Geology and characteristics of disseminated nickel mineralization in the Ban Phuc ultramafic intrusion

Abstract

This paper presents new research results on the geological characteristics and disseminated Ni-sulfide mineralization in the Ban Phuc ultramafic intrusion, based on the application of traditional geological methods combined with modern rock composition analysis techniques and several geological mathematical methods. The study yields the following key findings:

The deposit is characterized by a syncline, with the core composed of ultramafic rocks of the Ban Phuc intrusion. Its limbs consist of rocks of the Nam Sap Formation (D_1-2ns) and partly of the Ban Cai Formation.

The Ban Phuc ultramafic rocks are charactered by high magnesium content (40–47% MgO), very low titanium, chromium and alkali elements. The Ban Phuc ultramafic intrusive body is strongly differentiated into three zones from top to bottom (UB1, UBT, and UB2). UB2 is the lower zone consisting mainly of dunite. UB1 is the upper zone consisting mainly of peridotite. UBT is an intermediate transition zone.

Fractures within the Ban Phuc ultramafic body are mostly oriented northeast (30°) with dip angles of 70°–80°. This orientation is consistent with the structural trend of the ultramafic body boundary and the disseminated sulfide zone located in the southern part of the Ban Phuc ultramafic intrusion.

The disseminated Ni-sulfide ores in the Ban Phuc ultramafic intrusion occur in two types: primary Ni-sulfide ores and weathered Ni ores. The weathered ores include partially oxidized sulfide ores and completely oxidized sulfide ores.

The primary Ni-sulfide ore bodies are large in scale, mainly distributed in UB2 and to a lesser extent in UBT. They have complex morphologies, occurring as layers, lenses, bands, branching bands, as well as basal and hanging wall types. They extend about 865 m in length and 350–400 m in width. Thickness ranges from 0.75 m to 143.98 m, with an average of 27.29 m and high variability (Vm = 73%). The rock interlayer thickness varies from 1.8 m to 76.82 m, averaging 15.66 m. The main ore minerals are pentlandite, violarite, and valleriite.

Partially oxidized ore forms a transitional zone from primary Ni-sulfide ore bodies, occurring above the primary disseminated Ni-sulfide ore and below completely oxidized ore. This zone has a length like that of the primary ore body, with thicknesses ranging from 1.1 m to 44.28 m (average 14.83 m). The Ni-bearing minerals, including pentlandite, violarite, and valleriite, are oxidized by about 1–10%.

Completely oxidized ore is the uppermost zone of the disseminated Ni-sulfide ore body, directly adjacent to the ground surface. Thickness ranges from 2.5 m to 41.72 m, averaging 18.47 m. The Ni-bearing minerals include limonite, goethite, and saponite.