http://en.wikipedia.org/wiki/Gold
Jewellery. Because of the softness of pure (24k) gold, it is usually alloyed with base metals for use in jewellery, altering its hardness and ductility, melting point, color and other properties. Alloys with lower "k", typically 22k, 18k, 14k or 10k, contain higher percentages of copper, silver or other base metals in the alloy. Copper is the most commonly used base metal, yielding a redder metal.
If you want find gold but your LRL detects alloys of it 50/50... it actually doesn't find GOLD ! You cannot call it GOLD.
Max
No, the important is the presence of gold in the alloy since this point of view: When a LRL manufacturer claim that his product is capable to detect only gold, this means a jewell or object 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 karats, because the gold is in the metal.
I think this copper or silver mixed with the gold can creates the famous "halo". Or pure gold in presence of mineralized ferrous soil in combination with wet, pH, etc., maybe make the difference and can produces the "halo".
Regards
Nihil Roma Maius