Section 4. Mines with gold as principal product or as by-product. —

Newly-mined gold produced as principal product or as by-product may be sold either to the Government through the Central Bank in the form of refined gold at the current official price per ounce plus the assistance provided in section six hereof or in the domestic free market without benefit of the assistance, at the choice of the gold producer: Provided, however, That at least seventy-five per centum of all newly-mined gold produced by each mine either as principal product or as by-product shall be sold to the Government in accordance with the provisions of this Act: And provided, further, That mines producing gold as by-product shall only be entitled to the assistance given to mines over the marginal classification.