Gold prices rallied on Friday, gaining roughly $40 an ounce, or about 2%, in mid-day New York trading. The advance lifted bullion back toward the upper end of its recent range after a series of muted sessions. The move fed through to the SPDR Gold Shares exchange-traded fund, which tracked the metal’s gains, while silver prices also firmed. Traders cited renewed demand for perceived safe-haven assets as investors looked for hedges against market volatility.