泰国湾西部表层沉积物重矿物分布特征：对物质来源和沉积环境的指示

SPATIAL DISTRIBUTION OF HEAVY MINERALS IN THE SURFACE SEDIMENTS FROM THE WESTERN GULF OF THAILAND:IMPLICATIONS FOR SEDIMENT PROVENANCE AND SEDIMENTARY ENVIRONMENT

对泰国湾西部159个表层沉积物进行了碎屑重矿物鉴定分析，其中57站重矿物颗粒数少于300粒，有效数据102站。结果表明，泰国湾西部表层沉积物中重矿物含量低，平均为1．628％，变化范围在0．016％～20．755％之间；共鉴定出重矿物29种，包括菱铁矿、褐铁矿、普通角闪石、自生黄铁矿、赤铁矿、绿帘石、磁铁矿、白钛石、锆石、紫苏辉石、石榴子石、黑云母、水黑云母、普通辉石、透闪石、白云母、钛铁矿、白云石、榍石、磷灰石、绢云母、蓝晶石、十字石、阳起石、电气石、金红石、棕闪石、独居石和透辉石，其中优势矿物为菱铁矿、普通角闪石和褐铁矿，平均含量（颗粒百分含量，下同）分别为41．6％，15．6％和13．4％，最高含量分别为99．7％，72％和94％；特征矿物为自生黄铁矿、磁铁矿、赤铁矿和锆石，平均含量分别为6．8％，4．1％，3．9％和0．9％，最高含量分别为97．3％，26．0％，91．7％和11．O％。利用数学聚类方法并结合矿物分布，在泰国湾西部划分出6个重矿物分区，基本以从北到南垂直岸线的东西向带状分布，矿物分区与物质来源和沉积环境具有较好的对应关系。重矿物来源主要为陆源和白生源：前者以普通角闪石、磁铁矿、赤铁矿、钛铁矿以及紫苏辉石等为代表性矿物，陆源物质输入较为明显的海区有3处，即曼谷湾的罗勇外部海区、春蓬和敦沙之间的近岸区域以及宋卡和北大年之间近岸区域，其中春蓬和敦沙之间的近岸区域表层沉积物中黑色金属矿物含量高，与附近有铁矿床分布相符合；后者以菱铁矿、自生黄铁矿以及菱铁矿风化形成的褐铁矿为代表性矿物，主要分布在陆架中南部、中东部和中北部。依据自生源矿物分布特征在陆架中部划分出不同的沉积环境，在陆架中南部和中北部为轻度还原环境，局部出现较强的还原环境，而在陆架中南部的轻度还原区域内出现氧化沉积环境。

The Gulf of Thailand, a shallow arm of the South China Sea, is bordered by Vietnam, Cambodia, Thailand, and Malaysia. The covered area of the gulf is about 320, 000km^2. Its mean water depth is about 40m, and the maximum depth is 86m. Mae Khlong, Tha Chin, Chao Phraya and Bang Pakong rivers actual annual supply approximates 20 million tonnes of chiefly muddy sediment into the gulf. 159 surface sediment samples were collected during 2011 - 2012 in the western Gulf of Thailand. Heavy minerals for these samples have been separated by bromoform （CHBr3, sepecific gravity of 2.89）. Composition, assemblage and distribution characteristics of heavy minerals in the sediment were studied. The results show that mean weight content of heavy minerals is lower, with an average of 1. 628%, ranging between 0. 016% and 20. 755%. A total of 29 kinds of heavy minerals are indentified, including siderite, common hornblende, limonite, authigenic pyrite, hematite, epidote, magnetite, leucosphenite, zircon, hypersthene, garnet, biotite, hydrobiotite, augite, tremolite, muscovite, ilmenite, dolomite, sphene, apatite, sericite, kyanite, staurolite, actinolite, tourmaline, rutile, barkevikite, monazite and diopside. The dominant minerals are siderite, common hornblende and limonite, and the average content （percentage of grains, the same below） are 41.6%, 15.6%, 13.4% and the highest content are 99.7%, 72%, 94% respectively. The characteristic minerals are authigenic pyrite, magnetite, hematite and zircon, with the average content of 6.8%, 4. 1%, 3.9% and 0.9% and the highest content 97.3%, 26.0%, 91.7% and 11.0% respectively. Six mineral provinces have been indentified in the study area using cluster analysis method based on the distributions of heavy minerals, and the division boundaries are generally perpendicular to the north-south shoreline. The mineral provinces are generally in response to material sources and sedimentary environment. The heavy minerals in the study area are mainly derived from terrigenous and authigenic. The terrigenous sediment is characterized by enriched common hornblende, magnetite, limonite, ilmenite and hypersthene, which mainly distribute off the Bankok Gulf, coastal area between Chumphon and Don Sak, and nearshore area between Songkhla and Pattani. Black metallic minerals content is high in surface sediment where in coastal regions between Chumphon and Don Sak, there is consistent with nearby ferrous metal deposit occurred. While the authigenic are dominated by siderite, pyrite in foraminifera shell and limonite from weathered siderite, which concentrate in south, east and north central shelf. The sedimentary environment is distiguished according to the distribution of authigenic minerals. There ninfo, mild reducing environment is in the south and north central shelf, where local areas appear strong reducing environment, while oxidizing environment also occurs in south central shelf.