Connect VLF antenna to raspberryPi
If you are seeking to receive AM modulated broadcasts, you can make the situation better by adding a "One Chip AM Radio circuit" using something like the TA642 between your antenna and the MPC3008. Without a receiver circuit, you will have to do the demodulating within the Raspberry Pi software.

The Raspberry Pi SPI can clock at up to 20 Mhz as listed on Pi exchange
  However, the Raspien operating system running in background might need to be modified so as to not let the operating system consume too much of CPU time.

If not using a receiver (do your own software demodulating) then some signal conditioning will be still be needed (pre-amp) between the antenna (and capacitor tuning) and MPC3008. Depending upon signal strength (as questioned by Andy Aka in comments) the signal from antenna (capacitor) tuned circuit is likely to be in the realm of micro-volts (uV). Perhaps 600 uV or even 4000 uV (strong station). This small ( AC ) signal would have low resolution in a 3.3v or 5v ADC.