The Daping gold deposit contains one of the largest
resources in the Ailaoshan gold belt, the most economically significant Cenozoic gold belt in China. Most previous
researchers considered Daping
to be a reworked type gold deposit,
and the ore-forming fluid
to have been a mixture of magmatic
and meteoric water. Ore-forming materials were considered
to be derived mainly from the diorite host rocks or metamorphic basement by water-rock interaction. Detailed observation
and in-situ laser Raman analyses discovered a large amount of fine-grained, highly-crystallized graphite in the auriferous sulfide quartz veins with obvious O peaks
and very weak D peaks in the laser Raman spectrum, which suggests that those graphite grains were formed under granulite facies conditions. Iso
tope analyses show that the (
87Sr/
86Sr)
0 and εNd
(0) of the Daping scheelite, the earliest precipitated ore
mineral in the paragenesis, vary between 0.7088
and 0.7112
and − 8.43
and − 6.20, respectively,
and were projected in the lower crust field on a (
87Sr/
86Sr)
0 versus εNd
(0) diagram. Noble gases iso
topic compositions of fluid inclusions in Daping scheelites were performed by a high vacuum gas mass spectrum,
and the results show that the
3He/
4He ratios are (0.988–1.424) × 10
− 6 with an average of 1.205 × 10
− 6, corresponding
to R/Ra values of 0.706–1.018, with an average of 0.898. The
40Ar/
36Ar ratios are between 1801.8
and 2663.8, much higher than that of the air (295.5);
20Ne/
22Ne
and 21Ne/
22Ne are 9.600
to 11.56
and 0.028
to 0.0467, respectively,
and the respective
134Xe/
132Xe
and 136Xe/
132Xe are 0.394
to 0.692
and 0.301
to 0.462, demonstrating that ore-forming fluids
and materials of the Daping mine derived mainly from the transitional zone between the lower crust
and upper mantle. The δD compositions of fluid inclusions in Daping gold deposit are − 60.0‰
to − 85‰, with an average of − 74.5‰, whereas δ
18O
H2O are 2.39‰
to 7.59‰, averaging 5.68‰, indicating that the ore-forming fluids consist predominantly of metamorphic fluid, with
contributions from mantle-derived primary magmatic fluids. The δ
13C compositions of CO
2 in fluid inclusions of Daping gold deposit lie between − 3‰
to − 6.5‰, suggesting that most of the CO
2 in the ore-forming fluids was mantle-derived,
and part came from the lower crust in the Ailaoshan gold belt. In addition, primitive mantle normalized platinum group elements (PGE) patterns for the auriferous ores are similar
to that of the Cenozoic lamprophyre dykes in the gold deposit, but quite different from that of the diorite host rocks, also implying that the ore-forming materials in the Daping deposit derived mainly from the lower crust or even the upper mantle.
The new data imply that crust–mantle interaction may have played an important role in the mineralization of the Daping deposit. At around 33 Ma, the middle and lower crust in the Daping area suffered high-temperature and high-pressure metamorphism because of ductile deformation and upwelling upper mantle magmas. The ore-forming fluids enriched in CO2, 3He, 20Ne and 130Xe, generated by granulite facies metamorphism and baking by upwelling upper mantle magmas, were transported to the middle to upper crust along the ductile shear zone along with highly crystallized graphite grains, and finally precipitated auriferous sulfide-quartz veins in brittle structures because of declining temperature, pressure, and subsequent boiling. Daping is a typical ductile shear zone controlled orogenic gold deposit.
