The evolution of aqueous–carbonic fluids in the Amba Dongar carbonatite, India: implications for fenitisation

Abstract

Primary and pseudosecondary fluid inclusions hosted by apatite in calciocarbonatite at Amba Dongar, India, and primary fluid inclusions in quartz of the surrounding fenitised sandstones provide an important opportunity to reconstruct the evolution of fluids in a carbonatite setting and determine the composition of the fenitising fluids. Five types of fluid inclusions are present in apatite, namely aqueous liquid–vapour (LV), two-phase aqueous–carbonic (VL), three phase aqueous–carbonic (LLV), one or two solid-bearing aqueous (LVS), and multisolid-bearing aqueous (LVMS) fluid inclusions. Electron microprobe analyses of the solids in opened inclusions suggest that they comprise, calcite, strontianite, ankerite, arcanite (K 2SO 4), albite, K-feldspar and, possibly, pyrite, and hematite. Fluid inclusions in calcite of the calciocarbonatite are either secondary or of indeterminate origin and comprise LV, VL, and LLV types. Quartz-hosted inclusions are either secondary in detrital grains or primary, where they occur in overgrowths, and are mainly of the LV type; small proportions of VL and LLV inclusions are also present. Inclusions were rarely observed in K-feldspar (fenite), and where present are all secondary and similar to those in quartz. LV, LVS, and LVMS inclusions in apatite homogenise at temperatures from 142 to 342, 406 to 594 and 422 to 628 °C, respectively, while LV inclusions in calcite, quartz (primary), and K-feldspar homogenise at temperatures of 120 to 192, 122 to 262 and 183 to 228 °C, respectively; homogenisation temperatures of quartz-hosted inclusions increase with decreasing proximity to the carbonatite, i.e., with increasing intensity of fenitisation. Homogenisation temperatures were not determined for VL or LLV inclusions. All fluid inclusion types in apatite have similar ranges of salinity, from approximately 5 to 15wt.% NaCl eq. Those in calcite have a slightly different range of salinity, from 2 to 13 wt.% NaCl. eq., while inclusions in quartz and K-Feldspar have salinities of 7 wt.% NaCl eq. or lower. The carbonic phase of VL and LLV inclusions is CO 2-dominated, with the contents of other gases (CH 4 and/or N 2), being highest and lowest in apatite- and quartz-hosted inclusions, respectively. A carbonic phase is also present in some LVS inclusions, and contains the highest proportions of CH 4 and/or N 2. Analyses of decrepitate residues indicate that LVS inclusions have low Na/Na+K and high S/S+Cl ratios and contain minor concentrations of Fe. Residues of LVMS inclusions also have low Na/Na+K and high S/S+Cl ratios, albeit higher and lower, respectively, than those of LVS inclusions, but differ from LVS inclusions mainly in having high apparent concentrations of Si and Al, which together with Ca and Sr, likely represent contributions from daughter minerals or trapped solids. LV inclusion residues have relatively high Na/Na+K and low S/S+Cl ratios, particularly in quartz. The trend of Na/Na+K vs. S/S+Cl defined by fluid inclusion residues suggests that the fluids evolved in the sequence LVS⇒(LVMS)⇒LV (ap)⇒LV (qtz). This is supported by isochoric projections, which indicate that LVS fluids and accompanying carbonic fluids were the first to be trapped (at temperatures of >700 °C, the carbonatite solidus, and pressures>4 kilobars (kb)), LVMS fluids were trapped at similar temperatures, but pressures<2 kb, LV (ap) inclusions were trapped at lower temperature and LV (qtz) inclusions were trapped at temperatures of <250 °C and a pressure of about 100 bars. A model is proposed in which a CO 2–CH 4-bearing, S- and K-rich aqueous fluid, preserved as LVS fluid inclusions, exsolved from the calciocarbonatite at a depth of >10 km and a temperature≥700 °C. As this fluid rose to higher crustal levels, it exsolved its carbonic component, which was trapped as LLV and VL fluid inclusions, and mixed with oxidising meteoric or formational waters. The LVS fluid became more dilute and was trapped as LV fluid inclusions in apatite, while the carbonic fluid evolved by converting its CH 4 to CO 2. When the carbonatite magma reached a depth of 3–5 km, it exsolved its last aliquots of aqueous fluid, which were solute-rich, had intermediate K/Na and S/Cl ratios and probably contained significant concentrations of Ca, Al, and Si. These fluids, which were trapped as LVMS inclusions, invaded the surrounding sandstones and replaced quartz with K-feldspar producing potassic fenite. During the waning stages of hydrothermal activity, the system was dominated by a low salinity (NaCl) meteoric or formation water, which redeposited silica as overgrowths on corroded quartz relicts in the fenites.