Impact-generated Hydrothermal Activity at the East and West Clearwater Lake Impact Structures

English Abstract:

The Clearwater Lake complex is located in northern Quebec, Canada (56°10 N, 74°20 W). Originally thought of as a double impact structure dated at ~290 Ma, recent dating re-evaluated East Clearwater to have formed at ~470 Ma. Hydrothermal alteration has previously been noted at both structures but not investigated closely. We have examined drill core samples from the impact melt sheet of East Clearwater and field samples from West Clearwater to characterize and compare hydrothermal alteration and mineralization at both impact structures. The core from East Clearwater penetrates ~45 m of the impact melt sheet. Alteration is not common in the impact melt rock as a whole and mineralization is concentrated near the top of the melt sheet in a highly vesicular zone. Cavities are filled primarily with quartz mineralization and illmenite, galena, and millerite are common throughout. Sphalerite and chalcopyrite were identified closer to the middle of the sequence whereas magnetite and hematite are more commonly seen at greater depth. Calcite appears within the vesicular zone but is also more common at greater depth. At West Clearwater quartz-filled vugs and concentrated zones of vesicles occur within the impact melt rocks. Similar to the vesicular zone within the East Clearwater melt sheet, two outcrops within impact melt rock of West Clearwater have ~2-5 m highly vesicular zones. Vesicles here are also filled with quartz, calcite, and clay minerals yet the sulphide mineralization common in East Clearwater is not immediately evident at West Clearwater. Despite differences in the overall colour of the impact melt rocks at these locations the amygdales at both outcrops have pink, red, and dark purple haloes from their rims up to twice the diameter of the amygdale at the centre. A striking difference to East Clearwater however is the pervasive alteration evident in the impact melt rocks with their overall red colouration. The differences in colour between the impact melt rocks and the localised zones around the filled vesicles might represent different episodes of alteration. Further geochemical and fluid inclusion studies will be carried out to characterize the possible origin and evolution of the hydrothermal system generated by the impacts at this site.