Textures paragenesis and zoning of ores
and their significance

 

Textures

Ore microscopy involves not only the identification of individual mineral grains but also the interpretation of ore mineral textures, that is, the relationship between grains.  Ore mineral textures may provide information on:

  1. Process of initial ore deposition

  2. Post-depositional re-equiliberation or metamorphism

  3. Deformation

  4. Annealing

  5. Meteoric weathering

 The recognition and interpretation of textures is thus an important step in understanding the origin and post-depositional history of an ore.  The extent to which the ore minerals retain their composition and textures formed during initial crystallization varies widely:

a)    Oxides, disulfides arsenides and sphalerite are the most refractory ore minerals and hence are more likely to preserve evidence of their original conditions of formations.

b)   Cu-Fe sulfides and pyrrhotite are less readily re-equilibrated.

c)    Native metals, sulfosalte and argentite are among the most readily re-equilibrated ore minerals, and are thus least likely to reflect initial formation conditions.

 The variability in terms of equilibration rates in terms of time for various minerals is shown below:

 

Fig. 1. Equilibration times for various sulfides involved in solid-state reactions.  The field widths represent differing rates in different reactions as well as changes in rates due to compositional differences, and experimental uncertainty.

 

The textures observed in many polymetallic ores reflect the stages (sometimes numerous) in their development and post-depositional history.  Textural information is also important in the milling and beneficiation of ores.  Rarely does a single texture provide unequivocal evidence regarding the origin or history of an ore deposit.  Commonly, a variety of textures representing different episodes in the development and subsequent history of a deposit are observed.  With careful observation, common sense, and a little imaginative interpretation, much can be learned about the origin and post-depositional history of an ore from the study of ore textures.

 

Paragenesis of Ores

The term 'paragenesis' refers to the time-successive order of formation of a group of associated minerals within a particular deposit. Since the great majority of ore mineral occurrences have been formed by several distinct periods of mineralization, the complete description of the paragenesis of a deposit involves establishing the order in which the constituent minerals have been formed and the sequence of resorptions and replacements that have occurred. In order to establish the paragenetic sequence in a deposit, two broad approaches 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; open channel ways develop and layers or crusts of minerals may be deposited from successive pulses of fluid that pass through the fractures. By searching for variations in mineral grain size, symmetrical banding, and certain diagnostic structures (comb, cockade), one can recognize open-space filling and by studying the composition of sequential crusts along the walls of the vein, one can determine the paragenetic sequence. Three kinds of ore mineral deposition may be considered:

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

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

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

  • A full understanding of the sequence of deposition or PARAGENESIS can be obtained from a study of ore mineral textures as seen in polished sections under the microscope.

  • To determine the paragenesis of an ore, it is necessary to determine for each pair of minerals present, whether they were deposited simultaneously or one after another.

  • Sometimes, when two minerals do not show any relationship, it becomes necessary to establish their sequence of deposition indirectly.

  • Frequently the ninerals in an ore occur in groups, so that age relationships can be established between members of an individual group, and between groups as a whole.

  • The repitition of deposition of a group or groups of minerals does not necessarily imply an interruption or pulsation of the process.  In most cases it implies a changing character of the mineralizing solutions.

  • This change in the character of the mineralizing fluids is also revealed in the changing composition of the gangue minerals.

Zoning of Mineral Deposits

The time sequence of mineral deposition is known as the paragenesis of a deposit; the spatial distribution is described as zoning. The paragenesis, or chronological order of minerals, is determined by studies of mineral rela­tionships, with the emphasis on microscopic textural features.  Zoning pat­terns are manifested by mineralogical changes along both vertical and hori­zontal traverses across mineralized areas.  The zones may be defined by dif­ferences in mineral species, differences in types of metal, differences in sulfur content, or even subtle differences in the ratios between certain elements.  But whatever the relationship used to define a zone, in each case the zoning and the paragenesis will be cogenetic, because they are merely two different aspects of the same phenomenon.

The paragenesis of mineral formation in moving ore fluids produces changes in ore mineralogy along the course of deposition.  Such changes are described as zoning, and are found in sedimentary deposits as well as in magmatic and metamorphic ores.  In the  ideal case of a radiating hydrothermal or pneumatolytic fluid, changes in chemistry, temperature and pressure along the fissure result in the deposition of different minerals in con­centric zones at increasing distances from the magmatic source.  Syngenetic deposits, however, may be zoned parallel to a contemporaneous shore line or along a stream channel leading away from the source rock.  Any detection of a zonal pattern - epigenetic or syngenetic - is important to economic geology, because it helps to predict changes in mineralization as a deposit is developed and mined.

Zoning in ore deposits is conveniently divided into three intergradational classes, based upon size but independent of the origin.  These classes are:

1.  Regional Zoning:  Zoning on a very large scale, as exemplified by the Southern Piedmont region of the southeastern United States and by the ore deposits associated with the Sierra Nevada batholith.

2.  District Zoning:  Zoning shown by closely grouped mines, a category which includes the well-known  mining districts of Butte, Montana, Cornwall, England, and Bingham, Utah.

3.  Orebody Zoning:  Changes in the character of mineralization within a single ore body or a single ore shoot.  Many vein deposits in the volcanic rocks of Japan, as well as many single ore bodies within zoned districts, are in this category.

Notes & Handouts

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