BACKGROUNDThe composition of major and trace elements in arsenopyrite can be used to identify the occurrence of elements and explore the remobilization and migration behaviour of elements in different stages. The Haoyaoerhudong gold deposit in Inner Mongolia is a super large gold deposit hosted in the black shales of the Bayan Obo Group. Gold-bearing minerals such as arsenopyrite and loellingite are present. Previous researchers have used the traditional powder dissolution method to analyze the isotope of the ore and discussed the source of ore-forming materials, but the migration and enrichment mechanism of gold has not been unraveled.
OBJECTIVESTo understand the gold migration and enrichment process of this deposit.
METHODSBased on mineralogy, different types of arsenopyrite were analyzed by electron probe microanalyzer (EPMA) and inductively coupled plasma-mass spectrometry (ICP-MS). The data measured by EPMA was corrected by ZAF program, and the data measured by LA-ICP-MS was quantitatively calculated by "no internal standard-matrix normalized calibration".
RESULTSThe results showed that loellingite was developed in arsenopyrite. They can be divided into Apy-Ⅰ1, Apy-Ⅰ2, Lo-Ⅰ in progressive shear deformation stage and Apy-Ⅱ1, Apy-Ⅱ2 and Lo-Ⅱ in post shear deformation stage. The major element composition of arsenopyrite in different generations was stable, with a small amount of Co and Ni and a trace amount of Sb, Te, Bi, Pb, Au and Ag. Cobalt was higher in Apy-Ⅱ1 and Apy-Ⅱ2, whereas Au, Bi, Pb and Te were obviously enriched in Apy-Ⅰ1. Loellingite was rich in As (64.06%-67.87%), Co (0.33%-4.98%), Ni (1.23%-6.37%). Trace elements such as Au, Te, Bi, Pb and Ag were more enriched in Lo-Ⅱ.
CONCLUSIONSLo-Ⅱ is the most important gold-bearing mineral. The changes of temperature and sulfur fugacity lead to the precipitation of loellingite and native gold. Native gold is precipitated by remobilization and migration of "invisible gold" in early arsenopyrite and loellingite.