LITHOGEOCHEMISTRY OF ALTERED ROCKS AT THE NEW-INSCO VMS DEPOSIT, NORANDA, QUEBEC

The New Insco Cu-bearing massive sulphide deposit is located in volcan ic rocks of Archean age of the Abitibi greenstone belt in the Noranda district. The conformable ore lens lies within a sequence of calc-alka line andesites ( Zr/Y = 6.5; La-N/Yb-N = 5.3). Hydrothermal alteration has strongly affected these rocks on either side of the ore, converti ng them to assemblages rich in chlorite near the ore and in sericite /- calcite at the margins. Immobile Ti, Zr and Al were used to calcula te mass changes owing to alteration. Net changes of + 11 wt% consist o f large additions of Fe, Mg, K and S, and depletions Na, Ca and Si. Th e light- and middle-REE were also mobile in a very regular fashion. No rmative alteration minerals, cation bulk compositions, and mobile elem ent mass chan es illustrate geochemical and mineralogical trends in th e alteration zone. Rocks at the margin of;he zone were sericitized, ca rbonitized and locally silicified. Closer to the ore, Si, Na and Ca we re leached, Fe and Mg added, and the rocks were converted to quartz-ch lorite and chloritite. These geochemical trends can be used to disting uish hydrothermally altered rocks from other greenstones, and in evalu ating the potential for mineralization.