Information on the host rocks

 

Petrography
Petrochemistry
Structure

Sulfide Mineralization

Mineralogy of the Altered Zone around the Askot Sulfide Body

 

The terms metamorphic, secondary, and hydrothermal shall be used herein to indicate respectively original minerals of the country rocks, those that have been recrystallized, redistributed or altered by the hydrothermal activity, and those that have been emplaced as a result of metasomatic chemical additions.  It must be emphasized that it may not have always been possible to distinguish clearly between secondary and hydrothermal minerals on the basis of petrographic criteria alone.  Geochemical considerations were in some instances very helpful in differentiating.  Petrographic methods have been of little value in determining the mineralogy of feldspar alteration products.  The identification of clay minerals has not been possible, and pyrophyllite is very difficult to distinguish from sericite by optical means.

1. Quartz
In the fresh country rock, quartz occurs as medium sized, euhedral, and slightly strained grains.  Towards the zone of alteration it is progressively replaced by secondary quartz along grain boundaries and, fractures.  In the outer zone of alteration secondary quartz is very abundant and replaces metamorphic quartz.  The metamorphic quartz occurs as isolated porphyroblasts with embayed boundaries, surrounded by microcrystalline secondary quartz (fig. 1). 

Fig 1: Metamorphic quartz in the zone of hydrothermal alteration being replaced by microcrystalline secondary quatrz along grain boundaries and fractures. (+Nic)

 Metamorphic quartz is clear whereas the secondary quartz is diffused containing tiny inclusions of other minerals.  Quartz in the zone of silicification/feldspathization is generally derived from the breakdown of feldspars in the zone of argillic alteration.  This can be readily seen in Figure 7 wherein the SiO2 curve indicates a depletion of silica in the zone of argillic alteration and its enrichment in the zone of silicification/feldspathization.

 2. Sericite
Though the unaltered country rocks are sericitic, almost all the sericite in the altered zone is of secondary origin.  The mineral occurs as minute shreds as an alteration product of K-feldspars.  Small quantities have also been introduced by hydrothermal solutions as a result of potassium metasomatism.  Sericitization of the country rocks took place along S1 and S3 planes (fig. 2) along which the mineral occurs with no apparent orientation. 

Fig 2: Sericitization of the country rocks proceeding along the foliation planes S1 and S3.  Sulfide minerals occur as disseminations in sericite.

Together with silica, tourmaline, some epidote, and phlogopite, it constitutes the entire bulk of the altered country rocks in the immediate vicinity of the sulfide zone.  Within the sulfide zone, the sulfide minerals replace sericite, whereas in the zone of sericitic alteration, sulfides are seen as disseminations in sericite.  From the textural studies of the sulfide and altered zone it is interpreted that sericite generally formed earlier than the sulfides.  Minor amounts, however, may have formed even after the emplacement of sulfides, since some sericite appears to be replacing sulfide minerals.

 3. Chlorite
The unaltered country rocks are profoundly chloritic but in the zone of hydrothermal alteration the effects of bleaching suggest that the ferromagnesian minerals were broken down by the initial hydrothermal activity.  Biotite and chlorite encountered in the zone of alteration are of late hydrothermal origin.  Minor chlorite occurs as scaly masses in the outer fringe of the zone of sericitic, alteration and also in the zone of argillization and silicification.  It is occasionally an alteration product of biotite.  In the zone of silicification, garnet is seen altering to chlorite (fig. 3).  

Fig 3: Garnet crystals containing spiral trains of inclusions.  This garnet can be seen altering to chlorite.

The penninite variety usually occurs as an alteration product of garnet, while prochlorite is of hydrothermal origin.  Hydrothermal prochlorite is also noticed associated in appreciable quantities with epidote in veins that have undergone propylitic alteration.

4. Biotite
Biotite has been emplaced along the foliation planes of the country rocks.  Such biotite is of a secondary origin and can be readily recognised by its cross-cutting relationship with rock schistosity (fig. 4).  

Fig 4: Rock fabric disturbed by the growth of garnet crystals - post-tectonic crystallization.  Note the presence of later introduced biotite flakes that have cross-cutting relationship with the main schistosity.

In the zone of silicification/feldspathization biotite forms 2-10 percent of the total silicate minerals, whereas in the zone of sericitic alteration it is rarely more than 2 percent.

5. Epidote
Epidote occurs generally as granular aggregates of distinct elongate crystals.  This mineral invades the country rocks along fractures and foliation planes in the sericitized rocks.  Epidotization is most intense in the inner zone of sericitic alteration and in the outer fringe of the sulfide orebody.  Early formed epidote replaces the earlier sulfides and in turn is replaced by the later sulfides.  Some epidote is also seen replacing the second generation sulfides (fig. 5).

Fig 5: Early formed epidote (high relief) being replaced by the sulfide minerals.  Second generation epidote is seen replacing the sulfides.

 It is concluded that epidote was being introduced into the country rocks before the later sulfides were deposited, and continued to be added till the close of the hydrothermal activity.

6. Tourmaline
The unaltered schistose rocks do not contain tourmaline.  The entire tourmaline in the zone of alteration is hydrothermally produced.  Tourmalinization can be recognised in a 35-40 mts. wide band adjoining the sulfide orebody.  The mineral is intimately associated with epidote.  Two varieties of tourmaline can be recognised -- schorlite, and dravite.  Schorlite occurs in short prismatic crystals showing strongest absorption normal to the plane of the polarizer.  It is introduced into the country rocks along fractures and foliation planes (fig. 6).  

Fig 6: Tourmaline (schorlite) being introduced into the country rocks along foliation planes.  It occurrs as short prismatic crystals having angular relationships with the schistosity.

Dravite occurs as columnar and fibrous radiating aggregates.  It replaces and contains inclusions of several minerals.  In the sulfide and inner zone of sericitic alteration dravite is more abundant, while in the outer zones, towards the fresh rock side, schorlite is the more important member.

Fig 7: Hydrothermaly introduced apatite (light gray, upper left corner) replacing earlier sulfides and other metamorphic minerals.

 

 

 

 

7. Apatite
Apatite is quite abundant in the inner zone of sericitic alteration and the outer sulfide zone.  It occurs as medium sized hydrothermally formed crystals replacing earlier sulfides and other metamorphic minerals (fig. 7).

8. Fluorite
Fluorite is intimately associated with apatite and occurs as medium sized anhedral grains containing inclusions of apatite and epidote (fig. 8).  

Fig 8: Fluorite (anhedral central grain) replacing apatite (inclusions with moderate relief) and epidote (inclusions with high relief).

The distribution of fluorite is restricted only to a few meters in the innermost zone of wallrock alteration and in the sulfide zone.  It occurs as a typical hydrothermal mineral.

9. Phlogopite
Phlogopite occurs as scattered, yellow-brown to pale brown, highly pleochroic and short prismatic crystals of hydrothermal origin.  It is generally fresh, containing inclusions of sericite, epidote, and other minerals.  It occurs as a minor constituent in the innermost zone of sericitic alteration.  

Fig 9: Pleochroic haloes in phlogopite formed around tiny inclusions of unidentified radiocactive minerals.  Phlogopite is seen to be replacing epidote and some sulfide minerals.

The mineral contains tiny inclusions of unidentified radioactive minerals that have produced pleochroic halos in phlogopite (fig. 9).

10. Feldspars
Feldspars are present only in the outer zone of wall rock alteration.  Both plagioclases and K-feldspars are introduced in the silicified country rocks along the foliation planes.   These feldspars occur as poikiloblasts containing inclusions of quartz and micaceous minerals.  Plagioclases are more abundant than K-feldspars and both increase in amount towards the fresh rock side.  Plagioclases range from calcic oligoclase to labradorite, while K-feldspars are invariably orthoclase.  

Fig 10: Feldspars in the altered zone showing alteration to sericite and clay minerals along cleavages, fractures and grain boundaries.

The feldspars are untwinned and could only be recognised after staining following the method of Bailey and Stevens (1960).  Feldspars of metamorphic origin are quite abundant and show considerable signs of alteration (fig. 10).

11. Clays
Clays are residual products of the hydrolytic breakdown of feldspars releasing silica and potash.  In the outer zones of wall rock alteration relict feldspars can be seen surrounded by rims of sericite and amorphous clays.  They can also be seen hydrolysed along cleavage planes and fractures (fig. 10).  The peak development of clays appears between the outer sericitic and inner zone of silicification.

12. Andalusite
Andalusite occurs in the zone of alteration immediately adjoining the sulfide zone in the form of clear euhedral crystals and columnar aggregates.  It seems to have been emplaced prior to the sulfides, since it generally occurs as inclusions in the second-generation sulfides (fig. 11).  

Fig 11: Early formed andalusite being replaced by later sulfide minerals.  Note that the cleavage planes of andalusite are flexed, indicating a deformation of the rocks subsequent to the emplacement of the mineral.

Flexed cleavage planes of andalusite are evidence of a mild deformation of the rocks subsequent to the emplacement of the mineral.

13. Cordierite
Cordierite occurs as a typical metamorphic mineral only in the outer zone of the altered wall-rock.  It occurs as aggregates of porphyroblastic masses being corroded and replaced by secondary quartz and biotite along the grain boundaries and fractures (fig. 12).

14. Garnet
Garnet is also a metamorphic mineral and occurs only in the outer zone of wall rock alteration.  

Fig 12: Porphyroblasts of cordierite being corroded and replaced by quartz and biotite along grain boundaries and fractures. (+Nic)

Three genetic types are recognised - pre-, syn-, and post-tectonic (Fig. 3 & 4) and all these types are seen altering to chlorite.

15. Pyrite
Pyrite occurs as cubes, pyritohedra, and irregular grains.  It occurs as a typical hydrothermal mineral replacing the silicate minerals, and can be seen altering to limonite along grain boundaries.  Pyrite is quite abundant in the inner zone of sericitic alteration and decreases steadily in quantity towards the outer zones of wall-rock alteration.

 

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals

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