketch of pyrite grain and quartz appendage of plate 2; fig. 1. The optic orientation of the numbered grains was determined by Dr. Ronald Wilcox.

Fig. 5. Schmidt net point diagram, 30 optic axes of quartz from appendage of plate 2, fig. 1. Numbers serve to identify grains in figure 4.

430 Stanley A. Tyler and Elso S. Barghoom

described. The magnetite grains are about 30 microns in diameter, and the appendages are about .1 mm in length. The distal end of the appendages is rather irregular and does not record the profile of the chlorite-magnetite inter­face. The chlorite throughout the rock appears to have replaced carbonate, and thus the chlorite appendages may have been originally carbonate. Iron-bearing carbonate appendages may also be expected to be converted to magnetite, goethite, or hematite under appropriate conditions.

Quartz appendages on pyrite grains similar to those described have also been observed in sericitized plagioclase feldspar from the Butte monzonite.

ORIGIN OF THE PYRITE

The presence of pyrite and organic matter in the pyrite-chert facies of the basal Gunflint and Biwabik iron formations indicates that a reducing or euxinic environment existed on a regional scale at the time these sediments were deposited. The red algal jaspers, which have a restricted occurrence, are presumably the product of local oxidizing environments that existed as micro­environments within the euxinic environment. Much of the fine-grained pyrite (5 microns or less in diameter) in the black chert appears to have formed at the time of deposition of the enclosing sediment. Such features as discontinuous wispy lenses (.1 to 1 mm in thickness) of pyrite-bearing chert alternating with red jasper lenses and oolites with a jasper core surrounded by concentric laminae of pyrite-bearing chert are consistent with a primary origin for the pyrite. Partial to complete replacement of occasional chert oolites and car· bonate rhomhs by pyrite indicates that some of the larger grains and aggre­gates (.1 to 1 mm in diameter) developed at a later date, perhaps during the period of diagenesis.

OHIGIN OF THE APPENDAGES

The remarkable resemblance of the carbonate and quartz appendages of pyrite grains to similar forms described by Gruner (1923, fig. 1: 1925) from Soudan cherts and identified by Tilden as blue-green algae can not be fortui­tous. Unfortunately, these alleged algae were given generic designations and hence have become incorporated in the biological and paleontological literature. Gruner mentions the presence of magnetite and pyrite in the chert but does not identify the opaque objects at one end of at least four carbonate bodies in the figure mentioned above. The development of carbonate and quartz appendages on pyrite grains is almost certainly an inorganic process which appears to be related to the preferential growth of relatively coarse units of carbonate and quartz at the interface between pyrite grains and the enclosing chert-carbonate ground mass. The following discussion is based largely on ohservations made on the quartz appendages.

The character of the rock at the time the appendages were formed and the direction in which the appendages grew (whether the growth was localized at the pyrite-appendage interface or at the distal end of the appendage) is prerequisite to an understanding of the growth mechanism involved. Examina­tion of numerous appendages under the microscope reveals that: ( 1) the growth of the appendage took place at the pyrite-appendage interface. (2) the chert ground mass had attained its present grain size and texture prior to the

Ambient Pyrite Grains in Precambrian Cherts 431

growth of the appendage, ( 3) fractures passing from the chert matrix through pyrite aggregates have been offset by the growth of quartz in the appendage (pl. 3. fig. 3), and (4) quartz appendages have been observed to pass through oolites and granules of chert. Conclusions 1 and 2 are based upon the obsen·a­tions that: (l) when the appendage lies within the plane of the thin section. the distal end is essentially a direct image of the pyrite-appendage interface, and (2) the original pyrite-chert interface located at the distal end of the ap· pendage is often marked by opaque particlt"s less than a micron in diamt"ter, which together produce a dark line. The quartz and chalcedony (chert) grains of the enclosing chert pass through this boundary in optical continuity and be­come the outer rim of the distal portion of the appendage (pl. 2, fig. 6). The grain size of the quartz becomes larger away from this boundary and reaches its maximum size at the pyrite-interface. All the pyrite grains with quartz ap· pendages show the fine-grained border with the quartz grains in optical con­tinuity with the chert matrix.

A film of carbonaceous matter that ranges in thickness up to .l mm may be observed with reflected light on the leading faces of the pyrite crystals but is absent at the interface of the pyrite crystal and appendage. The carbonaceous film is barely perceptible on pyrite grains with short appendages but is very pronounced on those with relatively long appendages. The pyrite crystals have swept up the carbon pigment of the rock, accumulating it on the leading faces and leaving a clear quartz trail free of organic matter.

Pyrite crystals with attached quartz appendages were freed from the chert matrix by successive treatments with hydrofluoric acid. (The fine-grained chert is much more susceptible to solubilization than the relatively coarse­grained quartz in the appendages.) This procedure allowed a three-dimensional examination of the pyrite crystals and appendages. The carbonaceous film on the leading faces of the pyrite crystals and the quartz appendages were re­moved with a needle. The faces of the pyrite crystals thus uncovered were found to be highly lustrous and without evidence of pitting or etching.

The pyrite grains appear to have been propelled though solid rock by the force of crystallization of either quartz or carbonate in the appendage. Pres· sure-solution of the chert on the leading surfaces apparently provided the room for the advancing pyrite crystals. Some of the pyrite grains are ruptured, which suggests that the forces involved may have exceeded the strength of the pyrite crystals. If crystal growth continued in the appendage after rupture, the fragments of pyrite were propelled through the rock as entities. Certain critical attributes of the process may be deduced. It appears that: (1) the interface between pyrite grains and the enclosing chert matrix provided an environment favorable for the solution and recrystallization of both the carbonate and quartz of the appendages, (2) silica and carbonate were present in a mobile form to feed the growing crystalline units, and (3) the forces generated by the growing appendages promoted solution of the chert along the leading surfaces of the pyrite grains.

Independent evidence that both silica and carbonate were mobile at some time during the history of the rock is indicated by the following observations: (1) carbonate is commonly observed to replace chert along fractures, and (2)

432 Stanley A. Tyler and Elso S. Barghoom

carbonate rhomhs are oftE>n seE>n to he partially to eomph.·tely replaced by quartz.

The above discussion is based upon the concept that the force of crystal­lization played an important role in the development of the appendages. How­ever, the authors believe that it is likely that other physical chemical processes may be involved.

Questions that remain unanswered are: ( 1) Why is the interface between pyrite and chert a favorable site for the growth of coarse quartz and car­bonate'? (2) Why do some pyrite grains apparently lack appendages even though they occur in close proximity to grains with appendages?

It is almost certain that some pyrite grains have appendages that lie in planes perpendicular to the thin section. These appendages would not normally be observed. However, the abundance of pyrite grains without observable ap­pendages suggests that other factors may he involved. The accumulation of carbon or carbonaceous matter on the surfaces of the growing pyrite grains might well inhibit the continued growth of hoth the quartz or carbonate ap· pendagt>s.

In conclusion it should be noted that the structures described here are in no way genetically related to the morphologically distinct organic forms oc­curring in the Gunflint cherts (Tyler and Barghoorn. 1954). Continued study of the Gunflint cherts has yielded a large assemblage of diverse microorganisms. These have been the object of extensive morphological, petrographic, and chemical studies now nearing completion. It is fortuitous that these inorganic "pseudo fossils" represented as quartz and carbonate trails or appendages oc· cur in the same rock matrix as do well defined organically preserved organisms. As shown above. however, the "pseudo fossils., represented by the appendages occur also in rocks devoid of any evidence of organic form, and indeed, in one case in rocks of igneous origin. The presence of true algae in rocks older than the Animikie Series of the Middle Precambrian, hence. has not been demon· strated. and references to algal structures in the Lower Precambrian (Ar· chaean) should be reevaluated in the light of the phenomena here described. This reevaluation is of considerable theorPtical importance in proposed schemes of evolution and phylogeny, as it has been widely accepted in the literature that blue-green algae have been demonstrated as extant in Lower Precambrian (Archaean) time.

ACKNOWLEDGMENTS

This study was supported in part by a grant from the Wisconsin Alumni Research Foundation and the National Science Foundation. The authors wish to thank Dr. R. W. Marsden. J. T. :Mengel. Jr., and J. J. Finney for supplying some of the specimens. Credit is also due Dr. T. E. Hendrix and 1. T. Mengel for their able assistance in photographing the material and Dr. Ronald Wilcox, for universal stage studies of the quartz appendages.

REFERENCES

Gruner, J. W., 1923, Algae, believed to be Archean: Jour. Geology, v. 31, p. 146-148. ---- 1925, Discovery of life in the Archean: Jour. Geology, v. 33, p. 151-152. Tyler, S. A., and Barghoorn, E. S., 1954, Occurrence of structurally preserved plants in

pre-Cambrian rocks of the Canadian Shield: Science, v. 119, p. 606-608.