I think it can be attributed to the difference in their 'redeemability' for bullion.
The gold/silver ratio is now a bit over 65, showing that the markets are under some obvious and general liquidity stress, and as such, gold tends to offer a bit more 'safety.'
I am tending to view the selling in the equity markets as the effect of the Fed jawboning to let some of the air out of a growing asset bubble that was becoming overleveraged.
The professionals are using this as an opportunity to take the public and the real economy out for an old fashioned 'wash and rinse' in which they frighten people out of positions that up until recently they had been urging them to take.
There is money to be made in shorting, and then one buys the same assets all over again on the cheap. This is the fallacy of efficient market theory and the benefits of financialisation. It becomes, at its extremes of deregulation and moral hazard, little more than a wealth transferal scheme, which is a fancy word for a con game. And it can rise to and corrupt the highest levels of a society.
I may adjust my outlook if the September SP 500 futures do not hold at 1518 which is the 50% Fibonacci retracement level. Right now we are at 1553 which is about a 38.2% retracement from the highly controlled, almost straight line rally that began at the beginning of the year.