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Native Gold and Native Copper Grains Enclosed by Olivine Phenocrysts in a Picrite Lava of the Emeishan Large Igneous Province, SW China

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Native Gold and Native Copper Grains Enclosed by Olivine Phenocrysts in a Picrite Lava of the Emeishan Large Igneous Province, SW China American Mineralogist, Volume 91, pages 1178–1183, 2006 Native gold and native copper grains enclosed by olivine phenocrysts in a picrite lava of the Emeishan large igneous province, SW China ZHAOCHONG ZHANG,1,2,* JINGWEN MAO,3 FUSHENG WANG,2 AND FRANCO PIRAJNO4 1State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, 100083, China 2Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037, China 3Institute of Mineral Resources, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037, China 4Geological Survey of Western Australia, 100 Plain Street, East Perth W A 6004, Australia ABSTRACT A native gold bleb found in an olivine phenocryst in a picrite lava from the Emeishan Large Igne- ous Province (ELIP) may be the Þ rst documented case of the transport of gold as a distinct precious metal phase in a mantle-derived magma. Four picrite layers have been recognized in the lower part of the volcanic succession in the Lijiang area, in the western part of the ELIP. The native gold bleb was found enclosed in an olivine phenocryst in the second picritic layer of a basalt-picrite succession in the ELIP. The gold bleb is spheroidal, about 30 μm in diameter, consists of pure gold, and does not contain any other elements. In addition, native copper grains were also discovered in the serpentinized olivine phenocrysts, and native zinc and moissanite (SiC) were separated from an ~20 kg sample of picrite. The paragenesis of these minerals suggests that the primary magmas were S-unsaturated. The native gold and native copper grains are considered to be xenocrysts from the mantle, transported to shallow depths by a rising plume, and then captured by the picritic melts. The discovery of native gold and native copper grains provides direct evidence that the gold in the hydrothermal gold deposits and the native copper deposits in the ELIP lavas ultimately may be derived from a mantle plume. Keywords: Native gold, native copper, moissanite, mantle plume, olivine phenocryst, picrite, Emeishan large igneous province INTRODUCTION ince (ELIP). This is the Þ rst documented case of the occurrence The production of continental ß ood basalts (CFBs) has been of native gold and native copper in the olivine phenocrysts of attributed to the arrival of plume heads from a boundary layer picrites. We suggest that it may reß ect the signature of plume- deep within the earth (e.g., Richards et al. 1989; Hill 1991). This derived materials. has been suggested to be either the 670 km discontinuity or the GEOLOGY AND PETROGRAPHY OF PICRITES 2900 km D'' core-mantle boundary layer (Allegre and Turcotte 1985; Hofmann 1997). However, the chemical composition and The ELIP is a Late Permian continental ß ood basalt sequence mineralogy of mantle plumes is poorly known because of the lack covering large areas in the provinces of Sichuan, Yunnan, and of mantle xenoliths in the lavas from large igneous provinces Guizhou along from the eastern margin of the Tibetan Plateau to (LIPs). Highly magnesian, near-primitive picritic lavas in LIPs the western margin of the Yangtze Block in southwestern China 2 have been used to infer the characteristics of a mantle plume (Fig. 1). The ELIP covers an area of at least 250 000 km (Chung (e.g., Brugmann et al. 1987; Puchtel and Humayun 2000). This et al. 1998; Chung and Jahn 1995; but according to Xiao et al. is possible because of the speciÞ c conditions of picrite forma- 2003, this is a conservative estimate) in SW China. The western tion, including high-temperature partial melting, consistent boundary of the ELIP is the Ailao Shan-Red River Fault zone with formation of picritic magmas from mantle plumes (e.g., (ASSR), a major crustal structure separating the Yangtze Craton Campbell et al. 1989; Storey et al. 1991), sulfur undersaturation, from the Gandise and Yunnan fold belts. rapid adiabatic ascent, and eruption at temperatures close to the The ELIP consists of a succession of predominantly tholei- liquidus. These conditions ensure that little fractionation occurs ites, with minor picritic and rhyolitic lava ß ows. In addition to prior to eruption (e.g., Arndt et al. 1977; Herzberg 1995). Thus, lava ß ows, maÞ c-ultramaÞ c layered complexes, dikes and sills, the picrites should preserve some signatures of materials derived syenite, and other alkaline intrusions, are part of the ELIP (Xu et from a mantle plume. al. 2001; Boven et al. 2002; Zhang et al. 2004b; Xiao et al. 2004). A bleb of native gold and two grains of native copper were The maximum thickness of the lava ß ows is estimated at about discovered in the olivine phenocrysts from the same picrite 5400 m in the Binchuan region in the Pan-Xi rift, decreasing to sample from the Late Permian Emeishan large igneous prov- less than 500 m eastward from the rift (Xiao et al. 2003, 2004; He et al. 2003; Xu et al. 2004). The ELIP magmatism appears to be associated with Late Permian rifting of the Qiangtang terrane from the Yangtze block. * E-mail: [email protected] The ß ood basalts were subsequently uplifted, deformed, and seg- 0003-004X/06/0007–1178$05.00/DOI: 10.2138/am.2006.1888 1178 ZHANG ET AL.: NATIVE GOLD AND NATIVE COPPER IN OLIVINE PHENOCRYST 1179 mented by block faulting during the closure of the Tethys Ocean the thickness of each of the other three ß ows is similar to those in the Jurassic (Chung et al. 1998). The region was reworked in the Shiman section. Unlike the central and eastern parts of the further during the collision between the Indian and Eurasian ELIP, where plagioclase-phyric basalts are widely exposed, in continents (Leloup et al. 2001). the Lijiang region, plagioclase-phyric basalts are absent. The total eruptive volume is estimated to be between 0.3 and The native gold-bearing picrite (DJ03-35) is located in the 0.5 × 106 km3 (Yin et al. 1992; Jin and Shang 2000); however, second picritic layer of the Daju section (Fig. 2). The picrite given the large amount of erosion, and by comparison with other is highly porphyritic, and contains abundant forsteritic olivine continental ß ood basalt privinces, this volume may represent phenocrysts with minor clinopyroxene and Cr-spinel. Olivine less than 50% of the total amount of magma emplaced. The lava phenocrysts are generally euhedral to subhedral (Fig. 3a), rang- successions rest unconformably on the Early Permian Maokou ing from 0.2 to 4 mm in size; Strained, kink-banded crystals are Formation, composed chieß y of marine limestones, and are over- absent, but fractures usually occur in the olivine crystals. The lain directly by Late Permian and locally Early Triassic marine olivine phenocrysts contain scattered inclusions of glass, and strata. Lo et al. (2002) concluded from 40Ar-39Ar dating that the some enclose small, equant, euhedral to rounded, dark-brown main phase of ß ood basalt magmatism occurred at 251–253 Ma, Cr-spinel crystals, usually tens of micrometers across. Some Cr- more or less coeval with the Siberian Traps, which also contain spinel crystals are also present in the groundmass, which consists picrites (Sharma 1997). of Þ ne-grained unaltered olivine, diopside, and plagioclase. Picrites are present in the southern (Shiman) and northern Quenched olivine groundmass crystals are generally fresh or (Daju) portions of Lijiang county, Yunnan province. These two unaltered, and may be small (<0.1 mm), and near-equant “hop- occurrences are about 26 km apart. The geological characteristics of the two picrite occurrences are similar, with the picritic lavas being located in the lower portions of the ß ood basalt pile. In the Shiman section, three picritic ß ows are intercalated with pyroxene-phyric basaltic lavas, whereas four picritic ß ows oc- cur in the lower part of the Daju section. It is possible, perhaps likely, that the Shiman section corresponds to the middle part of the much thicker Daju section. Massive aphyric basalts and amygdaloidal basalts are dominant in the middle to upper part of the Daju sequence. In the Shiman section, the lowermost and the uppermost picritic ß ows are 3 to 5 m thick, and the middle picritic layer is 15 to 20 m thick. In the Daju section, the lower- most picritic ß ow varies from 20 to 50 m in thickness, whereas FIGURE 2. Stratigraphic column showing the succession of picrites and basalts, as exposed at Daju in the Lijiang area. Inferred relative stratigraphic locations and native gold- and native copper-bearing picrite (DJ03-35) are FIGURE 1. SimpliÞ ed map showing outcrops of Emeishan ß ood also shown on right side of each column. basalts (black areas; simpliÞ ed from Chung and Jahn 1995). The shaded Each shaded part represents an individual area marks the location of the Figure 2. lava ß ow. 1180 ZHANG ET AL.: NATIVE GOLD AND NATIVE COPPER IN OLIVINE PHENOCRYST FIGURE 3. Photomicrographs of the native gold bleb enclosed by the olivine phenocryst in the picrite. (a) Olivine phenocrysts and the groundmass of the picrite. The black grain enclosed by olivine phenocsyst is native gold. The blue circle is an ink mark. Cross-polarized light; (b) the entire outline of native gold bleb can be recognized under cross-polarized light; (c) only part of native gold bleb is exposed in the polished thin section, whereas the unexposed appears as a blurred image under reß ected light. sanite (SiC). To understand the paragenesis of these phases, we made 35 polished thin sections, and discovered a bleb of native gold and two grains of native copper in two of these sections, respectively.
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