Geology and He-Ar-S-Pb isotope constraints on the genesis of the Sidaogou gold deposit in Liaodong Peninsula, northeastern North China Craton

Abstract

The large Sidaogou gold deposit in the Liaodong Peninsula is a typical alteration-type gold system, hosted in Paleoproterozoic metamorphic rocks of Gaixian Formation. Ore paragenesis indicates Sidaogou mineralization occurred in four stages: (1) early ore stage S1 (quartz + albite), (2) main ore stage S2 (quartz + sericite + pyrite + gold), (3) secondary ore stage S3 (quartz + polymetallic sulfide ± scheelite ± biotite ± gold), and (4) late ore stage S4 (calcite + ankerite ± quartz ± sulfide). Associated hydrothermal alterations in wall rocks are dominated by albitization (S1), silicification (S1 to S4), sericitization (S2), sulfidation (S2 to S3), and carbonatization (S4), with minor biotitization (S3) developed locally. Gold mineralization was primarily occurred in S2 stage with intensive silicification, sericitization, and sulfidation in metamorphic wall rocks, and occurred to a lesser extent in S3 stage with weak alteration possibly due to fairly weak activity and small fluxes of the ore fluids.At gold precipitation stages (S2 to S3), three generations of ore-related pyrite (Py1, Py2, and Py3) are identified. NanoSIMS in situ S isotope analyses show that both Py1 and Py2 from the S2 stage have relatively broad ranges of δ34S values with significant intra-grain variations (Py1 = 7.7–16.5‰; Py2 = 3.7–14.8‰), varying between those of wall rocks (11.6–20.3‰) and typical magmatic sources, as well as the regional contemporary magmatic-hydrothermal gold system (CMHGS). This can be interpreted by a mixed process formed by magmatic-hydrothermal fluids leaching different proportion of sulfur from wall rocks during extensive fluid-rock interaction. Py3 in the S3 stage has relatively narrow range of δ34S values (Py3 = 0.6–3.7‰) similar to those of adjacent CMHGS, indicating a magmatic origin without notable wall-rock contamination due to weak fluid-rock interaction. The S2 stage gold-bearing pyrite (Py1-dominated) shows 206Pb/204Pb ratios from 17.882 to 18.938, 207Pb/204Pb ratios from 15.592 to 15.825, and 208Pb/204Pb ratios from 37.811 to 39.003, which are between those of nearby Early Cretaceous Sanguliu granitic pluton and wall rocks ((206Pb/204Pb)i = 17.958–20.085, (207Pb/204Pb)i = 15.664–15.976, and (208Pb/204Pb)i = 37.474–40.021), implying that the lead was originated from mixture sources.Ore fluids from the S2 stage pyrite (Py1-dominated) have 3He/4He ratios range from 0.21 ± 0.01 to 0.57 ± 0.03 Ra (average 0.34 Ra), indicating a crustal source with minor mantle contributions. Moreover, the 3He/4He ratios are similar to those of the adjacent CMHGS, demonstrating a similar magmatic origin for the ore fluids. The 40Ar/36Ar ratios range from 547 to 11994, indicating that the ore fluids contained 48 to 98% crust-derived radiogenic 40Ar* which most likely inherited from wall rocks. Combining the geologic evidences and isotopic results, we propose that the Sidaogou deposit was formed by magmatic-hydrothermal fluids exsolved from Sanguliu pluton or its deep magma. During fluid-rock interaction, sulfur, lead, and 40Ar* were partly scavenged from the wall rocks and then sequestered by ore-related pyrite. Nevertheless, gold was mainly of magmatic origin. Our study reinforces the previous conclusion that S-Pb isotope compositions of ore-minerals could be strongly modified by wall rocks during fluid-rock interaction.