The sources of base and precious metals in Kuroko deposits of NE Japan

The middle-Miocene volcano-sedimentary succession of Akita Prefecture, Northeast Japan, hosts a well-studied example of felsic volcanic-hosted massive sulphide deposits, known as “Kuroko”, which represent the type locality for this type of deposits worldwide. These have long been used as a model for understanding similar ore deposits occurring in other localities and across the geological time. The main commodities extracted from this type of ore are Zn, Cu and Pb, with locally significant amounts of Au and Ag as by-products.

In this study, we combine major and trace element analyses (including Au and Ag) with high-precision Pb isotope analyses of ore samples from Akita Prefecture and evaluate their co-variations in order to understand the source of base and precious metals in these deposits. We also compile previous Pb isotope analyses to obtain a wider view of the isotopic value distributions at the district scale. Lead isotope maps based on this dataset were compared with geological features, such as the orientation of main Miocene faults and basement depth to assess the possible effects of such features on Pb isotopic composition of hydrothermal deposits.

The highest values of Au (up to ca. 120 ppm) and Ag (up to ca. 7000 ppm) were observed in sphalerite-rich “black ore” samples from Matsumine and Shakanai deposits. Petrographic observations and mineral analyses in these samples indicate that the main host for precious metals are sulfosalts, such as tennantite-tetrahedrite and pearceite [Cu (Ag,Cu)₆ Ag₉ As₂ S₁₁]-polybasite [Cu (Ag,Cu)₆ Ag₉ Sb₂ S₁₁]. Electrum occurs at Au-Ag hosts phase of Nurukawa and Furutobe deposits, along with tennantite-tetrahedrite. In Matsumine and Shakanai samples, positive correlations in plots of Pb isotopic ratio ²⁰⁷Pb/²⁰⁴Pb vs Zn, Pb, Au and Ag point to contributions of these metals mostly from isotopically evolved sources (the pre-Miocene basement). Anticorrelation between ²⁰⁷Pb/²⁰⁴Pb and Cu indicates a relatively unradiogenic source for Cu (the Miocene volcanic rocks). The maps of ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb indicate a prominent N-S distribution of values, parallel to the orientation of the main Miocene faults and the elongation of Miocene rifts, reflecting the paths of hydrothermal fluid circulation. The Cretaceous Pb model ages of ore samples (ca. 80–140 Ma) are significantly older than the middle-Miocene formation age, and overlap with the ages of basement granites. In addition, a comparison of the map of Pb model ages at the district scale with the map of the basement depth indicates progressively older model ages occurring to the northeast, in areas where the basement becomes shallower. Lead with such “old” isotopic ratios was likely preserved in feldspar of Cretaceous basement granites, and remobilised during fluid circulation in the middle-Miocene. We propose a model that involves an isotopically juvenile source (the Miocene volcanic rocks) providing Cu and an isotopically evolved source providing much of Zn, Au and Ag to the mineralising fluids.