Hi Bopper - Part of the answer is all about how hard and/or abrasive the basalt is.  My guess is that it is both hard and abrasive.  If that is the case then I would avoid a VSI or impactor as they tend to have high wear rates and low availabilities in these applications.  The other question is in regards to how much spare capacity you have with your existing crushers.  More fines production will require more hp.  I would look at tightening up your jaw to get the minimum top size going to the screen, install a finer screen to direct more material to your cone, and then install a fine or extra-fine short-head liner (maybe you refer to it as a sand head) in your cone crusher.  Your circulating load will go up, and you'll draw more power on the cone, but you should generate more fines.  Whether you can completely avoid importing fines will remain to be seen.  I think this is the lowest cost and fastest approach to seeing if your current equipment can meet your production needs.

Best Regards

Andrew 

Is your feed 100% basalt? I mean.. isn't it an andesitic basalt? That would define how abrasive it is (because of it's silica content). Can you generate more fines from the quarry, adjusting your blasting parameters? I'm with Andrew, first you need to exhaust your posibilities with your current equipment. I've produced sand from andesite / andesitic basalts before, using an autogenous VSI.