A monzonite-diorite stock intrudes Devonian limestone of the Palliser Group. In 1987 over 480 metres of feldspar porphyry was first intersected by drilling. The feldspar porphyry was drilled from 1987 to 1999 by R.H. Stanfield (Bul River Mineral Corporation) in order to investigate the feldspar potential. Indicated and inferred resources are 300 million tonnes of monzonite-diorite, predominantly feldspar and syenite building stone (Bul River Mineral Corporation, 1998).

In 1994, R.H. Stanfield conducted an investigation of commercial feldspar resources on the Aspen 9, 10, 11, and 12 claims. Re-logging of 1987 drillcore from hole A11-1-87 was completed (Assessment Report 23602). The hole was initially drilled to a depth of 532 metres at the northeast corner of the Aspen 11 claim. In addition, in 1994, the approximate areal extent of the feldspar porphyry was determined by mapping of outcrops and two percussion drill holes were completed to determine the depth and grade of the feldspar porphyry intrusion. These holes (A10-1-94 and All-1-94) were drilled on the Aspen 10 and Aspen 11 claims respectively. Grab samples of the cuttings were collected for every two-foot (0.61-metre) interval. Composites of the cuttings were analyzed for major, minor and trace elements after complete fusion of minus 150 mesh pulverised pulps using atomic absorption spectroscopy. The alumina content must be in excess of 23 per cent and alkali content in excess of 14 per cent. The alumina content was just below that required.

In 1996 three percussion drill holes (F1,2,3,4 - 96) totaling 3576.11 metres were completed to investigate the area to the south of areas previously tested. (Assessment Report 24595). Later in 1996, eight rotary-drill holes (F5-96, F6-96, F7,96, F8-96, F9-96, F10-96, F11-96, F12-96) totaling 1083.4 metres were completed for R.H. Stanfield on the Aspen 11 and 13 claims (Assessment Report 25191). These holes were completed in order to add to previous work on the feldspar porphyry intrusion done in 1994, towards outlining a commercial feldspar resource.

A drilling campaign in 1998 extracted core rather than percussion chip samples (see Assessment Report 25871). This was done partly to get a better understanding of the alteration of the deposit. In 1998, NQ drilling totaled 1710 metres and percussion drilling totaled 93 metres. Tests were done for impurity removal.

Drilling in 1999 concentrated on the western portion of the monzonite-diorite intrusive body in order to correlate with the geophysical anomaly and to test the continuity of the alteration zones (Assessment Report 26203). A total of 841.3 metres were drilled 4 diamond-drill hole.

Refer to Bull River (082GNW002) for details of the greater Bull River property which contained the Aspen feldspar occurrence and about 25 other MINFILE documented mineral occurrences as of October 2022.