Geochemistry And Evolution Of Ore-Forming Fluids At Tongchang Porphyry Copper Deposit, Dexing County, Jiangxi Province

The Tongchang giant porphyry copper deposit is time-spatically associated with a granodiorite porphyry cupola emplaced into the Upper Proterozoic low grade metamorphosed tuffaceous phyllite in Dexing County, Jiangxi Province. Higher concentration and enrichment factors of ore-forming elements of granodiorite porphyry than those of wall-rocks, and distribution zoning of ore-forming elements all indicate that the mineralized metals of the Tongchang Cu deposit came mainly from the intermediate-acidic magma. Based on the detailed mapping of ore bodies, geochemical study of elements, alteration zoning, fluid inclusions, and H,O,Sr,Nd isotopes, this paper proposes that at least three types of hydrothermal fluids of different origins were involved in the Tongchang hydrothermal system: the magmatic fluid, the deep scated nonmagmatic fluid, and the meteroic water.During magma flowing upward, fractional crystallization and cooling, a high-temperature hydrothermal fluid of high salinity (31.0 to 63.3 wt percent NaCl equiv) and a CO₂-rich, low salinity (7.5wt percent NaCl equiv) were generated due to magmatic exsolution at 520℃ to 570℃ or higher. It was equilibrated with the residual silicate melt. At the main-stage, the high-temperature hydrothermal fluids of magmatic origin become immiscibly separated due to boiling at 360℃ to 480℃. It formed three subtypes of high- and/or middle- temperature (360℃ to 480℃) fluids: (1). fluid of high salinity(43.0 to 52.2 wt percent NaCl equiv) ; (2). fluid of low salinity (0.05 to 3.9 wt percent NaCl equiv) and vapor-rich; and (3).fluid of intermediate-, low- salinity (11.6 to 17.5 wt percent NaCl equiv) and CO₂-rich. Though not well-developed, main-stage potassic alteration, characterized by replacement of plagioclase and/or hornblende by biotite+K feldsder, formed in the lower central portion of the porphyry cupola due to prograding, saline, immiscible fluids along the axis of the cupola. These fluids were related to weak mineralization only.The sericitic-silicic-illitic alteration was caused by mixed fluids. While above-mentioned upwelling, cooling, saline immiscible fluids of magmatic origin circulated in the convective cell of the deep cupola along wall-rock contact at depth of ca. 2.5km, they mixed with the saline, nonmagmatic deep formation water (such as metamorphic and/or interlayer and/or tectonic water derived from meteoric water and/or seawater ) heated by the Tongchang porphyry body. As they upwelling, the mixed fluids might leach Cu and Fe from the deeper granodiorite porphyry body and/or the wall rock near the contact, and might have contributed centain amount of Cu, Fe in the ore zone. The temperature of mixed fluids might last from 360 °C to 260 °C or even still lower. While the mixed fluids at the point of transition from heating to cooling of nonmagmatic fluid at depth of ca. 1.6km to 1.7km (at present elevation of 130m to 50m, coincident with most intense mineralization and alteration), the bulk of the copper (and sulfides) in the ore zone was precipitated directly from the mixed hydrothermal fluids flowing vertically upward along the contact. This was the main ore forming period in the Tongchang system.At a later stage of the fluids circulation, the meteoric water was involved and consequently adjusted the physical-chemical condition (temperature, pH, Eh and chemical compositions etc.) of the main-stage ore-forming fluids in convective cell of shallow cupola. This would simultaneously favor the further process of ore mineral deposition. At the latest stage, a meteoric water dominated, low temperature and low salinty fluid (170°C to 240 "C, ca.1.6 wt percent NaCl equiv) caused widespread illitic-chloritic-carbonate alteration in the near surface condition, and superimposed on the main-stage alteration-mineralization and might redistribute copper. Some of the Cu is found in the latest quartz-sulfide and carbonate veins.Three general categories of the oxygen isotopic syetematics are observed in the Tongchang porphyry body: (1) altered granodiorite porphyries at Onv—i-80m level which exhibit close to 'normal' 18O characteristics, (2) rocks above +80m which exhibit in some case 18O-enrichment and in other case 18O-depletion ( 8 180=5.43%o 13.1%o), and (3) rocks below 0m which generally exhibit 18O-depletion ( 5 i8O=6.8%> ±O.5%o), relative to primary magmatic 8 l80=8.36%o. The complexity of 8 i?O values of rocks at higher and lower levels is a result of different alteration environments which depend on temperature, water/rock ratios and the history of above-mentioned three exchanging hydrothermal fluids.At Tongchang system, the higher copper grades (>0.25wt%Cu) in the orebody are spatially associated with intense sericitic-silicic alteration, quartz veins, hydrothermal breccia, abundant quartz-sulfide veins, high Cu/Mo ratio sulfides, lower illite crystallinity value and saline immiscible fluid inclusions along the upper granodiorite porphyry-wall rock contact and in the shallow portion of the porphyry cupola. The hydrothermal fluids intensely circulated at the middle-upper portion of the contact and at the top portions of the cupola, where occurred oxygen isotope equilibrium exchange with granodiorite porphyry and precipitation of quartz and copper sulfides due to decreased solubility with cooling and pressure reduction, where also formed hydrothermal illite of lower crystallinity value also controlled by higher water/rock ratio.During the hydrothermal fluids flowing upward and laterally, some relatively active elements (e.g., Rb, Sr ect.) and LREE have been transported as the ultimate product of the fluid-rock interaction. Extensive variation in REE patterns and contents are observed in hydrothermal altered both granodiorite porphyry and wall-rocks near contact. The redistribution of REE is mainly effected by the hydrothermal fluids with lower REE abundances and (La/Yb) n<1 controlled by Cl^, F" complexation. There is, moreover, a mutual compensation between REE loss of altered wall-rocks and enrichment of sericitized, chloritized granodiorite porphyry. A study of the Sr and Nd isotope composition of the Tongchang system was undertaken in order to assess the transport of Sr and Nd that accompanied the hydrothermal fluids. The results suggest that Sr isotope, but not Nd, is significantly mobile in hydrothermal fluids associated with mineralization in the Tongchang porphyry copper system. The 87Sr/86Sr ratios of the altered granodiorite porphyry increased regularly from 0.70508 to 0.71092 from the cupola center toward the contact with the wall-rocks, indicating that metal material migration proceeded primarily from the cupola interior toward the contact zone, which is consistent with an orthomagmatic model for porphyry copper ore genesis. On the other hand, the lack of significant Nd disturbance during hydrothermal alteration suggests that the Nd isotope compositions of the altered porphyry rocks can be used to determine the characteristics of the magma source.Through the application of the principles and methods of fractal geometry to the study of copper grade and fracture systems in the Tongchang porphyry copper orefield, it is known that two ore-control factors are of self-resemblance. It indicates that the NE- and NWW-trending fracture systems play an important controlling role in ore-forming processes, and that the higher the D value, the more favorable the conditions of Cu ore formation and the larger the deposit tonnage. Also, the distribution of copper grade in seven drill holes possesses the statistical self-similar character, and there are dual fractal dimension structures in the hanging wall. It suggests the higher the D value of the copper grades, the stronger the fluid potential, the more favorable the ore-formation. The fractal data further quantitatively indicate that the bulk of the copper in the ore zone is of orthomagmatic derivation, with merely a small portion from wall rocks and exclusively in the lower low-grade part of the hanging wall of the porphyry body.Continued studies focused on hydrothermal fluids flow paths, root zones of porphyry system, fluids interaction and mixed mechanism will undoubtedly lead to further development of this three-fluid model and will shed further light on the source and deposition of components in hydrothermal systems.