By THAN HTUN (GEOSCIENCE MYANMAR)
EPISODE:60
JADEITE FROM BURMA
This article serves as a continuation of Episode 59, “Jadeite from Burma”, originally reported by HL Chhibber in 1934. It provides further insights into the regional and specific geology of the jadeite mining area. Due to space constraints, a summary of the report is included.
Spotted actinolite-zoisite-schist with pegmatitic injections.
A specimen collected from the Nangma hka just a little south of “h” of hka (92 C/6), consists of granular aggregates of actinolite, zoisite, epidote, and vesuvianite with a bit of augite, quartz, and felspar. Of these minerals, zoisite is the most interesting and forms a fair proportion of the rock. The schist is injected with fine pegmatite consisting of quartz and muscovite.
Mica-pegmatite: Mica-pegmatites were observed in the Sanhka Chaung, about 2 ¼ miles southwest of Kansi (25° 47’ 1”, 96° 22’ 48”). They consist of a granitoid aggregate of quartz, felspar, and muscovite, the last mineral occurring in elongated blades. It is also present as inclusions in feldspar, and in places, the felspar and muscovite form a micrographic intergrowth and enclose sections of quartz.
Hornblende-peridotite: Specimens of a very interesting rock were obtained from the first stream, en route to Sietaung, about half a mile north-east of the “1316” hill marked on the map (sheet 92 C/6). Macroscopically, the rock is a beautiful aggregate of glistening black crystals of hornblende and pale green olivine. A thin section consists almost entirely of bluish-green hornblende (smaragdite) with clear olivine, diopside, and greenish augite. The rock is a hornblende-peridotite near the hornblendite of Dana. A similar rock was discovered as an ejected block from the Twindaung crater in the Lower Chindwin region.
Granite-gneiss: A coarse porphyritic granite-gneiss, which is interbedded with the crystalline schists, is seen in the Sage hka, a stream that joins the Hkara kha about one furlong from the latter’s confluence with the Tanai kha on one-inch Survey of India Sheet 92 C/13. The gneiss shows a texture varying from that of a fine-grained schistose rock to that of a coarse granite gneiss.
Serpentinite Peridotites.
An extensive complex peridotite, which in places has been partially or wholly altered to serpentine, extends southwards from the Sanhka hka (on the one-inch sheet 92 C/5) and a tributary of the Namsai hka to the latitude of Haungpa (25° 30’48”, 90° 6’15”). A W G Bleeck depicted it as two separate outcrops, but it is one continuous mass, as seen in the Uru chaung below Mamon (25° 35’ 10”, 96° 15’ 57”). Another outcrop on the one-inch sheet forms the hills “1448” and “1660,” while another mass, slightly east-north-east of Kansi, forms hill “2162” (one-inch sheet 92 C/5), etc. The serpentinised dunite from Tawmaw with a density of 2.795-2.802 on analysis by M Raoult yielded the following results:
SiO2=34.34, Al2O3=0.37, Fe2O3=3.60, FeO=7.04, MnO=0. 15, MgO=42.58, CaO=0.54, Na2O=0.41, K2O=0.17, P2O5=tr, H2O+ = 10.12, H2O = 0.31, Cr2O3=0.49
Granites.
In the Myitkyina district of the Jade Mines area, granite occupies a wide expanse of country, forming thickly forested hills up to 1,500 ft. in height and scattered knolls. The wide extent of the granite suggests that it has the form of a batholith, and it may be regarded as complementary to the ultrabasic complex described above and of approximately the same date. In the main, the rock is a medium-grained pink or grey granite, a type containing biotite being dominant. In addition to this, however, two-mica granite, hornblende-granite, and microgranite, as well as more basic types, including monzonites, are well represented. At the same time, diorite xenoliths in the acid rocks may well be interpreted as digested fragments of even more basic, early types. Pegmatites and aplites represent magmatic residue, while the final product of consolidation was quartz, which frequently veins the granite. These rocks are of special significance in connection with the problem of the origin of the unique jadeite-albite rocks, considered in detail below.
Altered Picrites and Volcanic Breccias.
Locally volcanic rocks, associated with highly altered picrites, occur, and perhaps represent the extrusive phase of the same igneous cycle that gave rise to the ultrabasic and acid complexes described above. As they are of no significance in connection with the genesis and exploitation of the jadeite-bearing rocks. Some notes on Altered andesite-tuff, Quartz-hornblende-dolerite, and Siliceous injections are discussed in the book.
The Tertiary Rocks.
The author has classified the Tertiaries of this region as follows:
(2) Naming Series - -Mio-Pliocene.
(1) Hkuma Series - - O l i go-Miocene.
The Hukma Series.
Good sections of the Hkuma Series are exposed along the Shadu, Tagam, Hkuma, and Hkada streams, of sheet 92 C/13, but since they were first met in the Hkuman kha and also from the high hill, 4,982 feet above sea level, known as Hkuma Bum, the author has designated them the Hkuma Series. Sandstones with occasional interbedded layers of shale or argillaceous sandstone are predominant. In places, they are finely laminated, while in others, they are coarse and pebbly. The most striking feature is their extremely well-bedded character. They are greyish, greenish, whitish, pale-yellow, or reddish, sometimes with black carbonaceous streaks. In the Shadu kha alone, about 10,000 feet of these sandstones are exposed. In the Tagam hka, sandstones are interbedded in places with black carbonate shales, which are sometimes very friable. Both sandstones and shales contain well-preserved fossil leaves; those from the neighbourhood of Hwehka include Tetranthera hwekonsis.
The shales are generally of a greyish colour and thin sections show dark-brown carbonaceous matter together with a considerable amount of calcite, some quartz, and a little mica. A number of foraminifera belonging to the family Globigerinidae were observed in the shales from Hwehka (25° 29’ 3”, 96°16’43”), and this tends to prove that a small inlet of the deep sea existed in the neighbourhood. This must have undergone very rapid changes as is shown by the interbedding of conglomerates, containing boulders of jadeite, various types of serpentines, peridotite, hornblende, graphite-, mica-and quartz-schists, proving that these rocks were exposed when the Tertiary sediments were being deposited. Bands of finely jointed, black, carbonaceous limestone also occur. The occurrence of coal and lignite is noteworthy.
On sheet 92 C/13, their junction with the crystalline schists and serpentines is faulted, as shown in the Hkuma and the Hshamshing hkas. Similar rocks are met within the upper course of the Namjan kha, on sheet 92 C/5, and the upper portion of 92 C/6. They are freshwater equivalents of the Pegu Series met with farther south, but their base may touch the Upper Eocene, as the heavy-mineral assemblages of some of the specimens correspond with those of the Barial Series (Eocene-Oligocene) of Assam.
Namting Series.
Since the Upper Tertiary rocks containing fossil wood were first met near Namting (25° 38’, 96° 27’), they have been called the Namting Series. The type area lies between Namting and Lonkin (25° 39’, 96° 22’), which are eight miles apart. Their thickness must be considerable, since, in places, the strata are vertical. They consist of sandstones, shales, and conglomerates. The sandstones are of various colours, coarse to medium grain size, and sometimes pebbly. They contain mineral minerals, but quartz and feldspar are predominant. Grains of epidote, glauconite, chlorite, and serpentine are quite common, while muscovite, biotite, haematite, chromite and calcite also occur. Small grains of graphite and graphite schist are also present, while grains of jadeite prove that the intrusion of the jadeite-bearing rocks must have preceded the deposition of these rocks. Some of the sandstones are calcareous, but the majority are argillaceous. Small trunks of silicified dicotyledonous fossil wood were seen about a mile west of Namting, and small stumps of fossil trees, comprising both palms and dicotyledons, were found a little southeast of Namyong (25° 40’, 96° 26’). The inner portion of some is carbonized, but the outer is silicified.
Similar rocks are exposed in the Tarong hka near Tarongyang (25° 40’, 96° 45’), and the Tertiary rocks forming the low hills near Nanyaseik (25° 37’, 96° 35’) also belong to this series, which is equivalent to the Irrawaddy Series. Specimens collected from them yielded heavy minerals, which, with very few exceptions, agree closely with those from the Tipam Series of Assam.
The later-Tertiary Plutonic Rocks.
Gabbro.
Gabbro first appears about one and three-quarter miles west-south-west of Δ 5124; in the Namjan hka which has cut a gorge with cliffs as far as its junction with the Loimye hka. From the latter point, the rock ascends and is exposed at about 750 feet above the level of the stream and is overlain by the volcanic deposits of Mount Loimye. A little over half a mile north of Δ 5124 it again makes its appearance and forms the “4842” Bum-i-Bum, and “4858” hills, whence it extends northwards as far as the headwaters of the Namjan hka where the latter bifurcate into the Tertiaries; in places, lit-par-lit injection is seen, and xenoliths of the latter occur in the former. It is very finely banded locally, and towards its western boundary, easterly dips ranging from 30º-50º are observed. Similar banding has been recorded in the case of the Tertiary gabbro of Skye. East of Namjan hka, north of its confluence with the Loimye hka for about 2 miles, the gabbro comes into contact with greenish-black basaltic tuff and, as a result of the intrusion, the latter has been baked, hardened, bleached, and metamorphosed. Very probably, this marks the position of one of the older vents, which served as passages for the uprise of the basic eruptive rocks.
The form of intrusion near the eastern margin appears to be a concordant, inclined composite sill or sheet. The rock is generally greyish-mottled with ferromagnesium minerals and shows considerable variation in texture. The different modifications such as massive, coarse gabbro, microgabbro (porphyritic), quartz-augite-enstatite gabbro, micrographic gabbro, granodiorite, quartz-diorite, camptonite, quartz-porphyry are described briefly.
The Uru Boulder Conglomerate.
The Plateau Gravels of Upper Burma are represented in the north of the Myitkyina district by a boulder conglomerate named by the author the Uru Boulder Conglomerate, after the river responsible for its formation.
The outcrop of the conglomerate attains a maximum width of over four miles in the longitude of Mamon, which was formerly very famous for its jadeite workings, and thence it extends to the north and northeast as a belt with an average width of two miles on the right bank of the river. The thickness of the formation must exceed a thousand feet in places, as is evident from a traverse along any of the tributaries of the Uru, for example, the Sabyi or Mamon Chaungs. In the latter case, there is an abrupt descent from Balahka (25º 37’ 30”. 96º 17’1”), which is situated on the conglomerate at a little over 200 feet O D, to the valley of the Mamon (Mamaung) Chaung. The cliffs overlooking the stream are composed entirely of the conglomerate, and no other rock is seen until Hpakan (25º 36’ 38”, 96º 18’ 40”) is reached, near the 806 spot-level on the map, sheet 92 C/6, where crystalline schists are encountered.
The age of the conglomerate is probably Pleistocene to Sub-recent. The Uru Boulder Conglomerate is strikingly polygenetic, and the older rocks of the neighbourhood are represented by boulders ranging in size from a few inches to several feet. They comprise several varieties of crystalline schists, including mica-, quartz-, glaucophane- and anthophyllite-schists; plutonic rocks, of which the chief are granodiorite, diorite, and epidiorite; metallic ores, including limonite, haematite, and chromite; and most important from the present point of view, boulders of jadeite, for which the formation is worked. It is locally believed that the jadeite of the boulders in the conglomerate is more “mature” than that of Tawmaw. The obvious difference between the two types is due to the removal, during transport, to the skin of the weathered mineral in the former case.
References: 1. Chhibber, H L, 1934: The Mineral Resources of Burma, Macmillan, and Co Limited, St Martin’s Street, London.
2. Carter, T Donald, 1943: The Mammals of the Vernay-Hopwood Chindwin Expedition, Northern Burma, Bulletin Amer. Mus. Nat. Hist., Vol. 82.
