Geology and Genesis of the Unkurtash Intrusion-Related Gold Deposit, Tien Shan, Kyrgyzstan

Situated in the heart of the Tien Shan gold province, Unkurtash (3.5 Moz at 1.82 g/t) is a representative lode gold deposit that is hosted within the Carboniferous Andagul granodiorite in the Kassan district, western Kyrgyzstan. The Tien Shan gold province consists of porphyry and epithermal deposits, which are related to magmatic arcs, and lode gold deposits formed in the late stages of continent-continent collision that have been generally classified as mesothermal or orogenic type. Some of the lode gold deposits have been considered to be intrusion-related, but the genetic relationship between gold mineralization and the magmatic intrusions remains ambiguous in most cases due to lack of absolute timing of the gold mineralization. This paper addresses this problem through detailed geologic and petrographic studies of the Unkurtash gold deposit in conjunction with Re-Os, U-Pb, and 40Ar/39Ar dating along with S and Pb isotopes. The paragenesis of the Unkurtash deposit is divided into four stages. Stage 1 is related to pre-ore sheeted quartz veins outlined by narrow K-feldspar alteration envelopes. Stage 2 is characterized by quartz-molybdenite (+/- Au) veins with sericite and chlorite alteration assemblages. Stage 3 is the main gold mineralization stage and is characterized by gold-bearing quartz-ankerite veins and pervasive quartz +/- K-feldspar +/- rutile +/- titanite alterations, with development of auriferous pyrite, free gold, electrum, galena, pyrrhotite, and minor chalcopyrite and arsenopyrite. Stage 4 is defined by the presence of abundant arsenopyrite in barren quartz-calcite veins associated with sericite +/- muscovite +/- carbonate alteration. Molybdenite from a single stage 2 vein yielded a weighted mean Re-Os age of 306.5 +/- 1.7 Ma (n = 7). Auriferous pyrite from stage 3 yielded an Re-Os isochron age of 307.3 +/- 8.0 Ma (n = 5) and a weighted mean age of 307.4 +/- 3.1 Ma (n = 2; low-level highly radiogenic pyrite). 40Ar/39Ar ages of three hydrothermal K-feldspar samples from stage 3 veins returned younger plateau ages (ca. 293-289 Ma) that are possibly related to post-ore thermal events. The molybdenite and pyrite Re-Os ages are comparable with a newly obtained U-Pb zircon age of 305.1 +/- 2.7 Ma for the Andagul granodiorite, providing unequivocal evidence for synchronous gold mineralization and magmatism. The similarity in delta S-34(Vienna-Canyon) Diablo Troilite (V-CDT) between auriferous pyrite (5.7-7.1 parts per thousand, n = 10) and the Andagul granodiorite (7.0-8.9 parts per thousand, n = 4) and their similarity in Pb isotopes, which are significantly different from those of the pre-Silurian metamorphic rocks and regional Devonian sedimentary rocks, support the hypothesis that the Andagul granodiorite was the main source of metals and sulfur for mineralization. These results, together with concentric development of proximal skarn and metamorphic rock-hosted Au-As-Bi deposits and distal Sb-Au-As-Hg deposits around the Andagul intrusion, provide a strong case for an intrusion-related gold system. The establishment of an intrusion-related model for the Kassan district broadens the window for additional discoveries of gold deposits in the Tien Shan and elsewhere in the Central Asian orogenic belt. Situated in the heart of the Tien Shan gold province, Unkurtash (3.5 Moz at 1.82 g/t) is a representative lode gold deposit that is hosted within the Carboniferous Andagul granodiorite in the Kassan district, western Kyrgyzstan. The Tien Shan gold province consists of porphyry and epithermal deposits, which are related to magmatic arcs, and lode gold deposits formed in the late stages of continent-continent collision that have been generally classified as mesothermal or orogenic type. Some of the lode gold deposits have been considered to be intrusion-related, but the genetic relationship between gold mineralization and the magmatic intrusions remains ambiguous in most cases due to lack of absolute timing of the gold mineralization. This paper addresses this problem through detailed geologic and petrographic studies of the Unkurtash gold deposit in conjunction with Re-Os, U-Pb, and 40Ar/39Ar dating along with S and Pb isotopes. The paragenesis of the Unkurtash deposit is divided into four stages. Stage 1 is related to pre-ore sheeted quartz veins outlined by narrow K-feldspar alteration envelopes. Stage 2 is characterized by quartz-molybdenite (+/- Au) veins with sericite and chlorite alteration assemblages. Stage 3 is the main gold mineralization stage and is characterized by gold-bearing quartz-ankerite veins and pervasive quartz +/- K-feldspar +/- rutile +/- titanite alterations, with development of auriferous pyrite, free gold, electrum, galena, pyrrhotite, and minor chalcopyrite and arsenopyrite. Stage 4 is defined by the presence of abundant arsenopyrite in barren quartz-calcite veins associated with sericite +/- muscovite +/- carbonate alteration. Molybdenite from a single stage 2 vein yielded a weighted mean Re-Os age of 306.5 +/- 1.7 Ma (n = 7). Auriferous pyrite from stage 3 yielded an Re-Os isochron age of 307.3 +/- 8.0 Ma (n = 5) and a weighted mean age of 307.4 +/- 3.1 Ma (n = 2; low-level highly radiogenic pyrite). 40Ar/39Ar ages of three hydrothermal K-feldspar samples from stage 3 veins returned younger plateau ages (ca. 293-289 Ma) that are possibly related to post-ore thermal events. The molybdenite and pyrite Re-Os ages are comparable with a newly obtained U-Pb zircon age of 305.1 +/- 2.7 Ma for the Andagul granodiorite, providing unequivocal evidence for synchronous gold mineralization and magmatism. The similarity in delta S-34(Vienna-Canyon) Diablo Troilite (V-CDT) between auriferous pyrite (5.7-7.1 parts per thousand, n = 10) and the Andagul granodiorite (7.0-8.9 parts per thousand, n = 4) and their similarity in Pb isotopes, which are significantly different from those of the pre-Silurian metamorphic rocks and regional Devonian sedimentary rocks, support the hypothesis that the Andagul granodiorite was the main source of metals and sulfur for mineralization. These results, together with concentric development of proximal skarn and metamorphic rock-hosted Au-As-Bi deposits and distal Sb-Au-As-Hg deposits around the Andagul intrusion, provide a strong case for an intrusion-related gold system. The establishment of an intrusion-related model for the Kassan district broadens the window for additional discoveries of gold deposits in the Tien Shan and elsewhere in the Central Asian orogenic belt.