Thermochronology of economic mineral deposits; dating the stages of mineralization at Panasqueira, Portugal, by high-precision 40 / 39 Ar age spectrum techniques on muscovite
Thermochronology of economic mineral deposits; dating the stages of mineralization at Panasqueira, Portugal, by high-precision 40 / 39 Ar age spectrum techniques on muscovite
复制标题
DOI：
10.2113/gsecongeo.83.2.335
复制
发表时间：
1988-04
期刊：
影响因子：
5.8
通讯作者：
L. Snee;J. Sutter;W. Kelly
中科院分区：
文献类型：
--
作者：
L. Snee;J. Sutter;W. Kelly
13 个白云母的 40 Ar/ 39 Ar 年龄谱日期已被用来重建葡萄牙帕纳斯凯拉锡钨矿床的热历史（热年代学），该矿床在空间上与海西岩体带有关。白云母样本的年龄差异小至 2.2 m.y。 （0.7% 的年龄）在统计上是不同的。对于来自不同事件的多个样本的比较，统计数据甚至更好；也就是说，相差 0.9 m.y。 (0.3%) 可以在这个大约有 300 年历史的矿床中得到解决。主要的锡和钨矿形成阶段，即氧化硅酸盐阶段、主要硫化物阶段和灰铁矿化阶段，发生在 296.3 + 或 - 0.8 (1 Sigma ) 和 291.6 + 或 - 0.8 m.y 之间。 （1 西格玛）。氧化硅酸盐阶段的第一个子阶段是 296.3 + 或 - 0.6 m.y. 处的短暂热脉冲；负责的液体可能来自已知的花岗岩冲天炉。主要硫化物阶段在 294.5 + 或 - 0.9 m.y 处活跃。作为寿命稍长的脉冲，该阶段最古老的证据（295.8 + 或 - 0.6 m.y.）来自距已知冲天炉最远的区域，而最年轻的证据（293.5 +或 - 0.8 m.y.）最接近冲天炉。氧化物-硅酸盐阶段的第二个子阶段在 292.9 + 或 - 0.7 m.y. 发生为短暂的热脉冲，与冲天炉的灰色化和硅盖在 292.1 + 或 - 0.4 m.y. 的蚀变同步。根据所有 13 个白云母的年龄，氧化硅酸盐阶段、主要硫化物阶段、灰岩化和硅盖蚀变的活动持续时间大于 4.2 + 或 - 0.5 m.y。 （1 西格玛）。所有定年白云母的少量氩气损失都发生在后来的再加热过程中，可能是在寿命较长的磁黄铁矿蚀变阶段。单一中心，即已知的冲天炉，具有长期的作用，是主要硫化物阶段、氧化硅酸盐阶段 II、云灰化和硅盖蚀变以及可能的氧化硅酸盐阶段 I 和磁黄铁矿蚀变阶段的来源；然而，不能排除后两个阶段有单独的来源。这项研究是白云母 40 Ar/ 39 Ar 年龄光谱测年的新的有力应用的一个例子。由于这项研究证明了高精度，现在可以确定解决经济地质学中一些长期存在的问题所需的时间限制。除此之外，帕纳斯凯拉独特的地质情况使我们能够量化白云母的热特性。已发表的流体包裹体数据已用于估计在快速冷却或短暂再加热期间约325摄氏度的白云母氩封闭温度以及在缓慢冷却或长时间再加热期间约270摄氏度的温度。所有已过时的白云母显示的氩气损失模式是由于最初关闭后重新加热造成的；这种氩气损失的机制似乎是通过体积扩散进行的氩气传输。因此，白云母的 40 Ar/ 39 Ar 年龄谱测年可用于评估控制氩扩散的热条件以及年龄、持续时间和矿化发生次数。
40 Ar/ 39 Ar age spectrum dates for 13 muscovites have been used to reconstruct the thermal history (thermochronology) of the Panasqueira, Portugal, tin-tungsten deposit, a deposit spatially associated with a belt of Hercynian plutons. Muscovite samples with an age difference as small as 2.2 m.y. (0.7% of the age) are statistically distinct. Statistics are even better for comparison of multiple samples from separate events; that is, a difference of 0.9 m.y. (0.3%) can be resolved in this approximately 300-m.y.-old deposit. The major tin and tungsten ore-forming stages, which are the oxide-silicate stage, the main sulfide stage, and greisenization, occurred between 296.3 + or - 0.8 (1 Sigma ) and 291.6 + or - 0.8 m.y. (1 Sigma ). The first substage of the oxide-silicate stage was a short-lived thermal pulse at 296.3 + or - 0.6 m.y.; the fluids responsible may have emanated from the known granite cupola. The main sulfide stage was active at 294.5 + or - 0.9 m.y. as a slightly longer lived pulse with oldest evidence for this stage (295.8 + or - 0.6 m.y.) coming from areas farthest away from the known cupola and youngest evidence (293.5 + or - 0.8 m.y.) closest to the cupola. A second substage of the oxide-silicate stage occurred as a short-lived thermal pulse at 292.9 + or - 0.7 m.y., synchronous with greisenization of the cupola and alteration of the silica cap at 292.1 + or - 0.4 m.y. The duration of activity of the oxide-silicate stage, the main sulfide stage, greisenization, and alteration of the silica cap based on the ages of all 13 muscovites was greater than 4.2 + or - 0.5 m.y. (1 Sigma ). Minor argon loss from all dated muscovites occurred during later reheating, probably during the longer lived pyrrhotite alteration stage. A single center, the known cupola, had a prolonged role and was the source for main sulfide stage, oxide-silicate stage II, greisenization, and alteration of the silica cap and possibly oxide-silicate stage I and the pyrrhotite alteration stage; however, a separate source for these latter two stages cannot be ruled out.This study is an example of a new and powerful application of 40 Ar/ 39 Ar age spectrum dating of muscovite. Because of the high precision demonstrated in this study, it is now possible to establish time constraints necessary for solving some of the long-standing problems in economic geology. Beyond this, the unique geologic situation of Panasqueira has allowed us to quantify the thermal characteristics of muscovite. Published fluid inclusion data have been used to estimate a muscovite argon closure temperature of approximately 325 degrees C during rapid cooling or short reheating and a temperature of approximately 270 degrees C during slow cooling or extended reheating. Argon-loss patterns displayed by all dated muscovites resulted from reheating after original closure; the mechanism for this argon loss appears to have been argon transport by volume diffusion. Thus, 40 Ar/ 39 Ar age spectrum dating of muscovite can be used to evaluate thermal conditions controlling argon diffusion as well as age, duration, and number of episodes of mineralization.