The repair of clear defects on X-ray masks (2 mu m Si membranes) requires the localized deposition of metal films with high aspect ratios and high X-ray opacity. Laser-induced heavy metal deposition from organometallic gas atmospheres has been investigated with respect to these requirements. An experimental comparison of pyrolytic (W) and photolytic (esp. W, Sn) depositions on thin Si membranes was performed to find out which of these two mechanisms is more favourable for X-ray mask repair. Using a simple model, the laser-induced temperature distribution in the membrane was calculated which strongly effects pyrolytic deposition. Although photolytic deposition tends to yield X-ray opacities below corresponding bulk values, this process turned out to be more suitable due to its better spatial resolution in this case of Si membranes used as substrates.