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Introduction to Ore Microscopy II

Erich U. Petersen

MINERAL PARAGENESIS

The term 'paragenesis' refers to the time-successive order of formation of a groupof associated minerals within a particular deposit. Since the great majority of ore mineraloccurrences have been formed by several distinct periods of mineralization, the completedescription of the paragenesis of a deposit involves establishing the order in which theconstituent minerals have been formed and the sequence of resorptions and replacementsthat have occurred. In order to establish the paragenetic sequence in a deposit, two broadapproaches are useful:

1. the study of open-space fillings

2. the study of alteration reactions - replacement relations among the ore minerals

In near-surface regimes, rocks yield by fracturing rather than by flowage; openchannel ways develop and layers or crusts of minerals may be deposited from successivepulses of fluid that pass through the fractures. By searching for variations in mineral grainsize, symmetrical banding, and certain diagnostic structures (comb, cockade), one canrecognize open-space filling and by studying the composition of sequential crusts along thewalls of the vein, one can determine the paragenetic sequence. Three kinds of ore mineraldeposition may be considered:

a) simultaneous deposition (in which two or more minerals are formed from thebeginning to the end of the process) e.g., galena-sphalerite, tetrahedrite-tennantite-pyrite

b) overlapping deposition (in which two or more minerals have formation periodsthat overlap in part) e.g., sphalerite-pyrite

c) successive deposition (in which the formation periods of two or more mineralssucceed each other with practically no overlap) e.g., sulfide-carbonates

In this review we shall concentrate on the second approach - - the study of alterationreactions - replacement relations of ore minerals - - in determining the paragenetic sequenceof minerals present in polished sections. During the deposition of minerals, the physico-chemical conditions may change and this leads to the replacement of mineral phases whichbecome unstable by phases that are more stable in the new environment. After the finalchanges, the minerals which remain have survived together and will, to some extent,provide evidence of these last changes; one must not, however, conclude that all of themwere stable phases, even during these last changes, nor that they are really stable under theconditions in which they are now found. All textures that show evidence that the 'primary'mineral content resulting from deposition has been changed may be classed under theheading 'replacement-alteration textures'. On the basis of Schneiderhons (1935) work,three broad categories are distinguished: (i) deposition textures, (ii) exsolution textures,and (iii) replacement textures. The last two types will be considered next.

1. Exsolution Textures

A solid solution in which two different elements A and B are completely

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interreplaceable at high temperatures (in disordered mineral structures) but not at lowertemperatures (in ordered mineral structures) will tend to break down on cooling into twoseparate phase, one rich in A and the other in B. This breakdown of a homogeneous solidsolution is known as exsolution. The extraneous materials which are forced out of themineral structure upon cooling tend to accumulate along cleavage surfaces orcrystallographically controlled directions, in small blebs and blades. Lamellar textures areespecially diagnostic of exsolution. Slow cooling, anomalous mixtures, small foreigninclusions, and tectonic stresses are some factors that facilitate exsolution. Silicates andsome oxides which have stable and compact lattices exsolve with much greater difficultythan do the sulfide minerals.

Some exsolutions are chemically 'closed', that is, the previously simple solidsolution is chemically equivalent to the composition of the disintegration products. Anexample is perthite of 70 percent orthoclase and 30 percent albite lamellae that was earlier ahomogeneous high temperature feldspar with 70 percent Or and 30 percent Abcomposition. An ore example is hematite-ilmenite.

Other exsolutions are chemically 'open' in that a change in the stoichiometricrelationships is observed. For example, if a feldspar exsolves to yield orthoclase and smallhematite plates, then either the solid solution could not have been expressible by thefeldspar formula or some elements were added or removed. Another example is theilmenite-magnetite pair. No solid solution exists between the endmembers, but blebs ofmagnetite are found in ilmenite and vice versa - - a phenomenon attributed to the expulsionof iron from ilmenite and titanium from magnetite. This exsolution is not due to a solvusseparation. Note that a high-temperature solid solution may form disintegration(exsolution) products that are quite different in their chemistry and crystallography from theinitial solid solution. Such a complete re-mineralization is quite similar to the exsolutionphenomenon. Examples of re-mineralization are: decomposition of a hematite-ilmenite intorutile and magnetite ('tie line switch') and the decomposition of bornite into chalcocite orchalcopyrite.

Often two or more generations of exsolutions can be observed in a mineral, forexample, sphalerite will disintegrate to form chalcopyrite which in turn will containexsolutions of FeS (mackinawite). An exsolution bleb which is separated at hightemperatures is often itself still a complex and non-stoichiometric solution with which afurther decrease in temperature will tend to exsolve new compounds by exsolution inseveral stages. A different situation is observed in the FeTiO3-Fe2O3 system in which thestrong supersaturation of one component in the mineral leads to the formation of secondgeneration exsolution discs.

The forms of exsolution bodies are often quite similar to those of replacementtextures or to those of the simultaneous oriented crystallization. In a rigorous sense, onecould view exsolution as a type of replacement phenomenon, for in the regions in whichnew exsolved minerals formed, the parent mineral must have diffused outwards and theexsolved mineral inwards. This latter diffusive movement is, therefore, a type ofreplacement. Many oxide exsolutions will maintain their geometry (i.e. outline) after theirformation. However, in the case of sulfides, and especially soft sulfides, the surfacetension and velocity of migration of species is sufficiently large even at moderatetemperatures that sharp-cornered exsolution bodies will be rounded off, lamellae will flowinto rows of dots, and fine exsolution bodies will coalesce into larger globs. The resultingforms are quite similar to typical replacement textures and thus may not always beinterpreted correctly as primary exsolutions. Relicts of exsolution textures not destroyed insuch 'collectivization' may still be present in 'armored' inclusions, for example, quartz.

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Some forms of exsolution blebs are outlined below:

1. Variable in size and form, not uniformly distributed. If the high temperaturemineral was compositionally zoned with the highest concentration of the extraneouselement in the center of the grain, one might see a greater concentration ofinclusions in the core of the grain as opposed to the periphery.

2. Number and size different along different directions in the parent mineral. Cracks,twin lamellae, inclusions, lattice distortions often cause preferred orientation ofblebs.

3. Irregular forms - - spontaneous vs. coalesce. (147, 148, 167)

4. Dispersions of regular shaped particles, for example, cubic minerals show stars oroctahedra development. (151, 152).

5. Lamellar texture well developed among cubic and hexagonal minerals, (147,156,157, 158, 159, 160, 169).

6. Myrmekitic - - often thought to be exsolution products where lattice structure ofcomponents is similar. (171)

7. Flame structure of pentlandite in pyrrhotite. (166)

8. Multiple exsolution. (172)

9. Some complications messing up the picture - - first exsolved mineral maysubsequently become transformed; first exsolution mineral may disintegrate intoseveral later components.

The following table lists some exsolution textures. The source of the data is Ramdohr(1969).

Characteristic type Relative amount ofParent Mineral Exsolved Phase of intergrowth parent exsolved

1 bornite chalcopyrite individual lamellae 1:3

2 chromite hematite platelets 1:5

3 chromite ilmenite platelets 1:10

4 chalcocite bornite network 1:2

5 chalcocite chalcopyrite - -

6 chalcopyrite bornite - -

7 chalcopyrite pyrrhotite - -

8 enargite chalcopyrite - 1:10

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9 galena argentite dispersion 1:10

10 hematite ilmenite discs (0001) all

11 ilmenite magnetite thin plate-like bodies (111)1:5

12 ilmenite hematite discs (0001) all

13 ilmenite rutile plates 1:10

14 magnetite ulvspinel network (100) 1:3

15 magnetite ilmenite platelets 1:1

16 pentlandite pyrrhotite dispersion 1:4

17 pentlandite chalcopyrite cellular-net shaped 1:10(similar and more commonas replacement)

18 pyrrhotite pentlandite myrmekitic (flames) 1:5

19 pyrrhotite chalcopyrite - 1:4

20 pyrrhotite magnetite platelets (v. characteristic)1:10

21 sphalerite pyrrhotite - -

22 sphalerite chalcopyrite emulsion 1:4

23 sphalerite cpy-po-cub emulsion 1:4

2. Replacement Textures

When a mineral which formed early in the paragenetic sequence is chemicallyaltered to form a new phase, the parent mineral is said to have undergone 'replacement'.Such replacement of one mineral by another can be partial, in which case some remnants ofthe parent grains are preserved, or complete, wherein no trace of the original mineral can beobserved. The associated replacement textures show a considerable variety of form owingto the mobility and frequent transformations of the minerals involved. The geneticsignificance of some replacement textures will be outlined below, but first, a few generalremarks on the replacement phenomena are in order:

Time of replacement :

1) Immediate during precipitation of coexisting minerals. cf. analogy of crystallizationin a simple binary system in which the crystals that have been precipitated first arealtered by the remaining solution (melt) and are replaced by solid solutions(crystals) of different composition, or by another type of crystal with resultantexcellent replacement textures. Complete or partial resorption of phases that are

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initially precipitated.

2) Later hydrothermal - - minerals not in equilibrium with bypassing solutions.

3) Metamorphic attack and re-equilibration.

4) Weathering (oxidation and supergene enrichment).

Mechanism of Replacement :

Early minerals undergo replacement when altering solutions circulate and penetratealong fissures and cracks between individual mineral grains or along cleavage planes,twin planes and crystal defects in individual crystals. A front of alteration can often beobserved as solutions penetrate through a given rock. The later mineral may replacethe earlier around the periphery of the latter without penetrating it, or it may penetrateand then attack from the core outwards. Successive minerals may be deposited fromsingle evolving solutions with complete or partial resorption of phases that wereinitially precipitated.

Textures of repl acement :

The relative amounts of the later and the earlier mineral produce textures to whichdifferent names have been given - - cataclastic, pseudo-eutectic, dendritic, etc.Unfortunately, most replacement texture classifications have genetic connotationsassociated with them and are not based on an objective, descriptive approach. Suchgenetic classifications are dangerous and ought to be avoided for many textures canform in a variety of ways.

Form of replacements related to: hardness, cleavages, degree of cataclasis, chemicalstability of the lattice structure, T, pH, Eh, composition of altering fluids (are theysaturated with respect to components in the mineral that is being altered?)

The following capsule descriptions of textures are based on Rahmdor's proposedclassification (1969):

1. Filigree network -- (209) favors ore minerals lacking definite cleavage or showingcataclastic texture.

2. Inclusions -- Inclusions are small particles of one mineral included in the material ofanother, and they represent a kind of texture. Inclusions can be either the relicts of amineral that is being replaced, or else the early signs of a new mineral that isreplacing the old; otherwise they can consist of a phase that has been unmixed fromthe host crystal. Schneiderhon (1943) has established four types of relationsbetween an inclusion and a host crystal:

a) Older foreign crystal inclusions engulfed by a growing new mineral (e.g.,chromite in Pt) - No Reaction Relation.

b) Inclusions unmixed from the material of the host mineral after the primarycrystallization of this (e.g., sphalerite in galena) - Exsolution.

c) Transformed inclusions that have been altered (e.g., 'exploded bombs' of

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pyrite) - Replacement.

d) Inclusions which are later than the host (e.g., zones of cementation andoxidation).

Inclusions are often quite irregular in shape, but in other cases they show regularorientation in the host crystal, as was observed in the discussion of exsolutionphenomena. The relation of the inclusion to the host mineral is often of greatimportance in determining the paragenetic sequence in a section. A few examplesof inclusions and their host mineral are given below:

Inclusion Enclosing Mineral

argentite galenabornite chalcopyritebornite chalcocitebreithauptite niccolitecubanite cupritechalcopyrite sphaleritechalcopyrite enargitegold pyritehematite magnetiteilmenite magnetiteilmenite chromitepyrrhotite sphaleritepyrrhotite chalcopyritepentlandite pyrrhotite(211-217)

3. Shredded Texture -- resembles island form structures, typical concave replacementrelicts. (213)

4. Skeleton shaped textures -- rapid growth of corners and edges of crystals leads toincorporation of inclusions. (218)

5. Lattice forms -- develop in minerals with well-developed cleavage cf. pyritereplaced by sphalerite. (221)

6. Zonal replacements -- cf. chromite replaced by niccolite along fractures andpreferentially along its marginal zones. (218)

7. Dendritic textures -- replaced along cleavage directions and along lines ofintersection of these. Invasion of solutions along twin lamellae. (219, 221)

8. Real frontal Replacements -- mineral lacks cleavage, fractures, grain boundariesand inclusions and no zones of apparent inclusions. Thus, replacement 'fronts' setin as irregular, blob-like forms often developing uniformly rounded and smoothboundaries.

A rule of thumb that be helpful is that harder minerals are replaced by softer ones. Forexample, the sequence pyrite < sphalerite < chalcopyrite < galena < argentite is quitecommonly observed.

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Examples of Important Ore Mineral Parageneses

Often one has the problem of identifying an unknown mineral in a polished section(an understatement?). Sometimes certain textures or the nature of certain inclusion can giveindications which are useful in the determination of the unknown. One can generallyreduce the number of possibilities of what the unknown is by considering the assemblageof minerals with which it occurs, i.e. the paragenesis can help. Certain minerals typicallyoccur in association with other minerals of a particular class or paragenesis; for example,the assemblage py-po-sl-cpy-mt is common. Unfortunately, there are some minerals whichoccur very widely in various ore bodies, and thus are not especially indicative of aparticular assemblage or environment of formation. Hence, neither can use be made of thepresence of other minerals to identify these, nor can use be made of their presence toidentify other minerals accompanying them. It is apparent that paragenesis can be useful indetermination only when one is concerned with minerals of relatively uncommonoccurrence or with those that are associated with relatively rare minerals. Several examplesof important parageneses are presented below.

1. Titanomagnetite-ilmenite paragenesisDerivative of simatic magmaTitanomagnetite or ilmenite are early crystallization products from basaltic or gabbroicrocks. Exsolution phenomena common: in gabbroic rocks at high solidificationtemperatures. Titanomagnetites in which segregation lamellae of ilmenite occur are firstformed. At lower temperature, ilmenite or ilmenite borders to previously separatedmagnetite grains form. Both phenomena may be seen in the same rock.Ilmenite - shows twin lamellae commonly and hematite exsolution blebs.

2. Nickel-pyrrhotite paragenesisSudbury, CanadaAssociated with gabbroic to quartz dioritic rocks, often rich in Pt. Deposit occurs bothintramagmatically as segregations and as hydrothermal formations; deposits associatedwith parent rock as impregnations occurring in alien rocks (e.g. granites) as fissureinfillings.Association: main minerals:

pyrrhotite, pentlanditechalcopyrite always presentmagnetite, ilmenite frequently as magmatic segregations from the rockswhich were impregnated by the hydrothermal sequences.

Bulk of paragenesis is always formed from pyrrhotite. Pentlandite, which usuallyappears later, occurs: as (1) an intragranular film, from which it replaces the pyrrhotite,(2) a migration into the pyrrhotite from clusters; thus, pentlandite intrudes outwardfrom cluster in flame-shaped aggregates (3) may have the pentlandite forming primaryflame-shaped segregations. These are aggregates of irregular lens shape parallel to thebase of the intruded segregation particles of pyrrhotite.Silicates are chlorite, actinolite.Later alteration of sulfides: pyrrhotite to pyrite and pentlandite to bravoite into limonite.At pH less than 7, possible to form marcasite from pyrrhotite and at higher Eh, possibleto form magnetite from pyrrhotite, starting as a rule from grain boundaries. Note thatthis magnetite could not display exsolutions of ilmenite.Paragenesis fairly uniform except that rarely see late-stage hydrothermal solutionsdepositing other sulfides, cf. stibnite.The initial parageneses were formed at relatively high temperature, so that it waspossible for cubanite to form.

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3. Chrome-spinel paragenesisSegregation products of magma rich in Cr, and as a rule, peridotitic type.Paragenesis highly uniform, consisting essentially of chrome-spinels c.f. chromite (Fe,Mg) (Cr,Al,Fe)2O4 and FeAl2O4-MgAl2O4 spinels.Sulfides are rare with chromite -- pyrrhotite or arsenides. Due to affinity of As for Ni,and due to high Ni content of peridotite.Host rock of chromite deposits - olivine and pyroxene rich; serpentine.

One can use paragenetic reasoning to distinguish between two similar lookingminerals in a section. As an example, consider the minerals ilmenite and manganite(MnO(OH)). Both minerals are of generally similar properties and can easily bedistinguished in large grains. In very small grains, however, the paragenetic reasoning canbe useful, as they have two radically different modes of occurrence.

Ilmenite Manganite

Found especially in basic Characteristic of superficial andigneous rocks low temperature formations - cement

Typical paragenesis: Typical paragenesis:ilmenite manganitemagnetite goethitechromite psilomelaneplatinum limonitepyrrhotite sideriterutileniccolitepyrite

Clearly, just by observing the minerals in the section with which the 'unknown' mineraloccurs, one can without hesitation rule out several possibilities.