DETERMINATION OF THE CONDITIONS OF FORMATION OF SINIAN AND EARLY CAMBRIAN GRANULAR PHOSPHORITES OF THE YANGTZE REGION, CHINA

中國揚子區震旦紀和早寒武世顆粒磷灰岩的形成條件

唐天福(TIANFU TANG) ； E. L. SHKOLNIK ； 薛耀松(YAOSONG XUE) ； 俞從流(CONGLIU YU)

《Collection and Research》 10期 (1997/12) Pp. 117-132

摘要

China possesses large-scale deposits of marine sedimentary phosphorite. The mainly occur in Guihzou, Yunnan, Sichuan, Hubei, Hunan and Jiangxi Provinces in South China (i. e. the Yangtze region). In this phosphogenic region, the sedimentary sequence from the Doushantuo and Dengying Formations of the Upper Sinian Series to the Lower Cambrian Series is continuous. Various types of phosphorite have been discovered in the Yangtze region but within commercial ores by far the most important phosphorite is the granular type. We focused our efforts on resolving what we regarded as the main problem - the nature of the grains in the granular phosphorites. To resolve this problem we made extensive use of scanning electron imaging of a large collections of thin section of granular phosphorites from different deposit of Sinian and Cambrian age in the Yangtze region. SEM analysis showed that the phosphatic grains of this region phosphoritcs consist predominately of algal fragments with lesser amounts of bacterial-algal mats and, rarely, accumulations of phosphatizcd acritarchs and spherical green algae. Accessory components include occasional stromatolitic structures, oncolites and their fragments, rarer complete phosphatized microfossil tests and also fragments of various algae. Some phosphorites consist of phosphatized oncolitcs, with the grains often having oncolite-Iike cortices, of greatest significance, from both the practical and the theoretical standpoints, is the discovery that the overwhelming majority of phosphatic grains consist of fragments of phosphatized algae and bacterial-algal mats. Data retrieved from the present study of Sinian and Early Cambrain granular phosphorites of the Yangtze region show conclusively the formation was controlled by several factors and took place in several stage. We conclude that the formation of Sinian-Cambrian granular phosphorite deposits depended on the appearance of a strict succession of certain biological, sedimentological, Paleogeographic and tectonic conditions, and on carbonate shelves broadly similar to those existing during late Precumbraim-early Cambrian times in the Yangtze region. Uncovering the full spectrum of this special set to circumstances, which led to the production of substantial masses of sub-and supralittoral algal mats (together with certain planktonic organisms) that later underwent intensive phosphatization fellowed by physical breakup, reworking anti redeposition. Be that as it may, it is now clear that granular phosphorites were mainly the product of biological processes, with physical processes acting only to facilitate the concentration of the biological products in ore bodies.

關鍵字

Yangtze region ； Sinian-Cambrian ； Granular phosphorite

並列摘要

中國具有許多大型的海相沉積磷灰岩礦床。它們主要産於中國南方的貴州、雲南四川、湖北、湖南和江西等省。在這個磷灰岩形成區，自晚震旦世陡山沱組和燈影組至早寒武世，磷灰岩沉積序列具有連續性。目前，已被發現的磷灰岩類型甚多，但具有重要工業意義的礦床是顆粒磷灰岩。本文致力於解決顆粒磷灰岩中顆粒的特性。爲此，我們採用掃描電子顯微鏡對本區震旦紀和寒武紀不同礦床的大量的顆粒磷灰岩薄片進行了分析。其分析資料表明，磷灰岩的磷酸鹽顆粒主要是由大量的-菌藻席碎屑及一些磷酸鹽化的疑源類和球狀綠藻組成。次要組成包括疊層石、核形石及其碎屑、完全磷酸鹽化的微化石和各種化石藻類碎屑。某些磷灰岩由磷酸鹽化的核形石和似核形石殼層的顆粒組成。關於絕大多數磷酸鹽顆粒是由磷酸鹽化的藻類和菌-藻席碎屑組成的發現，具有重要的理論和實際意義。目前對本區震旦紀和早寒武世顆粒磷灰岩研究所獲資料表明，它們的形成確實受到多種因素的控制和發生於不同的時期。我們認爲，揚子區震旦紀－寒武紀顆粒磷灰岩礦床的形成依賴於一定的生物學、沉積學、古地理學和構造條件的出現和類似於當時揚子區寬廣的碳酸鹽岩陸棚的存在。暴露充塡的特殊環境導致潮坪和潮下帶藻席（並伴生著一定的浮游生物）大量繁殖，然後遭受強烈的磷酸鹽化作用，緊跟其後的是物理碎屑、沖刷、搬運和再沉積作用。當然，顆粒磷灰岩主要是生物作用的産物，物理作用往往是促進生物産物的富集。

並列關鍵字

中國揚子區 ； 震旦紀－寒武紀 ； 顆粒磷灰岩