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第一篇考了说金矿的形成，一共就一段，小半屏
说africa某个地方有大量的gold，科学家要研究它如何形成，来更好的引导发掘队寻找金矿
有两种theory，一种说是水里的金子沉淀出来的，一种说是和土里的iron反应产生的。
第二种要是对的话金矿里一定有iron，而iron很好探测（有题
第一种有个问题就是科学家觉得这个地方金矿储量太大了水里没有那么多金子可以沉淀（有题
主旨题选discuss a science controversy....对就四个字，其他选项看起来对但细读都有问题
https://www.newscientist.com/article/mg14619734-300-after-the-gold-rush/
是原文吗？
THE gold in your ring or your filling or your watch could well be South African. Nearly 40 per cent of all gold ever mined originated there, enough to make a golden sphere 16.6 metres high and worth around $5 billion billion. It all comes from a relatively small province known as the Witwatersrand (see Map). Nowhere else has so much gold in one place, and only a handful of similar gold fields have been found elsewhere in the world despite decades of searching. Extraordinary processes must have combined to form it but people are still wrangling about the nature of those processes despite a hundred years of research.
It’s not that the gold is all gone, but at almost 4 kilometres below the surface, many mines are approaching unsustainable depths. Now mining companies are looking to researchers to help them find shallower and more accessible ores. The problem is that geologists have split into two different, often acrimonious, camps with two rival theories that predict that new, shallower gold fields will be found in completely different places. In the absence of a consensus, mining companies are being forced to try out both predictions, or use more methodical – but expensive and time-consuming – search strategies.
There are some things that everybody agrees about. The Witwatersrand (known as the Wits) gold fields formed around 3 billion years ago when the Earth was very young, in a region about the size of Ireland that lay between high mountain ranges and an ancient sea. Wind and rain eroded the rising mountains, shedding debris into fast-flowing rivers. As they reached flatter lands, the rivers slowed and formed braid plains – places where many waterways joined and parted – and dumped sand and gravel in the channels. The land sank as a result of tectonic activity, and the amount by which it sank matched the rate at which sand and pebbles were deposited until a 7-kilometre thick layer of pebbles, gravel and sand had accumulated.
讲的是河床有啥pebble好像，然后iron不是铜和银
But the devil is in the details of where exactly the gold came in. Enter the rival theories. According to the “placer” theory, the gold was deposited at the same time as the pebbles. The idea is that among the debris from the erosion were “placers” – particles of sand, gravel and gold containing gold that were also carried to the Wits valley by the ancient rivers (see Diagram). The “epigenetic” theory, however, has the gold arriving several hundred million years later, when hot water containing dissolved gold squeezed its way through pores deep in the rock, reacted chemically with the iron and carbon in the pebble beds and consequently dumped its gold there (see Diagram). If the placer theory is right, new gold should be found in all pebble beds in the region. But if the epigeneticists are right, there is no point in looking at any pebble beds that do not contain plenty of iron, and gold might even be found among other ironbearing rocks near the pebbles.
Some 2.2 billion years ago, says the epigenetic theory, hot waters loaded with gold-sulphur complexes, arsenic, cobalt, nickel and sulphur invaded the Wits basin deep underground, moving through pores and cracks across an area bigger than Ireland. When these waters reached the river bed sediments, they began to react with the iron minerals, releasing dissolved gold and forming pyrite from the iron and sulphur.
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发表于 2018-9-28 21:38:07
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