Geochemistry of Sphalerite from the Permian Volcanic-Hosted Massive Sulphide (VHMS) Deposits in the Tasik Chini Area, Peninsular Malaysia: Constraints for Ore Genesis

The Bukit Botol and Bukit Ketaya deposits are two examples of volcanic-hosted massive sulphide (VHMS) deposits that occur in the Tasik Chini area, Central Belt of Peninsular Malaysia. The mineralisation is divided into subzones distinguished by spatial, mineralogical, and textural characteristics. The primary sulphide minerals include pyrite, chalcopyrite, sphalerite, and galena, with lesser amounts of Sn- and Ag-bearing minerals, with Au. However, pyrrhotite is absent from both deposits. This study presents the results of sphalerite chemistry analysed by using an electron microprobe. Two types of sphalerite are recognised: sphalerite from the Bukit Botol deposit reveals a range of <DL to 24.0 mole% FeS, whereas sphalerite from the Bukit Ketaya deposit shows a range of <DL to 3 mole% FeS. Significant variations are shown in Zn, Cu, Cd, and Ag levels. Although the sphalerite has a wide variation in composition, a discernible decreasing Fe trend is exhibited from the stringer zone towards massive sulphide. This compositional variation in sphalerites may in part reflect variable temperature and activity of sulphur in the hydrothermal fluids during ore formation. Alternatively, the bimodal composition variations suggest that mineral chemistry relates to contrasting depositional processes. The Zn/Cd ratios for sphalerite from both these deposits are similar to those exhibited by volcano−sedimentary deposits with a volcanic origin. Therefore, the consistently low Cd concentrations and moderate to high Zn/Cd ratios suggest mixing of seawater and minor magmatic fluids controlling the chemistry of sphalerite at both deposits during their formation.

HIGH-PRECISION CA-ID-TIMS AGE CONSTRAINTS ON THE NIBLACK Cu-Zn-Au-Ag DEPOSITS: A NEOPROTEROZOIC VOLCANIC-HOSTED MASSIVE SULFIDE DEPOSIT IN THE NORTH AMERICAN CORDILLERA

The Neoproterozoic-Cambrian Wales Group and Ordovician-early Silurian Moira Sound unit of Prince of Wales Island, Alaska, USA, host numerous volcanic-hosted massive sulfide (VHMS) deposits and occurrences, including the Niblack VHMS deposits. Previous attempts to determine the age of the felsic volcanic host rocks in the Niblack area have resulted in conflicting results and interpretations. We have utilized chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon geochronology to acquire highly precise crystallization and maximum depositional ages for a total of six samples of felsic volcanic and intrusive rocks from Niblack. This study establishes age constraints for the Niblack felsic succession of (1) crystallization ages of 565.1 ± 0.9 and 564.8 ± 1.0 Ma for coherent rhyolite flows, (2) maximum depositional ages of 565.3 ± 0.9 and 565.2 ± 0.9 Ma for felsic volcaniclastic rocks, (3) a crystallization age of 565.2 ± 0.9 Ma for quartz-feldspar-phyric subvolcanic sill, and (4) a crystallization age of 564.8 ± 1.0 Ma for a felsic dike that crosscuts the Niblack felsic succession. These results indicate that the ~200-m-thick Niblack felsic succession and VHMS deposits formed during one episode of felsic volcanism at ca. 565.1 ± 0.9 Ma and are thus confirmed as part of the Neoproterozoic Wales Group. Results of this study provide the first chronostratigraphic framework for felsic volcanism associated with VHMS deposit formation at Niblack and have implications for mineral exploration on Prince of Wales Island and elsewhere in the Alexander terrane.