Equity investors lost Rs 6.31 lakh crore in wealth on Monday as benchmark indices Sensex and Nifty slumped nearly one per cent amid a broad-based sell-off triggered by escalating tensions in West Asia, surging crude oil prices and weak global market cues. The 30-share BSE Sensex declined 719.08 points, or 0.97 per cent, to close at 73,524.26, after plunging as much as 924.40 points during intra-day trade. The NSE Nifty also dropped 243.70 points, or 1.04 per cent, to settle at 23,123.