Age and structural evolution of the orogenic gold deposits in Kullaa, SW Finland: Implications for fluid activity and gold precipitation during evolving orogeny

IF 3.2 2区地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY

Precambrian Research Pub Date : 2025-05-27 DOI:10.1016/j.precamres.2025.107828

Jaakko Kara , Jukka Manninen , Pietari Skyttä , Markku Väisänen , Hugh O’Brien , Kathryn Cutts , Paavo Nikkola

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Abstract

We have studied two gold deposits, Välimäki and Kultakallio, located in SW Finland, to understand the interplay between gold precipitation, fluid activity and structural evolution within the Paleoproterozoic Svecofennian orogen. The Välimäki deposit is hosted by paragneisses characterised by pervasive silicification and the occurrence of foliation parallel quartz veins. The Kultakallio garnet-bearing gabbro shows two generations of quartz veins associated with arsenopyrite and graphite. Zircons from the leucosome within Välimäki yield an age of 1887 ± 4 Ma interpreted as the first metamorphic event (M1) associated with partial melting. In contrast, monazites of the same sample give bimodal ages at 1817 ± 7 Ma and 1794 ± 5 Ma both reflecting hydrothermal activity. In-situ zircon dating in Kultakallio yielded three age peaks at 1885 ± 4 Ma, 1859 ± 5 Ma and 1805 ± 7 Ma interpreted to represent magmatism, a secondary metamorphic event (M2) and hydrothermal activity, respectively. In-situ dating of Kultakallio titanites gives an age of 1786 ± 58 Ma and garnet an age of 1809 ± 470 Ma, which reflect the timing of the fluid activity. Based on hydrothermal ages, gold mineralising events at (I) 1815–1805 Ma and (II) 1795–1785 Ma are recognised in the study area. Stage I is related to the injection of foliation-parallel quartz veins and precipitation of arsenopyrite in which gold occurs as inclusions. Stage II is characterised by precipitation of gold associated with Te- and Bi-minerals, abundant graphite, remobilisation of pyrite and breakdown of arsenopyrite within the brittle-ductile transition zone. The NW-SE −oriented transpressional setting formed the NE-SW −oriented gold critical structures originally at 1.89–1.88 Ga (D2), which were later reactivated in the 1.83–1.78 Ga WNW-ESE transpressional setting (D3). The NW-SE trending crustal scale shear zones are suggested to act as first order structures whereas the NE-SW oriented strike-slip structures form the second order setting. In the target scale, foliation parallel and intersecting quartz veins and narrow shear zones represent the third order structures hosting the mineralisations. We suggest the driver for the fluid activity at ca. 1.80 Ga is lower crustal delamination during crustal extension in a tectonic switching cycle.

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芬兰西南Kullaa造山带金矿床的年龄和构造演化：演化造山带流体活动和金沉淀的意义

本文对位于芬兰西南部的Välimäki和Kultakallio两个金矿床进行了研究，以了解古元古代斯韦芬尼系造山带内的金沉淀、流体活动和构造演化之间的相互作用。Välimäki矿床赋存于副长岩中，其特征为普遍硅化，产片状平行石英脉。Kultakallio含石榴石辉长岩显示与毒砂和石墨伴生的两代石英脉。Välimäki内白色小体的锆石年龄为1887±4 Ma，被解释为与部分熔融有关的第一次变质事件（M1）。相比之下，同一样品的独居石的双峰年龄分别为1817±7 Ma和1794±5 Ma，均反映了热液活动。Kultakallio的原位锆石测年获得了1885±4 Ma、1859±5 Ma和1805±7 Ma三个年龄峰，分别代表岩浆活动、次生变质事件（M2）和热液活动。Kultakallio钛矿的年龄为1786±58 Ma，石榴石的年龄为1809±470 Ma，反映了流体活动的时间。根据热液年龄，确定了(1)1815 ~ 1805 Ma和(2)1795 ~ 1785 Ma的金矿成矿事件。第1阶段与叶理平行石英脉注入和毒砂沉淀有关，其中金以包裹体形式赋存。第II阶段的特征是金与Te和bi矿物伴生的沉淀、丰富的石墨、黄铁矿的再活化和脆性-韧性过渡区内毒砂的分解。在1.89 ~ 1.88 Ga (D2)，北西—东南向的跨洋构造形成了北东—西南向的金临界构造，在1.83 ~ 1.78 Ga （D3）重新激活。NW-SE向的地壳尺度剪切带是一级构造，NE-SW向的走滑构造是二级构造。在目标尺度上，片理平行相交的石英脉和狭窄的剪切带代表了承载矿化的三级构造。我们认为约1.80 Ga流体活动的驱动因素是构造转换旋回中地壳伸展过程中的下地壳剥离。

本文章由计算机程序翻译，如有差异，请以英文原文为准。

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来源期刊

Precambrian Research 地学-地球科学综合

CiteScore

7.20

自引率

28.90%

发文量

325

审稿时长

12 months

期刊介绍： Precambrian Research publishes studies on all aspects of the early stages of the composition, structure and evolution of the Earth and its planetary neighbours. With a focus on process-oriented and comparative studies, it covers, but is not restricted to, subjects such as: (1) Chemical, biological, biochemical and cosmochemical evolution; the origin of life; the evolution of the oceans and atmosphere; the early fossil record; palaeobiology; (2) Geochronology and isotope and elemental geochemistry; (3) Precambrian mineral deposits; (4) Geophysical aspects of the early Earth and Precambrian terrains; (5) Nature, formation and evolution of the Precambrian lithosphere and mantle including magmatic, depositional, metamorphic and tectonic processes. In addition, the editors particularly welcome integrated process-oriented studies that involve a combination of the above fields and comparative studies that demonstrate the effect of Precambrian evolution on Phanerozoic earth system processes. Regional and localised studies of Precambrian phenomena are considered appropriate only when the detail and quality allow illustration of a wider process, or when significant gaps in basic knowledge of a particular area can be filled.