The coexistence of Pb‐Zn deposits and oil/gas reservoirs demonstrates that a close genetic connection exists between them. The spatiotemporal relationship between Pb‐Zn mineralization and hydrocarbon accumulation is the key to understanding this genetic connection. The Mayuan large‐scale Pb‐Zn metallogenic belt is composed of a number of Mississippi Valley‐type (MVT) Pb‐Zn deposits that were recently discovered on the northern margin of the Yangtze Block, China. It is hosted in the dolostone of the Sinian (Ediacaran) Dengying Formation (Z2dn). In addition to the abundant bitumen in the Mayuan Pb‐Zn metallogenic belt, the paleo‐oil reservoir and the MVT Pb‐Zn deposit overlap in space. In this study, two precise ages of 468.3 ± 3.8 Ma and 206.0 ± 6.5 Ma were obtained via the Rb‐Sr isotopic dating of galena and sphalerite from the Mayuan Pb‐Zn metallogenic belt, respectively. The early metallogenic age of 468.3 ± 3.8 Ma is similar to the previously published age of 486 ± 12 Ma. The age of 206.0 ± 6.5 Ma is consistent with the age of the metallogenic event that occurred at 200 Ma in the Upper Yangtze Pb–Zn metallogenic province of the Sichuan‐Yunnan‐Guizhou polymetallic zone, which is located on the southwest margin of the Sichuan Basin, suggesting that the metallogenic effects of this period were regional in scale in the peripheral areas of the Sichuan Basin. Previous studies have shown that two periods of hydrocarbon accumulation occurred in the oil/gas reservoir that coexists with the Pb‐Zn deposits in the study area. The Pb‐Zn mineralization at 468.3 ± 3.8 Ma occurred during the first period of hydrocarbon accumulation, while the second mineralization at 206.0 ± 6.5 Ma occurred during the transformation of the paleo‐oil reservoir to a paleogas reservoir. The spatial relationship between the paleo‐oil/‐gas reservoir and the MVT Pb‐Zn deposits and the temporal relationship between mineralization and hydrocarbon accumulation show that a close genetic relationship exists between the MVT Pb‐Zn mineralization and hydrocarbon accumulation. Analysis of metals in the source rocks forming the paleo‐oil/‐gas reservoirs show that source rocks which formed paleo‐oil/‐gas reservoirs may have provided metals for Pb‐Zn mineralization. Both the paleo‐oil/‐gas reservoirs and Pb‐Zn mineralizing fluids had the same origin.
The Pb-Zn deposits in Nanjiang-Mayuan area on the north margin of Sichuan Basin chiefly occurred in dolomites of Sinian Dengying Formation. Host rock of deposits distributed both on the two wings of composite anticline of Micangshan, the Mid-Late Proterozoic Huodiya group and Jinning-Chengjiang period magmatic rock located in the cores of the composite anticline. The Dengying Formation in the study area could be divided into four lithologic Member: the first Member (Zd 1 ) was mainly made up of rhythm of several botryoidal dolomite →snowflake dolomite→microlitic-minicrystal dolomite; the second Member (Zd 2 ) consisted of the interbed of hoary-gray microlitic dolomite, gray algal-laminated dolomite and dolarenite; sandstone, siltstone mingled with blue gray mudstone constituted the third Member (Zd 3 ); the fourth Member (Zd 4 ) chiefly included microlitic-finely crystalline dolomite, dolarenite and algal dolomite. The geological and geochemical evidences suggested that there existed two obvious paleo-karst interfaces between Zd 2 and Zd 3 , between Cambrian and Zd 4 . Dissolved pores and karst caves well developed in the dolomite under the interface of paleo-karst and some of them were infilled with galena, blende, fluorite, barite, dolomite, quartz and bitumen. Geological Characteristics of DepositsDeposits such as Pb-Zn deposits, fluorite deposits and barite deposits has been found in Dengying Formation in the study area, among which, MVT Pb-Zn deposits developed best. Mineralization characteristics of fluorite deposit and barite deposit are similar with that of MVT Pb-Zn deposits; therefore, they were grouped into a subclass of MVT Pb-Zn deposits (Rowan et al., 1996).Study showed that paleo-karstification played a significant role in the mineralization of Pb-Zn deposits, fluorite deposits and barite deposits, and all these deposits developed under the paleo-karst interface between Cambrian and Zd 4 . The Pb-Zn deposits often occurred in layers; the ore mineral mainly consists of blende and/or galena, and gaugue minerals mainly consist of dolomite, pyrite, quartz and bitumen. Sometimes zebra structure was formed among galena, blende and dolomite. Pb-Zn deposits, fluorite deposits and barite deposits can adjacently occurred and form independent cystic or stratified ore body respectively. The galena, blende, fluorite and barite etc. mainly infilled in fissure, dissolved pores or karst cave formed by supergene karstification, which indicated that dissolved pore and karst cave were both the channel of metallogenic hydrothermal migration and the room for metallogenic elements accumulation and deposit. Meanwhile, the distribution of Pb-Zn deposits, fluorite deposits and barite deposits were controlled by paleo-karst landform and metallogenic elements only concentrated and deposited on karst highland in the paleokarst belts selectively.Research indicated that MVT Pb-Zn deposits were superimposed with the paleo hydrocarbon reservoirs of the Dengying Formation with each other in the space. Based on investigation of bit...
<p><span><strong>Abstract</strong>:Trapiche describes a gem texture that is characterized by the symmetric six &#8220;arms&#8221; radiating outward from the cores in gemstone minerals. These is a unique growth pattern, which however is still poorly understood. Here, we document the Trapiche in Trapiche rubies, and discuss formation mechanism of the patterns. The six arms of the Trapiche ruby radiate from a hexagonal core, which separate a single crystal into six growth sectors with internal bandings. Microscopic observations indicate a dendritic growth of the arms.&#160;Main branches are dominantly formed by the tube-shaped inclusions, and a part of which exhibit solid minerals, which including graphites, sulfides, calcites. The tube inclusions spatially have a 30&#176;&#160;angle to the radiating direction of the arms, and pointing to the direction perpendicular to the hexagonal prism cylindrical {10&#298;0}.&#160;Our Raman and EPMA analyses suggest that the origin of the ruby may be related to marble.&#160;Original&#160;source&#160;of&#160;the&#160;inclusions is aluminum-rich&#160;fluid with a&#160;high amount of CO</span><sub>2</sub><sub>, </sub><span>which originated and evolved from magma. Our detail textural and chemistry on Trapiche ruby may suggest that during the ruby formation, a high crystallization driving force is necessary for arms to be a dendritic pattern which can overcome the growth interface of the bandings. &#160;&#160;</span></p> <p>&#160;</p>
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