LOW-TEMPERATURE MINERALIZATION OF THE SUB-TRIASSIC UNCONFORMITY SURFACE AND ALTERATION OF THE UNDERLYING INTRUSIONS OF SOUTHERN LEICESTERSHIRE, ENGLAND

The Croft diorite in central England belongs to a suite of Caledonian igneous rocks collectively known as the South Leicestershire diorite c omplex. Although the intrusions occupy separate outcrops, they are lin ked at depth to form a single pluton, buried beneath a cover of Triass ic sediments (Le Bas, 1972; 1982, Allsop and Arthur, 1983). Both the C aledonian diorites and the overlying sub-Triassic unconformity have be en affected by a complex history of alteration and mineralization whic h can be subdivided into four stages: (1) deuteric effects, caused by the release of volatiles during magmatic cooling; (2) albitization thr ough sodic enrichment; (3) formation of low-temperature laumontite, an alcime and calcite veins with associated wall-rock alteration to prehn ite and pumpellyite; and (4) sub-Trriassic unconformity-hosted base me tal, manganese and palygorskite mineralization. Zeolite mineralization occured some 200 Ma later than the intrusion itself, during post-Tria ssic times, as indicated by the presence of a single vein of laumontit e and microcrystalline calcite which cross-cuts the sub-Triassic uncon formity surface and enters the overlying Triassic sediments. Evidence from fluid inclusion work indicates that two fluids were involved in t he deposition of the zeolite veins. One fluid was initially of relativ ely high temperature (similar to 100 degrees C to 320 degrees C) and l ow salinity (similar to 0.2 to 5.9 wt% NaCl equiv.), and was probably meteoric in origin, whereas the other was of relatively low temperatur e (similar to 41 degrees C to 165 degrees C) and high salinity (simila r to 0.4 to 16.72 wt% NaCl equiv.), and is interpreted to represent a basinal brine. During Triassic rifting, thinning and fracturing of the crust, with the possible rise of associated magmas (Halliday and Mitc hell, 1984), could have increased permeability and heat flow, initiati ng the circulation of hydrothermal fluids. Triassic unconformity-hoste d base-metal mineralization in Central England is similar to other Tri assic-Jurassic mineralization in Europe (Mitchell and Halliday, 1976). (C) 1998 Canadian Institute of Mining, Metallurgy and Petroleum. Publ ished by Elsevier Science Ltd. All rights reserved.