Except as provided in subsections (b) and (d), there shall be allowed as a deduction in computing taxable income all expenditures paid or incurred during the taxable year for the development of a mine or other natural deposit (other than an oil or gas well) if paid or incurred after the existence of ores or minerals in commercially marketable quantities has been disclosed. This section shall not apply to expenditures for the acquisition or improvement of property of a character which is subject to the allowance for depreciation provided in section
167, but allowances for depreciation shall be considered, for purposes of this section, as expenditures.