Geology and Origin of the Dongping Alkalic‐Type Gold Deposit, Northern Hebei Province, People's Republic of China

Abstract

: The Dongping deposit, located near the center of the northern margin of the north China craton, is one of the largest gold deposits in China. It is spatially, temporally, and genetically associated with the shallowly‐emplaced Hercynian Shuiquan‐gou alkaline intrusive complex. The complex intrudes high‐grade metamorphic rocks of the Archean Sanggan Group along a deep‐seated fault zone within the north China craton. Four major ore bodies (Nos. 1, 2, 22, and 70), consisting mainly of a set of en echelon lenses and veins, have been delineated at the Dongping deposit. Hypogene hydrothermal activities can be divided into four periods from early to late including: (1) gold‐bearing K–feldspar–quartz stockworks and veins; (2) disseminated sulfide and gold zones; (3) gold‐bearing quartz veins, and (4) barren calcite‐quartz veins. Individual veins and stockwork systems can be traced along strike for 125 to 600 m and downdip for 100 to 600 m; they range from 0. 5 to 3 m in thickness. The mineralogical composition of the ore in the first three hypogene periods is relatively simple. It is composed of pyrite, galena, sphalerite, magnetite, specularite, chalcopyrite, native gold, electrum, calaverite, and altaite. Gangue minerals include K–feldspar, quartz, sericite, chlorite, epidote, albite, and calcite. Ore grade averages 6 g/t Au, but varies between 4. 14 and 22. 66 g/t Au. Gold is generally fine‐grained and not visible in hand specimen. Fluid inclusions in ore‐bearing quartz of periods 1, 2, and 3 are CO2–rich, variable salinity (2. 5–21 wt% equiv. NaCl), and have variable homogenization temperatures of 195° to 340°C. Quartz in the gold‐bearing K–feldspar–quartz stockworks (period 1), disseminated sulfide and gold zones (period 2), and the gold‐bearing quartz veins (period 3) has calculated δ18OH2O values between –1. 7 and 6. 9%, and δ values of fluid inclusion waters between –101 and –66%. All these isotope data of the ore‐forming fluids plot between the magmatic fluid field and the meteoric water line. Sulfide minerals disseminated in host rocks show positive δ34S values of 1. 9 to 3. 5%. Pyrite separates from he gold‐bearing K–feldspar–quartz stockworks and veins (period 1) have a δ34S range of –4. 3 to 0. 5%, whereas δ34S values of pyrite, chalcopyrite, galena, and sphalerite from the disseminated sul‐fide and gold zones (period 2) and the gold‐bearing quartz veins (period 3) vary from –5. 3 to –13. 4%. Gold ores are also characterized by relatively radiogenic lead isotope compositions compared to those of the alkaline syenite host rock. The data are interpreted as indicative of a mixing of lead from the alkaline intrusive complex with lead from Archean metamorphic rocks. The combined fluid inclusion measurements, sulfur, oxygen, hydrogen, and lead isotope data, and petrological observations indicate that the Dongping deposit was formed from the mixing of these magmatic fluids with meteoric waters. The deposit is, therefore, believed to be a product of Hercynian alkaline igneous processes within the north Chi...

Intrusion‐Related Gold Deposits of North China Craton, People's Republic of China

“…Although the fields of lead isotopic compositions of the sulfide separates from the intrusion, gold ores and Precambrian wall rocks overlap to some extent, most of the sulfide separates from the intrusion-related gold deposits plot about halfway between the intrusions and Precambrian amphibolite. Similar Pb-isotope signatures spread out between values of Phanerozoic intrusions and surrounding Precambrian metamorphic rocks have been reported for other type gold deposits (metamorphic rock-hosted) occurring within the NCC (Lin and Guo, 1985;Wang et al, 1990;Trumbull et al, 1992;Nie and Bjørlykke, 1994;Nie, 1998;Nie et al, 2002. These data are best interpreted as a mixing line between lead derived from the Precambrian metamorphic rocks and from the Hercynian or Yanshanian intrusions.…”

“…Alkaline-type gold mineralization, occurring within the NCC and its marginal belts, has the following common features: (1) gold deposits are hosted by or closely related spatially to alkaline intrusive complexes of dyke swarms, which are distributed along the ancient continental rift or marginal deep-seated faults; (2) the ores are characterized by relatively high contents of K-feldspar, low contents of sulfides and low base metal/gold ratios (telluride is a common trace component associated with native gold); (3) altered rocks around orebodies are geochemically enriched with Au, As, Sb, Bi, Cu and Pb, which may serve as indicators for mineral exploration; and (4) typical alteration is vertically and laterally zoned, with K-feldspathization, silicification, sericitization and carbonatization (Nie and Wu, 1998;Nie, 1998;. Generally, the deep-seated faults are thought to be important in generating mantle-derived alkaline magma.…”

“…Gold ores are also characterized by relatively radiogenic lead isotope compositions compared to those of the alkaline syenite host rock. The data are interpreted as indicative of a mixing of lead from the alkaline intrusive complex with lead from Archean metamorphic rocks (Nie, 1998).…”

“…It is also one of the best studied gold deposits in the NCC, with extensive geological descriptions by Nie (1997bNie ( , 1998, Hart et al (2002), Mao et al (2003) and . The Dongping deposit is located in the middle section of the Inner Mongolia Axis, and spatially, temporally and genetically associated with the shallowly emplaced Hercynian Shuiquangou alkaline intrusive complex.…”

“…3, 2004 Intrusion-related Gold Deposits of North China Craton 313 oxygen, hydrogen and lead isotope data, and petrological observations indicate that the Dongping deposit was formed from the mixing of these magmatic fluids with meteoric waters. The deposit is, therefore, believed to be a product of Hercynian alkaline igneous processes within the NCC (Nie, 1998).…”