Tuesday, December 13, 2011

Exploration and Assessment of Thoshe Iron Deposit in Ramechhap District, Central Nepal


Krishna  P. Kaphle and Hifzur  R.. Khan
Department of Mines and Geology, Lainchaur, Kathmandu, Nepal

INTRODUCTION

Thoshe Iron deposit lies in between latitude 27o 33’ 00’’ to 27o 35’ 00’’ North and longitude 86o16’00’’ to 86o 17’ 30’’ East. It extends from Singate in southeast to Arubote and Danga Dinger village in northwest in Thoshe VDC, Ramechhap district, Central Nepal. The elevation of the deposit site varies from 8500ft at Arubote to 9600ft above msl. at Singate. The area can be accessed from Jiri to Thoshe Megchan by rough seasonal road and from Thoshe Megchan to Singate on foot. Jiri is linked with Kathmandu by a 198km black topped road. The nearest airport is in Manthali, the District Headquarter of Ramechhap. A small airstrip and helipad also exist at Jiri.

Jhingran (1951) prepared the first technical report on hematite of Thoshe area. Upadhyaya (1956), Suwal (1965) and Manandhar (1963) described about the old workings in Thoshe iron deposit. Weise (1960) prepared the mining scheme. Good (1963) recommended for mine development. Rana (1965) did preliminary feasibility study for 600 tons finished iron per year. Talalov (1972) did reconnaissance geological survey for the first time and concluded that it is not of commercial value. Poudyal & Shrestha (1977) carried out regional geological mapping of the area.  From all these information it is known that Thoshe and its surrounding area is represented by low to high grade metamorphic rocks. Hematite mineralization occurs within gently dipping chloritic mica schist overlain by calcareous schist and siliceous marble with some quartzite and underlain by sericitic chlorite mica schist

Kaphle and Khan conducted Preliminary investigation and Follow up exploration and assessment of Thoshe Iron deposit in FY2051/52 and FY2052/53 respectively. They prepared regional geological map of 100sq km area covering Thoshe and surrounding regions in 1:63,360 scale, semi detail geological map of Thoshe Iron prospect and adjacent area in 1:16,000 scale and a detail geological map of 2sq.km area of Thoshe iron deposit in 1:2500 scale. Northwest – southeast extending two hematite mineralization bands were traced by outcrop mapping and trenching/ pitting, which has helped to find out their extension length and thickness. 50 chip, 15 bulk and 30 grab samples were collected from the ore body and analyzed to find the iron content in them. Similarly 35 rock samples from various rock units were collected for petrographic and mineralogical studies. Few old mine pits (old workings) were cleaned up and examined. Field investigations were successful to trace 1 - 3m thick (combined) and about 4km long hematite ore body at Thoshe. The grade of iron ore varies considerably from 20% to 66% iron in them. Follow up geochemical stream sediment survey covering the same 100sq.km was able to trace minor one copper and two zinc anomalous bodies but none of them deserved further investigation. This paper is based on these field investigations and their results.

Old Mining and Ore Processing Activities
From the available literatures/ reports and local miners it is known that small-scale domestic mining of Thoshe iron deposit was started as early as in 1865 (1922 BS) and continued till 1966 (2022 BS). Smelting of mined ores was continued till 2028 BS. In those days, 15 mines (Fig.1) in Thoshe e.g. (1) Singate East (2) Singate West, (3) Ghyang Danda (4) Barappu (5) Kuwako Pakha (6) Sallyani (7) Bhosbhose (8) Pokhari, (9) Jhoreni (10) Pahare  (11) Kopu (12) Patale (13) Arubote (14) Dangadinger (15) Sotre), other three mines in (16) Majuwa Khani and (17) Soleban Khani in Priti and one mine in (18) Ghunsa were in operation. In early days the annual production was around 50 tons and later increased up to 150 tons ore per day. It is learnt from old miners that they use to hand crush the ore materials to minus 20 to 40 mesh size fraction and wash them in a slow running water with the help of wooden vessels to get the clean concentrate and upgrade the ore for smelting. The concentrated ores were used to smelt in 29 smelting places (Affars) located near by. Charcoal prepared from Khashru, Phalant, Banset, Gurans, Angari, Bulu and Uttis trees were used for smelting in which red clay was used as flux. It is also known from them that smelting procedure use to take 12 – 16 hours to smelt about 250kg ore into 100 to 125 kg sponge iron depending on the quality of the ore. Blacksmiths used to make various agricultural tools like spade, axe, knife (Khukuri), sickle (ansi), phali, hammer, nails, chains and cooking vessels like Tapke, Dadu, Paniyo etc. from the sponge iron. In those days, based on Thoshe iron, the government had also established a small gun factory at Thoshe Megchan, which was capable to produce nine barrels of the gun every day. The factory closed after 2007 BS but its remnants still exist in the school compound at Thoshe Megchan. The mining activity was closed in 2022 BS due to government’s new policy, shortage of fuel wood, technical difficulty in mining at depth and easily availability of finished iron in the free market.

OBJECTIVE


  • Compile the existing information, carry out Preliminary and Follow up investigation and quick assessment of the deposit.
  • Prepare regional geological map of 100sq.km area in 1:63,360 scale, semi detail geological map of 10sq.km area in 1:16,000 scale, Detail geological map of 2sq km in 1:2500 scale and trace the hematite mineralization bands in different sections and find out their thickness, lateral extension, the grade and  also roughly estimate the Tonnage of the ore deposit (i.e. geological reserve).
  • Excavate few trenches and open old mining pits to expose the mineralization bands to collect chip, bulk and grab samples for petrographical mineralogical studies and chemical analysis.
  • Make overall assessment of the prospect/ deposit.
  • Recommend for further detail investigation to exploit the ore in near future.

 

FIELD INVESTIGATION


Existing data were compiled and a quick field assessment of Thoshe iron deposit was carried out in 1995. A Regional Geological Map of 100sq.km (scale 1:63,360) and Semi Detail Geological map of Thoshe Iron prospect area covering over 10sq.km in 1: 16,000 scale (Fig.1) were prepared. Follow up stream sediment survey covering the same 100sq.km area at a sample density of 1 sample/sq.km. and heavy mineral concentrate sampling with a sample density 1 sample/sq.km. were carried out. In 1996 a detail geological map of 2sq.km area of Thoshe iron deposit in 1: 2500 scale (Fig. 2A & 2B) was prepared. The mineralized horizons were traced by shallow trenching. All together 50 chip, 15 bulk and 30 grab samples from the ore bodies, 35 rock samples from different rock units, 102 stream sediment samples and 20 heavy mineral concentrates from different streams were collected for laboratory investigations and chemical analysis (Table–2). All the chip, bulk and grab samples were analyzed for iron content and some selected samples analyzed for Cr, Ti, Ni and Ag content in them.

 

METHODOLOGY


- During field investigation/ survey a topobase map was used to locate the outcrops, samples and other features.
- Geological maps were prepared with the help of Brunton Compass and tape survey and identified the rock types, host rocks and trace the ore bodies.
- Survey compass, tape and altimeter were used to carry out the survey of the deposit and detail mapping.
- Chisel, hammer etc. were used to take the chip and bulk samples from the ore body.

 

REGIONAL GEOLOGY AND STRUCTURE


Poudel and Shrestha (1977) prepared the regional geological map of this area. Kaphle and Khan (1995) had also mapped some parts of Thoshe and Khimti Khola area. This area is represented by low to high grade metamorphic rocks which is broadly divided into three main Litho-Tectonic Units (Complexes) e.g. Low grade metamorphic rocks of (A) Nawakot Complex and High grade metamorphic rocks of (B) Central Crystalline (Crystalline Complex, Table-1) separated by Main Central Thrust (MCT) and (C) Lesser Himalayan Crystalline Gneiss (Ulleri gneiss ?) which occurs towards southern part of the investigated area (Fig.1). It is separated from Nawakot Complex by another thrust. Three transverse faults as (1) Khimti Khola fault, Kuthme fault and Singate fault are traced in the field. Part of the Khimti Khola is controlled by Khimti Khola fault. There exists a Dorje Khola anticline (Fig.1). The general trend of the rock units is NW – SE direction and dip 11 o – 40o towards north except in Khimti Khola gneiss that occurs towards southward of the area and dips towards south. Hagen (1965) had correlated these two groups of rock with Waling Schist and Darjeeling Gneiss respectively. Poudyal and Shrestha (1977) had divided the rock into two zones as (1) Carbonaceous and Argillaceous zones with calcareous horizons and (2) Schistose and Gneissic Zone. Present authors compiled the various litho-tectonic units as follows (Table-1).

Table.1: Litho-tectonic units in Thoshe and surrounding areas.
Complex
Formation

Lithological description

Higher Himalayan Crystalline Complex




------ MCT ------
 VII. Lumsa Formation
Medium to coarse grained garnet mica schist, Quartzo felspathic schist, augen gneiss, migmatites and granite bodies…
VI. Mélange Formation
Medium to coarse grained biotite rich quartzofelspathic mica schist, garnet mica schist, gneiss, and banded gneiss.
V. Dhunge Khola Formation


----------- MCT ----------
Medium to coarse grained kyanite bearing garnet mica schist occasionally with micaceous quartzite, kyanite schist and gneiss bands.
-------------------------------- MCT -------------------------------
Nawakot Complex (?)














-------Thrust ------
IV. Yalung Khola Formation
Medium to coarse grained chloritic garnet mica schist, calc schist  and silicified marble bands..
III.  Dorje Khola Formation









----------- Thrust ------------
(B) Fine to medium grained gray to light greenish gray chloritic biotite schist, calcareous schist with crystalline dolomite (marble) bands at the base. At few places small irregular Amphibolite bodies. Hematite bands located at the base of crystalline dolomite.
- - - - - - - - - - - - - - - - - -  - - - - -  - -    -  - - - - - - - - - - -  - - -
(A) Fine grained gray to dark gray carbonaceous slaty phyllite and greenish gray chloritic sericitic phyllite and chloritic mica schist.
-----------------------------Thrust ---------------------------------
II. Thoshe Megchan Quartzite

-----------Thrust -------------
Fine to medium grained sericitic white quartzite. At places feebly calc. quartzite bands and silicious magnesite/ dolomite with talc lenses
-----------------------------  Thrust -----------------------------
Lesser Himalayan Gneiss
I. Khimti Khola Gneiss
(Ulleri Gneiss ?)
Coarse grained porphyroblastic augen gneiss with quartzo felspathic schist and quartzite bands

Lesser Himalaya Gneiss (Ulleri Gneiss ?)
Coarse to very coarse grained Khimti Khola augen gneiss is possibly the part of crystalline nappe. It is similar to Ulleri type gneiss in its texture and mineral composition.

 

Nawakot Complex

It is represented by low to medium grade metamorphic rocks like (1) Sericitic white quartzite (2) Gray to greenish gray phyllite and Carbonaceous phyllite (3) Chloritic quartz mica schist with hematite followed by Calcareous schist and silicified Marble and (4) Chloritic garnet mica schist and calc mica schist.

Crystalline Complex
It is represented by (1) Kyanite bearing garnet mica schist and micaceous quartzite (2) Quartzo feldspathic garnet mica schist, gneiss and banded gneiss and (3) Garnet bearing quartzo-feldspathic schist, gneiss and migmatitic augen gneiss.

GEOLOGY OF THOSHE IRON DEPOSIT
 
Kaphle and Khan (1995) prepared a fairly detail geological map (scale 1:16,000, Fig.1) of the Thoshe and surrounding area. Thoshe area is a part of Lesser Himalaya. It is represented by low to medium grade metamorphic rocks of Nawakot Complex (?). High grade metamorphic rocks occur further north (beyond the present investigated area) and separated from low to medium grade rocks of Nawakot Complex by a prominent linear structure that is know as Main Central Thrust (MCT). The rocks of Nawakot Complex are represented by fine grained sericitic quartzite, fine to medium grained gray to light greenish gray chloritic mica schist and quartzite. At places minor amphibolite bodies are also recorded in the area. Various lithological units are given in Table-1 and briefly described below.

Thoshe Magchan Quartzite

This unit is separated from Khimti Khola gneiss by a prominent thrust. The rock unit is a part of Nawakot Complex (?). It is very well exposed right at the suspension bridge site in Thoshe Magchan at the confluence of a small tributary coming from the right side of Khimti Khola (Fig.1). It is a fine to medium grained, poorly foliated sericitic white to grayish white quartzite. Towards uppermost part, specially in Sikri Khola and Jiri Khola section it is calcareous and gradually grades into silicious dolomite and magnesite with talc lenses. Quartzite is mainly consists of quartz (80-90%), sericite (5-10%) and calcareous materials and iron oxide etc (2 –5%).

Dorje Khola Formation
This formation overlies the Thoshe Magchan quartzite and separated by a local thrust. It is well exposed in Dorje Khola, Sotre Khola, part of Khimti Khola, Sikri khola and Jiri Khola sections. The thickness of this unit varies considerably in different section (Fig.1). On the basis of lithology this unit is subdivided into two members: (2A) Fine grained gray to dark gray (black) slaty phyllite, quartzite and green to greenish gray chloritic mica schist and (2B) Fine to medium grained chloritic biotite schist, calc mica schist and crystalline dolomite. Hematite bands are located at the base of crystalline dolomite (marble) and calc schist with chloritic biotite schist. At places mica schist is poorly garnetiferous. Minor amphibolite bodies are recorded only at few places. Hematite mineralization bands occur towards the lower part. Barren white quartz veins are common in this unit. Quartz + Ilmenite and Quartz + Pyrite + Pyrrhotite and Ilmenite veins are recorded close to or within the hematite mineralization bands in Arubote and Sotre mine area. Isolated dolomitic limestone blocks and bands are also recorded in Patale, Arubote, and Ghunsa (Fig.1).

Yalung Khola Formation
This formation overlies the Dorje Khola Formation conformably. It is well exposed in the Yalung Khola, Shamble Khola and a small part of Khimti Khola section and east of Jiri on the ridge top. It is represented by medium to coarse grained calcareous mica schist (at places chloritic) silicious marble (crystalline dolomitic limestone), garnet mica schist and quartzite bands. Few sill like basic rock bodies (amphibolite) are also recorded within this unit. Ferrugenous calc schist and quartz mica schist are recorded in the upper part of Sotre Khola. Except very few quartz and quart + chlorite veins no other mineralized veins are recorded in this unit.

MINERALIZATION

 

Preliminary field investigations followed by follow up exploration and assessment of the deposit revealed that Thoshe Iron deposit occur within the low dipping chloritic mica schist (host rock) of Dorje Khola Formation. It is overlain by calcareous mica schist and silicious marble and underlain by green chlorite sericite mica schist and carbonaceous chloritic schist (Fig.1). There exist at least two (possibly three) mineralization horizons (ore bodies) separated by less than 25 cm to almost 6 m thick poorly mineralized ferrugenous sericitic quartz mica schist and almost green chloritic mica schist bands. Both the mineralization bands extend from Singate in southeast to Arubote in the northwest (about 4km) in strike length. The mineralized bands lie on the upper most part of the chloritic sericitic quartz biotite schist close to the contact with calcareous schist and marble (crystalline dolomite). Minor mineralizations are also recorded in Barappu, Salyani, Dangadinger, Sotre and Ghunsa (Fig.1) but none of them were studied in detail.

 

In Thoshe three types of hematite mineralization are observed as (1) massive hematite with few crystalline magnetite (2) massive to foliated mixed hematite and very few magnetite with few thin sericitic quartz lamina and (3) Incipient mineralization along the foliation of the host rock. Khimti Khola fault and Singate fault might have provided some hydrothermal fluids, which could be one of the sources of Pyrite, Pyrrhotite and Chalcopyrite mineralization in some parts of Hematite bands in Arubote and Sotre area. Both the mineralized bands are well exposed in Jhoreni Khani, Pahare Khani, Kopu Khani, Patle Khani and partly in Arubote Khani. However, in other places they are covered with old mine dump and slided debris. Mineral concentration within the ore body is not uniform rather it is irregular. As a result iron content in the ore samples varies considerably (Table-2).

 

The upper mineralization band: It is 30cm to 1.5m thick and consists of medium to coarse grained, massive to poorly foliated shining silvery brownish gray hematite with cherry red streak and minor amount of shining steel gray to silver gray magnetite with gray streak. In Arubote and Sotre sections few pyrrhotite, pyrite and magnetite are also recorded along with quartz veins and lenses in it.  Iron content in the mineralization bands is not uniform as the analytical results revealed 20 to 66% iron in them. The average iron content is below 45% i.e. around 42%. Study of hematite ore under the ore microscope revealed that it consists of mainly hematite (60 – 80%), magnetite (<5 – 8%), iron sulphide (1 – 3%) and gangue minerals (8 – 15%). Hematite is partly replaced by magnetite. Gangue minerals are mainly quartz and mica.


The lower mineralization band: It is separated from the upper mineralization ore body by less than 25 cm to 6m thick, foliated, incipiently mineralized ferrugenous sericitic mica schist and almost barren chlorite mica schist. It is 80cm to 1.6m thick and well foliated, medium grained, shining gray micaceous hematite with or without magnetite. At places quartz + magnetite with or without ilmenite veins or lenses are also recorded. The thickness and concentration of the ore minerals in this band (ore grade) varies considerably (from 20 to 45% iron) in different sections. However, the ore minerals remain the same. There exists few, very thin, thread like interlayers of quartz and quartz mica schist within the mineralization bands. Generally the concentration of ore minerals is comparatively rich towards the upper part. In addition to this in trench no 8 a 70 to 80cm thick mineralization band consisting reddish brown hematite is also recorded at the basal part of the lower mineralization band. In hematite, quartz and mica are the two main gangue minerals. The combined thickness of both the mineralization bands becomes 2 – 3 m. From the analytical results of the chip samples the average grade appears to be around 40% and from bulk samples it comes less than 40%.

According to the old miners there exists a 1.5 to 2m thick third mineralized body. It is comparatively rich in grade at Gairi Khani and Bhosbhose Khani where they have mined the ore in later part of mining activity (during 1960 – 1966). Because of foliated and brittle nature of mica rich hematite ore, mining was fairly easy. However, thick overburden of the overlying loose materials, old mine dumps and the overlying rocks it is not possible to expose the mineralization band by shallow trenching. Only 35 to 100m deep drill holes could confirm the existence of the third mineralization bands (ore body). The combined geological ore reserve (tonnage) and average grade are calculated only on the basis of the upper and lower mineralization bands.


Table 2: Chemical Analysis Results of ore and rock Samples
(Cp = Chip sample Blk = Bulk sampl  S = Grab sample   LG = Low grade  MG = Medium grade   HG = High grade)
               

Sample Number
(Chip/Channel samples)
Fe (%)
Location/ Remarks
RM/2052/53
KK/Th/Jh/Tr-1/Cp-1

32.5

Jhoreni Khani / LG ore
“     “                Cp-2
40
Jhoreni Khani/ LG ore
‘’       “        Ph/Cp-3
37.5
Pahare Khani/ LG ore
“     “   Ph/Tr-2/Cp-4
42.5
Pahare Khani/ MG ore
“    “       “    “   Cp-5
37.5
Pahare Khani/ LG ore
“     “     “       Cp-6
45
Pahare Khani/ MG ore
“     “    “         Cp-7
40
Pahare Khani/LG ore
“    “       Tr-3 Cp-8
47.5
Pahare Khani/ MG ore
“    “      “       Cp-9
45
"                "         "
"    " /Kp Tr-5 Cp-10
30
Kopu Khani/ LG ore
"   " /Kp.Tr-5Cp.11
40
Kopu Khani/ LG ore
“    “’     “     Cp-12
32.5
Kopu Khani/ LG ore
“     “   “ Tr-6/Cp-13
47.5
Kopu Khani / MG ore
“   “   “   Tr-7/Cp-14
47.5
Kopu Khani/ MG ore
“    “  “  Tr-7/Cp-15
32.5
Kopu Khani/ LG ore
“      “  “  Kp/Cp-16
47.5
Kopu Khani/ MG ore
“    “   “   Tr-8/Cp-17
32.5
Kopu-BhaluKhani/LG ore
“    “   “  Tr-8/Cp-18
32.5
"                "                  "  
“     “   “ Tr-8/Cp-19
40
"                "                  "
“   “    “ Tr-8/Cp-20
52.5
"               "/  HG ore
 "    " Tr-9/Cp-21
32.5
"                "/ LG ore
“   “    “ Tr-9/Cp-22
37.5
"                "/ LG ore
“  “    “ Tr-10/Cp-23
30
"                "/ LG ore
“  “    “ Tr-10/Cp-24
45
"               "/ MG ore
“   “      Dk/Cp-25
35
Danda Kharka/ LG ore
“   “      Dk/Cp-26
35
"               "/ LG ore
“   “      Dk/Cp-27
45
"              "/ MG ore
“  “  “ Tr-12/Cp-28
40
Sano "     " / LG ore
“  “   “ Tr-10/Cp-29
37.5
"        "     "/ LG ore
“  “  “ Tr-10/Cp-30
35
"        "    "/LG ore
“  “  “ Tr-10/Cp-31
37.5
"        "     " / LG ore
“  “  “ Tr-10/Cp-32
40
"      "      " / LG ore
“  “  “   Pt-1/Cp-33
50
Silili Khani/ HG ore
“ “  “   Pt-1/Cp-34
52.5
"      "  / HG ore
“  “  “   Pt-2/Cp-35
27.5
"       " / LG ore
“ “   “ Tr-14/Cp-36
52.5
Arubote Khani/ HG ore
“ “  “      Cp/36A
33.7
"        " / LG ore
“  “  Tr-15/Cp-37
55
"        " / HG ore
“   “       “   /Cp-38
45
"        " / MG ore
“  “    “    /Cp-39
55
"         " / HG ore
KK/Th/Cps-16A
40
"         " / LG ore
KK/Bml/Cps-40
47.5
Ghunsa Khani/ MG ore
KK/Th/Sii/Cp-41
47.5
"          " / MG ore
KK/Th/Sii/Cp-42
27.5
Barmasthali/ LG ore
KK/Bml/Cps-43
55
"          " / HG ore
Cp-44
NA
"          "    / (?)
Cps-45
37.5
"           " / LG ore
Bulk Samples

Location/Remarks
KK/Th/Jr/Tr-1/Blk-1
31.5
Jhoreni Khani/ LG ore
"     " Ph/Tr-2/Blk-2A
42.5
Pahare Khani /MG ore
"     "   " /Tr-2/Blk-2B
40
Kopu Khani/ LG ore
"     "   Kp/Tr-5/Blk-3A
42.5
"        " / MG ore
"     "  Kp/Tr-5/Blk-3B
40
"        " / LG ore
"    "/ Jr/Tr-1/Blk-4A
35
Kopu Bhalukuwa/LG ore
"    " Jr/Tr-1/Blk-5A
32.5
"         " /LG ore
"    " /Jr/Tr-1/Blk-5B
22.5
"         " / LG ore
"    " /Jr/Tr-1/Blk-5C
45
Kopu Division/ MG ore
"    " /Jr/Tr-1/Blk-6A
28.7
Kopu Division/ LG ore
"   "/Jr/Tr-1/Blk-6B
25
Danda Kharka / LG ore
"    " /Jr/Tr-1/Blk-7
37.5
Danda Kharka/ LG ore
"   " /Jr/Tr-1/Blk-8
37.5
Arubote Khani/ LG ore
"    " /Jr/Tr-1/Blk-9
22.5
Arubote Khani/ LG ore
"    " /Jr/Tr-1/Blk-10
NA
Arubote Khani/ ?
Grab Samples


RM/2051/052
               KK/S-6A

66
Arubote Khani/ Very High grade ore
    “       KK/S-6B
45
Arubote Khani/ MG ore
    “       KK/S-8A
57
Pahare Khani/ HG ore
    “       KK/S-8A-1
48
Pahare Khani/ MG ore
    “       KK/S-8B
31
Pahare Khani/ LG ore
    “      KK/S- 23A
30.5
Bhosbhose Khani/ LG ore
    “      KK/S- 23B
49.5
Bhosbhose Khani/MG ore
   “       KK/S- 23C
50
Bhosbhose Khani/MG ore
   “      KK/S- 25
45
Singate Khani/ MG ore
    “     KK/S- 26
45
Singate Khani/ MG ore
    “     KK/S- 26A
34
Singate Khani/ LG ore
    “     KK/S- 28
38
Barrapu Khani/ LG ore
  “      KK/S - 30
31.5
Salleni Khani/ LG ore
  “      KK/S - 30A
26.5
Salleni Khani/ LG ore
   “     KK/S - 31
33.5
Pahare Khani/ LG ore
   “     KK/S - 32
29
Pahare Khani/ LG ore
  “     KK/S - 34
32
Jhoreni Khani/ LG ore
  “     KK/S - 35
40
Jhoreni Khani / LG ore
  “     KK/S - 36
43
Kuwa Khani/ MG ore
“       KK/S - 38B
34.5
Pahare Khani/ LG ore
“       KK/S - 39A
42
Darim Danda/ MG ore
“       KK/S - 39B
40
Darim Danda/ LG ore
“       KK/S - 41
29.5
Kopu Khani/ LG ore
“       KK/S -42A
37
Kopu Bhalukuna/ LG ore
“       KK/S -42B
55.5
Kopu Bhalukuna/ HG ore
“       KK/S -43
38
Kopu khani/ LG ore
“      KK/S -66
40
Silili Khani/ LG ore
“      KK/S -67
49.5
Sano Dandakharka/MG ore
“     KK/S- 67A
35
Sano Dandakharka/LG ore
“    KK/S- 70
31.5
Danger Dinger/ LG ore
“    KK/S- 78
NA
-
“    KK/S- 001
-
-
“    KK/S- 117
41.
- MG ore
“    KK/S -141B
22.5
- LG ore

Ore Genesis
Field study of the ore bodies and laboratory investigations of the ore samples and the nature of the mineralization revealed that it is a synsedimentary hematite deposit, which was later affected by medium grade regional metamorphism. As a result the sedimentary hematite ore has been recrystallized into a less foliated to well foliated micaceous silvery gray hematite with minor amount of magnetite and almost massive hematite with few magnetite and very rarely pyrrhotite and pyrite (only in Arubote and Sotre). Minor hydrothermal effects has been indicated by the presence of few small quartz + magnetite + ilmenite veins and lenses as well as quartz + pyrite + pyrrhotite lenses within the hematite bands or in the host rock very close to the contact with the mineralization bands. The grade of the ore varies considerably from 20 to 66% iron with few Titanium, (<800 to 1600pm), Chromium (<20 to 50ppm), Nickel (<20 200ppm) and rare amount of Silver (<1 - 2ppm).

Geochemical Survey
Reconnaissance geochemical survey was carried out by Adhikary in 1980. The authors carried out Follow up Geochemical stream sediment survey in 100sq.km area around Thoshe covering some parts of Dolakha and Ramechhap district. 102 stream sediment samples with a sample density of 1 sample/sq.km and 20 Heavy mineral concentrate samples with a sample density of 1 sample per 5sq.km area was collected in the field. All of them were analyzed in the chemical laboratory. Analytical results are plotted in the anomaly maps of Copper, Lead and Zinc. No significant anomalous body for Cu, Pb and Zn was detected in this area.

 

FINDINGS


- Regional geological map of about 100sq km area of Thoshe and surrounding area in 1: 63,360 scale and semi detail geological map of 10sq.km area around Thoshe iron deposit were prepared in 1: 16,000 scale and the host rock of the hematite mineralization was clearly defined (Fig.1)
-    Compass – Tape survey (map in 1:2500 scale) was able to delineate a 2–3 m thick two mineralization bands/ bodies in a 4km strike length from Singate to Arubote. (Fig.2A and 2B).
-    Two mineralization bands were traced by excavating 15 shallow trenches and cleaning 2 old working pits. 50 chip samples, 15 bulk samples and 30 grab samples were collected from different parts of the ore bodies (Fig. 2A and 2B). All the samples were analyzed and petrographic studies were made in DMG labs.

-    Rough estimation of geological reserve based on the assessment data indicates that there exist about 10 million tons of iron ore with an average grade of about 40% iron. However, separate calculation of comparatively high grade ore is also made. According to this the ore mineralization bands were divided into four sectors. Each sector was recalculated for their average iron content (Fig. 2A and 2B).

Sector 1:  Arubote Khani (Fig. 2A)
This part is about 450 m long in strike length. The thickness of the mineralized body (combined) is 1.5 to 2.00m but the comparatively better mineralized parts only 1 to 1.5m thick. The iron content in this sector varies from 50 to 55% in chip samples and the average grade is about 52% only.

Sector 2: Sano Danda Kharka (Fig. 2A)
This sector is about 600m in its strike length. The thickness of the mineralized body (combined) is 1.5 to 2.6m. But the better mineralized part is only 1 to 1.5m thick. The iron content in this part is 37.5 to 45% in chip samples. The average grade is around 41% only.

Sector 3: Kopu Khani (Fig. 2B)
This part is about 350m in strike length. The thickness of the mineralized body (combined) varies from 2 to 3m but the better mineralization part is only 1 to 1.5m thick. Iron content in this part varies from 37.5 to 47.5% in chip samples. The average grade is around 42%.


Sector 4: Pahare Khani (Fig. 2B)
This sector is about 500m in strike length. Its thickness varies from 1.5 to 2.6m. In this area upper horizon is rich in iron content than the lower one. Iron content varies from 37.5 to 47.5% with an average of 42%.

 

 

 

CONCLUSION AND RECOMMENDATION


Thoshe iron ore deposit is the oldest known iron old working (mine) in Nepal. It was mined (in small scale) for about 102 years by local miners. The nature of the mineralization, its extension length, combined thickness and overall tonnage of the deposit appears quite interesting. But because of the non uniformity in the concentration of the ore minerals and low overall average grade of the ore, unsuitable condition for open cast mining (except in Singate and part of Arubote), unavailability of basic infrastructure like road networks, electricity and low price of iron, this deposit is categorized as a low grade, medium tonnage iron deposit, which does not appear to be an economic deposit at this stage. However, price increase of iron, development of all infrastructures and application of high metallurgical techniques may help to upgrade the quality of ore and then only it could be feasible for mining in future.

Geophysical / magnetic and gravity survey is recommended to confirm the shape, size and depth extension of the ore bodies. Such survey supported by selective test drill holes (6 – 10 holes) may help to confirm the grade, tonnage of the deposit and also trace the possible third mineralization horizon (body).

 

Selective mining of the comparatively rich ore may be possible but smelting of hematite ore into a sponge iron needs high amount of energy (coal or electricity).

 

ACKNOWLEDGEMENT


The authors would like to express their sincere thanks to Mr. N.R. Sthapit, Director General, Department of Mines and Geology for giving permission to make use all the existing reports and maps to prepare the paper. Their special thanks also goes to Mr. R.S. Thapa, Senior Chemist (retired), and his colleagues for their kind help to analyze the ore samples and Mr. R.S. Hada, Senior Cartographer and his colleagues for their help to draft the maps in suitable size.

REFERENCES:

Adhikary, P.P., 1980; Reconnaissance geochemical and geological mapping of a part of Dolakha and Ramechhap 72 I/2 and I/6. (MEDB/ DMG unpub. report GC/131/80).
Good, P.C., 1963; Report and Recommendation. Thoshe iron mines development. (Unpub. report).
Hegen, T., 1969; Geology of Nepal. (Uunpub. Report, DMG).
Jhingran, A.G. (1951): Report  on hematite in Thoshe area. (Uunpub. reportDMG).
Kaphle K.P, and Khan, H.R., 1995; Geological Report on Preliminary asssessment of Thoshe iron deposit, Ramechhap district, Central Nepal  pp.11 (Unpub. Report, DMG).
Kaphle K.P. and Khan, H.R., 1996; Field report on Preliminary follow up Exploration and assessment of Thoshe Iron Deposit, Ramechhap District, Central Nepal. pp 25 (Unpub. Report, DMG).
Manandhar, G.R., 1963; Field trip report of Wapsa copper mine Solu District, East No. 3 and Thoshe Iron mine, Thoshe east no. 2. (Unpub report, DMG).
Poudyal, K.R. and Shrestha, J.N., 1977; Report on geological and geochemical works in parts of Dolakha, Ramechhap and Solukhumbu area.(Unpub. Report, DMG).
Rana, M.N., 1965; Preliminary project report on Thoshe iron works. (Uunpub. Report, DMG).
Suwal, R.N., 1965; Iron old workings in Thoshe. (Uunpub. Report, DMG).
Talalov, V.A.; 1972; Geology and ores of Nepal, vol-II, pp.41-46 (Uunpub. Report, DMG).
Upadhyaya, R.P., 1956; Report on Thoshe iron deposit. (Uunpub. Report, DMG).
Weise R.O., 1960; Iron foundary at Thoshe. (Uunpub. Report, DMG).


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