Brecciation and epithermal mineralization in the Baia Mare ore district (Romania): The Wilhelm deposit

Abstract The Wilhelm deposit is part of an intermediate-sulfidation polymetallic Zn-Pb-Ag-Au vein system in the Baia Mare ore district of the Eastern Carpathians, Romania, where left-lateral tectonic displacement in an E–W-dominated fault system controlled the epithermal mineralization. The Wilhelm deposit consists of tens of vein sets, including the sub-parallel Complex veins (1–7), and is hosted by the ca. 11.6 Ma Ulmoasa dacite. The 7 Complex vein is composed of sub-vertical breccia, including marginal fine-grained rock flour breccia (Fgb), which is matrix-supported and cemented by minor interstitially precipitated minerals. This breccia is cut and re-brecciated multiple times by a clast-supported breccia (Csb) cemented by four hydrothermal mineralization stages comprising the economic ore, i.e., gray quartz–pyrite (M1); sphalerite–galena–pyrite–freibergite (M2); white calcite–electrum–freibergite (M3); and gray-blue quartz–freibergite with an unnamed Ag-mineral (Ag,Cu)8SbS5 (M4). Each mineralization stage follows its distinct brecciation event (B1 to B4). This clast-supported breccia is comminuted near contacts with unbrecciated dacitic host rock to a matrix-supported breccia with rock flour enclosing small ore and host-rock fragments (Ofb). The rounded shape of fragments originating in the underlying sedimentary rock basement indicates the upward transport of Fgb by incipient phreatomagmatic activity. The dominant ore and gangue minerals of Csb suggest dominant hydrothermal fluid flow during ore formation. The gradual contact of Csb with Ofb and its clasts indicate that Ofb was formed by tectonic movement during the waning stage of hydrothermal activity, because both breccias are cut by pink calcite veins (mineralization stage M5). A similar evolution from phreatomagmatic breccias through repeated hydrothermal brecciation and mineralization to late tectonic brecciation is characteristic of the entire Baia Mare district. Its consistent relation to coeval calc-alkaline magmatism indicates that magmatic fluids played an essential role in ore formation. Resulting steep thermal gradients with or without mixing with surface water created a metal zonation with deep Cu-rich zones, intermediate Pb-Zn zones, and Au-rich bonanza zones due to shallow fluid boiling. The local presence of Au-rich ores also at greater depth indicates fluid de-sulfidation as an alternative mechanism for Au precipitation.