Geology of Sedimentary Rock-Hosted Disseminated Gold Deposits in Northwestern Sichuan, China

Abstract

Sixteen sedimentary rock-hosted disseminated gold deposits (e.g., Dongbeizhai, Qiuluo, Manaoke, Laerma, Gala and Lianhecun) and about 50 prospects have been discovered in northwestern Sichuan Province, China since the late 1970s. They are hosted in complex rock series such as fine-grained clastic rocks, subvolcanic rocks, and ophiolitic melange, mainly of Triassic age. Mineral associates include pyrite, arsenopyrite, marcasite, realgar, orpiment, stibnite, cinnabar, quartz, cal-cite, and barite, and the elements gold, arsenic, antimony, mercury, thallium, and barium. In addition, scheelite, uranite, and bismuth minerals are present in some deposits. Submicron- and micron-sized gold is disseminated within black, carbonaceous, pelitic to sandy clastic and volcaniclastic rocks. Hydrothermal alteration includes fine-grained silica, calcite, dolomite, and siderite. Organic material occurs in most clastic host rocks and ores. Ore bodies are obviously controlled by fractures. Fluid inclusion data show that the ores were formed at temperatures ranging mostly from 150 to 250°C, at pressures less than 400 bars, and from fluids with salinities from 3 to 14 wt% NaCl and densities from 0.82 to 1.00 g/cm3. Isotopic data reveal that the ore-forming ages are from the middle-late Mesozoic to Tertiary and that the ore fluids vary from −117.8 to −52.9%c (average −83.6 + 16.5‰) in 5D value and from −11.1 to 16.2‰ (average 11.1 ± 9.2‰) in δ18O values. Sulfur isotopes from epigenetic sulfide change from −25.6 to 7.8‰, mostly between ‰6.6 and −1.2‰. In view of the element associations, sedimentary rock-hosted disseminated gold deposits in northwestern Sichuan can be divided into six subtypes as follows: Au, Au-As, Au-Sb, Au-Hg, Au-W, and Au-U. These deposits have similar features to those in Nevada in the western United States, including tectonic setting, mineral association, element association, gold occurrence, alteration (such as silicification and carbonatization), ore-forming temperature, metallogenic age, as well as spatial relationship with igneous rock bodies. But differences exist among them. Scheelite, uranite, and bismuth minerals are present only in some of the deposits of northwestern Sichuan. High-angle normal faults and regional oblique-slip (thrust) faults are responsible for the localization of mineralization in the Nevada and northwestern Sichuan deposits, respectively. Known northwestern Sichuan deposits are smaller than most United States analogues. In Nevada, the gold deposits occur chiefly in impure carbonate rock series of Paleozoic age, but in northwestern Sichuan they are hosted in complex rock series mainly of Triassic age. In the northwestern Sichuan deposits, decarbonatization and shallow acid-leaching alterations are much weaker than those in Nevada. Estimated pressures at the northwestern Sichuan deposits are lower than those for the Nevada deposits. The ore fluids in the northwestern Sichuan deposits have relatively higher salinities and slightly lower 8D values than those in the Nevada deposits. A possible model for the northwestern Sichuan deposits involves ore-forming components scavenged mainly from fine-grained clastic rocks by polygenetic fluids consisting of connate and meteoric water of moderate salinities. Regional geothermal anomalies were caused by tectonic and igneous activities during the Yanshanian intracontinental orogeny. The fluid, in which gold was transported mainly as Au (HS)-2, rose along deep fractures. At shallow depths, temperature and pressure decreased, the fluid was neutralized and oxidized, and ore was deposited.