Brecciation and Hydrothermal Evolution of a Cretaceous Hydrothermal – Epithermal System in Shui Koushan, China.
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. The Laiyang-Linwu Tectonic Mineralization Belt lies in a Permian fold zone in the central China that was modified by shearing during the Yanshanian orogeny (185 - 70 M.Y.). At the northern end of this belt clusters containing about a hundred breccia bodies are located in a 200 km2 area that comprises the Shui Koushan Mining District. In most places, the breccias are hosted by carbonate rocks, sandstones, siltstones and shales of Permian age; but minor Carboniferous carbonate rocks, Jurassic sandstones, granodiorite and skarn also occur. Individual breccia bodies, which are various in form but generally elliptical or irregular in plan, range in diameter from as little as I meter to hundreds of meters. Their vertical extents are from several meters to about 300 meters. Generally, the breccias contain angular to subangular clasts and, in some places, tabular fragments of host rocks; minor rounded pebble clasts are present in several bodies. The distribution of most breccias is controlled by S-N oriented folds, faults and an unconformity plane. Rarely, breccias were emplaced at the contact zone of a granodiorite intrusion of Late Jurassic age and its skarn. There are six different types of breccias, l) the Lao Yazao breccia. This occurs at the contact of igneous rocks and skarns. Most clasts composed of skarn and marble are angular, but some granodiorite clasts are round. This breccia was cemented by rock flour and has been carbonatized. 2) the Kang Jiawan breccia; most breccias at Shui Koushan belong to this type. Its clasts and matrices consist of sedimentary rocks. All breccias of this type have been silicified. 3) a pebble breccia. This occurs only locally. It is slightly altered and cemented by rock fragments. 4) a conglomerate breccia. This is a sedimentary rock which is conformably overlain by Cretaceous sandstone. It is hydrothermally altered. 5) fault breccia, and 6) karst breccia, both of which are almost unaltered. The six different types of Shui Koushan breccias were formed during three brecciation events as a consequence of magmatic hydrothermal and tectonic activity in a period between the Late Jurassic and Cretaceous. The first brecciation event, which is interpreted as having been of tectonic origin, occurred as a result of folding and then fracturing after the Lao Yazao intrusion, i.e., it is younger than 140 M.Y. Most silicified breccias, including the Lao Yazao pebble breccia (carbonatized breccia) belong to this event. The second brecciation event was characterized by veining and hydraulic fracturing which took place sometime between 127 - 80 M.Y. when the Xin Mengshan andesite extruded. This brecciation event most affected the previously altered breccias. The conglornerate breccia was hydrothermally altered at some time between the first and second brecciations. The unaltered pebble breccia, karst and fault breccias clearly formed after hydrothermal alteration and mineralization occurred. Almost all breccias were hydrothermally altered but only a few were Pb-Zn-Au-Ag mineralized. Four hydrothermal alteration sequences have been identified: l) an early replacement stage (the main stage) produced aggregates of albite + calcite + epidote + chlorite + quartz in skarn fragments, and calcite + illite / smectite + chlorite in the cement of the carbonatized breccia (Lao Yazao). Quartz + hydromica ± illite + chlorite replaced all silicified breccias. This early replacement stage was followed by - 2) an open-space filling stage (the second stage): minerals formed then were quartz + calcite + illite + chlorite± fluorite, 3) a post-base metal deposition stage. characterised by a kaolinite, halloysite + dickite + quartz assemblage, Which was restricted to local fractures. Finally, 4) oxidization. This was concentrated in the top parts of some igneous bodies and breccias. The representative minerals formed were limonite + opal + hematite + Au minerals. The occurrence of illite and fluorite is confined to the mineralized breccias. Among these types of altered breccias, four are mineralized. The ore minerals deposited at the early replacement stage were pyrite + sphalerite + galena + chalcopyrite + Au-Ag inclusions. The vein filling stage is characterised by galena + sphalerite + pyrite + chalcopyrite + Au-Ag inclusions. In the oxidization stage there occurred small scale secondary enrichment of Au-Ag. Generally. the intensity of hydrothermal alteration in the different breccias was nearly uniform throughout Shui Koushan, however, differences in their hydrothermal alteration can not be used to explain differences in their mineralization. Measurements made on pseudo-secondary and secondary inclusions in quartz, calcite. sphalerite and fluorite crystals from different stages of hydrothermal alteration show very wide ranges of apparent salinities and temperatures. The early (Early Cretaceous) fluid was hot (- 290 – 310 °C) and saline (21-23 eq wt% NaCl), Rare daughter minerals and freezing temperature results show a Ca-Mg salt present beside NaCl. ?18OH2o values of +7.86 ‰ suggest this water had a magmatic source. Slightly later, dilute meteoric water descended into the system in places, and heated up to - 290 °C at a depth of 1.5 km (minimum). The incomplete mixing of these two fluids (magmatic and meteoric) may have caused the early stage Pb-Zn deposition, The circulating fluids then cooled below 250 °C and became even slightly more dilute (5 - 8.2 eq wt % NaCl) after the system had uplifted and more meteoric water infiltrated into it. Later a large amount of warm (100 - 200 °C), dilute meteoric water ,with ?18O values in the range from -10.61 to -3.46 ‰, ? D -70 to -47‰,infiltrated into the system and mixed with the more saline water, causing the second stage of base metal deposition; this coincided with the second alteration stage. Cooler (<120 °C) but saline (18.4 eq wt% NaCl) fluid trapped in a few breccias suggests a brine source existed but its genesis is unknown. The apparent salinities measured in samples from all unmineralized breccias are nearly uniform, but saline fluids were trapped in a few breccias of which only two were mineralized. Generally, small breccias were not mineralized, even those that trapped saline fluids. The geology, hydrothermal alteration and fluid inclusion studies thus suggest that the Shui Koushan magmatic hydrothermal system started to form in the Late Jurassic as the Lao Yazao granodiorite intruded at about 3 km depth with an initial temperature of - 700 °C. Skarn formed at its contact at a temperature range of 600 - 450 °C. Following the extrusion of andesite at Xin Mengshan, the system rose steadily a further 600 metres, and a typical epithermal system developed sometime 80 M.Y. ago, in which a large amount of meteoric water circulated. Boiling occurred along some fractures which created a local acidic environment but without mineralization. Comparion of the geology and geochemistry characteristics of the Shui Koushan system to hydrothermal systems in the Colorado Mineral Belt shows that they are similar to the overprinted epithermal system that developed in the shallow part of a porphyritic system although a porphyritic system it either does or does not may not directly underlie Shui Koushan, but it is not unusual in the Laiyang-Linwu Tectonic Mineralization Belt.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1995