The company completed a resource enhancement reverse circulation drilling programme at the project during 2013 and undertook a series of screen fire assays during late December last year and early January this year.
Thor said the upgrade averaged 57% more gold over the full 34 samples submitted in the latest batch for screen fire assaying.
The company noted that samples were selected from conventional fire assay results below 2.0g per tonne and the first batch for screen fire assaying had been from samples above 2.0g per tonne of gold.
Spring Hill is situated south of Darwin. Thor owns a 51% equity interest in the project and is exercising its rights to increase its stake to 80%, from Western Desert Resources.
Thor Mining executive chairman Mick Billing said the additional results are significant, with the upgraded values spread across all four zones of the Spring Hill resource, rather than one particular zone. If this trend can be confirmed, then a substantial uplift in the value of the resource is probable.
"The upgraded results from conventional screen fire assays reported on 14th January, on assays of over 2.0g per tonne, averaged a 37% increase in grade. This latest upgrade on assays first assayed at below 2.0g per tonne, is 57%," Billing said.
"Subject to a further field programme to confirm this trend, we expect to find increases in the grade of the resource, the number of contained ounces, and the proportion of the Spring Hill resource which could be incorporated into an ore reserve and mining plan."