HDG TRACK DEV WITH APCH TO WRONG RWY BY A HAWKER JET CREATED AN NMAC BTWN IT AND THE RPTR'S ACFT ON SHORT FINAL APCHING RWY 8R AT MIA.

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FLT WAS CLRED FOR THE VISUAL APCH TO RWY 8R AT MIA. MIA TWR CALLED TFC; A HAWKER JET; BEHIND US ON APCH TO RWY 8L AND WE WERE TOLD THAT THE TFC HAD US IN SIGHT AND WOULD MAINTAIN VISUAL SEPARATION. WE SAW THE TFC ON TCASII BUT DID NOT HAVE VISUAL CONTACT. WE SAW THAT HIS TCASII TARGET WAS GETTING CLOSER TO US. I CONTINUED TO LOOK FOR THE TFC BUT DID NOT HAVE HIM IN SIGHT. WITHIN SECONDS; WE GOT A TCASII RESOLUTION ALERT INSTRUCTING US TO CLB AND THE ACFT'S TCASII TARGET SHOWED MERGED WITH US WITH A +1 (100 FT) DEV ABOVE US. I LOOKED BACK OUT THE L WINDOW AND STILL SAW NOTHING. I GLANCED BACK AT THE TCASII; WHICH STILL SHOWED THE TARGET +1 ABOVE. LOOKING BACK OUT THE L WINDOW; I SPOTTED THE HAWKER JET ROUGHLY 100 FT BELOW OUR L WING; MOVING UNDERNEATH US. THE FO (PF) CALLED FOR A GAR AND THE PROC WAS EXECUTED. I ADVISED THE TWR OF THE GAR AND HE SAID THAT THE HAWKER WAS ON A VISUAL APCH FOR RWY 8L AND HAD US IN SIGHT; BUT THAT MAYBE HAD OVERSHOT HIS APCH 'A LITTLE.' I TOLD HIM THAT THE ACFT WAS TOO CLOSE AND THAT WE WERE CONTINUING WITH THE GAR PROC. IT APPEARED TO ME THAT THE ACFT WAS HEADED TO RWY 8R. THE TWR INSTRUCTED US TO EXECUTE A TIGHT L 360 DEG TURN AND RECLRED US FOR THE VISUAL APCH TO RWY 8R. UPON COMPLETING 270 DEGS OF THE TURN; WE WERE TOLD THAT THERE WAS A DC8 ON A 9 MI FINAL FOR RWY 8L AND WE WERE INSTRUCTED TO 'KEEP IT TIGHT.' BOTH THE FO AND MYSELF GOT A VISUAL ON THE DC8 AND AGREED THAT IT WAS TOO CLOSE FOR US TO CONTINUE THE APCH SAFELY. I ADVISED THE TWR OF THIS AND WE WERE GIVEN A 320 DEG HDG TO AVOID THE DC8. WE WERE THEN INSTRUCTED TO CONTINUE THE R TURN AND RECLRED AGAIN FOR THE VISUAL APCH TO RWY 8R. AN NMAC RPT WAS FILED WITH THE MIA TWR CONCERNING THE HAWKER JET. INCIDENTALLY; AN OFF-DUTY ACR MECH SAW THE NMAC AND IMMEDIATELY CONTACTED THE MIA ACR CHIEF PLT'S OFFICE TO RPT WHAT HE SAW.