Geology of the Kochiu Tin‐Field, Yunnan A Preliminary Sketch

RÉSUMÉ With the foregoing brief description of the geological and mineralogical conditions of Kochiu, the geological history of the district and the genesis of the deposits can be interpreted. The structure of the Yunnanese arc or at least that of its western limb is of long standing dated back to pre‐Sinian times. Later movements only strengthen its framework and follow its old tectonic lines. At the close of the Paleozoic era, there was a long period of denudation which swept away most of the Paleozoic formations, thus later Triassic beds are sometimes seen rest uncomformably and directly on the Archean rocks. Simultaneously or slightly subsequent to the period of denudation there came a widespread volcanic eruption as manifested by the basalt layers underlying the Triassic beds. In the Triassic times, the immense Kochiu limestone and the red beds were deposited. Both the Ailao and the Wuliang ranges already rose to high mountains in this period. After this there came another period of orogenic movement and denudation, this brought forth the unconformable contact existing between the Kochiu limestone and the Huopachung Coal Series. Subsequent to these, there was a period of folding, overthrusting and batholithic intrusions belonging to the Yenshanian phase. The tin deposits were formed in the Yenshanian. Due to the granitic intrusions as batholiths there followed a period of readjustment which was materialized as a system of faults giving rise to the numerous lakes and their lacustrine deposits. Finally a concluding uplift gave rise to the present topography. The genesis of the Kochiu tin deposits is by no means an accidental affair. Such development is controlled by various factors. Primarily there must be a tin‐bearing magma; secondly a geotectonicly favorable location for the magma to differentiate; thirdly a good reservoir for the pneumatolytic minerals to deposit; fourthly local small receptacles in the form of anticlines or domes for the deposits to concentrate together into lodes larger enough for economic consideration. Places which are found to fulfil these four qualifications are rarely met with. Here Kochiu seems to be the rare case. The nature of the magma which generates the Kochiu tin deposits is in everyway similar to those of the Hsianghualing and the Kuposhan, as revealed by the characters of the granite and the mineral association of the tin ores. The immense thickness of the Kochiu limestone qualifies another condition, since it constitutes a very favorable receptacle for the tin deposits to form at the right high temperature and pressure. As to the tectonicly favorable location the apices of structural arcs seem to meet the requirement: Kochiu is found just at the apex of the Yunnanese arc, here tectonicly it is the place most disturbed and fractured and batholithic intrusions usually occur. In the tin‐producing areas of Kochiu, the tin lodes are found to localize in the axial portions of small anticlines or domes; the last condition is fulfilled. This short paper represents the effort of the late Dr. V. K. Ting in completion of his unfinished geological work at Kochiu. The writers (Meng and Chern) spent about 6 months in field work in the Kochiu district under Dr. Ting's personal direction. In appreciation of his advise and guidance this short paper is dedicated to his memory.