Dear Mr. Khashaba,

what is the size and distribution of your garnet XX? They are certainly not homogeneously concentrrated so as to constitute an alteration zone which can well be delineated in the host rocks. Granitic plutons do not rank among the crystalline rocks which concentrate garnet s.s.s. There is only an increase of spessartine-dominated garnet s.s.s. in pegmatitic rim zones or separate pegmatites of granites; none of them can be mapped by field evidence based on color variation only. The other option is grossular in contactmetamorphic zones around granites. All other common garnet s.s.s are unrelated to granites. The only accumulation of garnets which may be already recognized with the unarmed in the field are "red sands". It is a placer-type concentration of almandine-enriched garnet s.s.s. in continental (fluvial, lacustrine) and coastal marine depositional environments (e.g. Latvia, the Bay of Riga). Such areas of widespread occurence may under certain circumstances form a target for your studies.

H.G.Dill

Dear Dr. Khashaba,

I think you can't detect garnet by ASTER data for many reason such as repartion and can be confused with other sediments such as altered rocks; similarty compostion with other rocks...

Bejaoui

ASTER examines mineral bonds in the short-wave infrared spectra, such as OH, sulfate and oxide bonds. These bonds are not involved with garnet, and hence the basis of ASTER – SWIR detection of these bonds – will not see any signal from nesosilicates such as garnet.

Jeffrey Hendquist - that was my impression also.

There is gleam of hope via spessartine which is the only garnet present in significant amounts in highly differentiated granites in the apical parts. The mineral itself is not sensitive to the short-wave-infra red spectroscopy but the Mn phosphates, such as manganiferous apatite which are associated with the Mn-bearing garnet in pegmatitic zones. There exists a plethora of phosphates hydrated and non-hydrated which according to the physico-chemical laws should respond even better than the Mn garnet to the SWIR. The other option is the presence of garnet s.s.s. of the grossular-hydrogrossular s.s.s in which part of the SiO4 is replaced by (OH). This s.s.s. may occur in contac-metamorphic mineral assemblages of carbonate-bearing sediments at the exocontact of the granite. Due to its rarity it is rather a fancy of theoretical considerations than a real target in nature.

Exotic granites can be distinguished in quite contrasting ways by various processing methods. So, when using the methods of independent components or methods of the minemal noise fraction, alkaline small intrusions are well distinguished (there are other methods).

It can be assumed that garnet is formed in granite during the capture of carbonate rocks or in contact with them, while the intrusion should become more alkaline.

In my practice, I came across garnets in granite only once, they were small, bright red almandines in alaskite granites. Such granites were very well distinguished when processing satellite data by the method of principal components.

In other words, it is possible to narrow the search for certain intrusions presumably enriched in carbonates, or it is possible to search for the contact of intrusions with carbonate rocks, but it is impossible to determine the location of garnet in the intrusion using satellite data.

Almandine garnet in granite is more likely to result from melting of sili oclastic metasediments, not from incorporation of carbonates.

@Hedenquist, @Stevens, ASTER collects also TIR wavelength range and, therefore, can in theory be used to map garnets. However, you'd need probably a lot of garnet compared to other silicate minerals to be able to detect it with ASTER. I'd recommend TIR airborne hyperspectral sensors, such as SEBASS, which has been successfully used for remote mapping of garnet.

agrees with Brian P. J. Stevens that carbonates are not needed for the formation of almandine (Fe2 + 3Al2 [SiO4] 3) and contamination of metasediments is more likely. Thanks for the comment!

Why do look for the famous needle in the haystack which in case of granites is rather a "laboratory curiosity" in form of spessartine (pegmatitic affiliates of granite) and grossular (skarn)? Supergene and hypogene argillitization made up of kaolinite-group minerals, muscovite bearing Cr and Li and smectite (e.g. dioctahedral Fe-bearing s.s.s.) are widely associated in time and space with granites and have proven to be good targets. They respond to spectral methods be it mounted on airborne or satellite platforms.

The main issue is: What is the scope of that approach/ question ? Applied economic geology or throwing a stone in the water and being fascinated by the propagation of the waves.

H.G.D.