Xin-Fu Zhao

Re-Os dating of pyrrhotite from the Chaoshan gold skarn, eastern Yangtze craton, eastern China

The Chaoshan gold skarn, located in the central part of the Tongling Cu-Au-Mo district in eastern China, consists of 10 ore bodies occurring along an intrusive contact between the Early Cretaceous Baimangshan pyroxene diorite (BPD) and limestones of the Middle Triassic Nanlinghu Formation. Less commonly, ores occur in interlayered fractures within the calcareous wall rocks. Endoskarn alteration is well developed and manifested by the presence of massive skarn bodies consisting of garnet, scapolite, vesuvianite, epidote, actinolite, plagioclase, and lesser amounts of diopside. Toward the intrusion, the endoskarn alteration grades into potassic alteration composed of K-feldspar and biotite, suggesting that skarn formation, and thus gold mineralization, is directly related to the BPD. Metallic minerals are mainly pyrite, pyrrhotite, arsenopyrite, chalcopyrite, galena, and sphalerite, of which pyrrhotite, pyrite, and arsenopyrite are the dominant Au-bearing phases. Re-Os dating of auriferous pyrrhotite was performed to determine the age of mineralization of this deposit. Fourteen duplicate analyses from seven samples define a rough isochron on the 187Re/188Os versus 187Os/188Os diagram, yielding an age of 146 ± 47 Ma (2σ) and an initial 187Os/188Os of 0.97 ± 0.25 (MSWD = 17). A greatly improved Re-Os isochron was obtained by plotting seven analyses from four samples that have similar initial 187Os/188Os and thus are assumed to have been derived from the same source, yielding an isochron age of 141.7 ± 9.9 Ma (2σ) and an initial 187Os/188Os of 1.027 ± 0.058 (MSWD = 1.15). This age is consistent with the SHRIMP zircon U-Pb age (142.9 ± 1.1 Ma) of the BPD and molybdenite Re-Os ages (141.7 ± 2.5 to 136.9 ± 2.2 Ma) of Cu-Mo deposits throughout the Tongling district, and therefore is interpreted as the timing of gold mineralization. The extremely radiogenic initial 187Os/188Os implies a significant crustal component in the Os budget of the pyrrhotite samples, likely derived from an enriched mantle source and crustal contamination of the BPD magma during its ascent through the crust.

In situ elemental and isotopic analysis of fluorapatite from the Taocun magnetite-apatite deposit, Eastern China: Constraints on fluid metasomatism

Abstract Metasomatic alteration of fluorapatite has been reported in several iron-oxide apatite (IOA) deposits, but its effect on elemental and isotopic variations has not been well understood. In this study, we present integrated elemental, U-Pb, Sr, and O isotopic microanalytical data on fresh and altered domains in fluorapatite from the Taocun IOA deposit, Eastern China, to evaluate the timing and nature of the metasomatism and its effects on the ore-forming event. Orebodies of the Taocun deposit are spatially associated with a subvolcanic, intermediate intrusion, which displays zonal alteration patterns with albite in the center and increasing actinolite, chlorite, epidote, and carbonate toward the margin. Both disseminated and vein-type ores are present in the Taocun deposit, and fluorapatite commonly occurs with magnetite and actinolite in most ores. Fluorapatite grains from the both types of ores have been variably metasomatized through a coupled dissolution-reprecipitation mechanism. Many trace elements, including Na, Cl, S, Si, Mg, Sr, U, Th, and (REEs+Y), were variably leached from the fluorapatite grains during this process and the Sr and O isotopic signatures of the grains were also modified. The altered fluorapatite grains/domains have in situ 87Sr/86Sr ratios (0.70829–0.70971) slightly higher than those of the fresh fluorapatite (0.70777–0.70868), and δ18O values (–3.0 to +3.4‰) variably lower than the primary domains (+5.3 to +7.5‰). The Sr and O isotopes of the primary fluorapatite are consistent with or slightly higher than those of the ore-hosting intrusion, implying that the early-stage, ore-forming fluids were magmatic in origin but underwent weak interaction with the country rocks. U-Pb dating of the fresh and altered domains of the fluorapatite yielded indistinguishable ages of ~131 Ma, which are the same as the age of the ore-hosting intrusion. In combination with fluid inclusion data, we propose that the metasomatism of fluorapatite was induced by hydrothermal fluids at a late stage of the ore-forming event. The shifts to higher87Sr/86Sr ratios and lower δ18O values in the altered fluor-apatite indicate that the alteration was induced by fluids with more radioactive Sr and lighter O isotope signatures. The metasomatic fluids were likely dominated by meteoric waters that were mixed with the earlier magmatic fluids and interacted with sedimentary rocks. Our study highlights that elemental and isotopic compositions of fluorapatite can be significantly modified by hydrothermal fluids during ore-forming events. Thus, instead of traditional bulk-rock analysis, in situ microanalysis is important to provide accurate constraints on the magmatic and/or hydrothermal evolution of complex ore-forming systems.

Disturbance of the Sm-Nd isotopic system by metasomatic alteration: A case study of fluorapatite from the Sin Quyen Cu-LREE-Au deposit, Vietnam

Abstract The Neoproterozoic (840 Ma) Sin Quyen deposit in northwestern Vietnam contains replacement Cu-LREE-Au orebodies in Proterozoic metasedimentary rocks. In this deposit, LREE-bearing minerals include allanite-(Ce), monazite-(Ce), chevkinite-(Ce), and fluorapatite. Fluorapatite from orebodies has undergone variable degrees of metasomatic alteration. Samarium-neodymium isotopic analyses were conducted on altered fluorapatite, and also on allanite-(Ce) and monazite-(Ce), to investigate whether such metasomatism can affect the Sm-Nd isotope system. Allanite-(Ce) and monazite-(Ce) have 147Sm/144Nd ratios ranging from 0.0359 to 0.0549, and 143Nd/144Nd ratios from 0.51147 to 0.51172. Their initial 143Nd/144Nd values at the time of mineralization range from 0.51126 to 0.51148, but mostly cluster between 0.51135 and 0.51145. Thus, the primary ore-forming fluids were relatively homogeneous in their Sm-Nd isotopic compositions. In the 147Sm/144Nd vs. 143Nd/144Nd diagram, the compositions of allanite-(Ce) and monazite-(Ce) generally plot along a Sm-Nd isochron of 840 Ma, implying that the Sm-Nd isotopic systems of these minerals were either closed or only slightly modified. In contrast, altered fluorapatite crystals have 147Sm/144Nd ratios varying from 0.0667 to 0.1348, and 143Nd/144Nd ratios from 0.51160 to 0.51199. The calculated initial 143Nd/144Nd ratios range widely from 0.51114 to 0.51141, with most values lower than those of the allanite-(Ce) and monazite-(Ce). In the 147Sm/144Nd vs. 143Nd/144Nd diagram, their compositions mostly plot below the 840-Ma Sm-Nd isochron. Petrographic observations and trace elemental analyses show that metasomatic modification of fluorapatite grains led to increases of their Sm/Nd ratios. The unaltered domains in the grains have Sm/Nd ratios varying from 0.114 to 0.200, with an average value of 0.161; whereas the altered domains have Sm/Nd ratios varying from 0.111 to 0.254, with an average value of 0.183. The increased Sm/Nd ratios can cause the calculated initial 143Nd/144Nd ratios to be lower than actual initial isotopic ratios, and can also result in compositional deviations from the reference Sm-Nd isochron. This study demonstrates that the traditionally assumed inert Sm-Nd isotopic system can be metasomatically disturbed due to changes in the Sm/Nd ratio. Therefore, care must be taken when interpreting the Sm-Nd isotopic data from apatite/apatite-rich rocks that have undergone metasomatic alteration.