08-Mineral Characterization Paragenesis.pdf

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Identification (1) ... | Identification (2) ... | Characterization (1) ... | Characterization (2) ... |

Paragenesis ... | Review #2 ...

ParagenesisSession Headings:

Definitions

(See Summary for main points)

Paragenesis: any assemblage of ore minerals, with or without gangue, formed during thesame time (interval).

Paragenetic Sequence: chronological order of mineral deposition: includes the conceptsof contemporaneity, overlapping deposition and successive deposition.

Depositional Stages: recognizably distinct intervals of mineral deposition within thelonger depositional history of a complete paragenetic sequence. A depositional stagecommonly is characterized by a distinctive mineral assemblage.

Paragenesis deals with the sequence of mineral deposition. Here we will be concerned with

l the criteria by which sequential depostion can be established unequivocally

l diagrammatic means of summarizing established relationships

Why an Interest in Paragenesis?


The paragenetic sequence partly reflects changes in the general nature of the ore-forming

fluid.

Stages of deposition may be useful in determining age of formation of individual veins in avein camp and, more generally, might be useful in integrating vein evolution into thestructural history of an area. Processes superimposed on an original ore assemblage can beeither detrimental or beneficial from the point of view of beneficiation.

Paragenesis commonly is closely related to zoning, including (1) mineral or assemblagezoning, (2) textural zoning and (3) cryptic zoning. Cryptic zoning is not visiblemegascopically and thus depends on microscope investigations, assay information, etc. SeeGuilbert and Park (1986).

Minerals that are closely related in time, especially those formed by contemporaneousdeposition, are likely to form intergrowths i.e., paragenesis can be related to liberation

Professional DevelopmentPractical Ore Microscopy and Mineralography

Part 2: Mineral Identification and Characterization

Summary

Text

Definitions

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problems.

Paragenetic Sequence


Criteria for Determining Paragenetic Sequence

In older literature very definite paragenetic criteria commonly are interpreted as beinginfallible leading to very complicated and detailed sequences of mineralization beingoutlined. As Stanton (1972, p. 263) states, " It seems likely that at least many of the

structures ascribed to replacement up to about 1955 are due rather to simple grain growth, precipitation, annealing and so on." (See Stanton (1972)). During the past 40years experimental work with sulfide systems has cast doubt on the certainty of somecriteria and has shown that many textures are ambiguous in terms of determiningdepositional sequence; thus, it becomes important to evaluate individual textures critically.

For example, a variety of textures previously thought to be indicative of exsolution (e.g.,myrmekitic intergrowths of chalcocite and bornite) are now known to form by replacementand contemporaneous deposition as well as by exsolution. Even apparent cross-cuttingrelations must be examined in detail to ascertain their paragenetic meaning as they canarise by replacement, exsolution, open space filling, or may be relict features.

A few open space filling and replacement textures useful in establishing paragenesis areillustrated in Figure Sequence 1 and Figures 2 through 6. In brief, some of the usefultextures are:

l

Open Space: crustification, coxcomb, colloform, cockade, mineral inclusions along primary growth zones, etc.

l Replacement: generally divided into massive (no directional control) and oriented(directional control evident), eutectoid (development of an intergrowth of two solidsfrom a liquid) and exsolution (see special section); evidence includes pseudomorphs,cleavage control of a later mineral, twinplane control, various intergrowth textures, etc.





l Mineral stabilities in some cases can be used to indicate order of deposition.

Graphic Representation


Graphic Representation of Paragenesis

The depiction of a paragenetic sequence is through the use of two types of diagrams (1)

line diagrams, and (2) Vandeveer diagrams.

Simple line diagrams (Bastin (1950)) are illustrated in Figure 7, where minerals are listedalong the ordinate and relative position of mineral deposition (relative ages) is shownalong the abscissa (time axis). Two variations are shown in Figures 8 and 9, including acomposite line diagram for a mining district. The principal advantage of line diagrams istheir simplicity. Line diagrams show relative age of minerals, clearly demonstratecontemporaneity and overlap but have no implications of time duration for deposition of individual minerals.

Vandeveer diagrams (Robertson and Vandeveer (1952)) are somewhat more complicated but also depict more information, including some general textural implications. Minerals

are shown as circles arranged clockwise around a larger circle in order of younging (withrespect to paragenesis). The earliest mineral is located in the 12 o'clock position. Size ocircles (for minerals) is roughly proportional to the amount of mineral present, and thename and sequential number in the paragenesis can be shown beside each mineral circle.Identical numbers for two circles (minerals) indicates contemporaneous deposition. Theminerals in contact have their circles joined by a line, thus, mineral associations may standout on the diagrams.





Where replacement is obvious, an arrowhead is directed at the circle representing themineral that has been replaced. Variations in thickness of the lines joining mineralsinvolved in replacement can be used to indicate the relative intensity of particular replacement relations. Arrows in both directions on a single line indicate ambiguous or conflicting criteria for replacement. Dashed lines point to inconclusive criteria for the

process of replacement. Exsolution products are shown on a large arc with a line drawn between solute and solvent. Superimposed mineralogies (e.g., weathering products) areshown on a second, larger arc, further outward from the main mineral circle than is theexsolution arc, and connected by a line to the earlier minerals with which they areassociated.

The main problems with Vandeveer diagrams are (1) partly overlapping deposition of minerals is difficult to depict clearly, and (2) if many minerals are present that are incontact with each other the diagrams can become very confused. Apart from these

problems, the diagrams are particularly useful in applied mineralographic studies becausetheir features indicate mineralogy, relative abundances of minerals as well as implying

potential textural (replacement, intergrowth) and mineralogical problems.

Line diagrams do not show mineral associations clearly, do not relate subsequentalterations to primary minerals, can show only rough abundances and do not indicatereplacements and intensity of a replacement. Several of these limitations in line diagramsare better demonstrated using Vandeveer diagrams, particularly for studies directed towardmilling of ores. In practice, it is useful to generate both types of diagrams as part of anapplied mineralogical study.

Confirm your knowledge of "Part 2: Mineral Identification and Characterization" with - Review #2

OR

Continue the course with - Part 3: Exsolution, Metamorphism and Stabilities

Identification (1) ... | Identification (2) ... | Characterization (1) ... | Characterization (2) ... |Paragenesis ... | Review #2 ...

Practical Ore Microscopy and Mineralography - August 15 2003



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08-Mineral Characterization Paragenesis.pdf