Summary:
The Longnan Zijin comprises of the Liba, Dujiagou and Jinshan mines.
Ore deposit type - Para-Carlin.
The West Qinling fold belt area in the Gansu Province, hosting a group of sedimentary rock-hosted Au deposits, which include the Liba and Jinshan deposits. They are located near the Zhongchuan granite intrusion and hosted in Devonian phyllite.
Liba gold mine deposit is not only one of the most important gold ore mineralized concentrate zones in Gansu Province, but also one of the major gold deposits in Li-Min mineralization zone.
The Liba Au deposit is one of several Au deposits that lie in a regional-scale cluster of other sedimentary rock-hosted Au deposits in Gansu Province in the west Qinling area. The deposit differs from other deposits in the region because it is hosted in low-grade metamorphic Devonian and Triassic meta-siltstone, phyllite, and slate in the contact aureole of the Zhongchwan granite intrusive, about 2 km northeast from the granite contact.
The Liba Au deposit area contains more than 20 orebodies composed of complex vein lenses that branch, merge, swell, and pinch along steep-dipping faults that crosscut sedimentary bedding. The No. 5 and No. 6 orebodies are the main orebodies accounting for 80 percent of the total proven reserves of the deposit. The No. 5 orebody is 2,000 m long, 6 to7 m thick and 250 m deep; the No. 6 orebody is 600 m long, 5 to 6 m thick and 250 m deep. The average grade is 5 g/t and 4 g/t Au respectively.
Sedimentary rocks in the Liba Au deposit area are Middle and Upper Devonian, Carboniferous, and Permian in age. The Upper Devonian Dachaotan and Xihanshui, and the Middle Devonian Sujiaba Formations are low-grade metamorphic molasses that consist of coarse-grained alternating marine and continental sediments. The orebodies of the Liba Au deposit are hosted in the 1,106–m-thick Sujiaba Formation mainly in the second layer that is dominated by silty phyllite (80 percent) and metasandstone. Ore–in silty phyllite accounts for 78 percent of the ore, whereas ore in metasandstone and phyllite accounts for 22 percent. Carboniferous rocks consist of carbonaceous, clastic rock interlayered with multi-layers of limestone, conglomerate, and siliceous rocks. Lower Permian and lower Triassic sedimentary rocks are composed of shallow marine argillite, and clastic rock with interlayers of carbonate rock, especially at the top.
The 210 km2 Zhongchuan granite intrudes Middle Devonian to Middle Carboniferous sedimentary rocks south of the Liba Au deposit and is a multi-stage intrusion spanning the Hercynian (405 to 230 Ma), Indo-China (230 to 195 Ma), and early Yanshanian periods. The intrusive complex mainly is composed of six large late Mesozoic granitic intrusions, several small Middle Paleozoic basic rock stocks, and local andesitic porphyritic-dacite bodies, as well as Cenozoic alkalic to ultramafic volcanic rocks. Spessartite, diorite, oligoclase aplite, and granodiorite dikes also are present (Liu, M., 1994). The contact thermal metamorphism zones around the granite are: (1) chlorite-sericite, (2) biotite, and (3) andalusite+ cordierite. Some dikes are mineralized by Au. Gold orebodies mainly are in the biotite zone, but are closely associated with the minerals muscovite and sericite in this zone. Other elevated geochemical elements in the Mesozoic granitic intrusions are Sn, W, Mo, and Bi, but these elements are not commonly enhanced near the sedimentary rock-hosted Au deposits.
Typical Carlin-type ore deposit minerals are absent in the Liba Au deposit, such as stibnite, cinnabar, realgar, and orpiment. Instead the deposit contains a mesothermal mineral assemblage of pyrite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, and sulfate minerals, which indicates ore minerals reached temperatures higher than in most sedimentary rock-hosted Au deposits. Sericitization is closely associated with Au, where the alteration pattern is observed from inner to the outer zones of the ore-bodies from pyrite-sericite, to sericite, to chlorite. Grade and thickness of orebodies directly correlate with the intensity of sericitization (Liu, M., 1994). Igneous rocks in Liba Au deposit may have provided a heat source for the ore-forming system (Liu, M., 1994), and account for this different mineralogy.
Most ores are disseminated, but also are present in stockworks and bands. Oxide ore displays replacement, corrosion, pseudomorphic, and crushed textures. Pyrite is the main ore mineral in the primary zone and is accompanied by chalcopyrite, sphalerite, arsenopyrite, galena, pyrrhotite, rutile, native Au, minor bornite, magnetite, marcasite, and electrum. Pyrite, arsenopyrite, sericite, quartz, and calcite are the main Au carriers. Gold in Au–Ag compounds accounts for 97 volume percent of the ore. Gold in crystal lattices of sulfide minerals, primarily pyrite, accounts for only 2.7 volume percent. Gold lies mainly along the contacts between pyrite, quartz, sericite, and limonite crystals. Some Au is present along partings in pyrite and in quartz inclusions of pyrite. Limonite, hematite, and jarosite carry Au in the oxide zone. Muscovite, sericite, and quartz are the main gangue minerals with lesser feldspar, chlorite, biotite, carbonate minerals, graphite, organic carbon, minor tourmaline, zircon, apatite, zeolite, and clay minerals.
Fluid-inclusion studies of ore minerals from the Liba Au deposit by Liu, M., (1994) indicate ore fluid salinities of between 6.9 and 11.6 (average 8.4) equiv. weight percent NaCl, and densities of 0.65 to 0.86 g/cm3 (average 0.74g.cm3 ), which are compatible with igneousrelated ore processes, but also are consistent with thermally metamorphosed stratabound ore. The stratigraphic sequence near the Liba Au deposit in the Lixian-Mingxian area contains local horizons with anomalously high concentrations of Au (0.004 ppm), As (40 ppm), and Sb (2 ppm) (Liu, M., 1994). Although Li, Z.P. and Peters (1997) and Li, Z.P. and Peters (1998) included the Liba Au deposit as a sedimentary rock-hosted Au, this deposit may be intrusiverelated and not a Carlin-type deposit. Cheng, Y. and Zhang, W.D. (2001), however, suggest that Liba-style Au deposits in the Zhongchuan area are remobilized deposits from late Paleozoic source beds during Yanshanian (Late Mesozoic to Early Tertiary) magmatism. The Auassociated alteration assemblage of biotite-sericite and lack of other typical mineralogical indicators of Carlin-type Au deposits, such as arsenically zoned pyrite and realgar and orpiment, however indicate that additional work is needed to understand the genesis of the deposit. The similarities to other sedimentary rock-hosted Au deposits in the area, however are the late Paleozoic host rocks and As geochemistry.