The Porvenir polymetallic Sn-Zn-Ag (Tin-Zinc-Silver) project is located in the west central region of Bolivia, 65km southeast of the city of Oruro, in the Oruro department (Figure 1). The 11.25 sq-km property encompasses historical near surface open pit and underground workings, which exploited the deposit on a limited scale. The mineralised structures were primarily exploited for their high-grade Zinc (Zn) content.
Figure 1: Location of the Porvenir polymetallic Sn-Zn-Ag project.
A historical drill program was carried out by DOWA Mining, which included the completion of 88 diamond drill holes, for a total of approximately 25 000 m. Sporadic and inconsistent sampling of the drill cores indicated two distinct zones of polymetallic mineralisation (Figure 2), which include a central zone with elevated Sn as well as Zn mineralisation, as well as a Zn dominant, with minor Sn occurences in the NE portion of the property. It should be noted that selective sampling for high grade Zn mineralisation was carried out, and as such, sampling intervals are inconsistent due to entire drill cores not being sampled, as is evident from the cross sections below (Figures 3 and 4).
Figure 2: Property boundary at Porvenir highlighting the location of completed drill holes, the Sn and Zn rich zones, as well as the location of the two cross sections shown.
The mineralisation is hosted within near vertical NNW to NW trending structures which occur within Silurian age (423 – 419 Ma) clastic sediments comprising interbedded and layered units of sandstone and shale. The mineralisation comprises a stockwork framework formed by a series of mineralised stringers, < 1cm, as well as mineralised quartz veins which range in thickness from 1 to 30 cm.
Tin is hosted within cassiterite (SnO2), while sphalerite (ZnS) is the primary zinc ore mineral, with silver being hosted in tetrahedrite. Quartz, pyrite, siderite, barite, and kaolinite account for the majority of gangue and alteration mineral assemblages.
Figure 3: Cross section depicting the high-grade Sn intercepts in borehole PV-41, as well as high grade Zn intercepts. Note the selective and incomplete sampling intervals.
Figure 4: Cross section depicting the high-grade Sn intercepts in borehole PV-01, as well as high grade Zn intercepts. Note the selective and incomplete sampling intervals.
Table 1: Tin (Sn), Zinc (Zn), and Silver (Ag) assay results of selective boreholes and intercepts.