palaver2010 Newcomer

I question the seperation requirerment also. the situation is the heavy/b757 is in the VFR pattern. Approach is either clearing acft for the visual or putting acft on an ILS (only one runway @ the airport). what is towers seperation requirement? all that is written is radar (IFR)mileage. How can a tower controller provide radar seperation? The approach controller has not IDed the heavy so whats he to use, he may not even know about the heavy in the pattern. If the tower is working a small in the pattern with the heavy the twr just calls the tfc to the small prior to the small getting behind the heavy and advises the small to mantian visual sep. caution ect.The two pattern acft rules are in the book spelled out but when approach is running acft on final what is towers seperation requirerment?
ie... if the approach tfc checks in six mile final can I clear the heavy for the t/g and start a two mile base? whats the rule? sure its a good practice to follow the approach tfc but if they're lined up 20 miles out 5 to 8 mile gaps you have to get the t/g in some where.