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| 題 名 | Origin of the Nsuta Manganese Carbonate Proto-ore, Ghana: Carbon-and Oxygen-Isotope Evidence=非洲加納恩蘇達錳礦床之原始碳酸錳礦的成因 |
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| 作 者 | 葉學文; | 書刊名 | 中國地質學會會刊 |
| 卷 期 | 38:4 1995.10[民84.10] |
| 頁 次 | 頁397-409 |
| 分類號 | 357.28 |
| 關鍵詞 | 碳同位素; 氧同位素; 錳礦礦床; 元古代; 加納; 非洲; Carbon isotope; Oxygen isotope; Manganese deposit; Proterozoic; Rhodochrosite; Ghana; |
| 語 文 | 英文(English) |
| 中文摘要 | 非洲加納之恩蘇達(Nsuta)錳礦床為此類礦床世界上最有名者。恩蘇達礦 (Nsutite)即由此而來。該礦所處之沉積岩層為元古代的比利木安群(Biriman)。過去約 八十年以來該礦床所採者皆為氧化錳礦石。因此過去的研究都放在此表生礦(Supergene) 的成礦機制上。唯對其原始碳酸鹽礦石成礦機制的研究則幾乎不存在而不甚了解。由於表 生礦已近枯竭,目前已準備開採此原始碳酸鹽礦石。本文報導我們研究此原始碳酸鹽石成 礦機制的一些結果。 該原始礦石之礦物成份,除菱錳礦以外另有多種含錳碳酸鹽礦。此外還有方解石及白雲石。 因此,該礦石成份最少七種碳酸鹽礦物。唯礦石中只有菱錳礦及鈣菱錳礦兩種。在母岩中 則七種碳酸鹽礦都存在。 碳及氧的穩定同位素成份顯示:(1)菱錳礦形成時必有相當的有機碳參與反應,(2)菱 錳礦應是該原始礦石最初的原始礦物,(3)原始菱錳礦可能是在還原性海水中沉澱或從有 機物高之淺層沉機物中沉澱出來,(4)唯大部份之菱錳礦在形成後均曾經歷較高溫的影響 ,(5)其它碳酸鹽礦物則顯然為熱液礦物。 |
| 英文摘要 | The Birimian manganese deposits of Nsuta, southern Ghana, consist of Mn oxides and Mn carbonate. The Mn oxides are believed to be of supergene origin derived from the Mn carbonate proto-ore. While there is a general consensus on the origin of the Mn oxides, the origin of the Mn carbonate proto-ore is poorly understood. A carbon- and oxygen-isotope study of the carbonate minerals of the deposits have been made to gain some new insights into the origin of the proto-ore. This is a report of the results. The host-rock carbonates consist of at least seven different phases, while proto-ore carbonates consist of only rhodochrosite and Ca-rhodochrosite. Isotopically, the host-rock and the proto-ore carbonates are also different, providing evidence for their different origins. The δ□C□ values of the proto-ore carbonate, Ca-rhodochrosite and rhodochrosite, samples range from -5.0 to -15.9□. This reveals variable contributions of organic carbon to the carbonate (CO□). The δ□O□ values of the carbonate samples range from -12.6 to -14.9□ reflecting a relatively small range of temperature, and/or O-isotope compositions of the mineralization fluids. Nonetheless, these δ□O□ values are consistent with a hydrothermal origin of the carbonates. The δ□C□ values of most of the host-rock carbonates, excluding the rhodochrosite sample 993.28.1S1, are ≧ -3.6 □ indicating little or no organic carbon presence. The δ□O□ values of these host-rock carbonates, however, range from -14.4 to -17.2, with an average of -16.0□. These values are very different from those of Cenozoic marine calcites, which average about 0□ and suggest a secondary hydrothermal origin for the host-rock carbonate. The δ□C□ and δ□O□ values of the rhodochrosite sample number 993.28.1S1 are -8.8 and -0.8□, respectively. This provides some insight into the origin of this rhodochrosite sample as well as the proto-ore carbonate. It appears that this rhodochrosite sample was formed at sea-floor temperature under an anaerobic environment. It can be argued that this is the most likely representative of the proto-ore carbonate. It appears that the Nsuta Mn proto-ore carbonates formed initially as rhodochrosite via either sedimentary or early diagenetic processes under anaerobic conditions on and/or in the Early Proterozoic seafloor. Later on, hydrothermal activity altered much of the primary rhodochrosite. The hydrothermal activity also altered and produced various host-rock carbonates, including Mn-carbonates. Finally, the oxidation of proto-ore rhodochrosite and, perhaps, other Mn-carbonates produced the supergene oxide ore. |
本系統中英文摘要資訊取自各篇刊載內容。