Coal mining is one of the core industries that contributes to the economic development of a country but deteriorates the environment in certain ways. In the process of mining, huge amount of water is discharged on surface to facilitate the mining operation. The discharged water often contains high load of TSS, TDS, hard and heavy metals, which contaminate the surface and ground water. Sometimes it is acidic in nature and pollutes the water regime. The water is mainly rain water and mine discharge. The investigation was carried out to determine the pollution status caused by heavy metals of abandoned open cast mine pits through the assessment of phytoplankton diversity. The investigated sites were abandoned Harabhanga pit and Damalia pit in Harabhanga Village, Raniganj, West Bengal. The seasonal limnological study with respect to heavy metals was targeted to get uncommon biomass & algal biodiversity variation. Diatoms, followed by pollution indicator forms like Scenedesmus, Oscillatoria, Euglena, Phacus etc. were noted. The Damalia pit was found richer in comparison. Interestingly Dinoflagellate genera like Peridinium & Gymnodinium were noted. Although diatoms were recorded, dominant genera were Trachelomonas, Anabaena, Microcystis & Oscillatoria. Harabhanga pit represented a different composition of diversity having Merismopedia & Oscillatoria among blue greens, Melosira & Melosira among diatoms, Euglena & Lepocinlis among Euglenophytes and Coelastrum, Closterium & Lacunastrum among green algae. Most of the genera are pollution tolerant but some Prasinophytes are found in all water bodies. After this work it was found that rainy season is the most affective time in terms of heavy metals contamination, so phytoplankton diversity is high. The same status continues in post-monsoon season due to retention of the same metal substances in the pit water bodies. The lower rate of contamination in pre-monsoon season or dry hot summer months due to lack of surface runoff or other contaminations phytoplankton diversity appeared higher.