By virtue of Presidential Decree No. 461 dated May 17, 1974, the Bureau of Mines which was under the direct supervision and control of the Department of Agriculture and Natural Resources was transferred to the Department of Natural Resources. It was also on this date that the Mineral Resources Decree of 1974 (P.D. 463) was issued replacing Commonwealth Act No. 137 which provided, among others, for a modernized system of the administration and disposition of mineral lands. P.D. 463 also mandated the division of the Philippines into five (5) mineral regions administered by offices then called Mines Regional Offices (MRO) under direct supervision by the Bureau of Mines and Geosciences, Central Office. However, MRO Region IV was actually organized and started exercising its functions and authority only on October 1, 1976 with jurisdiction over mining activities (except energy) in 17 provinces comprising Region III (Central Luzon), Region IV (Southern Tagalog) and municipalities of the National Capital Region. Aquino issued Executive Order No. 192 which provided for a department-wide reorganization and integration. MRO-IV was merged with other Regional Offices of the Forestry, Environment, Lands and Research Sectors of the Department of Environment and Natural Resources (DENR) and was then officially known as the Mines and Geosciences Development Service (MGDS) of DENR Region IV headed by a Regional Executive Director. Its jurisdiction over mining affairs was then whittled down to 11 provinces of the Southern Tagalog Region(Aurora, Batangas, Cavite, Laguna, Marinduque, Occidental Mindoro, Oriental Mindoro, Palawan, Quezon, Rizal and Romblon). And again in 1991 its jurisdiction over small-scale mining (those operations covered by P.D. 1899 and R.A. 7076) was devolved to Local Government Units (LGUs) in compliance with R.A. 7160 (the Local Government Code), although the Regional Office continue to extend technical and staff assistance to LGUs through the Provincial Regulatory Boards (PMRBs). In January of 1994, then Secretary Angel A. Alcala signed DAO No. 230 which effected the split of DENR-IV into two Regional Offices, namely, Region IV-A covering the six (6) mainland provinces and Region IV-B over the remaining five (5) island provinces. This situation was, however, short-lived since the two sub-regional offices of MGDS including the MGB unit in NCR would later be merged and transformed to become MGB Regional Office No. IV under direct supervision and control by a now revitalized line bureau (Mines and Geosciences Bureau, Central Office) by virtue of R.A.. 7942, otherwise known as the Philippine Mining Act of 1995. Under this latest set-up, MGB Region IV exercises administrative and financial autonomy from its former mother unit, DENR Region IV while maintaining close coordination and inter-agency relationship in pursuit of common goals, objective and concerns. By virtue of DENR Special Order No. 2003-564 issued at the time of then DENR Secretary Elisea G. Gozun in compliance to Executive Order No. 103 signed by President Gloria Macapagal-Arroyo on May 17, 2002, Mines Regional Office No. IV was again split into two sub-regions known as CALABARZON (Cavite, Laguna, Batangas, Rizal and Quezon) and MIMAROPA (Mindoro Occidental and Oriental, Marinduque, Romblon and Palawan). At the same time, the province of Aurora was transferred from the Mines and Geosciences Bureau (MGB) Regional Office IV to Mines and Geosciences Bureau (MGB) Regional Office III implementing DENR Memorandum Circular No. 2002-07.
