Direct evolution of brine from a crystallizing silicic melt at the Questa, New Mexico, molybdenum deposit
Direct evolution of brine from a crystallizing silicic melt at the Questa, New Mexico, molybdenum deposit
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DOI：
10.2113/gsecongeo.89.8.1780
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发表时间：
1994-12
期刊：
影响因子：
5.8
通讯作者：
J. Cline;R. Bodnar
中科院分区：
文献类型：
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作者：
J. Cline;R. Bodnar
奎斯塔存款的西南矿体中的钼矿化与细晶斑岩结晶、爆发角砾岩和岩浆热液出溶有关。石英、钾长石、氟金云母和镁铝榴石从出溶水流体和胶结细晶斑岩和安山岩围岩碎屑中沉淀，形成岩浆热液角砾岩矿石。角砾岩基质中石英中的流体包裹体捕获了运输和沉淀钼的水流体。两个流体包裹体人口与镁橄榄石矿化被确定。最大的种群通常包含液体，蒸汽，石盐，钾盐，和一个或多个不透明的子晶体。这些夹杂物在200 ° C至500 ° C之间均质化，并在360 ° C至400 ° C下具有显著模式;盐度从31重量%至57重量% NaCl当量变化。低的首次融冰温度表明，可能存在额外的流体成分，可能是氯化钙或氯化镁。这些包裹体中大约80%通过岩盐溶解而均匀化;相平衡约束要求这些包裹体在无蒸汽的区域中捕获流体。剩余的盐水包裹体在高于岩盐溶解温度的温度下均质化为液体。值得注意的是，盐水流体没有与低密度的水相流体共存。第二个群体的包裹体包含的流体的盐度为2.7至25.0重量%的NaCl当量和均匀化的蒸汽泡消失，蒸汽泡膨胀，或临界行为。显微测温过程中的结构关系和相行为表明，这些包裹体构成了一个单一的群体，形成的压力和温度波动在近临界条件下，压力变化与角砾岩，其次是结晶和系统重新密封产生所观察到的流体。在一次主要的角砾岩作用之后，盐水被截留在系统的密封中。这些高盐度的流体产生的熔体，在压力下，从流体静力学增加到岩石静力学，分配到出溶水性流体的氯量增加。重新压裂和突然压力下降到静水条件下减少了氯分配到出溶流体的质量。低盐度流体在近临界条件下出溶，并被包裹体捕获，包裹体均匀化为液体、蒸汽或临界行为。缺乏同生的富含液体和蒸汽的流体包裹体和相平衡的限制表明，高盐度流体（≤ 57重量% NaCl当量）直接产生的结晶岩浆，而不是一个产品的水流体不可渗透性。
Molybdenum mineralization in the Southwest orebody at the Questa deposit is associated with crystallization of aplite porphyry, explosive brecciation, and exsolution of magmatic hydrothermal fluids. Quartz, K feldspar, fluorophlogopite, and molybdenite precipitated from exsolving aqueous fluids and cemented aplite porphyry and andesite wall-rock clasts forming a magmatic hydrothermal breccia ore.Fluid inclusions in quartz in the breccia matrix trapped aqueous fluids that transported and precipitated molybdenum. Two fluid inclusion populations associated with molybdenite mineralization were identified. The largest population commonly contains liquid, vapor, halite, sylvite, and one or more opaque daughter crystals. These inclusions homogenize between 200 degrees and 500 degrees C and have a prominent mode at 360 degrees to 400 degrees C; salinities vary from 31 to 57 wt percent NaCl equiv. Low first ice-melting temperatures suggest that additional fluid components, possibly calcium or magnesium chlorides, may be present. Approximately 80 percent of these inclusions homogenize by halite dissolution; phase equilibria constraints require that these inclusions trapped fluids in the vapor-absent field. The remaining saline inclusions homogenize to liquid at temperatures above halite dissolution temperatures. Significantly, the saline fluid did not coexist with a low-density aqueous fluid.A second population of inclusions contains fluids with salinities of 2.7 to 25.0 wt percent NaCl equiv and homogenizes by vapor bubble disappearance, vapor bubble expansion, or critical behavior. Textural relationships and phase behavior during microthermometry indicate that these inclusions constitute a single population which formed as pressure and temperature fluctuated at near-critical conditions.Pressure changes related to brecciation followed by crystallization and system resealing produced the observed fluids. Saline fluids were trapped as the system sealed following a major brecciation event. These high-salinity fluids were produced as the melt, under pressure that increased from hydrostatic to lithostatic, partitioned increasing amounts of chlorine into the exsolving aqueous fluid. Renewed fracturing and abrupt pressure decrease to hydrostatic conditions reduced the mass of chlorine partitioning to the exsolving fluid. Lower salinity fluids exsolved at near-critical conditions and were trapped by inclusions that homogenize to liquid, to vapor, or by critical behavior. The lack of cogenetic liquid- and vapor-rich fluid inclusions and phase equilibria constraints indicate that high-salinity fluids (< or = 57 wt % NaCl equiv) were generated directly by the crystallizing magma and were not a product of aqueous fluid immiscibility.