Mafic-Ultramafic Rocks, Metallogenetic Series And Prospecting Targeting In The Jinping-Song Da Rift

The paper has been focused on the genetic relation between the mafic-ultramafic magmatism in the Jinping-Song Da rift and the basaltic magmatism of Emeishan Large Igneous Province from the viewpoints of the geology of the rift, petrology, petrochemistry, REE and trace elements geochemistry, elements isotope and PGE geochemistry of basalts(kamatiite-basalts associations) and mafic-ultramafic intrusions, and whereas more study works have been concentrated on the characteristics of magmatic Cu-Ni(PGE) sulfide deposits and its genetic evolution relation with the ELIP magmatism and its hydrothermaUy modified features during post-Permian tectonothermal events based on field observation, the discussions over the ore features and ore deposit geochemistry(REE, trace elements and PGE geochemistry) and Ar-Ar dating of ore. The metallogenic model of magmatic liquation and differentiation and hydrothermally modification and comprehensive exploration model based on the advanced studies over the typical magmatic Cu-Ni(PGE) deposit of the Baimazhai and Ban Phuc deposits, in the Jinping-Song Da rift have been summarized in the paper, and mineralization Target areas have also been put forward for the magmatic Cu-Ni(PGE) sulfide deposits.Magmatic ore deposits are of two types: 1) Oxide ore deposits, which are the Cr, V, Ti and Fe deposits,genetically related to mafic-ultramafic(mainly layered) complex, such as the Panzhihua V-Ti magnetite deposit; 2) Sulfide ore deposits, which are the Ni, Cu, Co and PGE and associated Au and Ag deposits genetically related mantle-plume sourced mafic-ultramafic intrusions swarms. Magmatic sulfide Ni-Cu(PGE) deposits are generally situated in the stable platform and shield area, or in the rift in active margin of Craton area, and generally has older mineralization age. The Jinping-Song Da rift is actually south part of the Yanyuan—Lijiang—Dali—Jinping—Song Da Paleozoic rift as whole. The magmatic sulfide Ni-Cu(PGE) deposits, i.e. the Baimazhai and Ban Phuc deposit in the Jinping—Song Da rift are the typical magmatic sulfide ore deposit hosted by the Phanerozoic mafic-ultramafic intrusions rocks which genetically related to the magmatic liquation and differentiation in the mantle plume acting area.Jinping-Song Da area is a continental marginal rift which developed during the late Paleozoic era-early Triassic period and located in the junction between the SW margin of Yangtze craton and the east India-China plate(Fig.3-3). Spatially, the rift extends northwestward along SW side of the Ailaoshan metamorphic belts which situated in the SW margin of the Yangtze craton, it meets the Dali—Lijiang taphrogenic trough which is also located in the west margin of Yangtze craton, the Ailaoshan—Tentiaohe—Da Ma(Vietnam) suture zone forms its west boundary. Temporally, the rift started to be opened in the early Ordovician period(or late Cambrian period) and ended in the Triassic period. In its early stage of evolution, it exhibits the property of the passive continental margin with weak magmatism; in its middle evolution stage, it displays the features of the active continental margin with strong magmatism, especially it featured by the Emeishan basaltic eruption of the late Permian age and associated mafic-ultramafic intrusions. In the late stage of rift evolution, it is a passively sedimentary basin with continental source materials deposition and being of very weak magmatic activities only with thin rhyolite porphyry. Metallogenically, the mineral deposits in the rift can be categorized as 4 metallogenic series groups and 8 metallogenic series and 17 types of deposits(Table4-1).The late Permian basalts in the Jinping-Song Da rift present the thickest lava of up to 4536m in the Dalaotang profile, SE of the Jinping town. The rocks are dominant by lave, and the volcanic sequence can be divided into two erupting cycles with each begins with volcanic breccia, porphyric and amygdaloidal basalts, via massive basalt and trachybasalt, ended in basaltic tuff. The mafic and ultramafic intrusions occur in swarms and clusters or in belts as the Xinanli-Yinpanjie-Baimazhai intrusions belt and the Jiangjiaping-Niulanchong intrusions belt. To Vietnam in the south, the late Permian basalts mainly distributed in the fringe area of the Song Da rift, whereas the younger mafic-ultamafic intrusions with high MgO of the late Permian to the early Triassic age are distributed in the axis area of the rift.Both the Permian basalts in Jinping and in Song Da area in Vietnam share some petrochemical similarity and also some difference. Both have the petrochemical characteristics of the low Ti, low alkine, and content of Na₂O>K₂O, and both are belonging to the petrological series of sub-alkine, tholeiitic basalts. But in Song Da area, Vietnam, basalt(komatiite) exhibits extremely lower contents of TiO₂, Na₂O, K₂O and lower P₂O₅ and Al₂O₃ and higher MgO content and higher Mg# ratios((Mg#=82.22-92.58). The Mg# ratios of basalts in Jinping area is at the range from 45.33 to 65.85, which is more close to that of Emeishan basalts. In general, Basalts in Jinping area is similar to that in Panxi rift of the ELIP in petrology and petrochemical characteristics (low MgO and low TiO₂ content), while basalt in Song Da area, south section of the Jinping-Song Da rift, is similar in its high MgO and low TiO₂ content with that in the Yanyuan—Lijiang are of the ELIP, suggesting the similar genetic tectonic context of the basalts in the Jinping-Song Da rift and the basalts in the remaining area of ELIP. Basalts in Jinping section of the Jinping-Song Da rift exhibits a general similar variation trend in the primitive mantle normalized distribution patterns of trace elements with that of the ELIP basalts as whole(Fig.3-12 and Fig.3-13), both display a right-dipping type of the primitive mantle normalized pattern of the trace elements. Whereas the ELIP basalt have higher contents of HFSE elements such as Nb,Ta,Ti,Zr, Hf,Th etc. and higher contents of strongly incompatible elements, which is the products of partly melting from enriched mantle. But the basalts in the Jinping-Song Da rift show high Th content and low Nb, Ta ant Ti contents, and furthermore basalts from Jinping area and Song Da area share exact same distribution pattern of primitive mantle normalized trace elements, and both present extremely low content of Rb and strongly depleted P content and slightly enriched Hf contents over the primitive mantle value, although they have different distribution of primitive normalized REE elements(Fig.3-12), which are different with elements contents characteristics of the extremely low Rb content and high P and Ti contents of the ELIP basalt as whole showing weak contamination with substances from crust(Fig.3-13), while the features of trace elements disclose that basalts in Jinping-Song Da rift have contamination with materials from crust to some extent.The REE value characteristics of basalts in Jinping area are as follows:∑REE=102.48-288.15ppm;∑Ce/∑Y=1.23-4.05; (La/Yb)_N=3.03-13.4; (La/Lu)_N=2.95-12.93;δEu=0.94-1.1; which show the variable value of REE with LREE enriched and without apparent Eu anomaly, the primitive mantle normalized distribution pattern is typical right-dipping LREE enriched model.(Upper diagram of Fig. 3-14).The REE value characteristics of basalt-kamatiite series in Song Da area are as follows:∑REE=24.47-45.16ppm;∑Ce/∑Y=0.34-0.84; (La/Yb)_N=0.34-1.87; (La/Lu)_N=0.33-2.01, commonly lower than 1;δEu=0.75-1.52, mostly lower than 1, which show the extremely low REE values with badly differentiated REE and slightly enriched HREE values with slightly positive Eu anomaly, there no significant differentiation of the REE from the primitive mantle normalized distribution pattern (Lower diagram of Fig. 3-14).Except for slightly higher ratio of ²⁰⁸Pb/²⁰⁴pb, the range of the Pb isotope value of basalts in Jinping area is generally similar with that Red Sea area, Yongsheng and Binchuan area of the ELIP, which are of mantle plume genesis. But the isotope values of Pb of basalts in Jinping area are at odds with that of the Common Pb Isotopic Value (C composition), with only small number of samples being close to the C composition in Pb isotope value, which show that the basalts in Jinping area suffer contamination by materials from crust. The initial ratio of ⁸⁷Sr/⁸⁶Sr of basalts in the Jinping-Song Da rift (Table 3-7) indicate that it shares the similarity with that of the basalts in the main ELIP basalts and in Red Sea, Siberian and Hawaii which are genetically related to mantle plumes, suggestive of the similarity in the magma sources. From the diagram of ¹⁴³Nd/¹⁴⁴Nd versus ⁸⁷Sr/⁸⁶Sr(Fig. 3-15), it is conclude that basalts in Jinping, Yongsheng area and Binchuan area are genetically similar with each other, the primary magma of basalts may come from enriched mantle and experience contamination by substances from crust.The ratios of ¹⁴³Nd/¹⁴⁴Nd of the basalts in Jinping area range from 0.512269-0.512725, suggesting the crust source or contaminated crust source of the magma, while the ratios of ¹⁴³Nd/¹⁴⁴Nd of basalts in the Song Da area range from 0.512158—0.513184, indicating the magma originates mainly from mantle. The ^εNd values of basalt-kamatiite association are mostly more than O, and in big value, except for minor sample, it suggest that the magma come from strongly deplete mantle origin, while for the ELIP basalts, most of the ^εNd value are less than O, with minor more than O, indicating the magma origin being from mantle, but suffer contamination and assimilation to a great extent, and experience strong metasomatic action between mantle and crust. Almost all of ^εNd values of basalts in Jinping area are minus value excepting individual sample, suggestive of enriched mantle origin. The ratios of ¹⁴³Nd/¹⁴⁴Nd of basalts in Ban Phuc area, Vietnam, are commonly higher than that of basalts in Jinping, Yongsheng and Binchuan area, and moreover the ^εNd of the basalts in Ban Phuc area are in big positive value, it is suggested that the magma of basalts in Ban Phuc area is come from depleted mantle source with little contamination by crust materials.The typical ratios of the HFSE elements of basalts in Jinping-Song Da rift and ELIP area indicate that all the average ratios ofω(Ta)/ω(Hf)(Table 3-9) are bigger than 0.3, and ratios ofω(Nb)/ω(Zr) are bigger than 0.1, except for minor picrite sample, which are in accordance with that of mantle-plume originated basalts. From the west to the east in the ELIP, the ratios ofω(Ta)/ω(Hf) of basalts showing gradually decreasing trend, suggesting that the basalts in west ELIP have more geochemical features of mantle plume originated magma than that in the east ELIEComparatively, basalts in the Jinping-Song Da rift are more rich in Th, and relatively deplete in Ta, higher ratios ofω(Th)/ω(Ta)(Table3-9), lower ratios of (Ta)/ω(Hf) than that of basalts in ELIP or other mantle plume-related basalts, excepting that they all have similarity of ratios ofω(Nb)/ω(Zr). All types of ratios of basalts in the Jinping-Song Da rift are in line with that of basalts in intraplate extension area or initial rifting area, suggestive of richer Th content in magma source and suffering more crustal contamination than that of ELIP and mantle plume related basalts. From the ratios ofω(Th)/ω(Ta), it is indicated that the major part of the ELIP basalts originated mainly from primitive mantle with low grade of part melting and weak crustal contamination, while the origin magma of basalts in the Jinping-Song Da rift, which is spatially far away from the majority of the ELIP, experienced strong crustal contamination.The ratios of Nb/Zr and plots of Zr vs Zr/Y (Fig.3-16)show that the basalts in Jinping area in the Jinping-Song Da rift is originated in intraplate tectonic environment, and the basalts-komatiite association is mainly generated in mid-ocean ridge tectonic, with minor being in island arc, and these show that the two magma activity centers have different depth of rifting, The depth of rifting in the Jinping-Song Da rift probably reached to the shallow trough basin facies for Jinping section, and deep trough for the Song Da section, but the latter has not been developed to a stage forming the oceanic crust. The correlation diagrams of Sm/Eu vs Sr(Rb) (Fig.3-20c, d) show that the basalts in Jinping area is evolved mainly by the crystalline separation of pyroxene and olivine, which correspondent to that the phenocrystal minerals being mainly the pyroxene and olivine in basalts, but the basalts have a lower degree of crystalline separation, the variable Sm/Eu ratios reflects the crustal contamination to a certain extent.Differentiation degree of the PGE elements of the basalts in Jinping area is low and the its primitive mantle normalized patterns is of not-so-apparent Pt-Pd type, with the transition features of Ru-Os type and Pt-Pd type, steep left-dipping primitive mantle normalized distribution model with big positive slope.There are six categories of mineralized mafic-ultramafic intrusions in the Jinping-Song Da rift, all exhibit the characteristics of sub-alkaline, and fall into the types of low-Ti tholeiitic series and ferric mafic-ultramafic series. The Permian basalts and mafic-ultamafic intrusions and associated sulfide ores share geochemistry of REE and trace elements and PGE i.e. they all have the similar primitive mantle normalized REE distribution patterns(right-dipping light REE enriched distribution patterns), and PGE contents and its primitive mantle normalized distribution patterns(tower abundance value, bigger Pd/Ir ratios and left-dipping Pt-Pd enriched distribution patterns),and they all have the same primitive mantle normalized distribution spider diagrams and same elements abundance, which show there is an inevitable genetic relation between them in both the temporal and spatial evolution terms.There are also same REE and trace elements variations characteristics between the mafic and ultamafic intrusions distributed in the axis of the Song Da rift and the picrite-basalts association in the peripheral area, i.e. they all contain higher contents of Mg, Ni, Co, Cr, Yb, Lu and lower contents of Ti,Fe,Ca,Na,K,Rb,Sr,V,Nb,Ta,Zr and LREE.The magmatic Cu-Ni(PGE) sulfide metallogenic belt in the Jinping-Song Da rift is located in the SW margin of the ELIP metallogenic province which situated in the west margin of the Yangtze craton. The mineralized intrusions is generally emplaced in the same age as that of the intrusions of similar deposit in the adjacent area(Table 4-30), with the ore-bearing intrusions being the peridotite-pyroxenite associations of the ferric ultamafic rock series, and most of the intrusions-hosting country rocks contains carbonaceous shale with large number of the sedimentary pyrite. The size of mineralized intrusion is at small scale, and the better the differentiation of the intrusions, the better the mineralization of the intrusions. The Baimazhai and Ban Phuc Ni-Cu(PGE) deposits, which are the typical magmatic Ni-Cu(PGE) deposit in the Jinping-Song Da rift, share similar basic geological characteristics. Their rnineralised intrusions are all the differentiated ultamafic rocks, the sulfide ores are mainly located in the bottom or the center of the intrusions, the ore types are mainly the disseminated and massive ore, both the two typical deposits are of the same mineralogy as the pyrrhotite, pyrite, pentlandite and chalcopyrite, what is different is that the ores in Ban Phuc deposit contains more metal arsenide, telluride, antimonide and PGE minerals than that in the Baimazhai deposit while they are very rare.The distinguish features of the Baimazhai Ni-Cu sulfide deposit in Jinping, lies in that it is extremely rich in Cu and Ni(the massive ore-body No.1 take up 67.8% of the total Ni reserve with average grade of 3.68% of nickel and 1.99% of Cu) and relatively poor in PGE, with the massive ore being located in the center of the extremely well-differentiated and mineralized intrusion, all types of ores containing very low PGE elements(the average Pt+Pd contents of different ores range from 0.02 to 0.15g/t) and also the associated gold and silver are also very low (Table 4-32).As for the Ban Phuc Ni-Cu(PGE) sulfide deposit in the Song Da rift, it has similar basic geological features, ores associations and natural ore types with other deposits in the rift, but there is no bottom massive ores because of low grade of differentiation, the ore types are mainly the disseminated ore and massive ores in the veinlet ore body filled along the contact fracture zone. The sulfur isotope composition of the sulfides in the ores is close to that of the chondrite, there is not much sulfur mixture from country rocks during the ore-forming, the sulfur is mainly originated from the komatiite magma itself.The total average sulfur content in Baimazhai intrusions is 11.114%, which far higher than that of the global average ultramafic rocks(with sulfur content is 2850ppm). And the sulfur contents increasing greatly in the order from gabbro to peridotite, which shows the sulfur enriched features of the magma in later stage. Except for the intrusions in the Baimazhai ore deposit, the sulfur contents(0.285%) of other intrusions and basalts located in the Jinping rift are also higher than that of the global ultramafic rocks, which suggestive of sulfur enrichment in the primitive magma indicated by the mafic-ultamafic intrusions in the Jinping area. And the sulfur isotope value of ores in Baimazhai Ni deposit ranges from 6.68-7.59‰which is similar with that of the pyroxenite, basalts and the sedimentary shale in the Jinping area in terms of the positive sulfur isotope composition, and it suggesting they have the same sulfur source and the sedimentary shale has supply some sulfur source for the mineralization. And the Sr isotope composition also shows the contamination materials from the crust(including the sulfur).The chemical compositions of ores from both the Baimazhai and Ban Phuc Cu-Ni deposit are of similar basic geological features(Table4-32), showing that is enrich in Cu and Nickel while poor in PGE and associated gold and silver, with higher Ni:Cu:Co ratio and extremely low contents of Pt+Pd value, and low Pt/(Pt+Pd) ratios(＜0.5), it does not exhibit apparent enrichment of Pt. By comparison, the enrichment of Pt and Pd in the ores in the Jinbaoshan deposit is higher but with extremely low contents of Cu and Ni, while it shows Cu-Ni and Pt-Pd enrichments to some extent in the ores in the Yanliuping Cu-Ni sulfide deposit.Plots of chondrite-normalised REE(Fig.4-32A and B) shows the Baimazhai ores are slightly LREE enriched, with weak negative Eu anomalies. And the different types of ores and weakly mineralized gabbro in the Baimazhai deposit exhibit a similar primitive mantle normalized REE plots with that of the Kalangtoke and the Yangliuyping ores, the REE contents in Baimazhai deposit is 10-50 times chondrite values. But the total REE value of the ores in Kalangtoke and Yangliuping are higher than that of the massive ores(Fig.4-32A). The peddotite in Baimazhai intrusion is relatively LREE-enriched. And what it is important that The Yangliuping basalts, Hongge intrusions and Jinping basalts all show similar REE patterns(Fig. 4-32B).The Sr and Nd isotope compositions of the mafic-ultamafic intrusions in Baimazhai deposit indicate that the origin magma is contaminated with "materials from crust, while the Nd isotope composition of mafic-ultamafic intrusions in Ban Phuc deposit in the Song Da rift show that it was only slightly contaminated by crust matters.There are some apparent differences between the parent magmas of the mineralized mafic-ultamafic intrusions in the North(Jinping) section and south(Song Da) section, but intrusions and eruption in each area share genetic correlation with each other exhibited by the REE, trace elements and elements isotope geochemistry. There are two types of rock associations in the Jinping rift, i.e. the peridotite-pyroxenite-gabbro(Baimazhai Cu-Ni deposit and Yinpingjie Cu-Ni(PGE) deposit) and the pyroxenite-gabbro-diabase(Niulanchong and Jiangjiaping), all these rocks are the tholeiitic series, while in the south section of the Song Da rift, the mineralized intrusions in Ban Phuc nickel deposit is the dunite-peridotite association, and the erupted rocks is the basalt-komatiite association, which belong to the komatiitic-tholeiitic series(please refer to Chapter 3 for the details).The magmatic Cu-Ni(PGE) sulfide deposit in the Jinping-Song Da rift are formed in the late Paleozoic extension basin, ore-bearing mafic-ultramafic rocks occurs spatially in belts and swarms along structure zone, with ore deposits(occurrences) being located repeatedly in single deposits in different sections of the Jinping-Song Da rift. The single intrusion is composed of different rock facies with clear differentiation, the ore-bearing intrusions in Baimazhai deposit are the typical representatives among these intrusions, in which, there are at least five kinds of differentiated magma and ore magma decided by the spatial distribution of the different rock intrusions, i.e. the early gabbro magma, middle pyroxenite magma, middle-later olivine pyroxenite magma and pyroxene peridotite magma and the final sulfide highly enriched ore magma, there are other same types of intrusions with mineralization to different extent occurs in the surrounding area of the Baimazhai deposit, in which the mineralization is the results of part of the ore-bearing magma evolved from the unified ore magma represented by the ore-bearing intrusions in the Baimazhai deposit, the size of Cu-Ni mineralization depends on the amounts of the ore-magma evolved from the unified magma chamber. While in the Song Da area, the differentiated level of the intrusions are not so good by the known outcrops of the ultramafic rocks. During the formations of ore-enriched magma chamber evolved from the original magma source, several sub-sized magma chamber may be formed by the differentiations and liquation of the original magma due to regional tectonic events, and some sub-sized magma chamber may only inherit part of the magma from the middle-upper part of the parent magma chamber with low level of mineralization, which lead to the difference of the differentiation and mineralization of the intrusions. The comprehensive metallogenic model are showed in Fig.4-34.The enrich mechnism of the Cu-Ni sulfide in the Baimazhai and Ban Phuc nickel deposit is mainly evolved from the deep liquation and differentiation, but with great difference between each other, the former is mainly the Ni-Cu sulfide with rare PGE and no independent PGE minerals(which occurs in the chalcopyite and pyrite); while the latter has strongly hydrothermal modification, except for the Ni-Cu sulfide, there are also a great number of sulfosalt, telluride, Ni-Co sulfur arsenide and michenefite etc. But there also exists the pneumatolytic hydrothermal and hydrothermal fluids modification related to the mafic-ultramafic intrusions in the Baimazhai Ni-Cu deposit in Jinping-Song Da rift, which indicated by the contact metasomatism and hydrothermal metamorphism, and the hosting rocks are commonly hydrothermally metamorphosed, i.e. hornfelisic alteration etc. The hydrothermally-modified metallogenesis is showed in the ore-formations of Cu and PGE mineralization, by which the veinlets consisting of irregular or banded pentlandite and chalcopyrite and carbonate in the ores. It is common that the prrhotitie and pentlandite are replaced by the chalcopyrite, and massive ores are replaced by sideroritic ores forming the psedomorphic mixed disseminated ores in the contact zone between them, and metasomatism of disseminated ore to the rock-forming minerals. The occurrence of biotite, chlorite and amphibole inter-grown with sulphides is additional evidence of volatile activity. In general, these modified ores tend to be enriched in Cu, Au and Pd as indicated by the ores in the Baimazhai nickel deposit. The hydrothermally modified metallogensis is also verified by the Ar-Ar dating which indicate the mineralization age range from 160-170Ma.In addition, some rich veinlet Cu-Ni ore-bodies with small size are formed due to the superimposed hydrothermally modification metallogenesis, for instance, the veinlet rich massive Cu-Ni sulfide ores are common in the contact zone between the Yingpanjie-Mahuanggou mafic-ultramafic intrusions swarm. And the hydrothermally modification also lead to the further enrichments of the early disseminated ores and the enrichments of PGE elements. While the independent PGE minerals, sulfosalt, tellurides and irregular networks of sulfide associated with quartz are the results of the hydrothermally modified metallogenesis.Hydrothermal remobilisation at Baimazhai is probably linked to post-magmatic tectonothermal events in the region. These events produced hydrothermal fluids that were focused hydrothermal fluids into suitable structures and the same fluids also modified existing mafic-ultramafic sulphide systems. No detailed structural data are available from the Baimazhai region, but limited field structural observations show that mylonites are present in the ASRR fault zone and an overturned anticline affects the sedimentary rocks that are intruded by the Baimazhai intrusions. At Baimazhai, there is convincing evidence of hydrothermal alteration of the intrusive rock, during which the silicate minerals have been pervasively replaced by hydrous phases(Photoâ…¥-1,â…¥-2). The hydrothermal fluids may not have modified the entire body of massive sulphides, but it is possible that tectonic movements have affected its margins, forming zones of disrupted or brecciated sulphides. This would have facilitated the introduction of hydrothermal fluids resulting in disseminated and interstitial sulphide ores (Photoâ…¦-1—Ⅶ-5). The "alteration-modified" sulphides are interpreted as an example of this tectonic and hydrothermal activity. This is also indicated by the presence of sulphide phases, other than the normal pyrrhotite-pentlandite-chalcopyrite assemblage, in which metal redistribution of certain trace elements such as Bi and Pb occurred to precipitate exotic sulphides (e.g. parkerite). A schematic illustration of the model is shown in Fig.4-35. In this model we envisage two possibilities: 1) a massive sulphide body was deformed and its marginal zones disrupted and brecciated allowing access of hydrothermal fluids, which modified the sulphides and the host rock; 2) alternatively, a magmatic massive sulphide body was originally overlain by a zone of magmatic disseminated sulphides; upon deformation of the massive-disseminated sulphide system, hydothermal fluids penetrated the most permeable zones as represented by the disseminated zone. In both cases the contact between sulphides and wall rock is a zone of highest geochemical gradient as well as a favourable structural locale and the fluids are expected to be focused within this zone.