You are mistaken that minority rule is fundamental to Apartheid. It’s not simply ‘a system of oppression’, it is the establishment and maintaining of systematic oppression and domination of one racial group over another. Let’s look at Article II of the Apartheid Convention for one. We can also look into the definition from the Rome Statute.

Article II then lists specific inhuman acts that committed in this context amount to the crime under international law of apartheid, ranging from violent ones such as murder and torture to legislative, administrative and other measures calculated to prevent a racial group or groups from participating in the political, social, economic and cultural life of the country and deny them basic human rights and freedoms. The specific inhuman acts enumerated are: 

a. Denial to a member or members of a racial group or groups of the right to life and liberty of person:

(i) By murder of members of a racial group or groups;

(ii) By the infliction upon the members of a racial group or groups of serious bodily or mental harm, by the infringement of their freedom or dignity, or by subjecting them to torture or to cruel, inhuman or degrading treatment or punishment;

(iii) By arbitrary arrest and illegal imprisonment of the members of a racial group or groups;

b. Deliberate imposition on a racial group or groups of living conditions calculated to cause its or their physical destruction in whole or in part;

c. Any legislative measures and other measures calculated to prevent a racial group or groups from participation in the political, social, economic and cultural life of the country and the deliberate creation of conditions preventing the full development of such a group or groups, in particular by denying to members of a racial group or groups basic human rights and freedoms, including the right to work, the right to form recognized trade unions, the right to education, the right to leave and to return to their country, the right to a nationality, the right to freedom of movement and residence, the right to freedom of opinion and expression, and the right to freedom of peaceful assembly and association;

d. Any measures including legislative measures, designed to divide the population along racial lines by the creation of separate reserves and ghettos for the members of a racial group or groups, the prohibition of mixed marriages among members of various racial groups, the expropriation of landed property belonging to a racial group or groups or to members thereof;

e. Exploitation of the labour of the members of a racial group or groups, in particular by submitting them to forced labour;

f. Persecution of organizations and persons, by depriving them of fundamental rights and freedoms, because they oppose apartheid.