Faces beamed at the Interior Department's Bureau of Mines yesterday, after director Robert Horton announced that he had been able to resolve the "personal matters" that prompted his letter of resignation last month. To agency insiders, that means Horton won an important showdown with the forces of Interior Secretary James G. Watt over a new effort to roust out employes suspected of less-than-conservative bent. On Horton's side was the mining industry, which funneled letters of support through the office of Sen. Paul Laxalt (R-Nev.) . . . . But somebody did leave the Interior Department: Roy H. Sampsel, deputy assistant secretary for Indian affairs, who quit, effective yesterday, "for personal and financial reasons." Indian groups around town speculate that Sampsel wanted to drop the "deputy" off his title, but finally gave up waiting for Wattto nudge out assistant secretary Kenneth L. Smith . . . . Stephen Hanauer, the Nuclear Regulatory Commission's plain-spoken expert on reactor safety, is leaving tomorrow to join an energy consulting firm in Arlington. "Opportunity knocked," is Hanauer's explanation for the move after 17 feisty years with the NRC and its predecessor, the Atomic Energy Commission. No outpouring of industry support is expected for Hanauer, who once responded to industry calls for deregulation with a characteristically blunt remark: "What have you people been smoking?"