Wallrock alteration and mineralization of the A and the H West prospects, Chatree gold deposit, Changwat Phichit and Phetchabun, Thailand

Abstract

The Chatree Gold mine is presently the major gold deposit in Thailand located about 280 km north of Bangkok, on the border of Phetchabun and Pichit Provinces. This study was focussed mainly on wall rock alteration in relation to mineralization of the A and the H West Prospects. Even though many workers have previously studied on mineralization and wall rock alteration on these Prospects, those studies are still considered sporadic or focused on small areas, particularly on the wall rock alteration, of those Prospects. This study is therefore aimed at systematically characterizing in more details on the host rock alteration in relation to the mineralization by using staining technique, standard petrographic studies of thin sections, polish sections and polish thin sections in combination with electron probe microanalysis of altered and ore minerals. The rock units in the Chatree gold deposit comprise 4 units as Unit 1: fiamme breccias, Unit 2: epiclastic and fine volcaniclastic sedimentary facies and rhyolite breccia facies, Unit 3: polymictic and Monomict ic andesite breccia, and Unit 4: andesite porphyry. The A Prospect is reported only Units 1 to 3 while the lowest unit (Unit 4) is omitted due to the insufficient depth of drilled holes. The H West Prospect consists of three units as Unit 1, Unit 3, and Unit 4, while the volcaniclastics of Unit 2 is absent in this area. Based on previous and this studies, the A and H West Prospects possess similar nature of mineralization and six vein stages have been identified in these two Prospects, namely. Stage I: gray chalcedony, Stage II: quartz–euhedral pyrite, Stage III: major gold mineralization, Stage IV: quartz–carbonate±prehnite veinlets, Stage V: non-ferroan calcite veinlets, and Stage VI: laumontite. The A and H West Prospects comprise several mineral alteration halo, including silicification, potassic, and phyllic alterations that are proximal to the veins, whereas propyritic alteration is distal from the veins. The alteration minerals selectively replaced in the vein host rocks. The silicification was related to the Stage I mineralization. The gold-bearing Stage III mineralization was related to the potassic alteration proximal to the ore zone, while sericite±quartz or phyllic alteration was found next away from the ore zone in the Stage IV mineralization. Chlorite or porpylitic alteration was found distal to the ore zone in the Stages III-V. The late carbonate is assumed to occur in the stage V. The alteration halos in the H West Prospect are rather narrow (normally about 1-2 mm scale or may be up to 20 cm wide around veins system) as compared with those found in the A Prospect (normally 5 – 30 m scale).