Calculating the thermal conductivity of an alloy is not done by taking an average (or weighted average) of its elemental constituents. When two metals are mixed, their properties dramatically change, often in ways which would be difficult to speculate.
For example, the thermal conductivity of silver metal is approximately 429W•m−1•K−1 (according to Wikipedia) and tin metal is 73W•m−1•K−1. Both of these values are very high. Surprisingly, the conductivity of Indalloy #121 (96.5Sn 3.5Ag) is a considerably lower 33W•m−1•K−1.
This may not seem logical, but is correct. The thermal conductivity of an alloy is impacted more by the chemical bonding structure between the inclusive elements than by the measured conductivity each element exhibits alone. The bonding orientation of the metals affect the rate which heat can pass through the material.
Thermal conductivity of common metal alloys can be found in Indium's Solder and Alloy Directory.