Geology. In whatever way it is regarded, the Arakan-Naga province stands alone in Burma. Its structure, at any rate as far as the southern portion is concerned, is that of a great anticlinorium containing, according to E. H. Pascoe, a number of tight subordinate folds1. Moreover, it is complicated by igneous intrusions and certainly not simplified by its participation in comparatively late earth movements. It is not my purpose here to summarise or to attempt to correlate the descriptions of the rock groups met with on the various traverses that have been made from time to time into the Arakan-Naga region, but merely to point out that amongst the crushed, hardened and altered rocks which form its core a few fossils both of Triassic and Upper Cretaceous age have been found. Along part of its eastern margin in the districts of Minbu and Pakokku there is a thick band of hardened Eocene conglomerates followed by the Tertiary sequence of the Irrawaddy and Chindwin rivers, and the Tertiary deposits of its western flanks, that is, those laid down in the "Assam Gulf," though synchronous, are not identical. It follows that the folding which raised the Arakan Yoma and the associated mountain ranges of the Arakan-Naga region was initiated in the late Cretaceous and that part, if not the whole of it existed as land throughout the Tertiary period, forming a barrier between the two gulfs. Further, there is evidence that the geoanticline of the Arakan-Naga region was subjected to elevatory movements during the Tertiary period while the corresponding geosyncline of the Pegu gulf suffered secular depressions. In the extreme north, T. H. D. LaTouche found Tertiary rocks occupying the whole valley of the Dehing river and it may well be that here they stretch right across from Upper Assam to Burma, lying unconformably upon the crystallines of the Frontier ranges2. Stuart crossed the Patkoi Range recently3 from the head of the Assam valley to the Hukong Valley in Burma and although he describes the structure of the Patkai as that of a broad syncline, the axis of which is approximately coincident with the crest of the range, and across which sandstones of Upper Tertiary age extend, his map shows shows a broad broad expanse of pre-Tertiary deposits in a parallel ridge, nearer Burma, with the Tertiary deposits dipping away from them at high angles to the north-west and 1 E. H. Pascoe: (56), p. 250. 2 T. H. D. LaTouche: (37), pp. 111-115. 3 M. Stuart (64), pp. 398-411. south-east. Stuart correlates these with the pre-Tertiaries of the core of the Arakan Yoma far to the south-south-west. would So little is known of the Arakan-Naga Region that, it appear premature to discuss its minero-genetic features, yet the few outstanding facts which we do possess are of sufficient importance to Igneous Rocks of the Arakan-Naga Province. warrant certain definite conclusion. These are as follow: (1) Throughout the whole region so far as it has been examined not a single exposure of an acidic igneous rock of the granitic type has been recorded. (2) Intrusive serpentines have been found on practically every traverse. (3) The scanty occurrences of metallic minerals that are known, and none of them have proved to be of economic importance as yet, are such as would be expected to occur in an ore-province characterised by the predominance of ultra-basic rocks. They are chromite, native copper and chalcocite with the nonmetallic minerals of potential value,-steatite, chrysotile, and magnesite. In the Andaman Islands the chief igneous rock is a dark green Distribution of the serpentine with veins of chrysotile; the other Serpentines of the types are a bronzite-olivine-picotite peridotite Arakan-Naga Province. and a diorite. Serpentine pebbles often form part of the Eocene conglomerates of North Andaman Island. Serpentines, gabbros and diorites also occur in the Nicobar Islands. G. H. Tipper believes that the int usions are of upper Cretaceous age.1 Serpentines are common in the foothills of the Arakan Yoma of the Henzada, Promie and Thayetmyo districts. In Prome the rock frequently passes into gabbro with porphyritic crystals of bronzite. It is intersected by veins of gold-coloured chrysotile, or sometimes of magnesite. To the west of Thayetmyo town there is a serpentine intrusion some 5 miles long. In the western part of the Henzada district there are 21 distinct and isolated occuriences scattered over a length of 26 miles from north to south.2 The steatite deposits of eastern Kyaukpyu, of Minbu and probably those of Pakokku too, occur in serpentine. "The steatite occurs in veins traversing the dark green intrusive serpentine which is found in such quantities in the Arakan Yoma" wrote Sir Henry Hayden.3 The beds 1 G. H. Tipper : (66), pp. 10-11. 2 W. Theobald : (65), pp. 189-359; R. D. Oldham : (53), pp. 146-147. below the basal Eocene conglomerate of the Minbu and Pakokku. districts are ashy and there are numerous outcrop of serpentine along this horizon according to G. de P. Cotter.1 In the Manipur Hills, R. D. Oldham mapped a serpentine band 40 miles long and a mile or two in breadth, which is so similar to the rocks described by Theobald that Oldham deliberately chose the former's words to describe it. This example is, like so many of the others, confined to the eastern limits of the hill rocks.2 Corresponding to this band, E. H. Pascoe found serpentine further north in the Naga Hills. Thus between Puchinui and Karauni they form "a line of conical crag-crowned hills". The rock consists of a confused mass of massive and fibrous serpentine. In other places the serpentine with its associated chrysotile and chlorite has to a large extent become schistose. Boulders of the following rock types, believed to be at least in part responsible for the formation of the serpentine, were collected by Pascoe:-hornblende-enstatiteolivine gabbro, diallage gabbro, serpentinized lherzolite, spinellid peridotite altering into serpentine. In his brief notes of the geology of the Upper Chindwin valley the late H. S. Bion showed how the Tertiaries dip steeply off the complex of pre-Tertiary rocks in Manipur, and in the north come to an abrupt end against a deeply dissected mountainous area occupied by cleaved sediments. These lie exactly on the strike of similar strata found by Pascoe around Sarameti Peak in the Naga Hills. Further, a great many large boulders of serpentine occur in the upper reaches of the Chindwin River. There can "be no doubt", wrote Bion, "that the belt of intrusive serpentine which occurs along the boundary of the so-called Axials of the Arakan Yoma, Manipur and the Naga Hills, extends to the west of the Tertiary basin and crosses the Chindwin river a few miles above the Kyaukse rapids". To the far north in the valley of the Dehing river La Touche observed large, transported blocks of serpentine,5 while Stuart found serpentine intrusions in the preTertiary rocks of the Patkai Range between Upper Assam and Burma. It appears to be very generally believed that the serpentines are of Upper Cretaceous age and almost all the occurrences Types of Minerals found with the Serpentines-Chromite and Platinum. one. are on the eastern or Burmese flanks of the watershed. Oldham has stated that not a single intrusion has been detected in the unaltered Nummulitic rocks, but at a later date, Stuart maintained that certain intrusions of the Henzada district traverse coal-bearing sandstones which he correlates provisionally with the Laki Stage (Middle Eocene).2 The occurrences of chrysotile and steatite have already been alluded to. Chromite appears to be frequent in the ultrabasic rocks and to have been found wherever it has been especially searched for. It was reported from the vicinity of Port Blair in the Andaman Islands as long ago as 1883,3 and it is known from various localities in the Arakan Yoma and its extensions to as far north as the jade mines of Tawmaw which I regard as belonging to a sub-province of the same type as the Arakan-Naga H. S. Bion found rhombohedra of magnesite in a boulder of serpentine from the Upper Chindwin river. He also discovered small pockets of chromite in the serpentines near Sibong in Manipur. In discussing the origin of the platinum and osmiridium, which occur in very small quantities, but almost universally with the placer gold of the Chindwin and its tributaries, he drew attention. to the well-known, world-wide association of platinum with chromite segregations and serpentine. He mentioned that a belt of serpentine does extend from the Andamans, more or less continuously, through the Arakan Yoma, Manipur, the Naga Hills and the upper part of the Chindwin drainage system, and he concluded as follows:-"It is not impossible, therefore, that somewhere to the north there may be workable deposits of chromite associated with serpentine intrusions, and that such chromite deposits may prove to be the source of the platinum and platinoid metals occurring in the older gravels of the Upper Chindwin”.4 Native copper and copper sulphides have been obtained from the hills bordering the Kabaw valley in south Native copper eastern Manipur and the rocks surrounding copper ores. this tract belong to the altered pre-Tertiaries and contain serpentine.5 "In Burma", writes Dr. Morrow Campbell, and 1 R. D. Oldham : (52), p. 147. 2 M. Stuart: (62), p. 252. 3 F. R. Mallet: (46), p. 204. 4 H. S. Bion: (3), p. 247. R. D. Oldham: (52), p. 25; V. Ball: (2), p. 278. referring to the Pakokku Hill Tracts and Naga Hills, "considerable areas of serpentine occur in which native copper and chalcocite are associated with chromite. The conditions under which the copper ores occur lead to the belief that their formation was contemporaneous with serpentinization. Their deposition coincided with the period of ejection of the acid extract of the original peridotite. This acidic material is to be seen now in veins traversing the serpentine in the form of coarse quartz-hornblende pegmatite, and also chalcedonic quartz veins. At its periphery it is common to find a band of highly felspathic rock, evidently an acidic differentiate the result of a previous leaching: the origin of this was evidently prior to that of serpentinization. This serpentine occurs in the midst of Cretaceous rocks, and beyond the periphery very numerous fissures in the sediments are filled with crystalline quartz. There is no granite or other igneous rock in the vicinity. Dr. Morrow Campbell has kindly supplied the writer with some further notes on these interesting occurrences, from which it appears that while associated the chromite and copper minerals have no genetic connexion, the former being primary and residual in the strictest sense, whereas the copper seems to have been introduced into cracks in the serpentine, perhaps by deposition from the aqueous matter which brought about serpentinization. The native metal and the chalcocite are usually, if not always, in separate veins, and the former is invariably associated with cuprite (Cu2O). This mineral is probably due to oxidation and purely secondary as are also the malachite, azurite and calamine (ZnCO) of the cracks of the ferruginous gossans.2 may or Bion found that the serpentine belt crosses the Chindwin a few Serpentine miles above the Kyaukse rapids, further than outliers : the Tawmaw sub-Pro- that it has not been traced that it has not been traced continuously. In vince. the Kachin hills however A. W. G. Bleeck has partially mapped four large serpentine masses which may not be in direct geographical connection with the belt. In one of these, which has a length of at least 20 miles and a maximum breadth of 15 miles occurs the intrusive jadeite-albite dyke in which the famous Tawmaw jadeite mines are situated. These have supplied the Chinese market with jadeite for centuries. Boulders of the mineral are common at certain places in this neighbourhood 1 J. Morrow Campbell: (18), p. 14. 2 J. Morrow Campbell: in a letter to the writer dated June 22nd, 1923. |