Capitalizing on a strong refinance market and heavy consumer demand for fixed-rate mortgages, the nation’s thrift institutions originated some $74.3 billion in 1-4 family mortgages during the first quarter of this year. This was the second highest quarterly volume ever posted by the thrift industry, the Office of Thrift Supervision reported in releasing the new government data this week. It also represented a big 87 percent jump over thrift mortgage originations a year earlier.