Mineral Features of Surface Sediments and Analysis of Solid-State Mineral Resources in Southeastern China Sea

INTRODUCTION

In the past years, a few researches have been made into the Quaternary surface sediments in vast range of the Southeastern China Sea. In November 1997, the Guangzhou Bureau of Marine Geology Survey, Ministry of Land and Resources, started the regional geological surveying and mapping of China Sea ("Shantoufu, F-50"), and the marine geology and geophysics investigation in the Southwest Taiwan basin, and the Eastern Pearl River Mouth basin and their adjacent areas. In these investigations, the Guangzhou Bureau have obtained 19 cores, 64 grab samples, 10 box samples, 2 big gravity cores (15.86 m), 4 shallow wells (totaling 152 m long), 2 multi-metal nodule drag-net samples, and 560 km shallow single-channel seismic reflection sections. All these data have been analyzed in terms of grain size composition, mineral composition and chemical composition, following the marine-geology investigation regulations stipulated in 1992. Then these results and the conclusions from the "Marine Engineering Geological Investigating Project in the Pearl River Mouth Basin in the South China Sea (1∶200 000) " completed in 1990, and the regional geological data in the study area all have been used to analyze the mineral features of surface sediments and also the solid-state mineral resources in this area, as shown below.

TYPES AND DISTRIBUTION OF SEDIMENTS

Mineral is found in all kinds of sediments. However, the heavy mineral mainly exists in fine sand with the grain size of 0.125-0.063 mm. Therefore, the distribution of heavy minerals is closely related to the types and distribution of sediments.

According to udder-went-worth grade scale and Sheppard diagram, the sediments in this area are divided into 7 types as shown below.

Gravelly sand The fine gravel, whose grain size is more than 2 mm is the coarsest in the study area, is found only in the northeast of Taiwan Bank at water depth of about 30 m.

Coarse sand The coarse sand, whose grain size is 0.5-1 mm in diameter, is mainly distributed in the Taiwan Bank, Penghu Islands and in the inner shelf of Jiulongjiang depression.

Medium sand The medium sand, whose grain size is 0.25 mm, is mainly distributed in the inner shelf area off Nan'ao Island and in the upper slope of Dongsha upwelling area.

Fine sand The fine sand, whose grain size is 0.125-0.063 mm, is the major type in this area, because it is widely distributed in the shelf area.

Silty fine sand The silty fine sand, mainly composed of fine sand with silty sand of 25%-50%, is mainly distributed in the gulf and coast and at the edge of shelf.

Clayey silt The clayey silt, mainly composed of silt with clay of 25%-50%, is mainly distributed along the upper slope. The clayey silt is mainly composed of bioclast, secondly of quartz and a little of feldspar.

Silty clay The silty clay, mainly composed of clay with silt of less than 50%, is mainly distributed in the down slope and at the edge of sea basin.

MINERAL FEATURES OF SEDIMENTS

The minerals in the sediments vary with their grain sizes and origins. The minerals in the study area are divided into 3 major types: terrigenous detrital mineral, clay mineral and authigenic mineral (arranged in the descending order of their individual contents). The terrigenous detrital mineral is widely distributed in the shelf and upper slope. The clay mineral is mainly distributed in the down slope and at the edge of sea basin. The authigenic mineral occurs in the whole study area.

Terrigenous Detrital Mineral

The detrital minerals, whose grain sizes are greater than 0.063 mm in the study area, are mainly composed of terrigenous detrital mineral, a little of bioclast and authigenic mineral. In terms of chemical ingredient, the terrigenous detrital mineral in this area is divided into 5 types: silicate, oxide, phosphate, sulfide and carbonate, with the first two types acting as the dominant elements and the remaining three types as the minor elements (Table 1). The value of their specific gravity (with 2.88 as the dividing value) is used to classify the detrital minerals as 2 types: light minerals and heavy minerals.

Table  1.  DETRITAL MINERAL COMPOSITION OF SURFACE SEDIMENTS IN SOUTHEAST CHINA SEA

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Light mineral

Quartz In the shelf area, the quartz, whose highest content reaches 93%, is a dominant mineral with a mean content of 62%. In the upper slope, the content of quartz usually decreases with both the thinning of sediments and the increasing of bioclast.

Feldspar The feldspar comprises 24%, but locally up to 70% only takeay up one third of the total content of the quartz. In the shelf and upper slope area, the content of feldspar is relatively stable. However, the content of feldspar decreases distinctively in the down slope and at the edge of sea basin. In a word, the feldspar in this area is unevenly distributed, but is abnormally enriched in some local areas.

Weathered mineral The weathered minerals in this area mainly including feldspar and black mica comprise 4-7%, but locally up to 13%. The weathered minerals occur mainly in the shelf area, whose contents decrease gradually from land to sea and even disappear. The decrease of the weathered minerals with the increase of the offshore distance indicates that the sediments in the area come from the west land.

Heavy mineral

Heavy minerals, mainly distributed in the shelf area, amount to 1.5%-2.8% of the total mineral, locally to 5.7%. The contents and types of the heavy minerals in the slope area are distinctively reduced. The following heavy minerals are commonly observed in the study area.

Ilmenite The ilmenite whose content is the highest in this study comprises 12% of the total heavy minerals, and locally up to 39%. Ilmenite is mainly distributed in the continental shelf, especially in the inner sublittoral zone to the east of the Dongsha Islands where the ilmenite is relatively concentrated. In addition, the ilmenite is also commonly observed in the continental slope, but its content is very rare.

Zircon The zircon, comprising 5% of the total heavy minerals in this area, is mainly distributed in the continental shelf. But in the outer continental shelf to the west of Nan'ao Islands, the zircon is relatively concentrated and comprises 25% of the heavy minerals. However, the zircon is seldom found an the slope.

Anatase The anatase is widely distributed in the shelf area and occasionally found in the slope. The anatase comprises 1.5% of the total heavy minerals, and locally up to 5.25%.

Leucoxene The leucoxene, also widely distributed in the shelf area, comprises 0.5% of the total heavy minerals and locally up to 1.5%.

Epidote The epidote, widely distributed in the shelf area, comprises 5%. It is locally enriched in the littoral area to the east of the Dongsha Island, with its highest content reaching up to 16%. But the epidote is seldom found in the slope area.

Magnetite The magnetite is mainly distributed in the shelf and seldom found on the slope.

Amphibole The amphibole comprises 2.6% of the heavy minerals in this study area, and locally up to 6%. It is mainly distributed in the shelf area and occasionally on the continental slope.

Rutile, garnet and tourmaline These three minerals, often found in the continental shelf area, comprise a very low percentage. Only several grains can be observed in an ordinary sample.

Clay Mineral

The clay mineral is a kind of hydrous bedding silicate, whose grain size is generally less than 2 mm.

The clay mineral, mainly distributed on the slope and at the edge of sea basin, ranges from 30% to 60% in content. The lithology of the clay mineral is silty clay or clayey silt. Secondly, the clay mineral in the inner shelf and coast zone comprises 10%-23% with its lithology being silty fine sand. The content of the clay mineral in the outer shelf area is very low, with its highest content no more than 5%. The X-ray diffraction analysis of the clay mineral in this area concludes that the clay in this study area is mainly composed of illite, kaolinite, chlorite and montmorillonite. The content of illite, a dominant element, ranges from 54% to 79%, with an average value of 71%. The content of kaolinite ranges from 10% to 21%, with an average value of 12%. The content of chlorite ranges from 11% to 20% with a mean value of 13%. The content of montmorillonite ranges from 2% to 11% with an average value of 5.5%. The distribution patterns of these four clay minerals mentioned above follows the order shown below: from north to south, the content of illite increases gradually; from land to sea, the content of kaolinite decreases gradually; the content of chlorite decreases with the increasing of the transportation distance; the content of montmorillonite is low and changes within a small range. The content of montmorillonite locally reaches up to 15%.

Authigenic Mineral

The authigenic minerals in this area mainly contain manganese nodule (crust), glauconite and pyrite.

Manganese nodule (crust) The manganese nodule (crust), mainly distributed on the slope from Dongsha Islands to the west of 118°30′E, usually coexists with marine bioclast (such as foraminifera, shell, radiolaria and coral fragment). In general, these manganese nodules (crusts) occur in the form of incrustation on the bioclast, debris, rock block and marine basement rock, while some others occur in the forms of pellets and living organisms. For example, sample ST1, black/dark brown and 2 cm thick, whose surface layer is associated with the nodules protruding to 2-8 mm, is attached to the surface of strongly weathered basalt. This kind of manganese structured as a single layer contains relatively distinctive striation layers within the shell and therefore is defined as the alternate layers of incrusting Mn and Fe marine clays. According to the result of X-ray diffraction analysis, the surface layer contains quartz, feldspar, calcite, clay mineral, vernadite and todorokite, while the inner layer contains quartz, feldspar, calcite and clay minerals. The analysis of sample SP8-b2 with the same method also shows that the sample is similar in mineral composition to sample ST1. Moreover, sample SP8-b2 mainly consists of vernadite, todorokie and manganese, a conclusion obtained with EM method. According to the chemical composition of manganese nodule and its features, the colloid chemigenic and the ferromanganese matter in nodule originated from the weathered matter of magnetic rock and metamorphic rock in the land (Su and Wang, 1990), which were transported, in the forms of solution or suspended substance in the water, onto the slope near the Dongsha Islands where volcanic activities took place frequently and living organisms were abundant. In the area of growing living organisms, the ferromanganese matter was absorbed by marine sediments, resulting in the ferromanganese nodules.

Glauconite Glauconite is a kind of mineral which is distributed in the widest range. Therefore, the glauconite comprises the greatest percentage of the region, and its distribution pattern varies greatly in this region. In the shelf area, only a few samples have a relatively high content of glauconite amounting to 20%. However, in the upper slope, the average value of glauconite reaches 30%, even up to 92%. This sample with a high content of glauconite is associated with sediments containing a high percentage of bioclast. Therefore, the glauconite was inferred to be formed in two ways: biogenic and terrigenous.

Pyrite Pyrite is mainly found in the inner shelf area where the content is higher than those of outer shelf and slope areas.

MINERAL RESOURCES IN SURROUNDING AREA

The abundance of the deposition in the land area is a prerequisite for marine mineralization. The transporting distance of heavy mineral is often no more than 100 km, ranging from several kilometers to tens of kilometers. In this sense, the placers in the offshore region originated mainly from the land. Consequently, more heavy minerals precipitated in the primary rocks in the terrigenous area resulted in more corresponding heavy minerals in the sediments in the offshore area.

Regional geological data show that the repeated intrusions and eruptions of magnetic rocks originated from the multi-phase tectonic activities in southeast China Sea area and in the surrounding areas since Meso-Cenozoic. For example, granitite, granodiorite, adamellite, quartz porphyry, quartz syenite, intermediate-acid lava and pyroclastic rock occurred widely in the third, the forth and the fifth stages of Yanshan movement in these areas. The tectonic activity in Himalaya movement led to the local formation of gabbro, diabase and pyroxenolite. From Xiamen to Shantou, the magnetic rocks occupy 50% of the whole area and most of the magnetic rocks are composed of acid intrusive rocks in Yanshan movement (Fang and Jin, 1984). Abundant mineral resources have been located in this area so far, such as ilmenite, monazite and quartz sand ore deposit/spot in Zhangzhou-Dongshan ore-forming zone (Bureau of Geology and Mineral Resources of Fujian Province, 1994). In the Haifeng-Lufeng ore-forming zone, cassiterite, zircon, monazite and ilmenite are enriched, resulting in, for example, Jishuimen-Changpu mine of Sn-Pb-Zn, Niutoushan cassiterite placer and Dishui zircon placer of Lufeng. In Huilai-Shantou ore-forming zone, there are abundant zircon, monazite, ilmenite and rutile, resulting in, for examples, Tuyang ilmenite mine, Moshikeng monazite mine of Puning and Jiazi zircon mine of Lufeng. In the east land of studied area, the sedimentary area of west Taiwan, zircon, monazite and quartz sand are abundant, resulting in, for examples, Wangyegangshan, Qingshangangshan and Wangziliaoshan mines of monazite and zircon as well as Miaosulaotianliaodong quartz sand placer (Tan and Sun, 1988). All the data mentioned above indicate that the continental resources and ideal geological settings are both favorable for the marine mineralization.

MARINE SOLID MINERALS

Because the offshore is the natural extension of the land, the heavy minerals in the surface sediments in the neritic zone are similar to those in the mother rocks in the coastal sand mines and the surrounding rocks. The analyses of the types, distribution features and mineral features of the surface sediments in the offshore of this study area and of the distribution and development of the mineral resources in the surrounding area conclude that the following three kinds of solid-state mineral resources are present in the littoral and neritic zones of this study area.

Littoral/neritic placers About 25 kinds of heavy minerals are present in the littoral/neritic placers within the studied area. Such heavy minerals as ilmenite, zircon, anatase, leucoxene and rutile, locally enriched, comprise 15%-25% of the total heavy minerals. Furthermore, similar kinds of ore deposits and occurrences occur in the land. Therefore, it can be inferred that these heavy minerals might have been deposited into high-content mineral occurrences or anomalous placer mineral zones. Simultaneously, there may be some large-scale sea-beach placers and river-mouth submerged delta placers to the west of Dongsha Islands and to the east of Nanao Island as well as the Honghaiwan coastal zone. It is suggested that placer geological surveying should be carried out as soon as possible in this area.

Littoral/neritic sandy gravel According to quartz sand resources located in the surrounding area, the sediments in both the coastal area and the shelf are composed of coarse sand, medium sand and fine sand. Especially, coarse sand and medium sand are mainly distributed in and near Taiwan Bank, in the inner shelf off Dongsha Islands and Nan'ao Island as well as the upper slope of Dongsha Islands. Quartz sand, a kind of major composition of sand in the study area, amounts up to 80%, indicating that the littoral/neritic sandy gravel could be used as a good building material.

Ferromanganese nodule (crust) The ferromanganese nodule (crust) might be potentially enriched in this area. The surveying data indicated that surface sediments in the slope from Dongsha Islands to the west of 118°30′E are rich in ferromanganese nodule (crust) resources. However, the analytical data show that the contents of Fe, Mn in these samples, obtained from this surveying, are relatively lower. Therefore, further reconnaissance surveying and detailed investigation should be conducted to determine the economical value for further exploration and mining in this study area.