Abstract-The well-preserved state and excellent exposure at the 39 Ma Haughton impact structure, 23 km in diameter, allows a clearer picture to be made of the nature and distribution of hydrothermal deposits within mid-size complex impact craters. A moderate-to low-temperature hydrothermal system was generated at Haughton by the interaction of groundwaters with the hot impact melt breccias that filled the interior of the crater. Four distinct settings and styles of hydrothermal mineralization are recognized at Haughton: a) vugs and veins within the impact melt breccias, with an increase in intensity of alteration towards the base; b) cementation of brecciated lithologies in the interior of the central uplift; c) intense veining around the heavily faulted and fractured outer margin of the central uplift; and d) hydrothermal pipe structures or gossans and mineralization along fault surfaces around the faulted crater rim. Each setting is associated with a different suite of hydrothermal minerals that were deposited at different stages in the development of the hydrothermal system. Minor, early quartz precipitation in the impact melt breccias was followed by the deposition of calcite and marcasite within cavities and fractures, plus minor celestite, barite, and fluorite. This occurred at temperatures of at least 200 °C and down to ∼100-120 °C. Hydrothermal circulation through the faulted crater rim with the deposition of calcite, quartz, marcasite, and pyrite, occurred at similar temperatures. Quartz mineralization within breccias of the interior of the central uplift occurred in two distinct episodes (∼250 down to ∼90 °C, and <60 °C). With continued cooling (<90 °C), calcite and quartz were precipitated in vugs and veins within the impact melt breccias. Calcite veining around the outer margin of the central uplift occurred at temperatures of ∼150 °C down to <60 °C. Mobilization of hydrocarbons from the country rocks occurred during formation of the higher temperature calcite veins (>80 °C). Appreciation of the structural features of impact craters has proven to be key to understanding the distribution of hydrothermal deposits at Haughton.

No abstract

Hydrothermal alteration at Rhynie, Aberdeenshire, Scotland, is concentrated along a fault zone, which juxtaposes surface deposits and the mineralised feeder zone to the Rhynie hotspring system. Mineralisation consists of breccias and veins filled with quartz, chert, calcite, K-feldspar and pyrite. Associated pervasive alteration comprises a high-temperature K-feldsparquartz-illite facies (formed at 250–350°C), a medium-temperature mixed layered illite/smectitequartz-K-feldspar-chlorite-calcite facies (formed at 150–200°C) and a low-temperature mixed layered illite/smectite-chlorite-calcite facies (formed at 100 to +150°C). The fluids responsible for mineralisation were mainly moderate- to high-temperature (Th =91–360°C), low-salinity (<0·2 to 2·9 wt.% NaCl eq.) H2O-NaCl-heated meteoric fluids comparable to modern and ancient hot-spring systems. The migration of these fluids was mainly restricted to a major fault zone bounding the Devonian basin. Fluids responsible for mineralisation, alteration and cementation elsewhere in the basin were low-temperature (Th 57 to 161°C), low- to high-salinity (<0·2 to 18 wt.% NaCl eq.) H2O-NaCl fluids, which resemble basinal brines.

Veins of red dolomite occur extensively in the Dalradian rocks of Argyll, Scotland and adjacent areas. The veins represent brittle extensional deformation, preferentially reactivating Caledonian quartz veins. The dolomite is associated with reddening of the adjacent Dalradian country rock, which it partially replaced. Dolomite was also precipitated in overlying Old Red Sandstone, and probably dates to late Carboniferous–early Permian. Fluid inclusion studies show that the veining involved moderate- temperature (75 to 115 °C) fluids. Stable isotope data suggest that these fluids were basinal brines. Traces of chalcopyrite, paragenetically late in the veins, may reflect the mineralization which occurs more widely in the Dalradian rocks of Argyll. The red colour of the dolomite is due to abundant haematite crystallites that grew in the dolomite crystal fabric. Palaeomagnetic analysis yields a consistent late Permian–early Triassic age for the haematite growth in the dolomite veins and the reddened Dalradian country rocks. This age represents the time of haematite precipitation from iron-rich dolomite that may have been related to deep oxidizing weathering. Gold anomalies associated with reddened basement rock must be of this age or younger.