Geology and Metallogenesis of the Sawayaerdun Gold Deposit in the Southwestern Tianshan Mountains, Xinjiang, China

Abstract

The Sawayaerdun gold deposit, located in Wuqia County, Southwest Tianshan, China, occurs in Upper Silurian and Lower Devonian low-grade metamorphic carbonaceous turbidites. The orebodies are controlled by a series of NE-NNE-trending, brittle -ductile shear zones. Twenty-four gold mineralized zones have been recognized in the Sawayaerdun ore deposit. Among these, the up to 4-km-long and 200-m wide No. IV mineralized zone is economically the most important. The average gold grade is 1 -6 g/t. Gold reserves of the Sawayaerdun deposit have been identifi ed at approximately 37 tonnes and an inferred resource of 123 tonnes. Hydrothermal alteration is characterized by silicifi cation, pyritization, arsenopyritization, sericitization, carbonatization and chloritization. On the basis of fi eld evidence and petrographic analysis, fi ve stages of vein emplacement and hydrothermal mineralization can be distinguished: stage 1, early quartz stage, characterized by the occurrence of quartz veins; stage 2, arsenopyrite -pyrite -quartz stage, characterized by the formation of auriferous quartz veinlets and stockworks; stage 3, polymetallic sulfi de quartz stage, characterized by the presence of auriferous polymetallic sulfi de quartz veinlets and stockworks; stage 4, antimony -quartz stage, characterized by the formation of stibnite -jamesonite quartz veins; and stage 5, quartz -carbonate vein stage. Stages 2 and 3 represent the main gold mineralization, with stage 4 representing a major antimony mineralization episode in the Sawayaerdun deposit. Two types of fl uid inclusion, namely H 2 O -NaCl and H 2 O -CO 2 -NaCl types, have been recognized in quartz and calcite. Aqueous inclusions show a wide range of homogenization temperatures from 125 to 340°C, and can be correlated with the mineralization stage during which the inclusions formed. Similarly, salinities and densities of these fl uids range for each stage of mineralization from 2.57 to 22 equivalent wt% NaCl and 0.76 to 1.05 g/cm 3 , respectively. The ore-forming fl uids thus are representative of a medium-to low-temperature, lowto medium-salinity H 2 O -NaCl -CO 2 -CH 4 -N 2 system. The ␦ 34 S CDT values of sulfi des associated with mineralization fall into a narrow range of −3.0 to +2.6 ‰ with a mean of +0.1 ‰ . The ␦ 13 C PDB values of dolomite and siderite from the Sawayaerdun gold deposit range from −5.4 to −0.6 ‰ , possibly refl ecting derivation of the carbonate carbon from a mixed magmatic/sedimentary source. Changes in physico-chemical conditions and composition of the hydrothermal fl uids, water -rock exchange and immiscibility of hydrothermal fl uids are inferred to have played important roles in the ore-forming process of the Sawayaerdun gold -antimony deposit.

Evolution of ore-forming fluids in the Sawayaerdun gold deposit in the Southwestern Chinese Tianshan metallogenic belt, Northwest China

“…Previous studies show the Sawayaerdun gold deposit shares many similarities with the Muruntau deposit and other orogenic-type Au deposits, with respect to geological setting, alteration assemblages, paragenetic relationships and ore-forming fluids (Yang et al, 2007;Liu et al, 2007). Initial investigations indicate that hydrothermal fluids of the Sawayaerdun deposit are CO 2 -rich and probably evolved through multiple stages (Yang et al, 2007), consistent with most orogenic-type deposits (Chen et al, 2007). These characteristics suggest that the Sawayaerdun gold deposit could provide an excellent opportunity to study in detail the anatomy of an orogenic-type hydrothermal system.…”

“…The Sawayaerdun gold deposit is approximately 30 km northnorthwest of Wulukeqiati Township, Wuqia County, Xinjiang (Yang et al, 2007). It is located in the Paleozoic passive-continental marginal sedimentary belt of the southern Tianshan orogenic belt, which has contributed to the northwest margin of the Tarim plate (Fig.…”

“…As the largest orogenic-Au deposit in the Chinese Tianshan belt (Liu et al, 2007), the Sawayaerdun deposit contains >3 Moz Au with an inferred resource of at least 10 Moz (Rui et al, 2002). Previous studies show the Sawayaerdun gold deposit shares many similarities with the Muruntau deposit and other orogenic-type Au deposits, with respect to geological setting, alteration assemblages, paragenetic relationships and ore-forming fluids (Yang et al, 2007;Liu et al, 2007). Initial investigations indicate that hydrothermal fluids of the Sawayaerdun deposit are CO 2 -rich and probably evolved through multiple stages (Yang et al, 2007), consistent with most orogenic-type deposits (Chen et al, 2007).…”

“…Five alteration mineral assemblages formed during the main stages of mineralization can be recognized, including native gold-arsenopyrite-pyrite-quartz, native gold-pyrite-jamesonite (stibnite)-quartz, native gold-jamesonite (stibnite), native goldquartz-siderite and native gold-pyrite-pyrrhotite-quartz (Yang et al, 2007). According to mineral associations and their occurrence, the primary ores can be classified into four types: (i) auriferous quartz veinlet; (ii) auriferous quartz stockwork; (iii) auriferous altered carbonaceous phyllite; and (iv) auriferous silicified siltstone (Yang et al, 2007).…”

Decratonic gold deposits

“…Arsenic in orogenic gold deposits, however, is often enriched and occurs as arsenopyrite, as best illustrated in the Yindongpo gold deposit and the Yangshan gold deposit in the western Qinling orogen of China (Qi et al, 2003), the Sawaya'erdun gold deposit (Yang et al, 2005) and the Muruntau gold deposit in Kazakhstan (Drew et al, 1996) of the central Asian orogen, and the Bendigo gold deposit in Australia , among others. Arsenic enrichment in these deposits indicates that the metamorphic devolatilization process of sedimentary rocks or volcano-sedimentary rocks is essential for mineralization of orogenic gold deposits.…”