Dear Mr. Tom,

I think you expect too much as you are looking for an all-round method in exploration for REE and RM ( = rare metals ??) in soil and stream sediments. Some elements and their pertinent host minerals are very vulnerable to weathering , whereas others are very resistant to weathering and show up as heavy minerals in the clastic halo around the potential mineral deposits. There is no jack of all trades in chemial and mineralogical exploration. Firstly describe your objective of study (e.g. REE minerals, Nb-Ta, Be...), then make a terrain analysis which is a mixtum compositum of climology, lithology, hydrology and pedology. Do not forget the means you can avail of yourself. Have you got only a petrographic microscope for heavy mineral analysis or can you apply a SEM-EDX/WDX with MLA? There are more questions than answers.

Sorry for these blunt statements. It is not helpful to promise you anything which nobody can guarantee.

Best regards

H.G.Dill

Dear Mr. Dill,

I appreciate your direct answers to my vague query. I would like to stress upon the nature of investigation. I am in a preliminary stage of evaluating the REE and RM potential of a per-aluminous granite (468 Ma). The granite intrudes metasedimentary rocks and show evidences of mixing and mingling with basic magmas. Mafic sheet (diorite) is showing higher MREE in bed rock samples (10-15 times when normalized with UCC). During Regional Geochemical sampling, few anomalous zones were reported (up to 1500 ppm: Ce and LREE dominant). In the host granite, diorite and late aplite dykes, we could identify sphene and allanite up to 1% mode in host granite and aplitic dykes. Accessory phases are zircon, monazite and apatite having specific gravity > 5 < 7.  I am having petrography, SEM and EPMA facilities to identify the REE components in host phases. The area is part of Shillong Plateau and enjoys sub-tropical high land climate according to Köppen's climate classification and heavy rainfall (one of the highest in the world: Cherrapunjee, Meghalaya, India).

Both residual and transported (eluvial) soils are recorded in the area. I hope the information will make it easy for you and other readers to share the expertise.

Dear Mr. Tom,

apart from apatite which is highly susceptible to the impact of weathering, sphene, allanite and monazite are relatively resistant to weathering and may show up in the proximal part of the clastic halo around the rock targeted upon. I studied them in a savanna landscape under comparable climatic conditions. Prior to any areal survey I would test the vertical distribution of these minerals in the weathering profile and select a reference drainage system to see how these heavies show up as a function of distance (proximity indicator). Zircon should be used as a morphological tracer to get an impression on the temperature of formation. It is an ultrastable heavy mineral which well preserves its grain morphology even during longer transport (see publication).

The papers attached may give you an idea how you can proceed.

To be outspoken LREE alone are not a first-class target. You should have also higher values in the range of MREE.

Best regards

H.G.Dill

Dear Mr. Dill,

Thank you for your valuable advice. The publications are very useful and informative in my work. If you have any self- publication in the field or REE and RM, please do share. Could you give a good reference for the different soil types/ horizons developed in different climatic zones (preferably with photographs). Would post the progress / further queries in this regard. WIll stay in touch.

Dear Mr. Tom,

I have no further data released to the public.

I hope sharing my expertise with you was helpful for your work and I wish you good luck.

H.G.Dill

Dear Mr. Nibin Tom,

If you are looking for sampling for the exploration of REE and RM in the soil etc then this information may help you.You can go for  test pits ( about 1m x1m)  in a suitable grid pattern based on your level of exploration (G4/G3) and extent of soil/ eluvial placers. After test pits you will have a fairly good idea of drawing a sample for analysis. As you mentioned allanite, monazite  etc are also present in the area then a portable scintillometer will also aid you in sample selection. If the area is environmentally sensitive then test pits should be replaced by auger holes using a hand held auger. The soil/eluvial placer then can be subjected to the study of heavy minerals as most of REE minerals are heavy minerals  followed by chemical analysis of non-magnetic heavies and the grades in the soil/eluvial  can be back calculated if you are looking for a potential source of REE and RM in soil/ eluvial. In case of stream sediments and other eluvial placers the heavy mineral separation and their study  with analysis should be prompted. However, for the REE mineralogy only SEM/QEMSCAN and/  or EPMA study will be useful. As far as hard rock is concerned there should be no problem of sample selection. But as Mr. Dill pointed out , the values of Nd, Eu, Dy, Tb, Y ,Nb- Ta, etc would be deciding factors for the project.

Thanks

O.P.SOMANI. 

Peraluminous granites are probably not the most prospective for REE exploration. By the way, it is a bit unusual that you mention sphene and alllanite in the supposedly peraluminous granite; both minerals are calcium-bearing, whereas peraluminous granites typically have a deficiency of calcium (hence their peraluminosity). So those Ca-bearing minerals may be more likely be associated with the mingled mafic magma than with the peraluminous granite itself.

Dear Mr. Dill,

before to begin a research on the residual soils it would be necessary to have an evaluation of the quantity of these soils, to understand if they are exploitable from a mining point of view. In India heavy sands are exploited for REE, but the quantity of heavy sands is higher than soils. If the concentration of REE is very high it is possible to exploit them directly from the rocks. In to the Zambesian pegmatites of Mozambique the content of Ta, Nb e REE is so high that some mines are opened on the rocks. The applied technology is very simple, in fact all the mass is crushed and by means concentrators heavy minerals are separated from the rest.

Dear all,

Sorry for the delay in response due to field work. Thanks Mr. Om Prakash for your guidance and suggestions. Marlina is right about the xenoxrystic origin of sphene and other REE hosts. The granite ASI index too varies from per-aluminous to meta-aluminous. So two types of sphenes are recorded; other being the altered type. Thank you Francesco and all others for active participation.