** Australian gold stocks .AXGD rise as much as 3.1% to its highest since May 8
** Sub-index extends gains to a second straight session
** Bullion prices climbed amid growing concerns over U.S. government's growing debt and lukewarm demand for 20-year Treasury bonds GOL/
** Genesis Minerals GMD.AX and Northern Star Resources NST.AX rise 4.5% and 4.1%, respectively; among top gainers on ASX 200 index .AXJO
** Sub-index last up 2.9%, takes this year's gains to 43.2%, as of last close
(Reporting by Sherin Sunny in Bengaluru)
((Sherin.Sunny@thomsonreuters.com;))