Mattel shares jumped about 25% Thursday to near $15.80, putting the stock on track for its biggest one-day gain since 1996, after reports that brand-licensing company Authentic Brands Group had privately discussed a takeover offer for the toy maker. The potential bid could top $20 per share and value Mattel at around $6 billion or more, roughly 58% above Wednesday's close of $12.66. Mattel was worth about $3.6 billion before The Wall Street Journal reported the discussions, while its shares had declined more than 33% this year before Thursday's advance. Talks remain preliminary, a person familiar with the matter told CNBC. The approach comes as Condé Nast CEO Roger Lynch prepares to become Mattel's chairman on Friday and assume the CEO role within the next month, succeeding Ynon Kreiz, who is leaving to become co-CEO of Paramount. There is no guarantee Mattel will accept the approach or that a transaction will be reached, and a formal sale process is not underway.