| Cenozoic is the era of strong volcanic activity in northern Hainan Island. Duowen-periodic activities of intermittent volcanic eruption have been occurring from Paleogene Period to Holocene Epoch. Volcanic rocks are widely distributed and outcropped like a shape of "7" from north to east of Hainan Island in south Qiongzhou Strait. They are mainly basic rocks, with an area of more than4000km2which is19.5%of the whole island. In regional structure, they distributed in both sides of Wangwu-Wenjiao discordogenic fault, and mainly in the northern side.Northern Hainan island, locating at about20degree north latitude, belongs to the area of tropical oceanic monsoon climate. In this region, elements such as magnesium and silicon etc. are leached for strong weathering, and a mass of modern volcanic rock weathering crust composed of gibbsite, esmeraldaite, goethite, kaolinite, illite, montmorillonite, chlorite, etc. are formed. Some sectors of this region formed commercial valuable deposits such as bauxite, limonite, etc. According to related statistic data, the explored reserves of gibbsite-type bauxite, limonite, and cobalt formed in volcanic rock region in Hainan Island are25million, a hundred million, and more than ten thousand tons, respectively. Meanwhile, many valuable elements such as nickel, gallium, chromium, cooper, etc. are associated in these deposits. These mineral resources formed of weathered and leached volcanic rocks in Cenozoic, are typical and still forming. And the ore-forming process is clearer. Thus, they are natural laboratory of studying on weathering mineralization. However, researches and findings in the past mainly focused on the regional geology, environmental geology, regional mineral (mainly on combination gas), and engineering geology. Research on regional volcanic rock weathering mineralization is really rare, and some key problems are still unresolved.Previous studies put much empHasis on the region geology, environmental geology, regional resource (most on oil and gas) and engineering geology. However, less attention has been paid to the regional volcanic weathered mineralization and some critical issues remain unresolved. In this paper, we carry out a detailed study of the Cenozoic volcanic rocks (e.g. field geology, mineral composition, geochemistry and tectonic setting) and ore deposit related to their weathered crust (e.g. the profile characteristics, mineral and chemical composition and controlling factors of the weathering crust, and the geological, geochemical characteristic, and formation process of the weathered mineral). We also discuss the mechanism for element enrichment during the weathering, the main factors that control the formation of the weathered mineral, and establish a new model for the research and exploration of the weathering ore deposit.1Cenozoic volcanic activity in northern Hainan Island was most intense, and eleven stages have been recognized. Paleogene and Neogene volcanic rocks except for Shimacun Formation were buried underground, while Quaternary ones except for Pleistocene were outcropped.Regional volcanic rocks were weathering under the condition of tropical monsoon climate. The weathering crust generally shows an alternate distribution of thick (the older) to thin (the younger) from east to west. The thickness of weathering crusts affected by time has positive correlation with the age of volcanic rocks. In addition, results show that the weathering crust is producing formation of weathering deposits such as limonite, bauxite and asbolite, etc.2Based on the ore-forming element association, mineralization mother rock, microtopograpHy and microrelief etc., together with the spatial distribution of deposits (ore occurrence), the regional weathering deposits were divided into two metallogenic belts. Boundary between them is Wangwu-Wenjiao fault. The north one was represented by limonite composed of weathered volcanic rocks from Duowen Formation. And the south one was represented by aluminium, cobalt and chromium deposits composed of tertiary rocks from Shimacun Formation to Shimengou Formaion among Penglai, Juding and Nanyang.Limonite distribute mainly in volcanic rock weathering crust from Duowen Formation in the north metallogenic belt. And from Lingao to Chengmai it has a large scale. Ore bodies are affected by landform. Thus, on mountaintop and at hillside they are thicker, while the submontane and low-lying ones gradually thin out. Ore bodies are mainly constituted by ledges of pisolite limonite and massiveness limonite. Concentration of total iron is usually30%or so, the highest being57%.Bauxite distribute mainly in volcanic rock weathering crust from Shimacun Formation to Shimengou Formaion in the south metallogenic belt in Penglai of Wenchang. The ore bodies affected by landform shape like duplex dishes. Thicknesses of ledges covered with1to2m laterite layer are usually30to60cm, the thickest being4to5m. Gibbsite are the main ore, with a mineralized rate of400kg/m3or so and the highest being more than1000kg/m3.Asbolite usually associated with limonite and bauxite, distribute in volcanic rock weathering crust from Shimacun Formation to Shimengou Formaion in the south metallogenic belt in Penglai of Wenchang, and mainly in Anding and Wenchang. The ore bodies consist of bauxite, limonite and asbolite are affected by landform. Note that the asbolite locates at bearing bed below. The mineralized rate varies greatly from5to15kg/m3, with a minimum value of1kg/m3and a maximum value of124kg/m. In addition, the asbolite with shapes like flaky, pisolite and coralloid, colloid, concretion form are usually0.5to3cm thick. Its major mineral component is allopHytin. And concentration of cobalt is3.13to8.45%.3The chemical compositions of the samples of the each layers of weathering profile (such as limonite, bauxite, earthy cobalt) mainly consist of totally72.18%-84.58%SiO2, Al2O3and Fe2O3. They contain SiO2=15.46%-50.46%, Al2O3=7.82%-36.56%and Fe2O3=7.82%-36.56%, with average contents of30.45%,19.94%and28.33%, respectively. The average content (28.33%) of the Fe2O3is much higher than the content (5%) of the upper crust, suggesting that laterization in the tropic area is intense. In contrast, some soluble components (such as CaO、Na2O、K2O and MgO), with a low content, all have contents of less than1%. This Fe-Al-enriched, associated with strong chemical weathering, is responded for the climatic conditions of the tropical regions. The geochemical factors of affect the process of rock weathering mainly include pHysical and chemical properties of the elements, temperature and water, PH value and oxidation-reduction potential (Eh value).Ferrous iron ion released by weathering of volcanic rocks will transform into ferric iron ion in the earth’s surface oxidation environment. When the pH value is less than three, the ferric iron ion will be traps in the mineralization of hydroxide, which finally switch into goethite and limonite. Silicate minerals (e.g. feldspar, pyroxene, ampHibole and micas) will be changed into clay minerals (e.g. illite, kaolinite, and montmorillonite) through weathering. In the damp and torrid tropics, these clay minerals will be further decomposed into silicon and aluminum. Silicon is washed away by underground fluids, and aluminum ions are trapped in the aluminum hydroxide when the pH values are5-9, forming gibbsite-type bauxite deposits. Bivalent manganese commonly exists in the rock-forming minerals. When they are weathered, bivalent manganese is released from the source and carried by the fluid. Manganese deposits are controlled by pH and eh values. For pH value less than seven, manganese ionic compound is carried in the fluid, and it will be leached for pH value more than eight. In the sedimentation, separation of iron and manganese is controlled by pH and eh values. Valence bond force between manganese and oxide is weaker than the one between iron and oxide. Hence, bivalent manganese ion content is more than ferrous iron ion content in the carrying fluid which makes them separate during hydrolysis. Therefore, in the red bed weathering shell, iron oxide is on the top of manganese oxide. Manganese oxide usually lye on the surface of the rocks or fill in the cracks, and they will catch the polymer of copper, cobalt, nickel, and zinc. In the top of the weathering shell, pH value is lower and eh value is higher. Ferrous iron ion is usually transformed into ferric iron ion and deposited in this oxidation environment. These minerals further form goethite and hematite through dehydration, forming iron cap on the top of the weathering shell. In contrast, reduced facies of cobalt, nickel, copper, and manganese continue to migrate downward and finally deposit, forming the vertical elements zonality.4REE patterns of weathering profiles show that distribution patterns of surface laterite, granular limonite layer with beans, bauxite layer, cobalt-containing soil seam, bauxite, aluminum soil sapropel layer are consistent, Only differences in REE contents, Are rightist, rich in light rare earth Substantially similar distribution patterns in the region of lavaLight and heavy rare earth differentiation gradually increase in the profile in the upper part of the surface laterite ore beds, display LREE enrichment greater than the heavy rare earth, higher degree of differentiation between the light rare earth elements and Low level of differentiation between the heavy rare earth elements.the degree of differentiation between Light and heavy rare earthis weaken in the the white corruption clay layer, half weathered layers, Differentiation between the inside of the light rare earths is weakened or even no differentiation while Different degree of heavy rare earth is significantly enhanced. Heavy rare earth elements in the weathering process is easy and light rare earth leaching is easier enrichment.δCe performance trend was first increased and then decreased from the surface to the deep, δCe performance trend was first increased and then decreased from the surface to the deep,δEu is a weak negative anomaly or no anomaly in the weathering profile, Description that overall environmental performance of profile is oxidizing environment. The performance of the degree of oxidation gradually increased from the surface to the middle of the profile, and then decreased to the deep changes in trends.Above characteristics of rare earth elements indicate that Substances ingredients of the profile layers have certain amount of inheritance from top to bottom and Has a genetic link between them, Are the bedrock by the product formed by the weathering process at different spatial pHase.Penglai bauxite, in the1meter whole weathering mud at the bottom of the weathering profile, the total REE is2306.08ppm, reaching the weathering type REE content of industrial grade and considered to be a bauxite deposit. This bauxite mining area may be a large REE metallogenic prospective areas, which needs more attention in the following works.5Different combinations of ore-forming elements. The main ore-forming elements are iron associated with a small amount of cobalt in the North belt, Lingao-Chengmai limonite is a typical representative of the deposits of the northern ore zone. The main ore-forming elements of the South ore zone is aluminum and cobalt, the representative ore deposits are Penglai bauxite and juding cobalt soil mineral. The main factors that control differences in North and South deposit have mineralization host rock, microtopograpHic landforms, groundwater and precipitation. Mineralization parent rock lithology and era is different. Mineralization main parent rock in the North belt is Quaternary volcanic rocks of multicultural group, outh ore zone mineralization parent rock the Neogene system Shima gumi-the Shimengou group of volcanic rock. The natural and geograpHical environment between The north-south belt is not the same. monite metallogenic belt are mainly distributed in the volcanic rock mesa peneplain terrain, Altitude between30m to80m, the relative gap is small, less affected by the typHoon, with little precipitation and the distribution is more concentrated. Southern belt aluminum, cobalt polymetallic ore belt formed in a volcanic basin topograpHy, the terrain drop relatively large, affected by typHoons more and more precipitation.6The weathering firstly started by the disintegration of the rock-forming minerals, such as feldspar and pyroxene minerals in the basalt, according to the principle of the Bowen reaction series. Then, the desilication of matrix material began. Thirdly, the leaching, separation, migration and deposition process of Fe2O3, Al2O3and SiO2, retained residual iron (e.g. ferric oxide hydrate) and aluminum (e.g. Gibbsite) in the eluvial weathered crust and resulted in the formation of the ferrallitic weathering crust (or Limonite and bauxite).The different layers of weathering crust contain different material composition, and show a random distribution of the gibbsite and goethite. Gibbsite could appear in the topsoil, binding layer, sapropel layer and semi-weathered layer. Goethite occurred mainly in the upper part, and usually above the bauxite. Kaolinites occurred in all layers and occupy a higher proportion, together with larger number of chlorite and traversite, suggest that weathering crust is not yet mature. The appearance of halloysite (kaolinite), nerchinskite and allopHone indicate that the weathering is still in process and not reach the mature weathering crust that manly including gibbsite.In the weathering process, the geochemical behavior of elements is critically depended on the ionic radii, valence state and isomorpHism. |
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