SECONDARY MINERALS IN HALF-MOON VESICLES OF THE 
MESOZOIC BASALTS OF THE POMPERAUG BASIN

	GARABEDIAN, James, A.,  GRAY, Norman,  and H., 	
		STEINEN, Randolph, P.,
		all at   Dept. of Geology and Geophysics, University of  
		Connecticut,  Storrs, CT, 06269-2045

The secondary minerals filling decimeter size, half-moon shaped gas 
cavities in the upper 5 meters of the lower Holyoke flow in the 
Pomperaug Basin of western Connecticut are characterized by several 
distinct parageneses.  The mineral assemblages and textural features 
in adjacent vesicles are uniform over lateral distances of 10¹s of 
meters but transitions from one paragenesis to another are abrupt and 
occur over distances of less than a meter.
	Thin (< 1mm), evenly spaced, originally horizontal ³rafts² of 
spherulitic ferroan carbonate ± pumpellyite appear to be the earliest 
minerals precipitated in the cavities.  These delicate carbonate septa, 
which record the locations of former water levels within the vesicles, 
are only preserved where encrusted by later chalcedony, prehnite 
and/or datolite.  Early, rhythmically banded chalcedony, accompanied 
in places by cm long orthorhombic needle and plate-like anhydrite (?) 
crystals (now only represented by crystal casts or empty cavities), is 
common and lines the walls of many cavities.  In rare cavities the 
early lining is represented by an almost continuous rind of centimeter 
size trapezohedral (analcite or wairakite?) pseudomorphs.  The banded 
chalcedony and analcite(?) subsequently became unstable and are now 
preserved in various stages of dissolution and replacement by 
porcelaneous datolite and/or fine-grained pumpellyite.  Vesicles 
containing late prehnite display the most complex paragenesis.  The 
Pomperaug basin basalts are well known by mineral collectors for 
their unique prehnite ³hearts²;  loose colloform masses with no 
evident connection to the walls of the cavities.  The prehnite in these 
aggregates nucleated on the surfaces of the datolite pseudomorphs of 
the early chalcedony-linings.  Subsequent dissolution of the datolite 
exposed new, outward facing surfaces on which later prehnite was 
able to nucleate and grow.  The resulting prehnite aggregates thus 
show evidence for growth in two directions from ³phantom surfaces² 
suspended in the interior of the cavity.  Centimeter size clear, 
museum-quality crystals of laumontite, apophyllite, chabazite, 
heulandite, drusy quartz and calcite were amongst the last minerals to 
crystallize in open spaces remaining in the interior of the cavities.  In 
some half-moons a dusting of fine grained, vitreous bitumen 
hydrocarbon, sphalerite and pyrite coat the surfaces of the late drusy 
crystals.
	The fine-grained character, the isopacous accretionary 
rhythmic banding and the extensive dissolution and pseudomorphic 
replacement of some of the early crystallizing minerals, suggest rapid 
precipitation and extreme fluctuations in fluid compositions 
(especially Boron) consistent with an early steam dominated 
hydrothermal system associated with the initial cooling of the lava 
flow.  The later, well crystallized, prehnite, calcite and zeolite fillings 
imply quieter conditions, associated perhaps with the burial diagenesis 
of the whole sediment-basalt pile.