INFRARED MICROTHERMOMETRY AND CHEMISTRY OF WOLFRAMITE FROM THE BAIA SPRIE EPITHERMAL DEPOSIT ROMANIA

摘要

The chemical and microthermometric characteristics of wolframite crystals from the Baia Sprie Au-Ag-Pb-Zn-Cu-W ore deposit (Baia Mare district, Romania) have been investigated using electron probe microanalysis (EPMA), X-ray elemental mapping, and infrared microscopy. The chemical composition of wolframite changes from a nearly pure huebnerite (MnWO_4) core to nearly pure ferberite (FeWO_4) rims. This zonation correlates with a decrease in infrared transmittance from core to rims of wolframite crystals as demonstrated by Fourier transform infrared spectra. The Fe content of wolframite may control its infrared transparency, just as the Fe~(2+) reversible Mn~(2+) substitution controls its transparency in visible light. Microthermometric measurements, performed with an infrared microscope, indicate salinities ranging from 4.6 to 6.4 wt percent NaCl equiv and homogenization temperatures of 154 deg to 240 deg C for primary fluid inclusions, all trapped in the Mn-rich zones of the studied crystals. Secondary fluid inclusions showed similar microthermometric characteristics (4.6-6.4 wt percent NaCl equiv; temperature of homogenization, 171 deg-232 deg C). These microthermometric data agree well with data from previous studies on transparent minerals (quartz, sphalerite) from the Baia Sprie ore deposit. However, it is notable that wolframite appears to have precipitated after scheelite and at lower temperatures. A possible magmatic contribution to ore has been invoked for epithermal gold deposits that contain wolframite, including Baia Sprie. The unexpected low temperatures and low salinities of ore-forming fluids responsible for forming wolframite at Baia Sprie may indicate a mixing of magmatic fluids with a dominant meteoric water component.