| 其他摘要 | Marine sedimentary phosphate rock has been investigated for more than one hundred years and many genetic hypotheses were raised to interpret the genesis of this type of deposits. Up to now, the genesis of phosphate deposits still remains as a disputed topic. For one phosphate deposit, researchers usually have divergent views on its genesis.
The dispute among the genetic hypotheses of sedimentary phosphate rock depends on our understanding on the formation mechanism of phosphate minerals. Phosphate rock was intensely studied by many researchers. However, these previous studies mainly focused on the macroscopic features of phosphate deposit, such as the time-space distribution of phosphate rock system, the rock facies and palaeogeography of sedimentary basin and the minor element, REE and isotope geochemistry of phosphate ores. Few detailed study was conducted to dissect the microtexture of phosphate components in phosphatic rock system on microscopic scale, which is key to solve the dispute on the genesis of phosphate rock. Apatite in sedimentary phosphate rock can be biogenetic, chemogenic and colloid- chemogenic. No matter what genesis it is, apatite minerals usually appear as micro-grained to fine-grained aggregates, which are homogeneous under polarized light microscope. For this reason, modern advanced probing and analytical techniques are need to study the genesis of phosphate rock.
In this work, we chose Zhijin Xinhua phosphate deposit as studying object. Electron probe microanalysis (EPMA) and Analytical Electron microscope (AEM) were employed to clarify the genetic types of phosphate components in phosphate rocks, measure the microarea mineral and chemical compositions of each genetic type, observe their microtexture, sedimentation and enrichment forms of phosphate, and nano-scale fine structure. On the basis of these investigations, the ore-forming mechanism of Zhijin Xinhua phosphate deposit are discussed.
According to the results of electron back-scattering images, secondary electron scanning images and microarea composition analyses obtained by electron probe microanalysis, as well as the results of transmitted-electron images, electron diffraction analyses and microarea energy-spectrum composition analyses obtained by analytical electron microscope (AEM), we divided the phosphate components in Zhijin Xinhua phosphate deposit into four genetic types, which are clastic apatite, amorphous phosphate component, bioclastic apatite and spherical phosphate. Each genetic type has its characteristics microtexture and genetic process. Such a genetic clarification breaks through the limitation of previous studies which only took the colloidal phosphate ore as the occurrence form of phosphate, and is the important basis to discuss the ore-forming process of Zhijin Xinhua phosphate deposit.
The ultramicro-texture of clastic apatite was studied by using AEM, and the electron diffraction patterns of fluor-apatite single crystals were obtained. Our study indicates the fluor-apatite crystals are hyper-hexagonal grains and shows perfect crystal form. The size of crystals varies from 60 to 160 nanometers. Clastic apatite is the aggregate of fluor-apatite crystals. The orientations of crystals in the aggregate are randomly arranged. These features prove that clastic apatite was formed by chemical crystallization of apatite in the early stage, surface physical adsorption to form clastic clusters in the late stage in sedimentary basin and deformation during diagenetic process.
By AEM analyses, we also studied ultramicro-texture of the amorphous (formless) phosphate component and catched the transmission electron images of apatite particles inside relic biological cells. The apatite particles inside the cells are round in shape and about 10 nanometers in diameter, representing the primary form of biological accumulation of phosphate.
This research reveals that Zhijin Xinhua phosphate deposit formed as a result of comprehensive action of several ore-forming factors and each genetic phosphate component has its own features of genesis. The features of these genetic components are described as following:
1. The fluorapatite in the clastic apatite component was formed by chemical crystallization. It is probably related to sea-floor hydrothermal activity during the rifting of Rodinia super-continent. The hydrothermal activity supplied soluble phosphate and ultramicro-grained fluorapatite crystals. In a semi-closed sedimentary environment, these apatite crystals gathered to form crumbs about 35-300 micrometers in diameter. After then, these crumbs deposited and solidified to form apatite clasts.
2. Formation of the amorphous phosphate component is closely related to biogenic phosphatization. It is found that this type of component is mainly composed of biogenic apatite and organic biological relicts. Phosphatized microbiological population in sea water deposited to sea floor, accomplishing the transfer and stabilization of soluble phosphate to sediments. Phosphate was preliminarily enriched in the early stage of diagenesis,. Biogenic apatite aggregates inside the amorphous phosphate component are about 40-200 nanometers in diameter, with an average diameter of about 100 nanometers. They are nano-size biogenic poly-crystal aggregates inside relic kerogen, showing irregular grain shape. Phosphatization in the diagenesis stage happened inside phosphate component, forming micro-size silky and netted phosphate clusters, which are composed of nano-size micritic irregular phosphate crumbs and contain some muddy, silicious and iron hydrxide components.
3. The bioclastic apatite was created by bio-mineralization. Such a component represents biogenic phosphatisation.
The spherical phosphate component can be identified to an inner core and a crust. The inner core represents sea-floor sedimentary phosphatic mud. Its composition and microscopic texture are complicated and diverse. It is inferred that the inner core is a product of the phosphate-accumulating sedimentary process of ore-forming microorganisms and the phosphatization in the early stage of diagenesis. Before solidification, sea-floor phosphatic mud was tossed into seawater to form muddy spheruls which consist of the inner cores of the spherical phosphate component. While suspending in seawater, the spherules adsorbed micro-grained (or ultramicro-grained) fluorapatite crystals of hydrothermal chemical crystallization to form the crust of the spherical phosphate component. The crust possesses the same features of composition and texture with the clastic apatite component.
For Zhijin Xinghua phosphate deposit, chemical crystallization and physical absorption, which produced clastic apatite, is the most fundamental ore-forming process. However, such a process only formed phosphate-poor ores. Only when this process was superposed by biological phosphate accumulation, ores can get further enriched. |
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