Hydrothermal fluid evolution of the Paleoproterozoic Kutemajarvi gold telluride deposit, Southwest Finland

Abstract

The Kutemajrvi gold telluride deposit is located about 200 km north of Helsinki in southwestern Finland. It occurs in the continental island arc-type Paleoproterozoic volcano-sedimentary Tampere schist belt situated in the central part of the Svecofennian domain (2.0-1.75 Ga) of the Fennoscandian Shield. The deposit, consisting of four distinct vertical ore pipes, is associated with a strongly hydrothermally altered zone in the northern part of the Tampere schist belt. The most common host rocks are intensively deformed sericite-quartz schist, quartz rock, and quartz veins. Very fine grained native gold occurs mainly along quartz grain boundaries but also in late fractures and often as symplectites with tellurides. Also, the host rocks contain minor Fe-Cu-Zn-Pb-As sulfides-sulfosalts and abundant Fe-Pb-Bi-Ag-Au tellurides. In 1994, the proven ore reserves were about 0.36 million metric tons (Mt) averaging 7 g/t Au.Postpeak metamorphic sulfide + or - gold deposition (380 degrees -320 degrees C/2.8-2.0 kbars) resulted from the interaction between the wall rocks and a low-salinity (<7 wt % NaCl equiv) H 2 O-CO 2 + or -CH 4 hydrothermal fluid, which percolated through permeable zones developed during the syn- to late-stage deformation (i.e., ductile to brittle). Phase separation (effervescence) due to local pressure fluctuations may have caused gold telluride + or - sulfide deposition at 330 degrees to 270 degrees C/1.6 to 0.7 kbars. Late brittle deformation and hydraulic fracturing opened pathways for large volumes of CO 2 -CH 4 + or - N 2 fluids, allowing a possible gas-phase transport of tellurium (and gold?). Later reheating of the deposit resulted in remobilization of early gold associated with sulfides by diluted aqueous solutions at lowered temperatures (<380 degrees C) and pressures (<1 kbar).