some scattered thoughts and notes.
gold
start with magma as homogenous as you want, as it cools, minerals start to separate out in Bowen's reaction series. Gold and Silver do not have much affinity for entering these minerals, especially gold. [why?] the early minerals are dense and tend to settle in the cooling melt. as more minerals form, the melt that is left becomes more and more volatile and silicate rich, i.e. water, sulfur, chlorine, and of course in gold and silver (copper). eventually this fluid can get forced through cracks in the host rock and the gold and silver are dissolved in it. as it cools in the cracks minerals like quartz crystalize out and the gold and silver (always an amalgam to varying proportions) come out of solution too.
I wonder if successive waves of water rich fluids can come through these cracks and either deposit more gold or dissolve the gold that is there and redeposit it, perhaps there are patterns that concentrate the gold further.
so gold ends up being concentrated in tight places and not finely mixed through all the rock.
as the host rock is lifted and weathered away, the gold does not weather or dissolve and since it is dense, gets concentrated further in placer deposits by streams.
so... is there an element as rare as gold that DOES mix higgledy piggledy with all the rock?
In general, the 96 elements on earth do not mix higgledy piggledy, homogenously, because, atoms are particular, responsive to each other, and mathematics gives a complex discrete but finite set of solutions to systems with constraints. you dont get a higgledy piggledy mix of regular polygons when you try to fit them together around a sphere, you only get 5 possible solutions.
ditto with atoms. they only fit together in a discrete number of ways. naively you might think that the number of ways to combine 2 dozen kinds of atoms into groups of 3 to 5 might be (ignoring geometry) something on the order of 24^3 to 24^5, which is 13,000 to 8million. tho why stop at 5? why can't all 24 kinds of atoms mix together? now we are talking about astronomical numbers of combinations! at any rate, we've found only about 4000 kinds of minerals so far! so mathematics and physics limits the number of solutions.
after that there are other properties of minerals to consider: solubility, density, melting point. these can further separate things, so that the 4000 minerals aren't all mixed with each other higgledy piggledy.
then there are dynamic processes, iterating rounds of processes can again separate minerals out and concentrate them. there is the basic processes of magmatic differentiation which ultimately creates continents of different mineral composition than ocean crust.
**
further note: in early formation of earth, accretion and radiogenic heating melts dense iron nickel, that sinks to center,
i suppose next up is olivines, in fact, as earth material begins to melt, olivines are still solid and dense and will also sink?
ok... i want melting points of minerals but u got eutectics and so melting points depend on what other minerals are also present, complicated! oh yeah also pressure.
anyway, other minerals tend to collect closer to surface
but hotter earth... will have even more convection... so that will add a dynamic.
so (Q) how will temperatures begim to evolve as a function of time AND depth? there will be feedbacks, as differentitation occurs, radioactives will collect in mantle and crust, and even if hotter at depth... more pressure, so melting points will rise...
what if we ignore accretion heating, start with homogenous sphere. radioactive heating begins. cools fastest at surface, so heat accumulates towards center,
In addition, some heavy metals such as gold,
lead, and uranium, which have low melting points or
were highly soluble in the ascending molten masses, were
scavenged from Earth’s interior and concentrated in the
developing crust. This early period of chemical differ-
NOTES
gold alwys found mixed with silver. completely solid solution
notes from earth maerials: min and petr
chapt 16 economic mimerals fr veins and pegmatites
pg 1450
from geo text:
magmatic differentiation:
basalt cools: heavy minerals settle to bottom. some metals: chromite, with other metals in Montana's stillwater complex or Bushveld complex in south africa with 70% worlds platinum.
Pegmatites:
granitic, dissolves heavy metals
water and volatiles do not crystalize with bulk
left for last stages
these stages fluid rich, ion migration enhanced, grows BIG crystals. cms to meters. pegmatites
feldspar xtals as big as houses, muscovites meters across, spodumene in the black hills as large as telephone poles!
also contain rare elements (Q WHY?) lithium, cesium, uranium, rare earths, beryl topas, tourmaline.
most pegs occur in large ig masses or dikes veins cut into host rock
not always granite. Kiruna Sweden has magma with 60% magnetite solidified. largest iron deposit
HYDROTHERMAL DEPOSITS
chemically alter host rock so called hydrothermal metamorphism, chapter 8
late stages of magmatic processs at top of magma chambers, leaves fluids, metals dissolve in it, and volatiles like sulfur, it moves thru cracks and as it cools precipitates sulfides, quartz, and native metals.
can produce copper deposits like michigan
Dissemination Deposits
not concentrated in veins dikes, distributed as minute masses thruout entire rock. WHAT MAKES DIFFERENCE? much of worlds copper is foudd like this. very UNCONCENTRATED. bad mines. WHY COPPER DO THIS AND GOLD NOT?
HYDROTHERMAL METAMORPHISM
229 284
low pressure variety from groundwater seeping down and being heated and dissolving minerals as it goes.
high pressure variety deeper is from fluids from late stages of magmatic cooling, at tops of batholiths it seeps thru porous host rock or cracks.
third enviornment is ocean floor near mid ocean ridges. fresh hot magma below as it cools forms a convection system of ocean water seeping into cracks and flowing down, heating, and being expelled when hot, bringing more cool water in. these hot fluids
serpentinize the mafic minerals in the crust, releasing H2 and oxidized Fe, Mg ions, Leaves serpentinite and soapstone.
and also dissolves metals such as iron, cobalt, nickel silver copper gold! as the fluids rise to the ocean, they cool and precipitate black smokers depositing metal rich sulfides and carbonates.
OK: how localized are these fields? what is their fate? as the ocean floor spreads... FIRST OF ALL, THEY GET BURRIED UNDER SEDIMENT OVER 10S OF MILLIONS OF YEARS, 100S. THEN THEY GET SUBDUCTED!
geologists think the copper deposits in cyprus come from these. LOOK IT UP.
MELTING
lets explore this melting thing. if you have a solid mineral assemblage, n minerals, they will share faces in pairs, edges in pairs or triples (or more i guess) and vertices with triples or quadrupals etc... i suppose the more minerals mixed the lower the eutectic temp? so as you raise the temp... the vertices will melt first! that's VERY little volume. then each different edge will begin melting depending on the combination of minerals i.e. if there are 3 at once.. last to melt will be faces. but not really. depends on teh eutictic temps of each combination we have. some double eutectics might be lower than some triples?
at any rate if it's slow enough... the crystals will slowly melt and settle as the melt rises. as we reach the eutectic temp, the melt
will be at eutectic composition and temp will not rise, as heat goes into melting, untill the entire fluid is at the eutectic composition. if heating stops here, the magma will be at that composition and rise to cool somewhere.
so most igneus rocks are similar compositions because of eutectic points!
differing compositions will come from higher temperature melts!
melting points
0 water
100 water boils
112 sulfur
444 sulfur boils
800 min temp of SiO2/H2O melting high pressure
1500 various forms of SiO2
1700 various forms of SiO2
3000 graphite/diamond
4000 graphite/diamond
