Information on the host rocks

 

Petrography
Petrochemistry
Structure
Rock Alteration

Gangue Minerals Asociated with Askot Sulfides

The term gangue shall be used herein to describe silicate minerals associated with the metallic sulfides. Mineralogical and textural studies of the gangue minerals and their relationship with the sulfides are based on a study of 120 polished thin sections. The gangue minerals are derived from two sources, viz.:

1. The hydrothermal activity (hydrothermal), and

2. Relicts of original country rock (metamorphic).

A study of the gangue minerals throws light on the manner in which the orebody was formed, and furnishes information as to the physico-chemical conditions during deposition. Most gangue minerals occur in two generations.

1. Quartz
Quartz is the most abundant gangue mineral. It occurs as anhedral porphyritic masses containing inclusions of earlier formed sulfides. The later minerals along grain boundaries and fractures replace it. All quartz in the zone of massive sulfide mineralization is of hydrothermal origin whereas in the zone of disseminated sulfides it is mostly of metamorphic origin, occurring as relicts of unreplaced country rock. Substantial quantities of quartz in this zone, however, are also of hydrothermal origin. Quartz appears to have been added ubiquitously and occurs in complex intergrowths with all other sulfide and gangue minerals. The process of silicification appears to have become profuse following every period of fracturing.

2. Actinolite and Calcite
Actinolite and calcite occur in intimate intergrowths with each other. Actinolite occurs as long prismatic crystals and columnar aggregates, whereas calcite occurs as aggregates of fine to medium grained euhedral crystals. The two minerals make up as much as 75 percent of the gangue minerals in the central part of the orebody. Actinolite and calcite occur in two generations. Intergrowths of first generation calcite and actinolite replace the first generation sulfides and are replaced by the second generation sulfides, while the second generation of these minerals replace the second generation sulfides.

Fig 1: Pleochroic haloes in phlogopite formed around tiny inclusions of unidentified radioactive minerals.  Phlogopite is seen to be replacing epidote and some sulfide minerals. (250X)

3. Phlogopite
Phlogopite occurs as medium grained, thick tabular and short prismatic crystals. The first generation phlogopite replaces first generation calcite and actinolite and is replaced by the second generation sulfides. The second generation phlogopite occurs contemporaneously with the second generation gangue minerals (calcite, actinolite, epidote, etc.) and replaces the second generation sulfides. Pleochroic halos, resulting from the inclusion of optically unidentifiable tiny radioactive minerals are very common in phlogopite (fig. 1).

Fig 2: Early formed epidote (high relief) eing replaced by the sulfide minerals.  Second generation epidote is seen replacing the sulfides (50X),

4. Epidote
Epidote occurs as granular to columnar aggregates of fine euhedral crystals. The first generation epidote replaces the first generation sulfides and is replaced by the second generation sulfides. The second generation epidote replaces the second generation sulfides and also some gangue minerals (fig. 2).

Fig 3: Tourmaline (schorlite) being introduced into the country rocks along foliation planes.  It occurrs as short prismatic crystals having angular relationships with the schistosity.

 

 

 

5. Tourmaline
Tourmaline is a hydrothermal mineral introduced along the foliation. It occurs as short prismatic crystals (fig. 3) and columnar aggregates. The first generation tourmaline contains inclusions of first generation sulfides and is replaced by the second generation sulfides. The second generation tourmaline occurs in minor amounts and replaces the second generation sulfides.

Fig 4: 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.

6. Andalusite
Andalusite occurs as aggregates of coarse columnar crystaxs. The first generation andalusite occurs in association with first generation actinolite and calcite and is seen as inclusions in first generation sulfides (fig. 4). The second generation andalusite is seen replacing second generation sulfides.

Fig 5: Hydrothermaly introduced apatite (light gray, upper left corner) replacing earlier sulfides and other metamorphic minerals.

 

 

 

7. Apatite and Fluorite
Apatite is a hydrothermal mineral and is found as medium to coarse prismatic crystals (fig. 5) while fluorite occurs as tiny anhedral crystals (fig. 6). Fluorite and first generation apatite are intimately associated with each other, replace the first generation sulfides, and are replaced by the second generation sulfides. The second generation apatite replaces the second generation sulfides.

Fig 6: Fluorite (anhedral central grain) replacing apatite (inclusions with moderate relief) and epidote (inclusions with high relief).

 

8. Chlorite and Sericite
Chlorite occurs as small scaly masses, whereas sericite occurs as aggregates of very fine, elongated crystals. Chlorite is intimately intergrown, and appears to be contemporaneous with the second generation gangue minerals. The first generation sericite occurs abundantly as replacement relicts in the first generation sulfides while the second generation sericite occurs as a minor phase replacing the second generation sulfide and gangue minerals. The relative abundance of various.gangue minerals through the sulfide zone is given in Fig. 7.

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals

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