The Afon Rhiw is over 20km long - please can you provide a more precise location, preferably an Ordnance Survey national grid reference (OS 1:50,000 and 1:25,000 maps are available for free on Bing maps).
The standard gazetteer (J.R.Foster-Smith "The mines of Mongomery & Radnorshire" (British Mining no.10), Sheffield, 1978, lists only two mines close to the Afon Rhiw:
Montgomery no.22: Adfa, NGR SJ 060/010, a trial for barytes made in 1939.
Montgomery no.23: Fachwen, NGR SO 106/997, trials comprising open pits and a shallow shaft made 1920-23 on strings of calcite with a little barytes, with traces of iron and copper.
'Spar' can refer either to barytes or to calc-spar; both the above trials accord in that respect. If the level is on old maps as being for lead then it suggests that it predates the 1920s-30s trails listed above, and could indeed have been made in the hope of finding lead as that mineral was of greater interest in the nineteenth century than barytes or calc-spar.
Historic OS six inch maps are available for free on the National Library of Scotland website: is it marked on these maps? If so, please can you quote the sheet number and give an approximate location within the sheet?
If you can provide a location people may be able to make some suggestions.