BUZZ-Pacific Rubiales Energy Corp: Alfa, Harbour drop takeover plan
** Canadian oil and gas producer's shares down about 42 pct at C$3.05
** Mexican industrial conglomerate Alfa SAB de CV and energy investment firm Harbour Energy Ltd drop plan to buy Pacific Rubiales
** Deal terminated due to proxy firms' opposition over valuation, companies say
** Up to Wednesday's close, stock had risen nearly 9 pct since deal was made public on May 5
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