Someone's already mentioned _Grundrisse_ as key text (which translator Martin Nicolaus noted is difficult read because of great length, repetition, disjointed narrative)...
Alienation in _Grundrisse_ is much as it is in _Economic and Philosophic Manuscripts_: workers from their work activity, their products, other workers, and their "species being." G is in language of M's economic analysis and discusses socioeconomic basis of various elements of alienation. Much of EPM, already distinguishable from Hegel, Feuerbach, etc., is philosophical, hence, the separation from an "authentic human community."
"Species-life" never completely disappears from Marx. Creative work, rational control of production, and sociability appear as differentiating human properties throughout his writings, but he increasingly focuses on contradictory aspects of capitalism. Maturity, no doubt played part, but one wonders how much more he could have said about earlier stuff he wrote.
Re. Althusser, he turns out to have been one more contributor to proliferation of "Marxisms" among "intellectually" sophisticated much as were Franfurters, Praxis school, Sartre... Michael Hoover