Friday, August 2, 2019

Secondary Mineralization In Deccan Basalts: Timing And Precipitation Environments

When and how did these minerals form?

These are the world famous zeolites and other secondary minerals (green apophyllite) that fill cavities and cracks in Deccan Basalt lava flows.  Zeolites and other secondary minerals like apophyllite are calcium, sodium, potassium bearing alumino-silicates with varying amounts of water and other volatile elements like fluorine (apophyllite). They are prized by mineral collectors and by petrologists who study them to understand the geologic conditions that affected the lava after their eruption. This has broader implications for understanding the initiation and evolution of fluid circulation systems during the burial and exhumation of the lava pile.

A recent study has taken a step towards understanding the timing and precipitation conditions of these secondary minerals in basalts.

Exceptional Multi Stage Mineralization of Secondary Minerals in Cavities of Flood Basalts from the Deccan Volcanic Province, India - Berthold Ottens, Jens Götze, Ralf Schuster, Kurt Krenn, Christoph Hauzenberger, Benkó Zsolt and Torsten Vennemann. 

The most exciting part of this study is the publication of  absolute ages of mineralization of apophyllite using Rubidium-Strontium (Rb-Sr) and Potassium-Argon (K-Ar) radiometric methods. As far as I know , these are the first ever published absolute dates of secondary mineralization in Deccan lavas.

The scientists studied lava flows from the famous Savda quarries near the town of Jalgaon and also from quarries near the town of Nasik, both in the state of Maharashtra.The photograph below shows a portion of the lava sequence that was studied in one Savda quarry. The occurrence of secondary minerals in lava cavities is shown in the lava profile to the right. All the following images in this post are from the paper linked to above.

This study concentrated on examining the mineral sequences found in the large cavities in the central portions of a lava flow. Smaller amygdules (fully filled cavities) and vesicles (partially filled small cavities) occurring at the bottom and top of lava flows won't contain the entire sequence of minerals.

The investigation revealed three broad stages of mineralization. Stage 1 sequence precipitated first. It consists of an iron-magnesium and potassium bearing clay layer (containing celadonite and smectites) coating the walls of cavities and microbial films and filaments.  Stage 2 sequence consists of calcite (calcite 1), fine grained zeolites (zeolite 1), and plagioclase, followed by a layer of chalcedony and quartz.  This is followed by a second generation of larger calcite (calcite 2) and zeolite (zeolite 2) crystals. The common zeolites, both in zeolite 1 and zeolite 2 stages are heulandite, and stilbite with additional mordenite observed in zeolite 1 assemblage. Stage 3 sequence is made up of a third generation of calcite (calcite 3) , along with apophyllite and rare powellite (calcium molybdate). 

This multi-generation mineralization sequence could only be ascertained by carefully noting down the mineral sequences appearing in hundreds of different large cavities. Such a broad examination is necessary since the entire sequence may not have crystallized in any one cavity. The two pictures below demonstrate this problem.

On the left is a cavity which shows two generations of calcite (Stage 2) grown on a clay-chalcedony substrate (Stage 1). On the right is apophyllite (Stage 3) directly overlying Stage 1 mineral encrusted bio-filaments. In both these cavities, the Stage 2 zeolites have not precipitated. In some other cavity one might encounter a sequence of Stage 2 large zeolites overlain by Stage 3 calcite, but not the earlier calcite and zeolites, nor the Stage 3 apophyllite. Reconstruction of the entire sequence thus requires an examination of a large number of cavities in order to work out the true order of mineral succession.

This reconstructed mineral paragenetic sequence is presented in the graphic below.

The researchers used fluid inclusion studies and carbon and oxygen isotope analysis to narrow down the temperature conditions during precipitation (fluid inclusion) and the source of mineralizing fluids (isotope analysis).

Fluid inclusions are tiny volumes of fluid that get trapped during crystal growth. The fluid may be a liquid or a vapor or both. For ascertaining temperature of entrapment, a bi-phase inclusion is selected. The sample is heated until it reaches a temperature where the inclusions change from a heterogeneous (bi-phase) state to a homogeneous (one phase) state. This homogenization temperature (range) is taken to be the temperature of fluid entrapment i.e. initiation of crystal growth.

Fluid inclusion analysis of calcite 1, calcite 2 and quartz indicated temperatures between 94 deg C to 173 deg C. Inclusions in stage 3 calcite and apophyllites indicated higher temperatures between about 140 deg C to 244 deg C.

The carbon isotopes of calcites (presented as the ratio of C13 to C12) showed depleted values due to enrichment of the lighter C12 isotope. This was taken to indicate a substantial biogenic contribution for the carbon (the lighter isotope is preferentially taken up by organisms during photosynthesis). The oxygen isotope values for calcite 1 and calcite 2 were enriched in the heavier isotope O18. Values of delO18 were + 14 - +15 for calcite 1 and +19 - +27 for calcite 2. DelO18 is a measure of the ratio of the two isotopes of oxygen O18 and O16. Such enriched values indicate magmatic source fluids, although the slightly lighter values of calcite 1 suggests mixing with meteoric water, which is enriched in the lighter isotope O16 as compared to magma and sea water (meteoric water is derived from rainfall and ends up percolating through rock as groundwater). Isotope analysis of calcite 3 is not presented in this study.

Rb-Sr and K-Ar geochronology of apophyllites shows that precipitation of apophyllite took place repeatedly as discrete events spread over a large time span. The Nasik sample yielded ages of 58 mya (million years ago) and 21 mya. Apophyllites from Savda quarry near Jalgaon yielded ages of 45 mya and 27 mya.

The observed mineral sequence along with data from fluid inclusions, isotopes and geochronology have enabled the researchers to propose the following sequence of events depicted in the schematic below.

After the eruption of a lava flow, interaction of the hot lava surface with meteoric water resulted in filling up of cracks and cavities with water, accompanied by the alteration of mineral olivine, plagioclase and volcanic glass.  Fe, Mg, Si, Al released from the rock was recombined to form clay mineral and iron hydroxide coatings on the walls of cavities and on microbial films and filaments.

As the lava layer got buried under younger lavas, the composition and temperature of the fluids evolved resulting in the precipitation of calcite 1, fine grained zeolites 1 and chalcedonay and quartz. The oxygen isotope values of calcite 1 indicates a mix of magmatic residual fluids and meteoric water. Continued burial resulted in a diminishing contribution from meteoric water. Zeolite 2 and calcite 2 phases precipitated from magmatic residual fluids as indicated by the oxygen isotope values which are enriched in the heavier isotope. Maximum burial temperatures at this stage have been estimated to be about 150 deg C.

This interpretation of Stage 1 and Stage 2 mineral assemblages having formed immediately following volcanism and during burial is in line with previous thinking regarding secondary mineralization in Deccan basalts. Data about the timing of Stage 3 however has thrown up a surprise.

Geochronology indicates that precipitation of stage 3 minerals like calcite 3 and apophyllite took place much later. Deccan volcanism ended by 64 mya across most of the province. Mineralization ages of 47 mya and 27 mya indicate that by this time considerable erosion of the lava pile would have resulted in exhumation of deeper layers and much lower burial temperatures. The fluid inclusions in calcite 3 and apophyllite indicate crystallization at temperatures between 144 deg C and 244 deg C. Some earlier work by Shrikantappa and Mookherjee on fluid inclusions in apophyllite from Savda indicate even higher temperatures reaching 280 deg C. Such boiling conditions at a shallow burial level implies the formation of a hydrothermal system. The presence of powellite, a calcium molybdate, indicates oxidizing fluids. Such a system must have formed repeatedly at widely separated time intervals. The study does not put forth an explanation of the geological events that could have triggered the formation of these high temperature fluid circulation systems.

In summary, Stage 1 and Stage 2 involves a locally formed circulation system. Elements were scavenged from adjacent regions of the lava flow and incorporated into growing secondary minerals. Stage 3 involves a larger circulation system. The concentration of vanadium in basalts is low, in the range of few tens of ppm (parts per million). In Apophyllite, the concentration of vanadium is on the order of 3000 ppm.  This suggests that fluids attained this element concentration by circulating and reacting with a large volume of basalts over a widespread area.

The researchers have stressed that their proposed explanation applies to the specific mineral sequence observed at Savda quarries near Jalgaon and the lava flows near Nasik and is not to be taken as a general explanation for secondary mineralization covering the entire Deccan basalts. That would require much more extensive sampling from different regions and stratigraphic levels. The observations from other studies and the mineral sequence I have personally observed in the Pune area though does suggest that Stage 1 and Stage 2 sequences at least are common everywhere, although the specific combinations of zeolites may vary. Early near surface reaction of hot lava with groundwater and then progressive burial with mineralizing fluids getting contributions from both meteoric water and magmatic residual fluids would have been a common trajectory of fluid rock interaction across the volcanic province.

Such a timing of mineralization, contemporaneous with volcanism and continuing after burial, has been noted from lava provinces in Iceland and Iran too. The Stage 3 event though is much younger and would depend on later geologic triggers that may differ from place to place. In eastern Iceland for example, a late stage of mineralization has been linked to heat provided by the intrusions of dikes ( sheet like bodies of magma injected along fractures). The geochronology of apophyllites in the Deccan Volcanic Province needs to be validated by more such work.

Some questions do remain.

First, nearly 20 to 40 million years after Deccan volcanism ended, what could be the source of heat for the initiation of the late stage fluid circulation systems that precipitated calcite 3 and apophyllite?

And secondly, the lack of carbon and oxygen isotope analysis of calcite 3 prevents us from identifying the source of precipitating fluids. That is a lacunae that future studies must aim at filling.


Srikantappa, C.; Mookherjee, A. Water, Aqueous, H2O-CO2 and Gaseous Inclusions in Cavity Minerals in the Basaltic Lava flows around Pune, India: Evidence for Boiling. In Proceedings of the Second Meeting of the Asian Current Research on Fluid Inclusions (ACROFI–2), Kharagpur, West Bengal, India, 12–14 November 2008; p. 176.



  1. Wow! It's gorgeous. Well, this is clearly my fav! :)

  2. Posting a comment via email from Dr. Sudha Vaddadi, ex Geological Survey of India, who has extensive field experience in the Deccan basalts -

    "Thank you. Interesting paper. Lot of unanswered questions though.
    As it has been mentioned, the work is limited to a small and limited area, considering the vast expanse of DVP.
    Apophyllite occuring as a Stage 3 event is interesting.
    The famous Rahuri green and the dull light moss green ones from Jalna, occur in areas where there are many lineaments delineated. Could there be any link??
    Again dowmstream of Yedgaon dam, Madhohal dam there are cavities with colourless apophyllite. From Yedgaon area Phadke and Mukherjee have found small quantities of cavansite. Here again there are numerous fractures and dykes.
    The cavansite and pentagonites from wagholi are associated with calcites, occassionally with tiny cavansite crystals within calcite. These then should relate to Stage 3.
    In the same quarry, pentagonite and cavansite directly on heulandite is also seen.
    Another thing I noticed is that there is no mention of zonation in the overall distribution, as mentioned by earlier workers.
    Scope for future work. Some young geologists should take it up".

    My reply -

    Thanks for those additional observations and I agree, as also stressed by the authors, that this study is limited. What I did like is the combined use of fluid inclusions, stable isotopes and geochronology, which has given valuable information on nature of fluids and timing. I wish they had presented isotope work on stage 3 calcite. It is puzzling why they missed out on that.

    Hopefully this is just the beginning. Your field observations indicate that there is a lot of work to be done, not just on specific mineral assemblages, but drawing connections between mineralization and regional geology/stratigraphy and structure.


  3. A comment on the point of zonation raised by Dr. Vaddadi-

    Regarding zonation , they do refer to previous work (table 1) which suggested that the common minerals occurred in all formations and levels giving no evidence of zonation. Also, laumontite which forms at higher temperature (>200 deg C) occurs both in the once more deeply buried lava in Nasik area, as well as in the thinner sequence in Malad (Mumbai)which was not deeply buried. Such a scatter may imply a role for local hydrothermal processes.