Abstract

The Ushkatyn-III deposit is located 300 km west of Karaganda (Central Kazakhstan). It is classified as a weakly metamorphosed Atasu-type hydrothermal-sedimentary ore deposit. Stratiform iron, manganese, and barite–lead orebodies occur in the Upper Devonian (D₃fm₂) carbonate sequence of this deposit. The Fe- and Mn-bearing rocks were studied. Iron ores are composed of hematite, calcite, and quartz. Manganese ores are divided into two (braunite and hausmannite) types. Braunite ores are composed of braunite, calcite, quartz, and albite; hausmannite ores are composed of hausmannite, rhodochrosite, Mn-calcite, tephroite (sometimes with sonolite and alleghanyite), and friedelite. The mineral composition of ores was formed during the burial of metal-bearing sediments containing oxides of Fe³⁺ and Mn³⁺/Mn⁴⁺. The nature and intensity of the postdepositional processes was controlled by the content of organic matter (OM) in initial rocks. Braunite ores formed under oxidizing conditions; hausmannite ores, under reducing conditions. The initial presence of OM is indicated by the carbon isotope composition in carbonates. High δ¹³C_carb values (from –0.3 to 2.1‰) in hematite ores, braunite ores, and host limestones are close to those in the seawater bicarbonate precipitated as a part of biogenic calcite (e.g., shell fragments). At the same time, low δ¹³C_carb values (average –12.3‰) in rhodochrosite and Mn-calcite from hausmannite ores indicate the influence of carbon from the OM buried in sediments. The general oxygen isotope composition in iron ores, manganese ores, and host rocks was determined by variations of the isotope ratio in the initial precipitates of ¹⁸O-rich calcite and ¹⁶O-rich Fe/Mn oxides. Conversion of the primary sedimentary Fe³⁺ and Mn³⁺/Mn⁴⁺ oxides to hematite, braunite, and hausmannite slightly changes the oxygen isotope composition of Fe–Mn minerals. Average values of δ¹⁸O_tot determined in the hematite (1.7‰), braunite (14.8‰), and hausmannite (12.7‰) ores are close to those in the sedimentogenic iron (0‰) and manganese oxides (11.5‰). The oxygen isotope composition suggests that initial manganese oxides did not participate in the formation of carbonates in braunite ores. Average oxygen isotope composition of carbonates in these ores (δ¹⁸O_carb 19.1‰) is close to that in host limestones (20.8‰). During the formation of hausmannite ores, in contrast, manganese oxides reacted with OM leading to the formation of rhodochrosite and silicates. Consequently, the light isotope oxygen in the starting oxides was fixed in carbonates (δ¹⁸O_carb 15.9‰) and rocks in general (δ¹⁸O_total 12.7‰). The oxygen isotope composition was not averaged during the formation of hausmannite ores even in the mineralogically similar aggregates in the adjacent rock layers.

中文翻译：

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哈萨克斯坦中部Ushkatyn-III矿床同位素组成（δ13 C和δ18 O）和含锰沉积物成因

摘要

Ushkatyn-III矿床位于Karaganda（哈萨克斯坦中部）以西300公里处。它被归类为弱变质的阿塔苏型热液沉积矿床。在该矿床的上泥盆统（D ₃ fm ₂）碳酸盐岩层中存在层状铁，锰和重晶石铅矿。研究了含铁和含锰岩石。铁矿石由赤铁矿，方解石和石英组成。锰矿石分为两种类型（褐铁矿和方锰矿）。褐铁矿矿石由褐铁矿，方解石，石英和钠长石组成。菱锰矿矿石由菱锰矿，菱锰矿，锰方解石，球铁矿（有时含方钠石和钙铝石）和方沸石组成。矿石的矿物成分是在掩埋含铁氧化物的含金属沉积物时形成的³⁺和Mn ³⁺ / Mn ⁴⁺。沉积后过程的性质和强度受初始岩石中有机质（OM）的含量控制。在氧化条件下形成的褐铁矿；还原条件下的苏曼石矿石。OM的初始存在由碳酸盐中的碳同位素组成指示。高Δ ^13个Ç_{碳水化合物}值（从-0.3至2.1‰）在赤铁矿，褐锰矿矿石，和主机灰岩接近那些在海水碳酸氢盐沉淀为生物方解石（例如，壳碎片）的一部分。与此同时，低δ ¹³ C ^_{碳水化合物}菱锰矿矿石中菱锰矿和锰方解石的平均值（平均–12.3‰）表明了埋藏在沉积物中的OM碳的影响。铁矿石，锰矿石和基质岩石中一般的氧同位素组成是由¹⁸种富O方解石和^16种Fe / Mn富氧化物的初始沉淀物中同位素比率的变化确定的。原始沉积Fe ³⁺和Mn ³⁺ / Mn ⁴⁺氧化物向赤铁矿，褐铁矿和钙锰矿的转化会稍微改变Fe-Mn矿物的氧同位素组成。δ的平均值¹⁸ Ø _TOT赤铁矿（1.7‰），褐铁矿（14.8‰）和苏曼石（12.7‰）矿石中的铁矿石含量接近于成矿铁（0‰）和氧化锰（11.5‰）矿石中的铁矿石。氧同位素组成表明，初始锰氧化物不参与褐铁矿矿石中碳酸盐的形成。在这些矿石（δ碳酸盐的平均氧同位素组成¹⁸ ö_{碳水化合物}19.1‰）接近，在主机灰岩（20.8‰）。相反，在菱锰矿矿石的形成过程中，锰氧化物与OM反应导致菱锰矿和硅酸盐的形成。因此，在起始氧化物的轻同位素氧固定在碳酸盐（δ ¹⁸ ö_{碳水化合物}15.9‰）和岩石一般（δ ¹⁸ ö_总计12.7‰）。即使在相邻岩石层中矿物学上相似的聚集体中，在苏曼石矿石形成过程中，氧同位素组成也没有平均。