U-Pb dating of hydrothermal monazite and xenotime from the Ikkari and Heinä Central Au deposits, Central Lapland belt, northern Finland

Abstract

The Ikkari deposit is the most significant mineral discovery in northern Finland and the largest gold discovery in Central Lapland since the Suurikuusikko deposit was found in 1986. Located in the Central Lapland belt (CLB), which is renowned for its rich mineral resources and active mining operations, the Ikkari gold deposit contains a total of 4.22 million ounces of gold with an estimated mining lifespan of over twenty years. Heinä Central, the largest of the surrounding satellite deposits, holds approximately 150,000 ounces of gold and possibly recoverable copper.

The purpose of this thesis is to study the age of the Ikkari and Heinä Central gold deposits. Phosphate minerals monazite and xenotime are widely used in geochronological studies, especially in U-Pb dating, because their properties enable the analysis of past geological conditions and events. In the Ikkari deposit, the results show pyrite-associated gold mineralization with an age distribution between 1.98–1.69 Ga, where two main age populations at 1.83 Ga and 1.77 Ga were found. In the Heinä Central deposit, gold is primarily associated with pyrrhotite and molybdenite. The age distribution in Heinä Central ranges between 1.80–1.72 Ga, with one main age population at 1.76 Ga. The narrower age distribution in Heinä Central implies that ore formation commenced later. However, for both deposits, the period around 1.77–1.76 Ga comprises most of the dated ages and therefore represents the main ore formation event. With Ikkari, the 1.83 Ga age suggests another significant and possibly the earliest ore formation event.

Gold formation in the CLB can be divided into two key periods: an early collision stage around 1.92 Ga and late- to post-orogenic stage between 1.83–1.76 Ga. Based on the results, the Ikkari and Heinä Central gold deposits formed during the late- to post-orogenic stage, representing the main gold-forming period in the evolution of the CLB.