The headword is a masculine noun (but also serving an adjectival usage here) in the nominative singular; see LSJ s.v. The word is evidently a compound of o)/ros (mountain) and xalko/s (bronze), but it was misinterpreted by the Romans to be aurichalcum, or gold-bronze; see OED s.v. aurichalcite. There is in fact no gold component; the mineral is a cuprous hydrozincite (Dana et al., p. 712).
[1] The headword is identically glossed in the Synagoge, Lexica Segueriana 320.5, and Photius' Lexicon (omicron453 Theodoridis, with other references there). Adler also cites the unpublished Ambrosian Lexicon (424).
[2] Quotation (again at tau 1164) unidentifiable.
[3] Greek Anthology 6.234.5-6 (Gow and Page, vol. I, pp. 250-1), attributed to Erucius. A Gallus (Gallos) was a priest of Rhea, the Minoan goddess equivalent to the Phrygian Kybele (Cybele), and the sister of Cronus. See OCD(4) s.v. Cronus; gamma 41; and Gow and Page, vol. II, pp. 285-6.
[4] The phrase is not otherwise attested in this precise form, but surely derives from Plato, Critias 119C: the laws of Poseidon are inscribed e)n sth/lh| ... o)reixalki/nh| in his temple at the center of the island of Atlantis (web address 1).