Sequential evolution of Sn–Zn–In mineralization at the skarn-hosted Hämmerlein deposit, Erzgebirge, Germany, from fluid inclusions in ore and gangue minerals

Abstract

Skarn-hosted deposits can be important high-grade resources for a variety of metals, but Sn skarns are still of subordinate importance for global mining because of their complex mineralogy and evolution. As part of recent exploration efforts, the economic potential of the Sn–Zn–In mineralization at the Hammerlein skarn-hosted deposit is currently being re-evaluated. The temporal and spatial evolution of the ore-forming hydrothermal system is still debated. We analyzed fluid inclusion assemblages (FIA) in ore and gangue minerals using conventional and infrared microthermometry and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We further estimated alteration temperatures with chlorite thermometry and compared the fluid inclusion (FI) record at Hammerlein with a mineralized greisen sample of the nearby Eibenstock granite. Cassiterite in skarn forms the major mineralization at Hammerlein during stage I and hosts FIs showing homogenization temperatures of up to 500 °C and salinities between 30 and 47 wt% NaCl eq. Cassiterite from schists and the additional greisen sample from the Eibenstock granite of the later stage II show lower homogenization temperatures (350–400 °C) and considerably lower salinities varying from 1.9 to 6 wt% NaCl eq. Despite the different homogenization temperatures and salinities, the chemical compositions of FIs hosted in both generations of cassiterite show that both (cassiterite in skarn and in schist and greisen) are similar, which points to a common source. The gangue minerals mainly contain low-temperature FIA (max. 330 °C and 2–9 wt% NaCl eq.) and are interpreted to form during further cooling of the system in stage III. Intergrown chlorite has compositions indicating similar temperatures of around 260 °C and is thus also related to stage III. FIA in sphalerite homogenize around 200 °C with salinities between 2 and 6.7 wt% NaCl eq. and show decreasing trace element contents despite having the same salinity range as the gangue minerals, indicating dilution of the ore-fluid during stage IV as a possible precipitation mechanism. Stage I inclusions are solely hosted in cassiterite from skarn, which shows the importance of fluid inclusion analyses in ore minerals, and record remarkable high mineralization temperatures, exceeding the typically temperature range reported in other studies by at least 100 °C. Our results suggest that this main ore stage is related to the early expulsion of a high-salinity brine phase from an underlying magmatic intrusion at depths greater than 3 km, which likely is a relatively short-lived event within the evolution of the hydrothermal system.