Metallogenic Conditions And Gold Enrichment Regularityin The Daping Gold Deposit, Yunnan Province, P.R. China

The Ailao-Shan Red-River (ASRR) structure zone which located eastern of the Tibetan plateau and western of the Yangtze Block is the most important Cenozoic metallogenic zone of mesothermal gold deposits, such as the Laowangzhai gold deposit, the Jinchang gold deposit, the Daping gold deposit and the Chang’an gold deposit, linearly located along the ASRR shear zone. Since Late Paleozoic, the western Yangtze block has undergone close of Tethys, collision of the India plate and Eurasia plate, suffered frequently and continuously volcanic magmatic activities, bringing mountains of endogenic deposits.The Daping gold deposit sits in the south of the ASRR structure zone and has the largest mesothermal gold reserves in this area; and minerogenetic conditions and mineralization enrichment rules have led to great academic interests of the geologists since 20 years ago. A lot of research workers concerned on the vague but intimate relations between local intrusions, like diorites, granites, and lamprophyres, with gold mineralization. And the characteristics, evolutions and ore-control regularities of local fault structures are the weaknesses of the former researches. The absent of mineralization enrichment research also constrain the forward exploration. This academic dissertation is based on the summarization of former research work and geology reports; field work, petrography methodology, geochemical research and mathematical geology et al. are also introduced into the research. We used major and trace elements analyze as well as Sr-Nd isotopic systems to lucubrate the geochemistry of local intrusions. Systematic fluid inclusions research and stable isotopic research of H, O, S, Pb have also been applied to research on the fluid genesis and evolution. In order to date the intrusive ages of diorite and granite, we used LA-ICP-MS zircon U-Pb dating method. We sampled two generation of lamprophyres in the Daping gold deposit, on emplaced earlier than gold mineralization and the other later. Phlogopite phenocrysts were collected to 40Ar/39Ar isotopic dating using step heating method. Thus gold mineralization age of the Daping gold deposit was constrained based on the emplaced ages of lamprophyres and the metallogenic model of the Daping gold deposit was established. We used structural mechanic analysis and structural discrimination & assemblage combined with regional structure evolution history to elaborate the three scales ore-control fault structures. Mathematic geology method was applied to research on the thickness and grade enrichment regularities in the eastern and western mineralized belts of the Daping gold deposit; enrichment centers and enrichment belts of the auriferous quartz veins VI-2-3 and V8-9-14 are also concluded during the research. We also concluded the lateral trending directions of the enrichment belts and the transporting directions of the ore-forming fluids which have positive suggestions to the further prospecting of the Daping gold deposit.This paper has these contributions as follows:1. Geochemistry characteristics of the intrusions in the Daping gold depositThe most extensive country rock of the auriferous quartz veins is diorite, which has been severely altered according to microphotography and high LOI (lost on ignition) value. Hydrothermal alteration has led to depletion of Si, Na and enrichment of K, Ca, Al et al. Related to the trace elements, the diorites are enriched in large ion lithophile elements (LILEs) such as K, Rb, Sr, Ba et al., and depleted in high field strength elements (HFSEs) like Ta, Nd and Zr; enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs), which most possibly link to the arc magmas. The variation diagram of Nb/Y vs. Ta/Yb tells us that the tectonic setting of the diorite’origin was related to volcanic arc granite.The granites in the Daping gold deposit are potassic calc-alkali peraluminous granite. Similarly, granites have arc magma related trace element characteristics, such as enrichment of LILEs and LREEs and depletion of HFSEs and HREEs. Tectonic discrimination diagram of Nb/Y vs. Rb/Y+Nb indicates the granites origin from syn-collisional or volcanic arc tectonic setting.Two generations of lamprophyres are mainly consisted of phlogopite and plagioclase (>80vol.%) with high content of Ni, Co elements and K2O (>4.39wt.%), MgO (>8.7wt.%) which related to mantle origin. Lamprophyres are enriched in LILEs and depleted in HFSEs (such as Ta, Nb) with M shape primitive mantle normalized trace element patterns. Related to rare earth elements, lamprophyres has right deviation Chondrite normalized REE patterns with highly differentiation of LREEs and HREEs. Compared with volcanic rocks of Tibetan plateau, lamprophyres in the Daping gold deposit have lower Dy/Yb values and La/Yb values, which is interpreted to origin from phlogopite-bearing spinel facies lithospheric mantle.2. Ore deposit geological characteristics and fluid inclusions of the Daping gold depositOre deposits in the Daping are lens shape thin auriferous quartz veins with commonly thickness of 0.2^0.3 m. Mineral compositions are pyrite, chalcopyrite, galena et al. sulfides and quartz as mainly non-metallic minerals. Hydrothermal alterations are mainly silicification, sericitization, albitization, chloritization, carbonization et al. Hydrothermal mineralization consists three stages, including quartz-pyrite stage (Ⅰ), quartz-metallic sulfidation stage (Ⅱ) and carbonation stage (Ⅲ).The fluid inclusions in the Daping gold deposit are mainly constituted of NaCl-H2O-CO2±CH4 systematic three phase inclusions and H2O-CO2 systematic two phase inclusions. The quartz-pyrite stage (Ⅰ) has homogeneous temperature mainly between 280℃ and 340℃ with peak value of 300℃ and salinity around 6.1wt.%NaCl eqv; and the quartz-metallic sulfidation stage (Ⅱ) has homogeneous temperature mainly between 240℃ and 280℃ with peak value of 260℃ and salinity around 3.5wt.% NaCl eqv. Gold mineralization mainly accrued during quartz- pyrite stage and quartz-metallic sulfidation stage. Usually stage Ⅱ mineralization could superimpose on stage Ⅰ. The homogeneous temperature and salinity declined during the mineralization with the fluid evolution.The gas composition in the fluid inclusions has more than 80mol% of H2O and CO2, with little N2, CO, O2, CH4 et al. composition. The liquid composition is constituted of Na+-K+-Ca2+-Cl--SO42- system with the dominatingly compositions of Na+ and Cl-. Hydrogen isotope value of δ18Dv-SMOW is between -96%o～-72%o and oxygen isotope value of δOH2O is calculated to be 0.98%o～5.83‰ from 18 Ov-SMOW value, implying hydrothermal fluids are mostly from primary magma fluid with little effect of meteoric water.3. Sources of petrogenetic and metallogenic materialsThe sulfide isotopic value of δ34Sv-CDT is between 1.5‰～10.7‰ with peak values between 2.0‰～6.0‰ according to histogram, implying a relationship with mantle origin with crustal contamination. The lead isotope values are 206Pb/204Pb ratios of 18.687～18.742,207Pb/204Pb ratios of 15.656～15.714 and 208Pb/204Pb ratios of 39.036～39.238, plotting on near the orogenic belt and the mantle evolution line from lead isotopic tectonic diagram, indicating close origin relation with crustal and mantle interaction.Lamprophyres in the Daping gold deposit have 143Nd/144Nd values of 0.512029～0.512496 and 87Sr/86Sr values of 0.706688～0.710030 while diorites have 143Nd/144Nd ratios of 0.512064-0.512598 and 87Sr/86Sr ratios of 0.705662～0.718739, indicating they were all origin from enriched mantle.Thus, the metallogenic fluids of the Daping gold deposit was interpreted origin from enriched mantle with interaction of crust and mantle. The precipitation of gold is interpreted that ore-bearing fluids suffered from cooling, depression and water-rock reaction, leading to decomposition of gold-sulfide complex.4. Timing of gold mineralization and intrusions emplacementLA-ICP-MS zircon U/Pb dating was implied to date the emplacement ages of diorite and granite in the Daping gold deposit. Results of dating are diorites emplacing around 761.6±7.4 Ma and granite emplacing around 731±18 Ma. It is related to the break of Rodinia supercontinent and the formation of Gondwanala supercontinent, which suffered a change of transtensional to trans-compressive tectonic setting which lead to frequently and continuously volcanic and magmatic activities and forming abundant of trans-lithosphere faults.Two generations of lamprophyres are dated to emplaced at 36.8±0.2 Ma and 29.6±0.2 Ma according to phlogopite phenocrysts 40Ar/9Ar step heating dating. Thus the timing of gold mineralization was well constrained.5. Ore-control structure system and evolutionThe Daping gold deposit is controlled by three scales of faults, including the ASRR fault zone, the Xiaoxinjie fault and its secondary faults. The spatial deposition, ore-body shape and gold enrichment are constrained by the three scales of faults.The major compressive force orientated northeast at about 56～44Ma as the continental collision of India and Eurasia, forming a lot of parallel northwest trending deep faults. From then on, the structural force system has changed four times.Fist stage:44～36Ma, the continental collision direction had changed from oblique forward collision to clockwise rotation, leading to Tibet arise and left-lateral slippery of the Red River. The sinistral slippery formed shear force in the Daping gold deposit leading the Xiaoxinjie fault to right-lateral slip. The dextral strike-slip of the Xiaoxinjie fault formed the NW and NNW trending compress-shear secondary faults zones.Second stage:36～21 Ma, India plate had occurred anticlockwise rotation. At the meantime, transtensional tectonics occurred attributed to slab break off and the extension of the lithosphere which facilitated upraise of the ore-bearing fluids. The ore-bearing fluids transported from the Red River fault, the Xiaoxinjie fault to secondary fault zones and metallic materials precipitated due to the water-rock reaction.Third stage:21-10 Ma, India plate had turned to fast clockwise spinning. The change of the tectonic force system led the three scales of faults occurring reverse movement. Some branch faults formed herein would break the auriferous quartz veins.Fourth, from around lOMa to nowadays, India plate has occurred anticlockwise rotation. The Red River fault experienced two stages of dextral strike movement and formed numerous denudational faults which would break the orebodies of the Daping gold deposit.6. Mineralization enrichment regularityOrebodies in the Daping gold deposit has lensed or bean shape with thickness centers or grade enriched centers. Linearly distributed mineralization enrichment centers make up of the mineralization enrichment belts. Our research demonstrated that, the major orebody of V1-2-3 in the eastern metallogenic belt of the Daping gold deposit has three mineralization enrichment belts and seven mineralization enrichment centers. The mineralization enrichments belts have southeast lateral trending with 13°～30°angles and the ore-bearing fluids transported from northwest to southeast. The orebody of V8-9-14 in the western metallogenic belt has three mineralization enrichment belt and six mineralization enrichment centers. The orientation of the enrichment belts is southeast with angles of 8°～22°. The transport direction of ore-bearing fluids in the western metallogenic belt is interpreted from southeast to northwest.The mineralization belts are constrained by the secondary faults of the Xiaoxinjie fault. As in the extensional area of the secondary faults provides enough accommodations for the ore-bearing fluids to precipitate the metallic minerals, the auriferous quartz veins are commonly seemed to thinker or lager with higher gold grade, which usually forming the mineralization enrichment centers. On the contrary, the compressive area of the secondary faults commonly provides constrict accommodation to gold mineralization and the auriferous quartz veins are commonly barren.7. Metallogenic model of the Daping gold depositSince lamprophyres and auriferous quartz veins have intimate spatial and temporal relationships and have similarly origin source characteristics, they were inferred to form from differentiation the same source magma. The later emplaced lamprophyres have closer relations with primary magma, for example, they have lower primitive mantle normalized incompatible trace element patterns and lower Chondrite normalized rare earth elements patterns, as well as more primitive Sr-Nd isotopic characteristics. It is interpreted to attribute to the intrusion of asthenosphere during the interval of the two generations lamprophyres’ emplacement, and the lamprophyres were most possibly origin from metasomatism of asthenosphere and previous enriched mantle.Cenozoic collision of India and Eurasia led to Tibet arise, Indochina block’s extrusion and the Red River’s slippery. At about 37Ma, the slab break off led to asthenosphere intrusion and the formation of transtensional tectonic setting in the ASRR structure zone. The metasomatism of asthenosphere and previous enriched mantle formed the source magma of lamprophyres and gold mineralization. The potassic magmas rise along the tans-lithosphere fault driven by the high expression, containing abundance of metallic materials and volatiles. The transport paths are inferred to be the Red River fault and the Xiaoxinjie fault. The differentiation occurred during upraise of the potassic magma, forming lamprophyres, potassic porphyries and gold-bearing quartz et al. Finally, precipitation of gold occurred when the hydrothermal fluids came to the branches of the Xiaoxinjie fault, due to the decline of temperature, compression and water-rock reaction.In conclusion, this dissertation solved several long-term academic controversial problems in the Daping gold deposit based on the metallogenic conditions and gold enrichment regularity research. We provided new insights on the metallogenic genesis, ore-control structural regularity and mineralization enrichment regularity of the Daping gold deposit, provided positive suggestions for further prospecting. This contribution could make a classical research model for the hydrothermal gold deposits in the ASRR structural zone with pronounced theoretical and practical values.