Genesis Of Metabasites And Their Relationship With Gold Mineralization In The Baolun Gold Deposit, Hainan Province

The Baolun gold ore deposit, located between the E-trending Jianfeng-Diaoluo fault and Jiusuo-lingshui fault and on the western side of NE-trending Baisha fault, is the largest, high grade gold deposit in Hainan province. This deposit has been considered to be a typical quartz vein deposit-type, orebodies of which are strictly controlled commonly by the Haogangling anticline and NNW-trending fracture zones developed in the crests of a series of secondary anticlines. In the research of Baolun deposit, gold veins were observed to be spatially associated with considerable metabasites in the mining district. To determine the possible provenance and petrogenesis, and associated tectonic environment, and thereby to evaluate the possible contribution to the Baolun gold mineralization, study of petrography, genetic mineralogy, bulk rock geochemistry, Sm-Nd isotope, U-Pb SHRIMP zircon age and bulk rock Au abundance analysis on these metabasites have been carried out based on previous study and field investigation. Some conclusions are made as follows:(1) The metabasites were divided into two groups based on occurrences and geochemical characteristics. As to rock occurrences, Group One are fine vines, mixed with wall rocks and quartz veins, with quartz lenses and wall rock lenses observed locally, while veins of Group Two are broader, more stretching and little contaminated. With respect to geochemical characteristics, REE content of Group One is higher, about 20~80 times the content of chonchrite, and shows strong enrichment of LREE and negative Eu anomalies(except BL-065 and 13BL-87 with slight positive Eu anomalies). Th, Ta, Nb and Ce are strongly enriched relative to N-MORB and show uplifts in the middle of the spider diagram. The εNd(230Ma) values mostly range from-6.62 to-4.45. REE concentration of Group Two is lower, about 14~40 times the content of the chondrite, and characterized by LREE depletion(except 13BL-44) and positive Eu anomalies(except 13BL-71 with a slight negative Eu anomaly). Content of Sr, K, Rb, Ba, and Th is enriched relative to N-MORB and shows a high LIL/HFS ratio. The εNd(230Ma) values range from +0.33 to +5.65. Both the two groups of the metabasites are mainly composed of silicate minerals such as plagioclase and amphibole, and metallic minerals like magnetite, pyrite and chalcopyrite, with Group One having more quartz and carbonate, as well as a few more sphalerite, while Group Two exceptionally containing residual pyroxene and alteration minerals such aschlorite, epidote, zoisite, sericite, biotite and carbonate etc..(2) It is indicated by the geochemical characteristics of the metabasites that magma sources of Group One are contaminated by enriched mantle mainly, as well as crustal to some extent but hardly any fluids from subduction zone, while magma sources of Group Two most probably are similar to those of island arcs which are affected by fluids from subduction zone, showing some contamination by crustal but hardly any contamination by enriched mantle. However, magma sources of both groups are mainly depleted mantle from a shallow source where garnets are not stable. Fractional crystallization is the main magmatic process for Group One, with a fractional mineral assemblage of clinopyroxene+ plagioclase±olivine, while partial melting is likely the main magmatic process for Group Two.(3) Group Two has been dated 231.6±2.6Ma through SHRIMP U-Pb zircon method, which is very close to the crystallization age of Jianfeng granite(208~249Ma) and metallogenic age of Baolun gold deposit(224.6±7.2Ma). What’s more, these metabasites is geographically near Jianfeng granite. Based on the geochemical characteristics of the metabasites and previous studies about tectonic setting of Jianfeng granite, we thus suggest that the studied metabasites should have been developed in a post-collisional, extensional tectonic setting. According to the geochemical characteristics and the emplacement age of the metabasites, as well as the previous studies of tectonic setting of South China, the Paleo-Tethys ocean crust subducted from North to South, and the post-collision stage of South China began in ca. 208~249Ma.(4) Au abundance analysis of the metabasites indicate that Au content of a few samples in Group Two is obviously higher than the Clark value and shows a high dispersion, which suggests that Au in metabasites might have been activated and migrated in ore-forming process. Geochemical data also indicate that magma of Group Two could be a good carrier for Au. Nonetheless, diagenetic age of Group Two is consistent with the age of metallogenic age of Baolun deposit within analytical error. Therefore, Group Two probably provided Au for the Baolun metallogeny.(5) A model for the petrogenesis of the metabasites and gold mineralization is proposed to best explain the phenomena in this paper based on the analysis above. Firstly, the two groups of metabasites developed in sequence under post-collisional tectonic setting and the group developed later carried Au from the deep of the earth to the shallow. After that, the basic magmatic hydrothermal fluids carring Au weremixed with acid magmatic hydrothermal fluids from Jianfeng granite and migrated on along faults and fissures. It was not until the carbonaceous phyllite of Tuolie Formation and meteoric water changed the chemical properties of the hydrothermal fluids that Au was enriched and mineralized.(6) This paper shows that the Baolun gold ore deposit formed in the post-collision stage, when the stress mechanism was transformed from compression to extention. The Jianfeng-Diaoluo Fault nearby should be a deep fault cutting through the lithosphere. Combining with the previous research, Baolun gold deposit should be classified in to an orogenic Au deposit-type.