Dear Shivani, there are various methods alongside different remotely sensed imageries for mapping hydrothermal alterations. The following articles may help you regarding your problem:

Article Hydrothermal alteration mapping using Landsat-8 data, Sar Ch...

https://www.sciencedirect.com/science/article/abs/pii/S0034425798000406

Article Hydrothermal alteration mapping using ASTER data in the Infi...

Article Mineral imaging with Landsat TM data for hydrothermal altera...

Article Investigating the capabilities of multispectral remote senso...

https://www.tandfonline.com/doi/full/10.1080/10106049.2017.1334834?src=recsys

As regards field work, alteration of feldspars into clay or sericite, micas and mafics into hematite and carbonate are good indications for metal mineralization as well as the presence of veins. Epidote is the characteristic mineral outside the area of mineralization (of metals like copper, tin, uranium and possibly zinc as well).

above are good suggestions. you can also use a visible to near infrared (VNIR) to shortwave infrared spectrometer (SWIR) on outcrops (that are free of vegetation and cover). this is because many of the alteration minerals are spectrally responsive in the VNIR to SWIR. This works best in arid terrains. needs to be done on a cloud-free, rain-free day. If you cannot measure outcrops, fresh hand samples or drillcores can be measured using a contact probe (light source).

Alteration zones should first be mapped on the ground in the granitic terrains under study and clearly defined in the field taking into account the supergene alteration overprinting the hypogene one. Subsequently it is mandatory to verify the minerals by studies in the laboratory by means of XRD, thin sections, EMPA etc…

A rough field guide is given below.

1. Color shades (brown = “limonite,” red = hematite, black = “manganomelane/ leucoxene”, green/ fatty = nontronite, green-to yellow uranyl mica)

2. Texture (earthy powdery white = kaolin with minor illite, smectite)

3. Structure such as amygdule, vugs and druses may help to identify topaz, tourmaline, Li phosphates and Li mica, porous zones may be due to “episyenitization” with calcite, dolomite, Fe carbonate and zeolite, massive and zoned parts can direct our thoughts to skarn, siliceous hard zones to greisens

4. The clastic halo (cassiterite, tourmaline, topaz, and scheelite) is worth to be sampled and the stream sediments panned

5. After having allocated special areas to certain alteration zones on the ground you can check them with hand-held devices (“capturing digital data in the field”) from gamma spectrometry through PIMA.

6. With ground-surveys using ratio meters the electromagnetic spectrum can be measured and you can bridge the gap between ground and air-borne and satellite-borne devices.

Starting with color-composite images and then seeking for interpretation will mostly end up in poking around in the fog or putting the horse behind the coach.

H.G.Dill

If you have the airborne gamma spectrometry map for the granitic area, you can use the Th/K map to delineate the alteration zones, where Th is immobile elements not affected by alteration, while K in feldspar is easily altered. this method is a good method to delineate the alteration zone.

I once did a lot of mapping in the Colorado mineral belt, where there were good geologic maps, showing the distribution of primary rocks types. Although som geologic maps showed veins, most published maps did not specify hydrothermally altered rocks (and those that showed only a stippled pattern, but did not identify alteration minerals).

The first thing I noticed was that rocks within mining districts are commonly coated with reddish, to orange-brown, to yellow hydrous iron oxides and sulfates, whereas rocks peripheral to mining districts are commonly coated with black manganese oxides.

In the upper parts of mining districts, the reddish- to yellowish-stained rocks contain pyrite (disseminated and in veinlets), the biotite and feldspars are commonly altered to sericite +/- clay minerals (in general, and especially around veins). With increasing depth, sericite gives way to secondary biotite and K-feldspar.

In the balck-stained zone peripheral to veins and mining districts, the black-stained rocks commonly contain chorite after biotite, and veinlets of specular hematite (black but with a red streak).

One of the first things I would do when I started work in a mining district was to find a high place, where I could get an overview, so that I could sketch the boundary between the limonitic zone and the manganese-oxide zone. Then I would make traverses across its boundaries for confirmation. Then I would concentrate most of my efforts within the limonitic zone (or zones).

Alteration products such as chlorite, and sericite are easy to identify in hand samples and thin sections, but it can be tricky to discriminate between magmatic, metamorphic, and hydrothermal biotites (unless there is a visible difference in their colors, optical properties, or textural characteristics. Staining sawed slabs for K and Ca can also be helpful for identification of K-feldspar and sericite versus plagioclase and clay minerals. However, identification of clay-mineral species may require instrumental analysis (by x-ray diffraction, for example).

HOW I CAN OBTAIN INFORMATION OF HYDROTHERMAL VENTS IN GALAPAGOS , MAPS, CHARACTERISTICS .ETC