The Secondary Minerals from the Pillow Basalt of Salsette-Mumbai, Deccan Volcanic Province, India

Abstract

Secondary minerals occur within the tholeiitic basalts of Salsette Island in the greater Mumbai region, as well as in other localities in the Deccan Volcanic Province (DVP). However, the secondary minerals of Salsette Island show remarkable differences with respect to their mineral speciation and precipitation sequence, which are both due to their unique geological environment. The greater Mumbai region is built up by the Salsette subgroup, which represents the youngest sequence of the DVP. It formed subsequently to the main phase of DVP activity in Danian time (62.5 to 61.5 Ma), in the course of the India–Laxmi Ridge–Seychelles breakup. The main part of the Salsette subgroup consists of tholeiitic basaltic flows with pillows, pillow breccia, and hyaloclastite, which formed in contact with brackish and fresh water in a lagoonal environment. In some places, intertrappeans are represented by fossiliferous shallow water sediments. On the top, trachytic and rhyolitic subaqueous volcaniclastics occur, and some dioritic bodies have intruded nearby. Due to differing fluid rock interactions, several distinctly different secondary minerals developed in the void spaces of the hyaloclastite breccia of the interpillow matrix and in the pillow cavities. The highly permeable hyaloclastite breccia formed an open system, where pronounced precipitation occurred in the early phase and at higher temperatures. In contrast, the pillow cavities were a temporally closed system and contained, for example, more low-temperature zeolites. The genesis of the secondary minerals can be summarized as follows: During initial cooling of the volcanic rocks at about 62 Ma, the first mineralization sequence developed with chlorite, laumontite I, quartz, and calcite I. Ongoing magmatic activity caused reheating and the main phase of precipitation at prehnite–pumpellyite facies conditions. During generally decreasing temperatures, in the range of 270–180 °C, babingtonite, laumontite II, prehnite, julgoldite, yugawaralite, calcite II, ilvaite, pumpellyite, and gryolite developed. The fluid contained SiO2 + Al2O3 + FeO + MgO + CaO, and minor MnO and Na2O, and was predominately mineralized by the decomposition of basaltic glass. Further temperature decreases caused zeolite facies conditions and precipitation of okenite I, scolecite, heulandite, stilbite, and finally chabazite I, in the temperature range of 180 °C to less than 100 °C. As FeO, MgO, and MnO were then absent, an interaction of the fluid with plagioclase is indicated. According to Rb-Sr and K-Ar ages on apophyllite-K, a third phase of precipitation with apophyllite-K, okenite II, and chabazite II occurred in the late Eocene to early Oligocene (30–40 Ma). The new hydrothermal fluid additionally contained K2O, and temperatures of 50–100 °C can be expected.